Pi-Bonding and Reactivity in Transition Metal Nitrosyl ... · Pi-Bonding and Reactivity in...

Pi-Bonding and Reactivity in Transition Metal Nitrosyl Complexes

by

KEVIN MICHAEL SMITH

B.Sc, The University of Toronto, 1992

A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF

T H E REQUIREMENTS FOR T H E D E G R E E OF

DOCTOR OF PfflLOSOPHY

in

T H E F A C U L T Y OF G R A D U A T E STUDIES

Department of Chemistry

We accept this thesis as conforming

to the requjr^d standard.

T H E UNIVERSITY OF BRITISH COLUMBIA

January 1998

© Kevin M . Smith, 1998

In presenting this thesis in partial fulfilment of the requirements for an advanced

degree at the University of British Columbia, 1 agree that the Library shall make it

freely available for reference and study. 1 further agree that permission for extensive

copying of this thesis for scholarly purposes may be granted by the head of my

department or by his or her representatives. It is understood that copying or

publication of this thesis for financial gain shall not be allowed without my written

permission.

Department of

The University of British Columbia Vancouver, Canada

Date

DE-6 (2/88)

11

Abstract

This Thesis investigates the effect of 7t-bonding interactions on the electronic

configuration and reactivity of Cr and M o organometallic complexes that contain C p '

( C 5 R 5 ; R = H (Cp), C H 3 (Cp*)) and N O ligands.

From metrical parameters obtained from X-ray crystallographic studies and

geometry optimization computations, the orbitals of CpCr-containing compounds in a

variety o f spin states are examined using Extended Hiickel molecular-orbital calculations.

The ligand effects are evaluated in the context o f Spectrochemical and Nephelauxetic

effects.

Reaction o f C p ' C r ( N O ) 2 C l with A g O T f yields Cp 'Cr (NO) 2 OTf . The covalently-

bound trifiate ligand in CpCf(NO) 2 (OTf) can be displaced by amines to afford

[CpCr(NO) 2 (NHR 2 ) ] [OTfj salts ( N H R 2 = N H 3 , N H M e 2 , N H 2

r B u , N H 2 P h ) . Treatment o f

basic, aqueous solutions o f C p C r ( N O ) 2 O T f with FLXL reagents (FLXL = acetylacetone,

salicylaldehyde, picolinic acid) results in the precipitation o f C p C r ( N O ) ( L X ) complexes.

These mononitrosyl complexes may be independently synthesized from [CpCr(NO)(p-

I)] 2 and the appropriate chelating ligand in either anhydrous organic solvents or aqueous

solution.

C p M o ( N O ) ( C H 2 P h ) C l reacts with R 2 M g (R = M e , Ph) or L i R (R = C=CPh)

reagents to form C p M o ( N O ) ( C H 2 P h ) R complexes. These products are stabilized by the

7] -benzyl ligand; the corresponding 16e C p M o ( N O ) R 2 species are too thermally unstable

to be isolated. The r | 2 - C H 2 P h interaction is disrupted by introducing other ligands capable

of multihapto-bonding, as evidenced by the synthesis o f C p 2 M o ( N O ) ( C H 2 P h ) . Reaction

of C p M o ( N O ) ( C H 2 P h ) C l with A g O T f yields CpMo(NO)(CH 2 Ph)OTf , which reacts with

P P h 3 or pyridine to afford [CpMo(NO)(CH 2 Ph)L][OTfJ salts.

Cp*Cr (NO)(CO) 2 reacts with PC1 5 or I 2 in N C M e to form [Cp*Cr(NO)(M--X)] 2

dimers via the C p * C r ( N O ) ( N C M e ) X adducts ( X = CI, I). The coordinating solvent is

required in order to avoid overoxidation and subsequent decomposition via N O loss.

Cr (NO)(N 'Pr 2 ) (0 2 CPh) 2 reacts with Mg(CH 2SiMe3) 2 to afford

Cr(NO)(N'Pr 2)(CH 2SiMe3) 2. The unusual nature o f this 14e, pseudo-tetrahedral,

monomeric, diamagnetic bis(alkyl) compound is in part accounted for by the 7t-bonding

properties of the ancillary amide and nitrosyl ligands.

iv

Table of Contents

Abstract • 1 1

Table of Contents i v

List of Tables viii

List of Figures x *

List of Schemes x ' v

List of Abbreviations x v

Acknowledgments x * x

Quotations x x

Chapter 1: Introduction 1

Reactivity in Organometallic Compounds 2

Importance of n Bonding 2

Synthesis and Theory 5

TI Bonding in Cp'M(NO)-Containing Complexes 7

Outline of This Thesis 9

References and Notes 11

Chapter 2: Theoretical Investigations of Cp Chromium Complexes 14

Introduction 15

Results and Discussion 20

[CpCrL 3]" 2 0

CpM(NO)Cl 2 (M = Cr or Mo). 24

CpCr(NO)(NH 2)X (X = CI or CH 3 ) 26

CpCr(NO)(CH 3)X (X = CI or CH 3 ) 31

Spectrochemical and Nephelauxetic Effects 34

Summary 40

Computational Details 41


Chapter 3: Pi-Bonding and NO Loss from CpCr(NO)2Y Species 48

Introduction 49


Synthesis o f Cp 'Cr(NO) 2 (OTf) 52

Reaction o f 3.1 with a-donor ligands 55

Pi-Bonding and N O loss 58

Hydrogen Bonding in CpCr(NO) 2 (OH) (aq) 62

Synthesis o f C p C r ( N O ) ( L X ) 64

Aqueous Chemistry o f [ C p C r ( N O ) ( H 2 0 > 2 ] + 67

Summary 69

Future Work :70

Chelate assisted N O displacement 70

Generation of C p C r ( N O ) Y species 70

Experimental 73

Methods 73

Synthesis o f C p C r ( N O ) 2 O T f (3.1) 73

Synthesis o f C p * C r ( N O ) 2 O T f (3.2) 73

Synthesis o f [CpCr(NO) 2 L][OTf] (3.3-3.6) 74

Synthesis o f C p C r ( N O ) 2 ( N 2 C 5 H 7 ) (3.7) 74

Synthesis o f C p C r ( N O ) ( L X ) (3.8-3.10) 75

Synthesis o f [CpCr(NO)(H20)2][BPh4] (3.11) 75

Synthesis o f [CpCr(NO)(p.-OH)] 2 (3.12) 76

Characterization Data 77

vi


Chapter 4: Synthetic Utility of the [CpMo(NO)(CH2Ph)]+ Fragment 85

Introduction 86


Direct Synthesis of CpMo(NO)(CH2Ph)R 90

Stabilizing Effects of ri 2-CH 2Ph 95

Synthesis of [CpMo(NO)(CH2Ph)L][OTfJ 96

Deprotonation of [CpMo(NO)(CH2Ph)(PPh3)][OTfJ 97

Summary 99

Future Work 100

Carbene transfer from CpMo(NO)(=CHPh)(L2) intermediates 100

le Reduction of [CpMo(NO)(CH2Ph)L][OTfJ 100

Ligand-based reactivity of CpMo(NO)(CH2Ph)R 101

Cp'M(NO)(CH 2Ph)Cl as Cp 2Zr(CH 3)Cl analog 102

Synthesis and reactivity of Cp 2M(NO)H 103

Experimental 105

Methods :.. 105

Synthesis of CpMo(NO)(CH2Ph)R (4.1-4.4) 105

Synthesis of CpMo(NO)(CH2Ph)(OTf) (4.5) .106

Synthesis of [CpMo(NO)(CH2Ph)L][OTf] (4.6-4.7). 106



Chapter 5: Towards CpCr(NO)R 2 116

Introduction 117


Oxidation of Cp'M(NO)(CO) 2 with Halogen Sources 120

Amine Elimination Reactions of Cr(NO)(N'Pr2)3 125

Synthesis of Cr(NO)(N'Pr2)(CH2SiMe3)2 (5.3) 128

Alternative routes to CpCr(NO)R 2 134

Summary.. 136

Future Work 137

le Oxidation of 17e Cp'Cr(NO)R 2 anionic complexes 137

Diamagnetic 12e tris(alkyl) species 137

Paramagnetic 16e Cr(0) compounds 138

Diamagnetic 16e M(NO)(NR 2)(L)R 2 bis(alkyls) of Cr, Mo, and W. 139

Alternative route to Cp*Cr(NO) 2Cl and [Cp*Cr(NO)(u-Cl)]2 140

15e Cp*Cr(NO)R species 141

Experimental 142

Methods ; 142

Synthesis of [Cp*Cr(NO)(p.-I)]2 (5.1) 142

Synthesis of [Cp*Cr(NO)(^Cl)]2 (5.2) 143

Synthesis of Cr(NO)(N'Pr2)(CH2SiMe3) (5.3) 143

Reaction of 5.3 with CsHe 144

Reaction of Cr(NO)(N'Pr2)(02CPh)2 with PhC0 2 H 144

Reaction of Cr(NO)(0'Bu)3 with Na(DME)Cp, CpSiMe 3, LiCp 145

Reaction of Cr(NO)(O fBu)3 with H X 145

Generation of Cp*Cr(NO)(CH2Ph)2 146

Reaction of Cr(NO)(0'Bu)3 with LiCH(SiMe 3) 2 146

Reaction of Cr(NO)(N'Pr2)(02CPh)2 with dppe and M 147


Appendix...... 157

vm

List of Tables

Table 1.1. Summary o f Thesis by Chapters 9

Table 2.1 Optimized geometries and energies for CpM(NO)Cl2 18

Table 2.2 Optimized geometries and energies for CpCr(NO)(NR"2)X. 18

Table 2.3 Optimized geometries and energies for C p C r ( N O ) ( C H 3 ) X 19

Table 2.4 E H M O frontier orbital splittings for singlet C p C r ( N O ) X 2 38

Table 2.5 Interelectron repulsion energy parameters for CpM(NO)X2 39

Table 3.1 Selected Bond Lengths and Angles for Cp*Cr(NO) 2 (OTf) (3.2) 54

Table 3.2 Selected Bond Lengths and Angles for CpCr(NO>2(3,5-Me2pyrazolyl)

(3.7) 57

Table 3.3 Selected Bond Lengths and Angles for CpCr(NO)(acetylacetonate)

(3.8) 66

Table 3.4 Numbering Scheme, Color, Yie ld and Elemental Analysis Data 77

Table 3.5 Infrared and Mass Spectral Data 77

Table 3.6 T i and 1 3 C N M R Data.. 78

Table 4.1 Selected Bond Lengths, Bond Angles, and Torsion Angles for

C p 2 M o ( N O ) ( C H 2 P h ) (4.4). 93

Table 4.2 Numbering Scheme, Color, Yie ld and Elemental Analysis Data 107

Table 4.3 Infrared and Mass Spectral Data 107

Table 4.4 ! H and 1 3 C N M R Data 108

Table 5.1 Selected Bond Lengths and Angles for [Cp*Cr(NO)(p.-Cl)] 2 (5.2).. 123

Table 5.2 M - M Lengths in Group 6 Dimers 124

Table 5.3 Selected Bond Lengths, Bond Angles, and Torsion Angles for

Cr(NO)(N'Pr2)(CH2SiMe3)2 (5.3) 130

Table A l Crystallographic Data for Complexes 3.2, 3.7, and 3.8 158

ix

Table A 2 Crystallographic Data for Complexes 5.2 and 5.3 159

Table A3 Fractional Coordinates of DFT-optimized geometry o f

C p M o ( N O ) C l 2 160

Table A 4 Fractional Coordinates of DFT-optimized geometry o f

C p C r ( N O ) C l 2 161


C p C r ( N O ) ( N H 2 ) C l 162


C p C r ( N O ) ( N H 2 ) ( C H 3 ) 163


C p C r ( N O ) ( C H 3 ) 2 164


CpCr (NO)(CH 3 ) 165

Table A 9 Fractional Coordinates and B e q for 3.2 166

Table A10 Fractional Coordinates and U e q for 3.7 167

Table A l 1 Fractional Coordinates and B e q for 3.8 168

Table A12 Fractional Coordinates and B e q for 4.4 169

Table A l 3 Fractional Coordinates and U e q for 5.2 170

Table A l 4 Fractional Coordinates and B e q for 5.3 171

Table A l 5 Bond Lengths for 3.2 173





Table A20 Bond Lengths for 5.3 178

Table A21 Bond Angles for 3.2 179

X






xi

List of Figures

Figure 1.1 a - and 7t-Bonding in organic and organometallic complexes 3

Figure 1.2 Frontier orbital effects of 7t-donor and 7i-acceptor ligands 4

Figure 1.3 Synthesis, structure and bonding in organometallic chemistry 6

Figure 1.4 Frontier orbitals of pseudo-octahedral Cp'M(NO)(ligand)2

complexes 7

Figure 1.5 Solid-state molecular structure o f CpCr(NO)(NPh 2 )1 8

Figure 2.1 Relative energies and D F T - B 3 L Y P optimized geometries of singlet and

triplet C p M ( N O ) X 2 17

Figure 2.2 E H M O energy-level diagram for [CpCr(CO) 3 ]" , [CpCrCl 3T~, and

[CpCr(NO)Cl 2 ] " 20

Figure 2.3 Pictorial representation of the 7t-bonding interactions o f the five Cr 3d

orbitals in [CpCrL 3 ]~ (L - C O , CI) 21

Figure 2.4 Pictorial representation of the Cp and N O 7i-bonding interactions with the

five C r 3d orbitals in [CpCr(NO)Cl 2 ] " 22

Figure 2.5 E H M O diagram for singlet and triplet C p M ( N O ) C l 2

( M = M o , Cr) 24

Figure 2.6 E H M O diagram for singlet and triplet C p C r ( N O ) ( N H 2 ) X

( X = C1, C H 3 ) 26

Figure 2.7 Pictorial representation o f the N H 2 7t-bonding interactions in singlet

C p C r ( N O ) ( N H 2 ) C l 27

Figure 2.8 Pictorial representation of the N O and N H 2 7t-bonding interactions in

triplet CpCr (NO) (NH 2 )C l 29

Figure 2.9 E H M O diagram for singlet and triplet C p C r ( N O ) ( C H 3 ) X

( X = C1, C H 3 ) 31

Figure 2.10 Pictorial representation o f the Cp and N O 7t-bonding and CH3 a -

bonding interactions in H O M O -5 o f triplet C p C r ( N O ) ( N H 2 ) C l 32

Figure 2.11 Qualitative energy-level diagram illustrating 7t-donor effects on orbital-

splitting energy and spin state in d 4 M L . 6 36

Figure 2.12 Qualitative energy-level diagram illustrating N H 2 7t-donor effects on

orbital-splitting energy and spin state in C p C r ( N O ) ( N H 2 ) X 37

Figure 3.1 O R T E P plot of Cp*Cr(NO) 2 (OTf) (3.2). Thermal ellipsoids o f 50%

probability are shown 54

Figure 3.2 O R T E P plot o f CpCr(NO) 2 (3,5-Me 2 pyrazolyl) (3.7). Thermal ellipsoids of

50% probability are shown for the non-hydrogen atoms 57

Figure 3.3 Pictorial representation of N O 7t-bonding interactions in

C p C r ( N O ) 2 C l 58

Figure 3.4 Pictorial representation o f N O and N R 2 7t-bonding interactions in

C p C r ( N O ) 2 ( N R 2 ) 59

Figure 3.5 Qualitative enthalpy diagram of 7t-donor-assisted ligand

dissociation 61

Figure 3.6 O R T E P plot o f CpCr(NO)(acetylacetonate) (3.8). Thermal ellipsoids of

50% probability are shown 66

Figure 4.1 r i 2 -Benzyl interaction in C p ' M ( N O ) ( C H 2 P h ) X species 89

Figure 4.2 O R T E P plot o f C p 2 M o ( N O ) ( C H 2 P h ) (4.4). Thermal ellipsoids o f 50%


Figure 4.3 18e CpM(NO)R-containing complexes 95

Figure 5.1 O R T E P plot of [Cp*Cr(NO)(p.-Cl) 2] (5.2). Thermal ellipsoids o f 50%


Figure 5.2 O R T E P plot of C r (NO) (N 'P r 2 ) (CH 2 SiMe 3 ) 2 (5.3). Thermal ellipsoids o f

50% probability are shown 130

x i i i

Figure 5.3 Qualitative energy-level diagram illustrating N O and NR2 Tt-bonding

interactions in tetrahedral Cr(II) complexes 131

Figure 5.4 * H N M R spectrum o f Cr(NO)(N'Pr2)(CH2SiMe3)2 (5.3) in C 6 D 6 133

Figure 5.5 Qualitative orbital splitting and amide rotation barriers in Cr (NO) and

Cr(N) species 134

xiv

List of Schemes

Scheme 3.1 NO loss reactivity postulated for CpCr(NO) 2Y species 50

Scheme 3.2 Hydrogen-bonding, protonolysis, and chelate-assisted NO loss 64

Scheme 3.3 Synthesis of [CpCr(NO)(H 20) 2] + 67

Scheme 3.4 Postulated importance of steric bulk in the generation of Cp'Cr(NO)(Y)X

species from CpCr(NO) 2Y precursors 71

Scheme 3.5 Possible synthetic routes to Cp'Cr(NO)Y complexes 72

Scheme 4.1 Application of CpMo-containing complexes to organic synthesis... 86

Scheme 4.2 Potential extension of known CpMo-based organic synthesis to C - H bond

activation of C - C bond formation reactions 87

Scheme 4.3 Planned reactions to investigate synthetic utility of [CpMo(NO)(CH2Ph)]+

fragment 88

Scheme 4.4 Possible reactivity modes of Cp 2M(NO)H 104

Scheme 5.1 Oxidative halogenation of CpM(NO)(CO) 2 (M = Cr, Mo, W) and

subsequent alkylation reactions 118

Scheme 5.2 Oxidation of Cp'Cr(NO)(CO) 2 with halogen sources 121

Scheme 5.3 Steric influences on the reactivity of Cr tris(amide) complexes 126

Scheme 5.4 Derivitization of Cr(NO)(N'Pr2)3 127

Scheme 5.5 Current synthetic routes to Cp*Cr(NO)-containing complexes...... 140

XV

List of Abbreviations

The following is a list o f abbreviations and symbols employed in this Thesis, most

of which are in common use in the chemical literature.

ne n valence electrons

A angstrom, 10~ 1 0 m

acac acetylacetonate

anal. analysis

atm atmosphere

B 3 L Y P three-parameter form of the Becke, Lee, Yang and Parr functional

br broad

"Bu w-butyl, - C H 2 C H 2 C H 2 C H 3

'Bu / -bu ty l , -CMe 3

1 3 C carbon-13

1 3 C { 'H} proton-decoupled carbon-13

°C degree Celsius

C3H5 al lyl , - C H 2 C H = C H 2

cal calorie

calcd calculated

C p ' Cp or Cp*

Cp cyclopentadienyl, C5H5

C P centroid of the C5H5 ring

Cp* pentamethylcyclopentadienyl, CsMes

8 chemical shift

d days, or doublet (in a N M R spectrum)

xvi

D deuterium, H

D B U l,8-diazabicyclo[5.4.0]undec-7-ene

DFT density functional theory

dppe diphenylphospinoethane, Pl^PCFfeCHsPPl^

D M E dimethoxyethane, CH3OCH2CH2OCH3

E H M O Extended Huckel molecular orbital

EI electron impact

eq equation

equiv equivalents

Et ethyl, - C H 2 C H 3

FAB fast atom bombardment

*H proton

h hours

HF Hartree-Fock

HOMO highest occupied molecular orbital

Hz hertz, s - 1

'Pr /so-propyl, -CH(CH 3 ) 2

DR. infrared

J joule, kgm2s~2

J coupling constant

K degrees Kelvin

k rate constant

L Lewis basic, 2-electron-donor ligand; liter, 10 - 3 m 3

L U M O lowest unoccupied molecular orbital

M metal (usually group 6); molar, mole/liter

m multiplet

xvi i

m/z mass-to-charge ratio

M e methyl, - C H 3

min minutes

mmol millimole, 10" 3 mole

M O molecular orbital

mol mole

M S mass spectrum

v stretching frequency

N M R nuclear magnetic resonance

no. number

O R T E P Oak Ridge Thermal Ell ipsoid Program

P + parent molecular ion

Ph p h e n y l , - C 6 H 5

ppm parts per million

py pyridine, C5H5N

R hydrocarbyl

s singlet (in a N M R spectrum); strong (in a IR spectrum)

S O M O singly-occupied molecular orbital

t triplet

O T f trifluromethanesulfonate, triflate, OSO2CF3

o-tol or/Ao-tolyl, 2-C6H4CH3

/7-tol para-to\y\, 4 -C6H4CH3

T H F tetrahydrofuran, C4H8O

U V ultraviolet

vis visible

vs versus

xviii

X halide or other anionic 1-electron-donor ligand

Y amide, alkoxide or sulfide ligand

xix

Acknowledgements

The efforts of many people were required to see this work through to completion.

Thanks to Peter, for his good humor, unflagging support, attention to detail, and

steadfast insistence on producing nothing but the finest.

Thanks to all the members of the Legzdins' Group, past and present, for showing

what is possible. I 'm especially thankful for the camaraderie of a pair o f Steves (English

(Sayers) and Vancouver (McNeil)) , and for the opportunity to work with two very

talented fourth-year undergraduate students (Vick i Tong and Jane Kuzelka).

Thanks to the staff at U B C , particularly M . Austria, P. Borda, and L . Darge; to

Drs. S. Rettig and V . Young for solving the crystal structures, and to Prof. R. Pol i for his

computational work and helpful discussions.

Thanks to my family, for unceasing encouragement and support.

Finally, a limoful o f cash and prizes to Teresa, insufficient thanks for your songs

and understanding and silliness and insight and love.

Quotations

"This skein o f unusual reactions must surely be thought fanciful, were it not

the fact that it renders comprehensible an otherwise inexplicable transformation."

Seth N . Brown and James M . Mayer

Inorg. Chem. 1995, 34, 3560-3562

Chapter 1: Introduction

Reactivity in Organometallic Compounds 2

Importance of 7t Bonding 2

Synthesis and Theory 5

7i Bonding in Cp'M(NO)-Containing Complexes 7

Outline of This Thesis 9


2

Reactivity in Organometallic Compounds

Organometallic chemistry involves compounds that contain a bond between a

carbon atom and a metal a tom. 1 - 4 The work described in this Thesis belongs to the specific

subcategory of monomelic, homogeneous, transition-metal organometallic chemistry and

is particularly concerned with the synthesis, characterization and reactivity of these kinds

of complexes. For a given transition metal, two important factors can influence the

reactivity of organometallic compounds. The first involves the properties of the metal-

bound ligands whose effects can be divided into steric and electronic components. The

second is the electronic configuration of the metal, which reflects both the oxidation state

and the spin state. The studies presented in this Thesis were initiated to investigate how

one type of ligand property, namely n bonding, influences the electronic configuration and

reactivity of one class of organometallic compound, namely complexes o f Cr and M o

containing N O and C p ' ( C 5 R 5 ; R = H (Cp), R = M e (Cp*)) ligands.5

Importance of n Bonding

In order to understand bonding in organometallic chemistry, instructive parallels

may be drawn to simple organic chemistry (Figure 1.1). The a bond that exists between a

metal and a methyl group is fundamentally similar to the C - C single bond in ethane.

Because a bonds are cylindrically symmetric along the internuclear axis, there is no

electronic barrier to rotation about this type of bond.

However, n bonds possess a single nodal plane that contains the internuclear axis.

Rotation about this axis disrupts the orbital overlap essential for the existence of the 7t

bond, leading to a barrier to rotation. There are two types of organometallic 7t-bonding

interactions, classified by the formal direction of electron donation in the metal-ligand

bond. The N R 2 group shown in Figure 1.1 is an example o f a 7t-donor ligand since

electron density is donated from the filled N p orbital of the amide ligand into the empty

3

metal d orbital. Since the empty TC* orbital of the H2C=CH2 group is accepting electron

density from the filled metal d orbital, the ethylene ligand is a TC acceptor! Note the

orientation of the amide and ethylene ligands required to overlap with the metal d orbital.

F i g u r e 1.1. a- and TC-Bonding in organic and organometallic complexes

While organometallic TC bonds are generally weaker than a bonds, re-bonding

interactions can exert a powerful influence over the reactivity of organometallic

compounds by modifying the relative energies of the frontier orbitals. The degenerate set

of dxy, 6x2, and dyz orbitals shown in the center of Figure 1.2 constitutes the frontier orbitals

for a hypothetical, octahedral d 1 M L 6 complex. The left side of Figure 1.2 illustrates how

the empty, high energy TC* H2C=CH2 ligand orbital can interact with the d^ orbital if one

of the L ligands is replaced by ethylene. While the antibonding combination of this

interaction is too high in energy to be considered one of the frontier orbitals (indicated by

the box in Figure 1.2), the bonding combination is slightly lower in energy than the

corresponding d^ orbital in ML6. The net overall effect is that %-acceptor ligands typically

lower the energy of d orbitals. The situation is reversed for the interaction with the d^

orbital with the low energy, filled N p orbital of the amide ligand, shown at the right of

4

Figure 1.2. In this case, the occupied, bonding combination is low in energy, and the

antibonding combination lies slightly above the remaining d^ and d_̂ orbitals, and so 7t-

donor ligands are considered to raise the energy o f d orbitals.

y **- . ^

\

H2CCH2sv

— M ;

Figure 1.2. Frontier-orbital effects o f 7t-donor and 7t-acceptor ligands.

B y influencing the orbitals that are near the dividing energy between occupied and

unoccupied orbitals, n-bonding interactions can have a dramatic impact on several

important aspects of the chemistry of organometallic species. These include the oxidation

state of the metal and spin state of the complex as well as the orientation, binding modes

and rotation barriers of the ligands.6 When several K ligands are present in a single

complex, the resulting competition for the available rc-bonding orbitals can also have

important consequences for the reactivity of the compound. 7 Consideration o f 7i-bonding

interactions alone does not guarantee a complete understanding of the bonding o f an

organometallic complex which often also involves important a-bonding effects, subtle

geometric distortions, and critical steric influences.8 However, establishing a working

description of the n bonding present in a compound provides both a conceptual

5

framework with which to explain many important experimental observations and an

excellent starting point from which a more complete picture of the bonding may be

achieved.

Synthesis and Theory

While attaining a fuller understanding of the fundamental factors which determine

the behavior of organometallic compounds may be a sufficient goal in and of itself for

theoreticians, chemists concerned with more practical matters may rightfully ask how %-

bonding considerations may be applied to the actual synthesis of new complexes. After all,

most of the remarkable achievements of modern synthetic organometallic chemistry have

been attained without resorting to any theoretical concepts more advanced than the 18e

rule. At the risk of imposing artificial order on the naturally messy business of scientific

discovery, the use of theory to assist synthesis can be divided into three progressive

stages:

1. Explanation: using theory to account for trends that have already been observed for a

known class of related compounds, such as stability or reactivity patterns that may

result upon systematically altering the ligands or the metal.

2. Prediction: extrapolating from known trends to assist in the identification of new

target molecules or reactivity modes.

3. Design: using theoretical insights to design a complex that can accomplish a specific,

predetermined application.

Figure 1.3 illustrates how synthesis, structural determination and theoretical

modeling might be harnessed to further the development of organotransition-metal

chemistry. Once a new compound has been made, its solid-state molecular structure can

be determined using single crystal X-ray diffraction techniques. The crystallographically

determined parameters can then be used as the basis for molecular-orbital calculations.

F r o m the resulting theoretical description, critical bonding features o f the molecule can be

identified that will help guide subsequent synthetic studies.

N e w

Target

Molecules

Synthesis

Characterization

Techniques

Theoretical Description

Bondi ng Theory

Structural Determination

Figure 1.3. Synthesis, structure and bonding in organometallic chemistry.

Whi le interactions between synthetic and theoretical organometallic chemistry have

been possible for decades, 9 recent progress in computer technology has created

spectacular new opportunities to employ this interdisciplinary approach. Improvements in

data acquisition and processing techniques have made X - r a y diffraction studies so much

more accessible that crystallographic structural determinations are now comparatively

routine. Computational software is now readily available commercially, ranging from

simple molecular mechanics and Extended H u c k e l molecular-orbital programs for use on

standard home computers to advanced ab initio and hybrid density functional theory

packages for powerful workstations. T h e insights afforded by theoretical analysis may then

be directed towards the development o f new target molecules, thereby completing the

feedback cycle between synthesis, structure and theory.

7

7t Bonding in Cp'M(NO)-Containing Complexes

A simple model is sufficient to describe many of the 7t-bonding and reactivity

relationships observed for Cp'M(NO)-containing complexes. Figure 1.4 illustrates the

frontier orbitals of a pseudo-octahedral, d 4 to d 6 Cp 'ML 3 complex ( L - M - L = 90°,

C N T - M - L = 125.6°) where the Cp ligand occupies three facial coordination sites. If one

L ligand is replaced with NO, two of the "t2g-type" orbitals are lowered in energy due to

the two orthogonal M - N O 7t-acceptor interactions. The third orbital, designated d^ if the

z-axis lies along the M - N O bond, is of critical importance to the Cp'M(NO)(ligand)2

complex.10 For M(II), d 4 compounds, this orbital will be empty.11 Much of the known

reactivity of 16e Cp'M(NO)R 2 species consists of nucleophilic attack of small molecules

such as H 2 , CO, or 0 2 at this L U M O , and subsequent insertion into the metal-alkyl

bonds.5 If the M(II) complex contains a single-faced 7C-donor ligand such as an amide or

an alkylidene, the ligand will align co-planar with the M - N O bond in order to 7i-donate

into the vacant d^ orbital.12 Similarly, in M(0), d 6 compounds the dxy orbital is occupied,

and a 7i-acceptor ligand will also adopt an orientation which maximizes the 7t-bonding

interaction.13

Figure 1.4. Frontier orbitals of pseudo-octahedral Cp'M(NO)(ligand)2 complexes.

8

The same pseudo-octahedral bonding description was employed by W . Stephen

M c N e i l to explain the ligand-loss reactions which accompanied the reduction or oxidation

o f Cr(I), d 5 complexes. 1 4 For the Cr(I)/Cr(0) redox couple in [ C p C r ( N O ) L 2 ] + / 0

compounds, loss or gain of an electron was observed to induce lability in the L ligands. 1 5

The N O ligand was shown to dissociate when [CpCr(NO)Cl 2 ]~ was oxidized from Cr(I) to

Cr(II) . 1 6 These studies laid the groundwork for much o f the content o f this Thesis.

Figure 1.5. Solid-state molecular structure of CpCr(NO)(NPh 2 )I .

Another source of inspiration came from the solid-state molecular structure of

CpCr(NO)(NPh 2 ) I (Figure 1.5). The 1979 report o f this X-ray crystallographic study

provided only cursory synthetic details: "rj-Cyclopentadienyl(diphenylamido)iodonitrosyl-

chromium was obtained from chloro(r|-cyclopentadienyl)dinitrosylchromium by reaction

with lithium diphenylamide ( M e l + L i + N P h 2 H ) " . 1 7 The structure itself, however,

provides eloquent testimony to the importance o f amide Tt-donation to the Cr(II), d 4 metal

center. The amide ligand is planar, and the N(ni t rosy l ) -M-N-C( ipso) dihedral angles are

less than 175.1° and - 1 0 . 1 ° , which places one of the phenyl groups in a sterically

unfavorable position with respect to the Cp ring. Consequently, this orientation must be

9

ascribed to an electronic interaction, namely that of the amide group functioning as a TC-

donor ligand to the empty orbital perpendicular to the Cr-NO bond.

Outline of This Thesis

On initial inspection, the focus of this Thesis may seem narrow and limited. In fact,

the investigation of 7C-bonding and reactivity in transition-metal nitrosyl complexes has

enabled me to conduct research that is of potential relevance to some of the most exciting

areas of current organometallic chemistry. Table 1.1 lists the appropriate area for each

chapter of this Thesis, as well as the desired goal and specific results from each study.

Table 1 . 1 . Summary of Thesis by Chapters.

Title Relevant Area Goal Results

2 Theoretical Investigation of Cp Chromium Complexes

Spin State and Reactivity in Open-Shell Organometallics

Application of E H M O to interpret CT- and TC-Bonding Effects

Paradigms of Coordination Chemistry are applicable to Organometallics

3 Pi-Bonding and NO Loss from CpCr(NO) 2Y Species

(a) Aqueous Organometallics (b) NO release in Aqueous Solution

pH-Dependant NO loss from CpCr(NO)2(OTf)

TC-Donor Assisted Ligand Loss via Stabilization of Dissociative Intermediate

4 Synthetic Utility of CpMo(NO)(CH2Ph)+

C-HBond Activation and C - C Bond Formation

Generate CpMo(NO)(CHPh) via Intermolecular Deprotonation

rjz-Benzyl Interaction Stabilizes reactive hydrocarbyl Ligands

5 Towards CpCr(NO)R2

Chromium-Based Olefin Polymerization Catalysts

Develop a synthetic route to CpCr(NO)R2

Species

Highly Unsaturated, Diamagnetic Complexes accessible from Cr(NO)(N'Pr2)3

Each chapter is divided into seven sections: Introduction, Results and Discussion,

Summary, Future Work, Experimental, Characterization Data, and References and Notes.

Tables, figures, schemes, equations and complexes are all numerically sequenced within

each chapter. An Appendix to the Thesis contains crystallographic details and tables of

atomic fractional coordinates for all compounds structurally characterized during this

work.

11

References and Notes

(1) For an introductory treatment of organometallic chemistry and 71-bonding, see:

Shriver, D. F.; Atkins, P.; Langford, C. H. Inorganic Chemistry, 2nd ed.; W. H. Freeman

and Co.: New York, NY, 1994.

(2) For advanced textbooks on organotransition-metal chemistry, see: (a) Collman,

J. P.; Hegedus, L . S.; Norton, J. R.; Finke, R. G. Principles and Applications of

Organotransition Metal Chemistry, University Science Books: Mill Valley, CA, 1987. (b)

Crabtree, R. H. The Organometallic Chemistry of the Transition Metals, 2nd ed.; Wiley-

Interscience: Toronto, 1994.

(3) For recent, multi-volume reference works on organometallic complexes, see:

(a) Comprehensive Organometallic Chemistry II, Abel, E. W., Stone, F. G. A.,

Wilkinson, G., Eds.; Elsevier: Oxford, 1995. (b) Dictionary of Organometallic

Compounds, 2nd ed.; Chapman & Hall: New York, 1995.

(4) For current research in organometallic chemistry, see: (a) Organometallics

(published biweekly by the American Chemical Society) (b) Journal of Organometallic

Chemistry (published biweekly by Elsevier).

(5) Legzdins, P.; Veltheer, J. E. Acc. Chem. Res. 1993, 26, 41.

\ (6) (a) Caulton, K. G. New J. Chem. 1994,18, 25. (b) Gibson, V. C. J. Chem.

Soc, Dalton Trans. 1994, 1607. (c) Poli, R. Chem. Rev. 1996, 96, 2135.

(7) (a) Kubacek, P.; Hoffmann, R. J. Am. Chem. Soc. 1981,103, 4320. (b)

Templeton, J. L . ; Winston, P. B.; Ward, B. C. J. Am. Chem. Soc. 1981,103,11X3. (c)

Brower, D. C ; Templeton, J. L.; Mingos, D. M . P. J. Am. Chem. Soc. 1987,109, 5203.

(d) Su, F. -M.; Bryan, J. C ; Jang, S.; Mayer, J. M . Polyhedron 1989, 8, 1261. (e) Huber,

S. R.; Baldwin, T. C ; Wigley, D. E. Organometallics 1993,12, 91. (f) Atagi, L. M . ;

Mayer, J. M . Organometallics 1994,13, 4794.

12

(8) (a) Wolczanski, P. T. Polyhedron 1995,14, 3335. (b) Heyn, R. H.; Macgregor,

S. A.; Nadasdi, T. T.; Ogasawara, M . ; Eisenstein, O.; Caulton, K. G. Inorg. Chim. Acta

1997, 259, 5.

(9) Hoffmann, R. Science 1981, 211, 995.

(10) (a) Schilling, B. E. R.; Hoffmann, R.; Faller, J. W. J. Am. Chem. Soc. 1979,

101, 592. (b) Legzdins, P.; Rettig, S. J.; Sanchez, L.; Bursten, B E . ; Gatter, M . G. J. Am.

