SYNTHESIS AND CHARACTERIZATION OF RUTHENIUM(II)
TETRAPHENYLPORPHYRIN WITH REDOX ACTIVE AXIAL
LIGANDS FOR MOLECULAR WIRES
A Thesis
SUBMITTED TO THE FACULTY OF
UNIVERSITY OF MINNESOTA
BY
Mahtab Fathi Rasekh
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF MASTER OF SCIENCE
Advisor: Dr. Victor N. Nemykin
May 2016
© Mahtab Fathi Rasekh 2016
i
Acknowledgements
I would like to thank Dr. Nemykin for all his support and guidance, also I would
like to thank Nemykin lab members during last three years. I would like to thank
my family for their support, I would not have been able to do this without them.
Chapter 1 was reprinted with permission from Inorg. Chem., 2015, 54, 10711-
10724, 10.1021/acs.inorgchem.5b01614. Copyright 2015 American Chemical
Society.
ii
Absract
Four new ruthenium(II) tetraphenylporphyrin with redox active axially coordinated ligands
including two new heterotrinuclear Fe-Ru-Fe complexes of the ruthenium(II)
tetraphenylporphyrin axially coordinated with a pair of isocyanoferrocene
((FcNC)2RuTPP) or 1,1'-diisocyanoferrocene (([C5H4NC]2Fe)2RuTPP) ligands [Fc =
ferrocenyl, TPP = 5,10,15,20-tetraphenylporphyrinato(2-) anion], and two new
isocyanoazulene containing complexes (2-CNAz)2RuTPP ( 2-CNAz = 2-isocyanoazulene)
and (6-CNAz)2RuTPP ( 6-CNAz = 6-isocyanoazulene) were synthesized and characterized
by UV-vis, MCD, NMR, and FTIR spectroscopies as well as by electrospray ionization
mass spectrometry and single crystal X-ray diffraction. Isolation of insoluble polymeric
{([C5H4NC]2Fe)RuTPP}n molecular wires was also achieved for the first time. The redox
properties of the new trinuclear (FcNC)2RuTPP and ([C5H4NC]2Fe)2RuTPP complexes
were probed using electrochemical (CV and DPV), spectroelectrochemical, and chemical
oxidation methods and correlated to those of the bis(tert-butylisocyano)ruthenium(II)
tetraphenylporphyrin reference compound, (t-BuNC)2RuTPP, as well as (2-CNAz)2RuTPP
and (6-CNAz)2RuTPP complexes. In all cases, the first oxidation process was attributed to
the reversible oxidation of the RuII center. The second and third reversible oxidation
processes in (FcNC)2RuTPP are separated by ~100 mV and were assigned to two single-
electron FeII/FeIII couples suggesting a weak long-range iron-iron coupling in this complex.
Electrochemical data acquired for ([C5H4NC]2Fe)2RuTPP complex are complicated by the
interaction between the axial 1-1,1'-diisocyanoferrocene ligand and the electrode surface
iii
as well as by axial ligand dissociation in solution. Spectroelectrochemical and chemical
oxidation methods were used to elucidate spectroscopic signatures of the
[(RCN)2RuTPP]n+ species in solution. In both (2-CNAz)2RuTPP and (6-CNAz)2RuTPP
complexes, the first oxidation process was attributed to the reversible oxidation of RuII
center and the second oxidation process was assigned to the reversible oxidation of
porphyrin core. The third and fourth irreversible processes were assigned to the oxidation
of two isocyanoazulene axially coordinated ligands. DFT and TDDFT calculations aided
in correlating spectroscopic and redox properties of complexes with their electronic
structures.
