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PT 546.722
1
Syntheses and Characterisation of Polychalcogenides
by
JOHN MICHAEL CUSICK
A thesis submitted as a
partial requirement for the degree of
Doctor of Philosophy.
School of Chemistry
University of New South Wales
1993
I
UNIVERSITY OF N.G.I:J.
LIBRIU:llf=:S
Certificate of Originality
I hereby declare that this submission is my
own work and that, to the best of my
knowledge and belief, it contains no material
previously published or written by another
person nor material which to a substantial
extent has been accepted for the award of any
other other degree or diploma of a university
or institute of higher learning, except where
due acknowledgement is made in the text.
(/ John Michael Cusick
II
ill
ACKNOWLEDGEMENTS
I would like to express my gratitude to Professor Ian G. Dance for his guidance and
support throughout this work and to Dr. James Hook for his expertise, encouragement
and friendship during the good and bad times. Thanks are extended to my fellow
students, technical and research staff who have provided assistance and friendship. I
would like to thank Sally Radford for her patience and support and to all 'the boys' for
their timely diversions when they were required most.
Finally, I would like to thank my family for their continual encouragement, support
and financial assistance which is greatly appreciated.
N
ABSTRACT
This thesis describes the first systematic approach to the synthesis and
characterisation of polyselenide (Sex2-) and polytelluride (Tei-) ligands and their metal
complexation. The key features of this approach include the unprecedented use of 77Se,
I25Te and metal multinuclear magnetic resonance to unambiguously characterise
uncoordinated and coordinated Sex2- and Tex2- ions in Dl'v1F solution. The consequent
development of two logical, reproducible and inexpensive syntheses for metal
polyselenide complexes has led to the isolation of numerous, and unique compounds
including; (Ph4Ph[M(Se4h] (M = Zn, Cd, Hg, Pb, Pd, Mn, Ni), (Ph4Ph[M(Se4h] (M
= Sn, Pt), (Ph4Ph[M4(Se4h(Ses)] (M = Cu, Ag), (Ph4P)3[Co3(Se4)6] and
(Ph4P)[CpMo(Se4)i). These novel metal polyselenide complexes have been
comprehensively characterised by single crystal X-ray diffraction determination and
multinuclear NMR.
Other reactions between Sex2- and white phosphorus have resulted in the isolation
of the (Bu4Nh[P2Seg] molecule. (Bu4Nh[P2Seg] has been characterised by 3Ip and
77Se NMR as well as by single crystal X- ray diffraction determination and has
significant potential in generating electrochemically active compounds with semi
conductor properties.
TABLE OF. CONTENTS
Title
Certificate of Originality
Acknowledgements
Abstract
Table of Contents
Abbreviations
List of Tables in Thesis
List of Figures in Thesis
Chapter One: INTRODUCTION
1. 0. Overview
1.1. Objectives
1. 2. Structure and organisation of the thesis
1. 3. Formation and characterisation of uncoordinated
polysulfides
1. 4. Formation and characterisation of transition and post-
transition metal polysulfides
1.4 .1. Metal polysulfide syntheses
1 .4 .2. General routes for the syntheses of polysulfide complexes
1. 5. Uncoordinated 'free-chain' polyselenides
1. 6. General routes for the syntheses of metal polyselenide
complexes
v
Page
I
II
III
N
v Xlli
XV
XVII
1
6
7
9
14
16
17
21
26
1. 7. Synthetic routes to polyselenide complexes of transition
and post-transition metals
1. 8. Formation and characterisation of uncoordinated
polytellurides
1. 9. Transition metal polytellurides
1.9 .1. Synthetic routes to metal polytellurides
1.10. Thermodynamic influence of solvent on the activities of
anions
1.11. Conclusion
1.12. References
Chapter Two: BACKGROUND ON MULTINUCLEAR MAGNETIC
RESONANCE
2.1. Introduction to 32S, 77Se and 123/125 Te NMR
2.2. Chemical shifts
2.2.1. Chemical shift patterns
2.3. Relaxation behaviour
2.4. Selenium coupling constants
2.4.1. nJ(l7Se _lH)
2.4.2. nJ(l7Se- 77Se) and nJ(l25Te- 125Te)
2.4.3. nJ(l7Se _ 31p)
2.5. "J (125Te - X)
2.6. Other relevant polyselenides
2.6.1. Organic polyselenides (R-Sex-RJ
2.6.2. Selenium sulfide ring molecules SexSes-x
2.7. Other nuclei
31
34
35
38
39
41
42
55
57
60
63
64
65
65
67
67
68
68
69
70
VI
2. 8. References
Chapter Three: EXPERIMENTAL TECHNIQUES
3. 0. Introduction
3 .1. Preparative experimental techniques
3. 2. General procedures
3.2 .1. Deoxygenation and the addition of solvents and reactants
3.2 .2. Solvent removal
3 .2 .3. Filtration
3.2 .4. Heating, cooling and dissolving
3.2 .5. Purification of solvents and reagents
3. 3. Crystallisation Techniques
3.3 .1. Solvent layering
3.3 .2. U-tube diffusion
3. 4. Instruments used for identification of products
3.4 .1. Single-crystal X -ray diffractometry
3 .4 .2 Elemental analysis
3.4 .3. Inductively coupled plasma (I CP) analysis
3.4 .4. Infra-red analysis
3.4 .5. Multinuclear magnetic resonance spectroscopy
3. 5. References
4.1
Chapter Four: SYNTHESIS AND CHARACTERISATION OF
UNCOORDINATED POLYSELENIDE, [Sex]2-, IONS
Introduction
74
80
80
82
82
83
84
85
85
86
86
87
88
88
88
88
89
89
92
93
vn
4. 2. Syntheses and characterisation of uncoordinated
polyselenides
4.2 .1. Synthetic approaches
4.2 .2. Nuclear magnetic resonance studies
4.2.2.1 Monoselenide anion, Se2-, and hydroselenide anion, HSe-
42.2.2. Diselenide anion [Se2]2-
4.2.2.3. Na2Sex in solution
4.3. (Ph4Ph[Ses] and (Bu4Nh[Se6]
4.4. [Se(Se5) 2] 2 -
4. 5. Discussion
4. 6. Preparation of starting materials
4. 7. Conclusion
4. 8. References
Chapter Five: METAL COMPLEXES OF POLYSELENIDES
5.0. Introduction
5.1. Expectations
5.2 Results
5.2.1. 77Se and metal NMR studies of reactions between Cd2+, Hg2+ and
Sn2+ and Na2Sex (x = 3-6) and (Bu4NhSe6
5.2.2. Cadmium polyselenide solutions- Cd(Sem)n
5.2.3. Mercury polyselenide solutions- Hg(Sem)n
5.2.4. Tin polyselenide solutions- Sn(Sem)n
5.2.5. Overview of results
5.2.6. Crystallisation experiments of polyselenide solutions prepared from
the 'NHJ' method containing Cd2+, Hg2+, or Sn2+ ions
vm
94
94
97
97
100
101
105
106
109
115
123
124
127
130
131
133
133
137
139
142
144
IX
5.2.7. Crystallisation experiments ofpolyselenide solutions prepared/rom the
'DMF' method containing Ccf2+, Hg2+, and Sn2+ ions 145
5.2.8. Cry~tal structures of(Ph4Ph!Cd(Se4hl, (Ph4Ph[Hg(Se4)2l and
(Ph4P)2[Sn(Se4)Jl 146
5.2.9. NMR studies of redissolved (Ph4Ph[Cd(Se4hl, (Ph4P)2[Hg(Se4hl
and (Ph4P}2[Sn(Se4)3] 147
5.2 .1 0. Crystallisation of related complexes by other workers 148
5. 3. Crystallisation experiments of polyselenide complexes
containing other metal ions using both the 'NH3 ' and
'D~F' procedures 150
5.3.1. Synthesis and characterisation o.f(Ph4Ph!Zn(Se4hl 150
5.3 .2. Crystal structure of ( P h4P h!Zn( S e4hl 151
5.3 .3. Syntheses of related compounds 151
5. 4. Syntheses and characterisation of (Ph4P)z[Ni(Se4)z] 151
5.4 .1. Crystal structure of (Ph4Ph!Ni(Se4hl 152
5.4.2. Syntheses of related compounds 152
5.5. Syntheses of (Ph4P)z[Pb(Se4 )z] 153
5.5 .1. Crystal structure of(Ph4Ph!Pb(Se4hl 154
5. 6. Synthesis and characterisation of (Ph4P)z[~n(Se4)z] 155
5. 7. Synthesis and characterisation of (Ph4P)z[Pd(Se4)z] 156
5. 7 .1. Syntheses of related compounds
5. 8. Synthesis and characterisation of (Ph4P)z[Pt(Se4)z]
5.8.1. Crystal structure of(Ph4Ph/Pt(Se4hl
5.8 .2. Syntheses of related compounds
5. 9. Synthesis and characterisation of
(Ph4P}J[Co3(Se4)6]. (D ~F) 2
156
157
157
158
158
5.9.1. Crystal structure of(Ph4P)J[Co3(Se4)6l.(DMFh 159
5.10. Synthesis and characterisation of (Ph4P)[CpMo(Se4hl 160
5 .11. Synthesis and characterisation of (Ph4P)z[Cu4(Se4)z(Se5)]
and (Ph4P)z[Ag4 (Se4)z(Ses)]
5 .11.1. Crystal structures
5 .11.2. Related molecules
5.12 Discussion
5.12 .1. Vibrational spectra
5.12.2. NMR spectroscopy
5.12 .3. Redox chemistry in metal polyselenide solutions
5.12.4. Metalpolyselenide ring sizes
5.12 .5. Effect of counterion
5.14. Experimental detail
5.15. Conclusion
5 .16. References
Chapter Six: POLYTELLURIDES
6.0. Introduction
6.1. Expectations
6.2. Results
6.2.1. Te2- and HTe-
6.2.2. [Te2]2-
6.2.3. [Te3]2-
6.2.4. {Te)2- X =4, 5, 6
6.3. Metal complexation
6.3.1. Reaction between Na2Te3 and CdC/2.512 H20
162
163
164
165
165
165
172
174
175
179
189
190
195
196
197
197
198
199
200
201
201
X
6.3.2. Reaction between Na2Te4 and CdC/2.512 H20
6.3.3. Reaction betweenNa2Tes and CdC/2.512 H20
6.3 .4. Reaction betweenNa2Te6 and CdC/2.512 H20
6.4. Reaction between Na2Te6 and ZnBr2
6.5. Interpretation
6.6. Discussion
6.7. Experimental preparations of solid compounds
6.8. Preparation of solutions for 125Te NMR
6.9. Conclusion
6.10. References
Chapter Seven: PHOSPHORUS CHALCOGENIDES
7. 0. Introduction
7 .1. Background - Phosphorus-sulfur and phosphorus selenium
cages and rings
7.1.1 Phosphorus-sulfur cage compounds
7.1.1.1. P4S2
7.1.1.2. P4S3
7.1.1.3. P4S4
7.1.1.4. P4S5
7.1.1.5. P4S7
7.1.1.6. p 459
7.1.1.7. P4S10
7. 2. Phosphorus-selenium cage compounds P 4Se0
7.2 .1. P2Se5
XI
202
202
203
203
203
204
209
210
212
213
216
217
218
218
218
219
219
220
220
221
222
223
7. 3. Cyclodiphosphadithianes and related ring compounds 224
7.3.1. Other phosphorus-sulfur ring compounds
7.4. Results
7.4.1. Solution studies
7.4.2. Crystallographic studies
7.4.3. NMR spectroscopy of (Bu4N)2[P2Sesl
7.5. Discussion
7.6. Experimental
7.7. Conclusion
7.8. Future work
7.9. References
Appendix A: Crystallographic Details of Structures
Appendix B: X-ray Powder Diffraction Pattern of
(Ph4Ph[Mn(Se4hl
Appendix C: List of Publications
XII
224
227
227
233
233
235
238
241
241
242
245
259
260
XIII
ABBREVIATIONS
0
Angstom A
Bu Butyl
ca. Circa
Cp Cyclopentadiene
D De utero-() Chemical shift
DMF N, N-Dimethylformamide
DMSO Dimethylsulfoxide
E Chalcogenide
Ex Polychalcogenide
en Ethylenediaminne
Et Ethyl
EtOH Ethanol
FID Free induction decay
FT Fourier transform
hr Hour
HDS Hydrodesulfurisation
Hz Hertz
IR Infra-red
J Coupling constant
K Kelvin
L Ligand
LB Line broadening
M Metal
m Medium
Me Methyl
MeCN Acetonin·ile
MeOH Methanol
min Minute
NMR Nuclear magnetic resonance
Ph Phenyl
R Organic group
RT Room temperature
s Strong
XIV
s Second
Solv Solvent
Sex General formula for polyselenides
Sx General formula for polysulfides
THF Tetrahydrofuran
Tex General formula for polytellurides
uv Ultra-violet
v Very
w Weak
X Halide
xan xanthate
XRD X-ray diffraction
LIST OF TABLES IN THESIS.
Chapter One.
Table 1.1. Typical reductant I solvent systems used for the reduction of Sg in the
syntheses of poly sulfide solutions.
Table 1.2. Crystalline salts ofpolysulfide ions.
Table 1.3. Monometallic homoleptic polysulfides and their syntheses.
Table 1.4. Crystalline salts of polyselenide ions.
Table 1.5 A chronological table of anionic transition and post-transition metal
selenides and related species.
Table 1.6 Crystalline salts of polytelluride ions.
Table 1.7. Anionic transition metal polytellurides.
Chapter Two.
Table 2.1. NMR properties of magnetic isotopes of sulfur, selenium and tellurium.
Table 2.2. Chemical shifts and coupling constants for seleniumand tellurium
compounds referenced to Me2Se and Me2 Te respectively.
Table 2.3. Values ofnJ(17Se-lH).
Table 2.4. Selected one-bond couplings between 125Te and various nuclei.
Table 2.5. 77Se chemical shifts (ppm) of dialkyl polyselenides R-Sex-R.
Table 2.6. 77Se chemical shifts (ppm) and 77Se-77Se coupling constants (Hz) of
SexSs-x ring molecules.
Table 2. 7. Salient nuclear properties of various nuclei.
Chapter Three.
Table 3.1. Solvents and their purification procedures.
Table 3.2. Nuclei table for multi-nuclear NMR.
XV
Table 3.3. T 1 values for some polychalcogenide species.
Chapter Four.
Table 4.1. Synthetic approaches to the reduction of elemental selenium.
Table 4.2. 77Se NMR data for solutions containing Se1 species.
Table 4.3. Solvent effects on small anions.
Table 4.4. Chemical shifts at 220K for [Sex]2- ions in DMF plus 38% (vol)
EtOH.
Table 4.5. 77Se chemical shift data for species XSe- and XSeH.
Chapter Five.
Table 5.1. Formation and NMR of cadmium polyselenide solutions of nominal
composition [Cd(Sem)n] in DMF.
Table 5.2. Formation and NMR of mercury polyselenide solutions of nominal
composition [Hg(Sem)nJ in DMF.
Table 5.3. Formation and NMR of tin polyselenide solutions of nominal
composition [Sn(Sem)nl in DMF.
Table 5.4. Torsional angles (0 ) for theMSe4 rings in [CpMo(Se4h]-.
Table 5.5. 77Se NMR chemical shift and coupling constants of metal
polyselenides in DMF at 298K.
Table 5.6. Comparison of the Se-Se bond distances in MSe4 containing
compounds.
Chapter Six.
Table 6.1. 125Te chemical shift data for species Tei- and TeH-.
Table 6.2. Preparative details for the syntheses of solutions of Na2 Tex (x = 2-6).
XVI
Table 6.3. Investigations of cadmium polytelluride solutions of nominal
composition [Cd(Tem)nl·
Chapter Seven.
Table 7.1. Selected one-bond couplings in Hz of selenium-phosphorus
compounds.
LIST OF FIGURES IN THESIS
Chapter One.
Figure l.la. Structure of the [Fe2Se2(Seshl2- ion.
Figure l.lb. Structure of the [W2Se4(Se2)(Se3)]2- ion.
Figure l.lc. Structure of the [W2Se4(Se3h)]2- anion.
Figure l.ld. Structure of [W2Se4(Se2)(Se4)]2- anion.
Figure l.le. Structure of [V2(Se2)4(Se5)]2- anion.
Figure l.lf. Structure of [NbTe10]2- anion.
Figure 1.2a. Structure of FePS3 viewed perpendicular to the layers.
Figure 1.2b. Structure ofFePS3 viewed approximately parallel to the layers.
Figure 1.3a. Structure of the [Ag(Sg)]- anion.
Figure 1.3b. Structure of the [(S7hBiS6Bi(S7)]4- anion.
Figure 1.3c. Structure of the [Ag2(S6h]2- anion.
Figure 1.3d. Structure of the [Pd2(S7)4]4- anion.
Figure 1.3e. Structure of the [Cu4(S5)3]2- anion.
Figure 1.3f. Structure of the [Cu6(Ss)(S4)3]2- anion.
Figure 1.4a. Helical chain structure of [Se6]2-.
Figure 1.4b. Structure of the [Se10]2- anion.
xvn
Figure 1.4c. Structure of the [Se(Sesh]2- anion.
Figure 1.4d. Structure of the [Se16]2- anion.
Figure 1.5a. Structure of the [H~ Te12]4- anion.
Figure 1.5b. Two viewsof the I'O[Hg2 Tes]2- anion.
Figure 1.5c. Structure of the [(C0)4Cr(Te4)]2- anion.
Figure 1.6. Relative thermodynamic activities of various anions in water,
acetonitrile and DMF relative to methanol.
Chapter Two
Figure 2.1. Pattern of 77Se chemical shifts.
Figure 2.2. Pattern of 125Te chemical shifts.
Figure 2.3. SexS8-x (x = 1-7) ring molecules where Se atoms are denoted by
closed circles, and S atoms by open circles.
Chapter Three
Figure 3.1.
Figure 3.2.
Vacuum I dinitrogen manifold.
Filtering techniques.
Figure 3.3a. The liquid diffusion method of growing crystals in an inert
atmosphere.
Figure 3.3b. U - Tube diffusion method for growing crystals.
Figure 3.4. T1 measurement ofNaHSe in EtOH at 300K.
Chapter Four
Figure 4.1. 77Se NMR of solutions of NaHSe (ca. 0.63M) in (a) H20, (b)
ethanol, (c) DMF, at 300K.
Figure 4.2. 77Se NMR spectra of "Na2Se5" in DMF at the temperatures marked.
Figure 4.3. Diagram of the 77Se NMR resonance positions and intensities for
XVIII
solutions of nominal compositions Na2Sex in DMF at 230K.
Figure 4.4. ·Diagram of the 77Se NMR resonance positions (at 230K) for a
solution containing a mixture of [Se3]2- and [Se4]2-.
Figure 4.5. Diagram of the 77Se NMR resonance positions (at 230K) for a
solution containing a mixture of [Se5]2- and [Se6J2-.
Figure 4.6. Structure of the [Ses]2- anion.
Figure 4.7. 77Se NMR spectrum of (Ph4PhSes in DMF at 220K.
Figure 4.8. Structure of the [Se(Se5h]2- anion.
Figure 4.9. 77Se NMR spectrum of (Ph4PhSe(Sesh in DMF at 210K.
Figure 4.10. Comparative chart of the 77Se chemical shifts in four classes of
polyselenium chains: (a) [Sex]2-, (b) Octn-sex-Octn, (c) M(Se4)2-
and (d) Ss-xSex.
Chapter Five
Figure 5.1a. The 77Se NMR of (Ph4Ph[Cd(Se4hJ in DMF at 300K.
Figure 5.1b. The 113Cd NMR of (Ph4Ph[Cd(Se4h] in DMF at 300K.
Figure 5.2a. The 77Se NMR of (Ph4Ph[Hg(Se4hJ in DMF at 300K.
Figure 5.2b. The 199Hg NMR of (Ph4Ph[Hg(Se4h] in DMF at 300K.
Figure 5.3a. The 77Se NMR of (Ph4Ph[Sn(Se4)3] in DMF at 300K.
Figure 5.3b. The 119Sn NMR of (Ph4Ph[Sn(Se4h] in DMF ~t 300K.
Figure 5.4. Schematic representation of the compositional range for the reactions
containing [M(Sem)n] (M = Cd2+, Hg2+, Sn2+) in DMF at ambient
temperature.
Figure 5.5. The two independent [Cd(Se4h]2- complexes in (Ph4Ph[Cd(Se4h].
Figure 5.6. ORTEP representation and labelling scheme of the structure of the
[Sn(Se4h]2- anion.
Figure 5.7. The 77Se NMR of (Ph4Ph[Zn(Se4h] in DMF at 300K.
XIX
Figure 5.8. The 77Se NMR of (Ph4Ph[Ni(Se4h] in DMF at 300K.
Figure 5.9. Structure of the [Ni(Se4h]2- anion.
Figure 5.10. (a) The structure of the [Pb(Se4h]2- anion and (b) with the principal
bond lengths and angles.
Figure 5.11. The 77Se NMR of (Ph4Ph[Pd(Se4h] in DMF at 300K.
Figure 5.12. The 77Se NMR of (Ph4Ph[Pt(Se4)3] in DMF at 300K.
Figure 5.13. The structure of the [Pt(Se4)3]2- anion showing the disorder present
in the crystal.
Figure 5.14. The molecular structure of [Co3(Se4)6]3-. Atoms are labelled for
one of the two independent centrosymmetric molecules in the crystal.
Figure 5.15. The 77Se NMR of (Ph4P)[CpMo(Se4h] in DMF at 298K.
Figure 5.16. The molecular structure of [CpMo(Se4h]-.
Figure 5.17. The 77Se NM;R of (Ph4Ph[Cu4(Se4h(Ses)] in DMF at 298K.
Figure 5.18. The molecular structure of [Cu4(Se4h(Ses)]2-.
Figure 5.19a. The structure of [M3(Sx)3]3-.
Figure 5.19b. Diagrammatic representations of atom labels and approximate
conformations for (a) the Se5 and (b) the Se4 rings which occur in
(Ph4Ph[Ag4(Se4h(Ses)] additional to those shown for
(Ph4Ph[Cu4(Se4h(Ses)] in Figure 5.18.
Figure 5.20. (A) Stable assembly of mutually screened Ph4P+ cations and
[Pd(Se4h]2- anions. (B) Substitution of large Ph4P+ forK+ results
Chapter Six
in short anion-anion contacts developing destabilising repulsive
interactions. (C) A stable assembly is possible by converting
chelated Sei-ligands to bridging.
Figure 6.1. 125Te NMR of 'NaHTe' in H2o at 300K.
XX
Figure 6.2. 125Te NMR ofNa2Te3 in DMF at 220K.
Figure 6.3. Diagram of the 125Te resonance positions (at 220 K) for [Te3]2- in
DMF/ ethanol in the proportions marked.
Figure 6.4. . Diagram of the 125Te NMR positions and intensities for solutions of
nominal compositions [M(TexhJ2- (M = Cd, Zn) in DMF at 260K.
Figure 6.5. Diagram of the 113Cd NMR positions and intensities for solutions of
nominal compositions [Cd(TexhJ2- in DMF at 260K.
Figure 6.6a. The 125Te NMR spectrum of Cd2+ + "Te32-" in a 1:2 ratio in DMF
at260K.
Figure 6.6b. The 113Cd spectrum of Cd2+ + "Te32-" in DMF at 260K.
Figure 6.7. 125Te NMR spectra of Cct2+ + "Te42-" in a 1:2 ratio in the
temperatures marked.
Chapter Seven
Figure 7 .1. Observations for the reactions between polyselenides and P 4·
Figure 7.2. 31p NMR of reactions between fSex]2- and P4 at 300K.
Figure 7.3a. 31p NMR spectrum ofNa2Se3 + P4 (1:1) in DMF at 300K.
Figure 7.3b. 3lp NMR spectrum ofNa2Se5 + P4 (1:1) in DMF at 300K.
Figure 7 .4a. 31 P NMR spectrum of (B u4NhSe6 + P 4 ( 1: 1) in DMF at 300K.
Figure 7.4b. 3lp NMR spectrum of (Bu4NhSe6 + P4 (1:2) in DMF at 300K.
Figure 7.5 Crystal structure of (Bu4NhP2Ses.CS2.
Figure 7.6a. 31p NMR of (Bu4NhP2Seg.CS2.in DMF at 300K.
Figure 7.6a. 77Se NMR of (Bu4NhP2Se8.CS2.in DMF at 300K.
Figure 7.7. Possible locations of the spin active 77Se atom in the P2Se82-
molecule.
XXI
"Determination can get you a long way -
but only if it used in the right direction."
-Catherine Elizabeth Cusick-
1. 0 Overview.
CHAPTER 1
Introduction
Prior to the commencement of this work in 1988, alkali metal
monochalcogenides, [A2EJ and polychalcogenides, [A2ExJ. (A= Na, K; E= S, Se,
Te), had been known for about 100 years. I The monochalcogenides formally contain
£2- anions, whilst the polychalcogenides contain Ex2- (x=2-6) ions, generally existing
as extended chains (referred to as catenation) in which there are E-E bonds. The
catenation ability of the chalcogenide ions is also observed in the elements themselves
with sulfur, selenium and tellurium forming rings and long chains in their elemental
forms.
Since 1988, extensive research by this author 2,3,4,5,6 and other workers 7,8,9 in
this area of chemistry has led to the characterisation of many other crystalline salts of
polychalcogenides. These 'uncoordinated' crystalline salts of polysulfide (Sx2-),
polyselenide (Sex2-), and polytelluride ( Tex2-), ions are listed in tables 1.2, 1.4, 1.6,
together with their preparative reactions and their characterisation data. Despite this large
amount of information about crystalline compounds, uncertainty surrounded knowledge
of these species and their interconversion in solution. This uncertainty occurred because
the electronic and vibrational spectra used to monitor the species in these solutions were
not definitive.lO
This thesis describes an investigation of the solution chemistry of the Sex2 and
Tex2 ions in dimethylformamide (DMF) solutions. The Nuclear Magnetic Resonance
(NMR) capability of 77Se (spin 1/2 natural abundance 7.6%) and 125Te (spin 1/2 nat. . abundance 7.0%) has provided the means for understanding the complex solution
chemistry of both polyselenides and polytellurides. This is contrary to recent statements
that 77Se NMR signals cannot be observed for 1Sex]2- in DMF solution because
paramagnetic monoanions are present. X
1
The NMR technique is unique in that it can probe individual Se and Te atoms. The
inherent advantages of using 77Se and 125Te NMR over existing vibrational and
electronic spectra are the wide chemical shift ranges available (ca. 3000 and 4000 ppm
respectively), allowing similiar molecules and atoms in molecules to be easily
distinguished. Secondly, the consequent dynamic range of lQ-2 to IQ-5 seconds enables
access to rapid interconversion reactions. Unfortunately the only sulfur isotope with a
nuclear spin 33S , has a quadrupolar nucleus with 1=3/2 making both measurement and
interpretation of spectra extremely difficult and beyond the scope of this thesis.
Homoleptic, anionic transition metal polysulfide complexes [M(Sx)w]z containing
only polysulfide chains as chelating ligands have also been known for about 85 years.ll
A homoleptic complex has only one type of ligand, for instance [Cd(S6)2]2- has S62-
ligands only. The coordination chemistry of these molecular transition metal polysulfides
has since been shown to be rich and versatile, due to the tendency of sulfur to adopt a
wide variety of coordination environments. As a result hundreds of novel structures
containing both transition and main group metals have been isolated and
characterised.l2,13,14,15 These molecular polysulfides are remarkable for their
extraordinary variable bonding characteristics and structural diversity. This stems not
only from the great propensity of chalcogens to bind to multiple centres simultaneously
but also from their ability to catenate. The general synthetic routes to molecular transition
metal polysulfides will be reviewed in section 1.6. Much of this chemistry has been
facilitated in the last two decades by the deployment of non-aqueous media. Non
aqueous media, particularly aprotic media, have been shown to increase the
thermodynamic activities of polysulfide ions in solution, thereby expanding the range of
complexes that could exist in solution. The use of aprotic solvents in the development of
polysulfide chemistry and subsequently that of polyselenides and polytellurides, has been
of fundamental significance and is presented in detail in section 1.1 0.
In contrast to the extensive polysulfide chemistry reportedl2,13,16, the
coordination chemistry of the polyselenides and polytellurides, had virtually been
2
ignored. In fact only five anionic metai polyselenides were known to exist in 1988,
(Ph4P)2[Fe2Se2(Ses)2]17, (Ph4P)2[W2Se4(Se2)(Se3)]l8,19, the two isomers of
(Ph4P)2[W2Se10], (Ph4P)2[W2Se4(Se2)(Se4)]l8,19 and (Ph4P)2[W2Se4(Se3)2]18,19,
and (NE4)2[V2(Se2)4(Ses)]20, with the three tungsten complexes being found in the
same crystal (Figures l.la-e). Only one anionic metal polytelluride,
(Ph4P)3[Nb(TeiQ)]21, was reported in the literature (Figure 1.1f).
Perhaps the notion that the chemistry of the heavier polychalcogenides would be
very similiar to that of the polysulfides was a reason for the lack of development of
synthetic procedures toward the formation of polyselenide and polytelluride solutions.
The most common traditional method of synthesising molecular metal
polysulfides22, was to prepare an aqueous polysulfide solution by combining H2S and
elemental sulfur in aqueous base and adding this to metal salts (typically halides),
(equation 1.1). More recently, the direct analogue of the above aqueous method using
nonaqueous solvents has been the predominant synthetic method to metal polysulfides.23,
24, 25, 26 The driving force for these two synthetic methods is the nucleophilicity of the
polysulfide toward the metal halide.
..1.1
An0ther less common method reacted metal oxyanions with H2S, forming a
particular metal sulfide e.g MoS42-, and then with Sg to form metal polysulfides.27
Whilst the use of gaseous H2S and NH3 in polysulfide preparations has led to
numerous numbers of polysulfide complexes being isolated, analogous preparations for
polyselenides and polytellurides have rarely been reported.28,29 The reason being that
both H2Se and H2 Te are far from ideal starting materials due to the stench, toxity and
cost of H2Se whilst H2Te is unstable above ooc and decomposes in moist air and on
exposure to light.30
3
Se(3)
S.(S)
Figure l.lb. Structure of the w2se4(Se2)(Se3)2- ion_l8,19
Se(4) Se(3}
Se(6C)
Se(3)
Se(10B)
Figure l.ld. Structure of [W2Se4(Se2)(Se4)]2- anion.J8,19
Se(12)
Sfl(3)
Figure l.le. Structure of the [V2(Se2)4(Se5)12- anion.20
Tef11
Figure l.lf. Structure of the [NbTe 10J3 anion.2l
This thesis describes two simple systematic procedures using readily available
chemicals, for the synthesis of transition metal and post transition metal
polychalcogenides. Typically, alkali metal polychalcogenides of the general formula
Na2Ex ( E = Se and Te, x =1-6) were prepared and reacted with a transition metal or post
transition metal precusor in the aprotic solvent DMF, forming soluble polychalcogenide
complexes. These reactions are driven by the enhanced nucleophilicity of the
polychalcogenide anion. The alkali metal cations can be readily metathesised with large
organic· counterions such as quarternary phosphonium or ammonium ions resulting in the
crystallisation of novel polychalcogenide complexes. The publication3,4,5 of these results
in 1989 preceded an increase in the number of researchers studying this area of chemistry
and consequently a vast number of polyselenide complexes and to a much lesser extent
polytelluride complexes are now known. A comprehensive list of these compounds are
found in Tables 1.5 and 1.7.
The nature of transition and post transition metal polychalcogenide solutions has
not been well understood. Cadmium, mercury and tin polysulfide solutions have only
recently31, 32,33 been shown to be complex equilibria of varying composition, although
this was believed8 (without direct evidence) to be the case for all polychalcogenide
solutions for some time. In many cases, the composition of a final crystalline product is
determined by the reaction conditions and the counterion present, and the composition of
the mother solution has no obvious effect. In any event the •nature of the heavier
polychalcogenide solutions had not been well investigated.
Therefore I have used multinuclear magnetic resonance (MNMR) spectroscopy to
probe solutions of cadmium, mercury and tin polyselenides and cadmium polytellurides,
as part of a systematic investigation of metal polychalcogenide species and their equilibria
in solution. The advantage of this approach is that along with selenium and tellurium, the
metals, cadmium, mercury and tin all have I = 1/2 nuclei, making the monitoring of these
polychalcogenide mixtures potentially more informative than the similiar studies used for
polysulfide complexes.
4
Many of the metal polysulfides previously synthesised have been subsequently
found to be of great interest due to the variety of anisotropic electrical, optical, and
magnetic properties they may possess.34 Polychalcogenide compounds in general are
useful materials. For example, polysulfide ligands have been implicated as reactive
species in the hydrodesulfurisation (HDS) of oil and are thought to be at the surfaces of
metal sulfide catalysts.35,36,37 Polychalcogenide glasses are important materials for non
linear optical, infrared wave guide, photoconductive, optical switching, and optical
information storage applications.38
The phosphorus derivatives of some metal chalcogenides have also been proven to
be useful materials. Metal phosphorus trichalcogenides of the formula MPS3 39 ( M=
Pb,V, Mn, Co, Ni, Pd, Zn, Cd, Fe, Sn) and MPSe3 (M=Ni, Fe, Mg, Mn, Cd, In, Sn),
are known to form a class of compounds that exibit a layered structure, similiar to that
found in many of the transition metal dichalcogenides including TiSz. The intercalation
chemistry of TiSz has been extensively studied because of its utility in secondary
batteries.40 Similiarly, it has been found that metal phosphorus chalcogenides readily
undergo intercalation reactions with organic amines, alkali metals and some
organometallic molecules generating electrochemically active compounds with semi
conductor properties.39 Interestingly, the interpretation of the structures of these useful
compounds has been by vi~ualising the molecules as salts of divalent metal cations of the
molecular hexathiohypodiphosphate anion PzS64-.39
Molecular hexathiohypodiphosphate anion PzS64-·
ss~~ ...... s-
I .. P--s s_,,, I
s-
5
Qs Q Fe
QP
Figure 1.2a. Structure of FePS3 viewed perpendicular to the layers.39
0. p
0 F.,
Qs
Figure 1.2b. Structure of FePS3 viewed approximately parallel to the layers.39
These layered metal phosphorus trichalcogenides (Figures 1.2a and 1.2b) have
generally been synthesised at high temperature, usually by reacting the stoichiometric
quantities of the elements (or metal sulfides ) in evacuated silica tubes or by vapour
sublimation in a temperature gradient. However more recent sulfide preparations have
used the reaction of M2+ salts with sodium hexathiohypodiphosphate (prepared from
Na2S and PC13 as descibed by Falius 41), in aqueous solution (equation 1.2).42
... 1.2
It was expected that anionic phosphorus selenide species, similiar to P2S62-,
should exist and have potential as precursors to novel metal phosphorus selenides. This
thesis describes an investigation of reactions between phosphorus and polyselenide
anions, aimed at uncovering new anionic phosphorus selenide species. Subsequent
reactions with metal compounds were expected to generate [My(PnSem)z]X- species and
are described.
1.1 Objectives
The research described in this thesis had the following major objectives:
1) The first objective was to investigate and develop the syntheses of the polyselenide
and polytelluride ions. The reaction type involved the reduction of chalcogen E (E = Se,
Te) first with limited reductant to polychalcogenide Ex2- and eventually complete
reduction to E2-.
E ~ Ep2- ~ Eq2- ~ Er2- ~ E2- (p>q>r)
Important considerations in these reactions were:
a) the solvent
b) the reductant
c) the cations present and thus the solubilities of the polychalcogenide salts
6
2) The second objective was to investigate the equilibria in solutions of these
polychalcogenides, by MNMR spectroscopy.
3) The third objective was to take the preparative and NMR information from work in
parts 1) and 2) and prepare metal complexes. It was expected that these reactions could
be monitored by 77Se, 125Te and metal NMR, providing unprecedented information on
the equilibria involved. Emphasis was placed on the selection of metals suitable for
NMR study, and consequently initial studies were based on spin 1/2 active metal nuclei
only.
4) The fourth objective involved the isolation and characterisation of species formed in
the above solutions. This included the development of procedures for crystal growth and
the generation of crystals suitable for X-ray diffraction.
5) The final objective of this thesis was to investigate systematically reactions between
polyselenides and phosphorus utilising 3lp and 77Se NMR. Here it was expected that
novel phosphorus /selenide species could be observed in solution, and then isolated.
These isolated species could then serve as precursors to potentially useful metal
phosphorus selenide species.
1. 2 Structure and organisation of the thesis.
This thesis is divided into eight chapters as described below.
Chapter one presents:
i) tabulated information about the uncoordinated crystalline salts of the polysulfides,
polyselenides and polytellurides
ii) a comprehensive history of the syntheses of uncoordinated polychalcogenide solutions
and the methods used to characterise them
7
iii) the general synthetic routes presently known for metal polysulfides, polyselenides and
polytellurides, with comprehensive listings of the last two
iv) the thermodynamic influence of solvent on the activities of anions- the significance of
the use of aprotic solvents in polychalcogenide syntheses.
Chapter two presents the literature background on multinuclear magnetic resonance
as it is used extensively in this thesis. The chapter includes the NMR properties of the
magnetic isotopes of sulfur, selenium and tellurium and relevant metals and lists relevant
literature compounds and details their chemical shifts and coupling constants.
Chapter three deals with the general experimental techniques used in this thesis.
This includes a description of the Schlenk manifold, preparation of solid samples,
purification of solvents, deoxygenation techniques, filtration techniques, instrumentation
used, sample preparation for NMR studies, and crystallisation techniques used.
Chapter four presents the experimental details for the synthesis and characterisation
of uncoordinated polyselenides.
Chapter five presents the experimental details for the synthesis and characterisation
of metal polyselenides and their solutions.
Chapter six presents the experimental details of the syntheses, characterisation, and
reactions of uncoordinated polytellurides.
Chapter seven presents a comprehensive literature background on phosphorus -
selenium compounds. The chapter also presents the specific experimental details of this
work relating to the reactions between polyselenides and phosphorus.
Chapter eight provides an overall summary of this thesis, discussing the results of
this work relative to the initial objectives. Chapter eight also provides an insight into
future work in this area of research.
8
1. 3 Formation and characterisation of uncoordinated polysulfides
Solutions ofpolysulfides of sodium and potassium in water and in ethanol have
been known and extensively studied for over 85 years. 43,44,45 Since this time many
other preparative routes have been used for the formation of polysulfide solutions
using various reductants and solvents and are summarised in Table 1.1.
The compounds with the fomulae from Na2S to Na2S s and K2S to K2S6
inclusive were claimed to exist as solids using thermal analysis as early as 1905,
whilst Bergstrom46 first generated the alkali polysulfides in solution in liquid
ammonia. However the first spectrophotometric characterisation of alkali polysulfides
in solution in liquid ammonia was not presented until 1966 by Nelson. Nelson
claimed that the absorption bands of a given polysulfide do not depend upon the alkali
cation in solution and that the S22- and S4 2- are the only polysulfides present in
solution in ammonia.
More recently 59the identification of the blue trisulfide radical S3- in DMF and
NH3 solutions of alkali polysulfides, the study 60,61,62 of the equilibrium between
S62- and S3- in various media, and the synthesis 63 of S4N- have significantly
advanced the understanding of the solutions of sulfur in liquid NH3.
Chivers 59 showed that the colour of blue ultramarine and the deep blue colour
observed in alkali polysulfides in DMSO or DMF solvents, was attributable to the s3-
radical and not as previously believed to the disulfur radical ion, s2-.64 The blue
species is characterised by a visible absorption band at 610-620 nm, which is
associated with a Raman band at 594 cm-1. 59 The S3- also exhibits a characteristic IR
band at 580 cm-1.59 S3- is formed under a wide variety of conditions. The
characteristic blue colour develops when Sg is heated with H20 and traces of some
basic salt 65, or by alkali polysulfides in basic solvents 60,63, as well as in dilute
solutions of sulfur in ethylenediaminne 66, and in the mineral lapis lazuli 67.
9
10
Table 1.1 Typical reductant I solvent systems used for the reduction
of S8 in the syntheses of polysulfide solutions.
Reductant Solvent Reference Year
Na NH3 Bergstrom 46 1926
EtOH Kuster43 1905
DMF Banda31 1989
K NH3 Bergstrom 46 1926
EtOH Kuster43 1905
Li NH3 Dubois 47 1988
LiEt3BH THF Gladysz 48 1978
Cs2S H20/EtOH Abrahams49 1952
Li2S THF Shaver 50 1982
K2S H20 Haradem 51 1977
Na2S H20 Teller 52 1983
DMF Clark 53 1978
HMPA Clark 53 1978
H2S MeOH Udpa54 1987
H20 /NH3 Krause 23 1982
DMF/NH3 Mi.iller24 1984
MeCN /NH3 Mi.iller24 1984
(NH4hS H20 Wickenden 55 1969
NaH DMF Henkel 56 1984
NaHS EtOH Klemm 57 1935
N2H4.HCI NH3 Dubois58 1988
The common feature of these conditions in which S3- ion is formed is the
simultaneous existence of conditions in which polysulfide ions, Sx2-, are either
present or can be easily formed. It is now known68 that S3- is in equilibrium with
s62- (equation 1.5), and that the equilibrium is temperature dependent.60
This observation of the reduced s 3- ion along with the observation of the
oxidised species, S4N- in ammonia solutions, led to the proposal 58,68,69 that the
known slow solubilisation of sulfur in liquid NH3 is a redox disproportionation
process described by Equations 1.3.and 1.4. The S4N- species has been isolated as
its PPN+ salt 63 and is characterised by a visible absorption band at 580 nm.
. .. 1.3
... 1.4
However (1.4) does not describe completely the S-NH3 solutions since Raman
spectroscopy experiments 68 at low temperatures (-40.C) have shown the presence of
small concentration of S3N-. This fact is not included in the above equation.
Dubois et al 48 reported the first Raman spectra of solutions of lithium
polysulfides, Li2Sx, in liquid ammonia, together with their absorption spectra. They
identified s2-, S22-, S42-, and S62- (in equilibrium with the· radical S3·) in liquid
ammonia. An important difference in liquid ammonia compared to aqueous solutions
was that in liquid ammonia S62- was shown to exist. The lithium salts of the S42- and
S62- polysulfides were very soluble in NH3 but Li2S2 and Li2S were not. No
evidence was found by Dubois et al for the existence of Ss2- or S32- in solution, or
for a species less reduced than S62-.
Other UV -visible spectrophotometric studies by Dubois et al 58 with ammonium
polysulfides, ((NH4hSx), in liquid NH3 (equation 1.1), have shown that S62- is the
least reduced ammonium polysulfide species, with no evidence of the ammonium Ss2-
11
or S32- species. In these experiments the radical anion s 3- was always observed in
equilibrium with s 62- for x > 1. The results are similiar to those presented in the
lithium polysulfide studies 47 except that in the presence of NH4+, HS- is the most
reduced species, whilst s2- is the most reduced in the presence of Li+.
The solutions of alkali metals in liquid ammonia can react to give various alkali
amides. This reaction is catalysed by trace impurities and must be minimised when
alkali metal solutions in liquid ammonia are used or otherwise it induces an error in the
stoichiometry of the prepared poly sulfide and makes the solution basic.47
Unfortunately, all the background on uncoordinated polysulfides presented here
relies heavily on the interpretation of electronic and vibrational spectra, that contain
multiple overlapping absorptions, which are consequently not definitive. The UV-Vis
absorption spectra and Raman spectra are virtually identical for all the proposed
polysulfide species in solution and the interpretation is hindered further by the
significant temperature dependence of the UV part of the absorption spectra of the
polysulfide ions.47 For Li2S6 in NH3 two UV bands at 290 and 330 nm are well
resolved at low temperature (215 K) but are broad becoming unresolved at 253 K.
However this work does confirm the complexity of polysulfide solutions that arise
from the chemical equilibria.
Despite this uncertainty surrounding the knowledge of the polysulfide species
and their interconversions in solution, a large amount of information about isolated
polysulfide compounds exists. The characterised crystalline salts of the known
polysulfide ions (not containing transition and post-transition metals) are listed in
Table 1.2, together with their preparative reactions and their characterisation data.
12
13
Table 1.2 Crystalline salts of polysulfide ions.
Polysulfide ion Preparative Crystallisation Characterisation Reference Year Cation reactions solvent data
[S2]2- Na+H2S + S EtOH Crystal 70 1958
Na+ inEtOH structure 71 1962
[S2J2- K+H2S EtOH Crystal 70 1958
K+ at 670 K structure 71 1962
[83]2- Not given Not given Crystal 72 1936
Ba2+ structure
[S4]2- BaS +S H20 Crystal 73 1931
Ba2+.H20 in H20 structure 74 1954
analysis 75 1969
[S4]2- Na2S + S EtOH Crystal 76 1973
Na+ at 300 K structure
[Ss ]2- KHS + S EtOH Crystal 77 1976
K+ inEtOH Structure
[Ss]2- Tl +Sat 250°C Nil Crystal 78 1975
TI+ Structure
[S6]2- CsOH + CsHS H20 Crystal 49 1953 -Cs+ +02inH20 Structure 79 1952
Analysis
[S6]2- THF Crystal 80 1990 Li2S2 +tmeda Structure
Li(tmeda)+ +THF
[S7]2-(E14N)2MoS9 +
Crystal Ph4PCl + CH3CN 81 1983 Sodium diethyl structure U. V.
Ph4P+ dithiocarbarmate trihydrate +
CH3CN
1.4 Formation and characterisation of transition and post-transition
metal polysulfides.
The chemistry of anionic transition-metal sulfides began more than 100 years
ago with the synthesis of the metal sulfides, MS42- [M = Mo, W] anions, that were
used as precursors to the formation of metal polysulfides. 82,83
Homoleptic metal polysulfide complexes [M(Sx)wJZ containing polysulfide
chains as chelating ligands have also been known for a very long time. 84,85 These
metallapolysulfane complexes are now known to be formed in diverse reactions,
which have been recently reviewed.l2, 13,16
The interest in this area originated mainly from: 1) the possible relevance of such
ligands to the chemical nature of the surfaces of heterogeneous hydroprocessing
catalysts86,87,88,89,90,91 and 2) the importance of some Fe-Mo-S molecules in the
modeling of nitrogenase.92,93,94,95,96,97
1) The hydrodesulfurisation reaction involved in the catalytic hydrogenolysis of
organosulfur compounds is an important process in the purification of petroleum
products. Catalysts based on Mo (or W) and promoted by Ni or Co are widely used
in the petroleum industry for the hydrodesulfurisation (HDS) of petroleum
feedstocks. 87 Interest in these catalyst systems has recently been heightened by the
perceived need for large-scale processing of liquids derived from coal or oil-shale at
some time in the future. However, in spite of their wide and longstanding use, a
complete explanation of the nature of the active sites and their mode of action has not
been formulated.
Because of the reaction environment, the operational forms of these catalysts are
like those of the sulfides. It is generally accepted that MoS2 (or WS2) is the catalyst,
with reactive sites modified in some way by the promoter atoms (Ni or Co). Various
14
models have emerged in the past decade describing the action of the promoter in these
sulfide catalysts. These have recently been reviewed by Pratt et al. 87
As a consequence of the interest in the HDS process, the basic coordination
chemistry of molybdenum with sulfur ligands has received considerable attention
with12,98,99 complex anions such as [Mo2(S 2)6]2- 100, [Mo3SCS2)6]2- 101,
[(S4hMoS]2- 27, [Mo2S10]2-102, [Mo202S2(S2)2]2- 103 being reported.
Intramolecular redox reactions within this molybdenum-sulfide chemistry,
proceed with a change in the formal oxidation state of the molybdenum. An
example96,103,104 is the formation of [Mo202S2CS2)2]2- from [Mo02S2]2-(equation
1.5)
H20
[Mo02S2J2- +NaOH --7 [Mo202S2CS2hJ2- ... 1.5
The following intramolecular redox processes are proposed for 1.5, where oxidation
of the sulfide ligands is by the metal coordination centre.
Mo(VI) --7 Mo(IV)
Mo(Vl) + Mo(VI) --7 2Mo(V)
2S2- --7 S22-
2) During biological nitrogen fixation, the nitrogenase enzyme system catalyses
the reduction of dinitrogen to the ammonia level which is metabolically useable. The
conventional nitrogenase system consists of two metalloproteins: an iron (Fe)-protein
and a molybdenum-iron (MoPe)- protein. Three distinct types of redox centres are
associated with these proteins: The MoFe-protein contains two types of centres, the
FeMo-cofactor93,94,105 and the P-clusters,106 and the Fe-protein dimer contains one
4Fe:4S cluster whose structure has only recently been reported.107 Because the active
site of nitrogenase is provided by the MoFe-protein, the redox centres of this protein
have attracted considerable attention and the consequent attempts to model this system
15
has resulted in the isolation of novel Mo-S complexes (described above) and Fe-S
complexes including [Fe2S2(SC6H5)4]2-I07, [Fe2S2(Ss)z]2- 108, [Fe4S4Cl4]2- 114
and Fe-Mo-S complexes, [(S5)FeS2MoS2]2- and [(PhS)zFeS2MoS2]2-114
These factors which motivated the exploration of polysulfide chemistry did not
exist for the heavier chalco gens Se and Te and as a result similar developments did not
keep apace.
Arguably the most important advance in the preparative and reaction chemistry
of metal polysulfides was the introduction of non aqueous and aprotic solvents, which
increase the thermodynamic activities of polysulfide ions in solution and therefore
expand the range of complexes that can exist in solution equilibria. The significance
and logic behind the use of aprotic solvents is discussed in detail in section 1.9.
1.4.1 Metal Polysulfide Synthesis
Although many polysulfide preparations are now known, there are several
general routes which cover the majority of metal polysulfide syntheses. These are
presented below in section 1.4.2 along with relevant examples of structures. The
main method for the formation of non aqueous (Sx)2- has been the direct analogue of
the aqueous method (equation 1.1), namely HzS(g) + S + NH3(g)·
Significantly in many of the preparations described in section 1.4.2 the type of
Sx2- ligand occurring in the metal complex does not necessarily have a high
abundance in the polysulfide solution. For example, precipitation of complexes from
cadmium polysulfide solutions , Cd2+ + n[SmJ2- (where n = 3 and m= 3-6) with
Ph4P+ cation yielded only (Ph4P)z[Cd(S6hl and (Ph4P)z[Cd(S6)(S7)].31 The crystal
lattices are cation dominated, in that the metal polysulfide anions are relatively small
and occupy cavities between the cations. The crystals select and include from solution
the cadmium polysulfide complexes that fit the cavity.
16
1.4.2 General Routes for the Syntheses of Polysulfide Complexes
1. Reactions of metal complexes with poly sulfide ions are the most convenient
methods for preparing metal polysulfides. Solutions containing polysulfide ions can
be generated in many and varied ways as shown in table 1.1. Typically polysulfide
ions are reacted in various solvents as preformed alkali metal salts (e.g Na2S6)
(equation 1.5), or prepared in situ by the reactions between Na + Sg (equation 1.6) or
N a2S + Sg (equation 1. 7). The reaction of metal halides with ammonium polysulfides
in various aprotic solvents (equation 1.1) has also extensively been used to prepare
metal polysulfides. However the use of gaseous reagents in this preparation has led to
substantial variability of the composition of the resulting (NH4hSx solutions, and
consequently introduced difficulties in attaining reproducible metallapolysulfide
stoichiometry.
These methods have been most successful in the syntheses of homoleptic
polysulfides (Table 1.3).
DMF
CdCl2 + 2Na2S6 ~ (Ph4P)2[Cd(S6hJ + 2NaCl(s) + 2NaBr
DMF
Cd(N03h + 2Na + lOS ~ (Ph4Ph!Cd(SshJ + 2NaN03
2Ph4PBr
DMF
CdS + Na2S + (10 /8) Sg ~ (Ph4Ph[Cd(S6hJ + 2NaBr
2Ph4PBr
... 1.6
... 1. 7
... 1.8
Metal chlorides, metal nitrates and metal thiolates are the most common metal
precursors but sulfides have more recently been used (equation 1.8).31,32 The
advantage of using metal sulfides is that no unnecessary anions are introduced into the
17
reaction system. Adventitious anions can become activated in aprotic solvents and
compete with the polysulfide ligand for the metal (see section 1.9).
More recently synthetic approaches resulting in more controlled generation of
nonaqueous polysulfide solutions have been reported.31, 32 Bu4N+ and Ph4p+ salts
of Sx2- such as (Bu4NhS6, are isolated which are then redissolved and deployed
preparatively in solvents of interest.
All of these preparations have led to an astonishing variety of metal polysulfides
being discovered. Illustrative examples (Figures 1.3 a-f) of this variety are [Ag(S9)]-
24, [(S7hBiS6Bi(S7h]4- 26, [Ag2(S6h]2- 109, [Pd2(S7)4]4- 110, [Cu3(Sx)3]3- (x =
4, 6) 111,112 [Cu4(Ss)m(S4)3_mJ2- (m = 0-3) 113, [Cu6(S 5)(S4)3]2- 23, and
[Re4S4(S3)6]4-26
2. Oxidative addition of elemental sulfur to an electron-rich metal complex which is
co-ordinatively unsaturated, provides a convenient method for preparing non
homoleptic polysulfide (Sx2-) complexes.