Chem. Soc. 1985,107, 1411. (c) Bursten, B. E. ; Cayton, R. H. Organometallics 1987, 6,

2004.

(11) Throughout this thesis, the formal oxidation states and d-electron counts of

nitrosyl compounds are arrived at by considering the nitrosyl ligand as NO + . The

electronic configurations of transition-metal nitrosyl compounds are more accurately

described using Enemark-Feltham notation, see. Enemark, J. H ; Feltham, R. D. Coord.

Chem. Rev. 1974, 13, 339.

(12) (a) Hermann, W. A.; Hubbard, J. L. ; Bernal, I.; Korp, J. D.; Haymore, B. L.;

Hillhouse, G. L. Inorg. Chem. 1984, 23, 2978. (b) Ashby, M . T.; Enemark, J. H. J. Am.

Chem. Soc. 1986,108, 730. (c) Hubbard, J. L.; McVicar, W. K. Inorg. Chem. 1992, 31,

910. (d) Legzdins, P.; Ross, K. J.; Sayers, S. F.; Rettig, S. J. Organometallics 1997,16,

190. (e) Kuzelka, J.; Legzdins, P.; Rettig, S. J.; Smith, K. M . Organometallics 1997,16,

3569. (f) Tran, E. ; Legzdins, P. J. Am. Chem. Soc. 1997, 119, 5071. (g) Legzdins, P.;

Sayers, S. F. Chem. Eur. J. 1997, 3, 1579.

(13) (a) Boone, B. J.; Klein, D. P.; Seyler, J. W.; Mendez, N. Q.; Arif, A. M . ;

Gladysz, J. A. J. Am. Chem. Soc. 1996, 118, 2411. (b) Gladysz, J. A.; Boone, B. R.

Angew. Chem., Int. Ed. Eng. 1997, 36, 550. (c) Burkey, D. J.; Debad, J. D.; Legzdins, P.

J. Am. Chem. Soc. 1997,119, 1139.

(14) McNeil, W. S. Ph.D. Thesis, University of British Columbia, Dec. 1995.

13

(15) Legzdins, P.; McNeil, W. S.; Batchelor, R. J.; Einstein, F. W. B. J. Am.

Chem. Soc. 1995,117, 10521.

(16) Legzdins, P.; McNeil, W. S.; Rettig, S. J.; Smith, K. M . J. Am. Chem. Soc.

1997, 119, 3513.

(17) Sim, G. A.; Woodhouse, D. I.; Knox, G. R. J. Chem. Soc, Dalton Trans.

1979, 83.

Chapter 2: Theoretical Investigations of Cp Chromium Complexes

I n t r o d u c t i o n 15

Resu l t s a n d D i s c u s s i o n 20

S u m m a r y 40

C o m p u t a t i o n a l D e t a i l s 41

References a n d Notes 42

15

Introduction

Unlike the subsequent chapters of this thesis, this chapter contains no synthetic

chemistry. Instead, it describes the Extended Huckel Molecular Orbital (EHMO)

calculations conducted as part of a larger, ongoing synthetic and theoretical exploration

of CpCr(NO)X2 (X = halide, amide, alkyl) species.

Some background information is necessary to put the E H M O study in context.

The impetus for the investigation of CpCr(NO)X2 complexes came in part from the rich

chemistry exhibited by related Mo and W compounds.1 For example, CpMo(NO)X2

compounds are derived from treatment of CpMo(NO)(CO)2 with I22 or P C I 5 3 and

subsequent metathesis reactions of the bis(halide) species.4 The 16e, diamagnetic

CpMo(NO)(alkyl)2 species are of particular interest due to their unusual bonding,5*

synthesis,5b thermolytic decomposition50 and subsequent derivatization.5d In contrast, no

CpCr(NO)X2 compounds are isolable from analogous halogenation reactions of

CpCr(NO)(CO) 2

6 Chemical or electrochemical oxidation of [NEt4][CpCr(NO)Cl2]

results instead in the isolation of products consistent with the loss of NO from the

initially formed neutral CpCr(NO)Cl2 intermediate.7 Complexes such as

CpCr(NO)(NPh2)I8 and CpCr(NO)(N'Pr2)(CH2SiMe3),9 however, have been

demonstrated to be stable.

It was proposed that these observations are best accounted for by spin-state

considerations. Unlike the stable, diamagnetic CpMo(NO)X 2 species, CpCr(NO)Cl2

might adopt a high-spin, triplet configuration. If this configuration involves either the

population of M - N O 7t-antibonding orbitals or the removal of electrons from M - N O 71-

bonding orbitals, the resultant weakening of the metal-nitrosyl bond may render the

complex prone to NO dissociation.7 The strong Cr-N(amide) 71-bonding interaction

observed for diamagnetic CpCr(NO)(NR.2)X species may prevent this mode of

decomposition by stabilizing the singlet state with respect to the triplet.9

A density functional study was undertaken by Professor Rinaldo Poli (U. de

16

Bourgogne) in collaboration with the Legzdins group in order to address the following

four questions: (1) What is the source of the discontinuity in CpM(NO)X 2 chemistry

between Mo and Cr? (2) Does CpCr(NO)Cl 2 have an accessible high-spin state? (3) Does

adopting a S = 1 configuration labilize the nitrosyl ligand? (4) How does varying the X

ligands affect the relative energies of the singlet and triplet spin states? The geometries of

six model compounds of the formula CpM(NO)X 2 (M = Cr, Mo; X = CI, N H 2 , CH 3 ) were

optimized at the B3LYP level in both singlet and triplet electronic configurations, and the

overall energies of the two spin states were compared. The optimized geometries and

relative singlet vs triplet energies of the six compounds are shown in Figure 2.1. The

geometric parameters of the complexes CpM(NO)Cl 2 (M = Mo, Cr), CpCr(NO)(NH 2)X

and CpCr(NO)(CH 3)X (X = CI, CH 3 ) are displayed in Tables 2.1, 2.2, and 2.3,

respectively.

The E H M O study presented in this chapter is divided into five subsections. The

investigation of CpCrL 3 anions was actually conducted prior to the density functional

computations of Prof. Poli and is included to establish the bonding effects of the nitrosyl

ligand in cyclopentadienyl chromium systems. The E H M O calculations of CpM(NO)Cl 2

(M = Cr, Mo) address how adopting a triplet configuration disrupts M - N O bonding

orbitals. The effect of amide TC-donation is assessed for CpCr(NO)(NH 2)X (X = CI, CH 3 ).

The unexpected singlet stabilization that accompanies alkyl ligation is considered for

CpCr(NO)(CH 3)X (X = CI, CH 3 ) . Finally, the effect of ligands on the relative singlet vs

triplet energies in CpM(NO)X 2 species is discussed in the context of spin-pairing and

orbital-splitting energies.

17

i f I P •: MO ': MO

• X * A E = 17.62 kcal/mol

•

' 1A' -. ' V ...

AE = 8.20 kcal/mol

• -Cr

| A E = 1.52 | kcal/mol

H ^ 1A' V

. AE = 2.37 I kcal/mol

_ _ _« —»- ~~

C r f * H Cr(

| A E = 0.95 1 kcal/mol

H(b-. O60H H O N

0C3H

t AE = 9.55 ..--*" I kcal/mol

Figure 2.1. Relative energies and DFT -B3LYP optimized geometries o f singlet and

triplet C p M ( N O ) X 2 .

18

Table 2.1. Optimized geometries and energies for CpM(NO)Cl2 (M = Mo, Cr)

Structural Parameter*

CpMo(NO)Cl 2 CpCr(NO)Cl 2

S = 0 S = 1 S = 0 S = 1 C N T - M 2.123 2.101 1.957 1.969 M-C(average) 2.449 2.432 2.305 2.318 M-Cl 2.395 2.405 2.272 2.296 M-NO 1.777 1.830 1.655 1.860 N-0 1.220 1.225 1.199 1.193 CNT-M-C1 112.70 119.93 114.96 121.38 CNT-M-NO 123.10 112.11 126.42 119.22 Cl-M-Cl 114.95 94.62 108.36 98.14 Cl-M-NO 95.90 103.74 94.34 95.32 M-N-0 173.39 169.77 177.18 177.50 E(hartrees) -420.9587 -420.9306 -439.6831 -439.6961 a C N T = Cp ring centroid.

Table 2.2. Optimized geometries and energies for CpCr(NO)(NH 2)X (X = CI, CH 3 )

Structural CpCr(NO)(NH 2)Cl b CpCr(NO)(NH 2)(CH 3) c

Parameter4

S = 0 S = 1 S = 0 S= 1 CNT-Cr 1.978 1.978 1.984 2.024 Cr-C(average) 2.324 2.391 2.329 2.364 Cr-X 2.330 2.329 2.071 2.057 Cr-NO 1.648 1.811 1.641 1.773 Cr-NH 2 1.796 1.927 1.803 1.924 N-0 1.210 1.210 1.224 1.216 CNT-Cr-X 113.01 119.53 111.66 117.60 CNT-Cr-NO 126.26 126.57 127.61 127.37 CNT-Cr-NH 2 118.36 125.69 121.26 125.65 X-Cr-NO 93.43 99.77 89.86 96.69 X-Cr-NH 2 104.22 92.81 100.70 91.87 ON-Cr-NH 2 97.09 82.86 98.85 88.65 Cr-N-0 175.94 174.80 175.85 175.97 ECrNH 2

360.00 358.55 359.93 358.50 H-N-Cr-N 11.39, -74.37, 7.52, -76.66,

-169.36 91.00 -169.11 88.35 | E(hartrees) -480.6751 -480.6766 -505.5853 -505.5701

a C N T = Cp ring centroid. b X = CI. C X = C H 3 .

Table 2.3. Optimized geometries and energies for CpCr(NO)(CH 3)X (X - CI, CH 3 )

Structural CpCr(NO)(CH 3)Cl b CpCr(NO)(CH 3) 2

c 1 Parameter"

S = 0 S = 1 S = 0 S = 1 CNT-Cr 1.959 2.004 1.983 2.026 Cr-C(average) 2.307 2.347 2.328 2.365 Cr-CH 3 2.057 2.049 2.029 2.073 Cr-Cl 2.254 2.298 Cr-NO 1.647 1.825 1.640 1.792 N-0 1.212 1.201 1.223 1.215 CNT-Cr-NO 126.04 121.95 127.77 137.38 CNT-Cr-CH 3 110.78 117.89 114.40 117.36 CNT-Cr-Cl 120.06 124.82 X - C r - C H 3 105.07 94.44 110.92 95.69 ON-Cr-Cl 100.41 94.12 ON-Cr-CH 3 88.85 96.78 93.09 90.61 Cr-N-0 175.30 174.45 175.99 176.91 E(hartrees) -464.5993 -464.6017 -489.4986 -489.4948

a C N T = Cp ring centroid. b X = CI. C X = C H 3 .

20

Results and Discussion

[CpCrL 3 r

The E H M O energy levels for [CpCr(CO)3]~ [CpCrCl3]" and [CpCr(NO)Cl2]" are

shown in Figure 2.2.7'10

-9.0

-10.0

-11.0 eV -12.0

-13.0-

xy COTC'Y'^V

xz yz

Cr e C o

o

mcnb

cr | ci

mcnb—f— z2

_ | L _ x V

CpNOTi % g

Figure 2.2. E H M O energy-level diagram for [CpCr(CO)3]~ [CpCrCl3]", and

[CpCr(NO)Cl2]".

For [CpCr(CO)3]~, the alignment of the z-axis along the pseudo-3-fold rotation

axis through the Cp centroid allows for maximum overlap of two orbitals (d^ and d )̂

with the two Cp Tc-symmetry orbitals, while the remaining three orbitals (d^, dX2-y2 and

dZ2) each form rc-bonds with all three carbonyl ligands (Figure 2.3). All five orbitals are

doubly occupied, with the CpCr Tc-orbitals slightly lower in energy than the CrCO

21

orbitals. These results agree well with previous molecular-orbital calculations o f

CpM(CO )3 compounds 1 1 The same orbital orientation holds for the [CpCrCfo] - complex.

The dxz and d^ orbitals are still filled and strongly n-bonding with the Cp ligand, but the

dxy, dx2-y2, and dZ2 are now high in energy, singly occupied, and weakly rc-antibonding to

the filled p-orbitals o f the three CI ligands (Figure 2.3).

Figure 2.3. Pictorial representation o f the rc-bonding interactions o f the five Cr 3d

orbitals in [CpCrL 3 ]~ (L = C O , CI).

In the case o f [CpCr(NO)Cl2]~, the orbitals mix and reorient in order to maximize

Cp and N O 7i-bonding at the expense of any interaction with the CI ligands. 1 2 Four o f the

five Cr d-orbitals combine with the two Cp and the two N O Tt-symmetry orbitals to result

in four low-energy, filled orbitals, each o f which are 7t-bonding to both Cp and N O

(Figure 2.4). The four 7t-bonding orbitals consist o f the in-phase and out-of-phase

combinations o f Cp n and N O % orbitals both in and perpendicular to the C N T - C r - N O

plane. The remaining high-energy, singly-occupied d-orbital is nonbonding to the strong

22

Tc-ligands, although there is some rc-antibonding interaction with the weak rc-donor CI

ligands.

Figure 2.4. Pictorial representation of the Cp and NO re-bonding interactions with the ;

five Cr 3d orbitals in [CpCr(NO)Cl2r.

Reexamination of the E H M O calculations previously conducted13 on

[CpCr(NO)(NH3)2]+ reveals that this cationic species possesses an identical "four43elow-

one" splitting pattern, including two additional Cp and NO Tc-bonding orbitals lower in

energy than the two previously reported. The SOMO is around 0.8 eV lower in energy

than the corresponding orbital in [CpCr(NO)Cl2]~. While the difference in charge

undoubtedly plays an important role in this variation between the two complexes, the

nature of the ligands (7C-donating chloride compared to the purely a-bonding ammine) is

likely also a contributing factor to this energy difference and the resulting ease of

oxidation of the dichloro anion over the bis(ammine) cation.7'13

The remarkable mixing of Cp and NO 7t-bonding illustrated in Figure 2 4 is due to

23

the ability of the strong re-acceptor nitrosyl to compete with the rc-donor cyclopentadienyl

for the available metal TC-symmetry orbitals. Similar extensive orbital reorganization

resulting from competition between two strong rc-donor ligands in four-coordinate

compounds has been reported by Schrock and co-workers, who concluded that in such

cases, the two different lo, 27C ligands should be treated as a single unit.14 Research on

such compounds has since been largely restricted to high-valent d° and d 2 complexes of

the early transition metals containing strong 7C-donors such as [rj'-CsRs]-, [NR] 2 - , and

[CR] 3 - ligands.15 The E H M O calculations described here suggest that a sufficiently

strong %-acceptor ligand may compete significantly with a %-donor ligand, leading to

M(1CT,2TC)2 electronic cores analogous to those found in pseudo-tetrahedral compounds

with two 7C-donor ligands. In other words, our calculations indicate that CpCr(NO) may

be considered to be a M(lrj,27t)2 fragment, comparable to C p ^ T i , 1 6 CpV(NR), 1 7

Cr(NR) 2 , 1 8 and Cp'Cr(CR). 1 9

Comparison of the relative energies, occupancies, and bonding character of the

orbitals in Figure 2.2 suggests an appealing explanation for the ligand control of

electronic stability in these cyclopentadienyl chromium complexes. It appears that

nonbonding d-orbitals in CpCrL3~ complexes can readily accommodate single, unpaired

electrons, thereby resulting in stable paramagnetic species having fewer than 18e. This

feature accounts for the relative stability of 17e [CpCr(NO)L 2] + and [CpCr(NO)X2]~

7 1̂

complexes, ' which possess one nonbonding orbital, compared to the 17e CpCr(CO)3

metalloradical, which has no such orbital.116 The fact that the SOMO of a stable 17e

species is non-bonding should not be surprising since, to a first approximation, singly-

occupied orbitals can only mix with other orbitals that possess unpaired electrons.20

24

CpM(NO)Cl 2 (M = C r or Mo)

Singly-occupied metal-centered non-bonding orbitals are also significant for the

16e CpCr(NO)Cl 2 neutral species. Instead of being based on X-ray crystallographic

parameters, the E H M O energy levels shown in Figure 2.5 are based on the DFT-

optimized geometries of high- and low-spin CpM(NO)Cl 2 (M = Cr, Mo) complexes

(Table 2.1 for geometric parameters, Figure 2.1 for singlet vs triplet relative energies):

Comparison of the S = 0 and S = 1 geometries and energies reveals that CpCr(NO)Cl 2 has

a triplet ground state, 8.20 kcal/mol lower in energy than the singlet state, and that the

Cr-NO bond of the high-spin species is over 0.2 A longer than the low-spin distance of

1.655 A. The significance of this lengthening is underscored by the constancy of

experimentally determined Cr-NO bond lengths, which fall in the narrow range of 1.65

A to 1.69 A for CpCr mononitrosyl complexes with a variety of ancillary ligands, overall

charges, and formal oxidation states.7"9'13'21 This 0.2 A difference is interpreted as

signaling a critical weakening of the Cr-NO bond in the triplet configuration, consistent

with the nitrosyl lability proposed for high-spin CpCr(NO)Cl 2. 7

-9.0-

-10.0-

-11.0—I eV

-12.0-H

-13.0-

-14.0-

- u -- u --ti-s = o S = l s = o

-tt-

S = l

Mo cK I ^ci

N o

C l ^ | CI N O

Figure 2.5. E H M O diagram for singlet and triplet CpM(NO)Cl 2 (M = Mo, Cr):

25

The qualitative shape, energies, and bonding characteristics of both the singlet

and triplet CpCr(NO)Cl2 Extended Huckel molecular orbitals are very similar to those

obtained for the doublet CpCr(NO)Ci2 anion (Figure 2.4). This is because the partially-

disordered7 solid-state molecular structure of CpCr(NO)Cl2~ closely resembles the DFT-

optimized geometries of the neutral species, and the low-level EFfMO calculations do not

account for the variation in orbital occupancy between singlet, doublet and triplet

compounds. Comparing the S = 0 and S=l occupancies, we see that in triplet

CpCr(NO)Cl2 an electron has been promoted from an orbital with Cr-NO re-bonding

character to a metal-centered non-bonding orbital.

While this difference is undoubtedly responsible for part of the Cr-NO bond

lengthening in the high-spin case, close examination of the C l - C r - C l and C N T - C r - C l

bond angles indicates a second possible factor. The triplet geometry more closely

approximates a pseudo-octahedral geometry, a trend that carries through all the DFT-

optimized S=l geometries (Tables 2.1-2.3). These geometry changes suggest that the

orbitals are no longer mixing to form four CpCr(NO) rc-bonding orbitals for the triplet

species. This would be consistent with the inability of singly-occupied orbitals to mix

with doubly-occupied orbitals,20 resulting in separate CpCr and Cr(NO) 7t-bonds. While

this effect is too subtle to be readily apparent in the low-level E H M O calculations, it

accounts for the long triplet Cr-NO distance since there remains only one doubly-

occupied Cr-NO 7t-bond.

26

CpCr(NO)(NH 2)X (X = CI or C H 3 )

The instability of CpCr(NO)Cl 2 with respect to NO loss effectively precludes its

use as a precursor to other CpCr(NO)X 2 species by metathesis routes analogous to those

used for the congeneric Mo compounds.4'5 CpCr(NO)(NPh2)I has previously been

synthesized in unreported yield from the reaction of CpCr(NO) 2Cl and LiNPh 2 in the

presence of Mel . 8 CpCr(NO)(N'Pr2)(ri1-02CPh) and CpCr(NO)(N'Pr2)(CH2SiMe3) were

recently synthesized from Cr(NO)(N'Pr2)3.9 The solid-state molecular structures of all

three CpCr(NO)(NR 2)X complexes display short Cr-N(amide) distances of 1.83 to 1.89

A and planar Cr-NR 2 groups aligned with the Cr-NO axis, thereby suggesting a strong

Cr-N(amide) TC-interaction. Consequently, we examined the model CpCr(NO)(NH2)Cl to

gauge the role of the Cr-N(amide) Tc-bond in conferring a low-spin configuration on

these CpCr(NO)(NR 2)X species.

-9.0

-10.0-

-11.0-eV

-12.0-

-13.0-

-14.0-

•NH 27i*

.CpNOrc

- f i -NH 2Tt s = o

•NH27t*

-j— mcnb

- } — N07t,NH27r*

C p N O n

-fj-NOrt,NH 2TC

S = 1 - f f -NH 2Tc S = 0

mcnb

— f - N C b t , N H > *

IF - H -- f j - N O T i , N H 27t S = 1

Cr H 2 N | ̂ C l

N O

^ C r ^ H 2 N | ^ C H 3

N O

Figure 2.6. E H M O diagram for singlet and triplet CpCr(NO)(NH 2)X (X = CI, CH 3 ) .

27

The optimized geometry o f singlet CpCr(NO)(NH2)Cl, shown in Figure 2.1,

conforms with the known molecular structures of CpCr(NO)(NR2)X compounds. The

Cr-NFfe unit is planar, with the sum of the angles around N being 360°. The Cr-NFfe

bond length of 1.796 A is slightly shorter than those determined experimentally, perhaps

due to steric interaction between the Cp ring and the bulky NR.2 (R = Ph or 'Pr) ligands of

the amide complexes. The alignment o f the amide along the C r - N O axis indicated by the

H - N - C r - N torsion angles is consistent with the experimental results. 8 ' 9 Figure 2.6 shows

the E H M O energy levels for C p C r ( N O ) ( N H 2 ) X ( X = CI, C H 3 ) species.

Figure 2.7 illustrates why the ON-Cr -NH2 group must be planar for a full Cr-

N(amide) 7t-bond to form in CpCr(NO)(NR2)X species. The orbital perpendicular to the

C r - N O axis is empty, and can accept 7t-donation from the filled amide N p orbital only

when the amide ligand lies coplanar with the Cr-nitrosyl bond. The 7i-antibonding orbital

shown in Figure 2.7 forms the L U M O of singlet CpCr (NO) (NH 2 )C l , while the C r - N H 2

7i-bonding orbital lies below the four CpCr(NO) 7t-bonding orbitals in energy. The

Cr-NH2 7i-bonding interaction is expected to raise the energy of the empty orbital

perpendicular to the C r - N O axis in C p C r ( N O ) ( N H 2 ) C l relative to the analogous orbital in

CpCr(NO)Cl2, leading to a comparatively larger H O M O - L U M O gap and an increased

preference for the low-spin, singlet electronic configuration.

H H

H

N O

N O

N O

LUMO HOMO -5

Figure 2.7. Pictorial representation o f the N H 2 7t-bonding interactions in singlet

C p C r ( N O ) ( N H 2 ) C l .

28

The planar arrangement of amide and nitrosyl ligands in CpCr(NO)(NH2)X

species is reminiscent of the ligand orientations observed in related d 4 and d 2 complexes

10 1 ^ O O 0/1 —

which contain both 7i-donor and 7i-acceptor ligands. ' ' The ability of 7t-donor

ligands to stabilize unsaturated species has previously been noted for specific

CpM(N0)X2 species,25 and has been reviewed for organometallic complexes in general.26

The present results demonstrate that ligand 7t-bonding effects can also affect the relative

energies of the spin states available to organometallic species through their influence on

the orbital-splitting energy.

The DFT-optimized geometry of triplet CpCr(NO)(NH2)Cl (Table 2.1 and Figure

2.1) is remarkably different than that of the singlet state. As in triplet CpCr(NO)Cl 2, the

Cr-NO distance has extended beyond the range observed experimentally. The Cr-amide

distance has also lengthened, and while the Cr-NFL; group remains essentially planar, the

ligand has adopted a conformation roughly perpendicular to the Cr-NO axis.

Comparison of the total energies of singlet and triplet CpCr(NO)(NFJ.2)Cl shows

that the two configurations are nearly degenerate. Although the formal replacement of CI

in CpCr(N0)Cl2 with N H 2 has increased the relative stability of the singlet vs the triplet

spin state, the calculation still favors the triplet state by 0.95 kcal/mol. The known

CpCr(NO)(N'Pr2)X complexes (X = ri 1-0 2CPh, CH 2SiMe 3), on the other hand, are

diamagnetic. Both compounds possess planar O N - C r - N C 2 moieties in the solid state (X-

ray crystallography), and the presence of four inequivalent amide C H 3 groups in the J H

NMR spectra (room temperature, CeDe) suggest that this orientation is retained in

solution.9 While S=l complexes of the formula CpCr(NR2)L2 are known,27 the long

Cr-NO bond (1.811 A) in triplet CpCr(NO)(NH2)Cl suggests that adopting a high-spin

configuration would render CpCr(N0)(NR2)X species prone to decomposition via NO-

loss. The discrepancy between experimental observations and theoretical results is

presumably due to the differences between the actual compounds and the simplified

complex we have chosen as a model. The presence of electron-donating alkyl substituents

makes N'Pr2 a better 7i-donor than the N H 2 ligand, enhancing the stability of singlet

CpCr(NO)(N'Pr2)X complexes to a greater extent than the calculations indicate.

Cr—NE'g

N O

CI N . O

SOMOT (HOMO)

...-H . .MH

o S O M 0 2 (HOMO - 1 )

Figure 2.8. Pictorial representation of the NO and N H 2 7t-bonding interactions in triplet

CpCr(NO)(NH2)Cl.

The relevant 7t-bonding orbitals for triplet CpCr(NO)(NH2)Cl are illustrated

schematically in Figure 2.8. The highest-energy singly-occupied orbital is orthogonal to

the nitrosyl, metal-centered, and non-bonding. The other singly-occupied orbital is TT

bonding to the NO and % antibonding to the N H 2 . A fully 7t-bonding combination can be

found at slightly lower energy than the CpCr(NO) 7t-bonding orbitals (i.e. HOMO -5),

while the L U M O is n antibonding to both NO and N H 2 . The source of the lengthening of

both the C r - N H 2 and Gr-NO bonds in triplet CpCr(NO)(NH2)Cl can be traced to the

lower-energy singly-occupied orbital. Since this orbital is C r - N H 2 antibonding, the

Cr-amide bond order is decreased. Like triplet CpCr(NO)Cl 2, there are only three

electrons in the Cr-NO 7t-bonding orbitals, which again leads to a weaker bond to the

nitrosyl ligand.

The change in amide orientation that accompanies the adoption of a high-spin

configuration creates a new metal-centered non-bonding orbital. This orbital forms the

HOMO of the complex, readily accommodating the highest-energy, unpaired electron in

30

a manner directly analogous to triplet CpCr(NO)Cl2, doublet CpCr(NO)Cl2~, and quartet

CpCrCfT. As a result of the conformational change of the amide ligand, no purely

metal-ligand antibonding orbitals are populated in triplet CpCr(NO)(NH2)Cl.

In order to more closely approximate the known CpCr(NO)(N'Pr2)(CH2SiMe3),

calculations were performed on CpCr(NO)(NH2)(CH3). The singlet geometry of the

model complex closely approximated the experimentally-determined structure,9 and

singlet CpCr(NO)(NH2)(CH3) was calculated to be 9.55 kcal/mol more stable than the

triplet state, as indicted in Figure 2.1. The difference in the relative singlet stability of

over 10 kcal/mol between the two CpCr(NO)(NH2)X model complexes required further

investigation. Indeed, formal metathesis of the CI ligand in CpCr(NO)(NH 2)Cl with a

C H 3 group unexpectedly resulted in a significantly greater relative change than the initial

replacement of N H 2 for CI in CpCr(NO)Cl 2 (7.25 kcal/mol). Calculations were thus

conducted on methyl-containing compounds in order to identify the source of the

enhanced singlet stability.

31

CpCr(NO)(CH 3 )X (X = CI or C H 3 )

As shown in Figure 2.1, triplet CpCr(NO)(CH3)Cl is calculated to be 1.52

kcal/mol lower in energy than the singlet state. While this constitutes an improvement in

relative singlet stability of more than 6 kcal/mol over the CpCr(NO)Cl2 species, the

change in energies is larger between CpCr(NO)Cl2 and CpCr(NO)(NH2)Cl, or

CpCr(NO)(NH2)Cl and CpCr(NO)(NH2)(CH3). Formal substitution of a second methyl

group leads to an even smaller relative difference. CpCr(NO)(CH3)2 is calculated to have

a diamagnetic ground state, but the singlet is only 2.37 kcal/mol more stable than the

triplet state.

-9.0

-10.0-

-11.0 eV -12.0

-13.0

-14.0 s = o S= 1

C r H 3 C ^ | X I

N O

s = o S = 1

T / C r

H 3 C ^ | > C H 3

N O

Figure 2.9. EFfivIO energy-level diagram for singlet and triplet CpCr(NO)(CH 3)X (X :

C1,CH 3).

While the 7T-bonding interactions of amide ligands can be discerned in the

32

planarity and orientation of the NR 2 group, the DFT-optimized geometries of the

CpCr(NO)(CH3)X species (X = CI, CH3) provide no structural clues to help explain why

alkyl ligands also favor the diamagnetic configuration. The discrepancy between

CpCr(NO)Cl 2 (triplet state more stable by 8.20 kcal/mol) and CpCr(NO)Me 2 (singlet

statefavored by 2.37 kcal/mol) in particular is difficult to reconcile with the known

ability of chloride ligands to stabilize unsaturated compounds through TC-donation.26

Examination of the bonding character of the Extended Huckel molecular Orbitals of the

CpCr(NO)(CH3)X species (Figure 2.9) reveals unusual features which suggest a possible

interpretation for these results. While the other CpCr(NO)X 2 complexes possess four

CpCr(NO) TC-bonding orbitals (Figure 2.4), CpCr(NO)(CH3)Cl and

CpCr(NO)(CH3)(NH2) have five of these orbitals, and CpCr(NO)(CH 3) 2 has six. These

"extra" orbitals are the result of mixing of the Cp and NO TC-bonding Orbitals with CH3 a-

bonding orbitals, as illustrated schematically in Figure 2.10.

Figure 2.10. Pictorial representation of the Cp and NO Tt-bonding and C H 3 a-bonding

interactions in H O M O -5 of triplet CpCr(NO)(CH 3) 2.

The effect of the inclusion of methyl a-bonding character in the frontier orbitals

on the relative spin-state energies of organometallic complexes has been previously

investigated. Hall and co-workers conducted a theoretical investigation of TiX2(dmpe)2

complexes in order to explain why the X = CI species28 is paramagnetic while the

33

isostructural X = C H 3 compound is diamagnetic, and they concluded that the variation

was due to differences in electron-electron repulsions between the two complexes.30 The

more electronegative CI ligands increase the effective charge on the metal atom, leading

to relatively contracted frontier orbitals and strong repulsions between paired electrons

which results in a triplet ground state. The more covalent CH3 ligands cause the orbitals

to be more diffuse, thereby decreasing electron-pairing energy and favoring the singlet

state.

34

Spectrochemical and Nephelauxetic Effects

E H M O calculations based on DFT-optimized geometries and singlet vs triplet

relative energies have been used to demonstrate that while CpCr(NO)Cl2 possesses a

triplet ground state which renders the complex susceptible to decomposition, formal

metathesis of the chloride ligands with either alkyl or amide groups improves the relative

energy of the singlet spin state, thereby rendering the complexes stable with respect to

nitric oxide loss. The two ligand types accomplish this stabilization in differing manners:

the NH2 7t-donation increases the orbital-splitting energy, while the covalent CH3 a-bond

decreases the interelectron-repulsion energy. These two parameters correspond exactly to

the Spectrochemical and Nephelauxetic effects. Because the underlying principles of

orbital-splitting and spin-pairing energies remain constant, the paradigms originally

developed in the 1960's to describe and explain how ligands influence the electronic

structures of coordination compounds may be successfully applied to current problems in

organometallic chemistry.

Early work in this area was conducted on pseudo-octahedral Werner-type

coordination compounds. Magnetic susceptibility measurements allowed the ground spin

state of these species to be determined, and UV/visible spectroscopy provided

information about excited electronic configurations. The predominantly ionic bonding

present in these compounds encourages the formation of mid-valent compounds with

accessible high-spin configurations. The orbitals are essentially metal-centered, allowing

electronic transitions to be readily assigned, and rendering the complexes amenable to

theoretical treatment using relatively simple crystal field or ligand field descriptions. The

systematic investigation of a large number of compounds demonstrated the effects of

ligands on the splitting energy between the t2g and eg orbitals, Ao, and the interelectron-

repulsion energy, represented by the Racah parameters, B and C . 3 1 Common ligands were

ranked in the Spectrochemical and Nephelauxetic series, which reflected their

empirically-determined effects on Ao and the Racah parameters, respectively.

35

In contrast, application of the concepts of orbital-splitting and spin-pairing

energies to organotransition-metal complexes has generally been neglected. This is in

part due to the covalent bonding and Tt-acceptor ligands typical of organometallic

compounds which tend to enforce a low-valent, diamagnetic configuration. The increased

ligand character of the frontier orbitals hampers the assignment of the electronic

transitions of the UV/vis spectra and limits the applicability of simple ligand field theory.

Thus, magnetic susceptibility and UV/vis measurements were supplanted by IR and

NMR spectroscopy as the primary analytical tools for organometallic complexes, with

subsequent investigations proceeding down avenues of inquiry suitable to the latter

techniques.32

Recent improvements in X-ray crystallography and theoretical methods have

contributed to the current reevaluation of the relevance of spin state to organometallic

chemistry. The dramatic increase in well-characterized high-spin organometallic

species33 can be attributed in part to the increased application of X-ray crystallography as

a characterization technique,34 and the attendant shift away from the diamagnetic bias

inherent in N M R spectroscopy. Open-shell compounds have demonstrated their utility in

olefin polymerization35 and dinitrogen cleavage36 reactions. While accurate evaluation of

energetically similar electronic states has long been a formidable challenge, advances in

ab initio37 and hybrid density functional38 computational techniques have helped

theoretical chemists address these near-degeneracy problems.39 With the application of

theoretical methods to the examination of unsaturated organometallic species too

transient or unstable to be observed experimentally,40 it is becoming increasingly

apparent that the relative energies of the spin states of these intermediates constitute a

critical but previously unappreciated factor in several important reactions.41 For example,

calculations indicate that a low-lying triplet excited state assists the intermolecular C - H

bond activation reactions exhibited by CpM(CO) (M = Rh, Ir) and related complexes, yet

because CpCo(CO) possesses a triplet ground state, it neither binds nor activates

36

20,42 alkanes.

The ability of TC-bonding amide ligands to increase the H O M O - L U M O gap of

CpCr(NO)X2 species and thereby stabilize the relative energy of the low-spin, singlet

state may initially appear to contradict the fundamental principles of ligand-field theory.

For octahedral, Werner-type coordination compounds, TC-donor ligands typically decrease

the H O M O - L U M O gap, which is reflected in their empirical ranking as "weak-field"

ligands in the Spectrochemical series. The effects of TC-bonding on the H O M O - L U M O

gap are illustrated for each case in Figures 2.11 and 2.12: the filled ligand Tc-donor

orbitals are shown on the right side of the figure, the metal-based orbitals prior to TC-

donation are on the left, and the new orbital-splitting energy, A, is shown for the resulting

complexes.

*2g £bjf:[

M-LTC

,--'̂ 11 (J L TC

Figure 2.11. Qualitative energy-level diagram illustrating 7t-donor effects oh orbital-

splitting energy and spin state in d 4 ML6.

Figure 2.11 demonstrates how multiple TC-donor ligands interact with the orbitals

of a pseudo-octahedral, d 4 coordination compound. The TC-symmetry t2 g orbitals are raised

37

in energy, which decreases the energy difference between the t2 g and e g orbitals and

stabilizes the high-spin, S = 2 configuration. For the pseudo-octahedral C p C r ( N O ) X 2 case

shown in Figure 2.12, the t 2 g-type orbitals have already been split by the strong 7t-

acceptor nitrosyl ligand, and so the important orbital splitting occurs "within the t 2 g set"

rather than between the t 2 g and e g orbitals. In the observed planar O N - C r - N H 2

conformation, the single-sided 7t-donor amide ligand raises the energy of the t 2 g-type

orbital that is not involved in C r - N O 7t-bonding. The resulting increase in A leads to the

relative stabilization o f the low-spin, singlet spin state.

II

mcnb—L-tf-

n - II

C r - N H 2 7 t

C r - N O =tp—-\-

H H N H 2 %

Cr-NH 27C

CpCr(NO ) (NH2^xl

Figure 2.12. Qualitative energy-level diagram illustrating N H 2 7i-donor effects on orbital-

splitting energy and spin state in C p C r ( N O ) ( N H 2 ) X .