iv
Table of Content:
List of Tables……………………………………………………………………….vi
Table of Figures……………………………………………………………………vii
1 CHAPTER 1 ................................................................................................................. 1
1.1. Introduction .......................................................................................................... 1
1.2. Experimental Section ........................................................................................... 3
1.2.1 Materials ........................................................................................................... 3
1.2.2 Synthetic Work ................................................................................................. 3
1.3. Instrumentation..................................................................................................... 5
1.4. Computational Details .......................................................................................... 6
1.5. X-ray Crystallography .......................................................................................... 7
1.6. Results and Discussion ......................................................................................... 8
Conclusions ........................................................................................................ 39
1.8. References .......................................................................................................... 41
2 CHAPTER 2 ............................................................................................................... 52
Introduction ........................................................................................................ 52
2.2. Experimental Section ......................................................................................... 53
2.2.1 Materials ......................................................................................................... 53
v
2.2.2 Synthetic work ................................................................................................ 53
2.2.3 Instrumentation ............................................................................................... 54
2.2.4 Computational Details .................................................................................... 55
2.3. Results and Discussion ....................................................................................... 56
2.4. Conclusions ........................................................................................................ 75
2.5. References .......................................................................................................... 76
vi
List of Tables:
CHAPTER 1
Table 1. Selected Bond Distances (Å) and Angles (deg) for complexes 1 and 2. ........... 18
Table 2. DFT-predicted molecular orbital compositions for (RNC)2RuTPP complexes. 22
Table 3. Half-wave potentials (V) for (RNC)2RuTPP in DCM/0.05M TFAB solution at
room temperature.a ............................................................................................................ 34
CHAPTER 2
Table 1. Selected bond distances (Å) and angles (deg) for (2-CNAz)2RuTPP and (6-
CNAz)2RuTPP complexes. ............................................................................................... 62
Table 2. DFT-predicted molecular orbital compositions in (RNC)2RuTPP complexes. . 67
Table 3. Oxidation potentials (V) for (RNC)2RuTPP complexes determined by
electrochemical experiments in DCM/0.05M TBAF system at room temperature.a ........ 73
vii
Table of Figures:
CHAPTER 1
Figure 1. Transformation of the 1H NMR spectra of the (OC)RuTPP complex upon
stepwise addition of the 1,1'-diisocyanoferrocene ligand to its solution in CDCl3 at 25 ºC
1 - (OC)RuTPP, no ligand; 2 - 1.2 eq. of ligand; 3 - 2.4 eq. of ligand; 4 - 3.6 eq. of ligand;
5 - 4.8 eq. of ligand; 6 - 6.0 eq. of ligand; 7 - 7.2 eq. of ligand; 8 - 8.4 eq. of ligand; 9 - 9.6
eq. of ligand; 10 - 10.8 eq. of ligand; 11 - 12.0 eq. of ligand. Labeling legend: a, b, and c
are -pyrrolic, 1,1’-diisocyanoferrocene, and meso-phenyl fragment signals for complex 2,
respectively; d are the resonances for free axial ligand. ................................................... 14
Figure 2. Experimental UV-vis (top) and MCD (bottom) spectra of complexes 4 (A), 1
(B), and 2 (C). ................................................................................................................... 16
Figure 3. Molecular structures of complexes 1 (left) and 2 (right) at 50% thermal
ellipsoids. The toluene solvent molecule of crystallization observed in the structure of
complex 2 is omitted. All hydrogen atoms are omitted for clarity. .................................. 19
Figure 4. DFT-predicted orbital energy diagrams for (RNC)2RuTPP complexes. .......... 21
Figure 5. DFT-predicted frontier molecular orbitals for (RNC)2RuTPP complexes. ...... 22
Figure 6. Experimental UV-Vis (top) and TDDFT-predicted (bottom) spectra of
complexes 4 (A), 1 (B), and 2 (C). ................................................................................... 30
Figure 7. DPV (red) and CV (blue) electrochemical data for complexes 4 (top) and 1
(bottom) recorded in DCM/0.05 M TFAB solutions. In all cases, the CV data were recorded
at 100 mV/s scan rate. ....................................................................................................... 31
viii
Figure 8. Spectroelectochemical oxidation of the reference complex 4 in DCM/0.15M
TFAB solution at room temperature. ................................................................................ 35
Figure 9. Spectroelectochemical oxidation of the complex 1 in DCM/0.15M TFAB
solution at room temperature. (A) First oxidation; (B) combined second and third
oxidations. ......................................................................................................................... 36
Figure 10. Spectroelectochemical oxidation of complex 2 in DCM/0.15M TFAB solution
at room temperature. ......................................................................................................... 37
Figure 11. NIR portion of the electronic spectra for transformation of [1]+ (black line) into
the mixed-valence [1]2+ (red line) and [1]3+ (blue line) in DCM/0.15M TFAB solution at
room temperature (left). One of several possible band deconvolutions of the NIR transitions
envelope (right). ................................................................................................................ 39
CHAPTER 2
Figure 1. ESI MS spectra of the (2-CNAz)2RuTPP (A) and (6-CNAz)2RuTPP (B)
complexes. The experimental and theoretical isotope distributions for molecular ions are
given as insets…………………………………………………………………………….58
Figure 2. Experimental UV-vis (top) and MCD (bottom) spectra of (2-CNAz)2RuTPP (A)
and (6-CNAz)2RuTPP (B) complexes…………….……………………………………...60
Figure 3. Molecular structures of complex 1 (left) and 2 (right) as 50% thermal ellipsoids.