Examples are:-
i) Cp Rh (PPh3) + Sg --7 CpRh(PPh3)(Ss)+ (Ph3PS )118
ii) ML4 (M = Pd, Pt; L = PPh3) + Sg --7 L2M(S4)119
iii) Cp2Ti(CH3)2 + 5/8 Sg --7 CP2TiS5120
iv) Cp2 Mo(S2) + 1/4 Sg --7 CP2Mo(S4) 121
v) Cp2 Mo(SH2) + 3/8Sg --7 CP2Mo(S4) 121
... 1.9
... 1.10
... 1.11
... 1.12
... 1.13
18
51
Figure 1.3a. Structure of the 1 Ag(S9)]- anion.24
Figure 1.3b. Structure of the j(S7)2BiS6Bi(S7)]4- anion.26
5{6)
Figure 1.3c. Structure of the 1 Ag~cS6 l 2 ]2- anion. I 10
Eigure 1.3d.
Figure 1.3e. Structure of th~l Cu4< S5)3j2- anion.ll4
19
Table 1.3. Monometallic hornoleptic polysulfides and their syntheses •
Formula Source of sulfide . Solvent Metal Reference Precursor
(Ef4N)2[Zn(S4)2] BzSSSBz MeCN (Ef4N)z[Zn(SPh)4] 114
(Ph4P)2[Ni(S4)2] BzSSSBz MeCN (Ph4P)z[Ni(SPh)4] 114
(Et4N)2[Ni(S4)z]
(Et4N)z[Pd(S4)2] NH3(g)ISs/H2S MeOH (Et4N)z[Pd(SPh)4] 114
(Ph4P)2[Hg(S4)z] NH3(g)ISs/H2S MeOH Hg(0Ac)2 25
(Ph4P)z[Hg(S4)2]Br2 Na2S I Sg DMF HgS 32
(Ph4P)2[Cd(Ss)2] BzSSSBz MeCN [Cd(SPh)4]2- 114
CBU4Nh[Pt(Sshl (NB4)2[Pt(Ss)3] H20 (NJ4)2[Pt(Ss)3] 55
(NJ4)2[Pt(Ss)z] 115
(K)z[Pt(Ss)z] 115
(NH4)z[Pt(Ss)3] .2H20 (NB4)2SISs H20 PtC4 11
(NJ4)3[Rh(Ss)3] (NH4)2SISs H20 RhCl3 116
(Ph4P)2[Zn(S6h] BzSSSBz MeCN (Et4N)z[Zn(SPh)4] 114
(Ph4P)z[Zn(S6)2] NH3ISsH2S MeOH Zn(OAch 25
(Ph4Ph[ Cd(S6)2] Na2S I Sg DMF CdS 31
(PNP)z[Cd(S6)z].CH3CN NH3ISsH2S EtOH Cd(OAc)2H20 25
(Bu4N)(Ph4P)[Cd(S6)2] (Bu4NhS6 MeCN CdCl2.SI2H20 31
(Et4N)z[Hg(S6h] NH3ISsH2S MeOH Hg(OAc)z 25
(PNP)[Ag(S9)] Sg/NH3/H2S MeCN AgN03 24
(Ph4As)[Au(S9)] (Ph4AshSx EtOH KAu(SCN)z 117
3. By reacting the metal sulfides of Mo, W or Re as for instance [MoS4]2-,
[MoOS3]2- and [ReS4]2- with sulfur, polysulfide complexes containing S4 chelate
ring systems such as:-[SM(S4)2]2- (M = Mo, Re) 95 and [OMo(S4)2]2- 95 have been
obtained.
Reacting metals or metal halides with Sg and/or S2Cl2 at high temperatures generate
disulfides.122 For example:-
MoC13 + Ss/S2C12 ~ Mo3S(S2)3Cl4 + Mo2CS2)2Cl6 ... 1.14
4. The formation of polysulfide complexes has also been achieved by use of
reagents containing Sx bonds such as ChSx, R2Sx 123 The compounds
[CpFe(C0)2 ]Sx have been prepared by several routes, at low temperature to avoid
redox chemistry .124
[CpFe(C0)2]- + SCl2 ~ Cp(C0)2FeS3Fe(Co)2Cp
[CpFe(C0)2Br] + Li2S2 I Li2S4 ~ Cp(C0)2FeS2 Fe(C0)2Cp
... 1.15
... 1.16
5. Conversions of pre-formed polysulfide complexes with sulfur abstracting
reagents have been reported.125,126
i) [Pt(S5)3]2- + CN- ~ [Pt(S5)2]2-
This reaction involves a two-electron reduction ofPt(IV) to Pt(II)
ii) [Pt(S5)3]2- + 12Ph3P ~ S2- + (Ph3P)2Pt(S4)2 + 10Ph3PS.
... 1.17
. .. 1.18
6. Intramolecular redox reactions of metal-sulfide complexes provide routes to
S22- complexes. An example is the formation of [Mo202S2CS2)2]2- from
[Mo02S2]2- as discussed previously (equation 1.5).96,102,103
7. The neutral sulfur atom transfer agent, dibenzyl trisulfide, (BzSSSBz) has been
used as a source of polysulfide. This reagent has been extensively used with metal
20
thiophenolate precursors, [M(SPh)4]2-, for the syntheses of metal polysulfide
complexes.114 In polar media the RS- catalysed dissociation of BzSSSBz results in
the generation of SxO fragments (x=2-6) that in the presence of suitable reducing
agents form the anionic Sx2- polysulfide ligands. The PhS....:. ligands in the
[M(SPh)4]2- complex anions are oxidised readily by SxO fragments according to
equation 1.19.114
... 1.19
1. 5 Uncoordinated 'free-chain' polyselenides
Hugot 128 was the first to investigate the action of alkali metal-ammonia
solutions on electronegative elements. His results on the sodium-selenium system
indicated the formation of Na2Se4 as the highest polyselenide. Further work by
Bergstrom46 revealed a series of sodium and potassium selenides, Sex2-, (x=2-4),
and a higher unknown polyselenide. Bergstrom's results were in good agreement
with those of Mathewson 129 who based his conclusions on the thermal analysis of the
resulting solids. The colours of the polyselenide ions in NH3 were described by
Bergstrom as transparent red for the Se22-, deep green for Se32-, red for Se42-, and
red-brown for a higher uncharacterised species.
Potentiometric titration data obtained by Zintl 130 on the sodium-selenium
system indicated the existence of Sex2-, (x=2,3,4,5,6). The colours of the ions
reported by Zintl do not correspond to those given by Bergstrom. Zintl attributed the
green colour to the Se42-species whereas Bergstrom attributed this colour to Se32-.
Also, Zintl reported the penta- and hexaselenides whereas Bergstrom did not identify
the highest polyselenide (but suggested that the hexaselenide was the highest selenium
containing species in solution).
21
Klemm and coworkers 57 prepared the potassium polyselenide solids K2Sex,
(x=2-5). A significant part of this work was the magnetic susceptibility investigations
on the solutions of the various polyselenides where they found no evidence to suggest
the presence of paramagnetic species, such as the formation of radical anions of the
type found in some polysulfide solutions.
Based on the investigations of the polyselenide ions in liquid ammonia up until
1977, the reported species included Na2Se2, Na2Se3, Na2Ses and Na2Se6. Sharp and
Koehler 10 in 1977 reported an UV -visible spectroscopy study of solutions of sodium
polyselenides in liquid NH3. Here a systematic approach of synthesis, via sodium
reduction in liquid ammonia, isolation and analysis of the Na2Sex salts (x=1-6)
followed by comparative ultraviolet-visible spectroscopy investigations of all the
isolated compounds and mixtures was reported. The results of the investigation were
that Na2Se3, Na2Se4 and Na2Se6 were isolatable and were spectroscopically
identifiable species. Na2Ses was found to be a 1:1 mixture of Na2Se4 and Na2Se6
and no evidence was found to indicate a less reduced polyselenide than Na2Se6.
Na2Se2 could only be prepared in equilibrium with Na2Se3 and Na2Se and was not
isolated in a pure state from solution.
In a homologous series such as the polyselenides, Sex2-, the electronic spectra
of the various Sex2- species show a high degree of similarity and are characterised by
multiple overlapping bands making interpretation of the spectra difficult. Spectra are
resolved into component bands using mathematical manipulation and the resulting data
statistically analysed to test the significance of the parameters in the above
manipulations.
The colours of the various polyselenide solutions of nominal composition Sex2-,
(x=3,4,6), observed by Sharp and KoehlerlO were consistent with those seen by
Bergstrom46, i.e. 'Na2Se3' - brilliant green', Na2Se4' - bright red and 'Na2Se6'
dark red/brown.
22
Despite this uncertainty surrounding polyselenide species in solution, a large
amount of information about isolated polyselenide compounds exists. The
characterised crystalline salts of the known polyselenide ions (not containing
transition and post-transition metals) are listed in Table 1.4, together with their
preparative reactions and their methods of characterisation.
It can be seen from Table 1.4 that whilst 'Se62-• has been the longest chain
polyselenide observed in solution, complex anions have been isolated from these
solutions containing Sex2- fragments where x= 7, 8, 9, 10, 11 and16. The conditions
leading to formation of these longer polyselenides are not understood in detail,
however important factors include reaction conditions such as solvent and temperature
in addition to the size, charge and shape of the counterion. For instance, the
tetraselenide [Ph3PNPPh3hSe4.4CH3CN is formed from Cs3TaSe4 in MeCN in the
presence of [Ph3PNPPh3]+ at room temperaturel31, whereas heating a DMF solution
containing Li2Se4 in the presence of [Ph3PNPPh3]+ affords the bicyclic decaselenide
[Ph3PNPPh3hSe10.DMF.142
Similarly, [Se11]2- is a product of oxidation of the pentaselenide solutions by
N03-, I2 or Au3+ ions in DMF when either Ph4P+ or NPr4+ countercations are
present.2,139,140
Given the existence of various Sex2- species (x=3-6), of which the longer
members are helical chain structures (Figure 1.4a.), the structures of the Se102- and
Se112- anions are intriguing. The Se102- ion can be regarded as consisting of a central
Se22+ fragment and two Se42- chelate ligands (Figure 1.4b.). Se112- consists of a
central Se atom chelated by two Ses2- ligands. Formally, the central four coordinate
selenium atom can be thought of as a Se2+ centre (Figure 1.4c.).
The fact that a extended chain polyselenide version of either Se102- or Se112-
does not arise, suggests the instability of such species in the solid state or even in
solution and may imply that, beyond a certain polyselenide chain length, internal
electron transfer within the chain is favourable, resulting in a more compact, chelated
23
Figure 1.4a. Helical chain structure of Se62-. I 31 .
Figure 1.4b. Structure of the [ Se 10]2- anion.l43
Se(4) Se(3)
I
Figure 1.4c. Structure of rhe fSe(SeshJ2- anion.2,140,141
and presumably more stable molecule. A more efficient delocalisation of the
dinegative charge over these cyclic molecules compared to the hypothetical chain
isomers may be partly responsible for the stability of them.
The cesium polyselenide Cs4Se16 .was prepared by the methanolothermal
reaction of Cs2C03 and Seat 16oOC and 13 bar. [Se16]4- (Figure 1.4d) fragmen~s
may also be considered as containing an Se6 ring with Cs symmetry and two Ses
chains related by the crystallographic mirror plane .143
The conclusion reached from this survey of uncoordinated polyselenides was
that polyseienide solutions are in general complex and not well understood Therefore
a deliberate study of the synthesis and characterisa:tion of Sex2- ions (x = 1-11) in
solution was undertaken in this work; and is described in Chapter 4.
Se1
Figure 1.4d. Structure of the [Se16]2- anion.l43
24
25
Table 1.4 Crystalline salts of polyselenide ions
Compounds containing transition metals and post-transition metals are excluded
Polyselenide ion Preparative Crystallisation Characterisation Reference Cation reactions solvent method [Se2]2- Se + Na, NH3 Crystal structure 71
Na+ in NH3
[Se3]2- N a2Se2 + lOSe, Ethanol CHN analysis 130
[CH3(CH2)IsN1V1e3]+ in ethanol
[Se4]2- Cs3TaSe4 CH3CN Crystal structure 131
Ph3PNPPh3+ [Se4]2- Ba + Se, in ethylenedia- Crystal structure 132
[Ba(en)4]2+.en ethylenediamine mine
[Se4]2- BaBiSe3 + ethylenedia- Crystal structure 133 [Ba(2,2,2-crypt)]2+ 2,2,2-crypt) mine
[Ses]2- Side product DMF Crystal structure 134
Ph4P+ from Na2Sex + PtCh + Ph4PCl
[Ses]2- Unplanned DMF /ether Crystal 135
Ph4P+ product from Zr structure, IR + Zr(OEt)4 +
Se + Ph4PCl + (Oct1V1e2Si)2Se
[Ses]2- Cs3TaSe4 CH3CN Crystal structure 131 [Cs(18-crown-6)2]+
[Ses]2- Cs + Se, in NH3 Crystal structure 136 Cs+ NH3
[Ses]2- Se + Na, Ill NH3 Crystal structure 137 Rb+ NH3
[Ses]2- Na2Se2 + lOSe, Ethanol CHN analysis 130 [CH3(CH2hsN1V1e3]+ in ethanol
[Se6]2- Na2Se6, in Water Crystal structure 138 BU4N+ H20 vibrational
spectra, electronic spectra
[Se6]2 Na2Se + lOSe, Ethanol CHN analysis 130 E14N+ in ethanol
[Se6]2 N a2Se + 1 OSe, Ethanol Crystal structure 130 [CH3(CH2)13N1Vle3]+ in ethanol
[Se7]2 Na2Se + Se, Ethanol CHN analysis 130 [CH3(CH2h sN1V1e3]+ in ethanol
[Ses]2 Na2Se + Se, Ethanol CHN analysis 130 [CH3(CH2)13N1Vle3]+ in ethanol
[Se9]2 Na2Se + Se, Ethanol CHN analysis 130 [CH3(CH2)13N1V1e3]+ in ethanol
[Se(SeshJ2- Na2Ses + l2, DMF Crystal 139
Ph4P+ inDMF structure, two Na2Ses + crystal forms,
AuCl3, spirocyclic inDMF [Se(Ses)2]2-
[Se(Se5hJ2- Na2Ses + DMF Crystal 2
Ph4P+ NaN03 structure
[Se(Se5hJ2- K2Se2 + Se DMF Crystal 141
NPr4+ structure, IR .,
[Te(SeshJ2- Na2Ses in DMF Crystal 142
Ph4P+ DMF structure
[Sei0]2- Li +Se DMF Crystal structure 143
Ph4P+ in DMF
[Se16J4- Cs2C03 + Se, MeOH Crystal structure 144 Cs+ inMeOH at Se6 cycle with
160°C two Ses chains connected to one ~uare ~anar Se
[Ses]2- K2Se2 + Se DMF Crystal Structure 141
[N(C2H5)4]+ IR
1/2Se6.Se7 also found in cell.
[Seg]2- Na2Sex+CeCI3+ DMF Crystal Structure 145 N a( 12-Crown-4 )+ 12Crown4
(Se6, Se7) also found in cell.
1. 6 General Routes for the Syntheses of Metal Polyselenide
Complexes
Prior to the commencement of this work in 1988 soluble transition-metal
polyselenides were rare. The known compounds were:- (Ph4P)2[(Fe2Se2(Ses)2]17,
(Ph4P)2[W2Se4(Se2)(Se3)]l8,19 and (Ph4P)2[W2Se4(Se3)2]l8 (found in the same
crystal) and (NE4)2[V2(Se2)4(Ses)]20. However in the last few years extensive
development by myself and other research groups has led to a vast number of new
polyselenide complexes being isolated. Table 1.5 is a chronology of the discovery of
soluble transition metal polyselenides together with their preparative reactions and their
characterisation data. The general synthetic procedures are reviewed after Table 1.5.
26
27
Table 1.5. A CHRONOLOGICAL TABLE OF ANIONIC TRANSITION AND
POST-TRANSITION METAL POLYSELENIDES AND RELATED SPECIES
Formula Source of selenide Solvent Metal precursor Characterisa Year, tion method Reference
[WSe4J2- NH4 + H2Se H20 (NHL()2W04 Nil 1927,
145
[MoSe4]2- NH4+ H2Se H20 (NH4)zMo04 SXRD,IR 1967,
146
[Fe2Sez(Se5)z]2- Ph4P+ Se/Na DMF FeCI2 SXRD,IR 1984,
17
[WgSe9]2- Ph4P+ (NH4)z[WSe4] CHgCOO (NH4)z[WSe4] SXRD, 1987, H/MeOH 77Se
18
[W zSe9]2- Ph4P+ (NH4)z[WSc4i McCN/ (NH4)z[WSe4] SXRD, 1987,
DMF 77Se
20
[W 2se10]2- Ph4P+ (NH4)z[WSc4l McCN/ (NH4)z[WSe4] SXRD, 1987,
DMF 77Se
20
[V 2se13]2- NEt4 + Bis(dimethyl MeCN/ NH4VOg SXRD 1987, octylsilyl)selenide
Et3N 19
[WSe(Se4)z]2- Ph4As+ (NH4)z[WSe4] DMF/CS2 [NH4]z WSe4] SXRD, 1988, I.R., 77Se
147
[MoSe(Se4)z]2- NEt4 + [NEt4)z[MoSe4] DMF/CS2 [NEt4J2[MoSe4] SXRD, 1988, I.R., 77Se
147
[WS(Se4)z]2- Ph4As+ (NH4)z[WSe4] DMF (NH4)z[WSe4] SXRD, 1988, I.R., 77Se
147
[MoS(Se4)zl2- NEt4+ [NEt4]z[MoSe4] DMF [NEt4J2[MoSe4] SXRD, 1988, I.R., 77Se
147
[Mo(Se)(Se4)z]2- (Ph4P)P+ (Ph4P)z[MoSc4] DMF/TH (Ph4P)zMoSe4 SXRD 1988, F
148
[Cr3Se24]3- Ph4P+ K2Sc3 DMF CrCiz SXRD, 1988, magnetic
149 moment
[Zn(Se4)z]2- Not reported Not Not reported Not reported 1988, reported
150
[Hg(Se4)z]2- Not reported Not Not reported Not reported 1988, reported
150
[AgSe4]-n Ph4P+ Na2sc5 DMF AgNOg SXRD 1989,
151
28
*Na2Ti2Se8 Na2Se Nil Timetal. SXRD 1988,
152
[In2Se12]4- Ph4P+ Na2Se5 DMF InCI3 SXRD, 1989, 77SeUV,IR
153
* K3Nb2,Sell K2Se/Se Nil Nb metal SXRD 1989,
154
[Au2 WSe4].DMF Ph4P+ (Ph4Ph[WSe4] DMF AuCl3.2H20 SXRD 1989
155
[M(Se4)z]2- M = Na2Se/Se EtOH M(02CCH3)z IR, SXRD 1989,
Zn,Cd,Hg 156
(15-Crown-5)Na+
[Cd(Se4hJ2- Li3(12-crown- [Li(l2-crown- EtOH Cd(OAch IR, SXRD 1989,
4)3(0Ac)2+ 4)]zSe6 157
[Hg(Se4hl2- K(18-crown- [K(18-crown- EtOH Hg(OAc)z IR, SXRD 1989,
6)+ 6)hSe6 157
[Ni(WSe4)z]2- Ph4P+ [NH4h[WSe4] DMF NiCI2(PPh3)z SXRD,77Se 1989, IR,UV
158
[Pd(WSe4)z]2- Ph4P+ [NH4h[WSe4] DMF PdCl2(MeCN)2 SXRD,77Se 1989, IR,UV
158
[Ni(Sez)(WSe4)]2- Ph4P+ [NH4h[WSe4] DMF Ni(acac)z SXRD,77Se 1989, IR,UV
158
[W (Se2C2(COOCH3)z)JJ2- [AsPh4]z[WSc9J DMF [AsPh4]z[WSeg] SXRD,77Se 1989,
.C7Sg.112 DMF Ph4As+ IR,UV 159
[W 2Se2(Se2C2(COOCH3)z) (PPh4][W2Se10J DMF (PPh4][W 2Se10J SXRD,77Se 1989, 4]2- Ph4P+ .2DMF IR,UV
159
[Ag4(Se4)z(Se5)]2- Ph4P+ Na/Se DMF AgN03 SXRD 1989,
This work
[Cu4(Se4)z(Se5)]2- Ph4P+ Na/Se DMF CuCI2 SXRD 1989,
This work
(CsHs)Mo(Se4)zJ- Ph4P+ Na/Se DMF Cp2Mo2(C0)6 SXRD 1989,
This work
[Sn(Se4)JJ2- Ph4P+ Na/Se DMF SnCI2 SXRD 1989,
This work
[Zn(Se4)zJ2- Ph4P+ Na/Se DMF ZnCI2 SXRD 1989,
This work
[Cd(Se4)zJ2- Ph4P+ Na/Se DMF CdCI2 or SXRD 1989, Cd(N03)z This work
29
[Hg(Se4h]2- Ph4P+ Na/Se DMF HgC12 SXRD 1989,
This work
[Ni(Se4h]2- Ph4P+ Na/Se DMF NiC12 SXRD 1989,
This work
[Pb(Se4h]2- Ph4P+ Na/Se DMF PbC12 SXRD 1989,
This work
[W2(Se6)]2- Ph4P+ (PPh4] [W 2Se10l DMF (PPh4)[W zSelO] SXRD, 1989, 77Se, IR,
160 uv [Pt(Se4)3]2- Ph4P+ Li2Se/Se DMF/ Pt(xanthate)z SXRD, 1989,
Et3N 77Se 161
*K3[Nb4Se22] K2Se + Se solid state Nb SXRD 162
[Rez(Se4)z(C0)6]2- Ph4P+ K2se3 DMF Re2(C0)10 SXRD 1989,
163
[Ru(Se4)z(CO)z]2- Ph4P+ Na2se5 acetone Ru3(C0)12 SXRD, 1990, 77Se
164
( [Ag(Se5)]-} oo Me4N+ Na2Se5 DMF AgN03 SXRD, 1989, 77Se
165
[Ag4(Se4)4]4- Et4N+ Na2se5 DMF AgN03 SXRD, 1989, 77Se
165
[Ag4 (S e4)3]2- Pr4N+ Na2se5 DMF AgN03 SXRD, 1989, 77Se
165
[ln3Se3(Se4)3]3- Et4N+ Na2Se5 MeCN InC13 SXRD, 1990, 77Se
8
[AuzSe2(Se4)z]2- PPN+ Na2Se5 DMF AuCN SXRD, 1990, 77Se, UV
8
[Mn2(Sez)z(C0)6]2- K2Se3 DMF Mn2(C0)10 SXRD, 1990
Ph4P+ IR 166
[Mn2(Se4)z(C0)6]2- K2Se3 DMF Mn2(C0)10 SXRD, 1990
Ph4P+ IR 166
[Mn(Se4)z]2- K2se3 DMF Mn2(C0)10 SXRD, 1990
Ph4P+ IR 166
[Pd(Se4)2]2- Ph4P+ NazSe2 + Se DMF PdC12 SXRD 1990,
167
[Pt(Se4)z]2- Ph4P+ NazSe2 + Se DMF PtC12 Micro 1990, analysis
167
30
[Zn(Se4)(Se6)2- Li2Se/Se DMF Zn(CH3C00)2 SXRD 1991
Rb( 18-Crown-6)+ IR 168
[Hgz(Se<03J2- Li2Se/Se DMF Hg(CH3COO)z SXRD, 1991
Cs(18-Crown-6)+ IR 169
[Hg(Se4)z]2- Ph4P+ [Sn(Se4hJ2- DMF Hg(CH3C00)2 SXRD, 1991
(Ph4P)+ IR 169
[Na Au 12se8]3- Na2Se MeOH AuCN SXRD· 1992
Et4N+ IR 170
*K2Ag12se7 K2Se4 + Se en Ag metal SXRD 1992
171
*KzPdSe10 K2se4 H20 PdClz SXRD 1992
172
[Ni4Se4(Se3)5(Se4)]4- LizSe + Se DMF Ni(S2COEt)z SXRD 1992
NEt4+ 77se NMR 173
IR
*Solid state preparation
SXRD single crystal X ray diffraction
1.7 Synthetic Routes to Polyselenide Complexes of Transition and
Post-Transition Metals
Just prior to this work, several methods had been developed to improve access to
complexes of the metal polyselenides. Historically, the first successful method175,176
involved preparation of ternary Zintl type phases of the type AxMySez, where A is an
alkali metal and M is a d- or p- block metal. These can either form crystalline phases
directly from the molten state,34 or can be extracted with a basic amine solvent in the
presence of 2,2,2-crypt to form metal polyselenide clusters.l76,177,178,179
Another development180 had been the introduction of bis silylselenides such as
R3Si-Se-SiR3 ( R= alkyl functional group). These compounds have the advantage that
variation of R can change the properties and reactivity of the reagent. They react with
metal halides and oxyanions by exploiting the high oxophilicity and halophilicity of
silicon. These reagents have led to several interesting compounds including
(NEt4h[V2(Se4)2(Ses)] 19 (equation 1.20) (Figure l.le.).
. .. 1.20
However recent syntheses of transition metal polyselenides have exploited the more
obvious reaction of alkali metal polyselenides, A2Sex (x=l-6)(A = Li, K and Na), with
transition metal precursors in aprotic solvents (equation 1.21). This synthetic approach
has been the most successful for the syntheses of transition metal polyselenides as shown
by Table 1.5. The alkali metal polyselenides have been readily prepared by several
methods, the most common being melting the elements together in a quartz tube, or by
dissolving the alkali metal in liquid NH3 (equation 1.21). The reduced chalcogen is then
isolated by removing the ammonia. The resulting polyselenide anions are most generally
reacted with metal halides in polar solvents like DMF. The syntheses involving reduction
of Se by Na in liquid ammonia has been developed in this thesis (Chapters 4 and 5). This
31
strategy is directly analogous to the synthesis of polysulfides, but the products have been
shown in this work and by others to be quite different.
A third procedure developed in this work (Chapter 5) prepared in situ polselenides
by reactions between Na and Se in DMF solvent usually in the presence of the metal
halide precursor (equation 1.22). This 'self-assembly' type reaction has been shown in
this thesis to be a very effective synthetic route to novel metal polyselenides.
NiClz + 2Na2Se4 -7 [Ni(Se4)2]2- + 2NaCl + 2Na+ ... 1.21
CdC12 + 4Na +SSe -7 [Cd(Se4)z}2- + 2NaCl + 2Na+ ... 1.22
These reactions are driven by the nucleophilicity of the polyselenide anion for
class B metals as well as the elimination of solid N aCl. Most often the resulting
product is an anion and large organic counterions such as Ph4P+ are used to crystallise
products.
Finally polyselenide salts such as (Bu4N)2Se6 and (Ph4P)zSe5 have been
isolated and then redissolved and deployed preparatively in solvents of interest.
Reactions with metal halides of these polyselenide salts have been shown in this thesis
to be a useful synthesis of transition metal polyselenides (Chapter 5).
Metal halides have been preferentially used in this work as the use of metal
nitrates has been shown to oxidise polyselenide solutions in DMF (Chapter 4).
Kolis and others9 have developed other very useful syntheses involving soluble
polselenide anions and transition metal carbonyl complexes (equations 1.23 and 1.24)
... 1.23
32
Heating a solution of [Mn2(Se4h(C0)6]2- in DMF at 90°C leads to
disproportionation of the metal centre and the production of [Mn(Se4)i]2- (equation
1.24)
90°C
[Mn2(Se4)2(C0)6]2- ~ [Mn(Se4)2]2- +1/2 Mn2(C0)10 + CO ... 1.24
DMF
Other less used synthetic procedures include the reaction of low oxidation state
metal complexes with Se or SeS2, involving oxidative addition to metal complexes to
give chalcogen rich compounds (equation 1.25).181
benzene
Os(C0)2(PPh3)2 + 2Se --7 Os(Se2)(C02)(PPh3)2
R.T.
... 1.25
Reaction of the terminal Se2- or bridging Se22- ligands of selenide or
polyselenide complexes with elemental Se is a convenient metod for the synthesis of
metal polyselenides (equations 1.26 and 1.27).9,148
... 1.26
... 1.27
Finally a few reactions of metal complexes with COSe, CSe2, S2Cl2 or Se032-
lead to the formation of polselenide complexes. Reaction of CpMn(C0)2(0C4H3)
with COSe leads preferentially to the binuclear complex Cp2Mn2(C0)4Se2182 whilst
(triphos)RhCl reacts with CSe2 to give [(triphos)RhCl (Tl2-CSe2)] which on treatment
with PEt3 and oxidation in air gives the dimer [(triphos)2Rh2( J.!-Se2)2]2+_183 The
Cp2Cr(C0)3· ion reacts with either Se2C12 or Se032- to give Cp2Cr2(C0)4(Se2) and
Cp2Cr2(C0)4Se respectively.184
33
1. 8 Formation and characterisation of uncoordinated polytellurides.
The reaction of sodium with tellurium in liquid ammonia was first reported in
1899 by Hugotl who observed the initial reaction product to be a colourless,
gelatinous precipitate, insoluble in liquid ammonia, which was presumed to be Na2Te.
With additional tellurium, this material went into solution and the final compound
formed was assigned the stoichiometry Na2Te3. A more comprehensive investigation
by Kraus and Chim185 in 1922 established the identities of the species sequentially
formed in liquid ammonia to be Na2Te (yellow solution, white crystalline precipitate),
Na2Te2 (red solution) and Na2Te4 (deep red solution). These authors attributed the
discrepancies between their results and those of Hugot to the presence of water in his
reagents which would have resulted in the coprecipitation of hydroxide and resulted in
the interpretation of the white solid as gelatinous Na2Te. In 1931 Zintl, Goubeau and
Dullenkopf 130 also investigated the sequential reactions of tellurium with sodium in
liquid ammonia and reported the compound formed to be Na2Te (white precipitate,
yellow solution). These authors questioned the formation of Na2Te4. Klemm,
Sodomann and Langmesser 57 also reported the synthesis of Na2Te using the same
procedure. Zintl, Harder and Danth186 reported the X-ray structure of Na2Te
prepared in liquid NH3 solution. Although the monotelluride had been shown to be a
crystalline material,186, Kraus and Chim 185 observed the product remaining when
the ammonia was evaporated from a polytelluride solution to be distinctly metallic in
appearance, although no free tellurium was present. All of the r_eported telluride and
polytelluride solutions and isolated species were unstable upon exposure to air.
In 1929 Kraus and Glass187 studied the specific resistance of a series of
sodium-tellurium alloys and deduced the existence of Na2Te6 as the longest chain
polytelluride. These authors also reported the sodium tellurium phase diagram 188 and
observed the species Na2Te, Na2Te2 and Na2Te6.
There is still disagreement as to the identity of the least reduced sodium
polytelluride formed in liquid ammonia solution. Schultz and Koehler 189 studied
34
liquid ammonia solutions of sodium polytellurides formed in situ, by using UV -visible
spectroscopy. The results of the investigation indicated that NazTez and NazTe3 were
distinct spectroscopically identifiable species and were stable in liquid NH3 solution.
An intermediate yellow solution prior to the formation of the characteristic red
polytellurides, was attributed to the presence of the radical Te- species, whilst NazTe
was shown to be insoluble in liquid ammonia. No polytelluride more oxidised than
Na2Te3 was observed.
More recently Schultz190 reported a similar study with potassium polytellurides.
Again U.V-visible spectroscopic investigation indicated that KzTez and KzTe3 were
stable species in liquid NH3 solution. KTe was proposed as being responsible for the
yellow colour observed in the ammonia solution. Again no polytelluride more
oxidised than K2Te3 was formed.
However as in the case of the polyselenides, the spectra of the various species
displayed a high degree of similarity and were characterised by multiple overlapping
bands. These results do not provide an unambiguous assignment of the species in
polytelluride solutions. Therefore a more comprehensive study of polytelluride
solutions using 125Te NMR was undertaken in this work; see Chapter 6. Consistent
with the lack of knowledge surrounding polytelluride solutions is the small number of
isolated compounds found. The characterised crystalline salts of the known
polytelluride ions (not containing transition and post-transition metals) are listed in
Table 1.6, together with their preparative reactions and their characterisation methods.
1. 9 Transition Metal Polytellurides
In contrast to the large number of discrete, metal containing polysulfide and
polyselenide complexes known, there are few characterised transition-metal
polytelluride complexes (Table 1.7). However in recent years there has been a
growing interest in the synthesis of new telluride and polytelluride complexes for use
35
36
as IR sensory materials 201, superconducting solids 202 and amorphous spin glasses ..
with tunable conducting properties. 203,204
Table 1.6 Crystalline salts of polytelluride ions
Polytelluride ion Preparative Crystallisation Characterisation Reference Year
Cation Reactions Solvent Data
[Te2]2- Mg+Te Nil Crystal Structure 191 1969
Mg2+ at 670K
[Te3]2- K +Teat670K Nil Crystal structure 192 1978
K+ C, H, N analysis
[Te3]2- Rb/Cs + Te NH3 Crystal structure 193 1980
Rb+ orCs+ inNH3
[Te3]2- K2Te +Te + en Crystal structure 194 1977
[K-2,2,2-crypt]+ 2,2,2-crypt + en
[Te3]2- en Crystal 195 1987
[Ba(enh.4 or s]2 structures
[Te4]2- K4SnTe4 + MeOH Crystal structure 196 1984
Ph4P+. 2MeOH Ph4PBr +
MeOH
[Te4]2- NaT1Te2 + en Crystal structure 197 1985
fN a-2,2,2-crypt]-t 2,2,2-crypt + en
[Te4]2- Na2Te3 + diethyl ether o Crystal structure 198 1991
Ph4P+ PPh4Cl+DMF THF IR
+MnCl2 or
FeCl2
[Tes]2- K2Te3 +H20 H20 Crystal structure 52 1983
Bu4N+ + Bu4NBr C,H,N analysis
Raman
[Te9]2- No Details No Details No Details 199 1985 Na+ Reported Reported Reported
[TeH]- K2SiTe3 or en Crystal structure 200 1989 PPh4+ K2GeTe3 + IR
Ph4PBr +en
37
Table 1.7 ANIONIC TRANSITION METAL POLYTELLURIDES
Formula Source of Solvent Metal precursor Characterisa Reference selenide tion data
[Nb(Tew)]3- . DMF PPh4+ KzTe4 DMF NbC Is X-ray 21 structure
[Pd(Te4hl2- PPh4+ KzTe4 DMF PdCI2 X-ray 205 structure
[Mo4Te16(e~)]2- K(Crypt)]+ KzTe4 en Moz(OzCMe) X-ray 206 structure
[H&4Telz)J4- BU4N+ KzHgzTe3 en KzHgzTe3 X-ray 207 structure
[HgzTes]2- Ph4P+ KzHgzTe3 en KzHgzTe3 X-ray 207 structure
[KAu9Te7]2- Ph4p+ K3AuTez McOH/DMF KzHgzTe3 X-ray 208 structure
[KzAU4Te4(en)4]2· Ph4p+ KAuTe en KAuTe X-ray 208 structure
[KAU4Te4(DMF)2(CH30H)2]2- K3AuTez MeOH/DMF KzHgzTe3 X-ray 208 Ph4P+ structure
[Cr3(Te4)6]3- Ph4P+ KzTex DMF CrCl3 Crystal 149 Structure
[OMo(Te4)2]2· Ph4P+ KzTcz DMF MoCls Crystal 209 Structure
I.R. 125Te NMR
[OW(Te4)2]2· Ph4p+ KzTcz DMF WCls Crystal 209 Structure
I.R. 125Te NMR
[Hg(Te4)2]2· (Ph3P)2N+ Not Not reported Not reported Not reported 8 reoorted
[PbzTe3]2- KPbTc en KbTe Crystal 210 Structure
(2,2,2-crypt )K+
[Ni4(Te4(T~)z(T~)4]4- LizTe + Te DMF Ni(SzCOEt)z Crystal 173 Structure
Et4N+ 125Te NMR
[Fe8 Te1 0(CO)z0]2- (Ph4P)zTe4 DMF Fe(CO)s Crystal 211
Ph4P+ structure IR
1.9.1 Synthetic routes to metal polytellurides
The ethylenediamine extraction of certain ternary Zintl-type melts of composition
Ax My Tez (where A is an alkali metal, M is a transition metal), has provided a
convenient preparative route to a variety of structurally diverse polytelluride clusters
and polymers such as [Hg4Te12]4- 208 (Figure 1.5a), oo[Hg2Tes]2- 208 (Figure 1.5b),
[K2AuTe4]2-.4 solvent 209, and [KAu9Te7]4- 209. These compounds have no
analogues in sulfur or selenium chemistry and suggest that the transition metal
polytelluride systems may be quite different from those of the other chalcogens.
Surprisingly alkali metal polytelluride ions such as K2Te4 redissolved in en or
DMF have been rarely used to generate transition metal polytellurides (equation 1.28).
The potential of this method is illustrated by Kolis and coworkers' recent report 21 of
NbTe103-, formed in the reaction between NbC15 and K2Te4 redissolved in DMF, in
which the Nb atom is encapsulated in a" birdcage-like" Te sheath (Figure l.lf).
DMF/cn
PdCl2 + 2K2Te4 + Ph4PBr ---7 Pd(Te4h(PPh4h + 2KC1 + 2KBr ... l.28
Products from the reactions between metal carbonyls and polytellurides are
different from those of polysulfides and polyselenides. No homoleptic polytellurides
have thus far been reported using metal carbonyl precursors. 9
One important reason for the apparent change in reactivity of polytellurides is the
increased reducing ability of polytellurides compared to polyselenides and
polysulfides. Evidence for this is that polytellurides can reduce functional groups like
metal-metal bonds to form metal carbonyl anions. Thus reactions such as equation
1.29 can occur.9
... 1.29
38
Figure 1.5a. Structure of the IH~ Te 12]4- anion.208
Figure 1.5b. Two views of the ool Hg2 Te5]2- anion. 208
Figure 1.5c. Structure of the I (C0)4Cr(Te4) )2- anion.9
Substitution does occur in several cases, leading to a variety of novel products.
With the simple M(C0)6 (M= Cr, Mo, W) carbonyls, an excess of polytelluride leads
to substitution with formation of a chelated tetratelluride (equation 1.30) (Figure
1.5c.).9
M(C0)6 (M= Cr, Mo, W) +excess Te42- ~ [(C0)4M(Te4)]2- + 2CO ... 1.30
1.10 Thermodynamic influence of sol vent on the activities of anions
Solvents vary widely in their solvating properties and can influence drastically
the physical and reactivity properties of solutes. One of the most pronounced effects
is on the activities of anions. For example the solid AgSPh(s) will not dissolve in H20
or in excess PhS- in water but will dissolve readily in aprotic solvents to form anionic
cluster complexes (equation 1.31).
AgSPh(s) + PhS-(soln) ~ [Agx(SPh)y]z-(soln) ... 1.31
The preference for aprotic solvents over protic solvents in chalcogenide reactions
may be explained by considering Figure 1.6.212 In Figure 1.6, the thermodynamic
activities of the anions are presented as logarithms of activity coefficients in solvent S
relative to a reference solvent, methanol. The activity of PhS- is comparable with that
of Br. It can also be observed that the relative activity of these two ligands is 107
times greater in DMF than in water and the activities of solutes with high anionic
charge densities are greatly enhanced in aprotic solvents which are unable to solvate
such anions. It can also be seen that the activity of acetate ion varies by 1Q12 between
H20 and DMF, whilst the chloride ion is almost 109 times more active in MeCN than
H20. The quantitive solvent activity coefficients for anions presented in Figure 1.6
39
agree with the qualitative observations by Parker et al 213 that small charged anions are
more strongly solvated by protic solvents than by dipolar aprotic solvents, whereas
large polarisable anions are more solvated by dipolar aprotic than by protic solvents.
From the above discussion the equilibria for polychalcogenides described in
equation 1.32 is likely to be shifted to the right in aprotic solvents as the activity of the
anion is increased, resulting in enhanced solubility, or to the left in protic solvents.
M(Ex)(s) + y(Ex)(solvl- ~ [M(Ex)y+1J(solvly
where E = S, Se, Te
... 1.32
This solvent dependency of the activities of anions is probably due to the
hydrogen bonding capabilities of the solvent ( i.e protic solvents have hydrogen
bonding effects to anionic species whereas aprotic solvents cannot) and to the anionic
charge density at accessible solute sites. 214,215,216,217
The choice of the starting material will also be influenced by solvent effects. For
example the addition of counterions and metal halides introduces an extra ligand which
also becomes activated in aprotic solvents and can compete with the polychalcogenide.
Another effect of these varied activities is that an increase in ion activity
decreases the solubilities of compounds. The solubility of reactants. and products was
probably the main consideration in choosing the solvent in almost all of my reactions,
aprotic solvents such as DMF or acetonitrile were used.
DMF was usually the choice of solvent for reaction mixtures as reactions carried
out in aprotic solvents other than DMF, with the chalcogenides, usually yielded thick
colloidal solutions (see chapter 4 for details). DMF also allowed most of these
reactions to be carried out as homogeneous solutions.
40
Figure 1.6. Relative thermodynamic activities of various anions in water, acetonitrile
and DMF relative to methanol.213
9
8
7
6
..-.. 5 en c-
:I: 4 ~ ~
0 3 cO 0 -
2
Warer
Solvent (S)
. Acetooittile DMF ( ,.
CH1CXX>-"""- ..
I I I
I I ~--·a--~
I I ,'I
I I -- ' BPhf I I r- _ -Br-
\ II I \ II I
' 'J I .... --i"---~ \ I I \ II I I y' I /
NC2 1/'1 11
0~~ -~~;L\ I ,...,.....,
.AgO-/ \ .../_ ~ 2....,
-NC2 I /( -- J
I \ --~
" \ AgBr2 vj -~ B~~-/-- \ h ..... ~f2 -'-B~V T BPh~ ci"ti 1
... ..... ~
g02 iCH3UJO-
9
8
7
6
5 PhS-
4
.3
2
1
0
-1
-2
-3
1.11 Conclusion
The three elements of group 16, S, Se and Te have a tendency to catenate. This
results not only in rings or chains of their elemental forms, but also in the formation of
polychalcogenide Ei- (E = S, Se, Te; x = 2-6) ions which have predominately been
characterised in the solid state (tables 1.2, 1.4, 1.6). The versatile chelating ability of
the polychalcogenide ions, particularly the polysulfides, has in the last two decades
received considerable interest resulting in the isolation of numerous, structurally
diverse metal polysulfides. This period of time corresponds to a shift away from
classical aqueous chemistry to the use of non-aqueous, aprotic solvent chemistry. The
reason for this as explained in section 1.1 0, is the large thermodynamic influence of
aprotic solvents on the activities of polychalcogenide anions.
However, prior to the commencement of this work in 1988, the chemistry of
metal polyselenides and polytellurides, was rare. One reason for this is that the
chemistry of the metal polyselenides and polytellurides was expected to parallel that of
the metal polysulfides and consequently little interest was paid to them.
Despite the large amount of information about crystalline polychalcogenides,
uncertainty has until now, surrounded knowledge of these species and their
interconversions in solution. This uncertainty occurred because the electronic and
vibrational spectra used to monitor the species in these solutions were not definitive.
I have investigated the solution chemistry of Sex2- and Tex2- ions and their metal
complexes, in DMF solutions utilising the NMR capablity of the active nuclei 77Se and
125Te. The NMR technique is unique in that it can probe individual Se and Te atoms
and its inherent advantages over existing vibrational and electronic spectra are
presented in chapter two.
41
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53
54
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55
CHAPTER 2
Background on Multinuclear Magnetic Resonance
2.1 Introduction to 32S, 77 Se and 123/125 Te NMR
Fourier transform nuclear magnetic resonance (Ff NMR) spectroscopy has been
used to great advantage to study the chemistry of many elements of the periodic table,
including nuclei of the elements sulfur, selenium and tellurium.l-6 Many of the earlier
results for these nuclei were obtained by indirect double resonance methods,l,2 but
modem multinuclear FT instrumentation has now developed to the point where direct
observation is possible.
For sulfur, the lightest and most chemically prolific of these three elements, the
only NMR isotope with a nuclear spin is 33S. Unfortunately, 33S has properties that
are far from ideal for NMR work, being a quadrupolar nucleus (I= 3/2) that suffers
from very low natural abundance and low sensitivity relative to 13C. Table 2.1
summarises the salient nuclear properties of the magnetic isotopes of sulfur, selenium
and tellurium. Quadrupolar nuclei generate broad resonances that are difficult to
observe. Nonetheless some hundreds of sulfur compounds have now been examined
successfully) These are usually ones with a fairly high degree of symmetry so as to
reduce the electric field gradient at the sulfur nucleus and hence the quadrupolar
broadening experienced. There is a brief review of 33S NMR in the literature)
77Se and 125Te nuclei on the other hand, both have 1=1/2 isotopes with sufficient
sensitivity to make their study readily accessible by FT NMR. As can be seen from
Table 2.1, both 77Se and 125Te have natural abundances and sensitivities which make
them more favourable than 33S for NMR study. Whilst 123Te also has I= 1/2, its low
abundance makes it uncompetitive with 125Te. Although 125Te has a negative
magnetogyric ratio, proton-tellurium distances are usually so great that nuclear
Overhauser enhancement (NOB) is nonnally negligible and standard spin 1/2 methods
56
(i.e. proton decoupling, moderate pulse angles, fast pulse repetition rates
(milliseconds to seconds ) can be used in the majority of cases for this nucleus and for
77Se.
However in most of the compounds in the earlier literature there is low atomic
proportions of Se or Te. Therefore in these compounds relaxation times can be rather
long, necessitating slow pulse repetiton rates in the absence of added relaxation
reagent. This was found not to be the case in the alkali and transition metal
polyselenide and polytelluride compounds studied here, which typically had relaxation
times of <ls (see chapter 3).
Finally, the wide chemical shift ranges for 77Se and 125Te NMR of ca 3000 and
4000 ppm respectively and the consequent dynamic range of 10-2- lQ-5 s for access
to interconversion reactions makes both nuclei fundamentally useful.
Table 2.1: NMR Properties of Magnetic Isotopes of Sulfur, Selenium,
and Tellurium
Nuclide Spin Natural Receptivitya Resonance Standard reference material
frequencyb
33S 3/2 0.76 0.097 7.670123 2M Cs2S04 in H20 at pH 7.5
77Se 1/2 7.58 2.948 19.071523 Me2Se
123Te 1j2C 0.87 0.89 26.169773 Me2Te
125Te 1/2C 6.99 12.5 31.549802 Me2Te
aRelative to that of 13C
bFor the reference compound in a magnetic field strength for which Me4Si has a
proton resonance frequency of 100 MHz.
CNegative y.
57
Prior to the commencement of- this work in 1988, most of the literature
concerning 77Se and 125Te NMR was based on organic compounds. This work has
been extensively reviewed, with considerably more work being done on selenium
NMR than on tellurium NMR.3,4,5,6 The 77Se NMR technique had .also been applied
successfully in the characterisation of some coordination compounds including the
Zintl anions- MSe22- (M=Hg, Cd, ), MSe32_ (M=Sn, Tl ), SnSe42·, Tl2Se22· and
Pb2Se32-,?,8 selenocations,9,10,11 metal selenocyanates,12 metal phenyl selenolates,
13,14,15 metal selenoether complexes,l6,17,18,19,20,21,22 metal diselenocarbamates,23
and metal thioselenocarbamate complexes.24 The only paper concerning the 77Se
NMR of metal polyselenides was reported by Wardle et aJ.25 77Se NMR had also
been applied to the study of heterocyclic selenium sulfides SexSs-x obtained from
molten mixtures of the elements.26
Tellurium NMR spectroscopy has more recently been used to determine the
structures of cationic tellurides, Se-Te mixed cations,27 a large number of
organometallic compounds 28 and very recently, a few soluble metal tellurides.29
2.2 Chemical Shifts
The chemical shift range now exceeds 3000 ppm for selenium and
approximately 4000 ppm for tellurium. The standard references used for the two
nuclei are Me2Se and Me2Te respectively. All chemical shifts in this thesis are
referenced to these two compounds. The chemical shift convention used is that a
positive sign signifies a chemical shift to a frequency higher than that of reference
compound and conversely a negative sign signifies a chemical shift to a frequency
lower than that of the reference compound. Other reference compounds have been
used including Se02(aq), SeOCl2, selenophene, and Te(OH)6. Selenium dioxide
solutions30 have the disadvantage of being strongly pH-dependent.31 The chemical
shifts and coupling constants (wherever available) for representative organic and
58
inorganic compounds are listed in Table 2.2, and their chemical shifts are correlated
with chemical functionality in Figures 2.1 and 2.2.
In considering the data presented below it should be noted that variations up to
ca. 10ppm with solvent and temperature have been reported for 77Se and about twice
this amount for 125Te. Detailed studies of this behaviour for MezSe and MezTe in 30
different solvents indicate that solvent effects in 77Se and 125Te NMR chemical shifts
are influenced primarily by dipolarity and little, if at all, by hydrogen bond donor
acidity (except possibly in the case of chloroform) or polarisability. 32
Table 2.2 Chemical shifts and coupling constants for selenium and
tellurium compounds referenced to Me2Se and Me2Te respectively.
Compound Solvent o a (ppm) J (Hz) Reference
77SeNMR
MezSe Liquid 0 33
MeSe-Na+ H20 -322 16
Me3Se+I· HzO 253 16
Li+ (SeSiH3)· D20 -736 34
SeF6 Liquid 610 30
SeC4 DMF 1154 30
CF3SeH 287 35
Ni(SezCNEtz)z CDCl3 386 36
Pd(Se2CNEt2h CDCI3 265 36
Zn(Se2CNEt2h CDCI3 648 36
K(Se2CNiBu2) DzO 582 36
Pd(Se2CNiB u2) CDCl3 378 36
Ni(Se2CNiBu2) CDCl3 401 36
Pt(SezCNiBuz) CDCl3 415 Pt-Se, 36
111.7
Zn(Se2CNiB u2) CDCI3 668 36
Cd(Se2CNnBu2h CDCl3 717 36
59
[MoSe4] (NE14)2 DMF 1643 37
[WSe4] (NlJ4)2 DMF 1235 W-Se, 52 38
[MoSe(Se4)2] (NE14)z DMF t, 2357 37
m, 1163
r, 403
[WSe(Se4)z](AsPh4)2 DMF t, 1787 37
m, 1034 W-Se,
r, 324 108
[MoS(Se4)z](NE14)z DMF m, 1122 37
r, 396
[WS(Se4)2](AsPh4)2 DMF m, 993 W-Se, 37
r, 313 106
[MoO(Se4)2](NE14)2 DMF m, 946 37
r, 380
[WO(Se4)2] (AsPh4)2 DMF m, 828 37
r, 280
HgSe22- en -142 Hg-Se, 39
2258
Pb2Se32- en -99.4 Pb-Se, 40
149
Zn(SePh)42- MeOH 15.7 41
HgSeTe2- en -30.5 Hg-Se, 39
2270
Se42+ 30% oleum 1958 42 Seg2+ 30% oleum -106.1 64,41 43
219.3 152, 65,
668.0 35
-231 248, 153,
-256.3 84
247' 86,
37
Me3PSe -235 44 (Me0)3PSe -396 44 Te2Se22+ 30% oleum 1638 42
125TeNMR
60
Me2Te Liquid 0 45
Te(OH)6 H20 207 46
HgTe22- en -726 Hg-Te, 39 6500
CdTe22- en -1159 Cd-Te, 39 2148
Pb2Te32- en -927.1 Pb-Te, 40 1070
HgSeTe2- en -1495 Hg-Te, 41 6470
b[MoO(Te4)2](PPh4)2 DMF 717 47 89
b[WO(Te4)2](PPh4)2 DMF -120 47 -903
Te62+ 30% oleum 152 42
Te42+ 30% oleum 2811 42 Te2Se22+ 30% oleum 3102 42
a t, Terminal; b, bridging; m, metal bound: r, ring. b Chemical shifts referenced to
2.2.1 Chemical Shift Patterns
It is evident from the data in table 2.1 that the ranges of chemical shifts for these
nuclei are very wide and the chemical shift patterns complex. However a number of
systematic trends have been identified in the chemical shift of these nuclei, but their
interpretation is often not straightforward. It is reasonably well established that
electron withdrawal from selenium or tellurium will lead to an increase in the chemical
shift, as exemplified by series such as MeSe-, Me2Se, Me3Se+, and also by the low
chemical shifts of R3P=E, which can be attributed to predominance of the R3P+-E
canonical form E = (Se or Te).44
Nonetheless it must be emphasised that there are significant deviations from this
behaviour, notably in some species p-XC6H4TeCl3 where greater electron release by
2000 1500
Figure 2.1 Pattern of 77
Se Chemical Shifts
*Na2Sex -----------
R2Sex
------ RSe-
Ss-xSex
*M(Sex)y2-
Selenophenes
1000 500 0 I: 77 u ( Se)/ppm
R = organic functional group M =transition and post-transition metal * = this work
-500
Figure 2.2 Pattern of 125Te Chemical Shifts
R = organic functional group M= transition and posl- transition metal *=this work
3000 2000
Tellurophenes
1000
*Na/KTeH
0 -1000
61
the aryl group (as predicted by the characteristics of X) increases 8(125Te),48 in the
patterns of chemical shifts in the series H3Se03+, H3Se03, H2Se03- and HSe032-
obtained from Se02 under various conditions31 and in various mixed MeO/OHIF
derivatives of Te(OH)6 where the more electronegative substituent (fluorine) produces
lower tellurium chemical shifts. 3
Thus attempts to use 8(17Se) or 8(125Te) to derive substituent electronegativities
may not always be correct, although the pattern of shieldings in the mixed clusters
Te4-xSex2+ and Te6-xSex2+ can be readily interpreted by invoking the greater
electronegativity of selenium.10,42
An chemical shift scale for anionic tungsten and molybdenum selenides has been
deduced recently where terminal Se o > 1000 ppm; bridging Se 1100 ::;; 8 ::;; 600 ppm;
metal-bound Se (in an MSex ring ) 1100 ::;; o ::;; 500; and ring Se ( in an MSex ring), 8
::;;4Q0ppm.