Such an increase is indeed evident in the orbital-splitting energy between

occupied and unoccupied orbitals in singlet C p C r ( N O ) X 2 species (Table 2.4). The amide-

containing complexes exhibit the largest A values, with C p C r ( N O ) ( N H 2 ) C l and

C p C r ( N O ) ( N H 2 ) C H 3 possessing splitting energies o f 2.86 and 2.77 eV, respectively, both

38

higher than the 2.63 eV for CpCr(NO)Cl 2. While E H M O calculations are not the

computational method of choice for exact qualitative results, the trend that these A values

represent is nevertheless encouraging. There is not, however, a simple correlation

between the E H M O splitting energy and the relative energies of the singlet and triplet

states as calculated by DFT. Specifically, substitution of CI with CH3 simultaneously

decreases the orbital splitting energy yet increases the relative stability of the singlet

state due to changes in pairing energy.

Table 2.4. E H M O frontier orbital splittings for singlet CpCr(NO)X 2.

NH 2/C1 N H 2 / C H 3 Cl/Cl CH3/CI CH3/CH3

L U M O

HOMO

-9.227

-12.09

-9.507

-12.27

-9.333

-11.96

-9.622

-12.05

-9.937

-12.30

A 2.86 eV 2.77 eV 2.63 eV 2.43 eV 2.36 eV

In order to quantify the effect of the methyl ligands on the pairing energy, the

Coulomb (J) and exchange (K) integrals were evaluated for the CpM(NO)Cl 2 (M = Mo,

Cr) and CpCr(NO)(CH 3)X (X = CI, CH 3 ) complexes according to the method of Hall et

a/. 3 0 These J and K integrals were calculated by Prof. Poli with the assistance of Drs. Ivo

Cacelli and Antonio Rizzo; 4 3 they are the "spin-pairing" relationships that the Racah

parameters were originally formulated to approximate.44 The overall "pairing energy" can

be roughly described by the relation Jn - J i 2 + K i 2 , which is the difference between the

coulombic repulsions experienced by the paired (Jn) and the unpaired (Ji2)

configurations, plus the exchange energy (Ki 2) (Table 2.5).

The comparison between Mo and Cr is straightforward in that the orbital-splitting

energy is greater for CpMo(NO)Cl 2 (A = 2.92 eV) than for CpCr(NO)Cl 2 (A = 2.63 eV),

while the spin-pairing energy is less for the Mo species (94.72 kcal/mol) than for the

39

lighter Cr congener (172.84 kcal/mol). Both of these trends indicate that the singlet spin

state in the Mo complex will be favored to a greater extent than in the Cr compound, an

observation that is in accordance both with the DFT calculation results and generally

accepted periodic trends.33 For the CpCr(NO)X 2 (X = CI, Me) complexes, as CI ligands

are replaced by CH3 groups the decrease in A is more than offset, however, by the large

decrease in the spin-pairing energy, as the values for Jn - J12 + K12 indicate:

CpCr(NO)Cl 2 (172.84 kcal/mol) > CpCr(NO)(CH3)Cl (74.73 kcal/mol) >

CpCr(NO)(CH 3) 2 (-138.92 kcal/mol). The net result is that alkyl ligation enhances the

relative stability of the singlet state. This effect of covalent bonding on spin-pairing

energy precisely mirrors the intuitive picture of electron "cloud expansion" (i.e. the

Nephelauxetic effect) initially developed to explain the same phenomenon in Werner-

type coordination compounds.

Table 2.5. Interelectron repulsion energy parameters for CpM(NO)X 2

(M = Mo, X = CI; M = Cr, X = CI, CH 3 ) . a

Mo/Cl/Cl Cr/Cl/Cl Cr/Cl /CH 3 C r / C H 3 / C H 3

Jll 268.80 1240.46 976.17 531.65

J12 194.24 1134.98 966.79 697.64

K i 2 20.16 67.36 65.35 27.07

J l l _ Jl2 + K i 2 94.72 172.84 74.73 -138.92

"All energies are expressed in kcal/mol.

40

Summary

Extended Huckel molecular orbital calculations were performed on a variety of

CpCr-containing complexes, based on the structural parameters obtained from X-ray

crystallography or density functional geometry optimizations. The salient points resulting

from these calculations include:

1. The four 7c-bonding orbitals o f the CpCrfNO) fragment resemble the M(la ,27c) 2

electronic cores found in pseudo-tetrahedral complexes containing two strong TC-

donor ligands.

2. The apparently anomalous stability o f paramagnetic 17e CpCr(NO)-containing

compounds may be due to the non-bonding nature of the metal-centered L U M O .

3. The discontinuity in C p M ( N O ) X 2 chemistry between M o and C r may be due to spin-

state differences attributable to the increased interelectron repulsion present in the Cr

species.

4. Adopting a high-spin, S = 1 configuration significantly lengthens the metal-nitrosyl

bond in C p C r ( N O ) X 2 species due to promotion o f an electron from a C r - N O TC-

bonding orbital. The concomitant weakening o f the C r - N O bond provides a rationale

for the nitrosyl-loss reactivity postulated for high-spin C p C r ( N O ) C l 2 .

5. Formal replacement of CI with an amide ligand stabilizes the singlet state because the

C r - N R 2 re-bonding increases the orbital splitting energy.

6. Formal alkyl-for-chloride metathesis stabilizes the diamagnetic configuration because

of a decrease in pairing energy.

41

Computational Details

The optimized geometries and relative energies o f the singlet and triplet

C p M ( N O ) X 2 species were calculated by Pro f Rinaldo Pol i using G A U S S I A N 94 4 5 on the

DEC/Alphastation 250 at the University o f Maryland in College Park and on the SGI

Power Challenge at the Universite de Bourgogne. These calculations employed a Density

Functional Theory (DFT) approach, using the three-parameter form of the Becke, Lee,

Yang and Parr functional ( B 3 L Y P ) . 4 6 The monoelectric H F analysis o f the C p M ( N O ) X 2

30

complexes was performed according to the method o f Simpson, Hal l and Guest.

The visualization of the orbital interactions and the orbital energy diagrams was

assisted by the use o f Extended Hiickel molecular-orbital calculations, which were

performed on the crystallographically-determined or B3LYP-opt imized geometries using

the commercially available HyperChem for Windows Release 3 and ChemPlus

extensions for Hyperchem. 1 3 A n unweighted Hiickel constant of 1.75 was used.

42


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Raghavachari, K.; Al-Laham, M . A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.;

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48

Chapter 3: Pi-Bonding and NO Loss from CpCr(NO)2Y Species

Introduction 49


Summary 69

Future Work 7 0

Experimental 73

Characterization Data 7̂


49

Introduction

In 1956, Piper and Wilkinson 1 reported the synthesis o f C p C r ( N O ) 2 C l , one of the

first well characterized organometallic complexes o f chromium. 2 The compound is slightly

water-soluble, and upon treatment with aqueous A g N 0 3 forms solutions of the

[CpCr (NO) 2 (H 2 0) ] + cation 3 (which was originally formulated as [CpCr(NO) 2 ] + ) . 1 The

remarkable water stability of the [CpCr(NO) 2 ] + fragment is demonstrated by the isolation

of C p C r ( N O ) 2 X ( X = halide or pseudo-halide) complexes upon addition of K X salts to in

situ generated [ C p C r ( N O ) 2 ( H 2 0 ) ] + followed by CHC1 3 extraction. 1

Subsequent metathesis studies o f C p C r ( N O ) 2 C l have been performed in anhydrous

organic solvents.4"7 Under these conditions, several alkyl complexes o f the type

C p C r ( N O ) 2 R (e.g. R = M e , 4 a E t , 4 a C H 2 P h , 4 b t i u , 4 c C H 2 S i M e 3 , 4 d and CFfcX 4 6 ) have been

synthesized. The electron-donating ability of the alkyl groups is evident in the shift o f

v(NO) from 1815 and 1710 c m - 1 for C p C r ( N O ) 2 C l to -1780 and 1670 c n f 1 for

C p C r ( N O ) 2 R as measured by solution IR spectroscopy (CH 2 C1 2 ) . 4 The results o f the

alkylation reactions contrast with the metathesis reactions o f C p C r ( N O ) 2 C l with Y sources

( Y = SR, N R 2 , or OR) . The outcome of these latter reactions depends on the electronic

nature o f the Y group. A l l known compounds o f composition C p C r ( N O ) 2 Y contain

strongly electron-withdrawing substituents (e.g. S C N , 1 S 0 2 C H 3 , 4 b S C F 3 , 5 a N ( S 0 2 F ) 2 , 5 c

OS0 2 C 6 H4Me, 5 d N C O , 5 b and N 0 2

1 5 e ) . However, i f Y contains electron-donating

substituents (e.g. V = SPh , 6 b O M e , 6 g O E t , 6 e 0 'P r , 7 b N M e 2 , 6 d and N P h 2

7 a ) , the isolated

products have lost one nitric oxide ligand.

50

N O

Scheme 3.1. N O loss reactivity postulated for C p C r ( N O ) 2 Y species

A s illustrated in Scheme 3 . 1 , the initial metathesis products CpCr (NO )2Y are

presumed to initially lose N O to form the intermediate [CpCr(NO)Y] (A) , which then

either dimerizes to [CpCr(NO)(p-Y)] 2 ( B ) , 6 or undergoes a subsequent one-electron

oxidation process to form the Cr(II) species C p C r ( N O ) ( X ) Y ( C ) . 7 The process depicted in

Scheme 3 . 1 initially seems counter-intuitive since the unexpected ejection o f a strong n-

acceptor nitrosyl ligand is triggered by increased electron donation from the Y ligand. The

known CpCr(NO )2R complexes display no propensity to lose N O despite the strong

s/g7wa-donor alkyl ligands. 4 The observed reactivity is limited to /?/-donor Y ligands

because [CpCr(NO)2] + is a 7t-loaded fragment. The cyclopentadienyl and nitrosyl ligands

each form one a and two tt interactions, leaving no empty orbital available on the

M ( 1 C T , 2 U ) 3 fragment to accept % donation from the Y ligand. 8 This unfavorable filled/filled

Tt interaction9 is relieved by loss o f nitric oxide, since intermediate A possesses an empty,

it symmetry orbital to accept donation from Y .

These observations are of potential significance for the rational design of

metallonitrosyl complexes capable of releasing N O in vivo10 Several features make

CpCr(NO )2X complexes particularly suitable as model compounds for the study of N O

release from transition metals. These Cr(0), d 6 complexes are diamagnetic with two

51

equivalents o f N O per Cr atom, and the paramagnetic Cr(I), d 5 mononitrosyl compounds

that would be the products expected upon loss of N O radical have been shown to be

unusually inert. 1 1 Given the stability of the [CpCr(NO) 2 ] + fragment in aqueous solution, 1

we hypothesized that treatment of aqueous [CpCr (NO) 2 (H 2 0) ] + with base should form

CpCr(NO) 2 (OH) . The strong 7t-donor hydroxide ligand might then induce loss of one of

the nitrosyl ligands under mild, physiologically-relevant conditions.

The investigation o f these concepts of n bonding and N O loss were performed in

collaboration with V i c k i Tong, who completed her fourth-year undergraduate research

thesis in the Legzdins group under my supervision. 1 2 This chapter describes the research I

conducted prior to and concurrent with V i c k i Tong's thesis project, and reference is made

to her studies of the aqueous [CpCr (NO) 2 (H 2 0) ] + system where appropriate. A n

integrated presentation of my work in this chapter and V i c k i Tong's thesis appeared as the

Legzdins group's contribution to a special issue of Dalton Transactions to commemorate

the first anniversary o f the death of Professor Sir Geoffrey Wilkinson. 1 3

52


Synthesis of Cp'Cr(NO) 2(OTf)

The slight water solubility o f C p C r ( N O ) 2 C l initially reported by Piper and

Wilkinson 1 is presumably due to hydrolysis of the C r - C l bond as illustrated in equation

3.1.

Similar reactions have been studied in detail for C p 2 M C l 2 complexes (M = T i , Zr,

V , or M o ) . 1 4 More recent investigations o f aqueous organometallic chemistry 1 5 have relied

on the trifluoromethanesulfonate (CF3SO3, OTf) ligand which dissociates more readily

from the metal center. 1 6 As shown in equation 3.2, CpCr(NO) 2 (OTf) (3.1) can be

synthesized by treatment o f C p C r ( N O ) 2 C l with A g O T f in a manner analogous to previous

chloride abstraction reactions with A g P F 6

1 7 and A g B F 4 . 1 8 The preparation o f 3.1 is usually

best performed in E t 2 0 , but larger scale reactions (>1 mmol) give optimum results when

conducted in a 1:1 mixture of E t 2 0 and C H 2 C 1 2 .

AgOTf ^ ' ( 3 2 )

CI CH 2 Cl 2 :Et 2 0 I ^ Q T f N

o 3-i Complex 3.1 is soluble in C H 2 C 1 2 , C H 3 C N , alcohols, water, ethers, and aromatic

solvents, and is isolated as large, air-stable, black crystals. The solubility o f 3.1 in solvents

with poor ion-supporting ability suggests that the O T f ligand is bound to the C r atom. The

single resonance in the ' H N M R spectrum at 8 5.82 (CDC1 3) corresponds to the

53

cyclopentadienyl ligand, and the higher-energy value of the IR v(NO) bands of 3.1 at 1836

and 1730 c m - 1 (CH2CI2) as compared to the CI precursor reflects the greater electron-

withdrawing properties o f the O T f ligand. 1 9 The Nujol mull IR spectrum also contains six

bands between 1000 and 1350 c m - 1 consistent with the presence o f a covalently bound

triflate ligand. 2 0

Attempts to obtain single crystals o f 3.1 from numerous solvent mixtures resulted

only in twinned crystals. In order to confirm the presence of a C r - O T f bond in 3.1, the

related Cp*Cr(NO) 2 (OTf) (3.2) was synthesized. A s expected, the

pentamethylcyclopentadienyl ligand imparted increased solubility and crystallinity to the

complex, 2 1 and a sample o f 3.2 suitable for X-ray crystallographic analysis was obtained

from diethyl ether. The solid-state molecular structure of 3.2 is illustrated in Figure 3.1,

and its intramolecular parameters are listed in Table 3.1. The O T f group is indeed

covalently bound to the metal center in 3.2; the C r - 0 distance of 2.030(2) A is consistent

with a C r - 0 single bond.

Figure 3.1. ORTEP plot of Cp*Cr(NO)2(OTf) (3.2). Thermal ellipsoids of 50%

Table 3.1. Selected Bond Lengths and Angles for Gp*Cr(NO)2(OTf) (3.2).

Bond Lengths (A) Bond Ang es O Cr-O(l) 2.030(2) Cr-N(l)-0(4) 165.7(3) Cr-N(l) 1.713(3) Cr-N(2)-0(5) 169.9(3) Cr-N(2) 1.712(3) N(l)-Cr-N(2) 95.3(2) Cr-CP 1.85 0( l ) -Gr-CP 117.3

55

Reactions of 3.1 with o-donor ligands

To test the ability of the O T f ligand to act as a leaving group, 3.1 was treated with

potential a-donor ligands. A s expected, the reactions of 3.1 with amines generated the

salts [CpCr(NO) 2 (L)][OTf] (L = N H 3 (3.3), N H M e 2 (3.4), N H 2 ' B u (3.5), N H 2 P h (3.6)) in

good to moderate yields as analytically pure, air-stable green powders. This technique was

subsequently employed by V i c k i Tong to form similar salts with the N-containing

heterocycles imidazole and 3,5-dimethylpyrazole. 1 2 This reaction is most conveniently

conducted in Et 2 0 since the resulting salts are ether insoluble and are readily isolated by

removal o f the supernatant solution via cannula. As indicated in equation 3.3, isolated

CpCr(NO) 2 (OTf) may be used in these reactions, or 3.1 may be generated in situ from

C p C r ( N O ) 2 C l and A g O T f in Et 2 0.

1— —I— O T f ® L

c:r L > J r ® O T t 3-3 N H 3 ( 3 3 )

Q N ^ I ^ O T f Et20 v f " I ^ L 3.4NHMe2

N N 3.5NH2'Bu O O 3.6NH7Ph

Compound 3.6 was treated with " B u L i or K O ' B u in T H F in an attempt to generate

the neutral dinitrosyl amide complex CpCr(NO) 2 (NHPh) at low temperatures by

deprotonation. Monitoring this reaction by solution IR spectroscopy did not reveal any

v(NO) bands consistent with CpCr(NO) 2 (NHPh) . Instead, the v(NO) bands o f 3.6 were

replaced upon slow warming with many weak bands in the region expected for

mononitrosyl products; the precise nature o f these products was not investigated further.

In contrast, the heterocycle-containing [CpCr(NO) 2 (L)][OTf] species 1 2 were

cleanly deprotonated as monitored by IR spectroscopy. The product of

[CpCr(NO) 2(imidazole)][OTf] and " B u L i or K O ' B u displayed new v(NO) bands at 1817

and 1712 c m - 1 (CH 2 C1 2 ) , but the presumed C p C r ( N O ) 2 ( N 2 C 3 H 3 ) product could not be

separated from the inorganic triflate salt. The neutral 3,5-dimethylpyrazolyl product

56

CpCr(NO )2 (N 2 C 5 H7) 3.7, however, could be isolated in a pure state due to its greater

solubility (equation 3.4).

Complex 3.7 is relatively air-stable in the solid state and is soluble in T H F , C H 2 C 1 2 ,

C 6 H 6 and E t 2 0 . The shift in v(NO) from 1839 and 1722 cm" 1 for cationic

[CpCr(NO) 2(3,5-dimethylpyrazole)][OTfJ to 1800 and 1686 cm" 1 (Nujol) for neutral 3.7

reflects the increase in electron density at the Cr center that accompanies the

deprotonation reaction. The solid-state molecular structure o f 3.7 is illustrated in Figure

3.2, and its intramolecular parameters are collected in Table 3.2. The Cr-N(pyrazolyl)

bond length is a relatively long 2.011(2) A, and the planar pyrazolyl group seems to be

oriented so as to avoid unfavorable steric interactions with the Cp ligand. This contrasts

with the short C r - N distance and the electronically dictated O N - C r - N R 2 coplanarity

recently observed for [CpCr(NO)(NR 2)]-containing species which possess a C r - N %

bond, 2 2 thereby suggesting that such an interaction is absent in 3.7.

57

Figure 3.2. ORTEP plot of CpCr(NO)2(3>Me2pyrazolyl) (3.7): Thermal ellipsoids of .

50% probability are shown for the non-hydrogen atoms.

. Table 3.2. Selected Bond Lengths and Angles for CpCr(NO)2(3,5-Me2pyrazolyl) (3.7)

Bond Lengths (A) Bond Ang es O Cr-N(3) 2.011(2) Cr-N(l)-0(1) 172.7(2) Cr-N(l) 1.707(2) Cr-N(2>-0(2) 171.8(2) Cr-N(2) 1.716(2) N(l)-Cr-N(2) 91.86(9)

58

Pi-Bonding and NO loss

The ligand-dependent reactivity observed upon treatment of [CpCr(NO)2(L)][OTf]

complexes with bases underscores the importance of TC bonding in [CpCr(NO)2]-

containing species. The stability of 3.7 is attributed to the aromatic nature of the 3,5-

dimethylpyrazolyl ring which allows the electron density of the filled N p orbital to be

delocalized over the heterocyclic N2C3 moiety. The NHPh ligand presumably formed upon

reaction of 3.6 with base could also potentially delocalize excess electron density from the

amide N to the aromatic Ph ring. However, this stabilizing interaction is apparently

insufficient to prevent the loss of NO and subsequent decomposition of the conjugate base

of 3.6, even when the neutral phenylamide complex is generated under relatively mild

conditions. This reactivity is entirely consistent with the substituent effects observed for

stable5 and unstable6,7 CpCr(NO) 2Y (Y = SR, NR 2 , or OR) complexes, as outlined in the

introduction of this chapter.23

In order to explain why CpCr(NQ)2Y complexes containing TC donors seem to be

unstable with respect to NO loss, the bonding of CpCr(NO) 2Cl was examined with the aid

of Extended Hiickel molecular orbital calculations.

Figure 3.3. Pictorial representation of the NO TC-bonding interactions in CpCr(NO) 2Cl

59

A qualitative bonding picture of the frontier orbitals of CpCr(NO)2Gl is shown in

Figure 3.3. 2 4 , 2 5 All three t2g-type orbitals are engaged in 7t-bonding to both NO ligands,

while the remaining out-of-phase combination of N - 0 TC* orbitals in the O N - C r - N O

plane forms the ligand-centered, non-bonding L U M O .

If the CI ligand in CpCr(NO) 2Cl were replaced with a N R 2 group, the filled amide

p orbital could interact with either the d „ or dyz orbital, depending on the amide

orientation. Figure 3.4 shows the orbital interactions that would be expected in

CpCr(NO) 2NR 2 if the N R 2 ligand lies in the yz plane (bisecting the O N - C r - N O angle).

The dxz orbital forms a low-energy, doubly occupied combination that is TC bonding to the

NR 2 group and to both of the NO ligands. The fully antibonding combination is high in

energy and unoccupied. Between these two molecular orbitals is a doubly occupied

combination that is TC bonding to the nitrosyl ligands, but TC antibonding to the amide. The

net result is no Cr-NR 2 TC bond and stronger Cr-NO bonds. From this analysis, it appears

that the ground state TC-bonding effects in CpCr(NO)2(NR2) do not account for the NO-

loss reactions attributed to these species.

Low Energy Medium Energy High Energy Doubly Occupied Doubly Occupied Empty Tc-Bonding to N O Tc-Bonding to NO TC-Anti-Bonding to Tc-Bonding to NR2 7i-/4«r/-Bonding to NR.2 NOandtoNR.2

Figure 3.4. Pictorial representations of the NO and NR 2 TC-bonding interactions in

CpCr(NO)2(NR2)

60

Fe Ru Re £ ^ | ^ S R M e 3 P ^ | P h 3 P ^ | Y

C P M e 3 N O O

Similar ligand loss reactions have previously been reported for the d 6 C p ' M L 2 Y

complexes shown above. 9 These complexes are qualitatively similar to the C p C r ( N O ) 2 Y

species dealt with in this chapter, and studies performed on these Group 7 and 8 species

may provide insight into the reactivity of C p C r ( N O ) 2 Y complexes. Ashby, Enemark and

Lichtenberger conducted Fenske-Hall molecular orbital calculations on CpFe(CO) 2 (SH) to

explore the interaction between the filled p orbital on the thiolate ligand and filled metal

orbitals on the [CpFe(CO) 2 ] + fragment.26 They concluded that these filled/filled repulsions

were responsible for both the enhanced nucleophilicity at sulfur observed for

CpFe(CO) 2 (SR) complexes and the tendency o f these complexes to dimerize and lose

C O . 2 7 A n excellent quantitative mechanistic study o f Y ligand n donation and ligand loss

from a series o f Cp*Ru(PMe3)2Y complexes was conducted by Bryndza, Bercaw and co

workers. 2 8 The acceleration of P M e 3 dissociation from Cp*Ru(PMe3)2(OFf) over

Cp*Ru(PMe3)2(CH3) by about six orders o f magnitude was attributed to the u-donor

properties o f the hydroxide ligand. Gladysz and co-workers demonstrated that chiral

CpRe(NO)(PPh 3 )Y ( Y = O R , 2 9 " N H R 2 9 b c ) complexes epimerize via P P h 3 dissociation. In

all of these d 6 , C p ' M L 2 Y systems, the Y group does not induce L ligand lability by any

negative influence on the M - L bond strength in the ground state, but Y n donation does

stabilize the C p ' M L Y species, thereby lowering the energy of the dissociative reaction

pathway.

61

A H

reaction coordinate Figure 3.5. Qualitative enthalpy diagram o f 7t-donor-assisted ligand dissociation.

The reaction enthalpy diagram presented in Figure 3.4 illustrates how Y 7t-

stabilization o f the unsaturated C p ' M L Y species enhances the lability o f the L l igand. 9 ' 2 8 ' 2

The top curve shows the relative energies along the dissociative pathway for C p ' M L 2 R

complexes. The unsaturated C p ' M L R moiety is a high-energy intermediate with only a

small barrier to the addition o f L to reform the starting material. The lack o f empty metal

orbitals on C p M L 2 Y prohibits 7t donation from the Y ligand, leaving the ground-state

energy o f this compound roughly equivalent to that o f the alkyl species. The process of

ligand L dissociation, however, creates an empty orbital that the lone pairs on Y can

interact with. This results in a lower energy for both the C p M L Y intermediate and the

62

dissociative transition state compared to the alkyl complex which lacks this potential for

71-donor stabilization o f the unsaturated species.9

Similar bonding considerations can be used to explain why [CpCr(NO)(u J -NHR)] 2

complexes are generated when CpCr(NO)(NH 2 R)I (R = 'Bu) or [ C p C r ( N O ) ( N H 2 R ) 2 ] + (R

= C H 2 C H = C H 2 ) complexes are treated with base. 3 0 Presumably the C p C r ( N O ) ( N H R ) L (L

= N H 2 C 3 H 5 or I") species are formed initially, but then eject L due to the amide it

stabilization o f the intermediate formed by ligand dissociation. Two equivalents o f

CpCr(NO)(NHR) then associate to yield the observed products. 3 0

The loss of nitric oxide from C p C r ( N O ) 2 Y is different from the related L

dissociations discussed above in two important respects. First, the loss o f the two-electron

donor ligands C O or P R 3 from C p ' M L 2 Y complexes does not result in a change in

oxidation state at the metal center, unlike the one-electron oxidation which formally

accompanies the loss o f N O radical. Second, the C p C r ( N O ) 2 Y complexes have the

potential to convert from a linear to a bent nitrosyl ligand. The suggestion o f 7t-donor

influence on ancillary ligand bonding does resemble the "ring slipped" (rj 3-

Cp*)Ir(PPh 3 )(Y)R intermediates invoked by Bergman and Glueck ( Y = OEt , R = H ; 3 1 a Y

= N H P h , R = M e 3 1 b ) . In the absence o f a detailed theoretical investigation o f C p C r ( N O ) 2 Y

complexes, however, the possibility of a bent nitrosyl intermediate remains purely

speculative.

Hydrogen Bonding in CpCr(NO) 2(OH) (aq)

"Speculative" initially appeared to be a remarkably charitable adjective with which

to describe all the bonding arguments presented in the preceding section once V i c k i Tong

began her investigation of CpCr(NO) 2 (OTf) in water. While 3.1 did form

[CpCr (NO) 2 (H 2 0) ] + when dissolved in water, and the aqua cation was deprotonated by

added OFT (pATa = 6.8), CpCr(NO) 2 (OH) proved to be disconcertingly stable in water; the

63

single *H N M R resonance for the Cp protons remained unchanged over weeks at room

temperature in N a O D / D 2 0 solution under N 2 . 1 2

The stability of basic aqueous solutions of 3.1 is apparently due to the interaction

o f the hydrogen atoms of the solvent water molecules with the oxygen atom of the

hydroxide ligand o f CpCr(NO) 2 (OH) . The Lewis basicity of the hydroxide oxygen in

CpCr(NO) 2 (OH) has previously been established by the isolation o f C p C r ( N O ) 2 ( H O " A )

adducts with Lewis acids ( A = B P h 3 or [CpCr(NO) 2 ] + ) 3 2 from the basic aqueous work up

of CpCr (NO) 2 (BF 4 ) . Also worthy of note is the IR monitoring of the reaction of EtO~ and

C p C r ( N O ) 2 C l in ethanol which indicates that the species CpCr(NO) 2 (OEt) is stable until

the protic solvent is removed in vacuo.6* Attempts to induce N O loss by generating

CpCr(NO) 2 (OF£) in aprotic solvents such as T H F by reaction of 3.1 with C s O H instead

leads to the formation of [(CpCr(NO) 2) 2(p--OH)][OTfJ, identified in solution by IR

spectroscopy by comparison with the known BF4~ and B P h f analogues. 3 2 In the absence

of a hydrogen-bonding solvent, the initially generated CpCr(NO) 2 (OH) species displaces

the O T f ligand in another molecule o f 3.1 instead o f inducing nitrosyl ligand dissociation.

M u c h research has been conducted recently on alcohol molecules engaged in

hydrogen bonding to the oxygen atoms o f late-transition-metal alkoxide species, several of

which have been characterized in the solid state by X-ray crystallography. 3 3 These

complexes provide an interesting comparison to the Cr nitrosyl complexes reported here,

in that in both cases no empty metal orbitals are available to accept % donation, and so

strong H-bonding interactions are established to relieve the resulting filled/filled

repulsions.9 O f particular interest are the studies o f Simpson and Bergman 3 4 who invoke

analogous hydrogen bonded species as intermediates in the exchange reactions o f

alkoxides by protonolysis.

64

Synthesis of CpCr(NO)(LX)

While hydrogen bonding prevents one potential route to nitric oxide loss by

attenuating the electron density at the hydroxide oxygen of CpCr(NO) 2 (OH) , it presents a

second possible route through protonolysis with potentially chelating ligands. When

treated with acids such as acetylacetone, salicylaldehyde, or picolinic acid ( H X L ) , in situ

generated solutions of CpCr(NO )2 (OH) react slowly to give the mononitrosyl products

C p C r ( N O ) ( L X ) ( L X = 0 2 C 5 H 7 (3.8), 0 2 C 7 H 5 (3.9), NCeHjO;, (3.10)). These compounds,

which slowly precipitate from aqueous solutions as analytically pure powders, are thought

to be formed by the reaction sequence illustrated in Scheme 3.2.

Scheme 3.2 Hydrogen-bonding, protonolysis, and chelate-assisted N O loss

Initial protonolysis o f the hydroxide ligand occurs via a H-bonded intermediate,

resulting in a dinitrosyl species with a new C r - X bond and a pendant Lewis basic

heteroatom. The subsequent chelate-assisted intramolecular N O displacement reaction

forms a water insoluble paramagnetic product with a six- (3.8 or 3.9) or five-membered

(3.10) metallacyclic ring. Equations 3.535 and 3.617 display previous examples o f

substitution of nitric oxide from CpCr(NO) 2-containing species by rj-donor ligands,

65

although these reactions were performed under thermolytic conditions in organic solvents.

These relatively harsh conditions contrast sharply with the room-temperature, aqueous

solution reactions reported here.

C 6 H 6 | "CI

• N O

r © P F 6 ® T , M® T

PF6

C r L - L 6^Cx L n 6\ Q N ^ I N C M e C H 3 N 0 2 Y R ^ L > K J V J

N N O O

To confirm the identity o f the paramagnetic chelate complexes, 3.8-3.10 were

independently synthesized from [CpCr(NO)(p>I)] 2

3 6 and K L X salts in T H F (equation 3.7).

Compounds 3.8-3.10 are air stable as solids, and their v (NO) (1647, 1659, 1666 cm" 1

(Nujol), respectively), solubility and other physical properties are similar to related, stable

17-valence-electron Cr(I) mononitrosyl complexes. 1 1 , 1 9 The solid-state molecular structure

of 3.8 was confirmed by X-ray crystallography, as shown in Figure 3.6. The

intramolecular parameters o f 3.8 are shown in Table 3.3. The molecule has a mirror plane

that passes through the nitrosyl ligand, the C r atom, one C atom of the Cp ring, and the

methine C atom of the acetylacetonate ligand.

? $ t r i l ^ C r „ _ ^ L ^ ^ C < * > (3.7)

g y g U M

66

Figure 3.6. ORTEP plot of CpCr(NO)(acetylacetonate) (3.8). Thermal ellipsoids of 50%

probability are shown.

Table 3 .3. Selected Bond Lengths and Angles for CpCr(NO)(acetylacetonate) (3.8).

Bond Len gths (A) Bond Ang e s H Cr-0(2) 1.959(2) Cr-N(l)-0(1) 169.5(4) Cr-N(l) 1.683(5) 0(2)-Cr-N(l) 98.6(1) N(l)-0(2) 1.201(5) N(l)-Cr-CP 122.6 Cr-CP 1.89 0(2)-Cr-CP 119.7

67

Aqueous chemistry of [CpCr(NO)(H20)2]+

Since paramagnetic compounds of formula [CpCr(NO)L 2]+ have recently been

demonstrated to be remarkably inert,11 it was sought to expand this class of complexes to

the bis(aqua) mononitrosyl cation [CpCr(NO)(H 20) 2] + which may be synthesized in two

ways, as shown in Scheme 3.3. Suspensions of [CpCr(NO)(p.-I)]2 in hot water slowly

react to give green solutions of [CpCr(NO)(H20)2]+, while [nBu4N][CpCr(NO)(OTf)2]1 9

dissolves in water at room temperature over 5 min to provide the same cationic species.

Addition of aqueous NaBPh* to concentrated solutions of [CpCr(NO)(H 20) 2] + causes the

precipitation of [CpCr(NO)(H20)2][BPri4] (3.11) as analytically pure microcrystals. The

analogous chromium bis(ammonia) tetraphenylborate salt has been structurally

characterized by X-ray crystallography.11

N O H 2

H 2 0

o s o £ o s o I ° I

C F 3 C F 3

NaBPh4

H 2 0

Scheme 3.3. Synthesis of [CpCr(NO)(H 20) 2] +

Unfortunately, the pKa of [CpCr(NO)(H 20) 2] + could not be ascertained due to the

instability of its conjugate base. At pH > 7, solutions of [CpCr(NO)(H 20) 2] + precipitate

[CpCr(NO)(p.-OH)]2 (3.12) in low yields (equation 3.8). Apparently, any hydrogen-

68

bonding interactions that may exist between the expected CpCr (NO)(H 2 0) (OH) product

and the aqueous solvent are insufficient to prevent loss o f H 2 0 and subsequent

expected upon formal substitution of a nitrosyl ligand with a water molecule; H 2 0 should

be a much better leaving group than N O . Solutions of [CpCr (NO) (H 2 0) 2 ] + also react with

acetylacetone, salicylaldehyde or picolinic acid to form compounds 3.8, 3.9, and 3.10,

respectively. These reactions proceed at a qualitatively faster rate than the dinitrosyl

reactions described above, again presumably due to the enhanced lability o f the aqua

ligand compared to the nitrosyl group.

aggregation reactions. This reactivity contrasts with the stability o f CpCr (NO )2 (OH) , 1 2 as

O N /

(3.8) N O O 3.12

69

Summary

The remarkable water stability of the [CpCr(NO)2(H20)]+ fragment has been

known since the original synthesis of this class of compounds by Wilkinson in 1956.1

Subsequent studies directed at the metathesis of the CI ligand of CpCr(NO)2Cl with TC-

donor ligands report only /wowo-nitrosyl products67 Prompted by these two observations,

it was postulated that TC donation from the hydroxide ligand of CpCr(NO)2(OH) might

trigger nitric oxide release under physiologically relevant conditions. This mode of NO

loss was not observed, as CpCr(NO)2(OH) remains unchanged for weeks in basic aqueous

solution.12 The build-up of electron density at the hydroxide oxygen is presumably relieved

through hydrogen-bonding interactions with the aqueous solvent.

While hydrogen bonding may prevent the spontaneous release of nitric oxide from

CpCr(NO)2(OH), it likely helps the loss of NO upon treatment with acetylacetone and

related acids. In reactions which likely consist of sequential hydrogen bonding,

protonolysis and chelate-assisted nitrosyl-displacement steps, NO is liberated at room

temperature in aqueous solution. The identity of the paramagnetic, water-insoluble

mononitrosyl products was confirmed by their independent synthesis from [CpCr(NO)(u.-

I)]2 in aqueous solution and organic solvents.

70

Future Work Chelate assisted N O displacement

The success of the reactions of CpCr(NO) 2 (OH) with H X L sources suggests that

the chelate effect may be helpful in other NO-liberating reactions. Robert Poe is currently

investigating analogues of CpCr(NO)2Cl containing Cp ligands with pendant Lewis basic

functionalities.3 7 Potential monoanionic bidentate X L ligands include theophyline (a

smooth muscle relaxant), 3 8 O N N ( 0 ) R ligands such as Cupferron (which are known to

generate N O in vivo)39 and amino acids (which potentially could form a single

C p C r ( N O ) ( L X ) diastereomer by stereoselective displacement of N O from the prochiral

[CpCr(NO) 2 ] + fragment).40 Also of interest would be the isolation of a C p C r ( N O ) 2 ( r i 1 - L X )

intermediate, prior to the coordination of the L group.