The toluene solvent molecule of crystallization observed in the structure of complex 1. All
hydrogen atoms are omitted for clarity……………………………………………….….61
ix
Figure 4. DFT-predicted orbital energy diagrams for (RNC)2RuTPP complexes….…...64
Figure 5. DFT-predicted frontier orbitals for (RNC)2RuTPP complexes………….…....65
Figure 6. Experimental UV-vis (top) and TDDFT predicted (bottom) spectra of (t-
BuNC)2RuTPP (A), (2-CNAz)2RuTPP (B), and (6-CNAz)2RuTPP (C)…………...……70
Figure 7. DPV (red) and CV (blue) electrochemical data for (2-CNAz)2RuTPP complex
in DCM/0.05 M TBAF solution. In this case CV data was recorded at 100 mV/s
rate…………………………………………………………………………………….….73
Figure 8. Spectroelectochemical oxidation of the (2-CNAz)2RuTPP complex in
DCM/0.15M TBAF system at room temperature…………………………………….….74
Figure 9. Spectroelectochemical oxidation of the (6-CNAz)2RuTPP complex in
DCM/0.15M TBAF system at room temperature……………………………….………..74
1
CHAPTER 1
1.1. Introduction
Polynuclear organometallic (especially ferrocene-containing) supramolecular systems with
adjustable redox, electron-transfer, and photophysical characteristics have been envisioned
as prospective building blocks for application in molecular electronics.1 Because of their
well-defined properties, many porphyrins,2 tetraazaporphyrins,3 phthalocyanines,4
corroles,5 subphthalocyanines,6 BODIPYs and azaBODIPYs7 with ferrocenyl substituents,
linked to a core -system via conjugated fragments, have been considered in this regard
within the last two decades. Such ferrocenyl-aromatic -system assemblies often exhibit
long-range metal-metal electronic coupling and are attractive as potential molecular
random-access memory components, catalysts for electro- and photocatalytic
transformations, active components for light-harvesting, redox-switchable fluorescence
markers, and molecular wires.8 For most of these applications, ferrocenyl substituents
should be connected to the parent -system either directly or through a -conjugated
linking group. On the other hand, reports on porphyrinoids and BODIPYs in which the
central atom is attached to a redox-active ferrocenyl moiety directly or via a conjugated
linking group are quite rare.9
In coordination chemistry, organic isocyanides can act as both strong -donors and good
-acceptors,10 which can easily support formation of axially coordinated complexes with
2
FeII and RuII porphyrins and phthalocyanines of general formula (P)ML2 or (P)MLX (P =
porphyrin or phthalocyanine, L = organic isonitrile, X = additional axial ligand).11 In a
series of publications, Hanack and co-workers explored formation of oligomeric
complexes between iron(II) phthalocyanine (PcFe) or ruthenium tetraazaporphyrin
(RuTAP) and diisocyanoarenes, which were found to have semiconducting properties.12
Such scaffolds, if the simple diisocyanoarene ligands are modified with redox-active units,
can be viewed as prototypes for molecular wires. In our first report, we explored an axial
coordination of PcFe with redox-active isocyanoferrocene and found a weak long-range
metal-metal coupling in the (FcNC)2FePc system (FcNC = isocyanoferrocene).13 One of
the serious drawbacks of the (FcNC)2FePc complex is that its first one-electron oxidation
involves the phthalocyanine macrocycle, whereas for an ideal axial coordination-based
molecular wire oxidation of the central metal ion or a redox-active axial ligand is more
preferable. It is well-known that the oxidation potential of a porphyrin macrocycle in
general is higher compared to the phthalocyanine core.14 Thus, in this report, we introduce
the first example of a porphyrin featuring axially coordinated isocyanoferrocene ligands.