Se II
-M-
terminal
Se / ' M M
bridging
Se-Se
M~ I Se-Se
Se-Se
M~ I Se-Se
metal-bound ring
The chemical shifts for metal bound and ring selenium nuclei in molybdenum
compounds are at higher chemical shift relative to the analogous tungsten compounds
(see table 2.1).37.49 In these molecules there is probably orbital overlap between
selenium and molybdenum which leads to a higher chemical shift. In both
molybdenum and tungsten complexes, the substitution of selenium by sulfur or
oxygen results in positive shifts of the remaining selenium resonances, i.e 8Se
MSe(Se4)22- > MS(Se4)22- > MO(Se4)22-. The availability of low lying vacant d
orbitals on selenium and sulfur atoms favours greater overlap with the metal atom.
In the series of SexS8-x molecules 26 the chemical shifts are easily distributed
into three groups (see below): the selenium atoms with two sulfur neighbours show
62
the chemical shifts in the region above 690 ppm, selenium atom has one sulfur and
one selenium atom as the nearest neighbours, the chemical shift observed is at 690-
620 ppm. The chemical shifts of selenium with two selenium neighbours lie below
620ppm.
s-se-s >690ppm
Se-Se-S
690-620 ppm
Se-Se-Se
< 690ppm
Within each group an additional trend is apparent. If there are selenium atoms in the
3-, 5-, or 7- positions with respect to the active nucleus, the signal is shifted toward
higher chemical shift. The effect seems to be cumulative. Also, selenium atoms in the
4- or 6- positions show a similiar though weaker effect (see table 2.6).
These results generally indicate a direct correlation between an increase in
electron-density and a more negative chemical shift. However the question of whether
the chemical shift derives primarily from the diamagnetic or the paramagnetic term
associated with it is still uncertain. Evidence for the importance of the latter comes
from correlations of shielding with electronic excitation energies in certain classes of
compound which can be regarded as arising from the ~E component in the shielding
equation 2.1.
aP = -(~E)-l(Q4p + Q4d)
crp = paramagnetic term
~ = mean excitation energy
... 2.1
Q4p and Q4d are related to electron imbalance in the p and d
orbitals of Se.
For example in the organic molecules with a selenium- carbon double bond, such as
(t-Bu)2CSe, the o(77Se) is found to correlate with the wavelength of then---) 1C *
63
transition for the C=Se group. 50 Additional evidence has come from the enhanced
temperature dependences of o(77Se) and o(125Te) in diselenides and ditellurides
respectively. 51 An important feature of the literature is that tellurium and selenium
shieldings run closely parallel in equivalent compounds 52, a plot of o(77Te) against
o(77Se) being linear with a slope of c.a. 1.8. McFarlane et al 44 have based this
rationalisation of the linear relationship between selenium and tellurium chemical shifts
upon the paramagnetic shielding term crP, described. by the Ramsey's shielding
expression (equation 2.2),53 as it makes the dominant contribution to the observed
chemical shift ranges of the heavy nuclei. 54
a= ad+ crP ... 2.2
2.3 Relaxation Behavior
There have been comparatively few measurements of 77Se and 125Te relaxation times,
and these have tended to indicate a range of values for T1. Values ofT1 vary from
under lsecond for H2Se in D20 55 and Me2 Te56 to ca. 6s in the series of SexS8-x ring
molecules,26 to over 45 seconds for Me2Se at -60. C. 55
The dipole-dipole relaxation mechanism is seldom important. 57 The lack of dipole
dipole relaxation can be attributed to large E-H distances arising from the large
covalent radii of selenium and tellurium. However, intermolecular NOEs (implying
some dipole-dipole relaxation) have been reported for aqueous selenious acid. 58.
For small freely tumbling molecules the spin chemical shift anisotropy may be
important as a contributor to relaxation.57,59,60 This is especially true for species with
P=Se and C-Se double bonds for which the selenium shielding anisotropy may be
large.59,60
64
2. 4 Selenium Coupling Constants
Many of the earliest measurements of coupling between 77Se and nuclei other than lH
or 19p were actually done indirectly by means of 1H-77Se 'spin-tickling' experiments,
a procedure ·that utilises the high sensitivity of the proton and also gives the sign of the
coupling in many cases.62 This was done primarily because of the low natural
abundance (7.7%) of 77Se. However with the advent of more sensitve FT
instrumentation, coupling to most nuclei can be as easily observed in the 77Se
spectrum. 63,64,65
Coupling between 77Se and other spin 1/2 nuclei fall into two categories. The
first category is when Se is coupled to another nucleus X with I = 1/2 and having
100% natural abundance e.g 31p, lH, I9f. The nse NMR spectrum shows one pair
of doublets in a 1:1 ratio with no satellites. The X NMR spectrum will show a central
uncoupled peak with symmetrically placed satellites flanking the main peak. The
intensities of the satellites are dependent on the natural abundance of 77Se and how
many 77Se atoms are coupled. If one 77Se then a doublet is observed each normally
having ca. 3.8% of the intensity of this peak i.e 1/2 x 7.8%.61 If two 77Se are
coupled then a doublet of ca. 7.6% intensity of the central resonance in a 1:1 ratio are
seen.
The second category is when Se is coupled to another nucleus X where I = 1/2
and the natural abundance is < 100% e.g 113Cd, 119Sn, 199Hg. For the case X-77Se
the 77Se spectrum will show a main peak flanked by symmetrically placed satellites
flanking the main peak, the size of which are dependant on the natural abundance of X
and how many X atoms are coupled. Similarly, in the X spectrum a main peak is
observed with symmetrically placed satellites flanking the main peak the size of which
are dependant on the natural abundance of Se and how many Se atoms are coupled.
65
2.41 DJ ( 77Se~1H)
This topic was last reviewed by H.C.E. McFarlane and W. McFarlane.4 Rather
little in the way of fundamentally new results have appeared since then. Some typical
values ofnJ(17Se-1H) are given in Table 2.3.
Table 2.3 Values of nJ( 77Se-1H)
n Compound nJ(Hz) Reference
1 H2Se 65.4 66
MeSeH 44 67
PhSeH 56 67
2 MeSe·K+ 6.6 67
Me2Se -10.5 67
Me3Se+I- 9.3 68
Me2SeBr2 10.0 67
Me2SeO 10.7 67
2.42 nJ (77Se~77Se) and nJ (125Te .125Te)
The one bond 77Se-77Se coupling constants reported for a large number of
diselenides 69,70, organopolyselenides 71, and SexSs-x species 26, are in general,
small, i.e 4-55Hz. Larger values for the tellurium coupling constants are observed in
diary! tellurides, 213-369 Hz for 1J(123Te-125Te) and 170-207 Hz for1J(125Te
I25Te).72 However, larger coupling constants have been reported between chalcogen
atoms of organodichalcogenide anions, REE- (E = Se or Te), i.e 267-323 Hz for Seb
Set. (b =bridging, t =terminal), 1637-2186 Hz for Teb-Tet. and 481-738 Hz for Teb
Set.73 This data indicates that the nature of Eb-Et- anion bonds is significantly
different from that ofEb-Eb bonds in neutral species.
66
The 2J(77Se-77Se) coupling constants in the series of SexSS-x molecules 261ie in
the range 96-114 Hz. This is consistent with the two bond coupling observed for
dialkyl polyselenides, R-Sex-R, which ranged between 112-117 Hz.7l
There is very little data for the longer range coupling. In mono-, di-,and
triseleno- substituted alkenes it was found that 3Jsese is related to the dihedral angle.74
In the cis isomer the coupling is in the range 77-117 Hz whereas the trans isomer the
coupling is found between 2-12Hz. Eggert et al 71 have suggested that the magnitude
of the coupling depends on the the extent of interaction between the p lone pairs of the
selenium atoms in question. The dihedral angles in the polyselenide chains and also in
the eight-membered selenium sulfide ring molecules are close to 900. Therefore the p
lone pair overlap between the adjacent Se atoms is minimised. The corresponding
lone pairs in the atoms i and i + 2 have approximately the same orientation and have
therefore a possibility for the p-orbital overlap. It can therefore be understood why
lJsese < 2JseSe· The scheme also explains why the longer range coupling can be
expected to be relatively small in these systems. The observed range of 3Jsese in
selenium sulfides is 3-10Hz and that of4Jscsc 3-19Hz.
The lJsese coupling constants in the SexSs-x molecules 26 also show a trend
depending on the chemical surroundings of the SeSe bond. The absolute value of the
coupling constant for the isolated SeSe bonds (the structural unit -S-Se-Se-S-) is ca.
50hz. The end bond of a longer selenium fragment (the structural unit -S-Se-Se-Se- )
has a somewhat lower 77Se-77Se coupling of ca. 35 Hz, and the bond surrounded by
SeSe bonds (the structural unit -Se-Se-Se.-Se- ) shows a still lower coupling of ca. 20
Hz.
The values of the 2Jsese coupling constants seem to depend on the identity of the
atom between the two selenium atoms in question. In case of sulfur (the structural
unit -Se-S-Se- ) the coupling constant is ca. 95 Hz but rises to ca. 110Hz if the
67
middle of the fragment is selenium (the structural unit -Se-Se-Se- ). For longer range
coupling there is not enough data to make any conclusion about the trends.
2.4.3 nJ (77 Se-31 p)
The background to phosphorus-selenium coupling is extensively dealt with in
chapter7.
2.5 nJ (125Te • X)
As for selenium, the coupling constants for nJ (125Te- X) have in the past most
readily been determined by measuring the positions of the 125Te satellites in the
spectrum of the other nucleus. Relevant examples of tellurium coupling with other
nuclei are listed in table 2.4.
Table 2.4
Various Nuclei.
Compound
H2Te
Me2Te
Et2Te
TeF6
(t-Bu)3PTe
[(t-Bu)2P]2Te
(Me3Sn)2Te
Te42+
Selected One-Bond Couplings between l25Te and
nJ(125Te-X) (Hz) Reference
1J(125Te-1H) -59 75
2J(125Te-1H) -20.7 68
3J(125Te-1H) -22.7 76
1J(l25Te-19p) 3688 77
1J(125Te_31 P) 1600 78
1J(125Te-31 P) 451 78
1J(125Te-119Sn) -2270 79
1J(125Te-123Te) ±608 42
68
2.6 Other Relevant Polyselenides
2.6.1 Organic Polyselenides (R-Sex- R)
While organic polysulfides (R-Sx-R) x::; 6 are well documented substances,80
little information on the corresonding selenium compounds existed prior to 1986.
During this year Eggert et al 71 alkylated solution of polyselenide ions with various
alkyl halides or tosylates and subjected the resulting mixtures of R-Sex-R to 77Se
NMR analysis. Some of the relevant 77Se chemical shifts for a series of dialkyl di
and polyselenides are collected in Table 2.5.
Table 2.5 77Se Chemical Shifts (ppm) of Dialkyl Polyselenides
R-Sex-R
n=2 m=3 m=4 R
m=5
o(77sea.) o(77sea.) o(77seP) o(77sea.) 8(77Se~) o(77sea.) o(77Se~) o(77Se'Y)
Octyl 316.1 465.7 562.3 482.4 712.5 488.7 708.3
Cyclohexyl 375.1 578.5 495.5 577.0 690.2
2 ethyl hexyl 294.7 432.5 601.9 450.3 730.7 457.1 728.9
a. = outermost Se atom, P = inner Se atom, Y = innermost Se atom
The chemical shift range for the various selenium nuclei presented in table 2.4
covers more than 500 ppm. The R-Se-Se-R species consisting of an Se2 fragment
was clearly in evidence. It is surprising then that signals attributable to an Se2
fragment were observed for a Na2Se2 solution (see chapter 4). The 77Se NMR
spectrum of the dicyclohexyl polyselenide mixture was temperature dependent. At
55"C the spectrum showed five signals corresponding to a dicyclohexyl diselenide,
triselenide and tetraselenide. When the temperature was decreased, all peaks
broadened, reached a maximum line width for the a.-selenium signals about -20°C. At
855.2
854.7
69
-70°C the five signals sharpened again. This temperature dependence was observed
only for cyclohexyl as the alkyl group and is ascribed to ring inversion of the
cyclohexyl ring.
2.62 Selenium Sulfide Ring Molecules SexSes-x
The formation of the eight-membered selenium sulfide ring molecules in the
molten mixtures of sulfur and selenium is well established.26,81 The characterisation
of the crystalline phases obtained from the melts is difficult because it is not possible
to study pure stoichiometric compounds, since different species crystallise together
forming solid solution of complex molecular composition 26 However Laitenen et a1
26 have shown that 77Se NMR spectroscopy is a useful tool to study the cbmplicated
binary system of sulfur and selenium. Their results indicate the SexS8-x (n = 0-8)
system comprises of 29 distinct molecules excluding the optical isomers. The series
of molecules of special interest are those with conserved selenium atoms in the ring as
shown in Figure 2.3. The 77Se NMR of the various nuclei are presented in table 2.6.
The identification of the various SexSs-x species was based on the combined
information from the natural-abundance spectra and from the spectra obtained by use
of selenium enriched with 77Se isotope .
.... ..,. ...... 7 \: ............
circles, and S atoms by open circles.
70
Tabe 2.6 77Se Chemical Shifts (ppm) and 77Se- 77Se Coupling
Constants (Hz) of SexSs-x Ring Molecules.
Molecul&l Equivalent Chemical 1Jsese 2Jsese 3Jsese 4JseSe nuclei shift
A3 2 654.2 34
1 560.6 34
B3 1 727.4 98 6
1 662.9 54 98
1 653.0 54
C3 1 723.7 12 19
1 662.6 51 19
1 619.7 51 12
A4 2 664.4 39,17 112 3
2 581.6 39,17 112 3
B4 1 722.4 96 8 16
1 669.0 37 96,114
1 641.6 35 114 16 1 588.9 35,37 8
D4 2 680.8 56 97 10 2 655.4 56 10 0.5
As 2 - 657.9 40 110 5 3
1 598.2 23 110
2 591.2 23,40 108 5
a Refers to molecules in Figure 2.1
2.7 Other Nuclei
Table 2.7 summarises the salient nuclear properties of various other nuclei used
in this work.
71
Table 2.7 Salient nuclear properties of various nuclei.
Nuclide Spin Natural Receptivity a Resonance Standard
Abundance Frequency_ b Reference
113Cd 1/2 12.26 7.6 22.178 Cd(Cl04)2C
199Hg 1/2 16.84 5.42 17.910 H_gMe2 31p 1/2 100 377 40.480 85%H3P04
119Sn 1/2 8.58 19.54 37.290 SnMe4 195pt 1/2 33.8 19.1 21.4617 Na2PtCl6d
a Relative to that of Be, b Relative to I H = 1OOHz, c 0.1 M solution, d 1M aqueous
solution
113Cd NMR
113Cd NMR spectroscopy has received considerable attention since 1976 ,
where its utility as a metallobioprobe was demonstrated by Armitage. 82 Several
reviews which include selected aspects of the 113Cd NMR literature have recently
appeared. 83,84,85,86 The efficacy of 113Cd NMR as a nuclear probe is due in part to
the ability of Cd (the oxidation state is +2 ) to form complexes with a wide variety of
configurations. Also Cd2+ possesses a filled d-shell making the nucleus diamagnetic.
In addition 113Cd has a spin of 1/2, and therefore has no quadrupolar contribution to
NMR relaxation which broadens NMR signals. The 113Cd nucleus has a relatively
high natural abundance of 12.26% (compared to 1.108% for 13C) resulting in a
receptivity relative to Be of 7.6. Finally 113Cd has a demonstrated chemical shift
range of over 900ppm.
Cadmium has a second spin 1/2 nuclei, lllCd, with properties suitable for NMR
measurements (natural abundance of 12.75%) However the receptivity of lllCd is
less than that of 113Cd resulting in the majority of NMR studies having been
performed with the latter nuclide.
72
A significant paper by Banda et al 87 published after this review, presented a
113Cd NMR study of cadium polysulfide complexes in solution. Here 113Cd NMR of
non-aqueous solutions of cadmium polysulfides revealed the existence of a series of
monocadium complexes, [Cd(SxhJ2- which were shown to be in slow exchange at
ambient temperature. I have used both 113Cd and 77Se NMR to follow the reactions
between Cd2+ and polyselenide ions in DMF solutions (see Chapter 5 ).
199Hg NMR
Mercury has two isotopes, 199Hg and 201 Hg with non-zero nuclear spins both
occurring in useful proportions. The more receptive nucleus, 199Hg, with spin 1/2
and natural abundance of 16.84% can be observed without too much difficulty. 201Hg
has on the other hand proved more difficult to observe presumably as a consequence
of the quadrupole moment associated with a spin 3/2. A number of 199Hg studies
have been reported and reviewed. 88,89
Mercury chemical shifts are very medium dependent. HgMe2 in DMSO
resonates at -108 ppm whilst in hexane resonates at 5 ppm.90 A range relative to
Hg(Me)2 of between ca. 500 ppm (organo-mercury compounds) to ca. -3000 ppm for
inorganic salts has been reported. 89
A relevant recent paper by Bailey et al 91 presented a 199Hg study on the non
aqueous solution chemistry of mercury polysulfides. Here the 199Hg NMR
spectroscopy revealed the existence of variable chain length [Hg(Sx)(Sy)]2-
complexes within a solution of nominally only one chain length, thus suggesting that
in solution mercury polysulfides are in equilibria and undergo chemical exchange that
is slow on the NMR time scale at ambient temperatures. A study of mercury
polyselenides in this work shows that similiar equilibria do not exist in non-aqueous
mercury polyselenide solutions. (see Chapter 5).
73
3lp NMR
The observation of 3lp NMR spectra is easy with the 31p nucleus having I= l/2
and 100% natural abundance.
Values of the chemical shift can be quite large, ranging up to i376 ppm in PN92
but a range from -224 ppm (P4S3) to 326 ppm (P4010) is more typical.89
Chapter 7 presents a comprehensive compilation of phosphorus sulfides and
selenides relevant to this work.
2. 8 References
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79
CHAPTER 3
Experimental Techniques
3.0 Introduction
Monochalcogenide and polychalcogenide solutions and solids are highly reduced
species and therefore susceptible to aerial oxidation. Oxidation of them results in the
formation of elemental chalco gen. Therefore atmospheric control was required in all
experiments. A range of methods for handling reactive air sensitve materials are
available including, glove boxes, glove bags, and Schlenk type equipment. These
methods are briefly discussed in this chapter along with the handling procedures and
physical techniques used throughout this work.
3.1 Preparative Experimental Techniques
i) The inert - atmosphere glove box can provide a straightforward means of
handling air- sensitive solids and liquids. In its simplest form, it consists of a gas
tight box fitted with a window, a pair of gloves, and a gas- tight door or tranfer port.
The entire box is flushed with an inert gas, after which samples may be manipulated in
the inert atmosphere. The glove bag is a simple and inexpensive variant of the dry
box. It consists of a large polyethylene bag fitted with a nitrogen inlet and a open end
which may be closed by rolling and clamping. The bag is purged by several cycles of
filling with inert gas and collapsing, or by a continuous flush. Manipulations are
accomplished with the integral polyethylene gloves.
The appealing simplicity of glove - box manipulations must be weighed against
the disadvantages, which are the slowness of operation and difficulty in maintaining a
water and oxygen free atmosphere. Dry box work is slow because of the difficulty of
transfers into and out of the box and a reduction of dexterity due to the gloves. The
problem of maintaining an air- free atmosphere is not only complicated by impurities
in the flushing gas and leaks in the glove box but also by the diffusion of moisture and
80
oxygen through the gloves. Because of this continual influx of impurities, it is
necesssary with lengthy experiments to rejuvenate the atmosphere by a recirculating
purification system or a constant flush.
The chemistry involved in this work required rapid and easy manipulation of
solutions and simple transfer of solids. Because of these factors, it was the opinion of
this author, that both glove boxes and glove bags were generally unsuitable.
Therefore a Schlenk manifold and Schlenk- type equipment (descibed below) which
fulfilled both of these requirements was used for the handling of air - sensitive
compounds in this work.
ii) The Schlenk Manifold (Figure 3.1)1 provided the means for manipulations
of highly air-sensitive species, utilising an inert atmosphere of high purity N2(g) (a).
A paraffin bubbler, fitted to the N2 line allows the monitoring of the N2 flow rate.(b).
A mercury manometer (c) serves as a pressure-release outlet and also as an indicator of
the pressure inside the reaction vessels open to the manifold. The source of vacuum
was an oil filled mechanical pump (j), which is protected from volatiles by two solvent
traps (h). A three-way stopcock (i) placed between the vacuum pump and solvent
traps serves as a method of opening to atmosphere.
Schlenk apparatus was employed for routine handling of compounds and
reactions to further reduce the chance of oxidation. The essential pieces of equipment
are the Schlenk flask which consisted of a round bottom flask fitted with a sidearm
with a ground glass two-way stopcock. All other glassware was fitted with quick-fit
ground glass joints which allowed them to be assembled in various configurations to
get the desired apparatus. These joints were well greased with high vacuum grease to
ensure a gas tight seal.
The side arms of the Schlenk flasks were connected to the manifold via three way
taps (e) for routine work. A third line (g), consisting only of a two way stopcock
connected to the vacuum line, was used strictly for removal of solvents to avoid
contamination within the dinitrogen line. A fourth line (d) consisted of a cannula fitted
81
f
0
c
Figure 3.1 Vacuum/ dinitrogen manifold.
on to a piece of vacuum tubing via a piece of shaped glass. This line was used only
for degassing of solutions by bubbling of N2 and for introducing positive pressure in
filtrations. This stopcock was connected only to the dinitrogen line thus preventing
accidental suckback of degassing solvents. Flasks were sealed with greased ground
glass stoppers or rubber septa.
3.2 General Procedures2
3.2.1 Deoxygenation and the Addition of Solvents and Reactants.
Reaction vessels and apparatus were usually assembled while hot and purged of
air by three cycles of evacuation and dinitrogen flushing.
Solvents and liquid reactants were purified (section 3.2.5) and deoxygenated by
one of three methods depending on the type of reaction. Crude deoxygenation, by
sparging with N2 (ca. 10 minutes), was employed onl:y for the preparation of NMR
compounds known to be less air-sensitive. For most preparations, deoxygenation
was achieved by three cycles of evacuation and N2 flushing. For more rigorous
degassing, a 'freeze-pump-thaw' method was employed)
Solvents, liquid reactants and solutions were transferred by either syringe techniques
or by cannulas.2 Syringes are purged by inserting the needle in a septum fitted on a
preflushed Schlenk tube under positive pressure. The positive dinitrogen pressure
was maintained and the N2 was withdrawn into the syringe and expelled. This
procedure was repeated several times. The reactant may then be pressurised into the
syringe and transferred. A small amount of N2 was sucked into the tip of the syringe
after the liquid in order to protect the liquid from the atmosphere.
Cannula techniques were used mainly for transferring large quantities of
solutions or solvents. The cannulas used were made from stainless steel tubing which
was ground to a point at one end and flat cut on the other. The flat cut end was
inserted through a prepunctured septum fitted on the vessel containing the solution to
be transferred. The sharpened end is inserted through a septum on the receiving flask.
82
Dinitrogen is allowed into the first vessel forcing the liquid through the cannula. A
needle is inserted into the septum of the receiving flask in order to vent it.
Solid reactants (when necessary) were addded against a strong flow of Nz
minimising entry of 02.
All reaction vessels were sealed with ground glass joints under a slight dinitrogen
pressure, secured with elastic bands and wrapped in para:film.
Pyrex test tubes were employed for some crystallisations and preparations of
NMR solutions. They were degassed by N2 sparging of the solution contained
within. Reactants were added against a strong flow of dinitrogen. These reactions
were sealed by a septum and wrapped in parafilm.
3.2.2 Solvent Removal.
Solvent removal, either to concentrate the solution or to change solvent
composition, was achieved by low pressure distillation in a closed system to a large
capacity low temperature solvent trap cooled with liquid nitrogen. The flask from
which the solvent was to be removed was connected directly to the solvent collection
trap. A three way stopcock between this trap and the rest of the manifold was then
adjusted so that the experiment is then isolated from the rest of the manifold. The trap
was pumped down, then opened to the flask, which was stirred in an oil bath
maintained at a temperature higher than the distillation temperature of the solvent in
question. The rate of distillation was controlled by the temperature of the continually
stirring solution, degree of immersion in the oil bath and the setting of the stopcock on
the flask.
The procedure is accompanied by a cooling effect on the reaction mixture which
provides a means of checking the solubility and crystallisation of products. If
crystallisation occurs, distillation is ceased and the reaction mixture heated in an
attempt to dissolve the solid and the resulting solution allowed to cool at room
83
temperature. On completion dinitrogen gas was readmitted before the system is
disconnected.
3.2.3 Filtration
Filtration of reaction solutions and products was achieved using one or a
combination of the methods listed below. Schlenk apparati were utilised for filtering
very air sensitive solutions of reduced selenium and tellurium.
(i) Schlenk filter: The filter consisted of a glass tube containing a sintered glass
fritte fitted (of appropriate porosity) at both ends with male ground glass joints (Figure
3.2a). One end of the filter was attached to a receiving flask, whilst the other was
attached to the reaction flask. The filtration assembly was then evacuated and flushed
with N2. Filtration occurred by slowly rotating the filtration assembly. The reaction
flask's stopcock was closed and the receiving flask opened to slight vacuum. The rate
of filtration was controlled by the receiving flask's stopcock. On completion N2 was
reintroduced to the assembly via the reaction flask's stopcock.
(ii) Cannula fitted with a filter stick: A filtering stick consists of a fritte attached
to a small glass tube. A cannula is inserted through the septum which is fitted onto the
open end of the tube. The procedure then is the same as transferring solvent through a
cannula, filtering being achieved by a pressure differential (Figure 3.2b).
(iii) Open Buchner system: filtration by this method is usually reserved for well
crystallised samples, which can be filtered rapidly, thus restricting oxidation.
Crystalline samples which are moderately air sensitive may be filtered this way with
the aid of an inverted funnel connected to the nitrogen line, 'blanketing' the filtration
apparatus with inert gas (Figure 3.2c).
All solids were washed with the crystallising solvent usually MeCN, before being
dried under vacuum. Samples were stored in sealed glass vials. This was achieved by
filling the storage vial inside a nitrogen glove bag and fitting a rubber septum to it. The
vial was then removed from the nitrogen bag and evacuated and refilled with nitrogen
84
Nitrogen out
Nitrogen in
•• _/ Frit
b) Filtration by Carmula and needles
Figure 3.2. Filtering techniques
a) Filtration using Schlenk techniques c) Inverted Funnel
through a needle connected to the manifold. The vial was then sealed at a pre-formed
constriction with a gas torch.
3.2.4 Heating, Cooling and Dissolving.
Temperatures below ambient were maintained at constant values by the use of
slush baths. Typically an acetone/dry ice bath (-780C) was used in condensation
reactions involving liquid ammonia. The heating of reaction flasks was achieved in a
paraffin oil bath using a variable temperature magnetic stirring hotplate.
A Bransson 2200 ultra-sonication bath was used for the sonochemical
acceleration of heterogeneous reactions including dissolution of slowly soluble
species.
3.2.5 Purification of Solvents and Reagents
All reagents were laboratory grade (Aldrich, Ajax, not less than 98% pure) except for
Te powder (Aldrich 99.999%) and sodium dithionite (Aldrich, ca.68% pure) and were
used as received. 'Wet' or 'old' NaBB4 (Aldrich) was purified by dissolving the
solid in dry diglyme at sooc.4 The solution was filtered and allowed to cool in an ice
bath. A crystalline complex, NaBH4.diglyme was obtained, filtered and dried in
vacuo overnight to remove the diglyme.
All solvents were laboratory grade and were stored over 4A type molecular
sieves. Solvents which were treated before use are listed in table 3.1.
Pure solvents were distilled under inert gas in properly designed stills, allowing
storage and withdrawal of the distilled solvent.
85
Table 3.1 Solvents and their purification procedures.
Solvent
Acetonitrile (Ajax)S
Diethyl ether (Ajax)
Ethanol (Ajax)6
DMF (Aldrich)
THF (Ajax)
Drying procedure
Contact with K2C03 for 24hrs. Distilled from P4010·
Distilled from sodium benzophenone ketyl.
Contact with MgS04 and then K2C03. Distilled from
Mg turnings (iodine catalyst).
Stored over 4A molecular sieves. Distilled from BaO.
Distilled from sodium benzophenone ketyl.
3.3 Crystallisation Techniques
In polychalcogenide chemistry it is nearly impossible to predict the structures of
the proposed complexes or elucidate their structures with vibrational or electronic
spectroscopic techniques. Therefore many compounds were characterised by single
crystal X-Ray crystallography. It was of fundamental importance then, that
crystallisation techniques be developed capable of growing single crystals with
approximate dimensions of between 0.1 mm and 0.3mm. These dimensions represent
respectively, the minimum and maximum sized crystal required for optimal data
collection on the Nonius CAD4 diffractometer used. Three crystallisation procedures
were used throughout the course of this work; 1) Evaporation of solvent (described
above), 2) Solvent layering and 3) U-tube diffusion.
3.3.1 Solvent layering
Most reactions required the addition of a counter- ion to initiate crystallisation as
the product in most of the reactions studied was expected to be anionic and therefore
soluble. Typically, Ph4P+ and Bu4N+ salts were used. This was usually achieved
either by preparing a solution of the counter- ion in a solvent, usually MeCN, and
86
then adding the solution to the reaction slowly, mixing the reagents resulting in
crystallisation. Alternatively, the counterion was present in the reaction mixture and
then a second, precipitating solvent was added. Crystal gowth via this method was
usually achieved inside a test tube (Figure 3.3a).
This procedure was used for two reasons. Firstly, many reactions were
performed in DMF which, due to its high boiling point, made the use of the
evaporation technique difficult. Secondly, most of the final products were soluble in
DMF. Therefore a precipitating solvent was added slowly changing the solvent
composition and resulting in the product becoming less soluble. Of course the
solubility of the product(s) would often be unknown and so a range of solvents would
be tested. This was achieved in situ by adding a small amount of test solvent,
observing various temperature effects on solubility, then pumping the solvent away
before adding another solvent to be tested. Typically it was found that acetonitrile,
diethyl ether or THF were most successful layering solvents when dimethylformamide
was used as the reaction solvent.
3.3.2 U-tube diffusion
Often solvent layering techniques produced fine microcrystals that were too
small for single-crystallographic X-ray studies. Therefore a crystallisation technique
with a slower rate of crystallisation was required so that larger crystals would form.
Glass U-tubes were used for this purpose. (Figure 3.3b). These were sealed at either
end by a rubber septum which enabled the system to be evacuated and filled with
nitrogen using cannula techniques. Several millilitres of the reaction mixture, (E),
was then added via a cannula (A), into one of the arms. An outlet needle (F) allowed
a pressure differential which was removed immediately after the addition of reactants.
A separating "plug" of pure solvent, (C), again several millilitres, was then added to
be used as a diffusing medium. Finally, a third solution (B), containing the counter
87
(i) (ii) (iii) (iv)
Figure 3.3a. The liquid diffusion method of growing crystals in an inen attnosphere.
(i) Solution of the compound to be crystallised. (ii) -(iii) slow addition of the
precipitating solvent containing counter- ion. (iv) Crystals form where interface was.
A Canula
B Counter-ion
Solution
c Solvent 'Plug'
6 6
F Outlet Needle
E Reaction Mixture
Figure 3.3b. U - Tube diffusion method for growing crystals.
ion was slowly added, so that the "plug"; C, was between the reaction·mixture, E,
and the counter ion solution, B.(Figure 3.3b). It was found that supporting this U
tube inside a dewar containing hot water and leaving overnight to equilibrate to
ambient temperature resulted in formation of crystals, inside the diffusion medium,
suitable for single crystal X-ray analysis.
3.4 Instruments used for identification of products
3.4.1 Single-crystal X-ray diffractometry
Data collection was accomplished with a Nonius CAD4 instrument operated by
Mr Don Craig, whilst crystal data refinement was carried out by Mr. Don Craig and
Dr. Marcia Scudder, of the Chemistry School of UNSW, using standard
crystallographic techniques. The geometrical information provided by them was
interpreted by this author.
3.4.2 Elemental Analysis
C, H, N analyses were performed by Dr P.Pham of the Microanalysis service
of the Chemistry School of UNSW.
3.4.3 Inductively Coupled Plasma (ICP) Analysis
ICP data were obtained on a 'plasmalab' Labtam International atomic emission
spectrometer, provided with a polychromator and scanning monochromator, operated
by Mr Richard Finlayson, of the chemistry school of UNSW. Samples were generally
prepared by digesting c.a. 100 mg of sample in 15M HN03 (10 mls) overnight.
Concentrated HCl (60 ml) was added to the resulting solution which was then made
up to 1 litre with demineralised H20. This procedure generally provided adequate
metal analysis however results for selenium analyses were consistently high.
88
3.4.4 Infra-red Analysis
I.R. spectra were obtained on a Perkin-Elmer 580B spectrometer, interfaced to a
computer for analysis and presentation of results. Samples were run in three parts.
From 4000-400 cm-1 a halocarbon mull between CsBr plates was used. From 1480-
200 cm-1 a paraffin mull between CsBr plates was used. Finally from 400-200 cm-1 a
paraffm mull in between polyethylene plates was used.
3.4.5 Multinuclear Magnetic Resonance Spectroscopy.
All 77Se, 125Te, 113Cd, 199Hg, 119Sn, 31p NMR spectra were recorded on
Bruker CXP 300 or ACP 300 pulse spectrometers at field strength of 7 .OT, using
standard Bruker software. 10 mm multinuclear probes were used throughout all NMR
studies with spectra routinely obtained without locking (field drift was observed to be
< lHz/hr). Reference standards were run on the same day as samples. The observing
frequencies are listed in Table 3.2 for the various nuclei and spectrometer used. The
number of free induction decays, (FID's) accumulated depended upon the
concentration and sensitivity of the nucleus under consideration. For the more
concentrated samples, 1000- 3000 scans were collected, while 40,000- 200,000
scans were used for less concentrated samples: typical concentration range was 0.10-
0.?0M.
For 31p, where greater resolution was required, FID's were usually accumulated
with 16K data points over a spectral width of 20kHz resulting in an acquisition rime
of approximately 0.4s. Natural line widths at half-height varied over the range 4-20
Hz. Data resolution was approximately 2.5 Hz/pt. Resolution enhancement of the
resultant spectra was done with Gaussian multiplication of the FID; followed by zero
filling.
For 77Se, 125Te, 113Cd, 199Hg, 1 19Sn, FID's were usually accumulated with
16K data points over a spectral width of 60kHz, resulting in an acquisition time of
approximately 0.13s. Natural line widths at half-height for these nuclei varied over
89
the range 13-lOOOHz. Data resolution was usually between 6-8 Hz/pt as wide sweep
widths (lOOOOOHz) were required for many samples. The resultant spectra were
usually processed with line broadening between 1-20Hz, however values of the order
of 100Hz were used for broad lines. The spectra were recorded in the temperature
range of -73·c to 2TC. Table 3.2 lists the 90o pulse measured for various standard
nuclei, along with their chemical shift and spectral reference. · · , · ·
The 77Se NMR were run with a typical pulse width of 10 l.lS that corresponded
to a 450 pulse with a repetition rate usually between 0.5-1s. The species NazSes and
(Ph4P)z[Hg(Se4)z] showed very fast relaxation times (TI) of the order of 50-200
ms, as did NazTe3 in the 77se spectrum, whilst those of NaHSe and NazSe03
showed much longer relaxation times of the order of 5-7s (see Table 3.3). The T1
measurements were performed using an inversion recovery sequence consisting of a
180"/variable delay/90" acquisition sequence. Data was then fitted to a curve using
Bruker software (SIMFIT) program (see Figure 3.4).
The 31p, 125Te, 113Cd 199Hg, and 119Sn spectra were run with typical pulse
width values of 10, 12, 10, 20, 20 l.lS corresponding to 40, 50, 45, 50, 50 degree
pulses respectively. The repetiton rate was typically between 0.5-1.5 s. In general,
these spectra were recorded without allowing for full relaxation, and the relative
intensities reported are approximate, although all spectra where relative intensities are
compared were run under the same conditions. Integration was done by comparing
the relative intensities of peaks within a given spectrum. Numerical values were
provided by the integration routine in the Bruker software.
Temperatures between 210-300 K, were controlled using a VT-100 on the
CXP-300 and a Eurotherm on the ACP-300. Temperatures were calibrated by a
professional officer, Dr. James Hook, using the chemical shift difference in the proton
spectra found for MeOH, as described by the Varian EM 360 manual.
90
Figure 3.4. T1 measurement ofNaHSe in EtOH at 300K.
\.... ... . .,..
I
--- •• l,A... ........ ..,..._
2s
Is
O.ls
l 20s ~
: 15s
I.a. lOs r
8s
6s
4s
I . ---r i I e.o to~o 12.0 u..o 16.0 ts.o 20.0 TAU
ll•J· A• Tl• RSS PER POINT•
104.65,27 .9341'
5.46563 .15621
I I
I
-90 1.....---------·-----·--------_j
91
Table 3.2 Nuclei table for multi-nuclear NMR.
Nucleus Observing Secondary 90° Pulse Chemical Primary Freqency External (J..LS) Shift(ppm) External
(MHz) Reference relative to Reference external = 0 ppm
reference.
77Se 57.24 Na2Se03/ 22 )lS 1253 Me2Se
H20 sat
125Te 94.69 Ph2Te2f 23)ls 422 Me2Te
dichloro-methane
31p 121.49 85% H3P04 24 )lS 0 85% H3P04
113Cd 66.56 (Me4N)4 22 )lS 668 I 585 Cd(Cl04)2
[S4Cd10(SPhh6J (0.1 M)
in MeCN
199Hg 53.72 Ph2Hg/ 42 )lS -742 Me2Hg
dichloro-methane
119Sn 111.92 Me4Sn (neat) 24 )lS 0 Me4Sn
Table 3.3 T 1 Values for some polychalcogenide species.
Compound Solvent Temperature (K) T1 measurement
Na2Se03 DzO 300 7.2 s
NaHSe EtOH 300 5.5 s
Na2Ses DMF 220 103ms
(Ph4P)2[Hg(Se4)z] DMF 300 189ms
Na2Te3 DMF 220 37ms
3. 5 References
1. Figure taken from the PhD thesis of G. S. H. Lee University of New
South Wales 1991
2. D. Scliriver, "The Manipulation of Air Sensitive Compounds", McGraw
Hill, New York, (1969)
3. A. Nolle, P. Mahendroo, J. Phys. Chern., 1960, 33, 863
4. H. C Brown, E. J. Mead, B. C. Subba Rao, J. Am. Chem. Soc., 1955,
77, 6209
5. D. R. Burfield, K. H. Lee, R. H. Smithers, .T. Org. Chem., 1977,42,
3060
6. D. D. Perrin, W. L. F. Armarego, D. R. Perrin, in "Purification of
Laboratory Chemicals", Pergamon, Oxford, 1980
92
CHAPTER 4
Synthesis and Characterisation of Uncoordinated
Polyselenide,[Sex]2-, ions.
4.1 Introduction
Numerous uncoordinated polyselenide anions have bee11 structurally
characterised in the solid state as shown in Chapter 1. Many of these crystalline
polyselenide anion salts were isolated serendipitously from polyselenide solutions that
contained main-group or transition-metals, originally aimed at generating metal
complexes. In other cases the polyselenide ion isolated was not even present in the
inital reaction mixture. These results clearly indicate that complex equilibria exist in
polyselenide solutions.l,2 In contrast to this large amount of information about solid
compounds, very little information was reported about the species occurring in
solution and their interconversions. This lack of information occurred, as described in
chapter 1, because the electronic and vibrational spectra used to examine the species in
solution were not unambiguously definitive.
I proposed that selenium NMR spectroscopy would be well suited for
identification of species, structure determination, and characterisation of equilibria
involving polyselenide anions in solution. Selenium has a naturally occurring NMR
active, I= 1/2 isotope, 77Se, with a reasonable natural abundance of 7.6%. (see
chapter 2). This technique has the advantage over electronic and vibrational
spectroscopy of being able to probe individual selenium atoms. It has a dynamic range
of 10-2 - 1Q-5s, which can access interconversion reactions likely to occur in
polyselenide solutions. It was also predicted that the ability to make measurements
over a substantial temperature range would be advantageous in changing the rates of
interconversion reactions. Together with the wide chemical shift range (ca 3000ppm)
of 77Se NMR, permitting dispersion of the different Se resonances, identification of
93
individual selenium atoms in distinct chemical environments was expected to be
possible.
Consequently, a systematic investigation of the syntheses of various solutions of
selenides and polyselenides was undertaken. The preparative procedures resulting in
the most reproducible polyselenide solutions were then monitored by 77Se NMR. It
was expected that once the solution· chemistry of the uncoordinated polyselenides was
understood, main-group and transition metals could be introduced and these reactions
monitored by both 77 Se and metal NMR resulting in the isolation and characterisation
of new, exciting and potentially useful metal polyselenide species.
4.2 Syntheses and Characterisation of Uncoordinated Polyselenides
Full experimental detail is found at the end of this chapter.
4.2.1 Synthetic Approaches.
Various synthetic methods for the formation of mono- and polyselenide
solutions were investigated initially in order to establish a reproducible procedure for
the reduction of selenium, that was suitable for NMR study. The following methods
outlined in Table 4.1 involved the attempted reduction of selenium by various
reductants in various solvents. A more detailed description is presented in section
4.62.
94
95
Table 4.1 Synthetic Approaches to the Reduction of Elemental
Selenium.
SOLVENT REDUCTANT TEMP/ RESULT COMMENT (added as solid) oc
HzO/OH- NazSz04 75 Red/brown solution Red /brown solution after 1/2 hr then a indicative of NazSex.
colourless solution and White solid analysed as white precipitate after NazSe.
1hr. THF Na 45-70 No obvious reaction Unreacted Na and Se
present THF/EtOH Na 30 Red/brown solution The solution colour
after 1/2 hr. No further indicated NazSex. change after 1 hr. Reduction worked most
effectively in 1: 1 ratio of solvents. Complete
reduction to NazSe was not obseved on addition
of excess Na. EtOH Na 30 No observable reaction Unreacted Se and Na
after several hours. present EtOH NazS204 30-60 No observable reaction Unreacted Se and
after several hours. Na2S204 present H20 NaOH 80 Red/brown solution Colour indicates
with unreacted Se NazSex. However all present even after the Se would not react stirring overnight. even with excess
NaOH. DMF NaOH 80 Green/brown solution Colour suggested
after 1/2 hr with brown different NazSex solid present. No formed. Brown solid
further colour change decomposed readily in after 1 hr. air to Se.
MeCN NaOH 80 Red/brown solution Colour suggested after 1/2 hr with brown NazSex formed. Brown
solid present. After solid decomposed 1.5hr no further colour readily to Se in air.
change. Acetone NaOH 30 Crimson/red solution Colour change indicated
after 1/2 hr with red different NazSex solid present. species formed. Red
solid decomposed readily to Se in air.
DMF NaBR4 25 Red/brown solution Solution colour after 15min No further indicated NazSex.
change after 1.5hr species. Not possible Excess NaBR4 to completely reduce to
changed solution to expected colourless dark green. solution containing
NazSe even with excess NaBH4.
96
MeCN NaBfi4 60 Brown solution after Solution colour 15min proceeding to a indicated NazSex. crimson solution after species. Not possible to
1 hr. Excess NaBfi4 completely reduce to changed solution to red. expected colourless
solution containing NazSe even with excess
NaBif4. EtOH NaBR4 0 Red/brown solution Red/brown solution
after lOrnin. Excess indicated NazSex NaBR4 resulted in a present. Colourless
colourless solution that solution shown to readily oxidised to Se contain NaHSe.
on exposure to air. HzO NaBR4 20 Red/brown solution Red/brown solution
after 1 Ornin. Excess indicated NazSex NaBR4 resulted in a present. Colourless
colourless solution that solution shown to readily oxidised to Se contain NaHSe.
on exposure to air. DMF Na 60-90 Dark green solution Colour of solution
formed after 1 hr indicated different Excess Na resulted in a NazSex species. Not
emerald green then possible to completely red/brown solution. reduce to expected
colourless solution containing NazSe even
with excess Na. NH3(1) Na -78 Initially blue after 2rnin Blue colour attributed to
then dark green, solvated electron (see red/brown, emerald text). White solid
,brreen, and finally after analysed as NazSe. 1/2hr colourless with a Other colours attributed
white precipitate. to various NazSex ~ecies.
Table 4.1 shows that many of the solvent I reductant mixtures provided
solutions indicative of reduced selenium. However only three preparations were
capable of providing complete reduction of Se to either NazSe or NaHSe. The first
method using Na2S204 as the reductant 3, had problems in being reproducible. The
second method, using NaBR4 as the reductant 4 was very useful in generating the
soluble species, NaHSe, which had not been previously characterised in solution.
The third method,5 using Na as the reductant in liquid ammonia, was the only
procedure that provided access to both polyselenides and NazSe, the most reduced
selenium species. The ease and reproducibility of this procedure along with the
ability to remove the ammonia and replace it with an aprotic solvent like DMF for
NMR measurement, made this procedure the method of choice to study polyselenide
solutions. However the NaBI4 method was used to generate the hydroselenide
species, NaHSe.
4.2.2 Nuclear Magnetic Resonance Studies6
4.2.2.1 Monoselenide Anion, Se2·, and Hydroselenide Anion HSe·
The 77Se NMR spectra of several solutions containing HSe- are shown in
Figure 4.1 and all chemical shift data is presented in Table 4.2. All solutions of this
ion were colourless and highly air sensitive. The spectrum of the aqueous solutions
obtained by BJ4· reduction of Se is a doublet at oSe = -528 (300 "K) (Figure 4.1a.).
The splitting of ca 26 Hz is presumed to be the 1J (Se-H) coupling because it is
comparable with the value of 60.8 ± 0.1 Hz for H2Se 7, and values of 38-61Hz for
RSeH.8 Addition of NaOH, in attempt to deprotonate the HSe· caused the
crystallisation of a white, air sensitive solid believed to be Na2Se. This is consistent
with the fact that Na2Se as prepared by reaction of Na and Se in NH3(1) had very little
solubility in water. Na2Se also had little solubility in EtOH and DMF. This property
has also been reported by Odom et a1.9 Addition of Et3N to the aqueous solution of
HSe- did remove the splitting, but moved the resonance to -519 ppm. However this
experiment is not decisive, because the amount of Et3N required to be equivalent to the
high concentration of HSe- is not miscible with the aqueous phase: the high
proportion of Et3N probably caused a medium effect in the chemical shift, as
discussed below. Further the Ka data for H2Se in water is K1 = 1.3 x 10-4 10 and K2
= 1Q-15, 11 and indicate that Et3N would not be sufficiently basic to deprotonate HSe
in water. Because K2 (HSe·) is expected to be even smaller in aprotic solvents than
the value of 1Q-15 in water, it is most probable that in all of the solutions the principal
species is HSe-. The resonances for solutions of HSe-/Se2- formed by other means
and in two other solvents, EtOH and DMF, (see Table 4.2) are all at least 20Hz wide
97
(a)
{b)
-510
(c)
-430 -440
-520 ppm
ppm -450
-530 -540
-460
Figure 4.1. 77Se NMR of solutions of NaHSe (ca. 0.63M) in (a) H20, (b) ethanol,
(c) DMF, at 300K.
at half-height, and the lJ(Se-H) coupling is generally poorly resolved, if at all (Figure
4.lb and 4.1c). It is recognised that some of the line broadening (up to 60Hz) may
be caused by minor amounts of Se2- in fast exchange with HSe-. However the
spectra have been reproduced many times, indicating that adventitious base in these
unbuffered solutions is not influential. In the absence of definite evidence to the
contrary, it is presumed that the species in all the fully reduced (colourless) solutions
is HSe-.
Bjorgvinsson et al12 have recently claimed to have observed only a singlet in
ethylendiamine (en) solutions of NaHSe, indicating that the HSe- is partially or
completely deprotonated in the more basic en solvent. Unfortunately no chemical shift
was reported.
The solutions of HSe- prepared by borohydride reduction in ethanol showed
only a sharp singlet at oSe = -519 ppm (300°K) while in DMF the resonance shifted
substantially to -447 ppm (see figure 4.1(c)]. This is a medium influence on the
chemical shift, which is similar in magnitude to the medium influences observed for
Br, and other halide ions (see table 4.3) 13,14 and the newly observed HTe- ion
(Chapter 6), and correlates with the thermodynamic activation of small anions in
aprotic media)5,16,17 The oSe data for HSe- in DMF/Ethanol and DMF/water
mixtures are intermediate between the extremes (see Table 4.2) confirming that these
variations in chemical shifts are due to anion solvation.
The temperature dependence of oSe for HSe- in DMF is also large, ca. 1 ppm
per degree.
98
99
Table 4.2 77Se NMR data for solutions containing Se1 species
Solution Temp(°K) Chemical shift Resonance
(ppm)a characteristics
NaHSe in water 300 -529 doublet, 1J(17Se-1H) =
(0.63 M) 26Hz
NaHSe in ethanol 300 -518 singlet, 30 Hz
(0.63 M) fwhhh
NaHSeinDMF 300 -447 singlet, 55 Hz
(0.63 M) fwhh
260 -404
ca 245 -397
ca 235 -393
NaHSe in ca 30% 300 -462 doublet,
ethanol I DMF 1J(77Se-1H) ca
16Hz,
fwhh ca 13Hz
NaHSe in ca 45% 300 -475 doublet, 1J(77Se-
ethanol I DMF lH) ca 19Hz
fwhh ca 20Hz
NaHSe in ca 50% 300 -482 fwhh ca 90Hz
ethanol I DMF
NaHSe in ca 30% 300 -477 fwhhca 54Hz
wateriDMF
"Na2Se2" in water 300 -527
"Na2Se2" in ethanol 300 -518 25Hz doublet
just resolved
260 -514 25 Hz doublet
resolved
ca 245 -511 25 Hz doublet
well resolved
ca 235 -509 25 Hz doublet
well resolved
a Reference SeMe2 b fwhh = full width at half height
100
Table 4.3 Solvent effects on small anions.
Solvent NaHSe (0.63 M) KBr (0.20 M) Nal (0.20 M)
o 77Se (ppm) o 79Br (ppm)13,14 o 1271 (ppm)14
H20 -529 0 0
EtOH -518
DMF -447 +110 +167
DMSO +154 +266
4. 2. 2. 2 Diselenide Anion [Se2]2-
This ion has not been well characterised in solution. The reaction of Se and
equimolar Na in NH3 yielded a bright red solution and a white solid, but not a
homogeneous product. A similar result was observed by Sharp and Koehler.l8
Literature procedures for generation of [Se2]2- in solution, including reduction of Se
by BB4- in water (equation 4.1) or ethanol (equation 4.2),4 or by reduction with Na
in DMF,19 were assessed in previous papers only through further reactions of the
solutions, which were consistent with substantial yields of [Se2]2- in the reduction
stage.
4NaBB4 + 2Se + 2H20 --7 2NaHSe + Na2B407 + 14H2
Na2B407 + 2NaHSe + 2Se + 5H20 --7 2Na2Se2 + 4H3B03
.. .4.1a
.. .4.1b
Therefore the 77Se NMR of the solutions formed in ethanol or water by BB4-
reduction were measured, and found that between the ranges -860 to +820, a single
resonance at the same Ose as the HSe- solutions described above was found in both
cases. This result is consistent with the observation (see Experimental) that the
101
amount ofH2Se formed is less (ca 1/3) than predicted for reaction (equation 4.2), and
the fact that the solutions deposited some elemental Se on standing, indicating
disproportionation (equation 4.3 ). The resonance from the ethanol solution at -518
ppm is a doublet with the same 26Hz splitting as observed for NaHSe in water. Thus
there is NMR evidence HSe- but not for [Se2]2- in aqueous or ethanolic solution.
However, when equimolar Na and Se were reacted in DMF at 90°C a dark red
solution was obtained, which showed two equal intensity Se resonances at 598 and
347 ppm (ca 220K). This is consistent with the presence of [Se4]2- (see below), and
thus it appears that there is disproportionation of [Se2]2- (equations 4.3 and 4.4) to
[Se4]2- and probably [HSe]- although this was not detected .
When the "Na2Se2" formed by reaction of the elements in NH3, was treated
after NH3 removal with DMF there was not complete dissolution but instead formation
of white and black solids which is interpreted as disproportionation to Na2Se and Se
(equation 4.3).
Na2Se2 ~ Na2Se + Se
Na2Se + H20 ~ NaHSe + NaOH
4.2.2.3 Na2Sex in solution
.. .4.3
.. .4.4
Solutions with nominal compositions Na2Sex, x = 3,to 11, and concentrations
of approximately 0.3M were prepared by reaction ofNa and Se in NH3(l) followed by
removal of solvent and redissolution in DMF. The "Na2Se4" solution was brown,
"Na2Se3" was emerald green in NH3 but green/brown in DMF, while the other
"Na2Sex" solutions were dark green in both solvents.
All solutions were highly susceptible to oxidation by 02, precipitating Se on
exposure to air of less than 5 sec. Therefore rigorous air-sensitive techniques were
used throughout (see experimental ).