Generation of CpCr(NO)Y species

Designing a route to monomelic C p ' C r ( N O ) Y complexes poses an intriguing and

potentially rewarding challenge. Stabilization of such species might be achieved through a

combination o f steric and electronic factors. As discussed above, Y 7t donation would be

Q O S O Q _

expected to stabilize these complexes as unsaturated species. ' ' The preference for

unsaturation might be further enhanced by utilizing sterically demanding Y and C5R5

ligands. This strategy has been used successfully to isolate and structurally characterize

monomelic C p * R u ( P R 3 ) Y compounds (PR 3 = P 'Pr 2 Ph, P C y 3 ; Y = OR, N H P h ) . 4 1

For the C p C r ( N O ) Y species, increased steric bulk is required to prevent

dimerization to [CpCr(NO)(p.-Y)] 2 complexes. 6 A n instructive comparison may be drawn

to the isolobal, paramagnetic Ti(III) C p ' 2 T i Y complexes; C5H5 species with small O R or

N R 2 ligands form [Cp 2 Ti(u\-Y)] 2 dimers, 4 2 while the use of bulky Y ligands 4 3 or C s M e 5

groups 4 4 permits the isolation of C p ' 2 T i Y monomers. 4 5

71

Cr A M e I T r Cr *• Cr *• C r \

0 N | NPhs / \ "-CH3" v I NPhs N N NPhs N

o 0 0

HOiPr T Cr

"-H-" 'PrO^ I ^O'Pr O'Pr g

Scheme 3.4. Postulated importance of steric bulk in the generation of Cp'Cr(NO)(Y)X

species from CpCr(NO)2Y precursors

Postulating that steric bulk might stabilize Cp'Cr(NO)Y intermediates provides an

appealing explanation for the unexpected synthesis of CpCr(NO)(NPh2)I and

Cp*Cr(NO)(OTJr)2 from dinitrosyl precursors.7 As shown in Scheme 3.4, both reactions

are prevented from forming [Cp'Cr(NO)(p:-Y)]2 dimers by the steric bulk of the NPh 2

7 a or

C 5 H 5

7 b groups, and instead homolytically cleave available R-I or O-H bonds. This type of

radical abstraction chemistry is common for other paramagnetic Cr species,46 but is rarely

observed for d5 CpCr(NO)-containing complexes,11 and constitutes an intriguing reaction

motif for this class of complexes.

72

Scheme 3.5. Possible synthetic routes to Cp'Cr(NO)Y complexes

Scheme 3 . 5 demonstrates possible routes to Cp'Cr(NO)Y complexes, including (i)

metathesis of Cp'Cr(NO) 2Cl, (ii) metathesis of [Cp'Cr(NO)X]2 (X = CI or I),1 9'3 6 (iii)

reduction of CpCr(NO)(N'Pr2)X,2 2 or (iv) sequential nitrosylation and reduction of

(C 5Me 4SiMe 2N'Bu)CrCl(THF). 4 7

73

Experimental

Methods

A l l reactions and subsequent manipulations were performed under anaerobic

conditions using an atmosphere of N 2 unless otherwise noted. General procedures

routinely employed in these laboratories have been described in detail previously. 4 8 The

complexes C p C r ( N O ) 2 C l , 1 , 4 9 C p * C r ( N O ) 2 C l , 1 8 [CpCr(NO)(p-I)] 2 , 3 6 and

[ n Bu 4 N][CpCr(NO)(OTf) 2 ] 1 9 were prepared by the published procedures. A l l other

reagents were used as received from commercial suppliers. Filtrations were performed

through Celite ( 1 x 2 cm) supported on a medium porosity glass frit unless otherwise

specified.

The color, yield, and elemental analysis data for the new compounds in this chapter

are listed in Table 3.4. Infrared and mass spectral data are collected in Table 3.5. Table 3.6

shows the lH and 1 3 C {Ti} N M R data for the diamagnetic dinitrosyl compounds (3.1-3.7).

Synthesis of CpCr(NO)2(OTf) (3.1).

To C p C r ( N O ) 2 C l (0.655 g, 3.08 mmol) and A g O T f (0.793 g, 3.09 mmol) was

added C H 2 C 1 2 (15 mL) followed by E t 2 0 (15 mL). The solution was stirred for 15 min and

then filtered to remove A g C l . The Celite pad was washed with E t 2 0 ( 4 x 5 mL). The

volume of the solution was reduced slightly in vacuo and then hexanes (15 mL) were

added. The volume of the solution was again reduced slightly in vacuo, and the mixture

was then cooled to -30°C overnight, to provide 0.874 g of black crystals o f

CpCr(NO) 2 (OTf) (3.1).

Synthesis of Cp*Cr(NO)2(OTf) (3.2).

To C p * C r ( N O ) 2 C l (0.110 g, 0.38 mmol) and A g O T f (0.100 g, 0.39 mmol) was

added E t 2 0 (30 mL). The solution was stirred for 15 min, and then filtered to remove

74

AgCl. The Celite pad was washed with Et20 (3x10 mL). The volume of the solution was

reduced in vacuo to ~5 mL and the final solution was then cooled to -30°C overnight to

provide 0.125 g of dark brown crystals of Cp*Cr(NO)2(OTf) (3.2).

Synthesis of [CpCr(NO) 2L][OTf] (L = N H 3 (3.3), N H M e 2 (3.4), NH 2 'Bu (3.5), NH 2 Ph

(3.6)).

The four amine adducts 3.3-3.6 were synthesized using the same general

preparative procedure outlined here for the ammine complex 3.3.

To a stirred solution of CpCr(NO)2(OTf) (0.334 g, 1.02 mmol) in Et 2 0 (35 mL)

was added an excess of CH 2C1 2 saturated with N H 3 (~5 mL). A green powder immediately

precipitated from solution. The pale yellow solution was removed by cannulation, and the

solid was washed with pentane (2x5 mL). Drying in vacuo provided 0.206 g of

[CpCr(NO)2(NH3)][OTf] as a green powder.

The other amine adducts were prepared using either a CH2Cl2-saturated solution of

the amine (3.4) or the neat amine (3.5 and 3.6) in place of N H 3 .

Synthesis of CpCr(NO) 2 (N 2 C 5 H 7 ) (3.7).

To [CpCr(NO)2(3,5-dimethylpyrazole)][OTf] (0.232 g, 0.55 mmol) and KO'Bu

(0.068 g, 0.61 mmol) was added THF (-10 mL) via vacuum transfer. The solution was

stirred for 80 min in an acetone-dry ice bath, and then the bath was removed and the

solvent was removed in vacuo. The residue was extracted with Et 2 0 (15 mL) and filtered

to remove KOTf. The Celite pad was washed with Et 2 0 (3x5 mL). The volume of the

combined filtrates was reduced in vacuo, and the solution was cooled to -30°C overnight,

to provide 0.070 g of CpCr(NO)2(3,5-dimethylpyrazolyl) (3.7) as black-green crystals.

75

Synthesis of C p C r ( N O ) ( L X ) ( L X = 0 2 C 5 H 7 (3.8), 0 2 C 7 H 5 (3.9), N C 6 H 4 0 2 (3.10)).

Method 1. To CpCr(NO) 2 (OTf) (0.038 g, 0.117 mmol) and C s O H H 2 0 was added

water (10 mL). The solution was stirred for 30 min, and acetylacetone (0.020 mL, 0.20

mmol) was added via syringe. The solution was allowed to stir for 2 d while a green

powder precipitated from solution. The solid was collected by filtration in air, washed with

cold water, and dried in vacuo to obtain 0.012 g of CpCr(NO)(acetylacetonate) (3.8) as a

green powder.

Method 2. To a stirred solution of [ n Bu 4 N][CpCr(NO)(OTf) 2 ] (0.118 g, 0.182

mmol) in water (5 mL) was added acetylacetone (0.025 ml, 0.243 mmol). Within 1 min, a

green solid precipitated from solution. After 1 h, the solid was collected by filtration in air,

washed with water and dried in vacuo to obtain 0.089g o f a green powder which was

identified as 3.8 by IR spectroscopy and mass spectrometry.

Method 3. T H F (~5 mL) was vacuum transferred onto [CpCr(NO)(u.-I)] 2 (0.083 g,

0.151 mmol) and potassium acetylacetonate (0.052 g, 0.377 mmol). E t 2 0 (5 mL) was

added, and the solution was stirred for 12 h. The solution was filtered through alumina (1

x 2 cm), and the column was washed with E t 2 0 ( 2 x 1 0 mL). The combined filtrates were

reduced in vacuo to one-third volume, and hexanes were added (10 mL). The solution

was cooled to -30°C overnight to obtain 0.051 g o f green crystals which were identified as

3.8 by IR spectroscopy, mass spectrometry, and single-crystal X-ray diffraction.

Compounds 3.9 and 3.10 were synthesized by the methods used for 3.8,

substituting acetylacetone with salicylaldehyde and picolinic acid, respectively.

Synthesis of [CpCr(NO)(H20)2][BPh4] 3.11.

A suspension o f [CpCr(NO)(p-I)] 2 (0.260 g, 0.474 mmol) in water (10 mL) was

stirred and heated to 70°C for 30 min. The clear green solution was filtered, and the

filtrate was treated with NaBPru (0.680 g, 1.99 mmol) in water (5 mL). A microcrystalline

76

solid immediately formed and was collected by filtration, washed with water (2x5 mL)

then with hexanes (4x5 mL) to obtain 0.280g of [CpCr(NO)(H20)2][BPh4] (3.11).

Synthesis of [CpCr(NO)(n-OH)] 2 3.12.

A suspension of [CpCr(NO)(p-I)]2 (0.260 g, 0.474 mmol) in water (10 mL) was

stirred and heated to 70°C for 30 min. The clear green solution was filtered, and the

filtrate was treated with 1 M NaOH until the pH was 8.6. Within 10 min, brown powder

started to precipitate from solution. After being stirred for 12 h, the solution was filtered

in air, and the collected solid was washed with water (2x5 mL) to obtain 0.020g of

[CpCr(NO)(u.-OH)]2 (3.12) as a gold powder.

77

Characterization Data Table 3.4. Numbering Scheme, Color, Yield and Elemental Analysis Data

Complex Cmpd #

Color (yield, %)

Elemental analysis found (calcd) Complex Cmpd #

Color (yield, %) C H N

CpCr(NO)2(OTf) 3.1 black (87) 22.28 (22.09) 1.57(1.55) 8.49 (8.59)

Cp*Cr(NO)2(OTf) 3.2 brown (82) 33.26 (33.33) 3.87 (3.81) 6.88 (7.07)

[CpCr(NO)2(NH3)][OTf] 3.3 green (60) 21.17(21.00) 2.23 (2.35) 12.21 (12.24)

[CpCr(NO)2(NHMe2)] [OTf] 3.4 green (42) 25.84 (25.88) 3.12(3.26) 11.21 (11.32)

[CpCr(NO)2(NH2'Bu)] [OTf] 3.5 green (52) 30.39 (30.08) 4.30 (4.04) 10.33 (10.52)

[CpCr(NO)2(NH2Ph)] [OTf] 3.6 green (80) 34.27 (34.37) 2.88 (2.88) 10.01 (10.02) CpCr(NO)2(N2C5H7) 3.7 green (47) 43.36(44.12) 4.33 (4.44) 19.84 (20.58) CpCr(NO)(02C5H7) 3.8 green (68) 48.49 (48.78) 4.96 (4.91) 5.57 (5.69)

CpCr(NO)(02C7H5) 3.9 brown (84) 53.64 (53.74) 3.79 (3.76) 5.15 (5.22)

CpCrCNOCNCeHtOz) 3.10 green (59) 48.92 (49.08) 3.36 (3.37) 10.41 (10.41) [CpCr(NO)(H20)2][BPh4] 3.11 green (53) 69.50 (69.34) 5.86 (5.82) 2.80 (2.80)

[CpCr(NO)(M)H)]2 3.12 gold (22) 36.35 (36.59) 3.30 (3.66) 8.38 (8.54)

Table 3.5. Infrared v(NO) and Mass Spectral Data

Complex m(cm_1) FAB/MS (m/z) Complex

Nujol CH2C12 P+ P+-NO P+-2NO

3.1 1834 1729

1836 1730

326 296 266

3.2 1794 1708

1801 1702 396 366 336

3.3 1824 1735

1830 1727

194 - -

3.4 1826 1709

1825 1723

222 192 -

3.5 1818 1700

1822 1720

250 220 190

3.6 1823 1710

1830 1728

270 240 210

3.7a 1800 1686

1808 1701

272 242 212

3.8 1647 1656 246 216 -

3.9 1659 1677 268 238 -

3.10 1666 1683 269 239 -

3.11 1692 - - - -

3.12 1595 - 328 298 268

" Solution Infrared spectrum recorded in Et 20 instead of CH2C12.

78

Table 3.6. 'H and 1 3C N M R Data

Compound (solvent) * H N M R (8) 1 3 C { !H} N M R (8)

3.1 (CDC1 3 )

5.82 (s, CsHs)

3.2 (CDCI3)

1.86 (CsMe5)

3.3 ((CD 3) 2CO)

3.72 (br s, 3 H , N// 3) 6.15 (s, 5 H , Cstfj)

104.3 (CjH 5)

3.4 ((CD 3) 2CO)

2.77 (d, 6 H , N H M ? 2 ) , 5.52 (br s, IH, N/flVfej), 5.96 (s, 5 H , CsHs)

3.5 (CDCb)

1.34 (s, 9 H , NH 2CM? 3), 4.16 (s, 2 H , N// 2CMe 3), 5.94 (s, 5 H , CsHs)

30.0 (NH2'Bu), 45.1 (NH2'£w)> 103.3 (CjH 5)

3.6 (CDCI3)

5.82 (s, 5 H , C5//5), 6.38-7.22 (m, 5 H , NH2Ph) 102.8 (CjH 5), 119.9 (NH 2 P/J) , 125.3 (NH2Ph), 129.5 (NH 2 /VJ)

3.7 (C6D6)

2.19 (s, 3 H , CH3), 2.53 (s, 3 H , CH3), 4.79 (s, 5 H , CsHs), 6.18 (s, IH, CH)

14.7 (CH 3), 15.0 (CH 3), 102.8 (C5H5), 105.3 ( C H ) ,

79


(1) Piper, T. S.; Wilkinson, G. J. Inorg. Nucl. Chem. 1956, 2, 38.

(2) Organometallic 7c-arene complexes of Cr(I) were synthesized as early as 1918

by the reaction of CrCl 3 and PhMgBr. While Hein and co-workers studied these products

in the 1920's and 1930's, the true nature of these species went unrecognized until more

advanced synthetic techniques, spectroscopic tools and theoretical models suitable for

organometallic compounds were developed in the wake of the discovery of ferrocene. For

an excellent historical essay describing Hein's "polyphenylchromium" complexes, see

Uhlig, E. Organometallics 1993,12, 4751.

(3) Wilkinson, G.; Cotton, F. A. Prog. Inorg. Chem. 1959, 7, 1.

(4) (a) Piper, T. S.; Wilkinson, G. J. Inorg. Nucl. Chem. 1956, 3, 104. (b) Hanna,

J. A.; Wojcicki, A. Inorg. Chim. Acta. 1974, 9, 55. (c) Hoyano, J. K.; Legzdins, P.;

Malito, J. T. J. Chem. Soc, Dalton Trans. 1975, 1022. (d) Legzdins, P.; Richter-Addo, G.

B.; Wassink, B.; Einstein, F. W. B.; Jones, R. FL; Willis, A. C. J. Am. Chem. Soc. 1989,

777, 2097. (e) Hubbard, J. L . ; McVicar, W. K. Organometallics 1990, 9, 2683.

(5) (a) King, R. B.; Welcman, N. Inorg. Chem. 1969, 8, 2540. (b) Bush, M . A.;

Sim, G. A. J. Chem. Soc. A 1970, 605. (c) Frobose, R.; Mews, R.; Glemser, O. Z.

Naturforsch. B 1976, 31, 1497. (d) Hames, B. W.; Legzdins, P. Organometallics 1982, 7,

116. (e) Hubbard, J. L. ; Zoch, C. R.; Elcesser, W. L. Inorg. Chem. 1993, 32, 3333.

(6) (a) Ahmad, M . ; Bruce, R.; Knox, G. Z. Naturforsch. B 1966, 27, 289. (b)

McPhail, A. T.; Sim, G. A. J. Chem. Soc, Dalton Trans. 1968, 1858. (c) Bush, M . A.;

Sim, G. A.; Knox, G. R.; Ahmad, M . ; Robertson, C. G. J. Chem. Soc, Chem. Commun.

1969, 74. (d) Bush, M . A.; Sim, G. A. J. Chem. Soc. A 1970, 611. (e) Kolthammer, B. W.

80

S.; Legzdins, P.; Malito, J. T. Inorg. Chem. 1977,16, 3173. (f) Chisholm, M . H.; Cotton,

F. A.; Extine, M . W.; Rideout, D.C. Inorg. Chem. 1979,18, 120. (g) Hardy, A. D. U.;

Sim, G. A. Acta Crystallogr., Sect. B 1979, 35, 1463.

(7) (a) Sim, G. A.; Woodhouse, D. I.; Knox, G. R. J. Chem. Soc, Dalton Trans.

1979, 83. (b) Hubbard, J. L. ; McVicar, W. K. Inorg. Chem. 1992, 31, 910.

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(9) Caulton, K. G. NewJ. Chem. 1994,18, 25.

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Legzdins, P.; Pang, C. C. Y.; Shaw, M . J. U.S. Patent 5631284, 1997.

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Chem. Soc. 1995,117, 10521.

(12) Tong, V. B.Sc. Thesis, University of British Columbia, Apr. 1996.

(13) Legzdins, P.; Rettig, S. J.; Smith, K. M . ; Tong, V.; Young, V. G., Jr. J.

Chem. Soc, Dalton Trans. 1997, 3269.

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F.; Fish, R. H. Organometallics 1995,14, 2806. (c) Svetlanova-Larsen, A.; Zoch, C. R.;

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1992, 29, 193.

(22) Kuzelka, J.; Legzdins, P.; Rettig, S. J.; Smith K. M . Organometallics 1997,

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(23) Another instructive parallel may be drawn to cationic [CpCr(NO)2(=CR2)]+

complexes. While X~ abstraction reactions from CpCr(NO) 2(CH 2X) precursors failed to

generate stable methylidene (R = H) complexes,46 the heteroatom-substituted carbene

complexes [CpCr(NO)2(=C(OR)NHCH3)][PF6] are stable.17

(24) In order to simplify the discussion, the observed Cp components of the

Cr-NO TC-bonding interactions have been neglected. The E H M O calculation is based on

the more recent of the two X-ray crystallographic studies25 of CpCr(NO) 2Cl. 1

(25) (a) Carter, O. L. ; McPhail, A. T.; Sim, G. A. J. Chem. Soc. A 1966, 1095. (b)

Greenhough, T. J.; Kolthammer, B. W. S.; Legzdins, P.; Trotter, J. Acta Crystallogr.,

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(26) Ashby, M . T.; Enemark, J. H ; Lichtenberger, D. L. Inorg. Chem. 1988, 27,

82

191.

(27) Many o f the known [CpFe(CO)(p.-SR)]2 complexes were synthesized by G .

R. Knox and co-workers in the 1960's and 1970's, the same U . o f Strathclyde research

group which generated the first [CpCr(NO)(p.-Y)] 2 compounds: 6 a" d g , 7 a (a) King , R. B . ;

Bisnette, M . B . J. Am. Chem. Soc. 1964, 86, 1267. (b) Ahmad, M . ; Bruce, R.; Knox, G .

R. J. Organomet. Chem. 1966, 6, 1. (c) Dekker, M . ; Knox, G . R.; Robertson, C. G . J.

Organomet. Chem. 1969,18, 161. (d) Bladon, P.; Dekker, M . ; Knox, G . R.; Willison, D . ;

Jaffari, G . A . ; Doedens, R. J.; Muir , K . W . Organometallics 1993,12, 1725.

(28) Bryndza, H . E . ; Domaille, P. J.; Paciello, R. A . ; Bercaw, J. E .


(29) (a) Saura-Llamas, I.; Gladysz, J. A . J. Am. Chem. Soc. 1992,114, 2136. (b)

Dewey, M . A . ; Knight, D . A . ; Arif, A . ; Gladysz, J. A . Chem. Ber. 1992,125, 815. (c)

Dewey, M . A . ; Stark, G . A . ; Gladysz, J. A . Organometallic 1996,15, 4798.

(30) M c N e i l , W . S. Ph.D. Thesis, University o f British Columbia, Dec. 1995.

(31) (a) Glueck, D . S.; Newman Winslow, L . J.; Bergman, R. G . Organometallics

1991,10, 1462. (b) Glueck, D . S.; Bergman, R. G . Organometallics 1991, 10, 1479.

(32) Legzdins, P.; Martin, D . T.; Nurse, C. R.; Wassink, B . Organometallics 1983,

2, 1238.

(33) (a) Bryndza, H . E . ; Fong, L . K . ; Paciello, R. A . ; Tarn, W. ; Bercaw, J. E . J.

Am. Chem. Soc. 1987,109, 1444. (b) Bergman, R. G Polyhedron 1995, 14, 3227. (c)

Kapteijn, G . M . ; Spee, M . P. R.; Grove, D . M . ; Koojiman, H . ; Spek, A . L . ; van Koten, G .


(34) (a) Simpson, R. D . ; Bergman, R. G . Organometallics 1992,11, 3980. (b)

83

Simpson, R. D.; Bergman, R. G. Organometallics 1993,12, 781.

(35) Fischer, E. O.; Strametz, H. J. Organomet. Chem. 1969,10, 323.


(37) Poe, R. J.; Legzdins, P. work in progress.

(38) (a) Kistenmacher, T. J.; Szalda, D. J.; Marzilli, L. G. Inorg. Chem. 1975,14,

1686. (b) Szalda, D. J.; Kistenmacher, T. J.; Marzilli, L. G. Inorg. Chem. 1975, 14, 2783.

(c) Sorrell, T.; Marzilli, L. G.; Kistenmacher, T. J. J. Am. Chem. Soc. 1976, 98, 2181. (d)

Norris, A. R.; Taylor, S. E. ; Buncel, E. ; Belanger-Gariepy, F.; Beauchamp, A. L. Inorg.

Chim. Acta 1984, 92, 271.

(39) (a) Middleton, A. R.; Wilkinson, G. J. Chem. Soc, Dalton Trans. 1981,

1898. (b) Legzdins, P.; Rettig, S. J.; Sanchez, L. Organometallics 1988, 7, 2394. (c) Yi,

G.-B.; Khan, M . A.; Richter-Addo, G. B. Inorg. Chem. 1995, 34, 5703. (d) Schneider, J.

L.; Young, V. G., Jr.; Tolman, W. B. Inorg. Chem. 1996, 35, 5410.

(40) Amino acids have recently been used as chelating ligands for zero-valent

group-six transition metals40a and [Cp'Mo(NO)I]+:4 0 b (a) Darensbourg, D. L.; Draper, J.

D. ; Reibenspies, J. H. Inorg. Chem. 1997, 36, 3648. (b) Maurus, M . ; Aechter, B.;

Hoffmuller, W.; Polborn, K.; Beck, W. Z. Anorg. Allg. Chem. 1997, 623, 299.

(41) Johnson, T. J.; Folting, K.; Streib, W. E. ; Martin, J. D.; Huffman, J. C.;

Jackson, S. A.; Eisenstein, O.; Caulton, K. G. Inorg. Chem. 1995, 34, 488.

(42) (a) Lappert, M . A.; Sanger, A. R. J. Chem. Soc. A 1971, 1314. (b) Samuel,

E. ; Harrod, J. F.; Gourier, D.; Dromzee, Y.; Robert, F.; Jeannin, Y. Inorg. Chem. 1992,

31, 3252.

(43) Cetinkaya, B.; Hitchcock, P. B.; Lappert, M . F.; Torroni, S.; Atwood, J. L. ;

84

Hunter, W . E . ; Zaworotko, M . J. J. Organomet. Chem. 1980,188, C31.

(44) (a) Pattiasina, J. W. ; Heeres, H . J.; Van Bolhuis, F. ; Meetsma, A . ; Teuben, J.

H . ; Spek, A . L . Organometallics 1987, 6, 1004. (b) Feldman, J.; Calabrese, J. C. J. Chem.

Soc, Chem. Commun. 1991, 1042. (c) Luinstra, G . A . ; Vogelzang, J.; Teuben, J. H .


(45) Lukens, W . W. ; Smith, M . R.; Andersen, R. A . J. Am. Chem. Soc. 1996, 118,

1719 and references contained therein.

(46) (a) Kochi , J. K . ; Powers, J. W . J. Am. Chem. Soc. 1970, 92, 137. (b)

Espenson, J. H . Prog. Inorg. Chem. 1983, 30, 189. (c) Tyler, D . R. Prog. Inorg. Chem.

1988, 36, 125. (d) Baird, M . C. Chem. Rev. 1988, 88, 1217. (e) Espenson, J. H . Acc.

Chem. Res. 1992, 25, 222. (f) Huber, T. A . ; Macartney, D . H . ; Baird, M . C.


(47) Liang, Y . ; Yap, G . P. A . ; Rheingold, A . L . ; Theopold, K . H . Organometallics

1996, 15, 5284.

(48) Legzdins, P.; Rettig, S. R.; Ross, K . J.; Batchelor, R. J.; Einstein, F. W . B .


(49) Hoyano, J. K . ; Legzdins, P.; Malito, J. T. Inorg. Synth. 1978,18, 126.

85

Chapter 4: Synthetic Utility of the [CpMo(NO)(CH2Ph)]+ Fragment

Introduction 86


Summary 99

Future Work 1 0 0

Experimental 105


References and Notes 1 0 9

86

Introduction The majority o f the complexes described in this chapter were derived from

C p M o ( N O ) ( C H 2 P h ) C l . 1 The Mo(U), d 4 , benzyl chloride species was used to synthesize

new C p M o ( N O ) ( C H 2 P h ) R and [CpMo(NO)(CH 2 Ph)L][OTf] complexes. The triflate salts

were treated with Bransted bases in an attempt to generate CpMo(NO)(=CHPh)L

benzylidene compounds. In order to put these reactions in their proper context,

background information is required on (a) the use of [CpMo(NO)(CO)(al lyl)] + cations in

organic synthesis,2 and (b) C - C bond forming 3 and C - H bond activating 4 reactions o f

C p ' M ( N O ) L ( M = M o , W) species.

The development o f transition-metal-based reagents is regarded as one of the

most important areas o f current and potential progress in organic synthesis.5 Among other

applications, cationic metal complexes are commonly used to bind to unsaturated organic

moieties, activating the bound group for specific attack by nucleophilic reagents. Scheme

4.1 depicts a well-studied CpMo-based system that illustrates this type of sequential,

stoichiometric, "ligand elaboration" reactivity.6

R

[ N O ] B F 4

[O] M o R . - B F 4

R R

R' R '

Scheme 4 . 1 . Application of CpMo-containing complexes to organic synthesis

87

The initial organomolybdenum species is synthesized by treating in situ-

generated CpMo(CO) 3(FBF 3) with a diene to form [CpMo(CO)2(r|4-diene)][BF4] (A).

Coordination to the cationic CpMo(CO) 2

+ fragment renders the diene prone to

nucleophilic attack, affording the CpMo(CO)2(Y|3-allyl) ( B ) species. In order to

regenerate a reactive cationic species, one of the carbonyl ligands of B is substituted with

N O + to obtain [CpMo(KO)(CO)(n3-allyl)][BF4] (C). Addition of a second nucleophile to

C creates a Mo(0), d 6 complex, CpMo(NO)(CO)(ri2-olefin) (D) . Oxidation or hydrolysis

is then employed to liberate the organic product from the metal center. Each of the

individual reactions outlined in Scheme 4.1 has been individually studied, with great

attention being paid to the isomeric possibilities, the potential scope of nucleophiles and

organic substrates, and issues of regio- and stereoselectivity. More recent work has

extended the CpMo-based chemistry to related CpW, 7 TpMo, 8 TpW, 9 and T p * W 1 0

Scheme 4.2. Potential extension of known CpMo-based organic synthesis to C - H bond

activation and C - C bond formation reactions.

Scheme 4.2 shows two additional synthetic steps that might be achieved if the

carbonyl ligand could be induced to dissociate from D to form the unsaturated, 16e

CpMo(NO)(n2-olefin) (E) moiety. Related Cp'M(NO)L (M = Mo, W) species have been

systems.

K

88

invoked to explain a wide range of C - H bond activation and C - C bond formation

reactions that initially afford complexes analogous to CpMo(NO)(alkyl) 2 (F) and

CpMo(NO)(metallacycle) (G), respectively. 1 1' 1 2 Unfortunately, the M o - C O bond is quite

strong due to the n bonding interaction with the filled, high-energy dxy orbital orthogonal

to the M o - N O axis, 1 3 and so C O loss from D is not expected to occur even under

thermolytic or photolytic conditions. In order for the desired reactivity to be realized, an

analog of D has to be synthesized that contains a less strongly bound, 7t-neutral, 2e a -

donor ligand (L) in place of the 7t-acceptor C O group. However, initial attempts to

generate the target CpMo(NO) (C 3 H 5 )L + cations by reaction of CpMo(NO) (C 3 H 5 ) (CO) +

or CpMo(NO)(C 3 Hs)(OTf) with PPh 3 were unsuccessful, presumably due to the

susceptibility of the allyl ligand towards nucleophilic attack. To avoid these difficulties, a

different precursor was required in order to explore the possibility of C - H bond

activation and C - C bond formation reactions involving CpMo(NO)-containing

complexes (Scheme 4.3).

© O T f 3

Base

Scheme 4.3. Planned reactions to investigate the synthetic utility of the

[CpMo(NO)(CH2Ph)]+ fragment

89

The CpMo(NO)(CH2Ph)Cl starting material was chosen for five reasons. First,

it is readily available in high overall yield from the [CpMo(NO)Cl(u.-Cl)]2 d imer 1 4 in two

steps, unlike other CpM(NO)(R)Cl complexes which are only accessible via

hydrogenation of CpM(NO)R.2 in N C M e , followed by treatment o f the resulting

CpM(NO)(N=C(H)Me)R species with H C I . 1 5 Second, the r| 2-benzyl interaction typically

observed in Cp 'M(NO)(CH2Ph)X complexes 1 (Figure 4.1) was expected to stabilize

CpMo(NO)(CH2Ph)(OTf), thereby allowing access to [CpMo(NO)(CH 2 Ph)(L)][OTfJ

salts in two steps from the benzyl chloride. Third, the orientation o f the Ti 2 -benzyl ligand

and the lack of substituents on the Cp ring render the CH2Ph ct-H's exposed for

intermolecular deprotonation, unlike related C p * W compounds which form "tucked-in"

Me4C5CH2 complexes upon treatment with strong bases. 1 6 Fourth, the r | 2 -benzyl ligand

was expected to stabilize the products of C - H activation. Fifth, it was anticipated that the

reaction of R~ reagents with the benzyl chloride precursor would afford the

CpMo(NO)(CH2Ph)R complexes directly. This independent synthesis would provide an

opportunity to assess the stability o f the CpMo(NO)(CH 2 Ph)R products and establish

their diagnostic spectroscopic properties prior to attempting to generate them via the

CpMo(NO)(=CHPh) (H) intermediate.

Figure 4.1.T | 2-Benzyl interaction in C p ' M ( N O ) ( C H 2 P h ) X species.

90

Results and Discussion Direct Synthesis of CpMo(NO)(CH 2Ph)R.

R 2 M g / T H F

cr | CI ' N

O

R = CH2'Bu,o4oryl, R Ph, CH2SiMe3 N

R ' 2 M g / T H F •

'C I R'= CH2'Bu, o-tolyl, R ' Ph, CH2SiMe3, Me

\v (4-1) | ^ R -

N O

Equation 4.1 illustrates the procedure that has previously been used to synthesize

Cp*W(NO)(R)R ' mixed bis(alkyl) compounds. 1 7 The initial alkylation must be

performed at low temperatures, otherwise a 50:50 mixture o f the dichloro starting

material and the symmetric bis(alkyl) complex wi l l be formed. The second alkylating

reagent can be added to isolated Cp*W(NO)(R)Cl , or the alkyl chloride intermediate can

be generated and used in situ.

This synthetic protocol unfortunately cannot be extended to the CpMo(NO)(R)R '

species due to the decreased solubility of the unsubstituted cyclopentadienyl derivatives.

Reactions of [CpMo(NO)(Cl)(p-Cl)] 2 with one-half equivalent o f R 2 Mgx(dioxane) result

in the formation o f C p M o ( N O ) R 2 , as the initially-generated CpMo(NO)(R)Cl species

reacts more readily with the Grignard reagent than the comparatively insoluble dichloro

dimer. As a result, none of the CpMo(NO)(R)R' analogues of the compounds illustrated

in equation 4.1 have yet been synthesized.

ci-M o

N O

X I

B z 2 M g

T H F

HCI

C H 2 C 1 2

(4.2)

The C p M o ( N O ) ( C H 2 P h ) C l precursor used in the current study was prepared as

shown in Equation 4.2. The dichloro dimer is treated with Mg(CH 2 Ph) 2 x(dioxane) , and

91

then the isolated bis(benzyl) product is dissolved in dichloromethane and treated with

an excess of H C I dissolved in E t 2 0 , which selectively cleaves just one Mo-CH2Ph bond. 1

R R " - xL 4.1 M e (4.3)

T H F r " I ^ R 4 ' 2 P h

4.3 C C P h 4.4 Cp

Even after the desired benzyl chloride precursor has been acquired, the decreased

solubility o f the C p M o ( N O ) ( C H 2 P h ) X complexes compared to the Cp*W(NO)(R)R ' or

Cp*M(NO)(CH 2 CMe3)X derivatives continues to hamper the synthesis o f

CpMo(NO)(CH 2 Ph)R complexes. The reaction shown in Equation 4.3 proceeds cleanly

in T H F as determined by monitoring the change in v (NO) during the course o f the

reaction by solution IR spectroscopy. The metathesis can be accomplished using a

diverse range of alkylating reagents, including organolithium and organosodium species

as well as the more typical Grignard complexes. Unfortunately, the lack o f solubility o f

the CpMo(NO)(CH2Ph)R complexes 4.1-4.4 in alkane solvents hinders their complete

separation from the ionic byproducts o f the reaction. While these impurities are not

apparent in the solution IR spectra of the crude benzyl alkyl products, they are quite

evident in their elemental analyses, which are consistently low in carbon content.

Analytically-pure, crystalline samples may be obtained by chromatography on Alumina I

using toluene as eluant, followed by recrystallization from toluene:hexanes solvent

mixtures.

The replacement of the electronegative CI ligand with more covalently-bound

hydrocarbyl groups has the expected effect on the electron density at the metal center.

This is indicated by the v(NO) bands in the Nujol-mull IR spectra of the

CpMo(NO)(CH 2 Ph)R compounds which are from 14 to 44 c m - 1 lower in frequency than

the 1620 c m - 1 value exhibited by C p M o ( N O ) ( C H 2 P h ) C l . 1 The magnitude o f this decrease

92

in v(NO) follows the trend C ^ C P h < Cp ~ Ph ~ C H 2 P h < C H 2 S i M e 3 ~ M e , which is

consistent with the electron-donating abilities o f these hydrocarbyl ligands. The IR

spectrum o f 4.3 also displays a v (C=C) band at 2091 c m - 1 . 1 8

In their T I N M R (C6D6) spectra, the C5H5 signals o f compounds 4 .1-4.4 lie in the

4.80 to 5.20 ppm region and are expected to be useful diagnostic indicators o f these

complexes. Only a single resonance for both Cp ligands is observed in the room

temperature ' H and 1 3 C N M R spectra of C p 2 M o ( N O ) ( C H 2 P h ) . Also significant is the

signal at -0.88 ppm characteristic of a M e ligand in the ! H N M R (CeDe) spectrum of 4.1.

The I 3 C N M R spectra o f complexes 4.1-4 .3 are important chiefly for the ipso C signals

of the C H 2 P h ligands which fall in the 110 to 113 ppm range indicative o f a rj 2-benzyl

interaction. 1 , 1 9

Complex 4.4 does not appear to possess a r) 2-benzyl ligand according to its ' H

and 1 3 C N M R spectra. The benzyl methylene lH N M R signal appears as a singlet at room

temperature, and the ipso C signal occurs at 152.3 ppm in the 1 3 C N M R spectrum o f 4.4.