Because of the low chemical stability of FeII porphyrins, a RuII ion in RuTPP (TPP =
5,10,15,20-tetraphenylporphyrin(2-) ligand) was employed as the central atom for
accommodating axial coordination. In addition, we also tested the 1,1'-diisocyanoferrocene
ligand in a similar coordination reaction as the first redox-active, potentially bidentate
fragment needed for the formation of polymeric molecular wires. The optical and redox
properties of the new (FcNC)2RuTPP (1) and ([C5H4NC]2Fe)2RuTPP (2) ([C5H4NC]2Fe =
1,1'-diisocyanoferrocene) complexes as well as their electronic structures were elucidated
using a variety of spectroscopic methods and X-ray crystallography as well as density
3
functional theory (DFT) and time-dependent DFT (TDDFT) calculations. We were able to
establish a convenient way to form insoluble polymeric {([C5H4NC]2Fe)RuTPP}n
molecular wires (3). The properties of (FcNC)2RuTPP and ([C5H4NC]2Fe)2RuTPP will be
compared to and contrasted with the earlier reported “reference” (t-BuNC)2RuTPP
complex 4.
1.2. Experimental Section
1.2.1 Materials
All commercial reagents were ACS grade and were used without further purification. All
reactions were performed under a dry argon atmosphere with flame-dried glassware.
Toluene was distilled over sodium metal. Dichloromethane (DCM) and hexanes were
distilled over CaH2. Tetrabutylammonium tetrakis(pentafluorophenyl)borate (TFAB,
(NBu4)[B(C6F5)4]),15 isocyanoferrocene (FcNC),16 1,1’-diisocyanoferrocene
([CNC5H4]2Fe),17 and reference porphyrin 418 were prepared according to literature
procedures.
1.2.2 Synthetic Work
Synthesis of (FcNC)2RuTPP (1). Under argon atmosphere, commercially available
(OC)RuTPP (0.10 g, 0.13 mmol) was added to a solution of the FcNC ligand (0.30 g,
1.42 mmol) in 12 mL of 1:1 (v/v) mixture of toluene and DCM. The reaction mixture was
stirred at room temperature for 4h and then all solvents were removed under reduced
pressure. The residue was washed several times with hexanes, dried under vacuum, and
recrystallized from a mixture of dry DCM/toluene to form analytically pure complex 2.
4
Yield: 0.043 g (28.1%). Elemental analysis: calculated for C66H46Fe2N6Ru·0.16 CH2Cl2:
C, 69.15; H, 4.06; N, 7.31. Found: C, 69.15; H, 4.17; N, 7.05. 1H NMR (20oC, 500 MHz,
CDCl3): δ 8.54 (s, 8H, β- pyrrole), 8.24 (dd, J = 6.6, 2.9 Hz, 8H, m-Ph), 7.69 (m, 12H, o-
Ph, p-Ph), 3.28 (t, 4H, β-H, C5H4NC, J = 2.0 Hz), 3.19 (s, 10H, C5H5), 2.68 (t, α-H,
C5H4NC, J = 2.0 Hz). 13C NMR (20oC, 125 MHz, CDCl3): δ 142.6 (α-pyrrole), 142.3
(Cipso, Ph), 133.3 (Cortho, Ph), 130.6 (β-pyrrole), 125.9 (Cmeta, Ph), 125.3 (Cpara, Ph), 120.3
(Cmeso), 68.9 (C5H5), 64.3 (β-C, C5H4NC), 64.2 (α-C, C5H4NC), 52.4 (ipso-C, C5H4NC).
Because of the often19 broad nature of the 13C NMR signal for the isocyano carbon atom,
it was not clearly observed in the spectrum of (FcNC)2RuTPP. UV-vis [DCM; λ, nm (log
ε, M-1cm-1)]: 419 (5.73), 534 (4.15). IR (KBr): (N≡C) 2094 cm-1. HR ESI MS:
Calculated for C66H46Fe2N6Ru: 1136.1543; Found: 1136.1547 [M]+.