102
None of these solutions showed 77 Se resonances at 300°K, and it was necessary
to reduce the temperatures to ca 240°K in order to obtain satisfactory spectra. This
temperature effect is illustrated in Figure 4.2 with the spectra from the solution
"Na2Ses" at different temperatures. There is a collection of resonances which appear
for solutions "Na2Se3", 11Na2Se4 11,
11Na2Ses 11 and "Na2Se6"· but the resonances for
the solutions "Na2Sex" (x>6) were not detected .. The chemical shift range was
scanned from 2100 to -500 ppm, at room temperature and low temperatures, with no
evidence of additional resonances. Although all of the solutions are in the slow
exchange regime, the slow exchange limit for polyselenide interconversions is below
the accessible measurement temperatures.
The collection of resonances for 11Na2Se3", 11Na2Se4 11, "Na2Ses" and 11Na2Se6"
are shown diagrammatically in Figure 4.3. Clearly there exists more than one [Sex]2-
ion in some of these solutions, but the resonances belonging to each of the species
[Se3]2-, [Se4]2-, [Ses]2- and [Se6]2- can be assigned unequivocally.
Referring to Figure 4.3, the resonances at 251 and 192 ppm are due to [Se3]2-
as their intensity ratio of 2: 1 corresponds to that predicted for the -se-Se-Se- chain.
Similarly, the lines at 591 and 301 ppm are due to [Se4]2- as their intensity ratio of2:2
corresponds to that predicted for the -Se-Se-Se-Se- chain. The observation of a 11 pure
11 sample of Na2Se4 confirms this assignment although there are relatively small
variations in the resonance positions due to differences in temperature and solvent.
Those resonances at 860, 603 and 448 ppm (relative intensities 1 :2:2) are due to
[Ses]2-, while the equal intensity resonances at 798, 681 and 497 ppm are due to
[Se6]2-.
Unfortunately nJ(Se-Se) coupling satellites that could assist with the assignment
of the resonances for each ion were not able to be detected as the intensities of the
satellites at natural abundance are low (ca 5%), and the expected coupling magnitudes
of 20- 120Hz 20,21 are less than the line-widths of the resonances observed that are
typically 150- 300Hz wide. Only for [Se3]2- will the relative intensity data allow
900 800 700 600 500 400 ppm
Figure 4.2. 77Se NMR spectra of "Na2Ses" in DMF at the temperatures marked.
* .
900 800 700 600 500 400 300 200 100
ppm
Figure 4.3. Diagram of the 77Se NMR resonance positions and intensities for
solutions of nominal compositions Na2Sex in DMF at 230K.
103
unambiguous assignment of resonances to the individual Se atoms in the [Sex]2- ions.
In order to obtmn experimental information on the assignments of the resonances in
the other [Sex]2- ions, solvent influences were deployed (Figures 4.4 and 4.5). As
already described 4.2.2.1, the chemical shifts of 77Se in HSe- anions are strongly
dependent on solvent proticity. Like all instances of solvent proticity on anion
activity,l6,17,18 this effect is proportional to the negative charge density on the affected
atoms. Therefore the solvent influences on chemical shifts of the resonances in
[Sex]2- ions indicate the charge densities at the various Se atoms. The chemical shifts
of the resonances in a solution containing [Se3]2- and [Se4]2- (Figure 4.4) and a
solution containing [Ses]2- and [Se6]2- (Figure 4.5) vary in different ways as the
proportion of ethanol in DMF is increased.
Data for solutions in DMF and 50% DMF/ethanol mixtures are presented in
Figures 4.4 and 4.5. The notation ex., ~. y, is used to signify positions of Se atoms
relative to the chain ends in [Sex]2-.
In [Se3]2- the 257 ppm resonance (ex.) in DMF shifts to 193 ppm with increasing
solvent proticity, while the 193 ppm resonance (~) moves slightly in the other
direction to 214 ppm: the effect is greatest on first addition of ethanol to DMF. This
is entirely consistent with the assignments based on resonance intensities, and with the
assumption that the negative charge density on [Se3]2- is greater on the terminal ex. Se
atoms. This validates the assignment principle that can be applied to other [Sex]2-
ions, namely that as the negative charge density on Se diminishes with distance from
the terminal Se atoms the chemical shifts (at least in protic media) become less
negative. For [Se4]2- the higher chemical shift decreases by only 8 ppm with
addition of ethanol, while the other ose decreases by 29 ppm, thereby identifying the
equal intensity lines as ~ and ex. respectively. For the [Ses]2- the highest ose line
increases by 7 ppm, the intermediate line decreases by 34 ppm, and the lowest ose
decreases by 89 ppm, indicating the assignments Se(y), Se(~), Se(a) respectively for
these lines. In the case of [Se6]2- the solvent effects are similarly impressive, and
600 500 400 ppm
Se4(a)
Se3(a)
300
1
' ' '
Se3(~)
\ I \ I 1 I ,. ' ' ' •' •' ' \ I \ I I
200
DMF
+ 17 %(v) EtOH
+ 38%(v) EtOH
100
Figure 4.4. Diagram of the 77Se NMR resonance positions (at 230K) for a solution
containing a mixture of [Se3]2- and [Se4]2-.
900 800 700 600 ppm
500 400 300
Figure 4.5. Diagram of the 77Se NMR resonance positions (at 230K) for a solution
containing a mixture of [Se5]2- and [Se6]2-.
104
allow similar assignments: the highest 8se line increases by 5 ppm and is assigned
Se(y), the intermediate 8se line decreases· by 47 ppm and is assigned Se(J3), while the
lowest 8se line decreases by 95 ppm and is assigned Se(a). Note that the relationship
8se('Y) > 8se(J3) > ose(a) applies for all [Sex]2- ions, but is valid for [Se3]2- only
when the chemical shifts are those for protic solvents. Chemical shift assignments
standardised to the solution in DMF containing 38% EtOH are presented in Table 4.4.
Table 4.4 Chemical shifts at 220K for [Sex]2- ions in DMF plus
38% (vol) EtOH
[SexP- ion 8se(a) ppm 8se(J3) ppm ose('Y) ppm
[Se~]2- 193 214 -[Se4]2- 256 581 -[Ses]2- 354 570 868 [Se6]2- 404 636 807
105
-In general the line-widths of the [Sex]2- resonances decrease as ethanol is added
and the solvent proticity increases. This is interpreted as indicating retardation of
polyselenide exchange reactions on addition of ethanol, consistent with the normal
lowering of•the kinetic activity of anions with increased solvent proticity.
There have been two reports of the crystal structure of (Ph4P)2[Ses], from
crystals obtained accidentally from an irrational reaction22 between Zr + Zr(OEt)4 + Se
+ (OctMe2SihSe in DMF and as a side product 23 from the reaction between PtC12 +
Na2Sex in DMF. A direct, logical preparation of (Ph4P)2[Ses] was developed in this
research. The air-sensitive black-brown needles were prepared by the addition of two
equivalents of Ph4PBr in ethanol to a solution of Na2Ses in DMF. The crystal
structure of this compound was determined before the publication of the results of
others on the same compound 22,23. The molecular structure is shown in Figure 4.6.
There are some distinctive differences between the unit cell dimensions obtained in the
three independent measurements, listed for references 21 and 22 and this work
respectively: a= 9.573, 9.615, 9.617; b = 16.945, 17.122, 17.092; c = 13.563,
13.788, 13.724A, ~ = 105.29, 105.18, 105.18°. The residual R is substantially
lower for our structure than for the other two. It is interesting to compare the Se-Se
distances in [Ses]2- as obtained in the three independent determinations: the four Se
Se distances along the chain, listed for references 21, 22 and this work, are 2.316,
2.355, 2.366, 2.314; 2.326, 2.345, 2.365, 2.321; 2.308, 2.360, 2.354, 2.310A.
Thus two of the determinations, including ours, are in agreement with the chemical
notion that the Se(a)-Se(~) bonds are slightly (ca 0.04A) shorter than the Se(~)-Se(y)
bonds. The 77Se NMR spectrum of (Ph4P)2[Ses] dissolved in DMF at 300 K shows
no NMR signals but at 220K contains six resonances at 354, 404, 570, 636, 807, and
868 ppm (see Figure 4.7) respectively. These resonances show that a mixture of the
[Ses]2- and [Se6]2- species is present, indicative of slight oxidation. However this
Figure 4.6. Structure of the [Se5]2- anion.
900 800 700 600 500 400 ppm
Figure 4.7. 77Se NMR spectum of (Ph4PhSes in DMF at 220K.
106
experiment has been repeated many times using rigorous air sensitive techniques
suggesting that the oxidation is not due to the presence of adventitious 02. The
presence of [Se6]2- might indicate that some disproportionation is occuring however
the spectra showed no evidence of [Se4]2- or other more reduced polyselenide ions.
Low solubility in DMF at low temperature restricted the measurement of the 77Se
NMR spectrum of (Bu4N)2[Se6] prepared using the procedure descibed by
Haushalter.2 The (Bu4N)2[Se6] salt was very air-sensitive and also .. quite
hygroscopic.
4.4 (Ph4Ph[Se(Seshl
The crystallisation of the Ph4P+ salt of this unusual polyselenide anion was first
observed during my attempts to prepare polyselenide complexes of oxidising metal
ions, including Au(III) and Bi(III), and with some metal salts containing oxidising
ligands such as Cr(N03)3. The [Se1 I]2- anion has been characterised
crystallographically in this work (see below) and recently by Krebs et al.24
During preparation and characterisation of metal polysulfide complexes in
aprotic media it was observed that nitrate salts in DMF caused elongation of the
polysulfide chain, an oxidative process.25,26 Therefore I tested NaN03 as an oxidant
for polyselenides in DMF, and found that it converted [Ses]2- to [Se(Ses)2]2-, and
can be used as a convenient oxidant in the preparation of (Ph4P)z[Se(Ses)2]. The use
of Au(III) or I2 for oxidation of [Ses]2- to [Se(Ses)2]2- has more recently been
reported by Kanatzidis and Huang.27
The structure of (Ph4P)2[Se(Seshl shown in Figure 4.8, consists of a central Se
atom (situated at an inversion centre) chelated by two Ses2- ligands. Formally, the
central four coordinate selenium atom can be thought of as a Se2+ centre. The bonds
to the Ses2-Iigands from this central Se2+ atom are long, with Se-Se(l) = 2.652(1) A
and Se-Se(5) = 2.688(1) A respectively. Crystallographic details for
(Ph4P)z[Se(Ses)2] are given in appendix A.
107
The fact that a chain polyselenide version of the [Se11J2- anion has not yet been
found similiar to those for the various Sex2· species (x =3,4,5,6), is indicative of the
instability of such species either in the solid state or in solution. The implication is
that, beyond a certain polyselenide chain length, internal electron transfer within the
chain is favourable, resulting in a more compact, chelated, and presumably more
stable molecule. The unsuccessful attempts by this work to observe Sex2- (x > 6) in
solution using 77Se NMR would suggest this to be the case.
In the preparations ( described in chapter 5) of metal polyselenide complexes in
DMF and other aprotic solvents it is important to recognise that [Sex]2- could be
readily oxidised by both oxidising transition metals and oxidising counter anions
present. For this reason the commonly used metal nitrates were avoided because of
the proven ability of nitrates to oxidise the polyselenide solution.
Initial studies of solutions containing the [Se(Ses)2]2- ion did not display an
informative 77Se NMR spectrum. Even at low temperature (ca 235K) in DMF there is
a broad (1000Hz) resonance at 720 ppm. This resonance also appears in solutions of
other [Sex]2- ions that have undergone partial oxidation (see Figure 4.2). Four NMR
distinct Se sites were expected, but despite extensive searches (2100 to -500 ppm)
only the one broad resonance was located. The one resonance observed moved to
lower frequency and sharpened slightly on addition of ethanol. The magnitude of the
solvent dependence of the chemical shift of this broad signal is similar to that of the
Se(~) atoms in the non-cyclic [Sex]2- ions. However, on reducing the temperature to
200K the four expected resonances are resolved (Figure 4.9). The four resonances
occur at 795, 708, 672, and 484 ppm in a ratio of 1.5:2:2:1.5 respectively. At 200K
the full width at half height for each of these resonances was ca. 725Hz thus
precluding the observation of nJ (77Se-77Se) satellites. This result along with the
unexpected intensity ratio of 1.5:2:2:1.5 makes the assignment of the four resonances
to particular atoms in the molecule difficult. However the close proximity of the three
resonances at 795, 708, 670 ppm, suggests similiar atoms are involved, whilst the
Se(4)
Figure 4.8. Structure of the [Se(Sesh12- anion.
800 700 600 PPH
Se(3)
Figure 4.9. 77Se NMR spectrum of (Ph4P)2Se(Sesh in DMF at 210K.
500
108
resonance at 483 ppm suggests a different atom to the other three is involved. This
observation is consistent with the structural properties of the [Se(Ses)2]2- ion, where
the central four coordinate selenium atom can be thought of as a Se2+ centre that is
chelated by two Ses2- ligands. The resonance at 483 ppm then is assigned to this
central selenium atom. Unfortunately no similar molecules appear in the literature for
comparison. However this assignment is consistent with the notion that the central
selenium atom in this molecule is surrounded by the electron density associated with
the two Ses2- ligands. Therefore the resonance is predicted to be to a lower chemical
shift of the remaining 'ring' atoms. The remaining Se atoms are assigned using a., ~
and"( notation to signify their positions relative to the central atom. TheSe atoms (a.)
bound directly to the central atom are expected to have a greater electron density
associated with them then the other 'ring' atoms resulting in a relatively more shielded
resonance. This is predicted from the results observed for the metal chelated Se42-
systems. Therefore the resonance at 670 ppm is assigned to these atoms. Similarly,
the ~ and"{ atoms are expected to have less electron density associated with them then
the a. atoms therfore the resonances at 708 and 795 ppm are assigned to ~and 'Y
respectively.
109
4.5 Discussion
Although Se2- and HSe· are the simplest selenide species, their 8S e
characteristics had not previously been well established. Odom et al 9 observed a
resonance a.t -511 ppm for a solution of H2Se plus excess NH3 in D20, which was
interpreted without supporting evidence as being Se2- in equilibrium with a minor
amount ofHSe-. Lyons and Young 28measured a single resonance at -670 ppm for a
3M solution of K2Se in O.lm KOH. As this thesis was being written and about the
same time as our publication,6 Bjorgvinsson et al 12 found a strong singlet resonance
at -435 ppm assigned to the solvated Se2- anion when K2Se was treated with 2
equivalents of 2,2,2 crypt in ethylenediamine. Bjorgvinsson et aJ12 dissolved NaHSe
in anhydrous C2HsOH and observed the expected doublet for HSe- in the 77Se
spectrum at -495 ppm. The coupling however disappeared in an en solution of
NaHSe indicating that HSe· is partially or completely deprotonated in the more basic
ethylenediamine solvent.
The 8Se data are discussed in the context of those in Table 4.5 for other species
in the general classes XSe- and XSeH along with direct comparison to the results
recently observed by Bjorgvinsson et ai.12
In general 8Se becomes more negative with the negative charge on Se.8,13,29
By extrapolation of ~8Se for deprotonation of RSeH from R = Pri = Et, Me a value of
Ose for aqueous HSe- would be estimated (from 8Se for aqueous H2Se) as (-288- ca
200) = ca - 490 ppm. This approximate estimate is entirely consistent with the
observations in Table 4.5, and with our interpretation of the sample of (N14)2Se,
prepared by the deprotation of H2Se in excess NH3 in D20 solvent,8 as containing
Nif4+ + HSe- + NH3.
My value of HSe- in ethanol of -519 ppm at 300 K compares favourably with
that observed by Bjorgvinsson 12 of -495 ppm at (297"K) in the same solvent. The 24
ppm difference can probably be attributed to concentration differences.
110
Table 4 .5 77Se chemical shift data for species XSe- and XSeH.
Compound State 8se Reference
H2Se liquid -226 6, 7, 30
H2Se l.OM in -288 8
D20
NaHSe H20 -528 this work,
6
NaHSe EtOH -519 this work,
6
NaHSe DMF -447 this work,
6
NaHSe EtOH -495 12
(NJ4)2Se, interpreted D20, prepared as -511 9
as NJ4+ + HSe- + solution of H2Se,
NH3 excess NH3 in
D20
K2Se in O.IM aqueous -670 27
KOH
K2Se en -435 12
MeSeH neat, -116, 8
CDCl3 -130
MeSe-Na+ H20 -332 8
EtSeH liquid 42 8
EtSe- Na+ H20 -150 8
PriSeH liquid 159 8
Prise- Na+ H20 9 8
NCSe- K+ 0.5M in -322 31
ethanol
(H3Si)2Se liquid -666 32
H3SiSe- Li+ -736 33, this species is
involved in an
unspecified fast
exchange
111
The question remains open about the chemical shift for Se2-, which is likely to
exist as such only in aprotic basic media. The solution of K2Se in aqueous 0.1M
KOH listed in Table 4.1 would be expected to contain the HSe- ~ Se2- equilibrium
with HSe- dominant since pK2 for H2Se = 15)0,11 By general extrapolation using
the H2Se, HSe-, MeSe- data in Table 4.5, 8se (Se2-) might be expected to be at least
as negative as -700 ppm, and probably closer to -800 ppm. As Na2Se showed virtual
zero solubility in H20, ethanol and DMF our experiments could provide no direct data
for the chemical shift. However Bjorgvinsson et al 12 observed a single resonance
attributed to the solvated Se2 anion in ethylenediamine at -435 pm. Whilst it is known
that the chemical shift of the Se2- anion is very solvent dependent13, this result does
seem anomalous. The chemical shift at -435 ppm for Se2- is far from being the most
shielded selenium environment observed thus far. Compared with Se(SiH3)2,
-666ppm 32 Li+ ·SeSiH3, -736 ppm 33. It is more likely the species being observed
is in fact HSe-. The source of the proton is unclear, but the simplest explanation
would be an acid/base reaction between Se2- and the solvent and/or deprotonation of
H20 present in the solvent.
Kanatzidis 34 has recently claimed that polyselenide ions dissociate to
paramagnetic polyselenide radicals (see equation 4.5) in solution generating a 610 nm
absorption in the visible spectrum, characteristic of;
... 4.5
However the assignment2 of the 610 nm absorption in solutions of polyselenide
ions to polyselenide radicals is probably incorrect; such electronic absorptions had
previously been attributed to the polyselenide ions. IS Although the poor NMR spectra
of the polyselenide solutions at ambient temperatures could be due to paramagnetic
broadening, there is no evidence of concomitant paramagnetic shifting, and it is
112
considered by this author that the dissoCiation (equation 4.5) in solution occurs to a
very small extent that does not interfere with the recording of 77Se NMR.
It has been established unambiguously by this work that the four polyselenide
ions [Se3]2-, [Se4]2- [Ses]2- and [Se6]2- exist in solution, dispelling earlier claims of
the non existence of [Ses]2-,18 There is no evidence for the existence of [Se2]2- in
DMF solution. These results have been further validated by the recent work of
Bjorgvinsson et al who have shown by 77Se NMR the existence of Se32- and Se42- in
ethylenediamine and liquid NH3 solutions respectively.
As expected polyselenide ions are responsive to solvent proticity in various
ways, including the Se exchange dynamics and the Se chemical shifts. The magnitude
of the effect is dependent on the location of the Se atom in the polyselenide chain and
its negative charge density. There is no evidence that the solvent dependence involves
dissociation into radicals.
It is interesting to compare Bjorgvinsson's results 12 for the [Se32-] and [Se42-]
anions and the results reported here. The green triselenide ion [Se32-] is observed by
Bjorgvinsson as two singlets in the ratio 1:2 in the 77Se NMR. The signals are
reported at 278 and 304 ppm respectively in an ethylenediene solvent with 2-
equivalents of the complexing ligand 2,2,2 crypt present at (273 K). The peak at 304
was attributed to terminal Se atoms in the Se32- chain. A coupling, lJ (77Sea- 77Sep)
of 262 ± 6 Hz was observed.
This work shows the resonances for Se32- were observed in DMF only at low
temperature (240 K). The resonances were seen in a 2:1 ratio at 193 ppm and 214
ppm respectively. The low chemical shift resonance at 193 ppm is assigned to the
terminal Se atoms in contrast to Bjorgvinsson work that assigns the higher chemical
shift resonance as the outermost Se atom.12 Although these results seem
contradictory, they can be attributed to the solvent effect seen to exist when ethanol is
added to DMF solutions of [Se32-] (Figure 4.4).
113
Here it is shown that the 257 ppm resonance (a) in DMF shifted to 193 ppm
with increasing solvent proticity, while the 193 ppm resonance (j3) moved slightly in
the other direction to 214 ppm. This was consistent with the assignments based on
resonance intensities and with the assumption that the negative charge density on
[Se32-] is greater on the terminal a Se atoms.
Similarly Bjorgvinsson et al 12 observed 2 resonances at 321 and 608 ppm
respectively for [Se42-] with the signal at 321ppm attributed to the (a) Se atoms. The
argument used for this assignment is dubious, using the similarity of the 321 peak to
the terminal atom resonance of 304 in the [Se32-] spectra!
My results for [Se42-] in DMF at 230K show two equal intensity signals at 256
and 581 ppm respectively. The assignment of the lower chemical shift resonance at
256 to the (a) on terminal Se atom is consistent with that of Bjorgvinsson et a1.12 The
effect of adding a protic solvent, ethanol, to the DMF solution has resulted in a
significant lowering of chemical shift of the resonance at 256 ppm relative to the
resonance at 581 ppm (see table 4.4). With the assumption that the negative charge
density of [Se42-] is greater on the terminal a Se atoms, the terminal atoms would be
expected to be influenced to a greater extent by the addition of a protic solvent. This
effect was observed.
Bjorgvinsson et al 12 find no evidence even at 198 K, of higher polyselenides
than [Se42-]. I have shown the existence of [Ses2·] and [Se62-] .
Finally, in Figure 4.10 are charted values for Se atoms in four types of
polyselenium chains: (a) [Sex2·]; (b) Oct0 - Sex - Oct0 which are acyclic,35 (c)
selected (dlO) M(Se4) chelate rings [this thesis]; and (d) cyclic Ss-xSex.36
The 8Se values for Se atoms in polyselenide chains [Sex2-] of various lengths
provide good comparison for comparable 8Se data from polyselenium chains R-Sex-R
with various terminating groups R (see chapter 2). Discussing the [Sex2-] data first, it
can be seen that the weighted average chemical shift moves non-linearly to higher
frequency as the length of the chain increases, consistent with decreased average
114
negative charge per Se atom. The changes in oSe(a) with X in the [Sex2-] series are
more regular than the changes in oSe(~). The oSe(a) values and the oSe(XSe-)
values (Table 4.5) are entirely consistent with the general notion that the negative
charge on the terminal Se atoms of [Sex2-] is about 0.5. It is not clear why oSe(y) in
[Ses2-] is more negative than oSe(y) in [Se62-], although different torsional angles at
the centre of the longer chain may be responsible. It should be remembered that these
Se chemical shift tensors are very probably quite anisotropic and that an average
chemical shift only is accessible here. Further, the influence of solvent environment is
sufficient to interchange the two average chemical shifts in [Se32-]. Therefore for a
completely accurate comparison these variables need to be standardised.
In the series Octn- Sex- Octn it is the internal Se atoms that move than oSe(a) to
larger chemical shift as X increases. It is also noted that the oSe patterns for
comparable chains [Sex2-] and Octn- Sex - Octn are similar, with the oSe for [Sex2-]
less positive, and with the differences more pronounced for the shorter chains.
In the four MSe4 (M = znii, Hgll, SnlY) chelate rings (Figure 4.10) the oSe(~).
are virtually the same (ca 580 ppm) as that in [Se42-] and the oSe(a) are metal
dependent. In the polyselenium compounds where the 8-membered cycle is completed
by S atoms there is little polarity and limited dispersion of theSe chemical shifts. The
oSe(~) (and oSe(y)) are close to 580 ppm as they are in the metal-chelate rings, but the
oSe(a) are anomalously larger. This could be due to the reversed polarity of the
Se(a)-S bonds compared with oSe(a)-M.
Ia!
( St'-Se-Seo-Sen )1-.--J L---.
It
I St'-St'-Sey-Sep-Sen) 1-
r---------~~ rJ (Se-Se-Se-Sey-Sep-Seal1
-
--------======:=d~-~~ I f ,--
9041 800 700 600 500 400 .100 lOO I 00
lcl
ppm
r--ZnlliJ---,
Se-Se-Sei}-Sea ~ L ___ _
.--- u. -,
r--Cdlliz----,
Se-Se-Sep--Sea
~ ---r-- "•'''J----,
Se-Se-Sei}-Sea ___..JL ----.--- .,
r--SniVIJ----,
Se-Se--Set}--Sea
~~
•no 300 700 600 500 400 JOO 200 tOO ppm
0
0
!b)
oct -Se-Sei}-Sea-oct
~~ oct-Se-Se-Sei}-St'a-oct
r ~ oct-Se-Se-Sey-Sei}-Sea-oct
~ J __:..J
r r 1
900 800 100 600 son •oo Joo zoo too
{d)
r
r
ppm
( ....:---s4-, Se-Se-Se11-Sea
.J J F-s,----.
Se--Se-Sey--Sei}-Sea ~ J ..J
r-r s ----.. ' 1
Se--Se-Se--St'y--Sell-Sea ~ .J I
r--tr
9oo soo 100 6oo soo no Jeo 200 too ppm
0
0
Figure 4.10 Comparative chart of the 77Se chemical shifts in four classes of polyselenium chains: (a) [Sex]2-, (b) Octn-sex-Octn, (c)
M(Se4)2-, and (d) Ss-xSex. The thick lines mark the chemical shift values.
4.6 Preparation of Starting Materials
a) Survey of reductants for Se.
i) Solvent- H20/0H- Reductant- Na2S204
115
Following the procedure of Ellis et al 37 a 250 mL Schlenk flask was charged with
selenium powder (1.0 g, 12.7 mmol), Na2S204 (5.0 g, 28.7 mmol) and 50 mL of a
10% w/w NaOH aqueous solution. The resulting solution was stirred at 80°C for 10
mins. The solution proceeded through a series of colour changes from dark red
brown to colourless. On cooling an amorphous white solid precipitated. According to
the literature this compound is Na2Se.37 It was filtered, washed with 10% w/w
NaOH solution and dried under vacuum. On drying, the white solid oxidised to a
purple solid. This preparation was repeated six times without ever being able to store
the pure solid Na2Se. Many problems were encountered in this experimental
procedure. These included a requirement of Na2S204 far in excess than that
reported.37 This was attributed to partial contamination of the Na2S204 used. The
experiment described above represents the best preparation with decomposition even
in an inert environment was unavoidable.
i i) Solvent - THF Reductant - N a
Into a 250 mL Schlenk flask was added Se powder (2 g, 25.3 mmol) and THF
(50 mLs). Na (1.15 g, 50 mmol) was then added slowly in small pieces with stirring.
No reaction was apparent at temperatures up to 10oc.
iii) Solvent- THF/Absolute ethanol Reductant- Na
To the above reaction at 45°C, EtOH (50 mL) was added slowly. The reaction
began to proceed slowly on addition of 10 mL of EtOH. Generation of a light brown
solution indicated the formation of Sex2-. H2(g) was evolved as the Na dissolved. 10
mL increments of EtOH were added with colour intensity of the solution being
optimised at 50 mL. The solution at this stage was dark brown/red with both reactants
116
appearing to have been dissolved. The solution was stirred for lf2 hour. The solution
remained dark brown/red, with no precipitation of Na2Se observed.
i v) Solvent- Absolute ethanol Reductant - Na+ OEr-
Into a 250 mL Schlenk flask was transferred absolute ethanol (40 mL) and small
pieces ofNa (2.47 g, 107.4 mmole) were added slowly with the evolution ofHz(g)·
On the addition of Se powder (1 g, 12.7 mmole) there was no apparent reaction. The
temperature of the mixture was increased to 30°C still without reaction. However the
addition of 40 mL of THF slowly resulted in the red/brown colour of Sex2-. This was
stirred for lf2 hour with increased colour intensity to dark brown. The expected
colourless solution and white precipitate of Na2Se did not eventuate.
v) Solvent - H20 Reductant- NaOH
Into a 250 mL Schlenk flask was added Se powder (3 g, 38.0 mmol) and a
solution of NaOH (5 g, 125.0 mmol) in H20 (50 mL). The mixture was stirred at
80°C for 20 mins, with the Se not visibly reacting. The further slow addition of
NaOH (5 g, 125.0 mmol) with stirring generated a red/brown solution. However not
all theSe was reacted after lf2 hour. Further NaOH (5 g, 125.0 mmol) was added
gradually. Selenium reacted completely providing a more intense brown solution,
which did not become colourless on a further hour of stirring.
vi) Solvent- DMF Reductant- NaOH
Into a 250 mL Schlenk flask was added Se powder (3 g, 38 mmol), NaOH (5g,
125 mmol) and DMF (45 mL). The mixture was stirred at 8QOC for lf2 hour. The
mixture became dark green/brown with all reactant appearing to have dissolved.
Continual stirring for a further 20 minutes resulted in a dark brown precipitate being
formed. On cooling a greater amount of precipitate developed. The mixture was
allowed to stand for 2 days in the fridge. On opening the flask, a pungent amine
117
odour, escaped. This may have been Me2NH from the hydrolysis ofDMF. The solid
was filtered, washed with DMF and dried in a desiccator. Over a period of one week
the dark brown solid slowly decomposed to blue/green then to yellow/green and then
to light brown solid. This decomposed product was air stable in the light brown form
only. Light brown solid was insoluble in MeCN, EtOH, MeOH, Acetone, THF, and
Pyridine.
vii) Solvent- MeCN Reductant- NaOH
Into a 250 mL Schlenk flask was added Se powder (3 g, 38 mmol), NaOH (5 g,
125 mmol) and MeCN (50 mL). The mixture was stirred at 800C for lf2 hour. After
10 minutes a deep red/brown solution was obtained (Sex2-) with a dark brown
precipitate resulting on cooling. This was collected and washed with MeCN but on
drying decomposed to the light product indicated in (vi). The dark brown solid was
thought to be Na2Sex.
viii) Solvent- Acetone Reductant- NaOH
Into a 250 mL Schlenk flask was added Se powder (3 g, 38 mmol), NaOH (5g,
125 mmol) and acetone (50 ml). The mixture was stirred at room temperature for lf2
hour, and became bright crimson with a bright crimson precipitate. This product,
considered to be Na2Sex, was filtered, washed with acetone and dried. It decomposed
rapidly on exposure to air to a mixture of grey solid-selenium and brown gelatinous
solid.
ix) Solvent- DMF Reductant- NaBH4
Into a 250 mL Schlenk flask was added Se powder (1 g, 12.7 mmol) and DMF
(20 mL). NaBH4 (0.75 g, 19.8 mmol) was added to this mixture, followed by
stirring at 70°C for 1h hour. The resulting solution was dark brown in colour
indicating Sex2-. The solution did not change colour after 1112 hour stirring and no
solid was formed.
· x) Solvent- MeCN Reductant- NaBH4
Into a 250 mL Schlenk flask was added Se powder (1 g, 12.7 mmol) and MeCN
(20mL). To this mixture was added NaBH4 (lg, 26mmol ). The mixture was stirred
at 70DC for 1/2 hr. The resulting solution was red/brown in colour indicating Sex2-.
The solution did not change colour after 11 h hour stirring and no solid was isolated.
xi) Solvent- N2If4. H20 Reductant- N2H4. H20
Into a 250 mL Schlenk flask was added Se powder (0.5g, 6 mmol) and
degassed N2B4. H20 (5mL, 13 mmol ). The mixture was stirred at room temperature
for 15 min. The resulting solution was red/brown. Addition of excess resulted in a
colour change to green but not to the colourless solution of Na2Se.
xii) Solvent- DMF Reductant- Na
Into a 250 mL Schlenk flask was added Se powder (lg, 13 mmol), Na (0.6g,
25mmol) and DMF (30mL). The mixture was stirred at 90DC for 1hr. The resulting
solution was red/brown with all reactants dissolving in this time. Addition of excess
Na resulted in a colour change to green but not the expected colourless solution of
Na2Se.
xiii) Solvent- NH3 Reductant- Na
A 2-necked flask was charged with 1.0 g Se powder (13 mmol), fitted with a
low temperature condenser, and NH3 (30 mL liquid) was condensed into the flask.
Na (0.6g, 26 mmol) was then added against a N2 stream, and the reaction mixture was
stirred at -60°C for about one hour until the dissolution of sodium was complete. The
solution colour changed from initial blue through various colours ranging from
red/brown to emerald green and finally colourless with a white precipitate. The NH3
was distilled off leaving the white solid which was dried in vacuo and analysed as
118
119
Na2Se by oxidising a known amount in the presence of air and detennining Se.( Yield
1.55g, 98% ).
This method provided the best procedure for the partial and complete reduction
of Se and was therefore chosen as the method for the synthesis of selenide and
polyselenide ions in solution (see experimental below).
b) Experimental - preparation of solid compounds
NazSe
Na2Se was prepared as described in 4.6.2. (a) above.
NaHSe4
Following the procedure of Klayman and Griffin 4 Se powder (l.Og, 13 mmol)
was stirred with absolute ethanol (20 ml) cooled with ice. Solid NaBI4 (l.Og, 26
mmol) was added gradually with stirring, causing the vigorous and exothermic
evolution of hydrogen. The mixture became brown and then colourless. On
completion, the ethanol was distilled out leaving a white solid.
(Bu4N)zSe6
Following the procedure of Teller et al 2, a flask fitted with a low temperature
condenser was charged with Se powder (2.0g, 25 mmol) and liquid NH3 (30 ml). Na
(0.20g, 8-7 mmol) was added and the mixture stirred until all sodium had dissolved to
form a dark green solution. After removal of the NH3, water (20 mL) was added to
form a dark green solution, which was cooled to o·c and treated with a cooled
solution of Bu4NBr (2.73g, 8-5 mmol) in water (10 ml). The dark green needles that
formed at o·c were filtered, washed with cold water, and dried in vacuum, yield 3.3g
(81 %). The solid, which is oxidised to Se after several hours exposure to air, was
identified by its powder diffraction. This reaction has been scaled up tenfold.
(Ph4P)2[Sesl
The procedure of the previous reaction was followed, using Se powder (5.0g,
63 mmol), NH3 (40 mL liquid) and Na (0.58g, 25 mmol), generating a dark green
solution after stirring for 40 min. After removal of the NH3 to leave a red-brown
solid, water (100 mL) was added to form a dark red solution, which was cooled to
Q°C and treated with a cooled solution of Ph4PBr (10.6g, 25 mmol) in ethanol (50
mL). The black-brown needles that formed at 0°C were filtered, washed with cold
water, and dried in vacuum. Yield 8.2g (88%). The compound was characterised
by single crystal diffraction analysis (see appendix A).
(Ph4P)2[Se(Seshl
To a flask fitted with a low temperature condenser was added Se powder (5.0g,
63 mmol) and liquid NH3 (40 mL). Na (0.58g, 25 mmol) was added and the mixture
stirred for 40 min, generating a dark green solution. After removal of the NH3 to
leave a red-brown solid, DMF (80 mL) was added and the mixture stirred to complete
dissolution. NaN03 (1.08g, 13 mmol) and Ph4PBr (5.3g, 13 mmol) were added,
and the stirring was continued for 30 min to generate an olive green solution. After
filtration, THF (80 mL) was layered onto the solution which was stored at ca 0°C.
After 2 days black crystals had formed. They were collected, washed with THF, and
vacuum dried. Yield 1.06g. The product was identified by powder X-ray
diffraction, and by complete crystal structure determination (see appendix A).
(c) Preparations of solutions for NMR
NaHSe in aqueous solution. Following Klayman et al 4 equation (4.la)
NaBR4 (l.Og, 26 mmol) in water (1 0 mL) was stirred into a suspension of Se (1.0 g,
13 mmol) in water (10 mL), at room temperature. After 10 min the H2 evolution had
ceased and theSe had dissolved completely to yield a pale grey solution.
120
121
NaHSe in ethanol solution Following Klayman et al 4 equation (4.2),
excess NaBI4 was used in this reaction. Cold absolute ethanol (20 mL) was added
with stirring to a mixture of Se (1.0 g, 13 mmol) and NaBI4 (0.58g, 15 mmol), and
the mixture cooled during the vigorous evolution of hydrogen. A colourless solution
resulted.
NaHSe in DMF solution The ethanol was distilled from the previous
solution, yielding a white solid which was dissolved in DMF (20 mL) to generate a
colourless solution.
"Na2Se2" in aqueous solution. Following Klayman et al 4 NaBH4
(l.Og, 26 mmol) in water (10 mL) was added with stirring to Se (l.Og, 13 mmol)
suspended in water (10 mL) at room temperature. After the initial vigorous reaction
had subsided, a further portion of Se (l.Og, 13 mmol) was added, the mixture stirred
for 15 min, and then briefly heated to complete the reactive dissolution of the Se,
yielding a dark red-brown solution. After storage at 0°C for 16 hrs a black precipitate
(Se) formed, but the solution was still strongly coloured.
"Na2Se2" in ethanol solution Following Klayman et al 4 absolute ethanol
(100 mL) was added with stirring to Se (3.0g, 38 mmol) and NaBI4 (l.Og, 26 mmol)
cooled with ice. After the initial reaction had subsided the mixture was heated at
reflux for 1.5 hr, while the N2 flush gas containing H2Se was passed through a
saturated aqueous solution of lead acetate, yielding PbSe (1.15g, 4.0 mmol) as a black
precipitate. The dark red-brown reaction solution formed a small amount of black
precipitate on storage at 0°C.
"Na2Sex" in DMF solution, x = 3, 4, 5, 6. The general procedure is
the same for all compounds, so the description of Na2Se4 only is presented in detail.
A 2-necked flask was charged with 1.0 g Se powder (13 mmole), fitted with a low
temperature condenser, and NH3 (30 mL liquid) was condensed into the flask. Na
(0.15g, 6.3 mmole) was then added against a N2 stream, and the reaction mixture was
stirred at <60°C for about one hour until the dissolution of sodium was complete.
122
The solution colour changed from initial blue through various colours to dark-brown.
The NH3 was evaporated, leaving a dark coloured solid which was dried in vacuo for
a further 1/2 hr. The solid was then dissolved in freshly distilled and degassed DMF
(15 mL) at room temperature, generating a dark brown coloured solution.
Preparative data for other solutions are presented as: nominal composition "NazSex";
amount of Na; amount of Se; volume (mL) of NH3 and colour; volume (mL) of
DMF and colour. "NazSe3"; 0.2lg (9.1 mmol); 1.0 g (12.7 mmole); 30, green; 15,
brown I red. "NazSe4", 0.15g (6.5 mmol); 1.0 g (12.7 mmole); 30, brown; 15,
brown. "NazSes", 0.12g (5.2 mmol); 1.0 g (12.7 mmole); 30, green; 15, green.
"NazSe6", 0.097g (4.2 mmol); 1.0 g (12.7 mmole); 30, green; 15, green. "NazSe7'',
0.17g (7.4 mmol); 2.0 g (25 mmole); 50. green; 20, green. "NazSeg", 0.15g (6.5
mmol); 2.0 g (25 mmole); 50, green: 20, green. "NazSe9", 0.13g (5.6 mmol); 2.0 g
(25 mmole); 50, green; 20, green. "NazSe10", 0.12g (5.2 mmol); 2.0 g (25 mmole);
50, green; 20, green. "NazSeu", O.llg (4.4 mmol); 2.0 g (25 mmole); 50, green;
20, green. Very small amounts of unreacted Se were removed
Attempts to use this procedure to generate a solution of NazSez in DMF generated
white solids that would not dissolve in DMF, and were abandoned.
123
4.7 Conclusion
Preparative reactions for the ions Se2-, HSe- and [Sex]2- in protic and aprotic
solvent systems are established, and these ions in solution have been characterised by
the most informative probe, 77Se NMR. Fully reduced solutions in DMF contain
HSe- for which 8Se ranges between -530 and -390 ppm (vs MezSe) depending
strongly on solvent and temperature: ~!J.t.Se-.H) ;:;:.3Hz. Although it is likely that HSe
is in fast equilibrium with Se2-, there is no direct NMR evidence for the conjugate
strong base Se2- in any of these solutions; and it is predicted that 8Se (Se2-) < -650
ppm in conflict with the reported value of -435 ppm reported by Bjorgvinsson et al.
There is no NMR evidence for [Sez]2- which is believed to disproportionate to HSe
plus [Sex]2- (x > 2) or Se(s)· The polyselenide ions [Sex]2- x = 3,4,5,6 all exist in
DMF, and the resonances for all Se atoms in each of these ions have been observed
and assigned. Using a., ~."(to signify Se positions relative to the equivalent ends of
the chain, the 8Se(a.), 8Se(~),8Se(y) data (in DMF plus 38% (vol) ETOH, 230"K)
are: [Se3]2- 193,214 [Se4]2-, 256, 581,-: [Ses]2-, 354, 570, 868: [Se6]2-, 404, 636,
807. The chemical shifts of Se(a.) in these ions are very responsive to the solvent
proticity, becoming more negative in the range DMF -7 ethanol, while the Se(j3)
atoms respond much less, and the effect at theSe("() atoms is small and can be in the
other direction. Protic solvents retard the Se exchange reactions of polyselenide ions.
Comparative analysis is made of the 8Se values for related compounds of type XSe- or
with chains of Se atoms. There is no NMR evidence for ions with x > 6. The
spirocyclic ion [Se(Se5)z]2- is formed from [Sex]2- with many oxidants, including
N03- in DMF, and possesses only one exchanging 77Se resonance at 720 ppm at
temperatures greater than 200K. At 200K four resonances are observed at 795, 708,
672 and 484 ppm. A logical preparation of the simple pentaselenide salt, (Ph4P)2Ses
is given. This molecule shows six 77Se resonances indicative of Ses2- and Se62- at
354, 404, 570, 636, 807, 868 in DMF.
4.8 References
1. N. E. Brese, C. R. Randall, J. A. Ibers, Inorg. Chern. 1988, 27, 940
2. R. G. Teller, L. J. Krause, R. C. Haushalter, Inorg Chern. 1983, 22, 1809
3. A. B. Ellis, S. W. Kaiser, J. M. Bolts, M. S. Wrighton, J. Am. Chern. Soc.
1977, 199, 91 2839
4. D. L. Klayman and T. S. Griffin, J. Am. Chern. Soc., 1973, 95, 197-9.
5. W. Brauer, 'Handbook of preparative inorganic chemistry' Vol1. (non
transition elements) Academic Press. 2nd Ed, 1963
6. J. Cusick and I. Dance, Polyhedron, 1991, 10, 2629
7. H. J. Jacobsen, A. J. Zozulin, P. D. Ellis and J.D. Odom, J. Mag. Res., 1980,
38, 219.
8. W. McFarlane and R. J. Wood, J. C. S. Dalton, 1972, 1397-1402.
9. J. D. Odom, W. H. Dawson, and P. D. Ellis, J. Am. Chern. Soc., 1979, 101,
5815
10. H. Hagisawa, Bull. Inst. Phys. Chem. Res. Tokyo, 1941, 20, 384, Chern.
Abs. 1942, 36, 1231;
11. R. H. Wood, J. Am. Chern. Soc., 1958, 80, 1559-62.
12. M. Bjorgvinsson, G. J. Schrobilgen, Jnorg. Chern., 1991, 30, 2540
13. J. Mason, Multinuclear NMR, Plenum Press, New York, 1987, chapter 17.
14. T. R. Stengle, Y-C. E. Pan, C. H. Langford, J. Am. Chern. Soc., 1972, 94,
9037.
15. A. J. Parker, Chern. Rev., 1969, 69, 1-32.
16. R. Alexander, A. J. Parker, J. H. Sharp, and W. E. Waghorne, J. Am. Chern.
Soc., 1972, 94, 1148
17. ~· Reichardt, Solvents and Solvent Effects in Organic Chemistry, 1988, VCH
Publishers, 2nd ed.
18. K. W. Sharp and W. H. Koehler, Inorg. Chern., 1977, 16, 2258
124
19. D. J. Sandman, G. W. Allen, L.A. Acampora, J. C. Stark, S. Jansen, M. T.
Jones, G. J. Ashwell and B. M. Foxman, Inorg. Chem., 1987, 26, 1664
20. P. Pekonen, Y. Hiltunen, R. S. Laitinen, and T. A. Pakkanen, Inorg. Chem.,
1990, 29, 2770
21. P. Pekonen, Y. Hiltunen, and R. S. Laitinen, Acta . Chem. Scand., 1989, 43,
914
22. C-N. Chau, R. M. W. Wardle, and J. A. Ibers, Acta . Cryst., 1988, C44, 883
23. G. Krauter, K. Dehnicke, and D. Fenske, Chemiker-Zeitung, 1990, 114, 7
24. B. Krebs, E. Liihrs, L. Stork and R. Willmer, abstract 08.2-5, Congress of the
International Union of Crystallography, Perth, Australia, 1987.
25. H. R. M. Banda, I. G. Dance, T. D. Bailey, D. C. Craig and M. L. Scudder,
Inorg. Chem., 1989, 28, 1862
26. T. D. Bailey, H. R. M. Banda, D. C. Craig, I. G. Dance, I. N. L. Ma and
M. L. Scudder, Inorg. Chem., 1991, 30, 187
27. M.G. Kanatzidis and S-P. Huang, lnorg. Chern., 1989, 28, 4667
28. L. E. Lyons and T. L. Young, Aust . .!. Chem., 1986, 39, 511
29. R. K. Harris and B. E. Mann, NMR and the Periodic Table, Academic Press,
London 1978.
30. T. Birchall, R. J. Gillespie, and S. L. Vekris, Canad. J. Chem., 1965, 43,
1672
31. W. H. Pan and J. P. Fackler Jr, .!. Am. Chern. Soc., 1978, 100, 5783
32. D. E. J. Arnold, J. S. Dryburgh, E. A. V. Ebsworth, and D. W. H. Rankin,
J. Chem. Soc. Dalton, 1972, 2518
33. S. Cradock, E. A. V. Ebsworth, D. W. H. Rankin, and W. J. Savage, J.
Chem. Soc. Dalton, 1976, 1661
34. M.G. Kanatzidis, Comments. Inorg. Chern., 1990, 10, 161
35. H. Eggert, 0. Nielsen, and L. Henriksen, .T. Am. Chern. Soc., 1986, 108,
1725.
125
126
36. R. S. Laitinen and T. A. Pakkanen, Inorg. Chem., 1987,26, 2598.
37. A. B. Ellis, S. W. Kaiser, J. M. Bolts, and M. S. Wrighton, J . Am. Chem.
Soc., 1977, 99, 2839
CHAPTER 5
Metal Complexes of Polyselenides.
5. 0 Introduction
At the commencement of this work in 1988, metal polyselenide complexes were
rare. Indeed, as reported in chapter 1, only four homoleptic polyselenides,
(Ph4P)2[Fe2Se2(Ses)2],l (Et4N )2[V 2S e 13],2 (Ph4P)2[W 2Seg],3 and
(Ph4P)2[W2Sew],3 had been synthesised. The two tungsten complexes were found
in the same crystal! This was despite the fact that homoleptic metal polysulfide
complexes, [M(Sx)wJZ, containing polysulfide chains as chelating ligands, had been
known for more than 85 years.4
Although aqueous polysulfide reagents had been used successfully for many
years in the syntheses of polysulfide complexes, the last two decades has seen a shift
away from the classical aqueous synthetic strategy with the introduction of
nonaqueous and aprotic solvents, representing the most important advance in the
preparative and reaction chemistry of metal polysulfides.
The use of nonaqueous and aprotic solvents, as shown in Chapter 1, increases
the thermodynamic activities of polysulfide ions in solution, thereby expanding the
range of complexes that can exist in solution. Consequently, the use of non-aqueous
solvents in the preparations of metal complexes has enabled the crystallisations and
structural characterisation of molecular homoleptic metal polysulfides with wide
structural variety, containing from one to six metal atoms.5,6,7,8
The principal method used for the synthesis of non-aqueous polysulfide
solutions was the direct analogue of the aqueous method equation 5.1, applied
principally by MUller et ai.9,10,11,12,13
Aprotic solvent+ Sg + NH3(g) + H2S(g) --7 (Nli4)2SxCsoiv) ... 5.1
127
However the use of the gaseous reagents, NH3(g) and H2S(g) in this
preparation led to variability of composition of the (NH4hSx solutions and introduced
difficulties in attaining reproducible metal polysulfide stoichiometry. More recently,
metal polysulfides have been synthesised using other non-aqueous methods.l4,15,16
These methods include the use of salts such as (BU4NhS6 and (Ph4PhS6 which are
soluble in aprotic solvents such as DMF and MeCN and the in situ. formation of
Na2Sx in DMF by the reactions of Na <;>r Na2S with Sg. A comprehensive review of
metal polysulfide syntheses has been given in Chapter 1.
Polysulfide solutions are complex and factors such as the cation, solvent and pH
determine what can be isolated as a solid. Previously, the extensive evidence of the
polysulfide chemistry suggested that isolated complexes need not be representative of
the complexes existing in solution. Furthermore, crystal structures do not provide
information about the species, reactions, and equilibria that occur in solution,
information which could be used to support rational syntheses and applications.
Banda et al during the course of their work, confirmed this using metal NMR studies
of the reaction mixtures of cadmium, tin and mercury polysulfides.14,15,16 Here it
was shown that the complexes that crystallise from these solutions are not necessarily
representative of the complexes in solution. For example, the 113Cd NMR of a
particular solution revealed evidence of four species; [Cd(Ss)2]2-, [Cd(Ss)(S6)]2-,
[Cd(S6)2]2- and [Cd(S6)(S7)]2-. However, the precipitation of complexes from the
cadmium polysulfide solution with the PPh4+ cation yielded only two species;
(PPh4h[Cd(S6hl and (PPh4)2[Cd(S6)(S7)). The crystal lattices of these complexes
were dominated by the phenyl-phenyl interactions of the counter-cations, with the
metal polysulfide anions occupying the relatively small cavities between the much
larger PPh4+ cations. Crystallisation from these solutions was determined by the
solubilities of the various species. That is to say, the species that crystallised from
solution was the least soluble and not necessarily the most abundant.
128
In contrast to this extensive polysulfide chemistry, the coordination chemistry of
polyselenides and polytellurides has been traditionally ignored. The reasons for this
seemingly lack of interest are not obvious, but perhaps a factor may have been the
notion that the heavier chalcogens would exhibit analogous and thus not new
chemistry to that of the polysulfides. Consequently a lack of synthetic development
led to the paucity of polyselenide complexes, as described in the opening paragraph of
this chapter.
It has been only since 1988 that I and others have independently developed
synthetic methods to improve access to complexes of polyselenides, that has now
resulted in an explosion in the numbers of characterised polyselenide complexes. The
comprehensive listing of anionic metal polyselenides now known to exist and the
various synthetic approaches is presented as part of the review in table 1.5, Chapter 1.
Significantly, of the few metal polyselenides that had been reported prior to this
work, only the two tungsten species, (Ph4P)2[W2Se9], and (Ph4P)2[W2Sew], had
been characterised by 77Se NMR. No attempt was made to monitor their formation in
the reaction mixtures. In all the cases the primary means of characterisation was by
single crystal X- ray diffractometry of the products which crystallise.
I have used the knowledge gained from the 77Se NMR investigation of
uncoordinated polyselenides, presented in Chapter 4, to establish simple, inexpensive,
and reproducible synthetic procedures for the isolation of novel polyselenide
complexes. This material was first published in 1989-1990 17,18,19 and preceeded
many publications by other workers in the same area. Many of the new molecules
originally published by me have since been reproduced by these workers using
similiar procedures to those developed in this thesis. The relevant preparations are
compared in the result section of this chapter.
129
5.1 Expectations
As a consequence of the successful 77Se NMR study of uncoordinated
polyselenides solutions, it was expected that NMR techniques (both 77Se and metal)
would be extremely valuable in the study of the metal polyselenide solutions,
providing unprecedented information on the equilibria involved and the necessary
insight to logical and systematic syntheses.
It was expected that the addition of a metal ion to the Se32-, Se42-, Ses2-, and
Se62- solutions described in Chapter 4, would show that many molecular structures
existed in solution similar to that shown to exist in some metal polysulfide
solutions.I4,15,16 Additionally, it was predicted from the 77Se NMR studies of the
uncoordinated polyselenides that selenium atom redistribution reactions would be
kinetically fast relative to the NMR time scale and that metal complexes with different
polyselenide ring sizes would exist in equilibrium as in the analogous polysulfide
systems.l4,15,16
Metals suitable for NMR were intially chosen so that both 77Se and metal NMR
could be used to characterise the reaction mixtures. Hg, Cd, and Sn salts were chosen
as their respective nuclei, 199Hg, 113Cd, and 119Sn, are spin 1/2 NMR active nuclei
(see chapter 2) and have wide chemical shift ranges for metals.20,21 The advantage of
large chemical shift ranges in these nuclei, was expected to be that the frequency
separations between various metal sites would be large enough to ensure slow
exchange NMR conditions at ambient temperature, thereby allowing full observation
of all species in solution.
Once the equilibria involving metal complexes in solution were well
characterised, it was expected that these solutions could be then be used to prepare
crystalline products by rational methods. The resulting products could then be
unequivocally characterised using X-ray crystallography. For comparison the
crystallographically characterised product would then be redissolved in a suitable
solvent and its NMR compared to the original mixture.