The presence o f a V-benzyl ligand in 4.4 was confirmed by an X-ray crystallographic

analysis. The solid-state molecular structure o f C p 2 M o ( N O ) ( C H 2 P h ) is shown in Figure

4.1, and selected bond lengths and angles are collected in Table 4.1.

93

Figure 4.2. O R T E P plot of C p 2 M o ( N O ) ( C H 2 P h ) (4.4). Thermal ellipsoids of 50%

Table 4.1. Selected Bond Lengths, Bond Angles, and Torsion Angles for

C p 2 M o ( N O ) ( C H 2 P h ) (4.4).

Bond Lengths (A) Bond Angles (") Torsion An eles (°) M o - C ( 4 ) 2.700(3) M o - N ( l ) - 0 ( l ) 174.1(2) C ( l ) - C ( 2 ) -

C(3)-C(4) 7.0(4) M o - C ( 5 ) 2.667(3) M o - C ( l l ) - C ( 1 2 ) 117.4(2)

C ( l ) - C ( 2 ) -C(3)-C(4) 7.0(4)

C(4)-C(5) 1.317(5) C P ( l ) - M o - C P ( 2 ) 119.8 C ( 3 ) - C ( 2 ) -C ( l ) - C ( 5 ) 8.0(4)

M o - C ( l l ) 2.258(3) N ( l ) - M o - C ( l l ) 87.66(10) C ( 3 ) - C ( 2 ) -C ( l ) - C ( 5 ) 8.0(4)

94

The nature o f the C p - M o bonds in C p 2 M o ( N O ) X species has been the subject

of debate since their initial synthesis almost 30 years ago. 2 0 , 2 1 I f these compounds

possessed two t | 5 -Cp groups and a linear nitrosyl ligand, they would have a formal

electron count o f 20e. Early suggestions that this unfavorable situation might be avoided

by adopting an asymmetric n 5 -Cp / r | 3 -Cp binding mode 2 0 a b was not supported by

subsequent X-ray crystallographic studies. The solid state molecular structures o f

C p 2 M o ( N O ) ( r i 1 - C p ) 2 0 c , C p 2 M o ( N O ) ( C H 3 ) , 2 0 d and C p * ( C p ) M o ( N O ) ( C H 3 ) 2 0 e all exhibited

essentially planar but skewed C p ' groups with all 10 multihapto ring C atoms lying

within bonding distance o f the M o center. This contrasts with the bonding in C p 2 W ( C O ) 2 ,

which possesses distinct r | 5 -Cp and r f - C p ligands. 2 2 The rj 3-Cp group contains a

localized C - C double bond, with the two unique C atoms bent 20° away from the plane

containing the three C s bound to the W center.

The solid state molecular structure of C p 2 M o ( N O ) ( C H 2 P h ) (4.4) shown in Figure

4.1 exhibits features intermediate between C p 2 W ( C O ) 2 and the previously-reported

C p 2 M o ( N O ) R structures. A l l ten Cp C s are within 2.336 to 2.700 A (average = 2.475 A)

of the M o center, and the difference between the two Cp groups is not as marked as in the

C p 2 W ( C O ) 2 structure. However, the C( l ) -C(5) ring exhibits characteristics more typical

of a r | 3 -Cp ligand than previously observed for any other structurally-characterized

C p 2 M o ( N O ) R species. The C(4) and C(5) atoms are 2.700(3) and 2.667(3) A away from

the M o atom, respectively, and the C(4)-C(5) bond length o f 1.317(5) A is significantly

shorter than the other C - C distances in the ring (1.392(5) to 1.407(5) A, average 1.401

A). The C ( l ) - C ( 2 ) - C ( 3 ) - C ( 4 ) and C(3 ) -C(2 ) -C( l ) -C(5 ) torsion angles o f 7.0(4)° and

8.0(4)°, respectively, are also reminiscent of, i f less pronounced than, those found in

C p 2 W ( C O ) 2 . While these distortions may be attributable to some inherent difference

between 4.4 and other C p 2 M o ( N O ) R complexes, they may also simply reflect crystal

95

packing forces, or the high quality o f the current structural determination relative to the

older X-ray crystallographic studies.

Stabilizing Effects of r j 2 -CH 2 Ph.

The coordinative and electronic saturation imparted by the rj 2-benzyl ligand is

deemed to be responsible for the stability o f compounds 4.1-4.3. O f the known

C p ' M ( N O ) R 2 (Cp ' = C5H5, C 5 M e 5 ; M = M o , W) complexes, the CpMo-containing

species are the most Lewis acidic, the most prone to M - C bond hydrolysis, and the most

thermally sensitive. 2 3 For example, CpMo(NO)(CH 2CMe3) 2 spontaneously undergoes

elimination of CMe4 via a - H abstraction at room temperature,2 4 while the

C p * W ( N O ) ( C H 2 C M e 3 ) 2 analog requires heating to 70°C for 2 days to duplicate this

mode of r e a c t i v i t y . u b e 2 5 Due to this increased reactivity, the methodologies developed for

other 16e C p ' M ( N O ) R 2 complexes may be used to generate C p M o ( N O ) R 2 compounds

with relatively small R groups (e.g. R = Ph, /?-tolyl, o-tolyl) at low temperatures, but

these species decompose at ambient temperature even in the presence o f trapping

agents.2 6

Figure 4.3.; 18e CpM(NO)R-containing complexes

The fragility o f the 16e C p M o ( N O ) R 2 species is in marked contrast to the robust

18e complexes shown in Figure 4.2. These compounds achieve a saturated electronic

configuration via additional a-donor ligation, 2 7 lower oxidation state,2 8 or multihapto

ancillary ligand bonding. 1 8 ' 2 0 A l l o f the species in Figure 4.2 are thermally stable at

ambient temperature, and several are remarkably tolerant to air and/or water. The range

96

of hydrocarbyl ligands available to these 18e complexes extends to groups which are

difficult to synthesize (e.g. Me , Ph) or entirely unknown (e.g. Et, 'Bu , C = C R , CeFs) for

the 16e C p ' M ( N O ) ( R ) R ' compounds.

Synthesis of [CpMo(NO)(CH 2Ph)L][OTfJ.

The stabilizing influence o f the benzyl ligand is also o f critical importance to

C p M o ( N O ) ( C H 2 P h ) O T f (4.5), synthesized in high yields as illustrated in equation 4.4.

^ A g O T f > ^ ( 4 4 )

| CI C H 2 C I 2 : E t 2 0 f | ^ O T f

P 8 ( ^ O 4.5

Silver salts have been previously shown to abstract halide ligands from

Cp'M(NO)-containing complexes. If this reaction is performed using AgBF4 in

acetonitrile, NCMe-solvated organometallic cations can be i so la ted . 1 6 a ^ 9 , 3 0 Treatment of

C p ' M ( N O ) 2 C l with A g B F 4 in C H 2 C 1 2 generates the reactive C p ' M ( N O ) 2 ( F B F 3 ) species 3 1

which have been used to synthesize lactones 3 1 6 and pyrones. 3 1 c Halide abstraction

reactions with A g 0 2 C R (R = 2-phenylbutyrate) 2 9 d or A g 0 3 S R (R = p-tolyl, camphor) 3 2 in

C H 2 C 1 2 yields neutral complexes with covalent M - 0 bonds.

L ike the reaction of C p ' C r ( N O ) 2 C l and A g O T f described in Chapter 3, the

synthesis of 4.5 is best accomplished in a 1:1 solvent mixture of C H 2 C 1 2 and E t 2 0 The

stability of CpMo(NO)(CH 2 Ph)(OTf) , which is isolable as a orange powder that can even

be handled briefly in air without deleterious effects, 3 3 can be contrasted with that o f

CpMo(NO)(r| 3-(Z)-crotyl)(OTs) (which was not isolated, but generated at low

temperature and used in situ)32* and that o f Cp* W ( N O ) ( C H 2 S i M e 3 ) O T f (which

decomposes upon being generated in non-coordinating arene or chlorinated solvents). 3 4

The existence o f a covalent M o - O S 0 2 C F 3 bond is inferred from the solubility o f 4.5 in

97

poor ion-supporting solvents such as toluene, C e D 6 and Et20, and by its IR spectrum

(Nujol) . 3 5 The highly electron-withdrawing nature of the O T f group in 4.5 is evidenced

by the v(NO) (Nujol) value of 1654 c m - 1 , much higher than that exhibited by the

hydrocarbyl products 4.1-4.4 or the benzyl chloride starting material.

O T f C H 2 C 1 2

O 8

© O T f 3 L (4.5) v L 4.6 P P h 3

N 4.7 N C 5 H 5

A s shown in equation 4.5, the rj 2-benzyl ligand in 4.5 proved to be less

susceptible to nucleophilic attack than the rj 3-allyl group in the corresponding

CpMo(NO)(C3H 5)(OTf) complex. Reaction of isolated or in-situ generated

CpMo(NO)(CH 2 Ph)(OTf) with P P h 3 or pyridine results in the formation of complexes 4.6

and 4.7, respectively. These [CpMo(NO)(CH 2 Ph)L][OTfJ salts display decreased

solubility compared to neutral complexes 4.1-4.5; 4.6 and 4.7 are only moderately soluble

even in T H F , and dissolve readily only in good ion-supporting solvents such as CH2CI2

and CH3NO2. Solid 4.6 and 4.7 appear to be relatively stable with respect to O2 and

water, since the yellow powders may be handled briefly in air without noticeable

decomposition. The decrease in the highest-frequency v(SO) band o f the triflate group

from 1321 c m - 1 in 4.5 to -1260 c m - 1 in 4.6 and 4.7 is consistent with the displacement of

the O T f from the M o center.3 5 The lU and 1 3 C N M R spectra of 4.6 and 4.7 help

demonstrate that only one L ligand is present in each molecule and that the rj 2-benzyl

interaction is preserved.

Deprotonation of [CpMo(NO)(CH2Ph)(PPh3)][OTfJ

In an attempt to form a benzylidene complex via intermolecular deprotonation, 1 6 0

cationic 4.6 was treated with K O ' B u in T H F . While this reaction did result in a decrease

98

in the frequency o f the v (NO) band to 1614 c m - 1 (THF) consistent with the formation

of a neutral species, the identical spectroscopic properties were obtained by treatment of

C p M o ( N O ) ( C H 2 P h ) C l with K O ' B u in T H F . The product o f both reactions appeared to be

CpMo(NO)(CH 2 Ph) (0 'Bu) , as indicated by signals attributable to benzyl and butoxide

ligands in the LH N M R spectra o f the crude reaction products (C6D6).24b'36

Substitution o f P P h 3 seemed to be avoided by using a more hindered base. The

anionic amide L i N ( S i M e 3 ) 2 reacted with 4.6 in T H F or C H 2 C 1 2 to form red solutions

which displayed a broad v(NO) band at -1600 c m - 1 . The red residue that remained after

the reaction solvent was removed in vacuo was readily dissolved in E t 2 0 , providing

another indication that a neutral species had been formed. Somewhat unexpectedly,

neutral amine bases such as H N ' P r 2 , N E t 3 , and D B U also reacted with 4.6 in C H 2 C 1 2 ,

resulting in complete consumption o f starting material, as determined by solution TR

spectroscopy, and the formation o f red solutions with the same broad v(NO) band at

-1600 cm" I

© O T f 9 N R ,

C H 2 C 1 2

PPh3 (4.6)

The transformation shown in equation 4.6 is surprising, since H N R 3

+ cations are

more acidic than the neutral R O H species which have been demonstrated to protonate the

neopentylidene ligand o f C p M o ( N O ) ( = C H ' B u ) 2 4 b It is conceivable, however, that the

replacement o f the ' B u group with the more electron-withdrawing Ph moiety might

enhance the acidity o f the methylene H ' s o f 4.6. Unfortunately, conclusive evidence that

the reaction of 4.6 with potential Brensted bases actually forms the desired benzylidene

complex has yet to be obtained. The ' H N M R spectra o f the crude reaction mixtures were

devoid o f signals characteristic of alkylidene H 's . None o f the attempts to purify,

99

chromatograph, crystallize or precipitate the nitrosyl-containing product resulting from

the attempted reaction shown in equation 4.6 resulted in any tractable material.

Summary The failure o f this synthetic study to generate CpMo(NO)(=CHPh) via

intermolecular deprotonation leaves the synthetic utility o f the [CpMo(NO)(CH2Ph)]+

fragment yet to be demonstrated. Nevertheless, the following properties o f these

complexes have been successfully ascertained:

1. The 3e-donor CFfcPh ligand is capable o f stabilizing complexes containing

hydrocarbyl ligands such as M e , Ph, and C = C P h that are otherwise too

reactive to isolate as 16e CpMo(NO)R2 species.

2. The rj 2-benzyl interaction can be displaced by the introduction o f other

anionic ligands capable of multiple-electron donation, as evidenced by the

solid-state molecular structure of Cp2Mo(NO)(CH 2Ph).

3. Displacement o f the O T f group o f 4.5 can be achieved without the hindrance

o f nucleophilic attack at the 3e-donor benzyl ligand.,

100

Future Work

C a r b e n e t r a n s f e r f r o m C p M o ( N O ) ( = C H P h ) ( L 2 ) i n t e rmed ia te s .

Although the intermolecular deprotonation reactions described in this chapter

were not successful, modification of the organometallic cation, Bransted base and/or

reaction conditions may eventually provide the desired reactivity. One possible alteration

is to employ a bidentate a-donor ligand such as dppe or bipy instead of PPh3 or pyridine.

These chelating ligands would force the benzyl group to adopt a ry1 -coordination mode,

and would be expected to render the complex less prone to reduction or nucleophilic

attack at the metal center. These properties should permit the use of more potent bases

(e.g. "BuLi) without attendant reductive decomposition or L ligand substitution. If

formed, the resulting benzylidene complex (I) would be expected to be highly reactive

due to the lack of an available Tc-symmetry orbital on the Mo atom.37 One possible mode

of reactivity of I is transfer of the "CHPh" moiety to an olefin to form a cyplopropane

derivative38 and the known3 9 Mo(0), d 6 CpMo(NO)(dppe) species (Equation 4.7).

l e R e d u c t i o n o f [ C p M o ( N O ) ( C H 2 P h ) L ] [ O T f ] .

A potential mode of reactivity for [CpMo(NO)(CH2Ph)L][OTf] salts is their

reduction to paramagnetic, 17e, Mo(I), d 5 species (Equation 4.8). Analogous

CpCr(NO)(L)R complexes have been investigated,40 and the synthesis of

[Cp*Mo(NO)(CH2SiMe3)2]~ has also been recently reported.160

101

A preliminary attempt to generate the neutral CpMo(CH 2 Ph) (PPh 3 ) was

unsuccessful. The reaction of 4.6 with sodium naphthalide in T H F resulted in the

consumption o f starting material without producing any nitrosyl-containing products, as

determined by the absence o f v(NO) bands in the IR spectrum o f the final reaction

mixture.

Ligand-based reactivity of CpMo(NO)(CH 2Ph)R.

M u c h of the chemistry that has been explored for C p ' M ( N O ) R 2 complexes has

involved "metal-based reactivity". Typically, small molecules form 18e adducts by

coordination to the metal center, often followed by reaction with the M - R o bonds. 2 3 In

fact, the chemistry o f the benzyl complexes remains comparatively underdeveloped

because the r | 2 - C H 2 P h interaction effectively saturates the complex, thereby hampering

adduct formation.

The facile exchange between r\l- and rj2-coordination modes, the stabilization of

unusual hydrocarbyl ligands, and the deterrence of nucleophilic attack at the metal center

all combine to make the benzyl group an excellent ancillary ligand to assist in the

exploration o f the "ligand-based reactivity" of C p ' M ( N O ) ( C H 2 P h ) R . This mode o f

reactivity is exemplified by an impressive series of papers from Templeton and co

workers describing the chemistry of Tp*W(CO)(RC=CR')-containing species. 4 1 These

studies exploit the ability o f the alkyne ligand to donate two or four electrons to the

W(II), d 4 metal center, 4 2 thereby maintaining a saturated 18e configuration while

coordinated ligands are elaborated via sequential nucleophilic and electrophilic addition

(4.8)

102

reactions. One potential reaction o f 4.3 is thus the attack o f R~ on the B - C o f the

acetylide ligand, as shown in equation 4.9.

Cp'M(NO)(CH 2 Ph)Cl as Cp2Zr(CH3)Cl analog.

Another means o f harnessing the characteristic properties o f the benzyl ligand to

develop the utility o f the C p ' M ( N O ) fragment is suggested by the extensive synthetic

methodology developed by Buchwald and coworkers employing the Cp2Zr(CH3)Cl

precursor (Equation 4.10). 3 c The chloride ligand of the zirconocene precursor is replaced

with a hydrocarbyl group bearing 3~H's via standard salt metathesis techniques.

Thermolysis o f this species induces the elimination of methane and the unsaturated

organic moiety thus formed, stabilized by coordination to the Cp2Zr fragment, is rendered

prone to a wide range of coupling reactions.

(4.10)

Extension of this methodology to Cp'M(NO)-containing complexes has been

pioneered by Sean L u m b , 1 2 d but this technique is currently restricted to the

Cp*W(NO)(CH 2 SiMe 3 )(H2C=CPh) species. Synthesis o f C p * W ( N O ) ( C H 2 S i M e 3 ) R

complexes with other hydrocarbyl ligands containing P ~ H atoms has been effectively

precluded by the apparent thermal instability o f these species. It seems possible that the

103

rj2-benzyl group might render Cp*W(NO)(CH 2 Ph)R compounds sufficiently stable to

permit their isolation, yet reactive enough to exhibit the required B - H elimination

chemistry. Indeed, the varying thermal stability profiles exhibited by C p ' M ( N O ) -

containing species may well be utilized to selectively tune the toluene-elimination

reaction to a specific temperature range.

Synthesis and reactivity of Cp2M(NO)H.

The synthesis and structural charaterization of 4.4 may incite renewed interest in

Cp2M(NO)X species. A n intriguing target complex in this class is the hydride species,

C p 2 M ( N O ) H , which may be accessible either from C p 2 M ( N O ) I 2 0 a ' 4 3 or from the currently

unknown C p 2 M ( N O ) C l precursors (Scheme 4.4). Unlike C p * W ( N O ) ( C H 2 S i M e 3 ) H ,

which decomposes in the absence o f trapping reagents due to the reductive elimination of

S i M e 4 , 3 6 c C p 2 M o ( N O ) H species are potentially "self-trapping" as the CpM(NO)(r | 4 -

C5H6) complexes. 4 4 The possibility of a dynamic equilibrium between the M(II), d 4

hydride and the M(0) , d 6 diene compounds has exciting implications o f the reactivity o f

these species with small molecules. Likely reactivity modes include (i) ligand

coordination to form a M(II), rfVn1 complex, 1 1* (ii) insertion o f unsaturated organic

species into the M - H bond, 3 6 c (iii) intermolecular C - H activation, 1 1 (iv) coupling

reactions, 1 2 and (v) ligand coordination to form a M(0), t|5/r|2 compound. 4 4" Selectivity

between these modes may be influenced by the identity o f the metal center or the type of

cyclopentadienyl ligand used, especially the indenyl and fluorenyl derivatives.

Scheme 4.4. Possible reactivity modes of Cp2M(NO)H

105

Experimental

Methods

A l l reactions and subsequent manipulations were conducted under anaerobic

conditions using an atmosphere of N 2 . The complexes CpMo(NO)(CH2Ph)Cl 1 and

CpMo(NO)(C3Hs)I,4 5 were prepared by the published procedures. A l l other reagents were

used as received from commercial suppliers. Filtrations were performed through Celite (1

x 2 cm) supported on a medium porosity frit unless otherwise specified. For low

temperature reactions, solvents were transferred via trap-to-trap distillation from the

drying reagent directly onto the reactants contained in a flask cooled by a liquid nitrogen

bath.

The color, yield, and elemental analysis data for the new compounds in this

chapter are listed in Table 4.2. Infrared and mass spectral data are collected in Table 4.3,

and Table 4.4 shows the *H and 1 3 C { ' H } N M R data.

Synthesis of CpMo(NO)(CH2Ph)R (R = Me (4.1), Ph (4.2), C=CPh (4.3), Cp (4.4)).

The synthesis o f C p M o ( N O ) ( C H 2 P h ) M e (4.1) is described as a representative

example. T H F (-25 mL) was vacuum transferred onto C p M o ( N O ) ( C H 2 P h ) C l (0.186 g,

0.59 mmol) and Me 2Mg-x(dioxane) (0.089 g, 0.59 mmol). The solution was allowed to

warm slowly to room temperature. After -1 h, the solvent was removed from the red

solution in vacuo. The residue was extracted with a 2:1 solvent mixture o f E t 2 0 : C H 2 C l 2

and filtered. The solvent was again removed in vacuo, and the residue was triturated with

E t 2 0 twice to afford CpMo(NO)(CH 2 Ph)Me (4.1) as an orange powder (0.110 g, 63%

yield). Analytically-pure samples were obtained by extraction of the crude powder with a

1:1 solvent mixture of THF:toluene, followed by chromatography using an Alumina I

column and toluene as eluant. The resulting orange eluate was then reduced in volume in

106

vacuo, hexanes were added, and the solution was chilled to - 3 0 ° C to obtain orange

crystals.

CpMo(NO)(CH 2 Ph)Ph (4.2), C p M o ( N O ) ( C H 2 P h ) C = C P h (4.3), and

C p 2 M o ( N O ) C H 2 P h (4.4) were synthesized from C p M o ( N O ) ( C H 2 P h ) C l in an analogous

manner, using Ph 2Mg-x(dioxane), L i C = C P h , and N a ( D M E ) C p , respectively, instead of

Me 2Mg-x(dioxane) as the alkylating reagent.

Synthesis o f C p M o ( N O ) ( C H 2 P h ) O T f (4.5).

First C H 2 C 1 2 (20 mL) and then E t 2 0 (20 mL) were added via syringe to

C p M o ( N O ) ( C H 2 P h ) C l (0.433 g, 1.36 mmol) and A g O T f (0.350 g, 1.36 mmol). After -10

min, a flocculent white precipitate had formed in the orange solution. The solution was

filtered, and the Celite plug was washed with E t 2 0 ( 3 x 5 mL). The combined filtrates

were taken to dryness in vacuo, and the residue was triturated twice with E t 2 0 (10 mL) to

obtain CpMo(NO)(CH 2 Ph)(OTf) (4.5) as an orange powder (0.529 g, 83% yield).

Synthesis o f [ C p M o ( N O ) ( C H 2 P h ) ( L ) ] [ O T f ] (L = PPh 3 (4.6), N C 5 H 5 (4.7)).

Both triflate salts may be obtained from either isolated or in situ generated 4.5.

The two-step synthesis o f [CpMo(NO)(CH 2 Ph)(PPh 3 )][OTf] from C p M o ( N O ) ( C H 2 P h ) C l

is described as a representative example. First C H 2 C 1 2 (10 mL) and then E t 2 0 (10 mL)

were added to C p M o ( N O ) ( C H 2 P h ) C l (0.115 g, 0.36 mmol) and A g O T f (0.092 g, 0.36

mmol). After - 1 0 min, the solution was filtered through Celite onto solid P P h 3 (0.096 g,

0.36 mmol). The solution was stirred briefly, then hexanes (10 mL) were added. The total

volume o f the solution was reduced to -25 mL, and chilled to - 3 0 ° C overnight to obtain

[CpMo(NO)(CH 2 Ph)(PPh 3 )][OTf] (4.6) as a yellow powder (0.198 g, 79% yield).

107

Characterization Data

Table 4.2. Numbering Scheme, Color, Yield and Elemental Analysis.

Complex Cmpd #

Color (yield, %)a

Elemental analysis found (calcd) Complex Cmpd #

Color (yield, %)a C H N

CpMo(NO)(Bz)Me 4.1 orange (63) 52.67 (52.54) 5.11 (5.09) 4.78 (4.71)

CpMo(NO)(Bz)Ph 4.2 orange (72) 60.03 (60.18) 4.75 (4.77) 3.89(3.90)

CpMo(NO)(Bz)C=CPh 4.3 orange (68) 62.73 (62.67) 4.45 (4.47) 3.66 (3.65)

Cp2Mo(NO)Bz 4.4 dk green (65) 58.29 (58.80) 4.84 (4.93) 3.91 (4.03)

CpMo(NO)(Bz)OTf 4.5 orange (83) 36.35 (36.21) 2.81 (2.80) 3.16 (3.25)

[CpMo(NO)(Bz)(PPh3)] [OTf] 4.6 yellow (79) 53.31 (53.69) 3.95 (3.92) 1.89 (2.02)

[CpMo(NO)(Bz)(py)][OTf] 4.7 yellow (82) 42.51 (42.36) 3.29 (3.36) 5.22 (5.49)

a Yield calculated from crude isolated product

Table 4 .3 . Infrared v(NO) and Mass Spectral Data.

Complex Cmpd

#

IR(v(NO) cm"1) F A B / M S (m/z) Complex Cmpd

# Nujol C H 2 C I 2 P +

CpMo(NO)(Bz)Me 4.1 1576 1591 299

CpMo(NO)(Bz)Ph 4.2 1592 1603 361

CpMo(NO)(Bz)f>CPh 4.3 1604 1620 385

Cp2Mo(NO)Bz 4.4 1592 1607 349

CpMo(NO)(Bz)OTf 4.5 1654 1657 284

[CpMo(NO)(Bz)(PPh3)] [OTf] 4.6 1655 1655 546

[CpMo(NO)(Bz)(py)] [OTf] 4.7 1643 1651

108

Table 4.4. lH and 1 3 C NMR Data.

Compound (solvent)

*H N M R (8) 13C CB) N M R (8)

4.1 (C<P6)

7.5-6.8 (m, Ph), 4.99 (s, 5H, C5//5), 3.19 (d, IH CHH), 2.21 (d, IH, CH//), -0.88 (s, 3H, CH3)

131.0-127.5 (Ph), 112.2 (Cipso), 99.3 (C5H5), 37.8 (CH2), 1.34 (CH3)

4.2 (C6D6)

7.2-6.4 (m, Ph), 5.04 (s, 5H, C5//5), 3.12 (d, IH C//H), 2.83 (d, IH, CHH)

196.9 (C ipso Ph), 140.4-124.2 (Ph), 112.1 (C ipso Bz ) , 100.1 (C5H5),

40.1 (CH2) 4.3

(CsDs) 7.6-7.0 (m, Ph), 5.13 (s, 5H, C5//5), 3.19 (d, IH CHH), 2.66 (d, IH, CHH)

136:1-131.4 (Ph), 112.7 (C ipso Bz ) , 108.8 (C=C), 106.1 (CsHs), 42.8 (CH2)

4.4 (Q>D6)

7.6-7.0 (m, Ph), 5.17 (s, 10H, C5//5), 3.45 (s, 2H CH2)

152.3 (CipsoBz), 128.0, 127.7, 123.8 (CPh), 109.0 (C5H5), 25.1 (C772)

4.5 (C6D6)

7.5-6.8 (m, Ph), 5.04 (s, 5H, C5//5), 3.12 (d, IH C//H), 2.83 (d, IH, CHH)

137.2-127.7 (Ph), 112.4 (Cipso), 102.7 (C5H5), 50.5 (CH2)

4.6 (CD3NO2)

7.8-6.5 (m, Ph), 5.78 (s, 5H, C5H5), 3.92 (d, IH CHH), 3.63 (d, 1H, CHH)

136.3-126.8 (Ph), 110.1 (Cipso), 103.1 (C5H5), 44.7 (CH2)

4.7 (CD 3N0 2)

7.9-7.0 (m, Ph), 6.18 (s, 5H, C5H5), 3.95 (d, IH CHH), 3.78 (d, IH, CHH)

156.1-127.8 (Ph, NC5H5), 112.7 (C ipso), 104.9 (C5H5), 50.2 (CH2)

109


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(9) (a) Mauthner, K . ; Slugovc, C ; Mereiter, K . ; Schmid, R.; Kirchner, K .

Organometallics 1996,15,181. (b) Slugovc, C ; Mauthner, K . ; Mereiter, K . ; Schmid, R.;

Kirchner, K . Organometallics 1996, 75, 2954.

I l l

(10) Frohnapfel, D . S.; White, P. S.; Templeton, J. L . ; Riiegger, H . ; Pregosin, P.

S. Organometallics 1997,16, 3737.

(11) C - H activation by C p * W ( N O ) L (L = P M e 3 , l l a C6H4, u b = C H ' B u , l l c f

H C = C P h , I l d P P h 3

U e ) : (a) Legzdins, P.; Martin, J. T.; Einstein, F W . B . ; Jones, R. H .

Organometallics 1987, 6, 1826. (b) Debad, J. D . Ph.D. Thesis, University o f British

Columbia, 1994, pp. 112-115. (c) Ross, K . J. Ph.D. Thesis, University o f British

Columbia, 1994, pp. 175-180. (d) Debad, J. D . ; Legzdins, P.; Lumb, S. A . ; Batchelor, R.

J.; Einstein, F. W . B . J. Am. Chem. Soc. 1995,117, 3288. (e) Debad, J. D . ; Legzdins, P . ;

Lumb, S. A . ; Batchelor, R. J.; Einstein, F. W . B . Organometallics 1995,14, 2543. (f)

Tran, E . ; Legzdins, P. J. Am. Chem. Soc. 1997,119, 5071.

(12) C - C formation by C p ' M ( N O ) L : (a) Christensen, N . J.; Legzdins, P.;

Einstein, F. W . B . ; Jones, R. H . Organometallics 1991, 20, 3070. (b) Christensen, N . J.;

Legzdins, P.; Trotter, J.; Yee, V . C. Organometallics 1991,10, 4021. (c) Debad, J. D . ;

Legzdins, P.; Young, M . A . ; Batchelor, R. J.; Einstein, F. W. B . J. Am. Chem. Soc. 1993,

115, 2051. (d) Legzdins, P.; Lumb, S. A . Organometallics 1997,16, 1825.

(13) (a) Schilling, B . E . R.; Hoffmann, R.; Lichtenberger, D . L . J. Am. Chem. Soc.

1979,101, 585. (b) Schilling, B . E . R.; Hoffmann, R.; Faller, J. W . J. Am. Chem. Soc.

1979,101, 592.

(14) (a) Seddon, D . ; Ki ta , W . G . ; Bray, J.; McCleverty, J. A . Inorg. Synth. 1976,

16, 24. (b) Dryden, N . H . ; Legzdins, P.; Batchelor, R. J.; Einstein, F. W . B .


(15) (a) Debad, J. D . ; Legzdins, P . ; Batchelor, R. J.; Einstein, F. W . B .

Organometallics 1992, 11, 6. (b) Brunei, N . ; Debad, J. D . ; Legzdins, P.; Trotter, J.;

Veltheer, J. E . ; Yee, V . C. Organometallics 1993,12, 4572. (c) Debad, J. D . ; Legzdins,

P.; Lumb, S. A . ; Batchelor, R. J.; Einstein, F. W. B . Organometallics 1995,14, 2543.

112

(16) (a) Debad, J. D.; Legzdins, P.; Rettig, S. J.; Veltheer, J. E . Organometallics

1993,12, 2714. (b) Legzdins, P.; Sayers, S. F. Organometallics 1996,15, 3907. (c)

Legzdins, P.; Sayers, S. F. Chem. Eur. J. 1997, 3, 1579.

(17) Debad, J. D.; Legzdins, P.; Batchelor, R. J.; Einstein, F. W. B.

Organometallics 1993,12, 2094. For the related synthesis of Cp*M(NO)(CH 2CMe 3)R

(M = Mo, W) asymmetric bis(alkyl) complexes, see ref. 16a.

(18) CpW(NO)(C 3 H 5 ) (CsCR): Ipaktschi, J.; Mirzaei, F.; Demuth-Eberle, G. J.;

Beck, J.; Serafin, M . Organometallics 1997,16, 3965.

(19) Legzdins, P.; Jones, R. H.; Phillips, E. C ; Yee, V. C.; Trotter, J.; Einstein, F.

W. B. Organometallics 1991,10, 986.

(20) Cp2Mo(NO)R: (a) King, R. B. Inorg. Chem. 1968, 7, 90. (b) Cotton, F. A.;

Legzdins, P. J. Am. Chem. Soc. 1968, 90, 6232. (c) Calderon, J. L . ; Cotton, F. A.;

Legzdins, P. J. Am. Chem. Soc. 1969, 91, 2528. (d) Cotton, F. A.; Rusholme, G. A. J. Am.

Chem. Soc. 1972, 94, 402. (e) Lauher, J. W.; Hoffmann, R. J. Am. Chem. Soc. 1976, 98,

1729. (f) de Jesus, E . ; Vazquez de Miguel, A.; Royo, P.; Lanfredi, A. M . M . ; Tiripicchio,

A. J. Chem. Soc, Dalton Trans. 1990, 2779.

(21) Cp 2Mo(NO)X: (a) Hunt, M . M . ; Kita, W. G.; Mann, B. E.; McCleverty, J. A.

J. Chem. Soc, Dalton Trans. 1978, 467. (b) Hunt, M . M . ; Kita, W. G.; McCleverty, J. A.

J. Chem. Soc, Dalton Trans. 1978, 474. (c) Hunt, M . M . ; McCleverty, J. A. J. Chem.

Soc, Dalton Trans. 1978, 480.

(22) Huttner, G.; Brintzinger, H. H.; Bell, L. G.; Friedrich, P.; Bejenke, V.;

Neugebauer, D. J. Organomet. Chem. 1978,145, 529.

(23) Legzdins, P.; Veltheer, J. Acc Chem. Res. 1993, 26,41.

113

(24) (a) Legzdins, P.; Rettig, S. J.; Veltheer, J. E . ; Batchelor, R. J.; Einstein, F. W.

B . Organometallics 1993,12, 3575. (b) Legzdins, P.; Veltheer, J. E . ; Young, M . A .

Batchelor, R. J.; Einstein, F. W . B . Organometallics 1995 ,74, 407.

(25) Thermolysis o f C p M o ( N O ) ( C H 2 P h ) 2 in C 6 D 6 at 95°C for 40 h resulted in a

slight color change (orange to red), but no difference in the IR or ' H N M R spectra of the

sample.

(26) Dryden, N . H . ; Legzdins, P.; Rettig, S. J.; Veltheer, J. E . Organometallics

1992, 11, 2583.

(27) CpMo(NO)(Me)(L)R: Alegre, B . ; de Jesus, E . ; Vazquez de Miguel , A . ;

Royo, P.; Lanfredi, A . M . M . ; Tiripicchio, A . J. Chem. Soc, Dalton Trans. 1988, 819.

(28) C p M o ( N O ) 2 R : Hoyano, J. K . ; Legzdins, P.; Mali to, J. T. J. Chem. Soc,

Dalton Trans. 1975, 1022.

(29) (a) Legzdins, P.; Nurse, C. R. Inorg. Chem. 1982, 21, 3110. (b) Chin, T. T.;

Legzdins, P.; Trotter, J.; Yee, V . C. Organometallics 1992,11, 913. (c) Legzdins, P.;

Rettig, S. J.; Sayers, S. F. J. Am. Chem. Soc. 1994,116, 12105. (d) Dryden, N . H . ;

Legzdins, P.; Sayers, S. F . ; Trotter, J.; Yee, V . C. Can. J. Chem. 1995, 73, 1035.

(30) McCleverty, J. A . ; Murray, A . J. Trans. Met. Chem. 1979, 4, 273.

(31) (a) Legzdins, P.; Martin, D . T. Organometallics 1983, 2, 1785. (b) Legzdins,

P.; Richter-Addo, G . B . ; Einstein, F . W . B . ; Jones, R. H . Organometallics 1990, 9,431.

(c) Legzdins, P.; M c N e i l , W. S.; Vessey, E . G;Batchelor , R. J.; Einstein, F. W . B .


(32) (a) Faller, J. W. ; DiVerdi , M . J.; John, J. A . Tetrahedron Lett. 1991, 32,

1271. (b) Faller, J. W. ; Nguyen, J. T.; Ell is , W ; Mazzieri , M . R. Organometallics 1993,

12, 1434. (c) Faller, F . W. ; Chase, Mazzieri , M . R. Inorg. Chim. Acta 1995, 229, 39.