Synthesis of ([C5H4NC]2Fe)2RuTPP (2). Under argon atmosphere, the (C5H4NC)2Fe ligand
(0.090 g, 0.381 mmol) was added to a solution of commercially available (OC)RuTPP
(0.050 g, 0.067 mmol) in 6 mL of DCM and toluene (1 : 1 v/v) mixture. The mixture was
stirred at room temperature while the reaction progress was monitored by TLC on alumina
plates. After 30 minutes, a precipitate formed, which was filtered, washed with small
amounts of cold hexanes, hexanes/DCM, and dried under reduced pressure. Elemental
analysis, calculated for: C68H44Fe2N8Ru · 0.45 CH2Cl2 C, 67.17; N, 9.16; H, 3.70. Found:
C, 67.17; N, 9.21; H, 4.35. 1H NMR (20oC, 500 MHz, CDCl3): δ 8.56 (s, 8H, β-pyrrole),
8.21 (m, 8H, m-Ph), 7.69 (m, 12H, o-Ph, p-Ph), 3.64 (t, 4H, J = 2.5 Hz, -H, non-
coordinated C5H4NC), 3.42 (t, 4H, J = 2.5 Hz, -H, coordinated C5H4NC), 2.81 (t, 4H, J =
2.5 Hz, -H, non-coordinated C5H4NC), 2.77 (t, 4H, J = 2.5 Hz, -H, coordinated
5
C5H4NC). 13C NMR (20oC, 125 MHz, CDCl3): δ 142.5 (α-pyrrole), 142.0 (Cipso, Ph), 133.3
(Cortho, Ph), 130.9 (β-pyrrole), 126.1 (Cpara, Ph), 125.5 (Cmeta, Ph), 120.4 (Cmeso), 68.7 (-
C, non-coordinated C5H4NC), 66.9 (β-C, non-coordinated C5H4NC), 66.9 (-C,
coordinated C5H4NC), 66.0 (-C, coordinated C5H4NC). UV-vis [DCM; λ, nm (log ε, M-
1cm-1)]: 417 (5.25), 536 (3.56). IR (KBr): (N≡C) 2112, 2090 cm-1. HR ESI MS: calculated
for C68H44Fe2N8Ru: 1186.1448; found: 1186.1617 [M]+.
Synthesis of {([C5H4NC]Fe)2RuTPP}n (3). A freshly prepared sample of complex 2 was
washed with dry hexanes, toluene, and DCM at room temperature until the yellow color of
the free isonitrile ligand was no longer observed in the washings. The precipitate was then
dried under reduced pressure. The resulting polymer was not soluble in any non-polar
solvents, aromatic hydrocarbons, DCM, chloroform, acetone, or THF. Elemental analysis,
calculated for: C56H36Fe1N6Ru1 x3H2O x1.65CH2Cl2: C, 60.52; H, 3.99; N, 7.35. Found:
C, 60.54; N, 7.96; H, 3.99. IR (KBr): (N≡C) 2081 cm-1.
1.3. Instrumentation
A Varian Unity INOVA NMR instrument was used to evaluate spectra taken at 500 MHz
for 1H and 125 MHz for 13C nuclei, respectively. The 1H and 13C chemical shifts are
reported in parts per million relative to TMS as an internal standard. All UV-Vis data were
obtained on a JASCO-720 spectrophotometer at room temperature. An OLIS DCM 17 CD
spectropolarimeter with a 1.4 T DeSa magnet was used to obtain all Magnetic Circular
Dichroism (MCD) data. IR-data were obtained on a Perkin Elmer FT-IR-Spectrometer
6
Spectrum 100 at room temperature for samples pressed in KBr pellets. Electrochemical
measurements were conducted using a CHI-620C electrochemical analyzer utilizing the
three-electrode scheme. Unless stated otherwise, platinum working, platinum auxiliary,
and Ag/AgCl pseudo-reference electrodes were employed in a 0.05 M solution of TFAB
in DCM for electrochemical experiments. In all cases, the redox potentials are referenced
to the FcH/FcH+ couple using decamethylferrocene as an internal standard.
Spectroelectrochemical data were collected using a custom-made 1 mm cell, a working
electrode made of platinum mesh, and a 0.15M solution of TFAB in DCM. HR ESI mass
spectra were recorded using a Bruker MicrOTOF-III system for freshly-prepared samples
dissolved in THF under ambient atmosphere. Chemical titration experiments were
typically conducted using 1.0x10-6 - 3x10-6 M solutions of porphyrin complexes and
~1.0x10-3 M stock solutions of the oxidant (Fe(ClO4)3 or "magic blue") added in 0.1 - 0.3
equivalent increments. Elemental analyses were performed by Atlantic Microlab, Inc. in
Atlanta, Georgia.
1.4. Computational Details
All computations were performed using the Gaussian 09 software package running under
Windows or UNIX OS.20 Molecular orbital contributions were compiled from single point
calculations using the QMForge program.21 In all single-point calculations, the TPSSh
exchange-correlation functional22 was used. The triple-zeta quality effective core potential
SDD basis set23 was used for all atoms in all calculations. Frequencies were calculated for
all optimized geometries in order to ensure that final geometries represent minima on the
potential energy surface. TDDFT calculations were conducted for the first 80 (complexes
7
1 and 2) or 50 (complex 4) excited states in order to ensure that all charge transfer (CT)
and * transitions of interest were accounted for.