130
It was expected that eventually other metal precursors that are not necessarily
NMR active would be added to the polyselenide solutions resulting in the isolation of
other novel metal polyselenide complexes.
The obvious advantage of this polyselenide study compared to other polysulfide
experiments was the capacity to monitor the selenium nucleus, and to compare the
uncoordinated polyselenide chains to the coordinated polyselenide complexes.
5.2 Results
The 77Se NMR investigation ofNa2Sex solutions presented in Chapter 4 clearly
showed that the polyselenide species Na2Se3, Na2Se4, Na2Ses and Na2Se6, all exist
in DMF solutions. The addition of metal ions to these and other polyselenide
solutions has led to some surprising results which are presented below.
The initial studies in this work were based on monitoring the reactions of Cd2+,
Hg2+ and Sn2+ salts with Na2Sex (x = 3-6) solutions by either 77Se and or metal
NMR. The results of these NMR studies containing Cd2+, Hg2+ and Sn2+ salts are
presented first (section 5.2.1) along with the related results of other workers.
Syntheses involving other metals are then presented.
Three different synthetic strategies were used in this work to generate metal
polyselenide complexes. The first method derived from the methods used for
uncoordinated polyselenides described in Chapter 4, where Se was reduced by Na in
liquid NH3. The NH3 was then distilled off and replaced by DMF. The metal salt
was then added as a DMF solution or directly as a solid depending on the solubility of
the metal salt. The resulting mixture was stirred at ambient temperature (ca. 250C)
usually for 1!2 hr generating a solution containing the metal polyselenide complex.
The resulting anionic complex was then crystallised using one of the crystallisation
techniques described in Chapter 2. This preparation is referred to in this work as the
'NH3 Method'.
131
The second method combined Na; Se, DMF and the metal salt directly together
in one flask. The mixture was then generally stirred at 60-90°C overnight (ca. 14Hr)
resulting in a solution containing the metal polyselenide complex. The temperature
range of 60-900C was optimised to permit faster solvation of Na and consequently
faster reaction time. Temperatures higher than 90°C decomposed the mixture fonning
gelatinous solids. Temperatures lower than 60°C made reaction times too long. This
preparation is referred in this work as the 'DMF Method'. Crystallisation occurred
similiar to that described above in the 'NH3' method.
The third method, studied to a lesser extent, involved the isolation of the salt,
(Bu4N)2Se6, which was then redissolved in DMF and deployed preparatively in
reactions of interest with metal salts.
The experimental conditions in the first and second methods were optimised
using different ratios of metal and various polyselenide chain lengths in order to
determine both the compositional range which the various metal polyselenide
complexes could be kept in solution and to monitor the species present in these
solutions ~y NMR. The variables m and n are used in this work to indicate the
nominal composition M + n[(Sem)l2- of these solutions.
In all syntheses metal chlorides were preferred to other metal precursors because
the formation of NaCI was expected to help drive the reactions to completion. The
availability of metal chlorides also made them desirable. Nitrates were avoided where
possible eecause, as was shown in Chapter 4, NOr readily oxidises Ses2- to
Se(Sesh2- in DMF solutions.
It is nearly impossible to predict the structures of complexes using only
spectroscopic techniques. Therefore isolated compounds were structurally
characterised by x-ray crystallography, and in tum their 77Se NMR spectra compared
to that of the reaction mixtures.
132
5.2.1. 77 Se and Metal NMR studies of reactions between Cd2+, Hg2 +
and Sn2+ and Na2Sex (x = 3-6) and (Bu4NhSe6
Initial NMR studies involved the reaction of polyselenide solutions made using
the NH3 method or the preformed salt, (Bu4NhSe6, with CdCI2.S/2H20, HgCl2, and
SnCb.H20. These studies monitored the compositional range of the reaction
mixtures in which the various metal polyselenide complexes could be kept in solution,
and the species present in these solutions. The compositional range was determined
by preset m and n values (described above) expected to give high and low ratios of
metal to polyselenide ligand. Experiments were then designed to isolate the observed
species under the conditions seen to give optimal concentration. Similar conditions
were then used to prepare solutions containing other metal ions. This series of
experiments preceeded crystallisation studies involving the DMF method, and are
presented first.
In the following description, the various resonances are labelled by their
chemical shift at 300K. For details of the NMR parameters used for the various nuclei
see chapter 2.
The results of the following reactions are recorded in Tables 5.1, 5.2 and 5.3.
A schematic representation of the scope of m and n from these experiments is
presented in Figure 5.4 at the conclusion of Tables 5.1, 5.2 and 5.3.
5.2.2 Cadmium polyselenide solutions - Cd(Sem)n
In the reactions of CdCl2.512H20 with Na2Sex (x=3,4,5,6) in DMF, the
CdCI2.512H20 was added as a powdered solid to freshly prepared DMF solutions of
Na2Sex (x=3,4,5,6), and stirred at ambient temperature until the CdCl2.512H20 had
completely reacted, requiring ca . 30 min. CdCl2.512H20 is insoluble in DMF,
however on the addition to the polyselenide solutions it reacted instantaneously
generating a variety of colours that eventually ranged from dark green/brown to
red/brown, depending on the ratio of metal to ligand (see Table 5.1). Many of these
133
reactions resulted in the precipitation of dark coloured amorphous solids that were
removed by filtration. Attempts were made to redissolve these solids in hot DMF,
MeCN, pyridine, EtOH, and H20, without success. Powder diffraction showed the
solids to be amorphous, and as a consequence they were not identified. The cadmium
concentrations achieved in these solutions were in the range 0.63 - 1.9 mM. The
value of m ranged between 3-6, whilst that of n ranged between 0.65-5. The
solutions or filtrates were examined and despite the variation in colours observed, in
each case only a single product was observed by both 77Se and 113Cd NMR (Figures
5.1a and 5.1b). This product was characterised by a sharp single resonance in the
range Ocd 745-749 ppm and two sharp resonances in the ranges ose 580-583 and 35-
38 ppm. In addition, the resonance at 0Cd 748ppm showed doublet satellites with nJ
(113Cd - 77Se) coupling of 270Hz. A doublet coupling of 270Hz was also centred
around the resonance at ose = 38ppm.
134
500 400 300 ppm
200 100
Figure 5.la. The 77Se NMR of (Ph4Ph(Cd(Se4h1 in DMF at 300K. Inset.
Expansion of ose 38 ppm.
755 750 745 740 735 ppm
Figure 5.lb. The 113Cd NMR of (Ph4Ph[Cd(Se4hl in DMF at 300K.
135
Table 5.1 Formation and NMR of cadmium polyselenide solutions
of nominal composition [Cd(Sem)nl in DMF.
m,n Polyselenide Metal Solvent Comments
solution precursor
3, 1 Na2Se3 CdC12.5/2H20 DMF Immediate red/brown precipitate
(2.5 mmol) (2.5 mmol) (lOmL) settling in colourless solution.
Amorphous solid. 113Cd NMR
not recorded 77Se NMR not
recorded.
3,2 Na2Se3 CdCI2.512H20 DMF Green brown solution with NaCl
(2.5 mmol) (1.25 mmol) (lOmL) precipitation. Filtered. 113Cd
NMR of filtrate showed single
resonance at 748 ppm. 77Se NMR
showed two resonances at 581 and
37 ppm. Coupling of 270 Hz on
both the 748 and 37 ppm
resonances assigned to lJ (113Cd-
77Se).
3,4 Na2Se3 CdCI2.5/2H20 DMF As for m, n = 3,2
(2.5 mmol) (0.63 mmol) (lOmL)
4, 0.65 Na2Se4 CdCI2.5/2H20 DMF Red/brown ppte settling ln
(2.5 mmol) (3.8 mmol) (10 mL) colourless solution. NMR not
recorded.
4, 1 Na2Se4 CdCI2.5/2H20 DMF Immediate red/brown precipitate (2.5 mmol) (2.5 mmol) (10 mL) settling in colourless solution.
Amorphous solid. NMR not
recorded
4, 1.3 Na2Se4 CdCI2.5/2H20 DMF Dark red/brown solid in colourless (2.5 mmol) (1.9 mmol) (lOmL) solution. NMR not recorded.
4, 1.5 Na2Se4 CdCl2.5/2H20 DMF Red/brown ppte in colourless (2.5 mmol) (1.7 rnmol) (10 mL) solution. NMR not recorded.
4, 1.7 NazSe4
(2.5 mmol)
4, 2 NazSe4
(2.5 mmol)
4, 2.5 NazSe4
(2.5 mmol)
5, 0.66 NazSes
(12.7 mmol)
5, 1 NazSes
5,2
5,4
(12.7 mmol)
NazSes
(12.7 mmol)
NazSes
(12.7 mmol)
6, 0.67 NazSe6
(2.5 mmol)
CdClz.5/2HzO
(1.5 mmol)
CdCI2.5/2H20
(1.25 mmol)
CdClz.5/2HzO
(1.0 mmol)
CdC}z.5/2H20
(19.1mmole)
CdClz.5/2H20
(12.7mmole)
CdClz.5/2HzO
(6.3 mmol)
CdCI2.5/2HzO
(3.2 mmol)
CdClz.5/2HzO
(1.68 mmol)
DMF
(lOmL)
136
Red/orange solution with small
amount of orange ppte. Filtered.
113Cd NMR of filtrate showed
single resonance at 746 ppm. 77Se
NMR showed two resonances at
583 and 36 ppm. Coupling of 270
Hz on both the 7 46 and 36 ppm
resonances assigned to lJ (113Cd-
77Se).
DMF As form, n = 4, 1.7
(lOmL)
DMF Red/brown solution. No obvious
(10 mL) unreacted solid or ppte. 77S e
NMR showed two resonances at
580 and 39 ppm. 113Cd NMR
showed single resonance at 745
ppm. Coupling of 270 Hz on both
the 745 and 37 ppm resonances
assigned to lJ (113Cd- 77Se).
DMF Instantaneous black/brown
(40 mL) precipitate. No NMR recorded.
DMF
(40mL)
DMF
(40mL)
DMF
(40mL)
DMF
(10mL)
Instantaneous brown precipitate in
colourless solution. No NMR
recorded.
Dark brown solution. 113Cd NMR
showed a single resonance at 7 49
ppm. 77Se NMR showed 2
resonances at 581, 38 ppm.
Dark brown solution with
evidence of Se precipitation.
Filtered. 113Cd NMR showed a
single resonance at 749 ppm. 77Se
NMR showed 2 resonances at
578,37 ppm.
Brown ppte in colourless solution.
No NMR recorded.
137
6, 1 Na2Se6 CdCl2.512H20 DMF Chocolate brown ppte in
(2.5 mmol) (2.5 mmol) (lOmL) colourless solution. Some
evidence of unreacted black Se
powder. No NMR recorded.
6,2 Na2Se6 CdCI2.512H20 DMF Chocolate brown solution. No
(2.5 mmol) (1.25 mmol) (lOmL) obvious ppte. 113Cd NMR
showed single resonance at 750
ppm. 77 Se showed 2 resonances at
582, 38ppm
6, 3 Na2Se6 CdCI2.5/2H20 DMF Chocolate brown solution. No
(2.5 mmol) (0.84 mmol) (10 mL) obvious ppte. NMR as for m,n =
6,2.
6, 1 (Bu4N)2Se6 CdCl2.512H20 DMF Chocolate brown ppte in
(1.1mmol) (1.1 mmol) (lOmL) colourless solution.
6,2 (Bu4N)2Se6 CdCh.5/2H20 DMF Chocolate brown solution. No
(1.1mmol) (0.52 mmol) (10 mL) obvious ppte. 77Se NMR showed
2 resonances at 581, 35 ppm.
5.2.3 Mercury polyselenide solutions - Hg(Sem)n
The reactions between HgCl2 and Na2Sex (x=3,4,5,6) m DMF are
homogeneous (the HgCh was dissolved in DMF before addition), and occur readily
at 300C (ca 30 min). As with the Cd(Sem)n reactions various m and n values of these
reagents were examined and a variety of colours ranging from dark green/brown to
red/brown, were observed (see Table 5.2) in the solutions not precipitating dark
solids. The mercury concentrations used in these solutions were in the range 0.84 -
2.5 mmol I lOmL. Two values of m, 4 and 6 were used, whilst n values ranged
between 0.75 and 3. The homogeneous solutions or filtrates were examined and
typically only a single product was observed by both 199Hg and 77Se NMR (Figures
2a and 2b). This product was characterised by a single resonance in the range DHg
138
-479 to -483ppm and two resonances at ose 567-571 and 52-55ppm. In addition, the
resonance at OHg -481ppm showed doublet satellites with 0 J (199Hg- 77Se) coupling
of 1260Hz, with similar coupling pattern centred around the resonance at 8se ,..,
54 ppm.
Table 5.2. Formation and NMR of mercury polyselenide solutions
of nominal composition [Hg(Sem)n] in DMF.
m,n Polyselenide Metal Solvent 0 bserva tions/Resul ts
solution precursor
4, 0.75 Na2Se4 HgCh DMF Colourless solution and a dark
(2.53 mmol) (3.4 mmol) (10 mL) brown amorphous solid.
4, 1 Na2Se4 HgCh DMF Initial red/brown Se42- solution
(2.53 mmol) (2.53 mmol) (10 mL) becoming colourless with a dark
brown amorphous precipitate
resulting. Reaction vessels walls
developed a metallic fllm.
NMR not measured.
4, 1.5 Na2Se4 HgCl2 DJ\1F Dark brown solid precipitated
(2.53 mmol) (1.7 mmol) (10 mL) slowly leaving colourless solution
4, 1.7 Na2Se4 HgCI2 DMF Dark brown/green solution with
(2.5 mmol) (1.5 mmol) (10 mL) some unreacted solid evident.
Filtered. 199Hg NMR of filtrate
showed single resonance at -483
ppm with doublet coupling of
1260Hz. 77 Se NMR showed two
resonances 571, and 52ppm.
:...
I t I I I I t I I I I I I I t
700 600 500 400 300 200 100 0 ppm
Figure 5.2a. The 77Se NMR of (Ph4Ph[Hg(Se4)z] in DMF at 300K. Inset.
Expansion of ose 52 ppm.
-480 -460 -440 -420 ppm
Figure 5.2b. The 199Hg NMR of (Ph4PhfHg(Se4hl in DMF at 300K.
4,2
4, 3
6, 1
Na2Se4
(2.53 rnrnol)
Na2Se4
(2.53 rnrnol)
6, 1.5 Na2Se6
(2.5 mrnol)
6, 2 Na2Se6
(2.5 rnrnol)
6, 3 Na2Se6
(2.5 mrnol)
6, 1 (B u4N)2Se6
(1.0 rnrnol)
6, 2 (B u4N)2Se6
(1.0 mrnol)
HgCl2
(1.26 mmol)
HgCl2
(0.84 mmol)
HgCI2
(2.5 mmol)
HgCI2
(1.7 mmol)
HgCI2
(1.26mmole)
HgC12
(0.84 mmol)
HgCl2
(1.0 mmol)
HgCl2
(0.52 mmol)
139
DMF Red/brown solution with no
(IOmL) obvious precipitation. 199Hg
showed single resonance at -479
ppm with doublet coupling of
1260Hz. 7-7Se NMR showed two
resonances 569, and 54 ppm, the
latter showing doublets with ca.
1260 Hz coupling.
DMF Dark brown/green solution.
(10 mL) 199Hg showed single resonance at
-482 ppm with doublet coupling of
1265Hz. 77Se NMR showed two
resonances 567, and 55pprn. The
latter showing doublets with ca. 1265Hz COU__Qlin_g_.
DMF Chocolate brown ppte m
(10 mL) colourless solution. No NMR
DMF (10 mL)
DMF (10 mL)
DMF (10 mL)
DMF (10 mL)
DMF (10 mL)
run.
Brown/black ppte. Brown
solution. NMR as above.
Chocolate brown solution. NMR
as above.
Chocolate brown solution. No
obvious _QQ_te. NMR as above.
Chocolate brown precipitate in
colourless solution ..
Chocolate brown solution. No
precipitate. 77Se NMR showed
two resonances at 570 and 53
J2Q_lll.
5.2.4 Tin polyselenide solutions - Sn(Sem)n
SnCl2.H20 was added in a powered form to freshly prepared DMF solutions of
Na2Sex (x=3,4,5,6). The reaction proceeded readily at RT (ca 30 min) with stirring.
As with the previous reactions various m ( 2 - 6) and n ( 1 - 3) values of these
reagents were examined and a variety of colours ranging from dark green/brown to
red/brown, were observed (see Table 5.3 ). As was the case with the Cd2+ and Hg2+
solutions when m < 1.7 precipitation of dark coloured amorphous solids occurs. The
tin concentrations used in these solutions were in the range 0.5 - 8.44 mMol. The
homogeneous solutions or filtrates were examined and in each case only a single
product was observed by both 119Sn and 77Se NMR (Figures 5.3a-5.3b). This
product was characterised by a single resonance in the range osn -720 - -723ppm and
two resonances in the ranges ose 582-587 and 421-426ppm. A doublet satellite
attributed to IJ (119Sn - 77Se) coupling of R 16Hz was observed in both the 77Se and
119Sn NMR spectra.
Table 5.3 Formation and NMR of tin polyselenide solutions of
nominal composition [Sn(Sen)m] in DMF.
m,n Polyselenide Metal Solvent Observations
solution precursor
2,2 Na2Se2 SnCI2.2H20 DMF Reacted at 8o·c for 1.0
140
hr.
(12.6 mmol) (6.33 mmol) (40 mL) Colourless solution with dark
brown amorphous ppte. NMR
not recorded. Note so·c required
to dissolve mixture.
3, 1 Na2Se3 SnCI2.2H20 DMF Dark brown precipitate in
(8.44 mmol) (8.44 mmole) (40 mL) colourless soltion. NMR not
recorded.
3, 2 Na2Se3 SnCl2.2H20 DMF Brown/green solution. 77Se NMR
(8.44 mmol) (4.22 mmol) (40 mL) of 10 mL aliquot shows two
resonances at 585 and 424 ppm
respectively in 1:1 ratio. 119Sn
NMR showed single resonance at
-720ppm.
141
4, 1 Na2Se4 SnCi2.2H20 DMF Chocolate brown precipitate in
(6.33 mmol) (6.33 mmol) (40 mL) colourless solution. No NMR
recorded.
4, 1.7 Na2Se4 SnC12.2H20 DMF Dark green solution with
(6.33 mmol) (3.7 mmol) (40 mL) green/brown precipitate. Filtered.
77Se NMR shows two resonances
at 587 and 421 ppm respectively in
1:1 ratio. 119Sn NMR showed
single resonance at -723ppm.
4,2 Na2Se4 SnCl2.2H20 DMF Brown/green solution. NMR as
(6.33 mmol) (3.2 mmol) (40 mL) for m,n = 4, 1.7
4,3 Na2Se4 SnC12.2H20 DMF Brown solution. nse NMR
(6.33 mmol) (2.1 mmol) (40 mL) shows two resonances at 585 and
425 ppm respectively in 1: 1 ratio. 119 Sn NMR showed single
resonance at -722ppm.
5,2 Na2Ses SnCI2.2H20 DMF Brown solution. NMR as for m,n
(5.07 mmol) (2.5 mmol) (40 mL) = 4,3.
6,2 Na2Se6 SnCl2.2H20 DMF Brown/green solution.77Se NMR
(4.22 mmol) (2.1 mmol) (40 mL) shows two resonances at 586 and
421 ppm respectively in 1:1 ratio.
119 Sn NMR showed single
resonance at -722ppm.
6,2 Na2Se6 SnCI2.2H:!O DMF Brown/green solution.77Se NMR
(1.04 mmol) (0.5 mmo!) (10 mL) shows two resonances at 585 and
422 ppm respectively in 1:1 ratio. 119 Sn NMR showed single
resonance at -724ppm.
6, 1 (Bu4N)2Se6 SnCi2.2H20 DMF Dark brown solid in colourlesss
(1.0 mmol) (1.0 mmol) (10 mL) solution
6, 1 (Bu4N)2Se6 SnCl2.lH20 DMF Dark brown solution. 77Se NMR
(1.0 mmol) (0.5 mmol) (10 mL) shows 2 resonances at 582 and
426 ppm
Figure 5.3a The 77Se NMR of (Ph4Ph(Sn(Se4)3] in DMF at 300K. ·
.·
650 600 550 500 450 400 350
Figure 5.3b. The_ll9Sn NMR of (Ph4Ph[Sn(Se4)3] in DMF at 300K..
-5oo -6oo -7oo -aoo -9oo -woo ppm (vs. Me4Sn)
-Figure 5.4. Schematic representation of the compositional range for the reactions
containing [M(Sem>nl (M = Cd2+, Hg2+, Sn2+) in DMF at ambient temperature.
6
5
m
4 c
3
1 2 3 4 5 n
A = Region where dark brown I black amorphous solid precipitates to give
colourless solution.
B = Region where some dark brown I black amorphous solid precipitates from
dark green or red-brown solution.
C = Region where no solid precipitates from a dark green or red-brown
solution.
5.2.5 Overview of results
From tables 5.1-5.3 it is apparent that in each of the three M/Sex2- systems a
single complex forms. The evidence from the above exploratory NMR experiments
suggests that when a metal ion, Cd2+, Hg2+ or Sn2+, in the presence of a
polyselenide solution occurs is expected to undergo progressive coordination to form
soluble or insoluble compounds. This can be represented by the following equilibria.
M2+ + (Sex)2- ~ M(Sex) (neutral and usually insoluble) ... 5.2
M(Sex)Y + (Sex)2- ~ [M(Sex)y+1]2- (anionic and usually soluble) ... 5.3
From Figure 5.4 it is seen that in solution when n::;; 1.7 and m ~ 3 that typically
a dark brown amorphous solid precipitates, most probably CdSe, HgSe or SnSe.
When n ~ 1.7 the preference of the metal ions for a particular Sex2-ligand drives
all equilibria towards formation of that complex. This is indicated by the formation of
a single species in each of the reactions between the metal and the various polyselenide
solutions. In each of the solutions containing the Cd2+, Hg2+ and Sn2+ ions the
equilibria observed for the uncoordinated polyselenide solutions are completely
dominated by coordination, as stable MSe4 metallacycle rings are formed. The rapid
exchange of [SexJ2- in the absence of metals is no longer apparent with well resolved
spectra being easily recorded at room temperature. Surprisingly, these rings in all
cases contain only Se4 atoms despite some of the starting polyselenide solutions
containing Se32-, Ses2-, Se62- ligands. This is shown by the existence of only two
equal intensity selenium resonances in each of the reaction mixtures. Therefore it was
predicted that the species being observed were [Cd(Se4)2]2-, [Hg(Se4hJ2-, and
[Sn(Se4)3]2-.
142
The 113Cd spectrum (Figure 5.1 b) of the solution containing Cd2+ and Se42- in
a 1:2 ratio shows doublet satellites with intensity 16.5% of the central resonance. The
calculated spectrum for Cd bonded to four Se at natural abundance 77Se(I=1/2)
(0.0758), 0Se(I=0)(0.9242) provide stronger evidence that the species observed is in
fact [Cd(Se4)]2-. The following statistical proportions of 77Se and Ose for all possible
satellites can be represented as:-
Cd(0Se)4
Cd(0Se)3(17Se)l
Cd(Ose )2(17 Se )2
Cd(0Se)1(17Se)3
Cd(17Se)4
---7 1 X
---7 4 X
---7 6 X
---7 4 X
---7 I X
(0.9242)4
(0.9242)3 X (0.0758)
(0.9242)2 X (0.0758)2
(0.9242)1 X (0.0758)3
(0.0758)4
= 0.7296
= 0.2393
= 0.0294
= 0.0016
= 0.0000
= 0.9999
In reality only the doublet was observed. Assuming the 0.7296 value to
correspond to the central line and 100% intensity, the doublet satellites have a
predicted intensity of:-
0.2393 X 100 = 16.4%
2 X 0.7296
This value compares favourably with the observed value of 16.5%.
Similarly this calculation applies to the satellites on the Hg spectra (Figure 5.2)
which also has ca. 16.5% intensity.
Finally, by comparison with uncoordinated I Se4]2- it can be seen that the <>seC~)
atom resonates at virtually the same (ca. 580 ppm) as the high chemical shift in the
three MSe4 (M = Cdll, Hgii, Snii ) chelates with the lower chemical shift being metal
dependant (see Chapter 4 Figure 4.1 0).
143
These studies clearly show the existence of single polyselenide complexes in
solution. Therefore crystallisation experiments were designed to precipitate the
species observed in the Cd2+, Hg2+, and Sn2+ reactions.
5.2.6 Crystallisation experiments of polyselenide solutions prepared
from the 'NH3' method containing Cd2+, Hg2+, or Sn2+ ions.
The initial crystallisation experiments were performed with solutions containing
CdC12.5/2H20, HgC12 or SnCl2.2H20 and the Se42- ligand in a 1:2 ratio in DMF.
The large organic counter cation Ph4PBr, was chosen to crystallise the anionic
complexes, based on its reported successful use in polysulfide chemistry. In some
cases the Ph4PBr salt was added to the reaction mixtures as a solid with constant
stirring applied (see experimental). In all cases no obvious colour change occurred
with the addition of Ph4PBr. After approximately 30 min, the mixtures were filtered
to remove precipitated NaCl and NaBr. To the resulting homogenous solutions were
then added various precipitating liquids, including MeCN, THF, and diethyl ether.
More often though the reaction mixture was filtered first removing NaCl and then a
homogenous solution of Ph4PBr in MeCN was slowly added. This was necessary as
the complexes formed in the reaction mixtures were soluble in DMF. A layering
technique was used so that the slowly diffusing precipitating liquid would allow good
crystal growth. This was done either in the reaction vessel or inside a glass U-Tube
apparatus designed for this purpose to which some of the solution had been
transferred (see Chapter 2). Temperature is an important factor in the growth of good
crystals. Therefore some solutions were kept at ca. ooc, some at ambient
temperature, and some at initially elevated temperatures that were allowed to slowly
equilibrate to ambient temperature.
The most successful crystallisation procedure (based on yield and quality of
crystal), for these reaction mixtures, was the slow addition of MeCN, at ambient
temperature (see experimental). In all three cases dark brown I black needles were
144
formed in good yield (ca. 50%). Subsequent single crystal X-ray diffraction revealed
three previously unreported polyselenide complexes: (Ph4P)2[Cd(Se4)2],
(Ph4P)2[Hg(Se4)2] and (Ph4P)2[Sn(Se4)3l. Their crystal structures are shown in
Figures 5.5-5.7 and are discussed in section 5.28.
The addition of Ph4PBr in MeCN to the reaction mixtures prepared with other
Sex2- chain lengths (x:t:4), all resulted in the crystallisation of the same three metal
polyselenide complexes but in lower yields. Metal complexes of the Se42- ligand
formed in every reaction, independent of the Sex2- ligand used in the preparation.
This is entirely consistent with the NMR results.
5.2. 7 Crystallisation experiments of polyselenide solutions prepared
from the 'DMF' method containing Cd2+, Hg2+, and Sn2+ ions.
This second synthetic approach was also used for the isolation of the complexes
described in section 5.2.6. This procedure generated metal polyselenide solutions
directly in DMF by reacting Na(s) and Se(s) in DMF at elevated temperatures in the
presence of CdC12.5/2H20, HgC12 or SnC1 2.2H20. The resulting solutions were
then filtered and to the filtrate MeCN solutions containing the Ph4PBr counterion
were added. As in the 'NH3' method a layering technique was used so that the slowly
diffusing precipitating solvent would allow good crystal growth. This was done both
inside the reaction vessel and also inside a glass U-Tube apparatus designed for this
purpose to which some of the solution had been transferred (see Chapter 2). This
simple, inexpensive and effective preparation was the direct analogue to that
previously used in the synthesis of metal polysulfides.
However reactions of these polyselenide solutions with CdCI2.512H20, HgCI2,
or SnC!z.H20 led only to the formation of the same species that were found in the
'NH3' preparations, i.e (Ph4P)21Cd(Se4)2], (Ph4P)2[Hg(Se4)2] and
(Ph4P)2[Sn(Se4)3] (see experimental). These results were confirmed using both X
ray powder diffraction patterns obtained from the crystal structure determination from
145
the earlier preparation, and by 77se NMR. The respective yields of these compounds
were generally lower using this ' DMF' procedure; ca. 35% compared with the "NH3•
procedure, typically ca. 50%.
The (ormation of the Sn4+ complex is surprising considering the starting
material used Sn2+. This would indicate that possible Sn2+-Sex2- (x > 1) complexes
are unstable towards internal redox chemistry.
5.2.8 Crystal structures of (Ph4Ph[Cd(Se4)z], (Ph4P)z[Hg(Se4)z] and
(Ph4Ph[Sn(Se4)3].
The molecular structure of (Ph4PhfCd(Se4)2] is shown in Figure 5.5. The
structure consists of two independent [Cd(Se4)2]2- anions. Both anions have an
approximately tetrahedral Cd centre, but the two anions differ in some interesting
ways. In particular the bond angles [see appendix A] at Cd(2) are significantly less
obtuse than those at Cd(l). All four CdSe4 rings are best described as having the
distorted envelope conformation with the "flap" opposite one of the M-Se bonds. The
existence of two independent, differently distorted anions in the unit cell presumably
rises from the effects of packing forces on these rather flexible anions. Selected
metrical data is presented in appendix A.
In the crystal structure of (Ph4P)2[Hg(Se4)2], the Hg atom possesses
unexceptional tetrahedral coordination as was found in (Ph4P)2[Cd(Se4)2]. However
in (Ph4P)2[Cd(Se4)2] the CdSe4 rings are fully ordered, while three of the four
crystallographically independent HgSe4 rings in (Ph4P)2[Hg(Se4)z] are
conformationally disordered.
The structure of the [Sn(Se4)3]2- anion (Figure 5.6) consists of three four membered
selenium chains chelated to a central Sn4+. The coodination geometry of the Sn4+ is
octahedral approaching D3 symmetry, with an average Sn-Se bond distance of
2.709(13) A,. and a Se-Sn-Se angle of approximately 9ooc. Both envelope (i.e
SnSe(l)Se(2)Se(3)Se(4)) and puckered (i.e SnSe(5)Se(6)Se(7)Se(8) and
146
Se1AA
Se3AA
Se188
Se3BB
Se3AB
Figure 5.5. The two independent [Cd(Se4h]2- complexes in (Ph4P)z[Cd(Se4)z].
Se(11)
Se(10)
Figure 5.6. ORTEP representation and labelling scheme of the structure of the
[Sn(Se4))]2- anion.
SnSe(9)Se(10)Se(11)Se(12)) conformations are adopted by the SnSe4 rings. It
should be noted that this complex can give rise to optical isomerism, and the isolated
product is a racemic mixture.
5.2.9 NMR studies of redissolved (Ph4 P) 2 [ C d ( S e 4) 2] ,
(Ph4Ph[Hg(Se4hl and (Ph4Ph[Sn (Se4)3].
The 77Se NMR of redissolved (Ph4Ph[Cd(Se4h] species in DMF showed the same
two resonances at 582 and 38 ppm of equal intensity (Figure 5.1a) observed in the
Cd2+ I Sex2- reaction mixtures. The lower chemical shift resonance at 38 ppm shows
a doublet satellite that has coupling of 270 Hz (lJ 77Se-113Cd). This value concurs
with that seen in the 113Cd NMR where a single resonance at 748.1 ppm shows a
doublet coupling to 77Se of 270Hz (lJ 113Cd-77Se). The observed satellite intensities
in the 113Cd NMR spectrum of ca. 16.5% relative to the central resonance correspond
well with the calculated intensity of 16.4% (see page 17). No 77Se-77Se coupling was
observed. The 113Cd resonance of I Cd(Se4h ]2- at 7 48.1 ppm, compares favourably
with the resonances for the cadmium polysulfide complexes, [Cd(S5)z]2-,
[Cd(Ss)(S6)]2-, [Cd(S6h]2- observed at 740, 695 and 648 ppm which show a shift to
higher chemical shift with decreasing ring size.14
Similarly the 77Se NMR spectrum for redissolved (Ph4P)z[Hg(Se4)z] in DMF
shows 2 resonances at 569.1 and 53.9 ppm respectively. The lower chemical shift
resonance at 53.9 has doublet satellites with a coupling of 1260Hz (lJ 77Se-199Hg).
The 199Hg spectrum shows a singlet at -481 ppm with doublet coupling of 630Hz (lJ
199Hg-77Se). The 199Hg resonance of [Hg(Se4)z]2- compares well to the the
resonance for the polysulfide anion, [Hg(S4)]2- observed at -180 ppm. The observed
satellite intensities in the 199Hg NMR spectrum of ca. 16% relative to the central
resonance correspond well with the calculated intensity of 16.4% (see page 17). The
(lJ 199Hg-77Se) seen was 1260Hz. No 77Se-77Se coupling was observed.
147
The 77Se NMR spectrum of (Ph4P)2[Sn(Se4)3] in DMF at room temperature
shows two resonances at 586.5 and 427.5 ppm, respectively, with the resonance at
427.5 ppm showing doublet satellites with coupling of 816Hz (lJ 77Se-119Sn). The
119Sn spectum shows a single resonance at -723.1 ppm was observed with doublet
coupling of 816Hz. No 77Se-77Se coupling was observed. The observed 119Sn
NMR chemical shift for [Sn(Se4)3]2- correlates with those of [SnSe3]2- and [SnSe4]4-
anions which occur at -264.3 and -476.6 ppm respectively.22 The successive shift of
the 119Sn resonance toward more negative values with increasing tin coordination
number is evident and suggests increased shielding on the tin centre.
The presence of only two resonances in each of the 77Se NMR spectra indicate
that all three Se42- ligands in the complex are equivalent and suggests the various
conformations of the SnSe4 five-membered rings interconvert rapidly in solution at
room temperature.
In the 77Se NMR spectra of all three of [Cd(Se4)2]2-, [Hg(Se4)2]2-, and
[Sn(Se4)3]2- it is the lower chemical shift resonance that displays (lJ 77Se-metal)
coupling. Similar results have been observed in other metal systems. Opposite
results i.e where the (lJ 77Se-metal) coupling is on the highest chemical shift
resonance have also been observed. The reasons for these differences are presented in
section 5.12.2.
5.2.10 Crystallisation of related complexes by other workers.
Other workers have more recently synthesised the three complex anions
[Cd(Se4)2]2-, [Hg(Se4)2]2-, and [Sn(Se4)3j2-, using variations of the preparations
descibed above. Adel et al 23 synthesised [Na(l5- crown-5)]2[Cd(Se4h] and
[Na(15- crown-5)]2[Hg(Se4h, by reacting the respective metal acetates with ethanolic
solutions of sodium polyselenides at ambient temperature. The sodium polyselenides
were formed by the reaction between Na2Se and Se, in the presence of 15-crown-5.
Crystallisation resulted while the reaction mixture stood overnight at ambient
148
temperature. Using an analogous procedure KrHuter et al 24 prepared [K(l8-Crown-
6)2]2[Hg(Se4)2]. Ansari et al 25 have synthesised both [Ph4Ph[Cd(Se4)2], and
[NEt4]2[Hg(Se4)2 by the reaction of the metal xanthates with a polyselenide solution
in CH3CN-DMF. The polyselenide solution was prepared from the reaction between
Li2Se and Seat ambient temperature. Crystallisation occurred by the addition of 2-
propanol to both reaction mixtures and allowing them to stand for two days at ambient
temperature. Magull et al 26 and Kanatzidis 27 have also reported the isolation of
(Ph4P)2[Hg(Se4)2] using other methods. Magull et al 26 prepared (Ph4P)2[Hg(Se4)2]
by the obscure reaction between (Ph4P)2[ Sn(Se4)3] and Hg(CH3COO)z in DMF at
100°C. The addition of diethyl ether was used to initiate crystallisation. No synthetic
details have been published by Kanatzidis.
Both Magull et al26 and Kanatzidis 27 have reported the syntheses of the related
mercury complex [Hg2(Se4)3]2-. Magull et al 26 · prepared [Cs(18-Crown-
6)2]2[Hg2(Se4)3] by the reaction of a lithium polyselenide solution in DMF with
Hg(CH3C00)2 in the presence of CsBr and 18-crown-6 at ambient temperature,
while Kanatzidis prepared (Ph4P)2[Hg2<Se4)3] by the reaction ofHgCl2 and Na2Se4
in a 2:3 ratio in DMF at RT.27
Kim et al 28 report that if shorter polyselenide ligands such as Na2Se2 are
reacted with HgCl2, a multinuclear cluster, (Et4N)4[Hg7(Se)s(Se2)] is produced.
Finally, S-P Huang et al 29 have only recently reported another pathway to
(Ph4P)z[Sn(Se4)3]. They reacted SnCl4 as well as SnCl2.2H20 with Na2Ses and
Ph4PCl in DMF at RT to generate (Ph-1P)2[ Sn(Se4)3]. Diethyl ether was used to
initiate crystallisation.
149
5. 3 Crystallisation experiments of polyselenide complexes containing
other metal ions using both the 'NH3' and 'DMF' procedures.
As a ~onsequence of the systematic and logical syntheses of the three complexes
(Ph4P)2[Cd(Se4)2], (Ph4P)2[Hg(Se4)2] and (Ph4P)2[Sn(Se4)3] described in the
previous sections, it was expected that the addition of other metal precursors into these
polyselenide systems would result in other complexes being formed. The results of
these experiments are described below.
5.3.1 Syntheses and characterisation of (Ph4Ph[Zn(Se4)2]
a) 'NH3' Method
The addition of either ZnCl2 or ZnBr2 to Sex2- (x=3-6) solutions resulted in the
formation of the zn2+ complex anion [Zn(Se4)2]2-, which was precipitated by a
similar procedure to those described in section 5.2.7. The best procedure utilised
ZnBr2 as the metal precursor as it is easier to handle and less hygroscopic than ZnCl2.
The product was isolated in ca.53% yield using a slight variation of the crystallisation
procedure described in section 5.2.7. Here the reaction mixture was filtered removing
precipitated NaCl and then a solution of Ph._tPBr in MeCN was added and the resulting
solution allowed to stand at RT overnight (see experimental).
Zinc does not have an NMR active nucleus therefore the reactions were
monitored only by 77Se NMR. In all reactions only two resonances were observed at
<>se 573 and 101 ppm respectively and were similar for the redissolved product
(Figure 5.7). No coupling was observed. In the absence of coupling data for
[Zn(Se4)2]2-, the resonances cannot be unequivocally assigned. However, by
comparison to the 77Se NMR spectra of !Cd(Se4)2]2- and [Hg(Se4)2]2- it is predicted
that the lower chemical shift peak at <>se I 01 ppm, corresponds to those Se atoms
which are metal bound.
150
1000 800 600 400 ppm
200 .o -200
Figure 5.7. The 77Se NMR of (Ph4Ph[Zn(Se4hl in DMF at 300K.
b) 'DMF' Method.
The 'DMF' method has also been used successfully to isolate
(Ph4Ph[Zn(Se4)2]. Both ZnCh or ZnBr2 were used as metal precursors. The
reaction time of 14hrs required for the reaction to be complete is considerably longer
than the 2hrs required for the 'NH3' method and the yield obtained by this 'DMF'
procedure (29%) is significantly lower than for the 'NH3' method.
5.3.2 Crystal structure of (Ph4P)2[Zn (Se4)2].
The crystal structure of (Ph4P)2[Zn(Se4)2] is isostructural with that of
(Ph4P)2[Cd(Se4)2] (Figure 5.1).
5. 3. 3 Syntheses of related compounds.
Other workers have reported the synthesis of the [Zn(Se4)2]2- anion. Ansari et
al 25 prepared (Ph4P)2[Zn(Se4)2] using a mixture of Li2Se, Se, Ph4PC1, CH3CN,
Et3N, Zn(xanh and DMF and then layering the resulting solution with 2-propanol to
initiate crystallisation all at RT. Adel et al 23 synthesised [Na(15-crown-
5)]2[Zn(Se4hl by reacting zinc acetate with ethanolic solutions of sodium
polyselenides at RT, formed by the reaction between Na2Se and Se, in the presence of
15-crown-5. Crystallisation resulted by allowing the reaction mixture to stand
overnight at RT.
Fenske et al 30 have very recently prepared [Rb(18-crown-6)]2[Zn(Se4)(Se6)]
by the reaction of a Li2Se6 solution in DMF with zinc acetate in the presence of
rubidium iodide and 18-crown-6. Diethyl ether was added to initiate crystallisation.
5. 4 Syntheses and characterisation of (Ph4Ph[Ni(Se4)2]
a) NH3 Method
(Ph4P)2[Ni(Se4)z] was prepared using the 'NH3' method by the addition of
NiCl2.6H20 to a DMF solution of Na2Se4. The product was crystallised the same
151
way as described in section 5.2. 7, generating black needles in 60% yield. Nickel like
zinc does not have an NMR active nucleus, therefore the solutions were monitored
only by 77Se NMR. Two signals were observed for the reaction mixture and the same
for the redissolved solid at 8se 815 and 720 ppm respectively (Figu~e 5.8).
b) DMF Method
(Ph4P)2[Ni(Se4)2] was also prepared using the 'DMF' method by reacting
NiCI2.6H20, Se and Na in DMF solution at 60°C for 13hrs. A yield of only 17%
was obtained using this procedure.
5.4.1 Crystal structure of (Ph4Ph[Ni(Se4)2].
The [Ni(Se4)2]2- ion crystal structure (Figure 5.9) shows the Ni2+ atom has
approximately square-planar co-ordination, with a slight tetrahedral distortion: the
donor selenium atoms lie 0.33 A to either side and the nickel atom lies within the least
squares plane through the NiSe4 set. Mean dimensions are Ni-Se = 2.297(7)A;
intrachelate Se-M-Se = 102.0(4)" and interchelate Se-M-Se = 80.0(1Y. Unlike the
homologous [Ni(S4)2]2- ion crystallised with E14N+ 31,32 and Ph4P+ 32 the Se42-
rings in [Ni(Se4)2]2- are not conformationally disordered.
5.4.2 Syntheses of related compounds.
Ansari et al 25 have synthesised I PEtPh3l2rNi(Se4)2] by reacting a mixture of
Li2Se, Se, Ph4PCI, CH3CN, Et3N, Ni(xan)2 and DMF and then layering the resulting
mixture with diethyl ether to initiate crystallisation all at RT (50% yield).
McConnachie et al 33 have as this work was being written prepared the novel
Ni(IV) cubane [NE14]4[Ni4Se4(Se3)s(Se4)l .xNEt4Cl (x = 0, 1), from the spontaneous
assembly reaction between Ni(S2COEt)2 with Li2Se and Se and NE14Cl. Diethyl
ether was used to initiate crystallisation.
152
ppm
Figure 5.8. The 77Se NMR of (Ph4Ph[Ni(Se4h] in O:MF at 300K.
Figure 5.9. Structure of the [Ni(Se4hJ2- anion.
5.5 Syntheses of (Ph4Ph[Pb(Se4)2]
a) DMF Method
The Pb(II) complex, (Ph4Ph[Pb(Se4)z] could be isolated only by the 'DMF'
method. P~Cl2 was used as the source of lead. In other metal syntheses using the
'DMF' method a ratio of M: Na: Se of 1:2:8 was shown to be the optimal conditions
for complete reaction to occur. In this case where PbCl2 was used as the metal
precursor an excess of Na was required in a 1:4:8 ratio to generate the dark green I
brown solution containing the lead polyselenide complex. If Na amounts less than
this ratio were used reaction mixtures proceeded initially with the usual colour change
to dark green after ca. 20min and then proceeded to precipitate black solid and Se
leaving a colourless solution.
Crystallisation of the 1:4:8 PbCI2:Na:Se reaction mixture, was achieved by the
slow diffusion of a solution containing Ph4PBr in MeCN inside a U-Tube apparatus
(see chapter 2). The U-tube was placed in a thermostat bath at 90°C and allowed to
equilibrate to ambient temperature overnight. The resulting brown/black block
crystals (26% yield) were washed with MeCN and dried. These crystals were
considerably more air sensitive than the crystals of any of the previously descibed
[M(Se4)2]2- complexes, oxidising to Se within 1.5 hr on exposure to the atmosphere.
Unfortunately no NMR was recorded for this complex as the sample was given
for single crystal X-ray analysis and unintentionally exposed to the atmosphere
overnight. Various attempts since have been used in an effort to reprepare this unique
compound. Unfortunately no subsequent preparation has resulted in the reisolation of
(Ph4P)z[Pb(Se4)z].
Other experiments in attempts to repeat the successful synthesis of
(Ph4P)z[Pb(Se4h] have resulted in the isolation of (Ph4P)z[Se(Seshl and (P~PhSes
polyselenide salts. The 77Se NMR spectra of these reactions at 300K either show no
signal or a broad (1000Hz) signal at ca. 760 ppm attributable to the exchanging
153
[Se(Ses)2]2- anion. Low temperatures confirmed the four resonances at Bse 795,
708, 672 and 484 ppm attributable to the [Se(Sesh]2- anion.
Other experiments using the DMF method with 1:4:8 ratios but with Pb(N03h,
PbBr, Pbl2, Pb(C2H302h and PbSe precursors all failed to generate
(Ph4P)2[Pb(Se4h]. In all cases but PbSe, reaction mixtures were shown by 77Se
NMR to contain [Se(Sesh]2-. In the case of PbSe no resonances whatsoever were
observed. Consequent crystallisation experiments with Ph4PBr in MeCN, of the
reactions containing Pb(N03)2, PbBr2, Pbi2, Pb(C2H302h resulted in the
precipitation of only (Ph4P)2[Se(Seshl· The PbSe reaction mixture resulted in the
crystallisation of (Ph4P)2Se5.
b) NH3 method
Attempts to prepare any lead polyselenide complex using the 'NH3' method
failed. Mixtures containing Na2Sex (x = 2-6) and the metal precursors Pb(N03)2,
PbCl2, and Pb(C2H302h in ratios of 1:1 and 1:2 were prepared. When the
metal:ligand ratio of 1:1 was used a black precipitate was observed in a colourless
solution in all mixtures. ICP analysis of the solid indicated the presence of lead.
77Se NMR at 300K of the green I brown reaction mixtures containing a 1:2
metal:ligand ratio showed in ali cases the broad signal ca. 760 ppm characteristic of
[Se(Sesh]2-. Crystallisation with PI}4PBr in MeCN using the U-Tube technique
resulted in the crystallisation of (Ph4Ph[Se(Seshl.
5.5 .1 Crystal Structure of (Ph4Ph[Pb(Se4)2]
The molecular complex [Pb(Se4)2]2- (Figure 5.10) shows irregular
stereochemistry. Although the Pb-Se bond lengths are distinctly different, three Pb-Se
bonds [to Se(1B), Se(4B) and Se(lA)"I are 0.3 - 0.4A shorter than the fourth, and
subtend Se-Pb-Se angles that are close to orthogonal. Therefore the geometry is
interpreted in terms of trigonal orthogonal primary co-ordination, with a longer
154
(
Se4B Se4B
0
2.91A
Pb Se1A
Se4A
Se3A
Figure 5.10. (a) The structure of the [Pb(Se4h]2- anion and (b) with the principal
bond lengths and angles.
secondary connection [to Se94A)] also in a pseudo orthogonal location, and with two
vacant orthogonal positions [trans to Se(lA), Se(lB)]. An alternative but less realistic
interpretation of the coordination stereochemistry in [Pb(Se4)2]2- involves a trigonal
bipyrarnid with one vacant equatorial position.
5. 6 Synthesis and characterisation of (Ph4P)z[Mn(Se4hl
I have prepared (Ph4P)z[Mn(Se4)2] by the NH3 method, with the addition of
MnC}z.4Hz0 to DMF solutions containing NazSe4 and Ph4PBr resulting in brown
needle crystals in ca. 62% yield (see experimental). A complete crystal structure of
this compound was not determined after the unit cell was shown to be identical to that
of (Ph4P)z[Zn(Se4)2]. Micro analysis was consistent with the formula being
(Ph4P)z[Mn(Se4)2]. No 77Se NMR signals were observed in the reaction mixtures or
in solutions containing the redissolved product. This is attributed to the paramagnetic
state of the Mn2+ metal centre.
Syntheses of related compounds.
Ansari et al25 prepared (Ph4P)2[Mn(Se4)2] from a reaction mixture containing
K2Ses, MnClz and Ph4PCl in DMF in ca. 22% yield. They report a broad 77Se NMR
signal in DMF at ca. 720 - 790 ppm assigned to the [Mn(Se4)2]2- anion. However I
have shown this broad signal to be attributed to the presence of the oxidised species
[Se(Ses)2]2- (see chapter 4). The [Mn(Se4)2]2- anion has also recently been
synthesised and crystallographically characterised by O'Neal et al 34 using
polyselenide anions to oxidatively decarbonylate Mnz(CO)IO· In this work by O'Neal
et al, a series of three manganese polyselenide anions has been isolated from these
reaction mixtures. The reaction of Mn:;(CO)Jo with one equivalent of K2Se3 in DMF
generated [Mnz(Se2)2(C0)612-. two equivalents of KzSe3 generated
[Mn2(Se4)2(C0)6]2· and the thermolysis of this compound resulted in the formation of
155
Of the three reported syntheses of (Ph4P)z[Mn(Se4)2] the one reported here is
simpler and generates more than twice the yield reported for the other two. In the case
of the work by Ibers, I suspect that oxidation has occurred in the solution generating
the now well characterised species [Se(Ses)2]2-, thus accounting for the broad 77Se
NMR signal observed at 720-790 ppm.
5. 7 Synthesis and characterisation of (Ph4Ph[Pd(Se4)2]
This compound was prepared the same way as (Ph4P)2[Zn(Se4)2] using
Na2PdC14 as the metal precursor and Na2Se4 as the polyselenide source. On filtering
the dark green Pd2+JSe42- reaction mixture containing Ph4PBr, black needles
crystallised (54% yield) immediately without addition of MeCN. This lack of
solubility in DMF is unique to this Pd polyselenide complex. All other anionic metal
polyselenide complexes are completely soluble in DMF at ambient temperature.
The 77Se NMR spectrum of redissolved (Ph4P)z[Pd(Se4)z] in DMF at 300K
gave rise to two Se resonances at 869 and 732 ppm respectively. No coupling was
expected.
A complete crystal structure of this compound was not determined after the unit
cell was shown to be identical to that of (Ph4P)2!Ni(Se4)2].
5. 7.1 Syntheses of related compounds.
Ansari et al 25 also synthesised (Ph.:~P)2[Pd(Se4)2] by reacting Pd(S2COEt)2
with 2 equvalents of Li2Se4 in the presence of PPh4Cl in DMF in 60% yield. The
77Se NMR reported by Ansari et al in DMF at 300K shows two resonances at 893 and
758 ppm.
The hydrothermal reaction of PdCI2 with K2Se4 in the presence of KOH and
H20 in the 1:5:5:40 ratio in a sealed Pyrex tube at 110°C for one day produced
K2[PdSe10].35 This compound is polymeric, featuring interpenetrating frameworks
formed by two macroanions of !Pd(Se4)212- and [Pd(Se6)2]2-. In these two
156
I I I I I I I I I I I I I I I I I I I I I I I I I I I I I
940 920 900 880 860 840 820 800 780 760 740 720 700 680
ppm
Figure 5.11. The 77Se NMR of (Ph4PhfPd(Se4h] in DMF at 300K.
macroanions, palladium atoms still have the square-planar geometry. However, both
Se42- and Se62-Iigands, instead of being chelated to the same metals, act as extended
zigzag chains to bridge different metal centres (see section 5.12.3).
5. 8 Synthesis and characterisation of (Ph4Ph[Pt(Se4)3]
(Ph4P)2[Pt(Se4)3] was prepared by reacting Na2Se4 and (Et3N)2Pt2Br6 in a
10:1 ratio in the presence of Ph4PBr in DMF at 70°C. The dimer (Et3NhPt2Br6 was
used in attempt to generate possible platinum clusters. Black needles crystallised
(60% yield) by adding a layer of MeCN and allowing the mixture to stand overnight.
Redissolved (Ph4P)2[Pt(Se4)3] in DMF gave a 77Se NMR spectrum showing
two Se resonances at 754 and 644 ppm respectively (Figure 5.12). 1J(195Pt-77Se)
coupling of 118Hz was observed on the 8sc 644 ppm resonance. As observed for the
[Sn(Se4)3]2- anion the presence of only two resonances in the 77Se NMR spectrum
(Figure 5.12) of [Pt(Se4)3]2- indicates that all three Se42-ligands in the complex are
equivalent in solution at room temperature. Unfortunately, no 195pt NMR spectrum
was recorded. Ansari and Ibers 36 report two 77Se NMR resonances at 8se 790 and
680 ppm for the same anion. Coupling was observed on the 680 ppm resonance but
no data was reported.
5. 8.1 Crystal Structure
Figure 5.13 shows the [Pt(Se4hJ2- anion. The anion features a six-coordinated
PtiV centre in a pseudooctahedral geometry chelated to three Se42- units. Each five
membered PtSe4 ring is in the envelope configuration with a Se atom occupying the
"flap" position. The ion is chiral, and presumably optical isomers could be resolved,
as was done for the analogous polysulfide complex, [Pt(S5)3]2-,37 Pt-Se distances
vary from 2.479 (4) to 2.491(4) A. The average Se(internal)-Se(internal) distance is
2.327 (8) A, and the average Se(external)-Se(internal) distance is 2.339 (2) A. The
Se-Pt-Se angles in Pt(Se4)32- are 99.9 (1), 99.9 (1), and 99.9 (l)O. These angles
157
I I I I I I I I I I I I I
780 770 760 750 740 730 720 710 700 690 680 670 660 650 640 630
ppm
Figure 5.12. The 77Se NMR of (Ph4P)2[Pt(Se4)3] in DMF at 300K. Inset.