114

(33) Exposure o f 4.5 to air for 5 days resulted in no visible change in the

appearance of the orange powder, but its Nujol IR spectrum changed significantly. The

v(NO) band o f the air-exposed sample had shifted from 1654 c m - 1 to 1646 c m - 1 , and the

highest-frequency O T f band shifted from 1321 c m - 1 to 1276 c m - 1 . One possible

explanation for these spectroscopic changes is the displacement of the O T f ligand o f 4.5

by atmospheric H 2 0 to form [CpMo(NO)(CH2Ph)(H 20)][OTf] in the solid state.

(34) Legzdins, P.; Sayers, S. F. unpublished results.

(35) Lawrance, G . A . Chem. Rev. 1986, 86, 17.

(36) Cp 'M(NO)(R) (OR ' ) : (a) Legzdins, P.; Lundmark, P. J.; Rettig, S. J.

Organometallics 1993,12, 3545. (b) Legzdins, P.; Rettig, S. J.; Ross, K . J.

Organometallics 1994,13, 569. (c) Debad, J. D . ; Legzdins, P.; Lumb, S. A . ; Batchelor,

R. I ; Einstein, F. W . B . Organometallics 1995,14, 2543.

(37) Bonding in CpML4 complexes: (a) Kubacek, P.; Hoffmann, R.; Havlas, Z .

Organometallics 1982, 1, 180. (b) Poli , R. Organometallics 1990, 9, 1892.

(38) Cyclopropanes from transition-metal-carbene complexes and olefins: (a)

Brookhart, M . ; Studabaker, W. B . Chem. Rev. 1987, 87, 411. (b) McVica r , W . K . ;

Hubbard, J. L . Organometallics 1990, 9, 2683. (c) Gunnoe, T. B . ; Surgan, M . ; White, P.

S.; Templeton, J. L . ; Casarrubios, L . Organometallics 1997,16, 4865.

(39) Brunner, H . J. Organomet. Chem. 1969,16, 119.

(40) (a) Legzdins, P.; Shaw, M . J. J. Am. Chem. Soc. 1994,116, 7700. (b)

Legzdins, P.; Shaw, M . J.; Batchelor, R. J.; Einstein, F. W . B . Organometallics 1995,14,

4721.

(41) Tp*W(CO)(RC=CR')-containing species: (a) Feng, S. G . ; Philipp, C. C ;

Gamble, A . S.; White, P. S.; Templeton, J. L . Organometallics 1991,10, 3504. (b)

Collins, M . A . ; Feng, S. G ; White, P. A . ; Templeton, J. L . J. Am. Chem. Soc. 1992,114,

115

3771. (c) Feng, S. G ; Templeton, J. L. Organometallics 1992,11, 1295. (d) Feng, S. G.;

Templeton, J. L. Organometallics 1992,11, 2168. (e) Feng, S. G ; White, P. S.;

Templeton, J. L. J. Am. Chem. Soc. 1992,114, 2951. (f) Caldarelli, J. L. ; White, P. S.;

Templeton, J. L . J. Am. Chem. Soc. 1992,114, 10097. (g) Feng, S. G ; White, P. S.;

Templeton, J. L. Organometallics 1993,12, 1765. (h) Feng, S. G ; White, P. S.;

Templeton, J. L. Organometallics 1993, 72, 2131. (i) Caldarelli, J. L. ; Wagner, L. E.;

White, P. S.; Templeton, J. L. J. Am. Chem. Soc. 1994,116,2%1%. (j) Feng, S. G.; White,

P. S.; Templeton, J. L . Organometallics 1995, 14, 5184. (k) Gunnoe, T . B.; White, P. S.;

Templeton, J. L . J. Am. Chem. Soc. 1996,118, 6916. (1) Francisco, L. W.; White, P. S.;

Templeton, J. L. Organometallics 1996,15, 5127. (m) Wells, M . B.; White, P. S.;

Templeton, J. L. Organometallics 1997,16, 1857. (n) Francisco, L. W.; White, P. S.;

Templeton, J. L. Organometallics 1997,16, 2547. (o) Gunnoe, T . B.; White, P. S.;

Templeton, J. L . Organometallics 1997,16, 3794.

(42) Templeton, J. L . Adv. Organomet. Chem. 1989, 29, 1.

(43) Cp2W(NO)I: Legzdins, P.; Martin, D . T . ; Nurse, C. R . Inorg. Chem. 1980,

19, 1560.

(44) While Cp'M(NO)(ri4-diene) complexes typically adopt a v^-trans

configuration, 1 2 a b ' 3 6 c ' 4 4 a c r\4-cis complexes are also known:4 4 b c (a) Hunter, A. D.;

Legzdins, P.; Nurse, C. N ; Einstein, F. W. B.; Willis, A. C. J. Am. Chem. Soc. 1985, 707,

1791. (b) Hunter, A. D . ; Legzdins, P.; Einstein, F. W. B.; Willis, A. C ; Bursten, B. E.;

Garter, M , G. J. Am. Chem. Soc. 1986,108, 3843. (c) Christensen, N. I ; Hunter, A. D.;

Legzdins, P. Organometallics 1989, 8, 930.

(45) Faller, J. W.; Chodosh, D . F.; Katahira, D . J. Organomet. Chem. 1980,187,

227.

116

Chapter 5: Towards CpCr(NO)R2

Introduct ion 117


Summary 136

Future W o r k 137

Exper imenta l 142


117

Introduction

Olefin polymerization is one of the most important commercial applications of

transition-metal chemistry. Industrial chemists have spent decades empirically optimizing

reaction conditions for multi-component, heterogeneous catalysts based on T i and Cr . 1

Recently, academic chemists have conducted many synthetic, mechanistic and theoretical

studies of single-site, homogeneous systems. These soluble, well-defined complexes are

more amenable to the spectroscopic monitoring, detailed analysis, and computational

modeling required to elucidate the fundamental principles which govern the

polymerization process. 2 The enormous potential for rational design of homogeneous

catalysts is a result of several factors: (a) chain growth occurs in the coordination sphere

of the metal, (b) the ligand environment exercises a powerful influence on the overall

polymer structure by directing the insertion of each monomer, (c) the physical properties

of the polymer depend heavily on the polymer structure. The combination of the low cost

of the feedstock monomers, the high activity and efficiency o f the catalysts, and the

potential commercial value of new classes o f polymers combine to justify the

considerable effort invested in the design o f elaborate ligand systems.

But while homogeneous Group 4 catalysts have received intense academic

scrutiny, comparatively little attention has been paid to homogenous, Cr-based olefin

polymerization. 4 Studies o f C r - C o bonds have been hampered by the paramagnetism of

monomeric, midvalent C r alkyl complexes. Unlike C p 2 Z r R 2 and related d°, Group 4

catalyst precursors, organometallic Cr(III) species are uniformly paramagnetic with

broad, shifted signals in their N M R spectra. 5 ' 6 A l k y l compounds o f Cr(II) are either 7 R Q 11

electronically and coordinatively saturated, dimenc, or paramagnetic.

It is in this context that the potential significance o f C p C r ( N O ) R 2 complexes can

best be appreciated. I f the synthesis o f these unsaturated, monomeric, diamagnetic Cr(II)

alkyl compounds could be attained, they would present intriguing possibilities for the

118

synthetic, spectroscopic, mechanistic and theoretical investigation of C r - C a bonds.

When the work described in this chapter was initiated, chromium alkyl nitrosyl

complexes were restricted to the +1 and 0 oxidation states. 1 2 ' 1 3 The molybdenum and

tungsten congeners of the desired Cr(II) bis(alkyl) compounds are accessible in two steps

from the zero-valent Cp 'M(NO)(CO )2 species, as shown in Scheme 5.1. 1 4 While the

analogous Cp 'Cr(NO)(CO )2 compounds are readily available, their oxidative

halogenation reactions diverge from those observed for M o and W (Scheme 5.1).

Scheme 5.1. Oxidative halogenation of C p M ( N O ) ( C O ) 2 ( M = Cr, M o , W ) and

subsequent alkylation reactions

The lack of a direct route to Cr(H) dihalo complexes required the development of

new synthetic strategies in order to obtain the desired Cp'Cr(NO)R .2 species. This chapter

presents my work towards this goal, as well as some unanticipated products and insights

that this investigation revealed. For example, while the study of the oxidative

halogenation o f Cp 'Cr(NO)(CO )2 complexes did not result in the isolation o f stable

119

C p ' C r ( N O ) X 2 products, it helped guide the synthesis o f new [Cp*Cr(NO)(u,-X)] 2 ( X

= I (5.1), CI (5.2)) dimers. The inorganic complex Cr(NO)(N 'Pr 2 ) 3 served as a precursor

to Cr(U)(NO) complexes via amine elimination and salt metathesis reactions. O f

particular interest is the highly unsaturated bis(alkyl) compound,

C r ( N O ) ( N ' P r 2 ) ( C H 2 S i M e 3 ) 2 (5.3). The final precursor species examined in this study was

C p * C r ( N O ) 2 C l , which was used to generate Cp*Cr (NO)(CH 2 Ph) 2 in two steps via a

methodology that bypasses the unstable dihalo compounds.

120


Oxidation of Cp'M(NO)(CO) 2 with Halogen Sources.

The first example of the oxidative halogenation of zero-valent C p ' M ( N O ) ( C O ) 2

complexes ( M = Cr. M o , W ) was reported in 1967 when K i n g reacted C p M o ( N O ) ( C O ) 2

with I 2 in C H 2 C 1 2 to form [CpMo(NO)I(p.-I)] 2 . 1 5 This reaction was extended to related

Group 6 compounds by the research groups of McCleverty, Royo, and Legzdins, and was

shown to be remarkably general for Cp and Cp* derivatives o f M o and W using I 2 , B r 2 ,

C l 2 and PCI5 as halogen sources. 1 6 ' 2 1 In contrast, no analogous neutral dihalo Cr(II)

complexes have been prepared. Only in the single, specific case illustrated in Scheme 5.1

(i.e. C p ' = Cp, X 2 = I 2) are any mononitrosyl chromium products isolable from these

reactions. 2 2 In all other cases, C p ' C r ( N O ) 2 X species are obtained in < 50% yield, and no

other N O - or CO-containing products are observed in the final reaction mixtures by IR

spectroscopy. 2 2 ' 2 3

The initial stages of my own investigation o f this reaction were focused on

generating Cr(II) nitrosyl dihalo species, particularly from Cp*Cr (NO)(CO) 2 which had

not been as intensely studied as the Cp analog. 2 3 The failure o f these reactions to generate

[ C p ' C r ( N O ) X 2 ] n products is consistent with the nitrosyl-ligand lability which was

subsequently attributed to these presumably high-spin Cr(II) species. 2 4 When combined

with the qualitative reaction rates that can be inferred by IR spectroscopy, the postulated

NO loss from triplet Cr(U) intermediates helps explain the lack of generality o f the

reactions o f C p ' C r ( N O ) ( C O ) 2 with X 2 sources in non-coordinating solvents.

Examination o f the published 2 2 IR monitoring of the reaction o f CpCr (NO)(CO) 2

with I 2 in C H 2 C 1 2 reveals how the relative rates of reaction o f the various chromium

species in solution with iodine favor the clean formation o f [CpCr(NO)(u,-I)] 2. The key

feature is that only after all the CpCr (NO)(CO) 2 is consumed does any excess I 2 present

react with Cr(I) species to generate absorptions attributable to C p C r ( N O ) 2 I . 2 2 In other

121

words, I 2 reacts with CpCr (NO)(CO) 2 faster than with CpCr(NO)(CO)I or

[CpCr(NO)(p-I)] 2 , and so the presence o f the dicarbonyl Cr(0) reactant prevents the

formation of CpCr (NO)I 2 and the subsequent NO-transfer reactions. Clean generation of

other [Cp 'Cr(NO)(p-X)] 2 complexes by the same synthetic methodology is not possible

because the relative rates of reaction that favor the formation of [CpCr(NO)(p>I)] 2 do not

extend to related systems. Thus, when CpCr (NO) (CO) 2 is treated with PCI5, C l 2 , or B r 2 in

C H 2 C 1 2 , 2 2 or when Cp*Cr (NO)(CO) 2 is treated with any halogen source in a non-

coordinating solvent, 2 3 IR bands due to C p ' C r ( N O ) 2 X species are observed before all o f

the Cr(0) dicarbonyl reactant has been consumed. In these cases, therefore, X 2 reacts with

one or more o f the Cr(I) species in solution at rates comparable to its reaction with

Cp 'Cr (NO)(CO) 2 , thus hindering isolation o f the Cr(I) halo-bridged dimers (Scheme 5.2).

Scheme 5.2. Oxidation of Cp 'Cr(NO)(CO )2 with halogen sources

Therefore, in order to isolate [Cp'Cr(NO)(p.-X)] 2 complexes from the reactions of

C p ' C r ( N O ) ( C O ) 2 with X 2 , the Cr(I) compounds in solution must be converted to species

more resistant to oxidation than the Cr(0) dicarbonyl reactant. This was accomplished by

performing the oxidative halogenation reactions in the coordinating solvent acetonitrile.

In N C M e , C p ' C r ( N O ) ( C O ) 2 reacts cleanly with I 2 or PCI5, with only minimal amounts of

C p ' C r ( N O ) 2 X byproducts being evident by IR spectroscopy. The coordinated N C M e is

122

easily removed by repeated washing/vacuum cycles from the C p ' C r ( N O ) ( N C M e ) X

intermediates to obtain the appropriate dimer, including the previously inaccessible

[Cp*Cr(KO)(p>X)]2 compounds ( X = I (5.1), CI (5.2)), as summarized in equation 5.1.

N X = I (5.1)

C r ^ ' ^ » C r • y7cr - ' ' l X " ' » C f / = CI (5.2)

0 ° I C G N C M E X I N C M e ' * J7 ( 5 . 1 )

O O O ^

The ability o f a-donor ligands (L) to stabilize Cr(I) nitrosyl complexes has been

previously established. 2 5 CpCr(NO)(L)I compounds display no electrochemical oxidation

features to the solvent limit in T H F . 2 3 CpCr(NO)(PR .3)(CO) complexes are cleanly

converted to CpCr(NO)(PPv3)X compounds by I 2 , B r 2 and C I 2 . 2 2 Previous attempts to

make C p * C r ( N O ) ( L ) X complexes were successful when a trapping ligand (L = P P h 3 or

pyridine) was added immediately after Cp*Cr(NO)(CO )2 was treated with I2 without an

attempt to isolate the iodo-bridged dimer. 2 3

O f the two new [Cp*Cr(NO)(u.-X)] 2 complexes, the monochloro dimer is

preparable in higher yields, is easier to obtain in an acetonitrile-free form, and is more

readily crystallized. Crystals of 5.2 suitable for X-ray crystallographic analysis were

obtained by recrystallization from toluene:hexanes solvent mixtures. The solid-state

molecular structure o f [Cp*Cr(NO)(p>Cl)]2 is shown in Figure 5.1, and selected bond

lengths and angles are collected in Table 5.1.

123

Figure 5.1. ORTEP plot of [Cp*Cr(NO)(u-Cl)]2 (5.2). Thermal ellipsoids of 50%

probability are shown.

C(6A)

Table 5.1. Selected Bond Lengths and Angles for [Cp*Cr(NO)(p:-Cl)]2 (5.2).

Bond Lengths (A) Bond Angles (°) C r - C r 3.124(5) Cl -Cr-Cl ( lA) 96.40(4) C r - N 1.709(4) Cr-Cl -Cr( lA) 83.60(4) Cr-Cl(l) 2.339(1) C r - N - 0 171.3(3) Cr-Cl( lA) 2.343(1) C l - C r - N 97.85(11)

124

The intramolecular C r - C r distance in dimer 5.2 is 3.124 A. Metal-metal

bonding interactions can rarely be definitively assigned solely on the basis o f X-ray

crystallography, so the significance o f this distance must be judged with due caution.

C r - C r bonding in particular is notoriously complicated and controversial. For example,

the "supershort" C r - C r distance o f 1.980 A in the [L i (THF) ] 4 [Cr 2 Me 8 ] dimer 8 a was

interpreted by Cotton to imply a very strong C r - C r quadruple bond, 2 6 yet Gambarotta and

co-workers have shown that the dimer readily dissociates to [CrMe4]2~ monomers upon

addition o f T M E D A . 8 b

Modeling C r - C r bonds has also posed a significant theoretical challenge for more

than 10 years. 2 7 While contemporary D F T techniques have been successfully applied to

M 2 ( 0 2 C H ) 4 and M 2 ( H N C H N F f f l ) 4 models o f Nb, M o , Tc, Ru , and R h complexes, 2 8 the

strong electron correlation effects in C r 2 ( 0 2 C H ) 4 render these D F T calculations

inaccurate. 2 9 Even the diatomic C r 2 molecule continues to resist qualitative modeling, 3 0

although the "chromium dimer problem" is no longer considered to have broader

ramifications for theoretical transition-metal chemistry in general. 3 1 ' 3 2

Table 5.2 M - M Lengths in Group 6 Dimers.

Complex M — M (A) ref. rw 2 ci 9 i 3 - 2.41 35b [CpCr(NO)(u-NO)l2 2.615 33d fMozClgl3" 2.65 35c MCpCrfNOXu-NMe^h 2.67 33c c-rCpCr(NO)(u-NMe2)l2 2.72 33c [CpCr(NO)(u-OMe)l2 2.882 33e fCpMo(NO)(u-Cl)l2 2.9098 38

lCpCr(NO)(u-SPh)] 2 2.950 33a fCr.CM 3 - 3.12 35a [Cp*Cr(NO)(n-Cl)h 3.124 5.3 [Cp*Cr(CH3)(n-Cl)l2 3.278 34d fCpCr(CO) 3l 2

3.281 39 rCpCr(CH3)(u-Cl)l2 3.287 34b [Cp*Cr(CH 2Ph)(u-Cl)l 2 3.343 34f [CpCrCl(u-Cl)]2 3.362 34a

125

Despite the difficulties inherent in determining the exact nature of the C r - C r

interaction in 5.2 in the absence o f supporting magnetic and/or theoretical investigations,

comparison with the solid-state molecular structures of related bimetallic species suggests

that [Cp*Cr(NO)(u.-Cl)]2 does not posses a C r - C r bond. The C r - C r bond lengths in the

[CpCr(NO)(M.-Y)] 2 dimers listed in Table 5.2 range from 2.615 to 2.950 A, all

significantly shorter than the 3.124 A distance found in 5.2. The original papers that

describe the solid-state molecular structures o f these compounds contain at best only

cursory synthetic and characterization information. 3 3 However, subsequent research

demonstrated that the [CpCr(NO)(p.-NR 2)]2 complexes display sharp, unshifted *H N M R

signals, consistent with the presence of a C r - C r single bond. 2 4 a C r - C r distances of > 3.0

A are typically found for Cr(UI) dimers with bridging CI ligands and no metal-metal

bond 4 a " 3 4 The congeneric series [M 2Cl9] 3~ ( M = Cr, M o , W ) is particularly instructive. 3 5

Magnetic measurements reveal that the Cr2 species contains no metal-metal bond, while

the M02 and W2 compounds contain single and triple bonds, respectively. 3 6 This is

reflected in the M - M distances following the trend C r > M o > W , despite the increase in

M(III) ionic radius expected for the heavier Group 6 metals. 3 7 The same trend is observed

for the C p M o congener o f 5.2: [CpMo(NO)(p.-Cl)]2 has a shorter M - M distance o f

2.9098 A , consistent with a single bond. 3 8 The longer C r - C r distance o f 3.281 A

observed for the weakly-bonded [CpCr(CO)3]2 dimer is presumably due to the increased

39 steric repulsion and lack of bridging ligands in this compound as compared to 5.2.

A m i n e E l imina t ion Reactions of C r ( N O ) ( N ' P r 2 ) 3 .

Faced with chemical, electrochemical and theoretical evidence o f the instability o f

CpCr(NO)Cl2 with respect to N O loss, 2 4 Cr(NO)(N'Pr2)3 was considered as a potential

precursor to CpCr(NO)R2 complexes. While this classical inorganic 16e tris(amido)

complex may seem an unlikely stepping stone to organometallic Cr(II) nitrosyl species,

126

recent improvements in the scope and selectivity o f amine elimination reactions have

dramatically increased the utility o f metal-amide bonds to synthetic organometallic

chemists ,

Amine elimination reactions were first employed to form Group 4 4 0 and actinide 4 1

organometallic complexes over 25 years ago. More recently, the research groups Of

Teuben, 4 2 Herrmann 4 3 and Jordan 4 4 demonstrated the applicability o f these reactions to

synthesize Group 4 metallocene catalyst precursors for olefin polymerization This

methodology has since been enthusiastically embraced by other researchers interested in '

high-valeht organometallic complexes of Groups 4, 5 and 6 4 5

The study of the reactions of Cr(NO)(N'Pr 2 )3 mirrors the same pattern o f early

discovery, prolonged neglect, and recent renaissance that characterizes amine elimination

reactions generally. While several Cr(NO)(NR2)3 complexes were initially reported in

-1.970,46 with the solid-state molecular structure of Cr(NO)(N(SiMe3)2)3 appearing shortly

thereafter,47 Cr(NO)(N'Pr2)3 remains the only member of this class o f compounds to be

used as a precursor to new species. 4 8 This appears to be due to the specific steric

properties o f the N ' P r 2 l igand 4 9 and the synthesis o f the tris(amide) nitrosyl complexes, as

shown in Scheme 5.3.

R = M e Et [Cr(NR2)3] — — * [Cr(NR2)2]n + Cr(NR2)4

/ unstable 'less

r, o i ' ; bulk C r C l 3 R = /Pr v j n vrv

+ *- Cr(NR 2 ) r^— Cr(NO)(NR 2)3-^U Cr(NO)(NR2)3.nXn

L i N R 2 \ \ more

X bulk R = S i M e 3 C r ( N R 2 ) 3 - ^ ^ Cr(NO)(NR 2 ) 3 -^* no reaction •

Scheme 5.3. Steric influences on the reactivity o f C r tris(amide) complexes

The initial step o f the synthesis involves the reaction o f the LiNR2 reagent with

anhydrous, polymeric C r C l 3 . The use o f amide ligands at least as bulky as N ' P r 2 leads to

127

monomeric, 13e Cr(NR2)3 compounds, while smaller amides such as NMe2 or NEt2

yield only Cr(Bi) and Cr(IV) products of disproportionation.30 The stable Cr(NR2)3

complexes react with nitric oxide gas to form Cr(NO)(NR2)3 species.46 Complexes

containing NR 2 ligands larger than N'Pr2, however, subsequently fail to react with protic

sources, presumably due to the inaccessibility of the amide N atoms for electrophilic

attack.

Cr(NO)(N/PT2)3

HOR

V(Mes)3(THF)

HOR Cr(N)(N/Pr2)3

ref.Sla

lutH+r

Cr(N)(N/Pr2)2I

RLi

Cr(N)(N/Pr2)2R ref.Slb

[Cr(NO)(OR)3]„ R = /Bu, n = 1 r e f 4 6 4 8

R = /Pr,n = 2 J

Cr(rT)(N/Pr2)2(OR) ref.51c

HOR (xs)

Cr(N)(N/Pr2)(OR)2

2 RLi

Cr(N)(N/Pr2)R2

ref.51c

Scheme 5.4. Derivatization of Cr(NO)(N'Pr2)3

Scheme 5.4 summarizes the reactivity of Cr(NO)(N'Pr2)3 that was previously

known. Complete protonolysis of all three amide ligands with 'BuOH yields the

monomeric tris(butoxide) complex, while a dimeric product is formed when the less

sterically demanding 'PrOH is used.48 Cummins and co-workers have recently reported

selective elimination of one or two amide ligands from the nitride derivative.51

The initial investigation of Cr(NO)(N'Pr2)3 as a potential precursor to

organometallic C r ( n ) nitrosyl compounds was conducted as a fourth-year undergraduate

thesis project by Jane Kuzelka.521 designed and helped oversee this project which

resulted in the reaction sequence illustrated in equation 5.2. The treatment of the

128

tris(amide) with benzoic acid led to the protonolysis of two amide ligands to form

Cr(NO)(N'Pr2)(02CPh)2. The sequential salt metathesis reactions of the benzoate ligands

performed by Jane Kuzelka led to the first well-characterized Cr(IJ.) nitrosyl alkyl

compounds. O N

Cr jPr 2N'

^ - N / P r 2

N/Pr 2

P h C 0 2 H

O N

NaCp

Cr /Pr 2 N'

N ^ 0 2 C P h 0 2 CPh

Cr / P r 2 * r | R

N O i

R 2 M g

Cr

/ P r 2 N ^ | ^ 0 2 C P h N O

(5.2)

The solid-state molecular structures of the CpCr(NO)(N'Pr2)X (X = n 1 -0 2 CPh,

CH2SiMe3, V-C5H5) complexes all contain short Cr-N(amide) bonds53 and a planar

C r - N C 2 moiety aligned parallel to the Cr-NO axis,54 indicative of a strong

chromium-amide re-bonding interaction. The room-temperature *H NMR spectra of the

CpCr(NO)(N'Pr2)X species (CtJJe) display four distinct doublets for the four inequivalent

amide methyl groups, suggesting that the O N - C r - N R 2 alignment persists in solution.

The stability of the CpCr(NO)(N'Pr2)X complexes of Jane Kuzelka, as well as the

previously reported CpCr(NO)(NPh2)I5 3 a and Cp*Cr(NO)(0'Pr) 2, 5 4 b may be attributed to

their low-spin, S = 0 configuration resulting from the increased H O M O - L U M O gap

induced by the combination of the strong TC donor amide and TC acceptor nitrosyl

ligands.24

Synthesis of Cr(NO)(N'Pr2)(CH2SiMe3)2 (5.3).

Amide TC donation renders the CpCr(NO)(N'Pr2)R alkyl complexes electronically

saturated.55 In order to generate an unsaturated Cr(II) alkyl complex,

129

Cr(NO)(N'Pr 2 )(0 2 CPh)2 was treated with M g ( C H 2 S i M e 3 ) 2 to form

Cr(NO)(N'Pr 2)(CH 2SiMe3) 2 (5.3), as shown in equation 5.3. The diamagnetic 14e

bis(alkyl) 5.3 was isolated in high yields as a spectroscopically pure red powder after

extraction of the crude reaction residue with hexanes and filtration through Celite to

remove the M g ( 0 2 C P h ) 2 byproduct. Rectangular blocks of 5.3 suitable for X-ray

crystallography were obtained by dissolving the crude material in hexamethyldisiloxane

and cooling to -30 °C. The solid-state molecular structure o f 5.3 is shown in Figure 5.2,

and the pertinent bond angles, bond distances, and torsion angles are collected in Table

5.3.

O N

O N 5.3 M g ( C H 2 S i M e 3 ) 2

(5.3) s \ - 0 2 C P h

0 2 C P h / C ^ C H 2 S i M e 3

C H 2 S M e 3

/ P r 2 N / P r 2 N

1 3 0

Figure 5.2. ORTEP plot of Cr(NO)(N'Pr2)(CH2SiMe3)2 (5.3). Thermal ellipsoids of

5 0 % probability are shown.

C ( 3 )

C ( 7 ) f / f t C ( 4 )

O(l)

C ( 6 ) C ( 5 )

C ( 8 )

C ( 1 0 )

C ( 13 ) C ( 14 )

Table 5.3. Selected Bond Lengths, Bond Angles, and Torsion Angles for

Cr(N0)(NT-r2)(CH2SiMe3)2 (5.3).

Bond Len gths (A) Bond Angles (°) Torsion Angles (°) C r - C ( l ) 2.026(3) N ( l ) - C r - N ( 2 ) 102.91(12) N ( l ) - C r -

N(2)-C(9) -1.2(3) Cr -C(2) 1.986(3) C ( l ) - C r - C ( 2 ) 115.72(12)

N ( l ) - C r -N(2)-C(9) -1.2(3)

C r - N ( l ) 1.616(2) C r - N ( l ) - 0 ( 1 ) 179.0(3) N ( l ) - C r -N(2)-C(12) -177.8(2)

Cr -N(2) 1.763(2) N ( l ) - C r - C ( l ) 102.85(12) N ( l ) - C r -N(2)-C(12) -177.8(2)

131

Figure 5.2 shows only one of the two crystallographically independent

molecules o f 5.3 found in the unit cell. The bis(alkyl) complex adopts a flattened pseudo-

tetrahedral structure, with N(amide) -Cr -C and C-Cr-C angles >112° and

N(ni t rosy l ) -Cr-C and N(nitrosyl)-Cr-N(amide) angles <104°, due to the greater steric

bulk of the amide and alkyl ligands compared to the small N O group. The SiMe3 groups

of the two alkyl ligands are oriented away from the C r atom and "up" towards the nitrosyl

ligand, also presumably to avoid steric interactions with the bulky groups. The

Cr-N(amide) bond distance is a very short 1.763 A, 5 3 and the

N(ni t rosyl)-Cr-N(amide)-C torsion angles indicate the near planarity o f the

O N - C r - N C z group (all six atoms are within 0.1 A o f the plane). 5 4 These geometrical

parameters are consistent with the existence of a Cr-N(amide) TC-bonding interaction.

While highly unsaturated, four-coordinate, monomeric Cr(II) alkyl complexes are

not uncommon, all previously known examples o f this class of compound were square

planar and paramagnetic (5 = 2 ) . I 0 I l c d In order to explain the anomalous diamagnetic

electronic configuration and tetrahedral geometry of 5.3, a qualitative bonding analysis is

required. A

->-x

0

L R

xy N R 2 TC

2 2» : -y-rf -r - - -

xz y

yz N O TC Figure 5.3. Qualitative energy-level diagram illustrating N O and N R 2 TC-bonding

interactions in tetrahedral Cr(II) complexes.

132

The orbital diagram at the left of Figure 5.3 depicts a hypothetical, pseudo-

tetrahedral Cr(L)Pv3 anion with the z-axis lying along the Cr-L bond.56 Note that the .

small orbital-splitting energy between the degenerate d^ and dyz orbitals and the dxJdX2.y2

pair is not expected to be sufficient to overcome the interelectron repulsion energy,

resulting in a high-spin S = 2 configuration. This is typical for pseudo-tetrahedral

complexes which usually adopt a high-spin configuration whenever possible. In the

absence of strong 7i-bonding ligands, the tetrahedral geometry shown in Figure 5.3 is

higher in energy for 4-coordinate, d 4 complexes than the square planar arrangement; this

is a well-known example of the Jahn-Teller effect.57

However, when the generic 2-electron cr-donor L ligand is replaced with the

strong 7t acceptor NO+, the d^ and dyz orbitals are substantially lowered in energy. The

tetrahedral ligand-field splitting (At) and nitrosyl n bonding can be viewed as working

together to create a large HOMO-LUMO gap, and a diamagnetic 12e Cr(NO)R-3 species

results.58 This is in contrast to pseudo-octahedral Cr(U)(NO) complexes, where the NO ti-

acceptor interactions occur within the t2g set, and are therefore not additive with the large

octahedral field splitting (A0) in enforcing a low-spin electronic configuration.24,59

Replacing one R ligand with a NR2 group increases the electron count from 12e to

14e due to the formation of a Cr-NR2 TI bond. While the existence of an amide 7t-donor

interaction is confirmed by the solid-state molecular structure of 5 . 3 , the Cr-NR2 K bond

does not increase the HOMO-LUMO gap of Cr(N0)(NR2)R2 compared to a hypothetical

Cr(NO)R3 complex. Thus, the Cr-NR2 K bond is not considered to be a critical factor in

enforcing a diamagnetic configuration in 5 . 3 . Instead, a combination of tetrahedral

geometry and NO TC bonding increases the orbital splitting energy, while the covalent

chromium-alkyl bonds decrease the interelectron repulsion energy,240 leading to a S = 0

ground state.

133

O N R

R — C r C i ^ C r — R

I /Pro I R 2 N

O

O N I

C r

O N

/ P r 2 N ' < ^ R

R

"** / P r 2 N — C r : I

L

- R R (5.4)

This orbital rationale does not explain, however, the apparent reluctance of 5.3 to

decrease its coordinative and electronic unsaturation either through dimerization or

interactions with potential o-donor L ligands (Equation 5.4). Undoubtedly, the relatively

large steric bulk of the amide and alkyl ligands is largely responsible for the monomeric

nature of 5.3. Nevertheless, it is noteworthy that no highly unsaturated (<16e) Mo(II) and

W(II) alkyl complexes are currently known, and that the "spin-stabilization" invoked for

the S = 2 Cr(II) alkyl species is not applicable to the diamagnetic 5.3. 55b

4 t "• I 1 1 1 1 I i 0 2 i.

,—ip—i—r—i—i—|—l—i—r—i—|—1—l—I TT - | — i 2 0 l b 10 0 6

ppm

Figure 5.4. 1 H N M R spectrum of Cr(NO)(N/Pr2)(CH2SiMe3)2 (5.3) in C 6 D 6

Interestingly, unlike the saturated CpCr(NO)(N 'Pr 2 )X complexes, the amide ' H

N M R signals of 5.3 are very broad at room temperature (Figure 5.4). Evidence that this

must be due to rotation about the C r - N R 2 bond rather than any paramagnetic broadening

includes (a) the sharp, equivalent resonances assignable to the MesSiCHa ligands, (b) the

position o f the amide peaks at 0.92 and 1.14 ppm for the amide M e ' s and 3.28 and 4.36

ppm for the amide C - H ' s , well within the expected ranges for these signals, 4 8 ' 5 1 and (c)

the variable temperature behavior of these resonances, which sharpen slightly without

shifting up-, or down-field as the sample is cooled to - 5 0 ° C .

134

Cr(N)(N/Pr2)3 1 broad signals

hindered rotation

Cr(N)(N7Pr2)(CH2SiMe3)2 2 sharp doublets

very slow or wo rotation

z2

N

Figure 5.5. Qualitative orbital splitting and amide rotation barriers in Cr(NO) and Cr(N)

species

The amide rotation is possible because unlike the CpCr(NO)(N'Pr2)X complexes,

5.3 has two orthogonal, metal-based orbitals which are capable of accepting TC donation

from the NR 2 group (Figure 5.5). Since the dZ2 orbital is slightly higher in energy than the

degenerate d^/d^.^ pair, the planar O N - C r - N R 2 orientation is preferred (as seen in the

solid-state molecular structure, Figure 5.3), and rotation is slightly hindered. If free

rotation was possible, all the N'Pr2 methyl groups would be equivalent and give one sharp -

doublet, as is observed for Cr(NO)(N'Pr2)3. As outlined in Figure 5.5, the nitride

complexes have slower rotation than the corresponding nitrosyl compounds.51 This is

presumably due to an increased orbital splitting between the dZ2 arid d ^ / d ^ i orbitals,

since the stronger Cr=N c-bond lies along the z axis.

Alternative routes to CpCr(NO)R 2 .

This section outlines my attempts to generate CpCr(NO)R2 complexes from

derivatives of Cr(NO)(N'Pr2)3: namely Cr(NO)(0'Bu) 3, 4 6' 4 8 Cr(NO)(N'Pr 2)(0 2CPh) 2, 5 2

and Cr(NO)(N'Pr2)(CH2SiMe3)2 (5.3). Complex 5.3 did not react with C5H6 to eliminate

F£N'Pr2. Treatment of Cr(NO)(N'Pr2)3 with excess P H C 0 2 H (or isolated

Cr(NO)(N'Pr2)(02CPh)2 with one equivalent of PhC02Ff) did not yield the desired

Cr(NO)(N/Pr2)3

1 sharp doublet fast rotation

Cr(NO)(N7Pr2)(CH2SiMe3)2 2 broad signals slow rotation

O N

Cr x 2-y 2

xy

135

Cr(NO)(0 2CPh) 3 product. No absorbances in the expected v(NO) region of the

spectrum were exhibited when the solid was dissolved in CH2CI2, only broad signals

were evident in its *H NMR spectra (C6D6), and the green solid displayed a parent peak

in its mass spectrum consistent with {[(PhC02)2Cr] 3(p.-0)}+. CpCr(NO)(OR)2 complexes

are an attractive alternative to the unstable CpCr(NO)Cl2 as potential precursor

complexes for CpCr(NO)R2 species. However, the tris(butoxide) complex

Cr(NO)(0'Bu) 3 did not react with CpSiMe 3 or Na(DME)Cp, and no diamagnetic

organometallic products were evident in the reaction with CpLi (*H NMR, CeDe). No

isolable, nitrosyl-containing products were obtained from the reaction of lutidinium

iodide or P h C 0 2 H with Cr(NO)(0'Bu) 3 or 'BuOH with Cr(NO)(N'Pr2)(02CPh)2.