1.5. X-ray Crystallography
Single crystals of complexes 1 and 2 suitable for X-ray analysis were obtained by slow
evaporation of toluene or toluene/DCM solutions of the corresponding compounds at room
temperature, respectively. Experimental data for all samples were collected using a Rigaku
Rapid II X-ray diffractometer with curved IPDS detector employing graphite-
monochromatized Mo-Kα radiation (λ = 0.71075 Å). The structures of the complexes 1 and
2 were solved by direct methods using the SIR-92 program.24 All non-hydrogen atoms were
located from analysis of a difference Fourier-map and refined through isotropic and,
subsequently anisotropic approximations. The toluene solvent molecule in the X-ray
structure of the complex 2 was found to be severely disordered and thus was removed using
the PLATON SQUEEZE procedure. All hydrogen atoms were placed in their geometrically
expected positions. The isotropic thermal parameters of all hydrogen atoms were fixed to
the values of the equivalent isotropic thermal parameters of the corresponding carbon
atoms using riding model constraints so that Uiso(H) = 1.2Ueq(C) for the hydrogen atoms.
Both structures reported herein were completely refined via the full-matrix least square
method using the Crystals for Windows program.25 Complete crystallographic information
is available in the CIF accompanying this article. CCDC 1411393, and 1411394 contain
the supplementary crystallographic data for all compounds. These data can be obtained free
of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from Cambridge
8
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-
336-033 or [email protected]).
Scheme 1. Synthetic strategy for preparation of the (RNC)2RuTPP complexes.
1.6. Results and Discussion
Synthesis, spectroscopy, and X-ray structures. The trinuclear complexes 1 and 2 were
synthesized following the synthetic strategy developed earlier for the preparation of other
(RNC)2RuTPP complexes (Scheme 1).11,13,18 We used a large excess of 1,1’-
diisocyanoferrocene in the synthesis of complex 2 in order to minimize formation of
oligomers of the general formula
{[(CNC5H4)Fe(C5H4NC)]RuTPP[(CNC5H4)Fe(C5H4NC)]RuTPP}n featuring bridging
1,1’-diisocyanoferrocene linkers. The known "reference" complex 418 was synthesized and
isolated using a similar protocol. While complexes 1 and 4 exhibit good thermal stability
in solutions, heating of complex 2 even in hydrocarbon solvents or treating this complex
9
with toluene or DCM leads to the formation of increasing amounts of a dark-colored
insoluble substance, which most likely represents a polymeric
{[(CNC5H4)Fe(C5H4NC)]RuTPP[(CNC5H4)Fe(C5H4NC)RuTPP]}n molecular wire 3. This
polymer was characterized by elemental analysis and IR spectroscopy.
Similar to other axially coordinated diamagnetic ruthenium and iron porphyrins and
phthalocyanines,26 the NMR signals for the axial ligands in complexes 1 and 4 with
monodentate isocyanides are shifted upfield compared to those documented for the
corresponding free ligands. In order to understand the origin of the relatively low stability
of complex 2 in solution, we probed its formation in CDCl3 using 1H NMR titrations. Two
titration experiments were conducted. In the first experiment, up to 5-fold excess (12
equivalents) of the 1,1'-diisocyanoferrocene ligand was gradually added to a solution of
(OC)RuTPP (Figure 1 and Figure S1). These NMR data clearly reflect a dynamic
equilibrium that involves reversible binding of the sterically crowded bidentate axial ligand
to the RuTPP core. Similar complexity in the NMR patterns was previously discussed for
the interactions of RuTPP and sterically crowded phosphorus- or nitrogen-based bidentate
ligands.27 Indeed, upon addition of the axial ligand to the initial (OC)RuTPP complex
(RuTPP : ligand ratio of 1 : 1.2), the -pyrrolic 1H NMR resonances of the latter disappear
and at least 8 different porphyrin-based species can be identified in the 1H NMR spectrum,
along with the signals for the free axial diisocyanide ligand (Figure 1, Scheme 2). Upon
further increase the axial ligand’s concentration (up to 5-fold excess), the 1H NMR pattern
simplifies to suggest the presence of two major species in solution. Thus, at the RuTPP :
ligand ratio of 1 : 12, which corresponds to the 5-fold excess of the axial ligand for complex
2, one of the two major species constitutes a bis-coordinated complex 2, whereas the other