Expansion of the resonance at osc 644 ppm.
Se2B
Se2C' Se2C
Figure 5.13. The structure of the [Pt(Se4)3]2- anion showing the disorder present in
the crystal.
differ significantly from the S-Pt-S angles of 92.0 (3), 92.7 (3), and 92.3 (3)0 in
[NH4h[Pt(S5)3].38
5. 8. 2 Syntheses of related compounds
Ansari and Ibers 36 published the synthesis and crystal structure of
(Ph4P)z[Pt(Se4)3].D.MF after I had synthesised and characterised (Ph4P)z[Pt(Se4)3].
Their synthesis involved reacting Pt(S2COEt)z with Li2Se5 in the presence of Ph4PCl
in a mixture of DMF and Et3N. Crystallisation ocurred by cooling a saturated DMF
solution.
[Ptii(Se4)]2- has reportedly been synthesised in situ by the reaction between
BH4- and [Pt(Se4)3]2-, although these claims have been made based on spectroscopic
data and elemental analysis.25,39
In the polysulfide system only [Pt(S5)J]2- has been shown to exist
crystallographically. 38
5. 9 Synthesis and characterisation of (Ph4P)3[Co3(Se4)6].(DMFh
A notable absence in the metals known to form homoleptic metal
polychalcogenide complexes is cobalt, for which there was no literature on homoleptic
polychalcogenides, and reports of only two organo cobalt polychalcogenides -
(MesCshC02S4 40 and (MesCs)2Co2Ses 41
I included cobalt in the investigations of the syntheses, of metal
polychalcogenide complexes and report the first homoleptic cobalt polychalcogenide
complex: [ { Co(Se4)3}Co{ (Se4)3Co }](Ph4P)3(DMF)2].
(Ph4P)3[Co3(Se4)6].(DMF)z was only prepared using the DMF method by
reacting CoCI2.6H20, Na and Se in a 1:2:8 ratio in DMF at 6QOC for llhrs. Dark
green I black needles were crystallised (31%) by layering a solution of Ph4PBr in
MeCN and storing overnight at ooc.
158
No 77Se NMR signals could be found for the redissolved complex in DMF in
the range 300 - 220K. This lack of signal is attributed to the weak paramagnetism
[J..LCo ca 0.8 BM] which is normal for low spin Co(III). Its dark green colour
distinguishes it from the normal red/brown of other homoleptic metal polyselenides.
5.9.1 Crystal structure of (Ph4P}J[Co3(Se4)6].(DMFh
The molecular structure of [ { Co(Se4)3} Co { (Se4)3Co ](Ph4fl)3 (DMF)2 is shown
in Figure 5.14. There are two independent [Co3(Se4)6]3- complexes in the crystal,
each sited at a centre of inversion. These two complexes are very similar both
adopting virtual c3 symmetry. The molecules are best formulated as
[{ Co(Se4)3 }Co{ (Se4)3~0} ]3- in which the two terminal cobalt atoms have tris
tetraselenide chelate coordination which is approximately octahedral, while the central
cobalt atom also has approximately octahedral (J.t-Se)6 coordination, achieved by
sharing one selenium atom from each of the six tetraselenide groups. The non
bridging Co-Se bonds average 2.373(5)A., while the bridging Co-Se bond are not
significantly elongated at 2.317(10)A. At the terminal cobalt atoms the intrachelate
Se-Co-Se angles are 100.6(7)" and the interchelate Se-Co-Se angles are 82(1)" for
atoms related by the pseudo-three fold axis. At the central cobalt atom the Se-Co-Se
angles are 82.0(6)" around the pseudo-three-fold axis and 98.0(6)" along the pseudo
three fold axis. The Co-Se-Co angles are 81.9(3)" and the Co-Co distances are
3.115(5)A. Within the Se4 ligands the three types of Se-Se bonds are clearly
differentiated in length, the Se(l )-Se(2) distances at the bridging end being longer,
2.390(3)A, than Se(3)-Se(4), 2.368(9)A and the central bond Se(2)-Se(3),
2.326(4)A.
Almost two years after the publication by me of (Ph4Ph[Co3(Se4)6].(DMFh42,
MUller et al 43 published the synthesis of (PPNh[Co3(Se4)6].(DMFh. This molecule
was prepared by reacting CoCl2.6H20 with Li2Se6 in DMF in the presence of PPNCl
at 100°C.
159
Se4CA
Figure 5.14. The molecular structure of 1Co3(Se4)6]3-. Atoms are labelled for one of
the two independent centrosymmetric molecules in the crystal.
The complex (Ph4Ph[Co3(Se4)6].(DMF)z is comparable with the chromium(ll)
polychalcogenide complexes reported - [Cr3(Se4)6]3- and [Cr3(Te4)6]3-.44 The
crystals of (Ph4P)3[Cr3(Se4)6] are isomorphous with those seen in the Co complex
but the analysis of the former did not detect the lattice DMF. Other similar structural
linear clusters are found in the amino thiolate complexes:
[{Co(HzN(CHz)nS)3}Co{(S(CHz)nNHz)3CoJ]3+ where n = 2 45,46,47 and n = 3. 48
The comparable linear trichromium (III) complex [Cr(SCHzCHz0)3
Cr(OCHzCHzS)3Cr]3-, has also been isolated.49 Clusters such as these are under
scrutiny as potential precursors to metal chalcogenide polymers.49
Finally, although there exists general structural analogies between complexes
with polysulfide ligands and complexes with a, ro-alkanedithiolate ligands,50 the only
known cobalt alkanedithiolate complexes are tetrahedral [Co(SCHzCHzS)z]2- and the
square-planar (S=l) Co(III) species [Co(SCHzCHzS)z]-,51 not octahedral as seen in
[Co3(Se4)6]3-.
5.10 Synthesis and characterisation of (Ph4P)[CpMo(Se4)z]
(Ph4P)[CpMo(Se4)z] is the first member of a new class of mono
( cyclopentadienyl)-bis(polychalcogenide) metallates, distinct from existing compounds
which are formulated (115-CsHs)z M2CEx).l (E = S, Se, x = 3,4,5).
(Ph4P)[CpMo(Se4)z] was prepared by the reaction of (115-C5Hs)2Moz(C0)6,
Na and Se in a 1:2:8 ratio in DMF at 6QOC for 12hrs. Crystallisation of dark green
needles (36% yield) was achieved by layering a solution of Ph4PBr in MeCN and
storing overnight at OOC.
The 77Se NMR spectrum (Figure 5.15) of (Ph4P)[CpMo(Se4)z] in DMF at
298K shows two resonances at ose 1175 and 560 ppm respectively. No coupling was
observed, as expected because 97Mo is a quadrupolar nucleus with low natural
abundance.
160
1200 1100 1000 900 800 500 ppm
Figure 5.15. The 77Se NMR of (Ph4P)fCpMo(Se4)2] in DMF at 298K.
The molecular structure of the (115-CsHs)Mo(Se4)2]- ion is shown in Figure
5.16. There is a small but significant asymmetry in the chelation of both Se42-
ligands:
Mo-Se(1A) = 2.463(1), Mo-Se(4A) = 2.533(1); Mo-Se(1B) = 2.474(1), and
Mo-Se(4B) = 2.542(1)A. The intrachelate Se-Mo-Se angles are both 88°, and
the interchelate angles Se(1A)-Mo-Se(4B) and Se(4A)-Mo-Se(1B) are 74.8 and
72.40, respectively.
The torsional angles shown below show that ring A adopts the envelope
conformation with the flap at Se(4A), ring B is between an envelope [flap at Se(4B)
and half-chair [two-fold axis through Se(lB)]
Table 5.4. Torsional angles (0 ) for the MSe4 rings in [CpMo(Se4h]-.
Ring Bond
M---Se(l )----Se(2)----Se(3)----Se( 4 )-----M
A -30.2 +0.3 +32.3 -56.6 +50.3
B -24.6 -9.5 +41.8 -63.4 +51.1
Despite a large amount of research on cyclopentadienyl metal polysulfide
compounds, 5,52 there is no report of a polysulfide analogue of (Ph4P)[CpMo(Se4hl.
or of any mono(cyclopentadienyl)-bis(polychalcogenide) metallate in the general class
[(775-CsRs)M(ExhF (E = S,Se). Reactions of a variety of cyclopentadienyl metal
carbonyls with sulfur or selenium yield instead, uncharged compounds, usually
dimetallic, with bridging E atoms or Ex groups,52 for example (7J5-C5Me5)Rh(J.L
S4hRh(7J5-C5R5).41 Reactions of (7]5-CsRshMXn precursors with polychalcogenide
ions in solution yield the bis-cyclopentadienyl derivatives [(7]5-C5RshMCEx)], such as
[(7J5-C5HshM(E5)] (M = Ti, Zr, Hf; E = S, Se)53-57 and [(7]5-C5H5hM(E4)] (M =
Mo, W; E = S, Se).58-61
161
Se3B
Se3A. Se4A
©~~ Se2B
Se1A
Se2A Figure 5.16. The molecular structure of I CpMo( Se4)z]-.
Adel et al 62 have recently synthesised the anionic Mo(II) product, [(7]5-
CsHs)Mo(CO)l(,u, 172-Se2)]- by a similar procedure to the one described here and
published earlier,17 to generate (Ph4P)[CpMo(Se4)2]. The reaction they use is
addition of (7]5-C5HshM~(C0)6 to (Et4N)zSe6 in ethanol at room temperature.
Thus (Ph4P)[CpMo(Se4)z] is unique as an anionic cyclopentadienyl metal bis
polychalcogenide although the Mo(IV) compounds, [XMo(E4)z]2- (X = 0, S, Se; E =
S, Se) 63-65 are similar.
5.11 Synthesis and characterisation of (Ph4P)2[Cu4(Se4h(Ses)] and
(Ph4Ph[Ag4(Se4)2(Ses)]
.Prior to the publication 19 by me of the formation of the two complexes
(Ph4P)2[Cu4(Se4)2(Ses)] and (Ph4P)2[Ag4(Se4)2(Ses)], polyselenide complexes of
copper and silver had not been reported. The syntheses of these compounds are
described.
a) DMF Method
The syntheses of these two polyselenide clusters involved the addition of either
CuCl2.2H20 or AgN03 with Na and Se in a 1:2:8 ratio in DMF at 60°C for twelve
hours. Brown block crystals ( Cu, Ag; 28% ,31% yield) were crystallised by adding
a solution of P~PBr in MeCN and allowing the mixture to stand overnight at ooc. The 77Se NMR spectum (Figure 5.17) of (Ph4P)2[Cu4(Se4)2(Ses)] redissolved
in DMF at 298K showed four distinct resonances at Ose 623, 611, 308 and 182 ppm.
Surprisingly no 77Se signal could be found for (Ph4P)2[Ag4(Se4)2(Ses)] under the
same conditions suggesting that the Ag complex is less stable at 298K than the Cu
complex, and rearranges in solution. However even at 220K no evidence of any 77Se
NMR signal was found.
b) NH3 Method
Only (Ph4P)2[Cu4(Se4)2(Ses)] was prepared using the NH3 method. This was
achived by reacting CuCl2.2H20 and Na2Se4 in the presence of Ph4PBr in DMF at
162
ppm
Figure 5.17. The 77Se NMR of (Ph4Ph[Cu4(Se4h(Ses)] in DMF at 298K.
700C for 2hrs. Brown block crystals ( 46% yield) were formed by filtering the
reaction mixture and layering the filtrate with MeCN and storing overnight at room
temperature.
5.11.1 Crystal Structures.
Although crystallographically isomorphous (Figure 5.18)
(Ph4P)2[Cu4(Se4h(Ses)] and (Ph4P)2[Ag4(Se4h(Ses)] differ in structural detail.
Both compounds possess a tetrahedral array of metal atoms, with the donor
atoms of three polyselenide ligands bridging the edges of the M4 tetrahedron, such that
each metal atom has approximate trigonal planar coordination. The linkage pattern for
the polyselenide ligands is the same as that in [Cu4(Ss)3-n(S4)n]2- (n = 0-2). 12 M(1)
is coordinated by three different Sex2-ligands, while M(2)- M(4) are each chelated by
one Sex 2-Hgand and connected to another. This structure can be regarded as derived
from [M3(Sx)3]3- (Figure 5.19a) by folding up the terminal donor atoms E' to
coordinate another M atom.
Variability of Sex2- chain length and the MSex ring conformation are
significant characteristics of (Ph4PhfCu4(Se4)2(Ses)J and (Ph4P)2[Ag4(Se4)2(Ses)].
Figure 5.18 shows that in ·(Ph4P)2[Cu4(Se4)2(Se5)], rings A and B are Se42-, while
ring Cis either Ses2- or Se42- (41 %). In (Ph4P)2[Ag4(Se4)2(Ses)] ring A is the same
as in (Ph4P)2[Cu4(Se4)2(Ses)], ring B is either Se42- (91 %) or Ses2- (9%) and the
ring C position is occupied by Ses2- (78%, same conformation as (9)), or two
different Se42- conformations (14%, R%). In the ring C region there is an
approximate mirror plane through the AgSes chair, normal to Ag(l)-Ag(4) and
containing Ag(3), which relates these two minor Se42- conformations. The additional .....
rings and conformations are shown diagrammatically 1n Figure 5.19b.
All the MSe4 rings approximate the envelope (Cs) conformation: the torsional
angles around the rings average +6(3), -31 (6), +47(10), -43(5), +22(3r (or with
inverse signs), with the smallest torsional angle always at one M-Se bond.
163
Se3A
Figure 5.18. The molecular structure of [Cu4(Se4h(Ses)]2-.
Figure 5.19a. The structure of [M3(Sx))]3-.
Ag~g;_ Ag3 ;f~ \1. SelB
~· \ SeJJ Se2B'
\\ Se4B'
(a)
Se4C \~ Ao4 Se3C' Se5C---- .,
\ __k-Agl
SelC' ........... \ Ag3
Se3C.. Ag4 I --SeSC'---
'~ . YAgl ""'SelC- Ag3
(b)
Figure 5.19b. Diagrammatic representations of atom labels and approximate
conformations for (a) the Se5 and (b) the Se4 rings which occur in
(Ph4P)z[Ag4(Se4)z(Ses)], additional to those shown for (Ph4P)z[Cu4(Se4)z(Ses)] in
Figure 5.18.
Comparison of chelate rings A and B in Figure 5.17 emphasises two different possible
orientations of the MSe4 envelopes relative to the pseudo three-fold axis of the
molecular framework. In ring A the flap atom is Se(2A) near the top of the molecule
while in ring B the flap atom is Se(3B). The ring A and ring B orientations of the
MSe4 rings are analogous to the ob and lei conformational isomers of an
M(XCH2CH2 Y)3 octahedron.
The MSes rings have the chair conformation.
Mean bond lengths (excluding minor ring components) for
(Ph4P)2[Cu4(Se4)2(Ses)] and (Ph4P)2[Ag4(Sq)2(Ses)] respectively are: M-M,
2.77(6), 3.08(7)A, and M-Se, 2.37(3), 2.60(4)A; Se-Se, 2.34(5), 2.35(3)A.
5.11.2 Related molecules
A variety of copper and silver polysulfide compounds have been described in
the literature. These include the molecular complexes [Cu6(S4)3(Ss)]2-,12
[Cu3(Sx)3]3- (x = 49 and x = 610), [(S6)Cu(Sg)Cu(S6)]4-, 9 [(S6)Ag(Sg)Ag(S6)]4-,
66 [Ag2(S6)2]2- 67 the molecule [Ag(S9)]· 67, and the one-dimensionally non
molecular structure oo{[CuS4]·} 68.
The only other copper polyselenide reported so far is (Ph4P)4[Cu2(Se4)(Seshl
prepared by reacting CuCl with two equivalents of Li2Se6 in the presence of Ph4PBr
in DMF at 120oc.69
The chemistry of silver polyselenides has proven to be versatile. Huang and
Kanatzidis 70 have recently shown that several compositionally and structurally
different species are readily isolatable from a common reaction solution by using
different counterions as shown in the following equation.
AgN03 + Na2Ses + R4NCl (or Ph4PCI) ~ [(R4N)xAgy(Sen)z] or ... 5.4
[(Ph4P)xAgyCSen)z]) + NaCl + NaN03
164
The complexes (Ph4P)[Ag(Se4)]0~ (Et4N)[Ag(Se4)]4, (Ph4P)[Ag(Se4)]0 , have
been synthesised by this procedure and belong to the general family [Ag(Sex)]0°-. Both (Ph4P)[Ag(Se4)] 0 and (Ph4P)[Ag(Se4)] 0 .have polymeric structures while
(Et4N)[Ag(Se4)]4 is a molecular structure.
5.12 Discussion
5.12.1 Vibrational Spectra
IR and UV/vis spectroscopy provide liitle information in the characterisation of
heavy metal polychalcogenides. Unless CO groups are present in these molecules, the
mid-IR region is dominated the organic counterions. In the low frequency IR region
(200-340 cm-1 ), M-Se and Se-Se often show absorptions but without isotopic
labelling the assignment of these bands is virtually impossible.
The M(Se4)22- anions (M = Zn, Cd, Hg, Ni, Pb, Pd, Mn) show no characteristic
stretching vibrations in the 400-250 cnrl region of theIR spectrum and show no
characterisatic absorption in the UV-Vis region
5.12.2 NMR Spectroscopy
I have used 77Se NMR to characterise the majority of the complexes isolated.
The exceptions were the complexes (Ph4P)3[Co(Se4)6] and (Ph4Ph[Mn(Se4hl which
have inherent paramagnetic properties associated with Co(III) and Mn(II), and that of
(Ph4Ph[Pb(Se4h] which was not reisolated.
The complex anions [M(Se4h]2- (M = Zn, Cd, Hg, Ni, Pd) and [M(Se4h]2-
(M = Sn, Pt) studied consist of two (or three) MSe4 rings that generally have four (or
six) crystallographically independent Se atoms in the solid state. However for any
[M(Se4hJ2- anion in solution, only two resonances in the 77Se NMR spectrum are
observed, one arising from the Se atoms bound to the central metal atom ("metal
bound") and the other from the Se atoms in the ring, not metal bound or "ring" atoms.
165
This is due to interconversions among the various conformations in solution at room
temperature.
The assignment of the metal-bound and ring resonances depends upon
observation of satellites from the NMR active central metal or from expected analogies
within a periodic triad. The assumption made in the interpretation of the 77Se NMR
spectra is that lJ(M-Se) > 2J(M-Se) and it is the lJ(M-Se) being observed. Table 5.4
tabulates a comprehensive listing of the 77 Se NMR chemical shifts observed for
homoleptic metal polyselenide complexes studied here and by others.
Within the zinc triad, both Cd and Hg have the NMR-active nuclei (spin=l/2,
113Cd and 199Hg natural abundance = 12.75% and 16.84%). The observation of
satellites from these nuclei allows definite assignment of metal-bound resonances in
these M(II) (d10) species.
From table 5.4 it can be seen that for the complexes [M(Se4hJ2- (M = Cd, Hg,
Zn) all three workers have reported the predicted two 77Se resonances in solution for
each of the species. Ansari 25 and myself report the necessary coupling for
assignment of these resonances. The chemical shift difference of ca. 20 ppm,
observed between the results is due to concentration effects. The coupling constants
reported are virtually identical.
The metal bound Se atoms, for both [Cd(Se4hJ2- and [Hg(Se4hJ2- are the
resonances (38 and 54 ppm) found at lower chemical shift, as indicated by the
presence of satellites. Conversely ring Se atoms are found to resonate at higher
chemical shift ( 582 and 569 ppm) respectively. By analogy the two resonances for
[Zn(Se4hJ2- found at 523 and 101 ppm are assigned as "ring Se" and "metal-bound
Se".
166
Table 5.5 77Se NMR chemical shift and coupling constants
of metal polyselenides in DMF at 298K.
Compound Metal B 77Se (ppm) J (Hz)
d electron
configuration
[Zn(Se4)2]2- ctlO 573, 101
598, 127
607, 137
[Cd(Se4)2]2- ctiO. 582,38a Cd-Se, 260
608, 62a Cd-Se, 255
617, 75 Not reEorted
[Hg(Se4hJ2- ct10 569, 54a Hg-Se, 1260
594, 76a Hg-Se, 1265
604, 86 Not reEorted
[Hg2(Se4hJ2- ctlO 604, 86 Not reEorted
[Ni(Se4hJ2- ct8 815, 720
820, 748
829, 759
[Ni4Se4(Se3)s(Se4)]4- ct6 804, 789, 773,
749, 726, 680,
660, 470, 411,
401, 387, 361,
289, 134, 103
[Pd(Se4hJ2- ct8 869, 732
893, 758
b[Pt(Se4h]2- ct8 727a,642 Pt-Se 384
167
Reference
This work
25
29
This work
25
29
This work
25
29
29
This work
25
29
33
This work
25
25
168
[Pt(Se4h]2- d6 754, 644a Pt-Se 118 This work
d6 790, 680 Not reported 36
[Sn(Se4)3]2- d10 587,428a 816 This work
618,459 Not seen 29
[WO(Se4hJ2- d2 828a, 280 W-Se98 65
[WS(Se4h]2- d2 993a,313, W-Se 106 65
1787C
[WS(Se4h]2- d2 1034a, 324 W-Se 108 65
[MoO(Se4hJ2- d2 946, 380 65
[MoS(Se4h]2- d2 1122, 396 65
[MoSe(Se4hJ2- d2 1163,403, 65
2357C
d[CpMo(Se4h]- d2 1175, 560 This work
[Cu4(Se4h(Se5)]2- d10 623, 611, 308, This work
182
[Cu4(Se4)3]2- d10 662, 646, 344, 70
218
e[ Ag(Se4) ]4- d10 637, 609, 590, 70
282, 117, 61e
e[Ag4(Se4hJ2- d10 734, 663, 631, 70
179,5
[In3Se3(Se4)3]3- d10 643, 197, -244 27
a Metal coupling observed on this resonance. d Contains cyclopentadiene ligand
b Complex not isolated but made in situ. e Spectrum recorded at -550C
c Terminal Se atom.
Of all the other dlO metal polyselenides only [Sn(Se4)3]2- has an NMR active
nucleus, 119Sn (spin 1/2, 8.6% natural abundance). Huang et al did not observe any
119Sn-77Se coupling for [Sn(Se4)3]2- however I observed coupling of 816Hz on the
lower chemical shift resonance at 428 ppm. This was confirmed in the 119Sn
spectrum. Therefore this resonance at 428 ppm is assigned as being "metal bound Se"
while the high chemical shift resonance at 587 ppm is assigned as "ring" Se atoms.
Within the Ni triad only Pt has an NMR-active nuclei (spin= 1/2, 195Pt, natural
abundance= 33.8%). Attempts to prepare [Pt(Se4)2]2- were unsuccessful and yielded
[Pt(Se4)3]2-, a Pt(IV) complex. This complex showed two resonances at 754 and 644
ppm in DMF. The resonance at low chemical shift displayed doublet coupling of 118
Hz and consequently is assigned as metal bound. Ansari et al 25 did not report any
coupling for this species. However they propose that the reaction of [Pt(Se4)3]2- with
excess BH4- affords in situ the [Pt(Se4hcJ2- species as shown by its two line 77Se
NMR spectrum (8se 727 and 642). The resonance at 727 ppm showed coupling (384
Hz) to 195 Pt and is assigned as the metal bound resonance. From this evidence they
assign the high chemical shift resonances for both [Ni(Se4h]2- (815 ppm) and
[Pd(Se4hJ2- (869 ppm), as being metal bound. This interpretation seems generous
considering the complex [Pt(Se4h]2- has never been isolated even though Ansari et al
25, Huang et al29 and myself have failed in attempts to synthesise it. Only Krauter 39
maintains to have isolated the rPt(Se4h]2- ion based on less than perfect
microanalysis.
In the [MQ(Se4hJ2- series (M = Mo, W; Q = 0, S, Se)65 the metal bound
resonances forM= W show satellites arising from coupling to 183W (spin 1/2 natural
abundance = 14.3%), and for both Mo and W these resonances shift more with
variation of the terminal Q atom than do the ring resonances. For these W(IV) and
Mo(IV) (d2) systems the metal-bound resonances occur at high chemical shift.
In the Zn triad a reversal of the relative positions is observed for the metal-bound
and ring resonances compared to the Ni and Mo and W triad complexes. In the Zn
169
triad the metal bound resonance is observed at low chemical shift. A possible
explanation for the reversal of the relative positions can be made on the basis of
electron density of the Se42- ring and the central metal. I have shown (chapter 4) that
in the free Se42-Iigand that the terminal (a) Se atoms resonate at low chemical shift
(Ose 256 ppm) and the internal (~) Se atoms resonate at high chemical shift (ose 581
ppm) as predicted in accordance with the formal charge assignments of these atoms.
On chelating to the d10 metals Zn, Cd, or Hg, the d shell is full and cannot accept the
electron density from the Se42- anion. Consequently, each of these metal complexes
should display the metal bound resonance at a lower chemical shift, which is
observed. For the complexes I'M(Se4)2]2- (M = Zn, Cd, Hg) the metal bound
resonances are found at Osc 101, 3R, 54 ppm respectively, while the ring resonances
remain virtually the same as the(~) Se atoms (Ose 581 ppm) in the free Se42- at Ose
573, 582, and 569 ppm respectively.
The Sn(IV) d10 complex appears contradictory to the above argument with the
metal bound resonance ocurring at 42R ppm, downfield of the terminal (a) Se atoms
in Se42-. One interpretation of this is that the formal +4 charge of Sn has a greater
attractive capacity for electron density than does the +2 charge of Zn, Cd, and Hg
effectively forming a bond with increased covalency with the terminal Se atoms.
Therefore it is expected that the metal bound Se atoms should effectively lose their
charge and become more like the (~ ) atoms in the free Se42- which is shown.
In the Ni triad the metals are dx and somewhat poorer in electron density.
Electron donation from the Se42- ligands to the metal apparently is sufficiently great to
reverse the trend seen for the Zn triad so that the metal bound resonance observed for
[Pt(Se4)2]2- and presumably Ni and Pd, occur at higher chemical shift of the ring
resonances. The Mo and W anions have electron poor ct2 metals and these would of
course display the same trend as the Ni triad, which is the case.
The lowest unoccupied orbitals on Se are the 5s and 4d electron shells. These
are the same orbitals being filled in second-row transition metals. It has been
170
proposed65 that the orbital overlap of S~ with second-row transition metals is ~etter
than that with either first-(4s, 3d) or third-row (6s, 5d) metals. As long as there are d
electron vacancies, the Se atoms donate electron density to the metal and are
deshielded resulting in a higher chemical shift of the resonance. Therefore the greatest
downfield shift should occur for the second row metal in any given triad. This effect
is seen in the Ni triad for the species [M(Se4h]2- (M = Ni, Pd, Pt) where the metal
bound 77Se chemical shifts are 815, 869 and 727,65 respectively. When less donation
of electron density from selenium to the metal is possible, as in the dl0 of the Zn triad,
decreased shielding of the second-row metal does not occur. In fact, the Cd complex
with the the supposed best overlap is at a lower chemical shift than either Zn or Hg.
Another general trend can be seen within a row as a function of the d-electron
count. As the number of d electons on the central metal increases across a periodic
row there is a shift towards lower chemical shift of the metal bound Se resonance e.g
MoSe(Se4h2- (1163 ppm, d2), Pd(Se4h2- (732 ppm, d8), and Cd(Se4h2- (38 ppm,
dlO).
Other interesting results presented in table 5.5 are the measured identical
chemical shifts for [Hg(Se4h]2- and [Hg2(Se4)3]2-, the fifteen Se resonances
observed for [Ni4Se4(Se3)s(Se4) ]4- and the five Se resonances reported for
[Ag4(Se4)3]2-. The measured identical 77Se chemical shifts for [Hg(Se4h]2- and
[Hg2(Se4)3]2- at 604 and 86 ppm respectively, indicate that solutions of
[Hg2(Se4h]2- dissociates to give fHg(Se4)2]2- and presumably other unidentified
species.29
The low spin Ni(IV) d6 octahedral diamagnetic complex [Ni4Se4(Se3)s(Se4)]4-,
is unique. It is expected that at room temperature the selenium rings in this complex
would be fluxional in the NMR time scale. Therefore on the basis of the crystal
structure a nine line spectrum would be expected and not the fifteen lines observed.
McConnachie et al 33 has suggested that the presence of fifteen lines could imply some
171
exchange among the rings, but this has not been observed in any of the other systems.
The presence of more than one species in solution seems the most likely explanation.
According to the structure of [Ag4(Se4)3]2- only four resonances would be
predicted in the 77Se NMR spectum and not the observed five. No explanation has
been given for this anomalous behavior.70 At room temperature or below (220K) no
well-defined signals were observed by me in the 77Se NMR spectrum of
[Ag4(Se4)z(Ses)]2- solutions. However the [Cu4(Se4)z(Ses)]2- anion, shows four
sharp peaks at 631, 615, 312 and 187 ppm respectively, thus suggesting a labile
nature for the Ag complex whilst the Cu complex is apparently more stable. The four
resonsances observed for [Cu4(Se4)2(Ses)]2- is consistent with the five inequivalent
environments expected to exist from the crystal structure. The fifth selenium atom in
theSes ring has only a 9% probability to exist at any given time. For this reason it is
not anticipated that the resonance would be observed.
5.12.3 Redox Chemistry in Metal Polyselenide Solutions.
It is now generally recognised that polyselenide ligands are more variable than
their sulfur counterparts towards the different oxidation states of metal centres to
which they are bound. In systems such as Fen+j Sex2- (n = 2, 3), Aun+JSex2- (n = 1,
3), Snn+j Sex2- (n = 2, 4), TJn+j Se/- (n = 1, 3), and Nin+j Sex2- (n = 2, 4), the metal
in either oxidation state can readily fom1 a stable compound with the polyselenide
ligands regardless of the value of x in Sex2-, while polysulfides often select one
oxidation state of the metal to form a complex. For example, the Ni(IV) complex
(Et4N)4[Ni4Se4(Se3)s(Se4)].xEt4NCI can be obtained by a spontaneous assembly
reaction involving a redox process between Ni(II) and Sex2-_33,72 The Ni(II) centre
in the nickel xanthate precursor undergo a two electron oxidation to Ni(IV) with
concomitant reduction of the hexaselenide. In this particular reaction the NEt4+ ion
appears to be a necessary part of the reaction mixture as the same reaction between
Ni(xanh and Li2Sex in the presence of PEtPh3+ affords [PEtPh3h[Ni(Se4)2]25.
172
Product dependency on countercation size has been observed extensively in the
chemistry of transition metal polyselenides and is discussed later. The product is not
dependent on the presence of xanthate because the complex forms when either
Ni(acach or Ni(OAch is used as a starting material and forms with variable x in
Sex2-,33, 72
The redox chemistry between Aun+ (n = 1, 3) and Sex2- is intriguing. Both
Kanatzidis and Huang73 and myself (chapter 4), have shown the reaction of Au3+
with Na2Se5 in DMF generates the oxidised species [Se(Sesh]2-. In order to avoid
the above redox process and so as to isolate the gold-containing polyselenide
complex, Kanatzidis and Huang73 used AuCN as the starting material. Surprisingly,
a reverse redox reaction reaction ocurrecl where Au+ was oxidised by the Se52- to
Au3+, yielding the dimeric complex IAu2Se2CSe4hJ2-. Furthermore when shorter
polyselenide Sex2- ligands were used three other compounds containing Au+ were
obtained.74 The complexes (Ph4Phf Au2CSe2)(Se3)] and (PPN)2[Au2(Se2)(Se3)]
were synthesised in. a similar manner to I Au2Se2(Se4h]2- but using Na2Se2 or
Na2Se3 while (Ph4Ph[Au2(Se2)(Se4) I was prepared by using Na2Se4.
The isolation of the above three Au complexes suggests the formation of Au+
over Au3+ is determined by the size of the Sex2- ligands. This is an exception to what
is generally observed where the ligand preference of the metal ions are more important
in determining structure than the sizes of the Sex2-ligands used.
Other sytems showing interesting redox chemistry include the formation of
thallium (Ill) complex, [Tl3Se3(Se.:1 l3l3- hy oxidation of TlCl by Na2Se5 27 and as
shown in this work the fom1ation of the Cu(l) species [Cu4(Se4)2(Ses)f- by reducing
Cu(II) with Na2Se4 or the synthesis of the Sn (IV) complex [Sn(Se4))]2- using either
Sn(ll) or Sn(IV) with either Na2Se4 used in this work or Na2Se5 used by Huang et
aJ29 and finally, the preparation in this \vork of the Co(III) complex [Co3(Se4)6]3- by
the oxidation of Co (II) with Na2Se4.
173
It is clear from these results that there is very little predictability concerning the
final products of these polyselenide reactions and highlights the importance of not
only the single crystal structure determination of the isolated product but also
monitoring the reaction mixture by NMR to observe what is going op. in solution.
5.12.4 Metal Polyselenide Ring Sizes
Unlike metal polysulfide chemistry where variability of the MSx (x = 1-9) ring
is common the predominant metal polyselenide ring in the compounds known to exist
is MSe4. Only four compounds are known to exist that contain Ses2- ligands. These
are (115- CsHshTi(Ses)5 7, Fe2Se2(Sesh2- 1, V 2(Se2)4(Ses)2- 2, and
In2(Se4)4(Se5)4-. 27 Only one complex has been reported with a Se62- ligand,
Zn(Se4)(Se6)2-.30
With the possible exception of the Au+ I Au3+ sytem described above, generally
the metal ions are more important in determining structure of a complex than the sizes
of the S~2- ligands used. That is to say, the metal ion selects what S~2- ring it wants
from solution. This has been clearly demonstrated by my experiments on the
reactions between Cd2+, Hg2+ and Sn2+ and Sex2-(x = 3-6). In this series of
experiments no matter what Sei- was used above the given ratio of 1 metal : 1.7
ligands, the same product was observed in all cases i.e [Cd(Se4hJ2-, [Hg(Se4h]2-
or [Sn(Se4)3]2-. Ratios less than which resulted in precipitation of intractable solids.
Despite using starting solutions that were known to be free of Se42-, as shown by the
77Se NMR, invariably Se42- ligands were formed. In these cases the Cd2+, Hg2+
and Sn2+ preferentially select Se42- after the labile polyselenide chains have rearranged
in solution. The propensity for the Se42- ligand of the metal systems is surprising
considering the diverse nature of the analogous polysulfides. The reasons for the
preference for Se42 chelates are not obvious although it is apparent that the five
membered MSe4 ring is very stable bonding to metals in various ways e.g half- chair,
envelope and unsymmetrically bridged as shown below. Unfortunately no ab initio
174
molecular orbital calculations have been done on these systems which would assist in
explaining this observation.
/Se-~e ·--~-----------
' I Se-Se
half- chair
5.12.5 Effect of Counterion.
se~'se/ / ,'
M ,/ Se ,;~s/ _, e
envelope
, , , Se-Se I I Se Se-M '\ I
M
unsymmetrically
bridged
The other major factor in determining the final product from these metal
polyselenide systems is the use of different types of counterion. As already shown in
equation 5.4 Huang and Kanatzidis 70 have found that in the Ag+ I Sex2- system
several compositionally and structurally different species are readily isolatable from
the same reaction mixture by using different counterions. Here as discussed previosly,
four different Ag species, [(Ph4P)Ag(Se4)]n, [(Et4N)Ag(Se4)]4, [(Me4N)Ag(Ses)]0
and [(Pr4N)Ag4(Se4)3] were isolated from essentially the same reaction mixture, i.e
AgN03 + Na2Ses in DMF, simply by varying the counterion. I had earlier isolated
(Ph4Ph[Ag4(Se4h(Ses)J under very similar conditions.19
The fact that the same anion (Pd(Se4)212- can be either a discete molecule, when
crystallised with Ph4P+ or polymeric 35 when crystallised in K2[Pd(Se4)(Se6)] with
K+ illustrates the importance of the counterion in determining the structures of many
polselenide complexes. However this fact itself does not explain why the K+ salt of
the same compostion is polymeric. This has been explained35 by the "ion screening
effect" in the crystal lattice of the two molecules. As can be seen in Figure 5.20,
when the large P~P+ cation is changed to the drastically smaller K+, the [Pd(Se4h]2-
anions can no longer be effectively screened, resulting in destabilising repulsions.
The system responds to such a change converting the Se42- ligands from chelating
175
8
8 (A)
(B)
(C)
Figure 5.20. (A) Stable assembly of mutually screened Ph4P+ cations and
[Pd(Se4h]2- anions. (B) Substitution of large Ph4p+ forK+ results in short anion-
anion contacts developing destabilising repulsive interactions. The latter are
represented by dotted lines. (C) A stable assembly is possible by converting chelated
Sex2-Hgands to bridging.35
one palladium centre to bridging two neighbouring Pd atoms, leading to elimination in . Coulombic repulsions and a decrease in lattice energy.
However in many cases variation of the cation makes no difference to the final
product as i~ shown by the access to the [M(Se4)z]2- (M = Cd, Hg, Zn) anions which
have been crystallised with various counterions by several research groups. The
counterions used have been Ph4P+ (this work),25, [(15-Crown-5)Na]+ 24, [Li3(12-
crown-4)(02CCH3)]+ 24 and [K(18-crown-6)]+ 24.
5.13 Se-Se bond distances in the MSe4 containing compounds.
Table 5.6 presents some Se-Se bond distances found in illustrative compounds
studied here and by others.
Table 5.6 Comparison of the Se-Se bond distances in MSe4
containing compounds
0 0
Compound d 1 and d2 , A d 3 , A Reference
[Cd(Se4)z]2- 2.342(4), 2.325(4) 2.340(4) This work
2.340(4), 2.319(4) 2.323(4)
2.332(4), 2.332(4) 2.340(8) 23
2.344(4), 2.344(4) 2.321(6)
[Hg(Se4)z]2- 2.296(3), 2.310(3) 2.347(4) This work
2.321(4), 2.295(4) 2.327(4)
176
177
2.313(4), 2.313(4) 2.341(5) 23
2.331(3), 2.331(3) 2.324(4)
[Zn(Se4h]2- 2.323(3), 2.323(3) 2.334(5) 23
2.338(3), 2.338(3) 2.321(4)
[Ni(Se4h]2- 2.367(2), 2.377(2) 2.329(3) This work
2.367(2), 2.350(2) 2.332(3)
2.344(1), 2.398(1) 2.321(1) 25
[Pb(Se4h]2- 2.325(2), 2.310(2) 2.347(2) This work
2.362(2), 2.312(2) 2.325(2)
[Pd(Se4h]2- 2.340(2), 2.352(2) 2.330(3) 25
2.353(3), 2.362(2) 2.342(2)
[Mn(Se4h]2- A: 2.325(5), 2.328(5), A: 2.339(5) 75
molecules A and B 2.318(6), 2.333(5) 2.346(5)
B: 2.336(5), 2.338(5) B: 2.340(5)
2.329(6), 2.365(8) 2.396(7)
[Pt(Se4)3]2- 2.330(5), 2.337(5) 2.342(2) 36
2.342(5), 2.343(5) 2.342(5)
2.338(5), 2.342(5) 2:315(5)
2.324(2), 2.337(4) 2.305(6) This work
2.257(6), 2.273(6) 2.243(6)
2.384(6), 2.363(6) 2.331(7)
[Sn(Se4)3]2- 2.327(4), 2.341 (4) 2.302(4) This work
2.320(4), 2.337(4) 2.328(4)
2.308(4), 2.319(4) 2.332(4)
[MoO(Se4h]2- 2.390(1), 2.446(2) 2.303(2) 65
2.399(2), 2.425(1) 2.304(2)
[MoSe(Se4)z]2- 2.384(5), 2.461 (5) 2.291(5) 65
2.395(5), 2.446(5) 2.307(5)
[CpMo(Se4)z]- 2.381(2), 2.399(1) 2.310(1) This work
2.365(1), 2.419(1) 2.343(1)
[Cu4(Se4)z(Se5)]2- 2.380(2), 2.389(3) 2.301(3) This work
2.374(2), 2.375(3) 2.324(3)
[A~(Se4)z(Se5)]2- 2.307(2), 2.359(2) 2.310(2) This work
2.373(2), 2.373(2) 2.342(3)
From Table 5.5 it can be calculated that the Se-Se bond distances in the above
polyselenide structures alternate within each MSe4 ring. Typically, the average
internal Se-Se bond is 2.327 ± 0.022 A. This average bond length is not statistically
shorter than the average external bond of 2.344 ± 0.032 A. This is very common for
five membered metal polychalcogenide rings although some are more pronounced.
For example, the complexes of d2 and d8 metals show a large Se-Se bond alternation
whereas dlO metal complexes do not. The average internal Se-Se bond for the d8
metal complexes is 2.330 ± 0.007 A, whereas the the average external Se-Se bond is
significantly longer at 2.361 ± 0.016 A. Similarly, in the d2 systems the average
internal Se-Se bond for the d2 metal complexes is 2.309 ± 0.016 A, whereas the the
average external Se-Se bond is significantly longer at 2.409 ± 0.029 A. By
comparison, in the dlO systems the average internal Se-Se bond is 2.325 ± 0.014 A
and the average external Se-Se bond is similar at 2.329 ± 0.012 A
178
5.14 ·Experimental Detail
5.14.1.A Preparation of (Ph4P)2[Zn(Se4)2] - DMF Meth_od.
A mixture of black Se (2.3 g, 29 mmol), Na (0.4 g, 15 mmol), ZnCl2 (0.25 g,
1.8 mmol) and DMF (30 mL) was stirred for 10 hrs at 60°C. The resulting dark
brown solution was cooled to ambient temperature then filtered removing NaCl, and
onto the filtrate was layered a solution of Ph4PBr (1.6 g, 3.8 mmol) in MeCN (30
mL). Brown needles collected after storing overnight at ambient temperature. The
crystals were washed with MeCN and dried in vacuo (1.1 g, 44% yield based on Zn).
The crystal composition was confirn1ed by X-ray diffraction.
Anal. calculated for C4s~oZnP2Seg: C; 41.90; H, 2.93; P, 4.50; Zn, 4.75;
Found: C, 39.69; H, 2.49; P, 4.41; Zn, 4.64;
5.14.l.B Preparation of (Ph4P)2[Zn(Se4)2]- NH3 Method
Into a 2-necked flask was added black Se (2.0 g, 25 mmol). The flask was
fitted with a NH3 condenser and using an acetone/C02 coolant 30-40 mL of NH3 was
condensed into the flask. A continuous N2 flow kept an inert atmosphere. Na (0.29
g, 12.6 mmol) was then added and the mixture was stirred generating a dark solution.
The NH3 was then distilled out and the resulting dark red/brown solid was dried in
vacuo at 50°C, DMF (40 mL) was added resulting in a dark brown solution after
stirring at R.T. for 15 min. ZnBr2 (0.72 g, 3.2 mmol) was added and the resulting
mixture was stirred for 2 hrs at 70°C generating a dark brown solution. Ph4PBr (2.7
g, 6.4 mmol) was added and stirring was continued for a further 30 minutes without
any observable colour change occurring. The mixture was cooled to ambient
temperature then filtered to remove NaCl, and to the filtrate was added MeCN (40
mL). Dark brown needles were collected after allowing the mixture to stand overnight
179
at ambient temperature. The crystals were then washed with MeCN and dried in
vacuo (2.3 g, 53%). The purity was confirmed by 77Se NMR.
5.14.2.A Preparation of (Ph4P)2[Cd(Se4)2] - DMF Method
A mixture of black Se (2.3 g, 29 mmol), Na (0.27 g, 12 mmol),
CdCl2.2.SH20 (0.44 g, 1.9 mmol) and DMF (30 mL) was stirred for 10 hrs at 7o·c
for 14 hrs. The resulting dark brown solution was cooled to ambient temperature then
filtered removing NaCl. Onto the filtrate was layered a solution of Ph4PBr (1.6 g, 3.8
mmol) in MeCN (30 mL). Brown needles were collected after storing the resulting
mixture overnight at ambient temperature, washed with MeCN and dried in vacuo
(0.81 g, 29% yield based on Cd). Crystal composition was confrrmed by X-ray
diffraction.
Anal calculated for C4sB4oCdP2Seg: C; 40.52; H, 2.83;
Found: C, 39.60 H, 2.89
5.14.2.B Preparation of (Ph4P)2[Cd(Se4)2] - NH3 Method
Black Se (2.0 g, 25.3 mmol), and Na (0.29 g, 12.6 mmol), were reacted in
NH3 (40 mL) generating a solid of nominal composiiton of Na2Se4 after removal of
NH3. The mixture was then stirred at room temperature for 15 min. CdC12.2.5 H20
(0.72 g, 3.2 mmol) was added and stirred for 2 hrs at 7o·c resulting in a dark brown
solution Pi14PBr (2.7 g, 6.4 mmol) was added and the reaction mixutre was stirred for
a further 30 minutes. The mixture was then filtered removing NaCl and to the filtrate
was added MeCN (40 mL). Brown needles were collected after storing overnight at
ambient temperature. The crystals were washed with MeCN and dried in vacuo (2.6
g, 58% yield based on Cd). Purity was confirmed by 77Se and 113Cd NMR.
180
5.14.3.A Preparation of (Ph4P)2[Hg(Se4)2] - DMF Method
A mixture of black Se (2.3 g, 29 mmol), Na (2.3. g, 29 mmol), Na (0.3 g, 14
mmol) HgCl2 (0.5 g, 1.8 mmol) and DMF (30 mL) was stirred for 10 hrs at 60oC for
10 hrs. The resulting dark brown solution was cooled to ambient temperature and
filtered removing NaCI. Onto the filtrate was layered a solution of Ph4PBr (1.6 g, 3.8
mmol) in MeCN (30 mL). Brown needles were collected after storing overnight at
ambient temperature. The needles were the washed with MeCN and dried in vacuo
(1.2 g, 43.4% yield based on Hg). Crystal composition was confirmed by X-ray
diffraction.
Purity confirmed by 77Se and 199Hg NMR.
5.14.3.B Preparation of (Ph4P)2[Hg(Se4)2} - NH3 Method
Black Se (2.0 g, 25.3 mmol), and Na (0.29. g, 12.6 mmol), were reacted in
NH3 ( 40 mL) generating a solid of nominal composition Na2Se4. The NH3 was
distilled off and DMF (40 mL) was added. The mixture was stirred at room
temperature for 15 min then HgCl2 (0.86 g, 3.2 mol) was added and the reaction was
stirred for 2 hrs at 70"C resulting in a dark brown solution. Ph4PBr (2.7 g, 6.4
mmol) was added and the reaction mixture was stirred for a further 30 minutes. The
mixture was cooled to ambient temperature then filtered removing NaCl and to the
filtrate was added MeCN (40 mL). Black/brown needles were collected after storing
overnight at ambient temperature. The isolated crystals were washed with MeCN and
dried in vacuo (2.88 g, 5.96% yield based on Hg). Purity was confirmed by 77Se
and 199Hg NMR.
5.14.4.A Preparation of (Ph4P)2[Ni(Se4)2} - DMF Method
A mixture of black Se (2.3 g, 29 mmol), Na (0.34 g, 14.8 mmol),
NiCl2.6H20 (0.9 g, 3.8 mmol) and DMF (30 mL) were stirred at 60oC for 13 hrs.
The resulting dark brown solution was cooled to ambient temperature then filtered to
181
remove precipitated NaCl. Onto the dark brown filtrate was layered a solution of
Ph4PBr (1.6 g, 3.8 mmol) in MeCN (30 mL). Black needles were collected after
storing the resulting mixture overnight at ambient temperature. The resulting needles
were washed with MeCN and dried in vacuo (0.9 g, 17% yield based on Ni). Crystal
composition was confirmed by X-ray diffraction.
Anal calculated for C4sl4oNi P2Seg: C; 42.09; H, 2.92;
Found: C, 40.18; H, 2.82
5.14.4.B. Preparation of (Ph4P)2[Ni(Se4)2] - NH3 Method
Black Se (1.0 g, 12.7 mmol), and Na (0.15 g, 6.5 mmol) were reacted in NH3
(30 mL) generating a solution of nominal composition Na2Se4. The NH3 was
distilled off and DMF (40 mL) was added. The mixture was stirred at room
temperature for 15 min then NiCl2.6H20 (0.37 g, 1.6 mmol) was added and the
reaction was stirred for 2 hrs at 70°C resulting in a dark brown solution. Ph4PBr
(1.34 g, 3.2 mmol) was added and stirring was continued for a further 30 minutes.
The mixture was cooled to ambient temperature then filtered removing NaCl. Onto the
filtrate was added MeCN(40 mL). Black needles were collected after storing reaction
overnight at ambient temperature, washed with MeCN and dried in vacuo (1.3 g, 60%
yield based on Ni). Purity was confim1ed by 77Se NMR.
5.14.5 Preparation of (Ph4Ph[Pb(Se4)2]
A mixture of black Se (2.3 g, 29 mmol), Na (0.68 g, 29 mmol), PbCI2 (1.1 g,
3.8 mmol) and 30 mL DMF was stiiTed at 60°C for 14 hrs. (Note: double the amount
ofNa was required to completely reduce the selenium in this reaction compared to the
previous syntheses). The resulting dark green/brown solution was cooled to ambient
temperature then filtered removing NaCI. As in the previous syntheses a layered a
solution of Ph4PBr (1.6 g, 3.8 mmol) in MeCN (30 mL) was added to crystallise the
final product. However this procedure did not result in crystallisation. Various
182
temperatures were applied from -7 8 ° c to 1 oo· c without resulting in crystallisation.
Eventually aU-tube apparatus was designed (as described in Chapter 2 ), where the
filtered reaction mixture was allowed to diffuse through a DMF medium and react
slowly with the counter-cation layer. The U-tube was placed in a bath at 9o·c and
allowed to cool to ambient temperature overnight. The brown/black block crystals
were isolated and washed with MeCN then dried in vacuo. (1.52 g, 26.4% based on
Ph). The crystals were easily the most air- sensitive of the metal polyselenides made
in this work, oxidising within lhr of exposure to the atmosphere. Crystal
composition was confirmed by X-ray diffraction.
Anal calculated for C4sli4oPbP2Ses: C,37 .95; H, 2.92;.
Found: C, 38.34; H, 2.81
Many attempts have been made to reisolate (Ph4P)2[Pb(Se4)2] unfortunately no
subsequent preparation has achieved this goal. The reasons for this are not obvious as
the subsequent preparations were performed under identical conditions applied in the
successful preparation.
Variations of the successful preparations using other lead precursors were then
tried. These preparations are reported below.
Using the procedure described above five other metal precursors, Pb(N03)z
(1.3 g, 3.8 mmol), PbBr (1.4 g, 3.8 mmol), Pbi2 (1.8 g, 3.8 mmol), Pb(CzH302h
(1.2 g, 3.8 mmol) and PbSe (1.1 g, 3.8 mmol) were used under the same conditions
but all failed to generate (Ph4P)2[Pb(Se4hl- In all cases but PbSe, reaction mixtures
were shown by 77Se NMR to contain fSe(Se5)z]2-. In the case of PbSe no
resonances whatsoever were observed. Consequent crystallisation experiments with
Ph4PBr in MeCN, of the reactions containing Pb(N03)z, PbBrz, Phiz, Pb(CzH30z)z
resulted in the precipitation of only (Ph4P)z[Se(Ses)z]. The PbSe reaction mixture
resulted in the crystallisation of (Ph4P)zSe5.
Alternoute routes using the NH3 method were tried. Black Se (2.0 g, 25.3
mmol), and Na (0.29 g, 12.6 mmol), were reacted in NH3 (40 mL) generating a solid
183
of nominal composiiton of Na2Se4 after removal of NH3. The mixture was then
stirred at room temperature for 15 min. PbCl2 (0.9 g, 3.2 mmol) was added and
stirred for 2 hrs at 70"C resulting in a dark brown solution. Ph4PBr (2.7 g, 6.4
mmol) was added and the reaction mixture was stirred for a further 30 minutes. The
mixture was cooled to ambient temperature then filtered removing NaCl and to the
filtrate was added MeCN ( 40 mL). Brown needles were collected after storing
overnight at ambient temperature. The crystals were washed with MeCN and dried in
vacuo (1.8 g). The 77Se NMR of the redissolved product showed the broad hump at -
300K characteristic of (Ph4PhSe(Se5)2 at 720 ppm. This was confirmed by the
observation of four resonances at 200K at Bse 795, 708, 672, and 484 ppm.
Using a similar method to the NH3 procedure described above, black Se
(l.Og, 12.6 mmol), and Na (0.29 g, 12.6 mmol), were reacted in NH3 (40 mL)
generating a solid of nominal composiiton of 'Na2Se2' after removal of NH3 (Note
this is not a homogenous product with both white and dark brown solids present after
distillation of the NH3. The mixture was then stirred at room temperature for 15 min
and PbCl2 (0.9 g, 3.2 mmol) was added and stirred for 2 hrs at 70"C resulting in a
dark brown precipitate in a colourless solution. The brown solid was filtered, washed
with MeCN and dried in vacuo. The powder pattern showed this product to be
amorphous. No further attempt at characterisation was made.