While a viable route to CpCr(NO)(0'Bu) 2 has yet to be devised, the Cp*

bis(propoxide) analog has previously been synthesized from Cp*Cr(NO)2Cl and Na(O'Pr)

in hot 'PrOH. 5 4 b Cp*Cr(NO)(0'Pr)2 appeared to react cleanly with Mg(CH 2Ph) 2, as

judged by solution ER. spectroscopy; over 90 min, the v(NO) of the bis(propoxide)

starting material at 1669 cm - 1 was replaced with a new absorbance at 1623 cm - 1 (THF).

The *H NMR spectrum of the crude reaction residue displayed a single Cp* resonance

(CeD6), but the presumed Cp*Cr(NO)(CH2Ph)2 product could not be isolated free of the

Mg(0'Pr) 2 byproduct (equation 5.5). Both the organometallic species and the inorganic

salt were soluble in hexanes, and only an amorphous green solid was deposited from a

cold hexamethydisiloxane:hexanes solution. Thus far, attempts to separate the two

species by chromatography, aqueous work-up, or addition of benzoic acid have been

unsuccessful.

zPrO I ^ O / P r N O

Mg(CH2Ph)2

**• THF

PhCH 2

X r CH 2 Ph

+ Mg(0/Pr) 2 (5.5)

N O

136

Summary

The initial attempts to synthesize CpCr(N0)R.2 complexes were based on the

methodology used to obtain the heavier Cp'M(NO)R.2 (M = Mo, W) congeners.

Oxidative halogenation of Cr(0) d 6 Cp'Cr(NO)(CO)2 compounds failed to yield the

required Cp'Cr(NO)X2 (X = I, CI) complexes, however, due to the instability of these

Cr(II) d 4 dihalo species with respect to loss of NO. Overoxidation of the initially-formed

Cr(I) d 5 products was avoided by conducting the reactions of Cp'Cr(NO)(CO)2 with I2 or

PCI5 in NCMe, thereby providing synthetic access of the previously unknown

[Cp*Cr(NO)(u,-X)]2 (X = I 5.1, CI 5.2) dimers. The solvent dependence of this reaction is

of general significance since controlled oxidations of transition-metal carbonyls with

halogens constitute an important route to metal-halide complexes, synthetic precursors to

a vast number of organotransition-metal compounds.60

Subsequent studies employed nitrosyl complexes which already possessed the

desired Cr(II) d 4 electronic configuration. Selective amine elimination reactions were

crucial in order to use Cr(NO)(N'Pr2)3 as a precursor to organometallic Cr(II)(NO)

compounds, including Cr(NO)(N'Pr2)(CH2SiMe3)2 (5.3). A diamagnetic, tetrahedral, 14e

bis(alkyl) complex that displays rotation about the Cr-N'Pr2 bond, 5.3 provides an

interesting example of how unsaturated organometallic species can be stabilized by a

combination of steric effects and synergic 7t-bonding interactions.

When attempts to generate CpCr(NO)(O fBu)2 from Cr(NO)(N'Pr2)3 were

unsuccessful, the known Cp*Cr(NO)(0'Pr)2 was used as a precursor to Cp*Cr(NO)R.2

complexes. While Cp*Cr(NO)(0'Pr)2 appeared to react cleanly with

Mg(CH2Ph)2x(dioxane) as judged by solution IR spectroscopy, the presumed

Cp*Cr(NO)(CH2Ph)2 product could not be separated from the Mg(0'Pr)2 byproduct.

137

Future Work

l e O x i d a t i o n o f 17e C p ' C r ( N O ) R 2 a n i o n i c complexes

While the reaction of Cp*Cr(NO)(0'Pr)2 and Mg(CH 2Ph) 2 seems to indicate that

Cp*Cr(NO)(CH 2Ph) 2 is a viable target molecule, a new synthetic route is required to

avoid byproduct separation problems. One such route involves the reaction of

[Cp'Cr(NO)(p-X)]2 with alkyl lithium reagents to afford [Li(THF)n][Cp'Cr(NO)R2]

compounds. These 17e "ate" complexes could then be oxidized by I 2 or [Cp 2Fe]+ to

provide the desired 16e Cp'Cr(NO)R 2 species (equation 5.6).

O N

•n I..,. I 2 LiR c ' r © oxidation c ' (5.6)

N fa? N N

o o ^ o vLi(THF)n

Similar "ate" complexes have recently been synthesized by Steve Sayers and

Brett Sharp, including the 17e [Li(THF)n][Cp*Mo(NO)(CH2SiMe3)2] analog of the target

Cr compounds.61 The known complexes are stabilized by isonitrosyl-lithium interactions,

THF solvation of the L i + cation, and occasionally by adopting a dimeric structure with

bridging, planar M-NO(p-Li ) 2 ON-M units. Other compounds related to the proposed

[Li(THF)n][Cp'Cr(NO)R2] species include ["Bu4N][CpCr(NO)Cl2],2 4 b

[Li][Cp*Cr(CH 2Ph) 3], 3 4 e and [Li(THF) 2][CpCrCl 3]. 6 2

D i a m a g n e t i c 12e t r i s (a lkyI ) species

The qualitative orbital diagram shown in Figure 5.3 suggests that amide 7t

donation is not required to maintain the low-spin state of Cr(NO)(N'Pr2)(CH2SiMe3)2.

This hypothesis may be tested by the synthesis of a Cr(NO)R3 complex containing very

bulky R groups. I attempted to generate such a complex by reacting Cr(NO)(0'Bu)3 With

3 equivalents of LiCH(SiMe 3 ) 2 . 6 3 While a hexanes-soluble species with a new v(NO)

138

band at 1685 cm - 1 (hexanes) was observed, no product was isolated from the dark

khaki green solution (which appeared red to transmitted light). Closer examination of the

literature revealed that the synthesis of this very compound had previously been

attempted over 20 years ago (equation 5.7).58 Lappert and co-workers treated the 9e

Cr(CH(SiMe 3) 2) 3 complex with NO gas to give Cr(NO)(CH(SiMe3)2)3 This species

decomposed in cold hexanes solutions and was only characterized by IR spectroscopy

(v(NO) = 1672 cm - 1 , no medium reported).

\ / O O \ f \{ N . N. ^ S i — ( S i —

R = CH(SiMe 3) 2 / \ T

Alternative Cr(NO)R3 target molecules might include those with alkyl groups

without a-H's, in order to avoid a-hydrogen abstraction reactions to form unstable

alkylidene intermediates (e.g. R = adamantyl, mesityl).64

Paramagnetic 16e Cr(0) compounds

The bonding rationale illustrated in Figure 5.3 shows that the Cr(U), d 4, He

Cr(NO)(N'Pr2)(CH2SiMe3)2 possesses two empty, nearly degenerate, non-bonding

orbitals. This orbital description suggests that Cr(0), d6,16e complexes of the formula

Cr(NO)(NR 2)L 2 should be paramagnetic, with one electron occupying each of the two

metal-centered, non-bonding orbitals. Such a compound could be synthesized by (a) 2e

reduction of Cr(NO)(N'Pr2)(02CPh)2 in the presence of trapping L ligands, or (b) amide-

for-halide metathesis of a suitable Cr(NO)(L)4X precursor,65 followed by L ligand loss

(assisted by amide TC donation and steric effects), as shown in Equation 5.8.

139

Initial attempts to generate Cr(NO)(N'Pr2)(dppe) by method (a) using Mg or

K/graphite as reducing agents were unsuccessful.

Q , v 0 O N (a) N 0>) N •

I -2e" I , + NR 2 ~ cU^-L R , N ^ \ ° 2 C P h R o N ^ \ L - 2 L (5.8) R 2 W 0 2 CPh R 2 N L X

Paramagnetic Cr(NO)(NR2)L2 species would be of interest for five basic reasons.

First, analogous Mo and W complexes might be accessible from known M(NO)(PMe3)4X

precursors.66 Second, the tetrahedral geometry, high-spin configuration, and coordinative

and electronic unsaturation of these species would be very unusual, since Cr(0), d 6

complexes typically contain several 7t-acceptor ligands which encourage the formation of

low-spin octahedral CrL6 compounds.67 Third, the ligand addition/substitution processes

of Cr(NO)(NR 2)L 2 compounds would provide an excellent test case to study the effect of

spin state on these fundamental reactions.55b'68 Fourth, selective tuning of the spin state of

M(NO)(NR2)L2 species may be achieved by modifying the orientation and 7t-donor

properties of the NR2 group, the 7t-acceptor abilities of the L ligands, and the identity of

the metal center. Fifth, the potential C - H bond activation reactivity of these

M(NO)(NR 2)L 2 complexes to generate M(II), d 4 M(NO)(NR2)(R)(H)L2 species could

also be evaluated.

Diamagnetic 16e M(NO)(NR 2)(L)R 2 bis(alkyls) of Cr, Mo, and W

Another potential class of organometallic nitrosyl compounds which includes all

three Group 6 transition metals are M(II), d 4, 16e M(NO)(NR 2)(L)R 2 species. The

possibility that a class of M(NO)(L)X3 complexes should be synthetically accessible was

initially forwarded by Cotton, Chisholm, and co-workers.69 A prototypal ligand system I

designed employs a bidentate amidopyridine, monoanionic ligand derived from the imine

140

product o f the condensation of H 2 N R and 2-pyridinecarboxaldehyde. The sp3-

hybridized C H 2 group separates the strong 7t-donor amide from the a-donor pyridine,

thereby preventing the Tt-donor properties of the ligand from being attenuated through

derealization.70 When the amide donor aligns cis to M - N O axis, the pendant pyridine

group should coordinate trans to the nitrosyl ligand (equation 5.9). The pyridine ligation

will thus prevent the amide from rotating (as is observed in 5.3), while the chelate effect

overcomes the trans effect of the N O group, which would otherwise labilize the pyridine

moiety.48 Initial steps towards Mo(NO)(ArNCH2C5H4N)R2 compounds have been taken

by Craig Adams. 7 1 p j R N

+ N H 2 R

N O . N R (5.9)

Alternative route to Cp'CrCNOfcCl and [Cp*Cr(NO)(n-CI)h

NCMe A

* " O C — C r — N

NCMe

Diazald

PC15 NCMe

Scheme 5.5. Current synthetic routes to Cp*Cr(NO)-containing complexes

141

Scheme 5.5 illustrates the route currently used to make the Cp*Cr (NO)-

containing complexes described in this chapter. The difficulty in this sequence lies with

Cp*Cr(NO)(CO) 2 , which is more difficult to synthesize than CpCr (NO)(CO) 2 , and is

obtained in poor overall yield from Cr(CO)6 (-20-30%). The original synthesis o f

C p C r ( N O ) 2 C l from in situ generated C p C r ( T H F ) C l 2 and N O gas suggests that the

recently reported [Cp*Cr(p>Cl)] 2 dimer 9 8 may be a useful precursor to Cr(I) mononitrosyl

and Cr(0) dinitrosyl derivatives (Equation 5.10). If such a synthetic route could be

attained, it would help encourage the development of the chemistry o f Cp*Cr (NO)-

containing species.

(5.10)

15e Cp*Cr(NO)R species

In chapter 3,1 outlined the possibility o f 7t-donor stabilized Cr(I), d 5 , 17e

C p ' C r ( N O ) Y complexes. If the synthesis o f these amide and/or alkoxide species is

successful, attempts could be made to generate 15e Cp*Cr(NO)R compounds with very

sterically-demanding alkyl groups (e.g. R = mesityl, adamantyl, CH(SiMe3)2,

C H 2 C M e 2 P h ) . Similar highly unsaturated (<16e), paramagnetic complexes have been

reported for Ti(ffl) (d 1 ) , 7 2 V(m) (d 2 ) , 7 3 and Cr(UI) ( d 3 ) . 4 0 0

142

Experimental

Methods

A l l reactions and subsequent manipulations were conducted under anaerobic

conditions using an atmosphere o f N2. The complexes LiCH(SiMe3)2, 6 3 N a ( D M E ) C p , 7 4

Cr(NO)(N 'Pr2) 3 , 4 6 ' 4 8 ' M a Cr(NO)(0 'Bu) 3, 4 6 ' 4 8 Cr(NO)(N'Pr2)(02CPh)2,52 and

Cp*Cr(NO)(0 'Pr) 2

5 4 b were prepared by the published procedures. C p * C r ( N O ) ( C O ) 2

7 5

was synthesized by treatment o f C p * C r ( C O ) 3 H 7 6 with Diazald in T H F . Lutidinium iodide

was prepared by treating distilled 3,5-Me2pyridine in ' B u O H with one equivalent of

Me3SiI in an inert atmosphere glove box . 5 1 c A l l other reagents were used as received

from commercial suppliers. Filtrations were performed through Celite (1x2 cm)

supported on a medium porosity frit unless otherwise specified. For low temperature

reactions, solvents were transferred via trap-to-trap distillation from the drying reagent

directly onto the reactants contained in a flask cooled by a liquid nitrogen bath.

Synthesis of [Cp*Cr(NO)(p>I)]2 (5.1).

N C M e (-30 mL) was vacuum transferred onto Cp*Cr (NO)(CO) 2 (0.123 g, 0.450

mmol), and the orange chromium complex dissolved as the solution was warmed in an

ice bath. Iodine (0.056 g, 0.441 mmol) was added, and the solution was stirred for 6 h

while being slowly warmed to ambient temperature, after which time only peaks

attributable to Cp*Cr(NO)(CO)2 remained in the v(CO) region o f the solution's IR

spectrum. The solution was heated with a 55 °C water bath for - 35 min, and the solution

was then taken to dryness in vacuo. The remaining green residue was dissolved in T H F

(15 min) and taken to dryness four times, and then triturated with Et20 (30 mL, also

removed in vacuo) four times. The residue was washed extensively with hexanes and

then extracted into toluene, filtered, and recrystallized at -30 °C from toluene:hexanes to

afford a green powder (0.096 g, 62% yield).

143

Anal . Calcd. for C2oH3oCrI2N202: C , 34.90; H , 4.39; N , 4^07. Found: C, 35.29;

H, 4.38; N , 4.36. (reported elemental analysis is the average o f three separate runs).

IR(Nujol): 1647 cm" 1 . IR(CH 2 C1 2 ) : 1650 cm" 1 . Fast Atom Bombardment M S : m/z 688

(P+).

Synthesis of [Cp*Cr(NO)(p . -Cl) ] 2 (5.2).

N C M e (-20 mL) was vacuum transferred onto Cp*Cr (NO)(CO) 2 (1.48 g, 5.41

mmol) and the orange solution was warmed to -30 °C. PCI5 (0.550 g, 2.64 mmol) was

added, and the reaction mixture was stirred at - 30 °C for - 20 min and then allowed to

warm slowly to room temperature. After 90 min, E t 2 0 (10 mL) was added to the green

solution, and the solution was filtered. The plug of Celite was washed with E t 2 0 ( 3 x 5

mL), and the combined filtrates were taken to dryness in vacuo. The remaining residue

was triturated with E t 2 0 (10 mL) and washed extensively with pentane (7 x 5 mL).

Remaining solvent was removed under static vacuum to obtain crude [Cp*Cr(NO)(p.-

C l ) ] 2 as a green powder (1.22 g, 89 % yield). Crystalline samples suitable for elemental

analysis and X-ray diffraction were obtained by recrystallizing the powder overnight at

- 30 °C from a -2:1 toluene:hexanes solvent mixture.

Anal . Calcd. for C 2 0 H 3 o C r C l 2 N 2 0 2 : C , 47.53; H , 5.98; N , 5.54. Found: C, 47.53;

H, 6.14; N , 5.49. IR(Nujol): 1648 cm" 1 . IR(CH 2 C1 2 ) : 1645 cm" 1 . Fast Atom

Bombardment M S : m/z 474 (P + -NO) .

Synthesis of C r ( N O ) ( N ' P r 2 ) ( C H 2 S i M e 3 ) 2 (5.3).

T H F (-25 mL) was vacuum transferred onto Cr (NO)(N 'Pr 2 ) (0 2 CPh) 2 (0.344 g,

0.803 mmol) andMg(CH 2 SiMe 3 ) 2 x(d ioxane) (0.249 mg, 0.803 mmol). The solution

changed from an orange suspension to a clear, red solution as it was allowed to warm

slowly to room temperature over - 30 min. The solvent was then removed in vacuo, and

144

the red residue was extracted with hexanes ( 5 x 1 0 mL) and filtered. The solvent was

removed in vacuo to obtain a red powder (0.242 g, 85% yield). Dark red rectangular

blocks suitable for X-ray diffraction were obtained by recrystallization o f this powder

from hexamethyldisiloxane at -30 °C.

Anal . Calcd. for Ci4H 36CrN 2OSi 2: C, 47.15; H , 10.17; N , 7.86. Found: C, 46.77;

H , 9.84; N , 7.66. IRfNujol): 1670 cm" 1 . lH N M R ( C 6 D 6 ) : 8 0.20 (d, 2FL CHW), 0.32 (s,

18 H, Si(CH3h), 0.92 (br s, 3FL C H ( C / k ) 2 ) , 1.14 (br s, 3FL CH(C#02), 1.99 (d, 2H ,

CH//*), 3.28 (v br s, I H , C ^ C H ^ ) , 4.36 (v br s, I H , C / / ( C H 3 ) 2 ) . 1 3 C N M R ( C 6 D 6 ) : 8

2.54 (Si (CH 3 ) 3 ) , 21.3 (br, C H ( C H 3 ) 2 ) , 28.1 (br, C H ( C H 3 ) 2 ) , 68.06 ( C H 2 ) , 49.0 (br,

C H ( C H 3 ) 2 ) , 55.9 (br, C H ( C H 3 ) 2 ) . L o w Resolution Electron Impact M S (probe

temperature 150 °C): m/z 356 (P + ) .

Reaction of 5.3 with C5H6.

C r ( N O ) ( N ' P r 2 ) ( C H 2 S i M e 3 ) 2 (0.020 g, 0.056 mmol) was dissolved in C 6 D 6 ( ~ l m L )

and freshly "cracked" C5H6 (~0.1 mL) was added. The lH N M R spectrum of this mixture

displayed resonances attributable only to the bis(alkyl) starting material, C5H6, and the

CioHn dicyclopentadiene dimer.

Reaction of Cr(NO)(N iPr2)(02CPh)2 with P h C 0 2 H .

C H 2 C 1 2 (5 mL) was added to Cr (NO)(N 'Pr 2 ) (0 2 CPh) 2 (0.058 g, 0.15 mmol) and

P h C 0 2 H (0.019 g, 0.16 mmol). The resulting orange-red solution was monitored by

solution IR spectroscopy. Over the course of several hours, the peaks due to the

bis(benzoate) species (1720 c m - 1 , s) and benzoic acid (1695 c m - 1 , m) slowly diminished

in intensity. After -18 h, the green solution exhibited only a very weak, broad peak at

1703 c m - 1 and stronger, broad peaks at 1625, 1600, and 1548 c m - 1 in its IR spectrum.

145

The solvent was removed in vacuo to leave a pale green solid, which displayed a m/z

peak at 898, assigned to {[(PhC0 2)2Cr] 3(p>0)} + (Low Resolution Electron Impact M S

(probe temperature 350 °C)).

Reaction of Cr(NO)(0'Bu)3 with Na(DME)Cp, CpSiMe3, LiCp.

T H F (-30 mL) was vacuum transferred onto Cr (NO)(0 'Bu) 3 (0.092 g, 0.31 mmol)

and N a ( D M E ) C p (0.055 g, 0.31 mmol). The solution was allowed to warm to room

temperature and was stirred overnight. The solvent was then removed in vacuo, the

residue was extracted with E t 2 0 and filtered. A n IR spectrum of the filtered solution

displayed a single v (NO) peak at 1707 c m - 1 , due to the unreacted tris(propoxide) starting

material. The lack of reaction was also confirmed by *H N M R ( C 6 D 6 ) , which showed

only the single resonance due to Cr(NO)(0 'Bu) 3 .

C r (NO)(0 'Bu) 3 was similarly unreactive with C p S i M e 3 in E t 2 0 , even after

heating at 60 °C for 4 days in a sealed vessel. Reaction o f Cr (NO)(O f Bu) 3 with L i C p in

T H F overnight did result in the slow disappearance o f the starting material peak at 1708

c m - 1 along with the concomitant growth of a new peak at 1622 c m - 1 . However, the ! H

N M R spectra o f the residue o f this reaction dissolved in CeD6 displayed only a single

resonance at 1.502 ppm and no C5H5 signal.

Reaction of Cr(NO)(0'Bu)3 with HX (HX = PhC02H, lutidinium iodide).

Cr(NO)(0 'Bu) 3 (0.020 g, 0.066 mmol) was dissolved in E t 2 0 (5 mL), and an

initial infrared spectrum of the orange solution was taken. P h C 0 2 H (0.010 g, 0.082

mmol) was then added, and a voluminous, pale green precipitate immediately formed.

The infrared spectrum of the orange supernatant was identical to the initial spectrum,

although the intensity of the v(NO) peak of 1706 c m - 1 had decreased in intensity. The

orange solution was removed form the pale green precipitate, the E t 2 0 solution was

146

removed in vacuo to leave an orange residue which sublimed readily at - 40 °C under

vacuum onto a water-cooled probe 4 8 The green precipitate dissolved readily in T H F , and

exhibited a m/z peak at 898, consistent with {[(PhC02)2Cr]3(u,-0)}+ (Low Resolution

Electron Impact M S (probe temperature 350 °C)).

Reaction of Cr(NO)(0 'Bu) 3 with one or two equivalents of lutidinium iodide in

CH2CI2 or T H F did not yield any new nitrosyl-containing products, as judged by solution

IR spectroscopy.

Generation of C p * C r ( N O ) ( C H 2 P h ) 2 .

A cold solution o f Mg(CH 2 Ph) 2 x(dioxane) (0.432 g, 2.79 mmol) in T H F (-15

mL) was added to a frozen suspension of Cp*Cr(NO)(0 'Pr) 2 (0.466 g, 1.39 mmol) in

T H F (-35 mL). The liquid N 2 bath was removed and the solution was allowed to warm

slowly, infrared monitoring o f the reaction showed the nitrosyl band o f the

bis(propoxide) at 1669 c m - 1 being replaced by a new v(NO) peak at 1623 c m - 1 . After

-100 min, the green solution (orange to transmitted light) was reduced to dryness in

vacuo. The dark green residue was extracted with hexanes and filtered. N o ionic

byproduct was evident on the Celite filter pad. The solvent was again removed in vacuo

to afford a green oil . Attempts to extract the green oil with hexamethyldisiloxane resulted

in a green solution, a green tarry residue, and a white precipitate along the walls of the

flask. Hexanes was added to the (Me 3Si)20 suspension, and the green solution was

filtered. Cooling the dark green filtrate resulted in the deposition o f an amorphous green

material. Subsequent attempts to purify portions of the solution or the tarry residue by

aqueous extraction, addition of PI1CO2H, or chromatography were unsuccessful.

Reaction of C r ( N O ) ( 0 ' B u ) 3 wi th L i C H ( S i M e 3 ) 2 .

147

T H F (-10 mL) was vacuum transferred onto Cr(NO)(0'Bu)3 (0.110 g, 0.365

mmol) in a Schlenck vessel. The flask was warmed, the tris(butoxide) dissolved to afford

an orange solution, and the solution was then frozen using a liquid N 2 bath. A solution of

L i C H ( S i M e 3 ) 2 (0.184 g, 1.11 mmol, 3.03 equivalents) in -15 m L T H F was cannulated

onto the frozen Cr(NO)(0 'Bu) 3/THF solution. The combined solution was allowed to

warm slowly to room temperature. After ~ l h , the solvent was removed in vacuo, the red-

brown oily residue was extracted with hexanes ( 4 x 1 5 mL) and filtered to give a dark

khaki green solution that appeared bright red to transmitted light. The solution was

reduced in volume in vacuo to - 7 mL, and its infrared spectrum was taken (v(NO) at

1685 cm" 1). The solution was again removed in vacuo and the red-brown residue was

extracted with hexamethyldisiloxane (-4 mL). The solution was filtered and stored at -30

°C. Over several days, a fine brown silt precipitated from solution.

Reaction of C r ( N O ) ( N ' P r 2 ) ( 0 2 C P h ) 2 wi th dppe and M ( M = M g , K/graphite) .

T H F (-25 mL) was vacuum transferred onto Cr(NO)(N'Pr 2)(0 2CPh) 2 (0.039 g,

0.091 mmol), dppe (0.036 g, 0.090 mmol) and M g powder (0.040 g, 1.7 mmol). The

orange solution was allowed to stir and warm to room temperature. The initial TR

spectrum showed only a peak at 1721 c m - 1 , corresponding to v(NO) o f

Cr(NO)(N'Pr 2)(0 2CPh) 2. After -24 h, the solution turned from orange to bright green,

with no identifiable v(NO) bands in the TR spectrum of the green solution. A control

reaction o f dppe and Cr(NO)(N'Pr 2)(0 2CPh) 2 in T H F in the absence o f M g resulted in the

same slow color change and loss o f nitrosyl peaks.

T H F (-10 mL) was vacuum transferred onto Cr(NO)(N'Pr 2)(0 2CPh) 2 (0.094 g,

0.22 mmol), dppe (0.088 g, 0.22 mmol) and K C 8 (0.075 g, 0.54 mmol). A s the solution

thawed, an immediate reaction occurred to produce a dark brown solution. The TR

spectrum of this solution showed only broad lumps in the v (NO) region.

148


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(49) While the N ' P r 2 ligand could still be described as "extremely bulky" in the

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156

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1993,72,2268.

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157

Appendix

C r y s t a l l o g r a p h i c D a t a 1 5 8

D F T - O p t i m i z e d G e o m e t r i e s 1 6 0

F r a c t i o n a l C o o r d i n a t e s 1 6 6

171 B o n d L e n g t h s 1 ' J

179 B o n d A n g l e s

158

Table A l . Crystallographic Data for Complexes 3.2, 3.7, and 3.8.

3.2 3.7 3.8

formula CnHs^rFsNjOsS CioHi 2CrN 402 CioH 1 2 CrN0 3

formula weight 396.30 272.24 246.21

crystal color brown green green

cryst. size (mm) 0.20x0.40x0.40 0.45x0.42x0.30 0.07x0.30x0.30

crystal system monoclinic monoclinic orthorhombic

space group P2i/n C2/c Pnma

a (A) 8.575(1) 18.3382(4) 17.373(2)

b(A) 8.642(1) 11.7774(3) 8.833(1)

c(A) 21.8626(9) 12.91020(10) 6.926(2)

a(degrees) 90 90 90

P (degrees) 91.760(7) 121.3860(10) 90

y(degrees) 90 90 90

V (A 3 ) 1619.4(3) 2380.31(8) 1062.8(7)

z 4 8 4

D c ai c(g/cm 3) 1.625 1.519 1.539

Fooo 808 1120 508

diffractometer Rigaku AFC6S Siemens SMART Platform CCD

Rigaku AFC6S

X (Mo K a , A) 0.71096 0.71073 0.71069

temperature (K) 294(1) 173(2) 294(1)

no. observations 2546 2072 787

no. variables 209 168 102

goodness of fit 2.27 1.021 1.59

R 0.039 0.0318 0.037

Rw 0.039 0.0815 0.033

159

Table A2. Crystallographic Data for Complexes 5.2 and 5.3.

4.4 5.2 5.3

formula CnHnMoNO C2oH3oCr2N202 Ci4H36CrN2OSi2

formula weight 347.27 505.36 356.62

crystal color green/black green red

cryst. size (mm) 0.50x0.40x0.10 0.25x0.10x0.10 0.20x0.45x0.60

crystal system orthorhombic monoclinic triclinic

space group Pbca P2x/n PI

a (A) 11.120(2) 7.7192(2) 11.2004(12)

b(A) 15.2821(3) 15.0848(4) 12.8210(9)

c(A) 16.8371(6) 10.0264(3) 16.979(2)

a(degrees) 90 90 91.035(7)

B (degrees) 90 106.132(2) 91.743(3)

y(degrees) 90 90 113.6640(8)

V(A3) 2861.4(3) 1121.53(5) 2231.0(3)

z 8 2 4

Dcaic(g/cm3) 1.612 1.496 1.062

Fooo 1408.00 524 776

diffractometer Rigaku/ ADSC C C D

Siemens SMART Platform CCD

Rigaku/ ADSC C C D

X (Mo Kct, A) 0.71069 0.71073 0.71069

temperature (K) 180(1) 173(2) 180(1)

no. observations 3639 1963 4520

no. variables 181 132 361

goodness of fit 1.53 1.030 2.25

R 0.030 0.0464 0.056

Rw 0.021 0.0930 0.047

160

Table A3. Fractional coordinates of DFT-optimized geometry of CpMo(NO)Cl 2.

CpMo(NO)Cl 2 (S = 0) CpMo(NO)Cl 2 (S = 1)

X y z X y z

Mo 0.000000 0.190450 0.000000 0.000000 0.163021 0.000000

CP 0.513256 -1.9311888 0.000000 -0.2589802 -1.9222444 0.000000

CI 1.268246 -2.060042 0.000000 0.957479 -2.045700 0.000000

C2 -0.935807 -1.826917 0.724666 0.113673 -1.960797 1.167768

C3 -0.935807 -1.826917 -0.724666 -1.239863 -1.821964 0.725643

C4 0.429998 -1.972781 -1.157595 -1.239863 -1.821964 -0.725643

C5 0.429998 -1.972781 1.157595 0.113673 -1.960797 -1.167768

HI 2.345722 -2.148656 0.000000 2.033854 -2.151079 0.000000

m. 0.765832 -1.977373 2.184814 0.450458 -1.984371 2.194003

H3 -1.809573 -1.792316 1.360190 -2.111234 -1.742680 1.361070

H4 -1.809573 -1.792316 -1.360190 -2.111234 -1.742680 -1.361070

H5 0.765832 -1.977373 -2.184814 0.450458 -1.984371 -2.194003

Cl l 0.874001 1.136367 2.019498 1.243732 1.217996 1.768110

C12 0.874001 1.136367 -2.019498 1.243732 1.217996 -1.768110

N -1.511760 1.24549 0.000000 -1.597493 1.055392 0.000000

0 -2.616468 1.642179 0.000000 -2.755916 1.453255 0.000000

161

Table A4. Fractional coordinates of DFT-optimized geometry of CpCr(NO)Cl2.

CpCr(NO)Cl 2 (S = 0) CpCr(NO)Cl 2 (S = 1)

X y z X y z

Cr 0.000000 0.149609 0.000000 0.000000 0.089812 0.000000

CP -0.407645 -1.7641532 0.000000 0.2902544 -1.8572206 0.000000

CI 0.751141 -2.150810 0.000000 1.493576 -1.651475 0.000000

C2 -0.048100 -1.888680 1.156117 0.660878 -1.807838 1.165255

C3 -1.346583 -1.446298 0.721814 -0.682030 -2.009476 0.727084

C4 -1.346583 -1.446298 -0.721814 -0.682030 -2.009476 -0.727084

C5 -0.048100 -1.888680 -1.156117 0.660878 -1.807838 -1.165255

HI 1.789523 -2.450725 0.000000 2.559484 -1.472351 0.000000

H2 0.279905 -1.961445 2.182599 0.989197 -1.728526 2.191053

H3 -2.185661 -1.209217 1.360492 -1.541494 -2.171345 1.363395

H4 -2.185661 -1.209217 -1.360492 -1.541494 -2.171345 -1.363395

H5 0.279905 -1.961445 -2.182599 0.989197 -1.728526 -2.191053

Cl l 1.100844 0.895396 1.842417 0.726568 1.406943 1.734847

C12 1.100844 0.895396 -1.842417 0.726568 1.406943 -1.734847

N -1.098043 1.388438 0.000000 -1.739598 0.748655 0.000000

0 -1.936381 2.245438 0.000000 -2.836078 1.219570 0.000000

162

Table A5. Fractional coordinates of DFT-optimized geometry of CpCr(NO)(NH2)Cl.

CpCr(NO)(NH2)Cl (S = 0) CpCr(NO)(NH2)Cl (S - 1)

X y z X y z

Cr 0.179940 -0.120375 0.258956 0.155841 -0.025860 0.126545

CP -1.68665 -0.090923 -0.3958494 -1.7945818 -0.1983268 -0.1523216

CI -2.063535 0.729330 0.427830 -1.964778 0.914908 0.318149

C2 -1.570631 1.011361 -0.891307 -1.744352 0.602914 -1.071777

C3 -1.239701 -0.222758 -1.529443 -1.590178 -0.810933 -1.191199

C4 -1.510431 -1.285569 -0.590850 -1.704051 -1.383852 0.131669

C5 -2.048952 -0.686979 0.604523 -1.969550 -0.314671 1.051550

HI -2.381499 1.465814 1.153071 -2.097568 1.905669 0.728543

H2 -1.422703 1.995688 -1.309704 -1.671689 1.323773 -1.873523

H3 -0.846316 -0.340498 -2.529160 -1.413874 -1.358298 -2.107304

H4 -1.405503 -2.344718 -0.780720 -1.658725 -2.437761 0.371030

H5 -2.305364 -1.227888 1.491003 -2.100153 -0.411941 2.120533

CI 1.322775 1.866161 -0.158810 1.207600 1.998265 -0.341661

N l 1.402034 -1.061370 -0.321940 1.433612 -1.175760 -0.444796

0 2.251912 -1.765224 -0.818629 2.26852 -2.006294 -0.739494

N2 0.467073 -0.436167 2.003794 1.072334 -0.221351 1.810266

H6 1.292471 -0.899210 2.382315 1.041972 -1.029439 2.434499

H7 -0.191384 -0.152688 2.729551 1.823676 0.438050 2.024705

163

Table A6. Fractional coordinates of DFT-optimized geometry of

CpCr(NO)(NH2)(CH3).

CpCr(NO)(NH2)(CH3) (S = 0) CpCr(NO)(NH 2)(CH 3) (S = 1)

X y z X y z

Cr 0.276935 -0.253682 -0.090082 0.260177 0.210833 0.051365

CP -1.5590238 0.49401 -0.018047 -1.6641252 -0.4092824 -0.0419234

CI -2.080923 -0.599806 0.152022 -2.017809 0.649412 0.453032

C2 -1.632990 0.286060 1.191474 -1.851198 0.390194 -0.946299

C3 -1.090316 1.461193 0.583538 -1.4028880 -0.964264 -1.098595

C4 -1.177758 1.293709 -0.844229 -1.339070 -1.558098 0.215438

C5 -1.813132 0.028894 -1.100852 -1.709669 -0.563656 1.166807

HI -2.537865 -1.569498 0.299231 -2.336909 1.582909 0.896139

H2 -1.688341 0.091984 2.253121 -2.007922 1.099846 -1.746446

H3 -0.678803 2.318461 1.097952 -1.206343 -1.470024 -2.034482

H4 -0.866681 2.012896 -1.589002 -1.046530 -2.576336 0.435832

H5 -2.016326 -0.381636 -2.080780 -1.744071 -0.689617 2.241023

N l 1.691961 0.572418 -0.174880 1.718721 -0.761915 -0.215391

0 2.707884 1.254216 -0.193440 2.711528 -1.456423 -0.317665

N2 0.687675 -1.620891 -1.190551 1.114818 0.987615 1.590634

H6 1.610786 -1.798451 -1.585162 1.219422 0.517810 2.492472

H7 -0.008865 -2.285496 -1.529809 1.701867 1.819885 1.515106

C6 0.853132 -1.043244 1.735861 0.809129 1.797014 -1.137656

H8 0.122548 -1.810560 2.014320 0.177586 2.659212 -0.883793

H9 1.840873 -1.488146 1.584686 1.865300 2.032649 -0.955460

H10 0.907632 -0.256438 2.496064 0.675333 1.531546 -2.194901

164

Table A7. Fractional coordinates of DFT-optimized geometry of CpCr(NO)(CH 3) 2.