10
is the free axial ligand (Figure 1). In addition, three minor species were identified in this
1H NMR spectrum. One of these compounds exhibits the -pyrrolic 1H resonance at 8.40
ppm, which has ~10% of the intensity compared to the that of the -pyrrolic 1H signal
observed for 2. Other features in the above 1H NMR spectrum include signals for
diastereotopic phenyl ortho- and meta-hydrogen atoms as well as four triplet resonances
corresponding to the axially coordinated ligand. The integrated resonance intensities
indicate a 1 : 1 RuTPP : isocyanide ratio in the complex, whereas the presence of four 1H
resonances from the isocyanide ligands and the diastereotopic nature of the phenyl
hydrogen atoms clearly suggest that this minor species is a penta-coordinated
([C5H4NC]2Fe)RuTPP complex. The remaining two minor species contribute less than 1%
to the resonance intensities of the entire 1H NMR pattern and, similar to the previous
report,27 can be tentatively formulated as oligomeric {([C5H4NC]2Fe)2RuTPP}n and hexa-
coordinated 2-([C5H4NC]2Fe)RuTPP complexes (Scheme 2). The contribution of the
former oligomeric species increases after storing the freshly prepared NMR sample for
several days, while the contribution of the latter complex rises upon decreasing
concentration of the axial ligand (Figure 1). In the second titration experiment, (OC)RuTPP
was titrated into a solution of the free ligand (Figure S2). In this case, the bis-coordinated
complex 2 was observed with less than 5% of the other porphyrins present in solution only
when a large excess of the free axial ligand was available. Both 1H NMR titration datasets
suggest that unlike complexes 1 and 4, complex 2 can only be stabilized in the presence of
an excess of the axial ligand in solution. Indeed, when several freshly prepared samples of
the bis-coordinated complex 2 were washed with cold hexanes (to remove excess of the
free axial ligand), dried, and re-dissolved in CDCl3, the 1H NMR spectrum of the resulting
11
solution was consistent with only ~55% of complex 2, the remaining ~45% of the
ruthenium porphyrin contribution being from the porphyrins with a single axial ligand
coordinated to the ruthenium center. Free 1,1'-diisocyanoferrocene was present in this
mixture as well. Such mediocre stability of complex 2 in solution and formation of the
polymeric molecular wire 3 can be attributed to the bidentate capability of the sterically
crowded axial ligand. Indeed, the torsion angle between the two isocyanide groups,
determined from the X-ray structure of complex 2, is ~58o and thus the non-coordinated
isocyanide fragment is still located in close proximity to the ruthenium center. Thus, axially
coordinated 1,1'-diisocyanoferrocene may act as an 2 bidentate ligand to form hexa-
coordinated 1 : 1 (RuTPP : ligand) complex and free 1,1'-diisocyanoferrocene (Scheme 2).
12
Scheme 2. 1H NMR titrations-based possible transformations of the (OC)RuTPP complex into
([C5H4NC]2Fe)2RuTPP and {([C5H4NC]2Fe)RuTPP}n compounds.
The C≡N stretching bands in the IR spectra of complexes 1 - 3 are shifted to lower energies
compared to the corresponding spectra of the free ligands.16-18 For 1, the (C≡N) band
occurs at 2094 cm-1, whereas the free isocyanoferrocene ligand16 absorbs at 2120 cm-1 (26
cm-1 difference). The IR spectrum of complex 2 features two (C≡N) bands. One is
significantly (28 cm-1) red-shifted from the (C≡N) band observed for the free ligand,17
13
whereas the other is close in energy (8 cm-1 red-shifted) to the (C≡N) of 1,1’-
diisocyanoferrocene. These observations clearly indicate that only one isocyanide group
(2090 cm-1) is coordinated to the ruthenium center in complex 2, while the second C≡N
substituent (2112 cm-1) remains non-coordinated. Because of the 1:1- bridging
coordination of the 1,1'-diisocyanoferrocene linker in polymer 3, there is only one
isocyanide stretching band present in its IR spectrum. Moreover, since both isocyanide
groups in this complex interact with the ruthenium ions, the C≡N stretching vibration in
polymer 3 undergoes the largest (37 cm-1) red shift compared to the free ligand. The ESI
mass spectra of complexes 1, 2 (recorded for samples containing an excess of the axial
ligand), and the reference complex 4 are shown in Figure S3 and confirm formation of the
bis(isocyanide) coordination to the ruthenium porphyrin core in these compounds.