5.14.6 Preparation of (Ph4P)2[Ag4)(Se4)2.1(Ses)o.91
A mixture of black Se (0.77 g, 9.7 mmol), Na (0.11 g, 4.9 mmol), AgN03
(0.22 g, 1.3 mmol) and 30 mL DMF was stirred at 60"C for 12 hrs. The resulting
dark brown solution was cooled to ambient temperature then filtered to remove
NaN03. The filtrate was transferred to aU-tube apparatus (Chapter 3, Figure 3.3b)
and layered initially with DMF and then a solution of Ph4PBr (1.6 g, 3.8 mmol) in
MeCN (30 mL). The mixture was then refrigerated at ca o·c overnight to allow slow
diffusion. Good quality brown block needles were formed in the DMF interface.
184
These crystals were were collected and washed with MeCN then dried in vacuo (0.85
g, 30.7% yield based on Ag). Crystal composition was confirmed by X-ray
diffraction.
Anal calculated for C4sl4oAMP2Se12.9 (2128.9): C, 27.05; H, 1.88
Found: C, 26.38; H, 1.86
5.14.7.A Preparation of (Ph4P)2[Cu4(Se4)2.4(Ses)o.6l - DMF Method
A mixture of black Se (2.4 g, 30.0 mmol), Na (0.24 g, 10.4 mmol),
CuCl2.2H20 (0.33 g 1.94 mmol) and 30 mL DMF was stirred at 6o·c for 12 hrs.
The resulting dark brown solution was cooled to ambient temperature then filtered to
remove NaCl. Onto the filtrate was layered a solution of Pl4PBr (1.6 g, 3.8 mmol) in
MeCN (30 mL). Refrigeration at ca o·c overnight resulted in brown/black block
crystals. These were collected, washed with MeCN and dried in vacuo. (0.95 g,
28% based on Cu). Crystal composition was confirmed by X-ray diffreaction.
Anal calculated for C4sf4oP2Se12.6 (1927.9): C, 29.88; H, 2.07
Found: C, 28.71; H, 1.95
5.14.7.B Preparation of (Ph4P)2[Cu4(Se4)2,4(SeS)0.6] - NH3 Method
Black Se (1.0 g, 12.7 mmol), Na (0.15 g, 6.5 mmol), were reacted in NH3
(30 mL) generating a solid of nominal composiiton of Na2Se4. The NH3 was distilled
off and to the dry solid was added DMF (40 mL). The mixture was then stirred for 15
min at ambient temperataure. CuCl2. 2H20 (0.27 g, 1.6 mmol) was then added and
the mixture was stirred for 2 hrs at 1o·c resulting in a dark brown solution. Pl4PBr
(2.7 g, 6.4 mmol) was added and the reaction mixutre was stirred for a further 30
minutes. The mixture was cooled to ambient temperature and filtered. To the filtrate
was added MeCN (40 mL). Brown block crystals were collected after storing the
mixture overnight at ambient temperature. These crystals were washed with MeCN
and dried in vacuo (1.4 g, 46% yield based on Cu). Purity confirmed by 77Se NMR.
185
5.14.8 Preparation of (Ph4P)J[Co3(Se4)6}.(DMF)2
A mixture of black Se (1.8 g, 23 mmol), Na (0.27 g, 12 mmol) CoClz.6HzO
(0.46 g, 1.9 mmol) and 30 mL DMF was stirred at 6o·c for 11 hrs resulting in a dark
green mixture. The mixture was cooled to ambient temperature and filtered removing
precipitated NaCl. Onto the filtrate was added a layer of Ph4PBr (1.6 g, 3.8 mmol) in
MeCN (30 mL). The resulting mixture was stored overnight at ambient temperature
and resulted in dark green/black needles precipitating. These crystals were collected,
washed with MeCN and dried in vacuo. (1.9 g, 31.1% based on Co). Crystal
composition was confirmed by X-ray diffraction.
Anal Calcd for C7sH74Co3N202 P3Se24 (3236.2), C, 28.92; H, 2.29; N, 0.87
Found: C, 28.97; H, 2.03; N, 0.85
5.14.9- Preparation of (Ph4P)[CpMo(Se4)2]
A mixture of black Se (2.3 g, 29 mmol), Na (0.34 g, 14.8 mmol),
Cp2Mo2(C0)6 (1.86 g, 3.8 mmol) and DMF (30 mL) were stirred for 12 hrs at 6o·c.
The resulting dark green mixture was cooled to ambient temperature and filtered. Onto
the filtrate a solution containing Ph4PBr (1.6 g, 3.8 mmol) in MeCN (30 mL) was
slowly layered onto the surface. The mixture was kept at o·c overnight resulting in
green needles. The green needles were collected, washed with MeCN and dried in
vacuo (1.56 g, 36% based on Mo). Crystal composition was confirmed by X-ray
diffraction.
Anal calculated for C29H2sMoPSes (1132.12): C; 30.74; H, 2.21; Mo, 8.47; P,
2.74; Found: C, 28.37; H, 2.33; Mo, 8.12; P, 2.71
5.14.10 Preparation of (Ph4P)2[Mn(Se4)2} • NH3 Method
Black Se (1.0 g, 12.7 mmol) and .Na (0.15 g, 6.5 mmol) were reacted in 30
mL NH3 generating a solid nominal composition NazSe4. The NH3 was distilled off
and to the dry solid was added DMF (40 mL). The mixture was stirred for 15 min at
186
ambient temperature to dissolve the Na2Se4. MnCl2AH20 (0.31 g, 1.6 mmol) was
then added as a solid stirred at 70"C for 2 hrs resulting in a dark green I brown
mixture. Ph4PBr (1.34 g, 3.2 mmol) was added and the reaction mixture was stirred
for a further 30 minutes at 700C. The mixture was then cooled to ambient temperature
and filtered. To the filtrate was added MeCN'(40 rnL). Brown needles were collected
after standing the mixture overnight at ambient temperature, washed with MeCN and
dried in vacuo (1.3 g, 62% based on Mn). Powder diffraction pattern consistent with
that for (Ph4P)2[Mn(Se4)2] (Appendix B).
Anal. calc. for C4sH4oMnP2Ses: C, 42.24; H, 2.95; Mn, 4.02; P, 4.54; Found: C,
42.16; H, 2.87; Mn, 3.90; P, 4.44;
5.14.11 Preparation of (Ph4P)2[Pd(Se4)2l
Black Se (2.0 g, 25.3 mmol) and Na (0.29 g, 12.6 mmol) were reacted in 40
rnL NH3 generating a solid nominal composition Na2Se4. The NH3 was distilled off
and to the dry solid was added DMF (40 rnL). In order to dissolve the tetraselenide
the mixture was stirred for 20 min at ambient temperature. To the resulting brown
polyselenide solution was added Na2PdCl4.(0A6 g, 1.6 mmol) and the mixture was
stirred at 70"C for 2 hrs resulting in a dark green I brown mixture. Ph4PBr (2.7 g,
6.4 mmol) was added and the resulting mixture was stirred for a further 30 minutes at
70°C. The dark green I brown mixture was then cooled to ambient temperature and
filtered. To the filtrate was added MeCN (40 mL). Black needles crystallised
immediately. These were collected, washed with MeCN and dried in vacuo (1.20 g,
54% based on Pd). Crystal composition confirmed by X-ray diffraction. Purity was
confirmed by 77Se NMR.
Anal. Calcd for C4sB4oP2PdSes: C, 40.65; H, 2.85
Found: C, 40.58; H, 2.74
187
5.14.12 Preparation of (Ph4P)2[Sn(Se4)3}
Black Se (2.0 g, 25.3 mmol) and Na (0.29 g, 12.6 mmol) were reacted in 40
mL NH3 generating a solid nominal composition Na2Se4. The NH3 was distilled off
and to the dry solid was added DMF (40 mL). The mixture was stirred for 15- 20
min at ambient temperature to dissolve the polyselenide. SnClz.H20 (2.7 g, 6.4
mmol) was then added and the resulting mixture stirred at 7o·c for 2 hrs generating a
dark brown solution. Ph4PBr (2.7 g, 6.4 mmol) was added and the reaction mixture
was stirred for a further 30 minutes. The mixture was then cooled to. ambient
temperature and filtered and to the filtrate was added MeCN (40 mL). Black needles
were collected after standing the reaction mixture overnight at ambient temperature,
washed with MeCN and dried in vacuo (2.2 g, 39 % based on Sn). Crystal
composition was confirmed by X-ray diffraction.
Anal. Calcd for C4sH4oSnPzSe12: C, 33.02; H, 2.29
Found: C, 32.94; H, 2.25
5.14.13 Preparation of (Ph4P)2[Pt(Se4)3}
NazSe4 was prepared by reacting black Se (2.0 g, 25.3 mmol) and Na (0.29
g, 12.6 mmol) were reacted in 40 mL NH3. The NH3 was distilled off and to the dry
solid was added DMF (40 mL). The mixture was stirred for 15 min at ambient
temperature to dissolve the Na2Se4. (Et3N)2Pt2Br6 (0.24 g, 0.23 mmol) was added
and the resulting mixture stirred at 7o·c for 2 hrs resulting in a dark brown solution.
Ph4PBr (3.9 g, 9.3 mmol) was added and the reaction mixture was stirred for a
further 30 minutes. The mixture was then cooled to ambient temperature and filtered
and to the filtrate was added MeCN (40 mL). Black needles were collected after
storing the reaction mixture overnight at ooc. The crystals were collected washed
with MeCN and dried in vacuo (1.3 g, 60% based on Pt). Crystal composition was
confirmed by X-ray diffraction.
Anal. Calcd for C4sH4oPtP2Se12: C, 31.64; H, 2.20 Found: C, 31.56 H, 2.16
188
5.15 Conclusion
I have shown in this chapter that the chemistry of metal polyselenides is rich
and versatile. The previous predictions by other researchers, that the chemistry of
metal polyselenides would parallel that of metal polysulfides has been shown to be the
exception rather than the rule, with metal polyselenide complexes forming unique and
diverse structures including (Ph4P)3[Co3(Se4)6], (Ph4P)2[Cu4(Se4)z(Ses)],
(Ph4Ph[Pb(Se4hJ and (Ph4P)[ Cp(Mo(Se4hl.
I have successfully used both 77Se and metal NMR to monitor the reaction
mixtures of M2+ I Sex2- (M = Zn, Hg, Cd; x = 2-6) by varying the metal to ligand
ratios. In all of these metal systems I have shown that the only metal polyselenide
ring existing in solution is MSe4 regardless of the value of x in the starting
polyselenide reaction mixture. The propensity for the formation of the Se42-ligand in
these metal I polyselenide systems is in distinct contrast to the metal polysulfide
chemistry where variability of the MSx ring is common. Isolation and X-ray
characterisation of these polyselenide species using large organic counter-cations has
confirmed the predominance of structures in the solid state containing MSe4 rings.
As a consequence of these NMR studies I have developed two simple,
inexpensive synthetic routes to a range of previously unreported homoleptic
polyselenide complexes i.e the 'DMF' and 'NH3' methods.
I have also shown that factors such as reaction solvent, redox processes and
counterion all significantly effect the outcome of the final product. Consequently, a
X-ray crystal structure determination of the final product is essential, although NMR
does provide valuable insight to structural possibilities.
189
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Allg. Chern., 1984, 511, 84
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194
6. 0 Introduction
Chapter 6
POLYTELLURIDES
In contrast to the polysulfides and polyselenides, few uncoordinated
homopolytelluride anions have been reported. These include MgTezl , Kz ( Crypt-
2.2.2)Te3.en 2, Ba(enh Te3 3, KzTe3 4,5, Rb2Te3 and Cs2Te3 6,
(Ph4P)zTe4.2CH3CN and (Ph4P)zTe4.2CH30H 7, Na2(Crypt-2.2.2) Te4 8,
(Ph4P)zTe4 9, (TMDH) Te4 10 and (Bu4N)zTe5 11. These compounds have
previously been structurally characterised in the solid state. Details of their syntheses
are presented in Chapter 1. The protonated telluride ion, TeH-, has only recently been
characterised by X-Ray crystallography, as its Ph4P+ salt,12 despite its preparation
being first reported in 1838.13
However, as with the selenides, very little characterisation of these species and
their expected equilibria in solution have been reported. Studies ofpolytelluride
solutions in ammonia involving UV Nis spectra have been reported 14,15 but as
discussed in chapter 1, these spectra displayed broad overlapping absorption bands
which do not distinguish the various species. Therefore unambiguous identification
of these species is not possible.
Prior to the commencement of this thesis the NMR chemical shift characteristics
of the simplest telluride species Te2- and HTe-, or of the polytelluride species, Tex2- (x
~ 2), had not been well established. It was obvious to this author from the previous
successful study of polyselenides, that NMR spectroscopy would be particularly well
suited for structurally characterising and studying the chemistry of the telluride and
polytelluride anions in solution.
Tellurium has two NMR active spin 1/2 isotopes namely, 125Te( Natural
abundance= 6.99%), and 123Te( Natural abundance= 0.83%). The higher natural
abundance of 125Te was expected to make observation of this nucleus easier than for
123Te, and 125Te was therefore chosen to examine the polytelluride solutions.
195
Whilst this thesis was being written Bjorgvinsson and Schrobilgen 16 published
a similiar , although not identical, NMR study of these species. Both sets of results
are compared in the discussion section.
New non molecular, telluride and polytelluride metal complexes have recently
been used as IR sensory materials 17, superconducting solids 18, and amorphous spin
glasses with tunable conducting properties.19,20 All of the telluride and polytelluride
metal complexes used were prepared by solid state techniques involving the fusion of
solids at high temperatures. However just prior to, and during the course of this work
several n~w polytelluride complexes were prepared using solution chemistry. These
complexes included the species;
NbTew3-,21 Mo4Te16(en)2-,22 Hg4Tel24-,23 [Hg2Tes]2-,24 Cr3Te243-, 25
Pd(Te4)2-, 26 and (Ph3P)2Pt(~ -Te)2Pt(PPh3h 26 whose synthetic details are
presented in Chapter 1. These molecules are potential precursors to other useful
metastable materials, that could not have been synthesised using solid state
preparations.
6 .1 Expectations
The 77Se NMR study of the uncoordinated polyselenides presented in chapter
4, provided unprecedented information about the fast exchange of polyselenide ions
in solution. It was expected that a similiar study into uncoordinated polytellurides
would uncover similiar results. It was proposed that the same synthetic approach be
undertaken for the polytelluride work that proved successful in the polyselenide study,
using 125Te NMR to monitor the resultant solutions. That is to say, the polytelluride,
(Na2T~), solutions would be generated by the reaction ofNa and Te in liquid NH3
followed by the addition ofDMF solvent. It was expected that the various Na2T~
species would exchange at RT and that low temperature studies would be required in
order to observe the individual species ..
196
It was expected that once the solution chemistry of the uncoordinated Na2 Tex
species was understood metal precursors could be introduced. Cadmium was the
metal of choice for initial studies as extensive data already existed concerning
cadmium polchalcogenide chemistry as well as 113Cd NMR. It was expected that by
monitoring these solutions that information toward the synthesis of other potentially
useful materials might be gained.
6.2 Results
6.2.1 Te2- and HTe·
Na2Te is a white, highly air sensitive solid that displays little to no solubility in
the solvents; liquid NH3, H20, EtOH and DMF. As a result no 125Te NMR
measurements could be made of this compound.
The protonated telluride ion, TeH-, has recently been prepared as its Ph4P+ salt,
by the reaction between K2SiTe3 or K2GeTe3 and (Ph4P)Br in ethylenediamine and is
the first example of a well characterised solid containing the TeH- anion.12 An
approximate equilibrium constant for the reaction H2Te + NH3 ~ ~+ + HTe- in
liquid NH3 was reported in 1950, but was complicated by the rapid decomposition of
H2Te near 250C.27 The extent to which the decomposition proceeded could not be
quantitatively determined. The assumption that the product was (NH4)TeH was not
confirmed by the analytical results as decomposition occurred during analysis. It is
apparent that neither of these synthetic procedures is easily reproducible, or desirable
(H2Te is highly toxic), and require uncommon starting materials. Therefore
procedures analogous to the synthesis of NaHSe described by Klayman and Griffin 28
using a NaB~ reduction of Te in aqueous and ethanolic solutions were attempted, to
generate NaHTe. The 125Te NMR spectrum of the colourless, stable aqueous solution
obtained by NaB~ reduction ofTe (Figure 6.1) is a strong, broad singlet at &re =
-1220 ppm (300"K). The resonance shows no splitting but displays a large natural
line width of 415Hz which presumably precludes the observation of the predicted
197
doublet (for NaHSe in water the IJ (77Se-1H) =26Hz; see chapter 4 ). The large line
width is probably due to small amounts of Te2- in fast exchange with HTe-. However
the spectrum is reproducible indicating (as in the case ofNaHSe) that adventitious
base in these unbuffered solutions is not influential. Unfortunately, low temperature
studies that may have resolved the exchanging species were not possible in the
aqueous solvent.
Attempts to generate HTe- using NaBR4 reduction of Te in ethanol or DMF
resulted in mixtures containing unreacted tellurium powder. The addition of excess
borohydride did not resolve this problem, nor did elevation of temperature to refluxing
condition. These experiments were repeated several times with similar results and
were consequently abandoned.
6.2.2 [Te2]2·
The reaction of Te and equimolar Na in NH3 yielded eventually a deep purple
(burgundy) solution with a small amount of white ppte in evidence. However as the
Na was added in small amounts a series of colour changes were obseved. On initial
addition the NH3 solution became deep blue characteristic of the solvated electron.
Then as moreNa was added over a 5 minute period and the reduction of Te
proceeded, the mixture changed rapidly to a pale green, pale yellow, pale blue, dark
blue then to the eventual deep purple. The deep purple colouration of this nominal
"[T~]2-" ammonia solution conflicts with earlier reports for [Te2]2- reported as being
a blue colour in the same solvent but using potassium as the reductant. IS However the
preparation was repeated five times with identical results. The ammonia was distilled
out producing a dark solid of nominal composition "Na2Te2" that was then
redissolved in DMF at ambient temperature producing a homogeneous red solution.
The 125Te NMR of these solutions were all consistent with those seen for Te32-
solution, that is, two resonances at -325 ppm and -364 ppm, in a 2:1 ratio
respectively (Figure 6.2). The samples were examined at room temperature and low
198
C I I I I I I l I I -1110 -1110 -1200 -1210 -1220 -1231 -1240 -12SO -1260 -1270
f'PM
Figure 6.1. 125Te NMR of 'NaHTe' in HzO at 300K
-s1o -sao -sgo
Figure 6.2. 125Te NMR of Naz Te3 in DMF at 220K.
temperatures, scanning the chemical shift range of 4000 to -4000 ppm, with no
evidence of additional resonances. The presence of a small quantity of white
precipitate suggests the formation of Na2Te whilst the red solution is characteristic of
Na2Te3 (see section 6.33). These results suggest that under the conditions of this
experiment the Na2Te2 disproportionated to Na2Te and Na2Te3 (equation 6.1),
(similar to the observation for Na2Se2).
. .. 6.1
6.2.3 [Te3]2·
The reaction ofTe and Na in a 3:2 ratio in liquid NH3 solution generated a deep
red/purple solution via the series of colours described in 6.2.2. Identical colouration
was observed in dimethyl formamide solution. The 125Te NMR of the sample in
DMF showed two strong singlets at -325 and -364 ppm (220K) respectively. No
signals were observed at room temperature. Spin lattice (longitudinal) relaxation times
(TI) for both signals were determined at 220K. It was calculated that the resonance at
-325 ppm had a T1 relaxation time of 37 ms, whilst the resonance at -394 ppm had a
T1 relaxation of 31 ms. This· very fast relaxation time enabled a rapid pulse (every 0.3
s) sequence to be used, and a good signal to noise ratio was achieved in a short period
of time. However the detection of DJ(Te-Te) coupling satellites was not expected to be
possible due to the large natural line widths [ca. 400Hz] of the resonances.
Whilst the relative intensity data allow unambiguous assignment of the
resonances to the individual Te atoms in the I Te3]2-, the addition ofEtOH provided
significant information that the chemical shifts of 125Te of [Te3]2- are strongly
dependent on solvent proticity. Like all cases of solvent proticity on anion activity
(including Sex2·), this effect is proportional to the negative charge density on the
affected atoms. The chemical shifts of the resonances in a dimethyl formamide
199
solution of [Te3]2- vary significantly as the proportion of EtOH in DlviF is increased
(Figure 6.3).
The notation a, ~is used to signify position of Te atoms relative to the chain
ends in [Te3]2- In [Te3]2- the -324 ppm resonance (a) in DlviF shifts to -368 ppm on
the addition of 17% EtOH, a large -44 ppm chemical shift, while the -364 ppm
resonance also shifts to -368 ppm(~) with only -4 ppm chemical shift. Interestingly
both signals merge to form a single resonance at -368 ppm. The effect is greatest on
the first addition of ethanol to DMF. Fmther additions of EtOH to above mixture,
resulting in a composition of 29% EtOH results in (a) to -390 ppm and(~) to -370
ppm corresponding to a total change in chemical for (a) of 66 ppm and(~) of 6 ppm.
A final composition of 38% EtOH resulted in (a) shifted to -407 ppm and(~) shifted
to -373 ppm corresponding to a change for (a) of -83 ppm and(~) of -9 ppm. This is
entirely consistent with the assumption that the negative charge density on [Te3]2- is
greater on the terminal aTe atoms, and follows the pattern observed in the Sex2-
series.
6. 2. 4 [Tex]2- x = 4,5,6.
The solutions formed for Tex2- x = 4,5,6 from Na and Te in liquid NH3 were
homogeneous with no evidence of unreacted Te powder. All of these NH3 solutions
are dark purple in colour and indistinguishable from one another. Similarly dark
purple solutions are observed in DMF solutions. The solutions are very air sensitive
depositing aTe mirror on the shortest exposure ( < 2s) to the atmosphere.
The 125Te of the DMF solutions were examined over the chemical shift range of
-4000 ppm to 4000 ppm, both at room temperature and lower temperatures (300-
2IOK). Surprisingly no resonance whatsoever is observed for any of the nominal
species, Tex2- (x = 4,5,6). Similarly when the salt (Bu4N)2Tes 11 was isolated and
redissolved in DMF, no resonance was observed over the same temperature and
chemical shift range.
200
. ..
Figure 6.3. Diagram of the 125Te resonance positions (at 220K) for [Te3]2- in
DMF/ethanol in the proportions marked.
I I I I ' I I I I I. I l I
-300
., ......
...... ...... .... .... .. ...... ...
DMF
+ 17% (v) EtOH
~~~_...,._,..
. I I I I I I I I I I I I I I I I I I I I !
\ ' ' \
I ' I I I I. I ' I I I I I I I I I I I I -350 -400
PPM
+ 29% (v) EtOH
+ 38% (v) EtOH
I I I I I. I I I I I I I I I -4.50
However the addition of Cd and Zn salts to these solutions clearly result in the
production ofTe42- chelate rings (see 6.4.4).
6 • 3 Metal Complexation
A series of reaction mixtures were prepared containing a 1: 1 or a 2: 1 ratio of
Na2Tex (x = 3, 4, 5, 6 ) and CdCl2.512H20 in DMF . In all the reaction mixtures
containing a 1:1 ratio dark brown purple amorphous solids in a colourless solution
were generated. The solids were insoluble in DMF, MeCN, EtOH and Pyridine and
therefore the NMR was not run. The 2:1 reaction mixtures all generated purple I
brown solutions, that were filtered to remove precipitated NaCI. The homogeneous
filtrates were then transferred to lOmm NMR tubes and measured by both 125Te and
113Cd NMR. The nominal 'Na2Te6' reaction mixture showed evidence of grey
tellurium precipitating inside the NMR tube. The reaction mixtures are highly 02
sensitive, oxidising< 5s in air and precipitating Te film on the glassware.
The various 125Te and 113Cd NMR resonances to be described and discussed
below are each labelled by their chemical shift at the specified temperatures and are
schematically presented in Figures 6.4 and 6.5.
6.3.1 Reaction between Na2Te3 and CdCh.S/2 H20
The 125Te NMR spectrum of the 2:1 reaction between Na2Te3 and CdC12.5/2
H20 at 300K showed a single resonance at "-277" ppm. However the signal-to-noise
ratio was poor after 3,000 pulses so it was decided to decrease the temperature in
order to enhance the signal-to-noise. At 290K the 125Te NMR spectrum had slightly
improved signal-to-noise ratio and showed evidence of two resonances at "201" and
"-479" ppm. At 280K, two resonances were again seen at 201 and -485 ppm
respectively, in a 1:1 ratio, also with enhanced signal-to-noise ratio. At 270K three
resonances were observed at 201, -290, and -487 ppm in a 3:2:1 ratio.
201
. ·
Figure 6.4. Diagram of the 125Te NMR positions and intensities for solutions of
nominal compositions [M(Tex>:zJ2- (M = Cd, Zn) in DMF at 260K .
200 100 0 -100 -200 -300 ·400 -500
200 100 0 ·1 00 -200 -300 -400 -500
.Cd2+ + "[Tes]2-"
200 100 0 -100 -200 -300 -400 -500
200 100 0 ·100 -200 ·300 -400 -500
200 100 0 ·1 00 -200 -300 ·400 -500
·.
Figure 6.5. Diagram of the 113Cd NMR positions and intensities for solutions of
nominal compositions [Cd(TexhJ2- in DMF at 260K.
800 700 600 500 400 300 200
800 700 600 500 400 300 200
800 700 600 500 400 300 200
800 700 600 500 400 300 200
The 125Te NMR spectrum (Figure 6.6a) at 260K was well resolved with three
sharp resonances at 200, -295, and -491 ppm in a 3:2:1 ratio. The resonance at
-491 ppm showed a doublet satellite with ca. 955Hz coupling. The other two
resonances showed no obvious coupling. The 113Cd NMR spectrum (Figure 6.6b) at
260K showed two sharp resonances at 286 and 680 ppm in a 2: 1 ratio. The
resonance at 286 ppm displayed a pair of doublets, with couplings of 265Hz, and
610Hz. The resonance at 680 ppm showed a poorly resolved doublet of ca. 4% of the
central resonance with coupling of ca. 955Hz.
6.3.2 Reaction between Na2Te4 and CdCJ2.S/2 H20.
The 125Te NMR spectra of the 2:1 reaction between Na2Te4 and CdCl2.512H20
in DMF at various temperatures is shown in Figure 6.7. At 300K three broad
resonances at 203,-277, and -482 ppm are observed. At 290K the 125Te NMR
spectrum also show three broad resonances at 204, -281, and -482 ppm. All of these
showed poor signal-to-noise. The I25Te NMR spectrum at 270K revealed only two
sharp signals at 201 and -488 ppm, however the expected coupling was not resolved.
At 260K again only two resonances were observed at 199 and -492 ppm respectively
with the resonance at -492 ppm displaying the same coupling pattern seen in the
Na2Te3 reaction at 260K. No coupling was observed at the 199 ppm resonance.
The 113Cd NMR spectrum at 260K showed two resonances at "680" and "286"
ppm. The chemical shift and coupling patterns observed for both these lines were the
same as those seen for the Na2Se3 reaction.
6.3.3 Reaction between Na2Tes and CdCI2.S/2 H20.
The 125Te and 113Cd NMR spectra of the 2:1 reaction between Na2Tes and
CdCI2.5/2 H20 in DMF at 260K were the same as those described above for the
Na2Te4 reaction.
202
Figure 6.6a. The 125Te NMR spectrum of Cd2+ + ''Te3~' in a 1:2 ratio in DMF at
260K. Inset, expansions of resonances at Ore -491 and -295 ppm.
I 300
' ' I I ' 710
'', I j I I I , , , , , , I , , , , 1 200
'I 100
I 0 -110 •110 -300 -400 -500 _,00
''" Figure 6.6b The 113Cd spectrum of Cct2+ + "Te32-" in DMF at 260K. Inset,
expansion of resonance at 8cd 286 ppm .
• , •.•••..••.•.••••••• , •••• , •••• ,. ''1''''1''''1' 700 150 100 SSt 500 450 400 351 SOO 250
PPM
I f I i
211
Figure 6. 7. 125Te NMR spectra of Q12+ + ''Te42-" in a 1:2 ratio in D.MF at the
· temperatures marked.
I I I I I I I I I I. I I I I I I I I I. I I I I I I I I I. I I I I I I I I I. I I I I I I I I I. I I I I I I I I I 200 0 -200 -400 -600
PPM
6.3.4 Reaction between Na2Te6 and CdCI2.S/2 H20.
The 125Te NMR spectrum of the 2:1 reaction at 260K between Na2Te6 and
CdCl2.512H20 showed the similiar two resonances at 199 and -492 ppm seen
previously with the expected 955Hz doublet on the -492 ppm reson~ce. The 113Cd
NMR spectrum of this sample showed only one resonance at 681 ppm. Unfortunately
the poor signal-to-noise of this sample, probably due to the presence of precipitated
tellurium, precluded the observation of satellites.
6. 4 Reaction between Na2Te6 and ZnBrz.
The 2:1 reaction mixture between Na2Te2 and ZnBr2 in DMF generated a dark
burgundy/brown solution that was filtered to remove precipitated NaBr and some
precipitated Te powder. The 125Te NMR spectum of the filtrate showed two
resonances in a 1:1 ratio at 190 and -371ppm respectively.
6.5 Interpretation
In examining the 125Te NMR of the full range of cadmium solutions, three
resonances at approximately 200, -295, and -491 are observed, whilst in the 113Cd
NMR two resonances at approximately 286 and 680 are observed. In the 2:1 reaction
at 260K between Na2Te6 and CdC12.5/2H20 only the 200 and 491 lines occur in the
125Te NMR spectrum along with the 680 line in the 113Cd NMR spectrum,
suggesting that these lines occur independently of the other resonances. The line at
-491 in the 125Te spectrum displays a 955Hz coupling identical to that seen at the 680
resonance in the 113Cd spectrum suggesting that the respective cadmium and tellurium
atoms are directly bound. The 200 line shows no coupling but is obviously related to
the other two resonances occuring simultaneously in all other spectra. A comparible
spectrum is observed for the [Cd(Se4)212- species with the 113Cd NMR spectrum
displaying a single resonance at 748 ppm and the 77Se NMR spectrum displaying two
resonances. Therefore the lines at DTe 200 and 491 and 8cct 680 are assigned to the
203
[Cd(Te4)2]2- species. The crystal structure of the Php+ salt of this anion has been
reported. 29
Interestingly, the coupling pattern of the proposed [Cd(Te4)2]2- species in the
125Te NMR spectrum shows the resonance at high chemical shift being coupled to
cadmium whereas in the 77Se NMR spectrum of [Cd(S~)2]2- the low chemical shift
resonance is coupled to cadmium (see chapter 5).
The 113ect NMR spectrum of the 2:1 reaction between Na2Te4 and CdCl2.512
H20 at 260K shows the existence of the 8cct 286 ppm line but the 125Te NMR at the
same temperature shows only a minor amount of the &re -295 ppm line but does occur
as a major resonance at 290K at &re -281 ppm and as a minor resonance at &re
-291 at 270K (Figure 6.7). Therefore this is consistent with the idea that the two ii,nes
at 8cct 286 and 8Te -295 are related. The 8cct 2R6 ppm shows a pair of doublet
satellites of ca. 6.3% of the central resonance. The innermost doublet showed
coupling of 265Hz, whilst the outermost doublet showed coupling of 610Hz.
However this is not observed on the resonance at 8Te -295. These resonances are
possibly attributable to a new chloro complex of the form [CdCl2(Tex) ]2-, as the 8cct
2861ine corresponds to chemical shifts of the complexes: [CdC4]2-, [CdCl2I2]2-,
[CdCII3]2- with 8cct 510, 389 and 278 ppm respectively, but assignment of it is not
definite. 30
6. 7 Discussion
It is not surprising that although Te2- and HTe- are the simplest telluride species,
their &re characteristics have not been well established. Dihydrogen telluride, H2Te,
is thermally unstable and decomposes rapidly even at ooc,3I whilst the recently
reported monohydrogen derivative, (PPh4)TeH, has also been reported to have limited
stability at 25°C.l2 Na2Te has been shown in this work, and by others,14 to be
insoluble in various solvents, with similiar results reported for K2Te.IS Some
confusion previously existed concerning the colour of the simplest telluride, as it had
204
Haushalter12 report the (PPh4)TeH molecule to be orange in colour, whilst
Bjorgvinsson and Schrobilgen 16 reportedly observe the 125Te NMR spectrum of the
HTe- anion in a violet coloured solution. Both of these colours conflict with the
reported colourless electrochemically generated TeH- species 34 and the colourless
(Nf4)TeH compound.27 Unlike the analogous HSe-, there has been no
straightforward preparation of HTe- reported in the literature. The preparation of this
anion has generally been achieved in the past by the reaction ofH2Te with NH3 to
form (Nf4)TeH,27 and more recently by the illogical methods of extracting K2SiTe3
or K2GeTe3 with en 12 or by extracting the alloy LiPbTeo.67Seo.33 with en in the
presence of 12-crown-4.16
Only Bjorgvinsson and Schrobilgen 16 have attempted to measure the 125Te
NMR of the the tellurides Te2- and TeH-. Their work was published after the
completion of the work presented in this thesis. I discuss my DTe results in the
context of that collected in Table 6.1 for related species.
205
Table 6.1. 125Te chemical shift data for species Tex2· and TeH·.
Anion 8(125Te), lJ(A-B), Hz Temp. Solvent Reference
. ppm K
K2Te -1430 297 en +2,2,2-crypt 16
KHTe -1095 140 (1H_125Te) 297 en + 2,2,2-crypt 16
NaHfe -1220 Not observed 300 H20 This work
H2Te 59 (1H_l25Te) Neat 35
M~Te 0 20.7 (2H_l25Te) 300 Neat 36
K2Te2 -1080 3645 ± 20 219 LiquidNH3 16
(123Te-125Te)
K2Te2 -1074 Not observed 297 en 16
K2Te3 -286, -367 2175 ± 6 323 en + 2,2,2-crypt 16
(125Te-125Te)
K2Te3 -298, -372 Not reported 297 en +2,2,2-crypt 16
Na2Te3 -325, -364 Not observed 220 DMF This work
K2Te4 7±4 Not reported 278 en + 2,2,2-crypt 16
K2Te4 19± 8 Not reported 297 en +2,2,2-crypt 16
B jorgvinsson and Schrobilgen 16 reported a strong singlet at Ore -1430 at 297K
for a solution ofKzTe treated with 2,2,2-crypt in ethylenediamine and assigned it to
the solvated Te2- anion. Similarly a weak doublet at OTe -1095 ppm (lJ(lH- 125Te)
=140Hz) at 297Kwas also reported in the above KzTe solution and in" much higher
concentration" in a violet coloured solution containing the extracts of
LiPbTeo.67Seo.33 + 12-crown-4 in ethylenediamine and assigned as the solvated HTe
anion. The concentrations of these solutions could not be reported owing to the large
amounts of crystalline solid present in the NMR samples.
206
I have attempted to synthesise NaHTe using an analogous procedure to the
synthesis of NaHSe described by Klayman and Griffin 28 using a NaBI4 reduction
ofTe in aqueous solution and monitor the reaction by 125TeNMR. The resulting
colourless solution showed a broad resonance (415Hz) at 0Te -1220 ppm at 300K.
The magnitude of this chemical shift seems consistent with the notion of an
electronegative species such as HTe-. The most negative 8Te recorded is the value of
-1214 ppm for (Me3Sn)2Te in CH2CI2.37 The absence of the expected doublet with
coupling of ca. 140Hz, is attributable to the large natural line width of 415Hz.
Unfortunately, the complete lack of data concerning similiar inorganic or organic
species in the literature precludes any attempt to extrapolate a predicted chemical shift
for HTe- or Te2-.
There is no evidence for the existence of [Te2]2-, reported to exist by
Bjorgvinsson and Schrobilgen,l6 or any other [Tex]2- (3<x:::;6) species.
Bjorgvinsson and Schrobilgen claim to observe a single resonance for a mixture
containg K2Te + Te in liquid NH3 ( -54°C) at 8Te -1080 ppm and a similiar result for
the same reaction in ethylenediamine at 8Te -1074ppm (240C). The proximity of
these two results does seem extraordinary considering the variation in solvent and
temperature. I have previously shown that a small increase in solvent proticity (17%)
in [Se3]2- and [Ses]2- solutions leads to a large change in chemical shift of 64 ppm.
Similarly, up to 44 ppm is observed with only a 17% increase in solvent proticity in
[Te3]2-. The temperature dependence of 8sc for HSe- in DMF is large, ca -1 ppm per
degree (see chapter 4). In my reaction containing nominal composition of"Na2Te2"
in DMF the l25Te NMR showed two resonances at 8Te -325 and -364 ppm,
characteristic of [Te3]2-. There was evidence of a white undissolved solid in the
reaction flask which is assigned as Na2Te. Therefore it is proposed that under these
conditions [Te2]2- behaves like [Se2r2- and disproportionates to [Te]2- and [Te3]2-
Bjorgvinsson and Schrobilgen also found two resonances at OTe-286 and -367 ppm.
for [Te3]2· in en in the presence of 2,2,2 crypt at sooc and at OTe -298 and -372 ppm
207
at 24oc. These results are consistent with the notion that the chemical shifts of the
polytelluride ions are responsive to solvent proticity. As was seen in the case of the
polyselenides the magnitude of the effect is dependent on the location of the tellurium
atom in the telluride chain, and its negative charge density. In all three results the
central Te atom (a), shifts only marginally from &re -364 to -372 ppm. Whilst the
outermost tellurium atoms(~), carrying the expected charge density, shifts between
&re -325 and -286 ppm with decreasing solvent proticity (Figure 3).
The lack of evidence for any longer chain length [Tex]2- speies in solution than
[Te3]2- is surprising as both [Te4]2- and [Tes]2- salts have been isolated and the
analogous [Se4]2-, [Ses]2-, [Se6]2- have been shown to exist in solution (see chapter
5). This observation is in agreement with Bjorgvinsson and Schrobilgen16 and
Schultz,14,15 whose UVNis results indicate [Te3]2- to be the longest chain
polytelluride in solution. It is suspected that the remaining [Tex]2- ions (x>3), must
be involved in rapid exchange and consequently do not give rise to an observable
125Te NMR resonance. The observation of them would probably be possible at lower
temperatures but unfortunately, in DMF the polytelluride ions become insoluble at ca.
215K and crystallise from solution.
However, there is definitive evidence in DMF solution for the Te42- ion when in
the presence of either Cd2+ or zn2+ metal precursors. In these solutions the metal
presumably coordinates allowing the rapidly exchanging Te42- chains to chelate. The
125Te NMR of the solutions containing Cct2+ and Tex2- (x = 3,4,5,6) (Figure 6.4)
clearly illustrates the presence of the 1: I ratio resonances of Te42- at 198 and -491
ppm. Similarly for the reaction between zn2+ and Te62- where the two resonances
are slightly shifted to &re 190 and -371 ppm respectively. The coupling of ca. 955 Hz
observed around the low chemical shift resonance in the Cd2+ I Tex2- solutions is
identical to that observed in the 113Cd NMR around the 8cd 680 ppm resonance. The
chemical shift of 8cd 680 ppm is very similar to that observed for the [Cd(Se4h]2-
208
anion at Bed 748 ppm (see chapter 5), although the 955Hz coupling of the proposed
[Cd(Te4h]2- species is considerably larger than the 130 Hz seen for [Cd(Se4)2]2-.
6. 7 Experimental Preparations of Solid Compounds
6.7.1 (Bu4Nh[Tesl
The procedure used for the synthesis of this compound was based on that
described by Haushalter.ll A flask fitted with a low temperature condenser was
charged with Te powder (2.0 g, 15.7 mmol) and liquid NH3 (30 ml). Na (0.24g,
10.44 mol) was added and the mixture stirred until all sodium had dissolved (1 hr)
generating a deep purple solution. The NH3 was then pumped off leaving a grey solid
with a nominal composition of Na2Te3. To the dry solid was then added 30 ml of
demineralised H20 and the resulting mixture was stirred for 20 mins at ca. soc in an
ice bath generating a purple solution. There was no evidence of undissolved solid. To
this was added a solution of Bu4NBr (3.4g, 10.6 mmol ) in demineralised H20 (30
ml) at ca. 5°C. Within seconds black crystalline solid precipitated. The solid was
collected, washed and diethyl-ether and dried in vacuo (yield 1.95g). The solid is
very hygroscopic and oxidised readily to Te after several minutes exposure to air.
This reaction has been successfully scaled up twofold. (Bu4N)2[Tes] is soluble in
MeCN (purple), DMF (purple).
6.7.2 NazTe
The procedure used for the synthesis of this compound was similiar to that
described by Brauer.l5 A 2-necked flask was charged with Te powder (l.Og, 7.8
mmol), fitted with a low temperature condenser and NH3 (30 ml) was condensed into
the flask. Na (0.36g, 15.6 mmol) was then added against an N2 stream and the
reaction mixture was stirred at -78°C for ca. 1 hr until the dissolution of sodium was
complete. The solution colour changed from the initial blue (due to solvated electron)
through pale green, yellow, then colourless with a white precipitate. The NH3 was
209
pumped off leaving the white solid. The white solid was observed to be insoluble in
H20, MeCN, and DMF. This solid was extremely 02 sensitive oxidising instantly on
exposure to air to purple then grey solid.
6.8 Preparation of solutions for 125Te NMR
6.8.1 NaHTe in aqueous solution
NaBf4 (0.87g) in water (10 ml) was stirred into a suspension of Te (l.Og, 7.8
rnrnol) in water (10 ml) at room temperature. Hydrogen instantly evolved from the
reaction which slowly changed colour from colourless to purple then back to
colourless over a 20 minute period. After 20 mins the hydrogen evolution had ceased.
A 10 ml aliquot was then transferred to a 10 mm NMR tube for study.
6.8.2 "Na2Tex" DMF Solution x = 2, 3, 4, 5, 6.
The general procedure is the same for all compounds, so the description of
Na2Te2 only is presented in detail. A 2-necked flask was charged with Te powder
(l.Og, 7.8 mml) fitted with a low temperature condenser, and NH3 (30 ml) was
condensed into the flask. Na (0.18g, 7.8 mole) was then added against a N2 stream
and the reaction mixture was stirred at -78"C for ca.lhr. The solutions changed colour
initially to blue, pale green, yellow, then pale blue, dark blue, then deep purple over a
time period ca. 5 mins. The NH3 was evaporated, leaving a dark coloured solid
which was dried in vacuo for a further 1/2 hr. The solid was then dissolved in freshly
distilled and degassed DMF (15 ml) at room temperature, generating a dark red
(burgundy) solution. A small amount of white ppte was present in the case of
"Na2Te2''.
Preparative data for other solutions are presented below in Table 6.2.
210
211
Table 6.2. Preparative details for the syntheses of solutions of Na2 Tex
(x=2-6)
Nominal Amount of Amount of Volume of Volume of DMF Composition Na(g) Te(g)
NazTez 0.18 1.0
NazTe3 0.12 1.0
NazTe4 0.09 1.0
NazTes 0.07 1.0
NazTe6 0.06 1.0
6.8.3 Reactions of Tex2· with Zn2+ and Cd2+
1. Reaction of "NazTez" with ZnBrz
NH3 (ml)/ (ml)/ colour colour
30 (purple) 15 (red/purple)
30 (purple) 15 (red/purple)
30 {Qt!!:£le) 15 {Q~le)
30 (purple) 15 (purple)
30 {Qt!!:£le) 15 {Q~le)
A DMF solution of a nominal composition of "NazTez" was prepared using the
procedure outlined in Table 6.2 To the resulting purple solution was added 1/2
equivalent of ZnBrz (0.44g, 1.95mmol ) and stirred at room temperature for ca.l/2
hr. The reaction mixture changed to a dark burgundy/brown colour. The mixture was
then filtered removing unwanted NaBr and small amount ofTe powder and taken for
125Te NMR (c125Te = 190,-371 ppm).
2. Reaction of "NazTex'' (x = 3,4,5,6) with CdClz.5/2 H20 (see table 6.3)
General. DMF solutions (20 mL) of nominal composition of "NazTex" were
prepared using the procedure outlined in Table 6.1. To the resulting red/purple
solutions was added 1/2 equivalent of CdClz . 5/2Hz0 and resulting mixtures stirred
at room temperature for 20 mins generating dark purple/brown solutions. The
mixtures filtered removing NaBr and precipitated Te powder in the case of the NazTe6
preparation. The samples were then taken for 125Te and 113Cd NMR.
Table 6.3. Investigations of cadmium polytelluride solutions of
nominal composition [Cd(Tem)nl
m,n Nominal Metal Precursor polytelluride (mmol)
solution (mmol) Na2Te3 CdC12 . 5/2H20
3, 0.5 (2.6) (1.3) Na2Te4 CdC12 . 5/2H20
4, 0.5 (1.96) (0.98) Na2Te4 CdCI2 . 5/2H20
5, 0.5 (1.57) (0.78) Na2Te4 CdCl2 . 5/2H20
6, 0.5 (1.3) (0.65)
6.9 Conclusion
Preparative reactions for the ions HTe- and Te32- in protic and aprotic solvent
systems are established and these ions in solution have been characterised by the most
informative probe, 125Te NMR. A broad ( 415 Hz) singlet at 8Te -1220 ppm (300K)
has been assigned to the HTe- ion in aquem~s solution. The large line width precludes
observation of the expected doublet (as observed for NaHSe in water (1J(77Se-1H) =
26Hz) and is probably due to small amounts of Te2- in fast exchange with HTe-.
There is no evidence for [Tev2- which is believed to disproportionate to HTe- plus
[Te3]2- or Te(s). Unlike in [Sex]2- where x = 3-6 were shown to exist in DMF
solution [Te3]2- is the only polytelluride observed in DMF. The 125Te NMR
spectrum of [Te3]2- shows two resonances at 8Te -325 (a) and -364 (f3) ppm at
220K. The chemical shift of Te(a) is very responsive to the solvent proticity,
becoming more negative in the range DMF ~ EtOH, while the Te(f3) atom responds
much less. The same effect was observed for the [Sex]2- ions. The lack of evidence
for any longer chain length [Tex]2- species in DMF has also been recently confrrmed
by a similar NMR study by Bjorgvinsson and Schrobilgen16. It is suspected that the
212
remaining [Tex]2- ions (x > 3), must be involved in rapid exchange and therefore do
not give rise to observable 125Te NMR signals. Complexation of nominal [Tex]2- (x
= 3-6) solutions in DMF, with Cd2+ clearly indicate the formation of only the
[Cd(Te4)2]2- species.
6.10 References
1. S. Yanagisawa, M. Tashiro, S. Anzai, .1. Inorg. Nucl. Chern., 1969, 31,
943.
2. A. Cisar, J.D. Corbett, Inorg. Chem., 1977, 16, 632
3. R. Zagler, B. Eisenmann, H. Schafer, Z. Naturforsch. 1987, 42B, 151
4. B. Eisenmann, H. Schafer, Angew. Chem, 1978, 90, 731
5. B. Eisenmann, H. Schafer, Angew. Chem. Int. Ed, 1978, 17, 684
6. P. Bottcher, J. Less-Common Met., 1980, 70 263
7. J. C. Huffman, R. C. Haushalter, Z. Anorg. Allg. Chem. 1984, 518,
203
8. L.A. Devereux, J. F. Sawyer, G. J. Schrobilgen. Acta. Crystallogr.
1985, C41, 1730
9. H. Wolkers, B. Schreiner, R. Staffel, U. MUller and K. Dehnicke.
Z. Naturforsch, 1991, 46B, 1015
10. K. W. Klinkhammer and P. Bottcher, Z. Naturforsch, 1990, 45b, 141
11. R. G. Teller, L. J. Krause, R. C. Haushalter. Inorg. Chern. 1983, 22,
1809.
12. J. C. Huffman, R. C. Haushalter, Polyhedron. 1989, 8, 531
13. A. Bineau, Annls. Chim. Phys. 1838, 67, 230
14. L. D. Schultz and W. H. Koehler, Inorg. Chem. 1987, 26, 1989
15. L.D.Schultz, Inorganica. Chim. Acta. 1990, 176, 271
16. M. Bjorgvinsson and G. J. Schrobilgen, lnorg. Chem. 1991, 30, 2540
17. R. Korenstein, W. E. Hoke, P. J. Lemonias, K. T. Higa and
213
D. C. Harris, J. Appl. Phys. 1987, 62, 4929
18. M. Potel, P. Gougion, R. Chevrel and M. Sergent, Rev. Chim. Minerale,
1984,509
19. R. C. Haushalter, C. M. O'Conner, J.P. Haushalter, A.M. Umarji, and
G. K. Shenoy, Angew. Chern. Int. Ed. Engl. 1984, 23, 169
20. R. C. Haushalter, D.P. Goshorn, M.G. Sewchock, and C. B. Roxlo,
Mat. Res. Bull. 1987, 22, 761.
21. W. A. Flomer, J. W. Kolis, J.Am Chem.Soc., 1988, 110, 3682
22. B. W. Eichhorn, R. C. Haushalter, F. A. Cotton, and B. Wilson, Inorg.
Chem.1988, 27, 4084
23. R. C. Haushalter, Angew. Chern. Int. Ed. Engl. 1985, 24, 433
24. R. C. Haushalter, Angew. Chern. Int. Ed. Engl. 1985, 24. 434
25. W. A. Flomer, S. C. O'Neal, W. T. Pennington, D. Jeter, A. W. Cordes, J.
W. Kolis, Angew. Chem.Int. Ed. Engl., 1988, 27, 1702
26. R. D. Adams, T. A. Wolfe, B. W. Eichhorn and R. C. Haushalter,
Polyhedron, 1989, 8, 701
27. A. B. Ellis, S. W. Kaiser, J. M. Bolts and M.S. Wrighton, J. Am. Chem. Soc.
1977, 99, 2839
28. F. F. Mikus and S. Carlyon, .T. Am. Chern. Soc., 1950, 72, 2295
29. D. L. Klayman and T. S. Griffin, .!.Am. Chern. Soc. 1973, 95, 197
30. M.G. Kanatzidis, Comments Inorg. Chern., 1990, 10, 161
31. R. Colton, D. Dakternieks, Aust.J.Chem, 1980, 33, 2405
32. N. N. Greenwood and A. Earnshaw, Chemistry of the Elements, Pergamon
Press, 1984
33. D. Gruen, R. McBeth, M. Foster, and C. Crouthamel, J. Phys. Chern., 1966,
70, 472
34. D. Bamberger, J. Young, and R. Ross, .1./norg. Nucl. Chern. 1974, 36, 1158
214
35. M. Pourbaix (Ed), Atlas of Electrochemical Equilibrium in Aqueous Solutions.
Pergamon Press, Oxford (1966)
36. C. Glidewell, D. W. A. Rankin, G. M. Sheldrick, Trans. Farad. Soc., 1969,
65, 1409
37. W. McFarlane, Mol. Phys., 1967, 12, 243
38. H. C. E. McFarlane and W. McFarlane, J. Chem. Soc. Dalton. Trans., 1973
2416
39. W. Brauer, 'Handbook of preparative inorganic chemistry' Voll.(non
transition elements), Academic Press. 2nd ed, 1963
215
7. 0 Introduction
CHAPTER 7
PHOSPHORUS CHALCOGENIDES.
216
In contrast to the extensive chemistry of phosphates, the chemistry of anionic
phosphorus sulfides and selenides has not been well developed. Most of the previous
work has focused on neutral cages of sulfur 1,2 and selenium.3.4,5 Athough some work
with anionic phosphorus sulfides has been reported, including the species P03S3-,
P02S23-, PQS3-, PS4-, 6 [P3Sg]S- 7 and [P2SsJ2- 8 and the cycle- species; [P4Ss]4-,
[PsSw]5-, [P6St2]6-.7 Corresponding work on anionic phosphorus selenides had not
been reported prior to the commencement of this thesis.
Recently, non-oxide phosphorus chalcogenide glasses, based on the sulfides, and
selenides outlined above, have gained much interest as materials for infrared- transparent
optical fibers, reversible conductivity switching devices, semiconductors,
photoconductors, photoresistors, and solid electrolytes for battery applications.
Phosphorus - selenium glasses are of particular interest because of their infrared
transparency, their comparatively high stablility in a moist atmosphere, and their
resistance to crystallisation.9
Metal phosphorus chalcogenides have been known for a long time, with the first
metal phosphorus trisulfides reported before the turn of the century.l0,11,12,13,14 Metal
phosphorus trichalcogenides15 of the fommla MPS3 ( M= Pb,V, Mn, Co, Ni, Pd, Zn,
Cd, Fe, Sn) and MPSe3 (M=Ni, Fe, Mg, Mn,Cd,In,Sn), are known to form a class of
compounds that exibit a layered structure, similiar to that found in many of the transition
metal dichalchogenides including TiS2. The intercalation chemistry ofTiS2 has been
extensively studied because of its utility in secondary batteries.16 Similiarly, it has been
found that metal phosphorus chalcogenides readily undergo intercalation reactions with
organic amines, alkali metals and some organometallic molecules generating
electrochemically active compounds with semi- conductor properties.15 These useful
compounds have structures consisting of divalent metal cations complexed with the
molecular hexasulfidohypodiphosphate anion PzSG'- (see Chapter 1, page 5).15
217
However these layered metal phosphorus trichalcogenides have generally been
synthesised by solid state methods, usually by heating the stoichiom~~c quantities of the
elements (or metal chalcogenides) in evacuated silica tubes or by vapour sublimation in a
temperature gradient.
It was expected that anionic phosphorus selenide species, similiar to PzS62-,
should exist and have enormous potential as precursors to novel metal phosphorus
selenides. Therefore in the expectation of generating new phosphorus selenide anions, an
investigation of the reactions between phosphorus and polyselenide anions was
proposed, this synthetic approach being a logical progression from the work established
in Chapters 4 and 5. Subsequent reactions with metal compounds were expected to
generate [My(PnSem)z]X· species.