CpCr(NO)(CH 3) 2 (S = 0) CpCr(NO)(CH 3) 2 (S = 1)

X y z X y z

Cr 0.000000 3.81150 0.000000 0.000000 0.281549 0.000000

CP 0.70922 -1.4710862 0.000000 0.3175912 -1.7198644 0.000000

CI 1.883182 -1.139789 0.000000 1.515850 -1.481532 0.000000

C2 1.074130 -1.369926 1.157525 -0.651080 -1.909265 0.717461

C3 -0.242671 -1.737895 0.719960 -0.651080 -1.909265 -0.717461

C4 -0.242671 -1.737895 -0.719960 0.687133 -1.649630 -1.160128

C5 1.074130 -1.369926 -1.157525 0.687133 -1.649630 1.160128

HI 2.918678 -0.825391 0.000000 2.578372 -1.280918 0.000000

H2 1.389776 -1.266015 2.186123 1.014485 -1.581168 2.188364

H3 -1.079161 -1.995703 1.354291 -1.510335 -2.077877 1.353372

H4 -1.079161 -1.995703 -1.354291 -1.510335 -2.077877 -1.353372

H5 1.389776 -1.266015 -2.186123 1.014485 -1.581168 -2.188364

N -1.569566 0.855582 0.000000 -1.405353 1.393943 0.000000

0 -2.762181 1.126724 0.000000 -2.316012 2.198245 0.000000

C6 0.436044 1.445636 -1.671323 0.905041 1.389661 -1.499485

H6 1.502113 1.343492 -1.905142 1.962124 1.097733 -1.527345

H7 -0.177790 1.170493 -2.535251 0.420615 1.154101 -2.457336

H8 0.193362 2.477667 -1.377559 0.812597 2.457295 -1.278459

C7 0.436044 1.445636 1.671323 0.905041 1.389661 1.499485

H9 1.502113 1.343492 1.905142 1.962124 1.097733 1.527345

H10 -0.177790 1.170493 2.535251 0.420615 1.154101 2.457336

H l l 0.193362 2.477667 1.377559 0.812597 2.457295 1.278459

165

Table A8. Fractional coordinates of DFT-optimized geometry of CpCr(NO)(CH3)Cl.

CpCr(NO)(CH 3)Cl (S = 0) CpCr(NO)(CH 3)Cl (S = 1)

X y z X y z

Cr -0.211457 -0.014328 0.105076 0.155932 -0.014571 0.182793

CP 1.7029544 -0.1303132 -0.2956746 -1.792489 0.0718936 -0.2776332

CI 2.047656 0.710178 0.519777 -2.034280 0.032015 0.923950

C2 1.917450 -0.700164 0.758324 -1.801195 1.216156 0.148109

C3 1.497781 -1.327673 -0.462453 -1.594803 0.812861 -1.230860

C4 1.346614 -0.292988 -1.453915 -1.640480 -0.604375 -1.282732

C5 1.705271 0.959081 -0.840106 -1.891687 -1.097189 0.053367

HI 2.317913 1.457300 1.253426 -2.241042 -0.002826 1.983656

H2 2.109755 -1.205932 1.693346 -1.866033 2.234639 0.507657

H3 1.346043 -2.386299 -0.619376 -1.414646 1.477660 -2.065014

H4 1.060374 -0.437882 -2.486045 -1.473143 -1.217405 -2.157311

H5 1.659859 1.928287 -1.316040 -1.989477 -2.136828 0.334335

N -1.376291 -1.051998 -0.422105 1.287554 1.379374 -0.141715

0 -2.161320 -1.881166 -0.829349 2.022366 2.272032 -0.468429

CI -1.292038 1.952441 -0.105964 1.620704 -1.602511 -0.601989

C6 -0.592630 -0.630246 2.030642 0.665025 -0.286869 2.148684

H6 0.049023 -0.044905 2.695842 0.233151 -1.239911 2.476285

H7 -1.645908 -0.349846 2.142799 1.757733 -0.335854 2.184994

H8 -0.465687 -1.704170 2.191526 0.291843 0.545466 2.758515

166

Table A9. Fractional Coordinates and B e q for 3.2.

atom X y

Cr(l) 0.62171(6) 0.32891(6)

S(l) 0.30990(10) 0.12690(10)

F(l) 0.1361(3) -0.0776(3)

F(2) . 0.1278(3) 0.1452(3)

F(3) 0.3261(3) 0.0014(3)

0(1) 0.3990(2) 0.2564(2)

0(2) 0.1813(3) 0.1771(3)

0(3) 0.4049(3) 0.0055(3)

0(4) 0.7496(4) 0.3830(4)

0(5) 0.7996(3) 0.0524(3)

N(l) 0.6823(4) 0.3516(3) .

N(2) 0.7159(3) 0.1565(3)

C(l) 0.7798(3) 0.5096(4).

C(2) 0.6377(4) 0.5841(3)

C(3) 0.5170(3) 0.5308(3)

C(4) 0.5821(4) 0.4224(3)

C(5) 0.7461(3) 0.4145(4)

C(6) 0.9381(4) 0.5388(5)

C(7) 0.6246(5) 0.7051(4)

C(8) 0.3516(4) 0.5817(4)

C(9) 0.4968(4) 0.3432(4)

C(10) 0.8603(4) 0.3280(5)

C(l l) 0.2213(5) 0.0450(5)

z B e ?

0.38638(2) 2.94(1)

0.36910(4) 3.72(2)

0.4194(1) 7.62(7)

0.4615(1) 7.71(8)

0.4769(1) 7.90(8)

0.3968(1) .4.11(5)

0.3318(1) 6.39(8)

0.3462(1) . 5.89(7)

0.5068(1) 8.56(10)

0.3684(2) 7.86(10)

0.4612(1) 4.93(8)

0.3762(1) 4.53(7)

0.3573(1) 3.29(7)

0.3744(1) 3.19(7)

0.3345(1) 2.96(7)

0.2931(1) 3.07(7)

0.3060(1) . 3.18(7)

0.3866(2) 5.30(10)

0.4230(2) 5.2(1)

0.3337(2) 4.54(9)

0.2414(1) 4.66(9)

0.2689(2) 5.5(1)

0.4358(2) 4.9(1)

Table A10. Fractional Coordinates and Ueq for 3.7.

X Y Z U(eq)

C r ( l ) '2018 (1) 4 (1) 2303 (1) 23(1) N ( l ) 2556 (1) -134 (2) 1557 (2) 34(1) N(2) 2881 (1) -476 (2) 3635 (2) 27 (1) 0(1) 2965 (2) -118 (2) 1104 (2) . 62 (1) 0(2) 3522 (1) -690 (2) 4536 (2) 42 (1)

C ( l ) 2029 (2) 1711 (2) 2999 (3) ; 46(1) C(2) 1406 (2) 1067 (2) 3083 (2) . 34(1) C(3) 774 (1) 767 (2) 1896 (2) 32(1) C (4) 1000 (2) 1205 (2) 1093 (2) 40(1) C(5) 1771 (2) 1797 (2) 1779 (3) 48 (1) N(3) 1349 (1) -1445 (2) 1665 (2) 25(1) N(4) 1108 (1) -1778 (2) 502 (2) 26 (1) C(6) 696 (1) -2767 (2) 318 (2) 29(1) C(7) 667 (2) -3077 (2) 1338 (2) 32(1) C(8) 1084 (1) -2219 (2) 2170 (2) 29(1) C(9) 340 (2) -3370 (2) -876 (2) 42(1) C(10) 1244 (2) -2085 (2) 3426 (2) 40(1)

Table A l l . Fractional Coordinates and B

atom X y

Cr(l) 0:45374(4) 0.2500

0(1) 0.4035(2) 0.2500

0(2) 0.3885(1) 0.0918(2)

N(l) 0.4174(2) 0.2500

C(l) 0.5500(3) 0.2500

C(2) 0.5573(2) 0.3763(5)

C(3) 0.5688(2) 0.3288(5)

C(4) 0.3187(2) 0.1102(3)

C(5) . 0.2825(3) 0.2500

C(6) 0.2755(3) -0.0313(5)

for 3.8.

z occ

0.1753(1) 2.83(2) 1/2

-0.2198(6) 5.3(1) 1/2

0.2874(3) 3.20(5)

-0.0499(7) 3.4(1) 1/2

0.3938(9) 4.9(2) 1/2

0.2769(7) 4.4(1).

0.0890(8) 4.26(10)

0.3392(5) 2.70(7)

0.3560(8) 3.2(1) 1/2

0.3841(7) 4.1(1).

169


atom y z Be,

Mo(l) 0.08242(2) 0.174875(14) 0.10364(2) 1.677(5)

0(1) 0.2723(2) 0.04584(12) 0.06261(13) 3.26(6)

N(l) : 0.1903(2) 0.09618(13) 0.07689(14) 1.91(6)

C(l) 0.0288(3), 0.2235(3) -0.0257(2) 4.54(11)

C(2) -0.0204(4) 0.1401(2) -0.0150(3) 4.80(12)

C(3) -0.1140(4) 0.1501(2) 0.0392(3) - 4.26(11)

C(4) -0.1299(3) 0.2401(2) 0.0532(2) 3.94(10)

C(5) -0.0468(4) 0.2833(2) 0.0135(2) 3.82(10)

C(6) 0.2373(3) 0.2644(2) 0.1512(2) 2.64(8)

C(7) 0.1964(3) 0.2139(2) 0.2146(2) 2.23(7)

C(8) 0.0807(3) 0.2434(2) 0.2347(2) 2.25(7)

C(9) 0.0539(3) 0.3156(2) 0.1863(2) 2.35(7)

C(10) 0.1470(3) 0.3265(2) 0.1333(2) 2.71(7)

C(H) -0.0068(3) 0.0705(2) 0.1781(2) 2.00(7)

C(12) 0.0635(3) 0.03364(15) 0.2457(2) 1.79(7)

C(13) 0.1632(3) -0.0209(2) 0.2334(2) 1.93(7)

C(14) 0.2284(3) -0.0541(2) 0.2959(2) 2.31(8)

C(15) 0.1970(3) -0.0354(2) 0.3735(2) 2.47(8)

C(16) 0.0975(3) 0.0163(2) 0.3869(2) 2.66(7)

C(17) 0.0320(3) 0.0494(2) 0.3248(2) 2.45(8)

Table A13. Fractional Coordinates and Ueq for 5.2.

X y z U(eq)

C r (1) 268 (1) 4258(1) 6124 (1) 20(1)

CI (1) 2192 (1) 5373 (1) 5761 (1) 26(1)

C ( l ) -1315 (5) 4726(3) 7590 (4) 27 (1)

0(1) 1728 (5) 2777 (2) 5033 (3) 54.(1)

N ( l ) 1115 (5) . 3404 (2) 5375 (3) 32(1)

C(2) -1517 (5) 3783(3) 7358 (4) 29(1)

C(3) 201 (6) 3399(2) 7877 (4) 29 (1)

C(4) 1461 (5) 4089(3) 8362 (4) 26 (1)

C(5) 491 (5) .4898 (2) 8219 (4) 24(1)

C(6) -2823 (6) 5392 (3) 7246 (5) 49(1)

C(7) -3260 (6) 3297 (3) 6784 (5) 52(1)

C(8) 604 (7) 2418 (3) 7954 (5) 53(1)

C(9) 3457 (6) 3975 (3) 9015 (4) 47(1)

C"(10) 1291 (7) 5801(3) 8700 (4) 44 (1)

171


atom X y

Cr ( l ) 0.14618(4) 0.54593(4)

Cr(2) 0.19977(4) 0.06207(4)

Si( l ) 0.23795(8) 0.58170(7)

Si(2) 0.39461(9) 0.61244(8)

Si(3) 0.27887(13) 0.06298(9)

Si(4) -0.10192(9) -0.03322(8)

0(1) 0.2372(2) 0.7855(2)

0(2) 0.1152(2) 0.2358(2)

N ( l ) 0.1972(2) 0.6824(2)

N(2) -0.0011(2) 0.5097(2)

N(3) 0.1517(2) 0.1624(2)

N(4) 0.3511(2) 0.1395(2)

C ( l ) 0.1212(3) 0.4901(2)

C(2) 0.2898(3) 0.5270(2)

C(3) 0.4080(3) 0.6475(3)

C(4) 0.1887(4) 0.6979(3)

•C(5) 0.2391(3) 0.4959(3)

C(6) 0.5369(4) 0.5721(5)

C(7) 0.4527(5) 0.7669(3)

C(8) 0.3020(4) 0.5796(4)

C(9) -0.0807(3) 0.5666(3)

C(10) -0.0056(4) 0.6517(3)

C ( l l ) -0.1229(4) 0.6249(3)

C(12) -0.0515(3) 0.3833(2)

z B e ,

0.26346(3) 2.190(10)

0.26731(3) 2.494(11)

0:45335(5) 3.12(2)

0.13013(5) 3.49(2)

0.07143(6) 5.44(3)

0.33496(6) 3.91(2)

0.28680(14) 4.78(6)

0.2249(2) 5.37(7)

0.27623(14) 2.93(6)

0.20772(13) 2.53(5)

0.2429(2) 3.36(6)

0.31820(14) . 2.75(6)

0.3751(2) 2.78(7)

0.2075(2) 2.85(7)

0.4181(2) 4.13(8)

0.4855(2) 4.85(10)

0.5409(2) 4.44(9)

0.1191(3) 7.97(14)

0.1562(3) 7.65(13)

0.0349(2) 5.79(11)

0.1749(2) 3.37(8)

0.1140(2) 4.92(10)

0.2412(2) 5.00(10)

0.1985(2) 2.83(7)

172

Table A14. Fractional Coordinates and B e q for 5.3 (continued).

atom

C(13)

C(H)

C(15)

C(16)

C(17) '

C(18)

C(19)

C(20)

C(21)

C(22) •

C(23)

C(24)

C(25)

C(26)

C(27)

C(28)

-0.0559(4)

-0.1799(3)

0.2075(3)

0.0610(3)

0.2314(6)

0.4604(5)

0.2263(7)

-0.2196(4)

-0.0952(5)

-0.1655(4)

0.4289(3)

0.3522(4)

0.4755(3)

0.3923(3)

0.3915(4)

0.5201(3)

y z

0.3451(3) 0.1127(2) 4.35(9)

0.3232(3) 0.2378(2) 4.63(9)

-0.0081(3) 0.1630(2) 3.82(8)

-0.0356(3) 0.3393(2) 4.03(9)

-0.0438(4) -0.0122(3) 9.5(2)

0.1224(5) 0.0831(3) 9.6(2)

0.1799(5) 0.0482(3) 10.7(2)

-0.1589(4) 0.3870(3) 7.30(13)

0.1004(4) 0.3842(3) . 6.94(13)

-0:0415(4) 0.2320(3) 6.45(12)

. 0.2581(2) 0.3425(2) 3.17(7)

0.3024(3) ' 0.3958(2) 4.43(9)

0.3312(3) 0.2727(2) 3.90(8)

0.0500(3) 0.3460(2) 3.60(8)

0.0413(3) 0.4351(3) 5.74(11)

0.0592(3) 0.3124(3) 5.51(11)

173

Table A15. Bond Lengths for 3.2.

atom atom distance atom atom distance

Cr(l') . 0(1) 2.030(2) Gr(l) N(l) 1.713(3)

Cr(l) N(2) 1.712(3) Cr(l) C(l) 2.176(3)

Cr(l) C(2) 2.226(3) CT(1) C(3) 2.253(3)

Cr(l) C(4) 2.211(3) CT(1) C(5) 2.211(3)

Cr(l) CP 1.85 S(l) 0(1) 1.475(2)

S(l) : 0(2) 1.419(3) S(l) 0(3) 1.428(3)

S(l) C(l l ) 1.810(4) C(l l) 1.330(4)

F(2) C(H) 1.317(4) F(3) ' C(l l ) 1.306(4)

0(4) •N(l) 1.169(3) 0(5) N(2). 1.167(3)

C(l) C(2). 1.438(4) C(l) . C(5) 1.413(4)

C(l) : C(6) 1.504(4) C(2) C(3) 1.409(4)

C(2) C(7) 1.498(4) C(3) C(4) . , 1.429(4)

C(3) C(8) 1.485(4) C(4) C(5) 1.428(4)

c(4): •. C(9). 1.493(4) C(5) C(10) 1.492(4)

*CP refers to the unweighted centroid of the C(l-5) ring.


C r ( 1 ) - N ( l ) C r ( l ) - N ( 3 ) C r ( 1 ) - C ( l ) C r ( 1 ) - C ( 2 ) N ( l ) -0(1) C ( l ) -C(5) C(2) -C<(3) C(4) -C(5) . N(3) -N(4) C(6) -C(7) C(7) -C(8)

1.707 (2) 2.011(2) 2.198 (3) 2.240(2) 1.168 (3) 1.392 (4) 1.402 (3) 1.401 (4) 1.383 (3) 1.393 (3) 1.380 (3)

C r ( l ) - N ( 2 ) C r ( l ) - C ( 5 ) C r ( l ) - C ( 4 ) C r ( 1 ) - C ( 3 ) N(2) -0(2) C ( l ) - C ( 2 ) C(3) -C(4) N(3) -C(8) N(4) -C(6) C(6)-C(9) C(8) -C(10)

1.716 (2) 2.192 (3) 2.212 (2) 2.245 (2) 1.173 (2) 1.423 (4) 1.399(4) 1.350(3) 1.340 (3) 1.503(3) 1.498(3)

175


atom atom . distance

Cr(l) 0(2) 1.959(2)

Cr(l) • C(l) 2.255(6)

Cr(l) . C(3) 2.199(4)

0(1) N(l) 1.201(5)

C(l) C(2) 1.384(5)

C(3) C(3)* 1.393(8)

C(4) C(6) 1.490(5)

atom atom distance

Cr(l) N(l) 1.683(5)

Cr(l) ..' C(2) 2.231(4)

Cr(l) CP 1.89

0(2) C(4) 1.275(3)

C(2) C(3) 1.381(6)

C(4) C(5) 1.391(3)

176


atom atom distance atom atom distance

Mo( l ) N ( l ) 1.757(2) Mo( l ) C ( l ) 2.377(4)

M o ( l ) C(2) 2.362(4) Mo( l ) C(3) 2.469(3)

M o ( l ) C(4) 2.700(3) M o ( l ) C(5) 2.667(3)

M o ( l ) C(6) 2.340(3) Mo('l) C(7) 2.336(3)

Mo( l ) C(8) 2.442(3) Mo( l ) C(9) 2.581(3)

Mo( l ) C(10) 2.477(3) Mo( l ) C ( l l ) . 2.258(3)

Mo( l ) CP(1) 2.23 Mo( l ) CP(1) 2.12

0(1) N ( l ) 1.217(3) C( l ) C(2) 1.398(5)

C( l ) C(5) . 1.407(5) C(2) C(3) 1.392(5)

C(3) . C(4) 1.407(5) C(4) C(5) 1.317(5)

C(6) C(7) 1.394(4) C(6) C(10) 1.414(4)

C(7) C(8j 1.404(4) C(8) C(9) 1.404(4)

C(9) C(10) 1.377(4) C ( l l ) C(12) 1.492(4)

C(12) C(13) 1.402(4) C(12) C(17) 1.398(4)

C(13) . ' C(14) 1.377(4) C(14) C(15) 1.382(4)

C(15) C(16) 1.378(4) C(16) C(17) 1.372(4)

* Here and elsewhere, CP(1,2) refer to the unweighted centroids of the

C(l-5) and C(6-10) rings, respectively.


C r ( 1 ) - N ( l ) . ' - : 1 .709(4) C r ( 1 ) - C ( 3 ) 2 .196(4) C r ( 1 ) - C ( l ) - . 2 .269(4)

. C r ( 1 ) - C I ( 1 ) 2 .3386(11) C l ( 1 ) - C r ( 1 ) # 1 2 .3434(11) C ( l ) -C(2 ) " 1.442 (5) 0(1) - N ( l ) 1.152 (4) C(2) -C. (7 ) 1 .499(5) 6 ( 3 ) - C ( 8 ) 1.508 (5) C ( 4 ) - C ( 9 ) 1 .507(6) ,

C r ( l ) - C ( 4 ) 2 191(4) C r ( l ) - C ( 2 ) • 2 212 (4) C r ( l ) - C ( 5 ) 2 274(3) C r ( l ) - C l ( l ) # l : 2 3434(11) C ( l ) - C ( 5 ) . 1 386 (5) C ( l ) - C ( 6 ) 1 504(5) C(2) -CU) 1 408 (5) C ( 3 ) - C ( 4 ) 1 415(5) C(4) -C(5 ) 1 418(5) C ( 5 ) - C ( 1 0 ) 1 518 (5),

178


atom atom distance

Cr(l) N(l) 1.616(2)

Cr(l) C(l). 2.026(3)

Cr(2) N(3) 1.637(3)

Cr'(2) C(15) 1.992(3)

Si(l) C(l) 1.859(3).

Si(l) G(4) 1.863(3)

Si(2) C(2) . 1.849(3)

Si(2) C(7) 1.859(4)

Si(3).' C(15) 1.852(4)

Si(3) C(18) 1.866(6)

Si(4) C(16) 1.836(3)

Si(4) • C(21) 1.865(4)

0(1)' N(l) 1.221(3)

..N(2) C(9) 1.464(4)

•N(4) C(23) 1.461(4)

C(9) C(10) 1.525(5)

C(12) . '' C(13) 1.520(4)

C(23) C(24) 1.516(4)

C(26) C(27) 1.520(5)

atom atom distance

Cr(l) N(2) 1.763(2)

Cr(l) C(2) 1.986(3)

Cr(2) N(4) 1.770(2)

Cr(2) C(16) 2.022(3)

Si(D C(3) 1.869(3)

Si(l) C(5) 1.869(4)

Si(2) • C(6) l'.876(4)

Si(2) C(8) 1.842(4)

Si(3) C(17) 1.863(5)

Si(3) C(19) 1.861(5)

Si(4) C(20) 1.879(4)

Si(4) C(22) 1.853(4)

0(2) N(3) • 1.207(3)

N(2) C(12) 1.491(3)

N(4) C(26) 1.477(4)

C(9) C(ll) 1.529(4)

C(12) C(14) • 1.516(4)

C(23) C(25) 1.498(4)

C(26) C(28) 1.518(5)

179

Table A21. Bond Angles for 3.2.

atom atom atom angle

0(1) Cr(l) N(l) 100.8(1)

0(1) Cr(l) CP 117.3

N(l) Cr(l) CP 118.0

0(1)'-. S(l) 0(2) . 112.8(2)

0(1) S(l) G(H) ; . 101.0(2)

0(2) S(l) 'C( l l ) • 104.2(2)

Cr(l) 0(1) S(l) 131.7(1)

Cr(l) N(2) 0(5) 169.9(3)

C(2) C(l) C(6) 125.1(3)

C(l) C(2) C(3) 107.8(3)

C(3) C(2) C(7) 126.7(3)

C(2) • C(3) ' • C(8) 126.4(3)

C(3) C(4) C(5)- 107.9(3)

C(5) '• C(4) C(9) 125.8(3)

C(l) • . C(5) C(10) 127.0(3)

S(D- 'C( l l ) F(i) '• '• 1.09.5(3)

S(l) C(l l) F(3) 111.7(3)

F(l) C (H) ' : F(3) 108.5(3)


0(1) Cr(l) N(2) 101.3(1)

N(l) Cr(l) N(2) 95.3(2)

N(2) . Cr(l) CP : .120.2

0(1) S(l) 0(3) ,114.1(1)

0(2) S(l) 0(3) 1.17.7(2)

0(3) S(l) C(H) 104.7(2)

Cr(l) N(l) 0(4) 165.7(3)

C(2) C(l) C(5) 108.2(3)

C(5) C(l) C(6) 126.5(3)

C(l) C(2) C(7) . 125.4(3)

C(2) C(3) C(4) 108.2(3)

C(4) C(3) C(8) 125.3(3)

C(3) C(4) C(9) 126.1(3)

C(l) C(5) C(4) • 107.8(3)

C(4) C(5) C(10) 125.1(3)

S(l) C(l l) F(2) 111.3(3)

F(l) C(l l ) F(2) 107.6(3)

F(2) C(l l) F(3) 108.1(3)

180


N ( l ) -Cr (1) -N(2) N(2) -Cr(1) -N(3) N(2) -Cr (1 -C(5) N ( l ) -Cr (1 - C ( l ) N(3) -Cr (1 - C ( l ) N ( l ) -Cr (1 -C (4) N(3) -Cr (1 -C(4) C ( l ) - Cr (1 -C(4) N(2) -Cr (1 -C (2) C(5) -Cr (1 ) -C(2) C (4) -Cr (1 )-C(2) N(2) -Cr (1 ) -C(3) C(5) -Cr (1 ) -C(3) C (4) -Cr (1 )-C(3) 0(1) - N ( l ) -Cr(1) C(5) - C ( l ) -C(2) C (2) - C ( l ) -Cr(1) C(3) -C(2) -Cr (1) C (4) -C(3) -C(2) C(2) -C(3) -Cr(1) C(3 -C (4) -Cr(1) C ( l -C(5) -C(4) C(4 -C(5) -Cr(1) C(8 -N(3) -Cr(1) C(6 -N(4) -N(3) N(4 -C(6) -C(9) C(8 -C (7) -C(6) N(3 ) -C(8) -C(10)

91.86(9) N ( l ) 100.29(8) N ( l ) 124.14(11)N(3) 115.61(11)N(2) 144.83 (9) C(5) 100.71(10)N(2) 98.21(9) C(5) 6 1 . 8 4 ( l l ) N ( l ) 98.61(9) N(3) 6 1 . 8 7 ( l l ) C ( l ) 61.52(10)N(1)

131.31(9) N(3) 61.33(10)C(1) 36.59(9) C(2)

172.7(2) 0(2) 108.1(2) C(5) 72.92(14)C(3) 71.98(13)C(1)

108.7(2) C(4) 71.60(13)C(3) 73.01(14)C(5)

108.4 (2) 72.2 (2)

132.2 (2) 105.7 (2) 119.8 (2) 105.4 (2) 122.3 (2)

C ( l ) C(8) N(4) N(4) C(7) N(3) C(7)

-Cr(1) -Cr(1) -Cr(1) -Cr(1) -Cr(1) -Cr(1) -Cr(1) -Cr(1) -Cr(1) -Cr(1) -Cr(1) -Cr(1) -Cr(1) -Cr(1) -N(2)--C ( D -- C ( 2 ) -- C ( 2 ) -- C ( 3 ) -- C ( 4 ) -- C ( 4 ) -- C ( 5 ) --N(3)--N(3)--C(6) --C(6) --C ( 8 ) -- C ( 8 ) -

-N(3) -C(5) -C(5) - C ( l ) - C ( l ) -C(4) -C(4) -C(2) -C(2) -C(2) -C(3) -C(3) -C(3) -C(3) Cr (1) Cr (1) C ( l ) Cr (1) C r ( l ) C(5) C r ( l ) Cr (1) N(4) C r ( l ) C(7) C(9) C(7) C(10)

95. 90.

134. 94 . 36.

156 . 37.

151. 108. 37.

136 . 85. 61. 36 .

171. 71.

106 . 69. 70.

107. 70, 71

109 117 .110 129 108 129

57(9) 28 (11) 99 (10) 81 (10) 98 (12) 47(10) 11(11) 45(10) ,44(9) ,38(10) .13(10) .10 (8) .42(9) .43(9) .8(2) . .3(2) .9(2) .70(14) .40 (13) .8(2) .7(2) .8(2) .9(2) .87 (13) .8 (2) .4 (2) .1(2) .6(2)

181



0(2) . Cr(l) 0(2)* 91.0(1)

0(2) . , .Gr(l) CP 119.7

Cr(l) ,' . 0 ( 2 ) • C(4) - 124.8(2)

C(2) C(l) . C(2) 107.4(6)

C(2) C(3) C(3)* 107.7(3)

0(2) C(4) C(6) . 115.5(3)

C(4) C(5) C(4)* 125.2(4)


0(2) Cr(l) • N(l) 98.6(1)

N(l) Cr(l) CP 122.6

Cr(l) N(l) 0(1) 169.5(4)

C(l) C(2) C(3) 108.7(4)

0(2) C(4) C(5) 124.5(3)

C(5) C(4) C(6) 119.9(3)

* Symmetry operation: x, 1/2-y, z

182



N ( l ) Mo( l ) C ( H ) 87.66(10)

N ( l ) M o ( l ) CP(2) 115.3

C ( U ) Mo( l ) CP (2) 106.6

Mo( l ) . N ( l ) 0(1) 174.1(2)

C ( l ) C(2) C(3) 106.1(3)

C(3) C(4) C(5) 108.5(4)

C(7) C(6) C(10) . 107.7(3)

C(7) C(8) C(9) 107.9(3)

C(6) C(10) C(9) 108.3(3)

C ( l l ) C(12) C(13) 121.8(3)

C(13) C(12) C(17) 116.2(3)

C(13) C(14) C(15) 120.9(3)

C(15) C(16) C(17) 120.8(3)


N ( l ) Mo( l ) CP(1) 116.8

C ( H ) Mo( l ) CP(1) 103.5

CP(1) Mo( l ) CP(2) 119.8

C(2) C ( l ) C(5) 107.3(3)

C(2) C(3) G(4) 108.1(3)

C ( l ) C(5) C(4) 109.3(3)

C(6) C(7) C(8) 107.8(3)

C(8) C(9) C(10) 108.1(3)

Mo( l ) C ( H ) C(12) 117.4(2)

C ( H ) C(12) C(17) 122.0(3)

C(12) C(13) C(14) 121.5(3)

C(14) C(15) C(16) 118.5(3)

C(12) C(17) C(16) 122.0(3)

183


N ( l ) - C r ( 1 ) -C(4 ) 105. 0(2) N ( l ) - C r (1 ) - C ( 3 ) 90 . 18(14) C (4) - C r ( 1 ) - C ( 3 ) 37. 64(14)N(1) - C r (1 ) - C ( 2 ) 112. 1(2) C (4) - C r ( 1 ) - C ( 2 ) 62. 55(14)C(3) - C r (1 ) - C ( 2 ) 37 . 27(14) N ( l ) - C r ( 1 ) - C ( l ) 149 2(2) C(4) - C r (1 ) - C ( l ) 6 1 . 41(14) C(3) - C r ( 1 ) - C ( l ) 61 87 (14)C(2) - C r (1 ) - C ( l ) 37 . 52(14) N ( l ) - C r ( 1 ) -C(5 ) 141 7(2) C(4) - C r (1 ) - C ( 5 ) 36. 98(13) C(3) - C r ( 1 ) - C ( 5 ) 61 58(13)C(2) - C r (1 ) - C ( 5 ) 61 . 30(13) C ( l ) - C r ( 1 ) - C ( 5 ) 35 52 (13)N(1) - C r (1 ) - C l ( l ) 97. 85(11) C (4) - C r ( 1 ) - C l ( l ) 97 97 (11)C(3) - C r (1 ) - C l ( l ) 134 92(11) C (2) - C r ( 1 ) - C I ( 1 ) 147 30(11)C(1) - C r (1 ) - C l ( l ) 110 91(11) C(5) - C r ( 1 ) - C I ( 1 ) 87 13(10)N(1) - C r (1 ) - C l ( l ) # l 98 51(12) C (4) - C r ( 1 ) - C I ( 1 ) # 1 150 40 (11)C(3) - C r (1 ) - C l ( l ) # l 126 27(11) C(2) - C r ( 1 ) - C I ( 1 ) # 1 92 12 ( l l ) C ( l ) - C r (1 ) - C l ( l ) # l 89 28(10) C(5) - C r ( 1 ) - C l ( l ) # l 118 7 4 ( 1 0 ) C 1 ( 1 ) - C r ( 1 ) -C1(1 )#1 96 40 (4) C r ( l ) - C l ( l ) - C r ( l ) # l 83 60 (4) C(5) - C ( l ) -C(2 ) 108 0(3) C(5) - C ( l ) - C(6) 126 6 (4) C(2) - C ( l ) -C(6 ) 125 4(4) C(5) - C ( l ) - C r (1) 72 4 (2) C(2) - C ( l ) - C r ( l ) 69 1(2) C(6) - C ( l ) - C r (1) 125 5(3) 0(1) - N ( l ) - C r ( 1 ) 171 3(3) C(3) - C ( 2 ) - C ( l ) 107 3 (3) C(3) -C(2) -C(7) 126 4 (4) C ( l ) - C (2 ) - C(7) 126 1(4) C(3) - C (2) - C r ( 1 ) 70 8 (2) C ( l ) - C ( 2 ) - C r (1) 73 4 (2) C(7) -C(2 ) - C r ( 1 ) 125 1(3) C (2) -C(3 ) - C(4) 108 .1(3) C(2) -C(3) -C(8) 125 6.(4) C (4) - C { 3 ) - C(8) 126 .3 (4) C(2) -C(3 ) - C r ( l ) 72 0(2) C(4 - C ( 3 ) - C r (1) 71 .0(2) C(8] -C(3) - C r ( l ) 124 .7(3) C(3 - C ( 4 ) - C(5) 107 .8 (3) C(3 -C(4) -C(9 ) 126 .0(4) C(5 - C ( 4 ) - C(9) 126 .1(4) C(3 - C (4) - C r ( l ) 71 .4 (2) C(5 - C ( 4 ) - C r ( l ) 74 .7(2) C(9 -C(4) - C r ( l ) 123 .1(3) C ( l - C ( 5 ) - C(4) 108 .7(3) C ( l -C(5 ) -C(10) 125 .6 (4) C (4 - C ( 5 ) - C(10) 125 .7(4) C ( l -C(5 ) - C r ( l ) 72 .0 (2) C (4 - C ( 5 ) - C r ( l ) 68 .3 (2) C ( 1 0 ) - C ( 5 ) - C r ( 1 ) 126 .5 (3)

184



N(l) Cr(l) . N(2) 102.91(12)

N(l) Cr(l) C(2) . 103.46(12)

N(2) Cr(l) C(2) 115.68(11)

N(3) Cr(2) N(4) 102.60(11)

N(3) Cr(2) • €(16) 103.73(13)

N(4) Cr(2) C(16) 112.57(13)

C(l) Sid) . C(3) •. 111.69(14)

C(l) Si(l) C(5) 110.33(14)

C(3) Si (!) C(5) 107.9(2)

C(2) Si(2) C(6) 109.5(2)

C(2) Si(2) C(8) 109.55(15)

C(6) Si(2) .' C(8) 107.2(2)

C(15> ' •Si(3) C(17) 109.7(2)

C(lo) Si(3) C(19) 111.7(2)

C(17) Si(3) C(19) 110.0(3)

C(.16) Si(4). C(20). 109.8(2)

C(16) Si(4) •C(22) 1117(2)

C(20). Si(4) C(22) 107.6(2)

Cr(l) N(l) 0(1) 179.0(3)

Cr(l) N(2) C(12) 103.2(2)

Cr(2) N(3) 0(2)- 179.5(2)

Cr(2) N(4) • C(26) 103.8(2)

Cr(l) C(l) . Si(l) 117.39(14)

N(2) C(9) C(10) 110.4(3)


N(l) Cr(l) C(l) 102.85(12)

N(2) Cr(l) 0(1) 113.72(11)

C(l) Cr(l) C(2) 115.72(12)

N(3) Cr(2) C(15) , 102.47(14)

N(4) Cr(2) C(15) 116.33(12)

0(15) Cr(2) C(16) 116.52(13)

C(l) Si(l) C(4) 110.0(2)

C(3) Si(l) C(4) 108.55(15)

C(4) Si(l) C(5) 108.3(2)

C(2) Si(2) C(7) 110.1(2)

C(6) Si(2) C(7) •'. 110.3(2)

C(7) Si(2) C(8) 110.2(2)

C(15) Si(3) C(18) 109.2(2)

C(17) Si(3) C(18) 106.9(3)

C(18) Si(3) C(19) . 109.3(3)

C(16) ' Si(4) C(21) 109.7(2)

C(20) Si(4) C(21) 108.9(2)

C(21) Si(4) C(22) 109.1(2)

Cr(l) N(2) C(9) 138.4(2)

C(9) N(2) C(12) 118.3(2)

Cr(2) . N(4) C(23) 136.8(2)

C(23) N(4) C(26) 119.1(2)

Cr(l) C(2) Si(2) 130.6(2)

N(2) C(9) C(l l) 109.9(3)

185

Table A26. Bond Angles for 5.3 (continued).

atom atom atom angle atom atom atom angle

C(10) C(9) C(ll) 111.2(3) N(2) C(12) C(13) 111.7(3)

N(2) C(i'2) C(14). 112.6(2) C(13) C(12) C(14) 113.0(3)

Cr(2) , ' C(15) Si(3) . 128.8(2) Cr(2) C(16) Si(4) 120.2(2)

N(4) C(23) C(24) 110.4(3) N(4) C(23) C(25) 111.3(3)

C(24) C(23) C(25) 111.5(3) N(4) C(26) C(27) 112.7(3)

N(4) • C(26) C(28) . 112.8(3) C(27) C(26) C(28) 112.8(3)

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