Two types of synthetic approach were considered :
I. Reaction of P4 dissolved in CS2, with various NazSex precursors (generated as
in Chapter4) in various P4/Sex ratios in DMF.
II. Reaction of P4 dissolved in CS2 with the preformed polyselenide salt,
(B04N)2Se6 in DMF.
All reactions could be monitored by 31 P and 77Se NMR. The obvious advantage
here over the NMR study of the uncoordinated polyselenides in Chapter 4, being the
100% natural abundance and inherent sensitivity of the 31p nucleus (see Chapter 2).
7 .1 Background
Phosphorus • Sulfur and Phosphorus - Selenium Cages and Rings.
There is an extensive series of phosphorus-sulfur cage compounds with structures
based on the successive insertion of sulfur atoms into the P-P bonds of the tetrahedron of
white phosphorus. One or more of the phosphorus atoms can also be oxidised with the
formation of exocyclic double bonds to sulfur. These series of phosphorous-sulfur cage
218
compounds include P4S2, P4S3, P4S4, P4S5, P4S7, P4S9, P4S 10 discussed below. One
notable absence in this series is P 4S6 This is surprising in view of the ease of formation
ofthe oxygen analogue P406.
In principle, selenium analogues of these compounds can be expected and P4Se3,
P4Se4, P4Ses, P4Se7, P4Seg, and P2Ses have been reported (see section 7.2). However
in general there are fewer P-Se compounds and their chemistry is less well investigated.
7 .1.1 Phosphorus-Sulfur Cage Compounds
The phase diagram between red phosphorus and sulfur has been re-examined to
show evidence for the six P4Sn species where n = 2,3,5,7,9 and 10. 17
7.1.1.1 P4S2
Little is known about this sulfide, but it has been identified in the P-S system and
results when P4S3 is treated with white phosphorus. IS
7.1.1.2 P4S3
P4S3 is the most stable compound in the P4Sn series and can be prepared by heating
the required amounts of red P and sulfur above 18QOC in an inert atmosphere and then by
purifying the product by distillation at 420°C or by recrystallisation from toluene.2 There
exist two solid state structures of this molecule. The high temperature form of P4S3 has
been prepared by sublimation of the nonnal forn1 at temperatures higher than 100°C. The
high temperature form of P4S3 shows little crystallinity and transforms irreversibly to the
normal form.19 IR and Raman data indicate that the cage structure of the solid state
normal form is retained in the melt and vapour phases.20,21 The retention of a P3 ring in
the structure is notable.
219
In solution, the chemical shift for the basal P atoms is particularly solvent-sensitive,
but the solvent effect is much less marked with the apical atom. The coupling constant
3J(P-S-P) is 71 Hz.22
Three forms of this sulfide are known. The a- and P-isomers can be obtained by
substitution of the iodine atoms in a and P-P4S3I2 respectively by sulfur using
[(Me2Sn)2S] in CS2 solution.23 The third form. y, for which structure below has been
proposed, is obtained by slowly cooling a 1:1 mixture of P4S3 and P4S5 that has been
heated to 700K or from a mixture of P 4S 1 o and red phosphorus heated to 4000C. 24
a-P4S4 P-P4S4 y-P4S4
P. P. P.
P./f"-s s/f"-s s/f"-s I~ I I s I I s I S\\S/P \t/p \\/P=S
p p p
P4S5 disproportionates below its mp to P4S3 and P4S7 and so cannot be obtained
by direct reaction of the melt. It is prepared by irradiating a solution of P4S3 and S in CS2
220
solution using a trace of iodine as catalyst. There are two forms of this compound. The a
form P4S5 has an ABCX-type 31p NMR spectrum conistent with the first structure below
while that for the f3 form shows an A2X2 system consistent with the later structure.25.26
P.
s/("-s I s I
/-\-\ s----P- -s
P4S7 is the second most stable sulfide in this series and can be obtained by direct
reaction of the elements. There are two crystalline forms of this compound. The
monoclinic (a) 27 and orthorhombic (f)) 28 forms of this sulfide have the same molecular
structure.
The compound P4S9 also exists in two forn1s which interconvert at 170-190°C.29
The low temperature form has a structure similar to that ofP409.
221
In agreement with this structure the 3lp NMR spectrum in CS2 solution shows signals at
57.3 and 62.9 ppm downfield from H3P04 with 2J(P-S-P) = 96 Hz.30
P4S9 has been prepared by the following methods.
a) heating a mixture of red phosphorus and P4S 10 to 400-450·c.31
b) treating either P4S3 or P4S7 with sulfur.29
c) crystallisation from carbon disulfide of the mixture obtained by heating P4S10.32
(d) hydrolysing a 1:1 mixture of SPCl3 and water in a sealed tube at 15o·c.33
(e) abstracting sulfur from P 4S 1 o with triphenylphosphine.29
P4S10 is commercially the most important sulfide of phosphorus and is fonned by
direct reaction of liquid white P4 with a slight excess of sulfur. Purification of
commercial P4S10 involves heating it in a vacuum at IOO"C to remove excess of sulfur
and at 14o•c to remove the P406S4 impurity.34 P4S10 has essentially the same structure
as molecular P4010.
s II P.
s/!"s Is I
s~P~~sj.::::::.s [,P.._s
S I s P4S10 is a source ofP2Ss units and is extensively used as a thialating agent for a wide
variety of organic compounds and in the preparation of many organothiophosphorus
compounds.35
7.2 Phosphorus-Selenium Cage Compounds P4Sen.
222
The most comprehensive reports on the number of phases observed in the P-Se
system point to the formation of three modifications of P 4Se3 and two forms of P 4Se4, in
addition to low and high temperature modifications of P4Sew.36,37
Transitions between the P4Se3 modifications occur at sz·c (<X-~) and 192·c (~y)36,37. The structure of P4Se3 is similar to that of P4S3.38
The tetraselenide P4Se4 can be prepared by heating a P4Se3 + Se mixture at 250-
30(tC, and there is a reversible transition between the a and ~ forms at 3oo·c_39 The
structure proposed is as below.
P4Se3 is converted to P4Ses by reacting bromine in CS2 solution, and has the same
unusual structure as a P4S5
p"' s(/ Se I Se I P-~-P
Se~ -P/ \\ Se
223
Although neither P4Se7 nor P4Seg has been observed in the P-Se phase system,
compounds containing these species can reportedly be produced by treating P4Ses with
nitrogen bases.40
P2Ses is prepared from red phosphorus and selenium. It was previously thought
to be an amorphous solid 40 although suggested structures had been advanc~d in favour
of non-molecular and molecular structures. Recently, Blachnik et aJ41, crystallised
P2Ses as a decomposition product of the monoiodide, P3Se4I. and identified its structure
as that below.
7. 3 Cyclodiphosphadithianes and related ring compounds
Much of the chemistry of monocyclic P-S systems is dominated by the four
membered cyclodiphosphadithianes (P2S2) and, although compounds apparently
224
containing the related P3S3 system are known, where structural information is available
they are based instead on P3S2 with the third sulfur atom in an exocyclic position.
Organic derivatives of the four-membered P2S2 system continue to be of great
interest from utility as synthetic intermediates for a range of organophosphorus
compounds with biological activity. One of the major preparative routes is the reaction of
a thiophosphonic dichloride with dry hydrogen sulphide (Equation 7.1).42
s II
R-P-S 2 RPSC12 + 2H2S --- 1 1 + 4H Cl ... 7.1 S-P-R
II s
Compounds of this type are very good thiolating agents for a wide range of organic
compounds and this aspect has been exploited particularly by Lawesson and his
coworkers.43,44
There are no selenium analogues reported in the literature.
7 .3.1 Other Phosphorus-Sulfur Ring Compounds
A number of compounds which from their molecular formulae might be expected to
contain P3S3 rings, have been shown by vibrational and 31p NMR spectroscopy to be
based on P3S2 systems with the third sulfur in an exocyclic position. This occurs with
the phenylated compound R =Ph (see below), obtained from K2(PPh)3 and S3Cl2.45
R R '\ I P-P=S I \ s, /s
p I R
225
The silver salt Ag2P2S6 obtained from stoichiometric quantities of the elements at 873K,
contains the cyclic anion [P2S6]2- 46
and this species together with the hexathiohypodiphosphate anion P2S64- occurs in the
ZI14cP2S6)3 structure.47 In Z114(P2S6)3 the Zn atoms are tetrahedrally coordinated. There
are two different P2S6 groups. One is analogous to P2S64- described above. In the
other, the two P atoms are linked by two S bridges.
Extensive studies of other metal phosphorus trisulfides of the formula MPS3,
whose structures can be considered as being salts of divalent metal cations with the
hexathiohypodiphosphate anion P2S64- have been reported.
A P2Sg2- ion, is found in the product (pY2HhP2Ss from a reaction of P4S10 with
refluxing pyridine.8 Moisture is probably important in this reaction.
s s-s ~I \ /s
P p -s/ \ I~ s-s s
The centrosymmetric ring has a chair conformation with endocyclic angles of 102.2 and
104.7" at phosphorus and sulfur respectively.
A rather different series of cyclic thiophosphate(III) anions [(PS2)nJn- has emerged
from a study of the reaction of elemental phosphorus with polysulfide. Anhydrous
226
compounds Ms[cyclo-PsSIO] and M6[cyclo-P6SI2] were obtained using red phosphorus,
whereas white P4 yielded [NH4]4[cyclo-P4Sg].2H20. This unique and stable P4Sg4-
anion is the first known homocycle of 4 tetracoordinated P atoms. 7
s" hs s~ P~ /s'\: P./ p
s-/ '-P./ ~ sij "'-s-P4Ss4-
Its 3Ip NMR spectrum CH20) displays a singlet at 8 = 121 (85% H3P04, external).
Amorphous precipitates of various colours are formed when [NH4]4[cyclo-P4Sg].2H20.
is reacted with the following cations: Pbll (red-brown), Snll (orange), Bi (brown-black),
Cull (yellow-brown), Cdll (white), Hgll (bright yellow), Ag (yellow, slowly becoming
brown and then black).
Figure 7.2. 31p NMR specrra of·reactions between [Sex]2- and P4 at 300K.
Ratio Chemical Shift (3 Jlp) (ppm)
100 80 60 40 20 0 -20
100 80 60 ~0 20 0
100 80 60 ~0 20 0 -20
Na2Se5 + P4 (1:1)
I 100 80 60 40 20 0 . 2 0
Ratio Chemical Shift (3 3lp) (ppm)
100 80 60 40 20
100 80 60 40 20 0 -2 0
100 80 60 40 20 0 -20
100 80 60 40 20 0 -2 0
--
100 80 60 JO 20 0 -20
227
7.4 Results
The results presented in the following section are preliminary investigations only.
Unfortunately time constraints did not permit a comprehensive investigation into many of
the species observed in the following solutions. In all solutions 31~ NMR spectra were
recorded but unfortunately 77Se NMR spectra were not. However, significant progress
was made with the isolation and identification of a novel molecular P-Se compound
discussed below.
7 .4.1 Solution Studies
3lp NMR has been used to characterise reactions between P4 and the polyselenides,
Na2Se3, Na2Ses, Na2Se6 and (Bu4NhSe6. A typical preparation is described.
Pre-weighed (under H20) white P 4 was rapidly transferred to a Schlenk tube and
pumped dry under vacuum (0.1 mmHg). Freshly distilled and degassed CS2 (5mL), was
then admitted to the tube via a cannula. The P 4 readily dissolved with stirring generating
a very pale yellow solution. This P JCS2 solution was then transferred dropwise with
stirring to a second tube containing freshly prepared polyselenide solution in DMF
(15mL)(see chapter 4). Generally, on addition of the P JCS2 solution a characteristic
colour change occurred. Typically, the initial green polyselenide solution became yellow
brown after the addition of ca. lmL of the PJCS2 solution and proceeded to red-brown
on complete addition (15 min). lOmL of the resulting mixture was then transfemed to a
degassed and septum sealed lOmrn NMR tube via a cannula. Samples were usually
made as concentrated as possible (usually;:: 0.5g polyselenide I lOmL) to acquire high
quality spectra in a relatively short time. NMR spectra were run immediately at 300K.
Reactions between [SexJ2- and P 4·
The synthetic reactions between P 4 and the polyselenide species are reported in
detail in section 7 .32. General observations of the resulting reaction mixtures are
summarised in Figure 7.1, whilst the characteristic resonances of their 31 P NMR spectra
228
are shown in Figure 7.2. In the following description, the various resonances are
labelled by their chemical shift (ppm) at 300K. In analysing the following 3lp spectra it
should be emphasised that in order to be clear and unambiguous about how many P
resonances ~here are for each species the variation of intensities of the resonances should
be considered.
Figure 7.1 Observations for the reactions between polyselenides and P4.
P 4 Molar Ratio
2:1
1:2
1:1
Orange/ Red
Red/ Brown
Clear/ Orange
Brown/ Green
Red/ Brown
Red/ Orange
Black ppte
Red/ Brown Black ppte
Black ppte
Red/ Brown
Red/ Brown
Red/ Brown
The 3lp NMR spectrum of the 2:1 reaction between Na2Se3 and P4 in DMF,
showed three sharp resonances at 24, 37, and 60 ppm in a relative ratio of 2:4:1
respectively (Figure 7 .2). The poor signal-to-noise ratio in this spectrum (ca. 5000
scans) precluded the observation of any nJ(31 P-77Se) coupling. The bright transparent
orange I red solution showed no precipitation .
229
The 1:1 reaction between Na2Se3 and P4 provided a well resolved 31p NMR
spectrum after only 1000 scans (Figure 7 .3a). Two sharp major resonances, at 30 and 39
ppm in a 1 :3 ratio, and a third much smaller resonance was recorded at 55 ppm. The
resonance at 39 ppm displayed doublet satellites of 4% intensity of the central resonance
with coupling of ca. 760Hz indicative of 1J(3lp_77Se) (see discussion). Again no
precipitation was observed in this red I brown reaction mixture. Whilst it can be see from
Figure 7.2 that the three resonances observed here have similar chemical shifts to those
observed in the 2:1 mixture above, the diminished intensities of the 55 and 30 ppm
suggest that the three resonances do not belong to one species. This is confirmed by
examining the 2:1 reaction between Na2Ses and P4 (described below) where the 37 ppm
resonance was observed independently of the other two resonances. Furthermore by
examining the 1:1 reaction between Na2Ses and P4 (described below) where the 40 and
30 ppm resonances are observed without evidence of a third resonace ca. 60 ppm.
The third reaction in this series, the 1:2 reaction between Na2Se3 and P4 generated a
detailed 31p NMR spectrum after ca. 2000 scans (Figure 7.2). Four sharp major
resonances, at 39, 49, 63 and 79 ppm were observed in a 1:8:4:1 ratio. A doublet
satellite (ca. 4% intensity) with a one bond coupling of ca.760 Hz, was observed on the
"63" resonance. This clear orange reaction mixture provided no precipitant. Here the
fourth resonance at 79 ppm is again unrelated to any other species observed thus far.
Reaction between Na2Ses and P4.
The reactions between Na2Ses and P4 typically provided reaction mixtures that had
a darker hue than the respective Na2Se3 reaction mixtures. No precipitation was
observed in the following series of reactions.
The 3I P NMR spectrum of the 2: 1 reaction between N a2Ses and P 4 provided a
single sharp resonance at 37 ppm that displayed a doublet satellite ( 4% intensity) of ca.
762Hz. Attempts to isolate this single species (see experimental) with the counterions
Bu4N+ and Ph4P+ were unsuccessful.
Figure 7 .3a.
31p NMR spectrum of Na2Se3 + P4 (1:1) in DMF at RT.
Figure 7.3b
31PNMRspectrumofNa2se5 + P4 (1:1) in DMFatRT.
70 60 50 40 30 20 10 ppm
100 80 60 40 20 0 ppm
230
The 1:1 reaction between Na2Ses and P4 displayed two sharp resonances in its 3lp
NMR spectrum (Figure 7.3b) at 40 and 30 ppm in a 3:2 ratio. The resonance at 40
displayed a similar coupling pattern to that of the 37 ppm resonance previously with a 4%
doublet doublet and 1J(3lp_77Se) coupling of ca. 760Hz.
Finally, the red/orange reaction mixture containing the 1:2 reaction between Na2Ses
and P4 showed three sharp major resonances at 32, 49 and 63 ppm. The resonance at
49 displayed doublet satellites of ca. 4% intensity of the central resonance with coupling
of 830 Hz, whilst the resonance at 63 ppm also displayed doublet satellites of ca. 8%
intensity of the central resonance with ca. 740Hz coupling.
Reactions between Na2Se6 and P4.
The reactions between Na2Se6 and P 4 were not as clean as those previously
observed. Precipitation of a black solid occurred in all reaction mixtures. In the
experiments with the ratios 1:1 and 2:1 the resulting supernatants were colourless and
gave no 31p NMR signal. The black solids were amorphous.
The 1:2 reaction between Na2Se6 and P4 generated a red/brown mixture in DMF.
One sharp resonance at 38 ppm was observed in the 31p NMR spectrum that displayed
doublet satellites of ca. 4.0% intensity of the central resonance with coupling of ca. 760
Hz coupling. The spectrum was difficult to obtain as finely suspended black solid
continuously formed. However after 50 000 scans a suitable spectrum was obtained.
Reactions between (Bu4N)2Se6 and P4.
In an attempt to observe other species being produced in the reaction between
[Se6]2- and P4 an alternate polyselenide precursor, (Bu4N)2Se6 was used. The 1:1
reaction between (B04N)2Se6 and P4 generated a red/brown solution in DMF without
evidence of a precipitate. The 31 P spectrum showed several major sharp resonances at
..
Figure 7.4a. 3lp NMR spectrum of (Bu4NhSe6 + P4 (1:1)
inDMFatRT .
100 80 60 40 20 0 ppm
Figure 7.4b. 3lp NMR spectrum of (Bu4N)2Se6 + ·
P4 (1:2) in DMF at RT.
70 60 50 40 30 20 10 0 -10 ppm
- 4.4, 22, 30, 37, 40, 46, 59, 63, 83 ppm respectively (Figure 7.4a). The resonance
at -4.4 ppm displayed two sets of doublet satellites of ca. 4% intensity of the central
peak. The outermost doublet showed coupling of 666 Hz, whilst the innermost
doublet showed coupling of 374Hz. No obvious coupling was obse.rved on the other
signals. It is clear from this spectrum that another new unrelated species at -4.4 ppm
not previously seen is present. This species has been isolated independent of the other
species in the reaction between (Bu4NhSe6 and P4 in the ratio 1:3 (see below)
In contrast, the 31 P spectrum of the 1:2 reaction between (Bu4NhSe6 and P 4
showed only two sharp resonances at -4.4, and 63 ppm (Figure 7.4b) although clearly
these two resonances do not belong to the same species as in the 1 :3 reaction described
next the -4.4ppm resonance occurs without the 63ppm resonance. The resonance at
-4.4 ppm was the same as that seen above, whilst the resonance at 63 ppm showed
doublet satellites of ca. 7.6% intensity of the central resonance with coupling of
740Hz. The intensities of the two resonances at -4.4, and 63 ppm are approximately
1:1.
The reactions between (Bu4NhSe6 and P 4 with ratios 1:3, 1:4, 1:5 all
generated red/brown solutions in DMF. Although their 31p spectra were not identical
they were very similar and showed only one major sharp resonance at -4.4 ppm with
the same coupling pattern as reported above. No precipitation was observed in any of
these reaction mixtures.
231
It is apparent that several intense resonances, indicating major species, and several
less intense resonances, indicating minor species, repeatedly occur throughout these
reaction mixtures (Figure 7.2). In some cases resonances are assigned as indicating the
same species, even though their chemical shifts differ by 2-3 ppm in the different reaction
mixtures.
The resonance assigned as "37" ppm in the 2:1 reaction between NazSe3 +P4
manifests itself in the some of the spectra of the other solutions listed below. It exists as
the only product of the reaction mixtures of the 2:1 reaction between NazSe3 +P4 (37
232
ppm) and the 1:1 reaction between Na2Se6 + P4, (38 ppm) and as the major product of
the 1:1 reaction between Na2Se3 +P4 (40 ppm), whilst it exists as a minor product in the
reactions between the 1:2 reaction between Na2Se3 +P4 (40 ppm) and the 1:1 reaction
between (Bt14N)2Se6 + P4 (39 ppm). It does not exist at all in the other reaction mixtures
described. The resonance is characterised by doublet splitting pattern with satellite
intensities approximately of3.8% and coupling of760 Hz. This spectrum suggests that
under these conditions a species is formed containing a phosphorus atom coupled to one
type of selenium atom. Attempts to crystallise this species from the 2: 1 reaction mixture
ofNa2Se3 + P4 by layering MeCN solutions containing Bu4N+ and Ph4f>+ cations did
not result in precipitation of solid (see experimental section).
The resonance assigned as "60" ppm in the 2:1 reaction between Na2Se3 +P4 also
manifests itself in the some of the other spectra. It exists as a major product in the
reaction mixtures of 1:2 Na2Se3 +P4 (63 ppm), 1:2 Na2Ses +P4 (63 ppm), and 1:2
(B04N)2Se6 + P 4 (63 ppm). The 60 ppm resonance also exists as a minor product in the
reaction mixture of 1:1 (Bu4N)2Se6 + P4 (63 ppm). The resonance is characterised by
doublet splitting pattern with satellite intensities of approximately 8.0% and coupling of
740Hz. The 8% doublet suggests that a species with phosphorus atom coupled to two
identical selenium atoms is formed. Attempts to crystallise this species from the 1:2
reaction between Na2Se3 +P 4 using the procedure outlined above failed, with no solid
precipitating (see experimental section).
The two resonances at 49 ppm in the 1:1 Na2Ses +P4 spectrum and the resonance at
-4.5 ppm, first observed in the 1:1 reaction mixture of (Bu4N)2Se6 + P4 also showed
coupling to selenium. The resonance at 49 ppm is characterised by doublet splitting
pattern with satellite intensities of approximately 3.8% and coupling of 835Hz, and
occured as a major product in the 1:3 reaction mixture of N a2Se3 + P 4 ( 49 ppm ), and a
minor product in the 1:1 reaction mixture of (Bu4N)2Se6 + P4. This suggests a single
type of phosphorus atom surrounded by one type of selenium atom. The resonance at
-4.5 ppm was only observed in the reaction mixtures containing the (B04N)2Se6 salt. In
Figure 7.5 Structure of the fP2Seg]2- anion.
---
Figure 7.6a. 31p NMR of (Bu4NhP2Ses in DMF at RT.
-2 -3 -4 -5 -6 -7 -8 ppm
Figure 7 .6b. 77Se NMR of (Bu4NhP2Seg in DMF at RT.
Inset. Expansion of doublet of doublets at 05e 732 ppm.
L .
100 750 710 550 100 550 PPH
233
all of these reactions the resonance at -4.5 ppm occurred as one of the major species. The
line was characterised by an interesting coupling pattern , best described as a central
singlet with three pair of doublets each approximately 3.8% intensity of the central
resonance. The splitting of each of these doublets is 4Hz. The oute~ost doublet
coupling was 666Hz, the next outermost being 374Hz and the innermost coupling being
34Hz.
Filtration of the 1 :2 reaction ~ixture resulted in the precipitation of a yellow
crystalline product characterised by single crystal X-Ray diffraction as (BU4N)2[P2Seg],
the first reported anionic phosphorus selenide species (see section 7 .4.2). Interestingly,
crystallisation of the species in the unfiltered reaction mixture did not occur, but was
initiated by filtration through the Schlenk frit (see discussion).
7 .4.2 Crystallographic Results
The structure of (Bu4Nh[P2Seg] (Figure 7.5), consists of a six-membered P2S~
ring with each phosphorus atom containing two terminal selenium atoms formally placing
each phosphorus atom in the pv oxidation state. The ring P-Se distances are
approximately 0.15 A longer than the terminal P-Se distances, as would be expected.4,5
The six membered ring is in the chair confom1ation with the phosphorus atoms 1,4
relative to one another. The structure is nearly identical with the analogous sulfide
molecule, P2Ss2-.8 [P2Seg]2- has crystallographic point group symmetry of l.
7 .4.3 NMR Spectroscopy of (Bu4N)2[P2Seg]
The 3lp NMR spectrum of (Bu4NhfP2Seg] in DMF, is the same as that observed
in the reaction mixtures - i.e a central singlet at 8 -4.5 ppm coupled to three pair of
doublets approximately 4% intensity of the central resonance. Each of these satellites is a
doublet with 4Hz splitting (Figure 7 .6a). The outermost doublet coupling is 666 Hz, the
next outermost being 374Hz and the innermost coupling being 34Hz. The larger
coupling of 666 Hz is assigned to the terminal phosphorus selenides due to the shorter P-
234
Se bond distance and increased double bond character (see discussion). The coupling of
374Hz is assigned to the lJ(P-Se) coupling between the "ring" selenium atom directly
bound to one phosphorus atom, whilst the coupling of 34Hz is assigned to the 2J(P-Se)
coupling between the second "ring" selenium atom and the same phosphorus atom. The
coupling of 4Hz is assigned to the 3J(P-P) coupling between the non-equivalent
phosphorus atoms.(see discussion)
The 77Se NMR spectrum of (Bt14N)2[P2Seg] in D:MF, displayed a doublet of
doublets at Ose 732 and Ose 432 (Figure 7.6b). The coupling between the two singlets
was 666Hz, whilst the coupling between the pair of doublets was 374Hz, for the
outermost coupling and 34 Hz for the innermost coupling. The assignment of these
coupling patterns is the same as those reported above.
7. 5 Discussion
Typical values for 1J(77Se 3lp) are listed below in Table 7.1. In the molecules with
a P=Se bond are associated with a substantially greater magnitude ( ca. 400Hz) than the
single bond. There is also a clear dependence on the electron - withdrawing ability of the
substituents attached to the P=Se link with greater substituent electronegativity leading to
an increase in 1J(3lp77Se). However the single bond 77Se-31p couplings are less well
studied and depend substantially upon the nature and orientation of adjacent
substituents.5,48,49,50,51
Table 7.1. Selected One-Bond Couplings in Hz of Selenium
Phosphorus compounds.
Compound 1J(77Se3lp) {Hz) Reference
MezPSeMe 205 (P-Se-Me) 48
MezP(S)SeMe 341 (P-Se-Me) 48
P2Ses 244, 284 (P-Se) 5
P4Se3 256, 316 (P-Se) 52
Me~PSe 684 (P=Se) 53
(MezNhPSe 805 (P=Se) 53
(Me0)3PSe 963 (P=Se) 53
(Et0)3PSe 935 (P=Se) 54
(2-MeCt#4)3PSe 708 (P=Se) 55
Longer range couplings between phosphorus and selenium have rarely been
reported. In P4Se3 the 2J(77Se3lp) is 57Hz. 56 Similiarly the 2J(17Se_3Ip) in
[PtSe2CN-i-Bu2)(PPh3)2]+ is 90Hz 57 and 17.0, 16.5 and 14.7 Hz in the respective
molecules P3Se4Cl, P3Se4Br and P2Ses 5
The P2Seg2- molecule can be considered to be a mixture of seven isotopomers,
with six of them containing one NMR active 77Se atom as shown in Figure 7.7.
235
The seventh isotopomer contains only NMR inactive 76Se atoms which, together with the
symmetry of the molecule makes the two P atoms magnetically equivalent. It is this
isotopomer that gives rise to the central singlet at -4.4 ppm in the 3Ip NMR spectrum
(Figure 7.6a).
Ignoring the fine structural detail for the moment the 3lp NMR spectrum of
(Bu4N)zP2Seg shows three sets of doublets around the central resonance all having an
intensity of 8% of the central resonance. The large 666Hz coupling is assigned to the
terminal phosphorus selenides which show a shorter P-Se distance and increased double
236
bond character. At room temperature there is fast exchange occurring between these
terminal selenides making them equivalent. No attempt was made to observe the
predicted fluxional behavior of these Se atoms at lower temperature. The magnitude is
consistent V{ith that seen for the P=Se bond in Me3PSe. Similiarly, the 374Hz coupling
is assigned to the Se atom bound directly to one of the phosphorus atoms forming a
single P-Se bond. The magnitude of this coupling is consistent with that observed for
the single P-Se bond in P4Se3 and P2Ses. There is insufficient evidence in the literature
to unequivocally assign the coupling of 34 Hz, although coupling between the same
phosphorus atom and the second 'ring' selenium atom would be a two bond coupling and
in P4Se3 the 2J(77Se-31p) is 57Hz. Therefore it is proposed that this coupling is
attributable to the 2J(77Se-31p ).
The intensities of the satellites observed in the 3Ip NMR spectrum of
(Bu4NhP2Seg can be confirmed by considering the following calculations.
If one 3Ip atom is coupled to one type of 77Se atom ( 7.6% abundance) a doublet
with lines each of 3.8% intensity of the central signal is predicted. If one 3Ip atom is
coupled to two Se atoms of the same type (arising from the symmetry of the molecule),
then a doublet is still expected, but the intensities will be double i.e 2 x 3.8% = 7.6% ca.
8%. This is what is observedin the 31p spectra (Figure 7.6a). The 1:2:1 triplet expected
for a 77Se-3lp_77Se coupling would have only intensities of 0.15%, 0.30%, 0.15%
respectively, i.e 7% x 7% I 4 = 0.15%. However, because of the low natural abundance
of 77Se, the triplet that is expected to be present is well below the detection limits of the
experiments conducted here.
Finally, the doublets in the 31p spectrum show a 4Hz coupling. This is assigned to
a 3J(3lp_3lp) coupling arising from the six isotopomers in Figure 7.7. Note that in each
of them one of the P atoms is directly bound to a 77Se atom, and thus the chemically
equivalent P atoms become magnetically inequivalent.
237
Figure 7. 7. Possible locations of the spin active 77 Se atom in the P2Seg2- molecule.
or
or
or
238
7. 6 Experimental
The molecular white phosphorus was used without further purification (M&B).
Chunks of the phosphorus were cut and weighed under water then transferred to a
Schlenk fl'!sk and pumped dry under vacuum (0.1 mm Hg). Residues were disposed of
by ignition in a sand bath placed in the fume hood. Manipulations of the P 4 were all
carried using forceps inside the fumehood. In all cases exposure of P 4 to the atmosphere
was minimised.
Other chemicals were used as previously described.
7.6.1 Synthesis of (Bu4N)2Se6
(BU4N)2Se6 was prepared as described in Chapter 4.
7.6.2 Synthesis of (Bu4NhP2Ses.CS2
A solution of white phosphorus (0.89 g, 29 mmol) in CS2 ( 40 ml) was added
slowly with stirring to a solution of (Bu4N)2Se6 (13.8 g, 14.4 mmol) in MeCN (100 ml).
The mixture was stirred for 10 minutes at room temperature generating a yellow/brown
solution. A small amount of black selenium powder ( < 0.5 g) precipitated during this
time. The reaction mixture was then filtered through a fast Schlenk filter and on standing
at room temperature for several minutes yellow needles crystallised. These were collected
and recrystallised from hot MeCN (40 ml) resulting in a bright yellow crystalline solid
(Yield 10.1 g, 56% based on (Bu4N)2Se6 ). The yellow needles appear relatively air
sensitive with complete oxidation to black Se observed after 2 hrs of direct exposure to
the atmosphere. Crystal composition was confirmed by X-ray diffraction.
Anal calcd for C33H72N2P2SesS2 ( M = 1253.68).C, 31.59; H, 5.74; N, 2.23
Found C, 32.41; H,5.63; N, 2.19
7. 6. 3 Solutions for NMR study
7.6.3.1 Reaction between Na2Se3 and P4.
a) Ratio 2:1
c) Ratio 1:2 Na2Ses: P4
Reaction was prepared as in (a) except that 0.16 g P4 (5.2 mmol) in CS2 (5 ml)
solution was added.
7 .6.4 Reaction between Na2Se6 and P4
240
Na2Se6 was prepared as described in Chapter 4 by reacting Se powder (1.0 g, 12
mmol) and Na (0.1 g, 4.4 mmol) in liquid NH3 (30 ml). The NH3 (1) was distilled off
and the resulting solid was dissolved in DMF (15 ml). A solution ofP4 (0.7 g, 2 mmol)
in CS2 (5 ml) was slowly added with stirring over a 15 minute period, resulting in a
colour change from dark green to red/brown. A 10 ml aliquot was transferred to a 10 mm
NMR tube. 31p NMR was then carried out on the sample.
7. 7 Reaction between (Bu4N)2Se6 and P4
a) Ratio 1:1
To a solution of (Bt14N)2Se6 (1.0 g, 1 mmol) in DMF (15 ml) was slowly added a
solution of white phosphorus (0.03 g, 1 mmol) in CS2 (5 ml). The mixture was stirred at
room temperature for 15 mins and changed colour from an initial dark green to
red/brown. A 10 ml aliquot was transferred to a sealed, nitrogen flushed 10 mm NMR
tube. 31 P NMR was then carried out on the sample.
b) Ratio 1:2
Reaction was prepared as in a) except with the addition of P4 (0.06 g, 2 mmol).
c) Ratio 1:3
Reaction was prepared as in a) except with the addition ofP4, (0.09 g, 3 mmol).
d) Ratio 1:4
Reaction was prepared as in a) except with the addition of P4 (0.12 g, 4 mmol).
241
7. 8 Conclusion
In summary it can be seen that these P 4 I Sex2- mixtures are generally complex
containing many species that are apparently unrelated as detennined by their variable
intensities. With the exception of (Bu4N)2[P2Seg] none of the other .species have been
isolated and without further experimentation structural characterisation of them is not
possible. However due to the simple nature of the coupling patterns observed around
some of the resonances the species are almost certainly not as complex as those described
at the start of this chapter.
7. 9. Future Work
Despite the large number of polyselenide compounds synthesised during the course
of this work by myself and others, most aspects of their reactivity remain unexplored.
Furthermore, virtually nothing is known about the mechanistic formation of metal
polyselenides or why MSe4 rings dominate this chemistry. These issues need to be
addressed in order to further understand the complex chemistry involved in metal
polyselenide formation.
The pioneering metal polytelluride studies presented here should be developed to
generate systematic syntheses for this under explored area of chemisty.
Finally, metal complexation of the [P2Seg]2- anion should be attempted in an effort
to syntheise potentially useful materials as well as attempts to isolate some of the
interesting species observed in the P 4 I Se/- reaction mixtures.
242
7.10 References
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245
Appendix A
Crystallographic Details of Structures
Table 1. Crystallographic data and numerical details of the ref~ement of the structure of
(Ph4P)2[Ni(Se4)2]
Formula, formula mass C48H40NiP2Se8, 1369.2
Space group P2I
a! A 10.934(6)
b!A 15.531(3)
c/A 14.313(7)
~/ 0 92.54(2)
VJA3 2428(2)
Temp/ °C 21(1)
z 2
Dcaldgcm-3 1.87
Radiation, AI A MoKa, .7107
Jl I cm-1 64.3 Scan mode 8/28
ro scan angle (0. + O.tan 8)
No of intensity measurements 4590
Criterion for observed reflection I/cr(I) > 3
No of independent obsd reflections 2557
No of reflections (m) and variables 2557, 291
(n) in final refinement R = z;mi.1FI/2:m1F0 1 0.037 Rw = [2:ffiwl.1f'l2;z:mwiF0 12] 1/2 0.041 s = r~:mwl.1f'l2/(m-n)] l/2 1.29 Largest peak on final diff. map/eA-3 0.5
Table 2. Numerical details of the solution and refinement of structure
(Ph4Ph[Pb(Se4)2]
Formula, formula mass C48H40P2PbSeg, 1517.7
Space group PT
a! A 10.349(3)
b!A 10.759(4)
c!A 23.034(9)
aJO 77.03(3)
~/ 0 83.52(3)
y/ 0 78.78(3)
V/A3 2445(2)
Temp/ °C 21(1)
z 2
Dcald gcm-3 2.06
Radiation, AI A MoKa, .7107
Jl I cm-1 94.7
Scan mode 6/26
ro scan angle (0. + 0. tan 6)
No of intensity measurements 6781
Criterion for observed reflection I/cr(I) > 3
No of independent obsd reflections 4354
No of reflections (m) and variables (n) 4354, 532
in final refinement R = I,ml~FI/I,miF0 1 0.037
Rw = [I,ffiwi~I2/I,mwiF012] 1/2 0.044
s = [I,mwi~F12J(m-n)] 1/2 1.38
Largest peak on final diff. map/eA -3 2.0
246
247
Table 3. Crystallographic data and numerical details of the refinement of the structure
of(Ph4Ph[Cu4(Se4)2(Ses)]
Formula, formula mass
Crystal description
Space group
a! A b/A c!A ~/ 0
VfA3 Temp/ °C
z Dcaldgcm-3
Radiation, 'AI A ~I cm-1
Crystal dimensions
Scan mode
28max• o
ro scan angle
No of intensity measurements
Criterion for observed reflection
No of independent obsd reflections
No of reflections (m) and variables (n)
in final refinement R = LmiL1FI/l:mlF 0 1
Rw = [LmwiL1FI2/ImwiF0 12] 1/2
s = [Lmwi~FI2/(m-n)] 1/2
Crystal decay
Max, min transmission coefficients
R for merging reflections
CU4(Se4)2.4(Ses)0.6 (PPh4)2 1927.9
(11-2)(-1-7-3){ 001 }(-223)(2-1-4)
P2}/C
12.779(5)
13.109(2)
34.47(1) 102.24(2)
5643(3)
21 (1)
4 2.27
MoKa, .7107
96.3
0.15 X 0.15 X 0.06
8/28 44
(0.45 + 0.35tan 8)
7427
I/cr(l) > 3
3030
3030, 338
0.043
0.049
1.49
1 to 0.96
0.58, 0.29
0.044 (77 refl)
248
Table 4. Crystallographic data and numerical details of the refinement of the structure
of(Ph4Ph[A~(Se4h(Ses)]
Formula, formula mass
Crystal d~scription
Space group
a! A b!A c!A ~/ 0
VJA3 Temp/ °C
z Dcald gcm-3
Radiation, AI A ~I cm-1
Crystal dimensions
Scan mode
28max• o
w scan angle
No of intensity measurements
Criterion for observed reflection
No of independent obsd reflections
No of reflections (m) and variables (n)
in final refinement
R = :Lm1.1FI/:Lm1F0 1
Rw = [:Lmwi6FI2/ImwiF0 12] 1/2
s = [:Lmwi6FI2/(m-n)] 1/2
Crystal decay
Max, min transmission coefficients
Range of indices
R for merging reflections
Ag4(Se4)2.1 (Se5)0.9 (PPh4)2 2128.9
{001 }(110)(-1-10)(1-10)(-5 11-2)
P2t/C
12.798(3)
13.348(1)
34.733(8)
102.38(1)
5795(2)
21(1)
4 2.44
MoKa, .7107
94.3
0.16 X 0.16 X 0.20
8/28
44
(0.45 + 0.35tan 8)
7623
I/cr(l) > 3
4130
4130, 343
0.042 0.051
1.68
I to 0.98
0.29, 0.16
-13 <= h <= 13
-14 <= k <= 0
0 <= 1 <=36
0.017 (93 refl)
249
Table 5. Crystallographic data and numerical details of the refinement of the structure of
(Ph4Ph[Cd(Se4hl 1
Formula, formula mass
Crystal description
Space group
a! A b/A c!A ~/ 0
VJA3 Temp/ oc z Dcald gcm-3
Radiation, AI A ·J..L/ cm-1
Crystal dimensions
Scan mode
28max• o
ro scan angle
No of intensity measurements
Criterion for observed reflection
No of independent obsd reflections
No of reflections (m) and variables (n)
in final refinement R = :Lmll1FI/:Lm1F0 1
Rw = [:Lmwli1FI2/:Lmw1F0 12] 1/2
s = [:Lmwli1FI2f(m-n)] 112
Crystal decay
Max, min transmission coefficients
C48H40CdP2Se8, 1422.9
{ 010} { 001} { 100}(110)(1111)(011)
P2I
10.185(5)
14.137(4)
34.39(2)
92.22(2)
4948(4)
21(1)
4 1.91
MoKa., .7107
63.6
0.12 X 0.22 X 0.05
8/28
40
(0.40 + 0.35 tan 8)
5232
1/cr(I) > 3
3459
3459,486
0.046 0.055
1.81
none
.70, .45
1 Crystal structure determination was made on crystal of (Ph4Ph[Cd(Se4)i)
prepared by H. Banda.
250
Table 6. Crystallographic data and numerical details of the refinement of the structure of
(Ph4P)z[Sn(Se4)3] 1
Formula, formula mass
Crystal d~scription
Space group
a! A b!A c!A ~/ 0
V/A3
Temp/ °C
z Deale! g cm-3
Radiation, AI A Jl I cm-1
Crystal dimensions
Scan mode
28max• o
ro scan angle
No of intensity measurements
Criterion for observed reflection
No of independent obsd reflections
No of reflections (m) and variables (n)
in final refinement R = z;mi.1FI/I:m1F0 1
Rw = rz:mwi.1FI2/I;mw1F0 12] 1/2
s = [l;n1wiL\FI2/(m-n)] 1/2
Crystal decay
Max, min transmission coefficients
C48H40P2Se12Sn, 1745.0
{012}{001}{01-1}{102}{11-2}{-112}
P21/C
13.307(3)
11.696(2)
34.699(9)
98.82(1)
5337(2)
21 (1)
4 2.17
MoKa, .7107
86.8
0.20 X 0.20 X 0.10
8/28
44
(0.40 + 0.35 tan 8)
6524
1/cr(I) > 3
4112
4112, 568
0.029
0.035
1.21
none
0.37, 0.21
1 Crystal structure determination was made on crystal of (Ph4P)z[Sn(Se4)3]
prepared by H. Banda.
251
Table 7. Crystallographic data and numerical details of the refinement of the structure of
(Ph4PhfHg(Se4)z] 1
Formula, formula mass
Crystal description
1-11)(0-11)
Space group
a! A b!A c!A wo V/A3
Temp/ °C
z Deale! g cm-3
Radiation, AI A J.L/ cm-1
Crystal dimensions
Scan mode
28max• o
ro scan angle
No of intensity measurements
Criterion for observed reflection
No of independent obsd reflections
No of reflections (m) and variables (n)
in final refinement R = LmiL1FI!LmiF0
Rw = [Lmwl.!lFI2/LmwiF0 12] 1/2
s = [Lmwlt1FI2/(m-n)] 1/2
Crystal decay
Max, min transmission coefficients
C4sH4oHgP2Seg, 1511
{ 110} ( 1-10} {001 }(7-1-2)(-101)(-
P21/C
23.128(7)
20.311(5)
23.956(8)
119.43(1)
9801(5)
21(1)
8 2.05
MoKo:., .7107
91.2
0.20 X 0.20 X 0.25
8/28
40
(0.50 + 0.35 tan 8)
9778
1/cr(I) > 3
4609
4609, 527
0.041 0.047
1.44
1 to 0.92
.28, .16
1 Crystal structure determination was made on crystal of (Ph4P)z[Hg(Se4)i)
prepared by H. Banda.
252
Table 8. Crystallographic data and numerical details of the refinement of the structure of
(Ph4Ph[Co3(Se4)6.(DMFhl
Formula, formula mass
Crystal description
Space group a! A b/A c/A a;o wo y/o
VJA3 Temp/ °C
z Dcalcfgcm-3
Radiation, AI A Jl I cm-1
Crystal dimensions
Scan mode
28max• o
ro scan angle
No of intensity measurements
Criterion for observed reflection
No of independent obsd reflections
No of reflections (m) and variables (n)
in final refinement R = ImiLWI/LmiF0 1
Rw = [LmwiLWI2/ImwiF0 12] 1/2
s = [LmwiLWI2/(m-n)] 1/2
Crystal decay
Max, min transmission coefficients
Lru:gestpeak in final diff. map /eA.-3
C7sH74Co3N202P3Se24. 3236.2
{ 1 00} { 0 1 0} { 00 1 }
Pi 13.851(8)
14.125(9)
25.60(1)
105.52(3)
91.86(4)
95.11(3)
4798(5)
21 (1)
2 2.24
MoKa, .7107
96.2
0.26 X 0.05 X 0.09
8/28
40
(0.50 + 0.35tan 8)
8955
l/cr(I) > 3
4821
4821, 550
0.044
0.051
1.57
none
0.62, 0.41
1.7
253
Table 9. Crystallographic data and numerical details of the refinement of the structure of
(Ph4Ph[Ses]
Formula, formula mass
Space group
a! A b/A ciA 1310
VJA3
Temp/ oc z Dcalcfgcm-3
Radiation, AI A Jl/ cm-1
Crystal dimensions
Scan mode
28max• o
ro scan angle
No of intensity measurements
Criterion for observed reflection
No of independent obsd reflections
No of reflections (m) and variables (n)
in final refinement R = Im1.1.FI/l:m1F0 1
Rw = [l:mwi.1.FI2JimwiF0 12] 1/2
s = [l:mwi.1.FI2f(m-n)] 112
Crystal decay
Max, min transmission coefficients
Largest peak in final diff map /eA -3
R for 380 multiple measurements
SesP2C4sf4o, 1073.6
P2I
9.617(3)
17.092(4)
13.725(5)
105.18(1)
2177(1)
21(1)
2 1.64
MoKa., .7107
42.7
0.30 X 0.17 X 0.17
8/28
45
(0.50 + 0.35tan 8)
2567
1/cr(I) > 3
3223
3223,495
0.026
0.032
1.13
1 to 0.53
0.45, 0.39
0.29
0.027
254
Table 10. Crystallographic data and numerical details of the refinement of the structure of
a (Ph4P)2[Se(Seshl
Formula, formula mass
Space gropp
a! A b!A ciA ~/ 0
v;A3 Temp/ °C
z Radiation, ')J A Scan mode
Criterion for observed reflection
No of independent obsd reflections
No of reflections (m) and variables (n)
in final refinement
Seu(PPh4)2, 1547.4
P2t/C
9.961(4)
13.883(4)
19.137(7) 107.93(2) 2518(1)
21 (1)
2
MoKa, .7107
9/29 1/cr(I) > 3
1920
1920, 277
R = L,mlilFIII,miF0 1 0.039
Rw = [L,mwltlFI2/L,mwiF0 12]112 0.044
s = [L,mwlilFI2/(m-n)] 1/2 1.46
Lar_gest peak on final diff. map/eA -3 1 .4
255
Table 11. Crystallographic data and numerical details of the refinement of the structure of
~ (Ph4Ph[Se(Sesh]
Formula, formula mass
Space group
a! A b!A c!A ~/ 0
VJA3 Temp/ °C
z Radiation, /J A Scan mode
Criterion for observed reflection
No of independent obsd reflections
No of reflections (m) and variables (n)
in final refinement
R = _LmltlFI/I,miF0 1
Rw = [2:mwltlFI2/LmwiF0 12] 1!2
s = [_Lmwi.1FI2f(m-n)]1!2
Se11 (PPh4)2, 1547.4
P21/C
12.869(5)
14.818(6)
15.780(6)
121.93(2)
2554(1)
21(1)
2
MoKa, .7107
8/28
1/a(I) > 3
2191
2191, 277
0.051
0.061
2.08
256
Table 12. Crystallographic data and numerical details of the refinement of the structure of
(Bu4NhP2Seg
Formula, formula mass
Crystal description
Space group
a! A b/A c/A 1310 V/A3
Temp/ °C
Dobsl g cm-3
z Dcald gcm-3
Radiation, ')J A ~I cm-1
Crystal dimensions
Scan mode
28max• o
ro scan angle
No of intensity measurements
Criterion for observed reflection
No of independent obsd reflections
No of reflections (m) and variables (n)
in final refinement R = L,ml~FI/L,mlF01
Rw = [L,mwl~FI2/L,mwiF012] 1/2
s = [L,mwi~FI2/(m-n)] 1/2
Crystal decay
Max, min transmission coefficients
Se4PC1sH39Nz, 630.3
{ 100){011 }{0-11)
P2J/C 9.680(6)
16.957(5)
16.185(11)
96.84(3)
2638(3)
21 (1)
4
1.59
MoKa, .7107
55.72
0.30 X 0.17 X 0.17
8/28
45
(0.50 + 0.35tan 8)
2567
1/cr(I) > 3
1565
1565, 226
0.039
0.048
1.63
1 to 0.53
0.45, 0.39
257
Table 13. Crystallographic data and numerical details of the refinement of the structure of
(Ph4P)fCpMo(Se4)2]
Formula, formula mass
Crystal description
Space group
a! A b/A ciA (J}O
~/ 0
y/ 0
V/A3
Temp/ °C
z DcaJcfgcm-3
Radiation, AI A ~I cm-1
Crystal dimensions
Scan mode
28max• o
co scan angle
No of intensity measurements
Criterion for observed reflection
No of independent obsd reflections
No of reflections (m) and variables (n)
in final refinement
R = rmt1FI/LmiF0 1
Rw = [Lmwi.1FI2/Imw1F0 12] 1/2
s = [Lmwi.1FI2/(m-n)] 1/2
Crystal decay
Max, min transmission coefficients
Range of indices
Mo(Se4)2(C5H5) (PPh4) 1132.1
{ 100) {001 }(0-1-1)(01-2)(010)
PI 8.778(3)
11.500(4)
16.710(6)
104.56(2)
97.02(2)
93.77(2)
1612.3(9)
21(1)
2
2.33
MoKa., .7107
94.1
0.20 X 0.25 X 0.08
8/28
50
(0.45 + 0.35tan 8)
5666
1/cr(I) > 3
3757
3757, 364
0.030
0.036
1.29
none
0.56, 0.30
-10 <= h <= 10
-13 <= k <= 13
-19 <= 1 <= 19
258
Table 14. Crystallographic data and numerical details of the refinement of the structure of
(Ph4P)}[Pt(Se4)3]
Formula, formula mass ·
Crystal description
Space group
a! A b!A ciA 1310 VJA3
Temp/ °C
z Dcald gcm-3
Radiation, AI A J.L/ cm-1
Crystal dimensions
Scan mode
28max• o
ro scan angle
No of intensity measurements
Criterion for observed reflection
No of independent obsd reflections
No of reflections (m) and variables (n)
in final refinement
R = 2:m1D.FI/:LmlF 0 1
Rw = [2:mwiD.FI2J:LmwiF0 12] 1/2
s = [2:mwiD.FI2J(m-n)] 1/2
Crystal decay
Max, min transmission coefficients
Largest peak in final diff map /eA. -3
R for 160 multiple measurements
Se12PtP2C4sf4o, 1073.6
{ 1 00} { 0 11 } { 0-11 }
P2t/C
13.131(4)
11.651(3)
34.92(1)
98.14(1)
5288(3)
21(1)
4 1.64
MoKa, .7107
42.7
0.30 X 0.17 X 0.17
8/28
45
(0.50 + 0.35tan 8)
2567
1/cr(l) > 3
3117
3117,371
0.058
0.071
2.15
1 to 0.53
0.45, 0.39
2.5
0.13
Appendix B
Table 1: X-ray Powder Diffraction Patternl of (Ph4Ph[Mn(Se4hl
28
8.1
8.7
8.9
9.5
10.0
10.3
10.8
11.1
11.8
12.1
12.6
13.0
13.6
16.2
crystallised from DMF I MeCN
d(A) Intensity 28 d(A)
11.04 m 18.4 4.82
10.16 w 1 R.6 4.77
9.93 vw 21.0 4.23
9.30 w 21.5 4.13
8.84 m 22.2 4.00
8.58 m 22.9 3.88
8.19 m 23.5 3.78
7.96 m 27.4 3.25
7.49 m 27.9 3.20
7.31 m 28.5 3.13
7.02 w 28.6 3.12
6.80 mw 29.4 3.04
6.51 mw 29.7 3.00
5.47 mw
1 Cu-K radiation used: 'A= 1.54056 A a
lntensi!Y_
m
m
s
m
m
w
s
m
m
m
m
m
s
259
Appendix C
List of Publications
The Characterisation of [HSe]- and fSexF- ions by 77Se NMR.
J. Cu~ick and I. Dance, Polyhedron, 1991, 10,2629
Synthesis and structure for the first Cobalt Polychalcogenide complex
[Co3(Se4)6]3-, crystallised with Ph4P+.
J. Cusick, M.L. Scudder, D.C. Craig, and I.G. Dance, Aust. J. Chern. 1990,
43, 209
Synthesis and structures of molecular copper and silver polyselenide complexes
[M4(Se4)x(Sesh-xJ2·.
J. Cusick, M.L. Scudder, D.C. Craig, and I.G. Dance, Polyhedron, 1989, 8,
1139
Synthesis and x-ray structures of molecular polyselenide complexes [M(Se4)i)2-.
M = Zn, Cd, Hg, Ni, Pb.
R.M.H. Banda, J. Cusick, M.L. Scudder, D.C. Craig, and I.G. Dance,
Polyhedron, 1989, 8, 1995
Syntheses and structures of anionic metal polyselenide complexes
[(CsHs)Mo(Se4)z]· and [Sn(Se4))]2-, crystallised with P~P+.
R.M.H. Banda, J. Cusick, M.L. Scudder, D.C. Craig, and I.G. Dance,
Polyhedron, 1989, 8, 1999
The formation, characterisation and reactions of fP2Ses]2-.
J. Cusick, J. Hook, and I.G. Dance, In Preparation
260
91.
West Ryde ltl4 Phone: 107 eo2e
L-/5