14
Figure 1. Transformation of the 1H NMR spectra of the (OC)RuTPP complex upon stepwise addition of the
1,1'-diisocyanoferrocene ligand to its solution in CDCl3 at 25 ºC 1 - (OC)RuTPP, no ligand; 2 - 1.2 eq. of
ligand; 3 - 2.4 eq. of ligand; 4 - 3.6 eq. of ligand; 5 - 4.8 eq. of ligand; 6 - 6.0 eq. of ligand; 7 - 7.2 eq. of
ligand; 8 - 8.4 eq. of ligand; 9 - 9.6 eq. of ligand; 10 - 10.8 eq. of ligand; 11 - 12.0 eq. of ligand. Labeling
legend: a, b, and c are -pyrrolic, 1,1’-diisocyanoferrocene, and meso-phenyl fragment signals for complex
2, respectively; d are the resonances for free axial ligand.
a bc d
15
The UV-Vis and MCD spectra of the complexes 1, 2, and 4 are shown in Figure 2. Similar
to our previously reported (RNC)2FePc complexes,13 the UV-Vis and MCD spectra of the
compounds reported herein are very similar to each other. Indeed, the Q-band region of the
UV-Vis spectra of all three complexes consists of a single band observed between 529 and
536 nm as well as a prominent shoulder with max at ~580 nm. These two features in the
UV-Vis spectra of 1, 2, and 4 are associated with two Faraday A-terms centered around 535
and 580 nm in the corresponding MCD spectra. The Soret band region in the UV-Vis
spectra of the (RNC)2RuTPP complexes is dominated by a single Soret band with max
around 418 nm, which is associated with a Faraday A-term in the corresponding MCD
spectra. As expected for the effective four-fold symmetry of the porphyrin core in the
(RNC)2RuTPP complexes, HOMO > LUMO (HOMO is the energy difference
between two highest energy occupied porphyrin-centered -orbitals and LUMO is the
energy difference between two lowest energy unoccupied porphyrin-centered -orbitals),
which is reflected in negative-to-positive (in ascending energy) sequence of the MCD
signals.28 The intense UV-Vis band and the MCD A-term signal of the Soret band can be
easily assigned to the porphyrin-centered * transition.29 Assignment of the Q-band
region in the UV-Vis and MCD spectra of 1, 2, and 4 is less straightforward. Indeed, the
MCD A-terms around 535 (stronger) and 580 (weaker) nm can be assigned as a vibronic
Q0-1 component and as a Q0-0 transition, respectively,29 or, alternatively, they can be
attributed to the Q0-0 transition and the MLCT (Ru → *, TPP) band. These two alternative
assignments are discussed below in the context of our DFT and TDDFT calculations.
Overall, the influence of the ferrocene-containing axial ligands on the UV-Vis and MCD
spectra of (RNC)2RuTPP complexes is negligible at best as these are very similar to the
16
UV-vis spectra of all earlier reported alkyl- and aryl-isocyanide complexes of ruthenium
tetraarylporphyrins.11,18
Figure 2. Experimental UV-vis (top) and MCD (bottom) spectra of complexes 4 (A), 1 (B), and 2 (C).
17
The molecular structure of the reference complex 4 is known,18 while the solid state
structures of the new complexes 1 and 2 were determined in this work by X-ray
crystallography (Figures 3, S4, and S5).23 After exploring several solvent systems for
crystallization, we found that room temperature evaporation of saturated solutions of
complexes 1 and 2 in toluene afforded good quality single crystals suitable for X-ray
experiments. Taking into consideration a relatively low stability of the latter complex in
solution, crystals of 2 were grown in the presence of excess free axial ligand. While the
solid-state structure of complex 1 is solvent-free, a toluene solvent molecule is present in
the X-ray crystal structure of complex 2. The CAMERON drawings of the molecular
structures of the complexes 1 and 2 are illustrated in Figure 3. Key crystallographic
information and selected metric data for these complexes are provided in Tables 1 and S1.
18
Table 1. Selected Bond Distances (Å) and Angles (deg) for complexes 1 and 2.
Complex 1
Ru(1)-N(1) 2.0674(4) N(1)-Ru(1)-N(2) 90.74(17)
Ru(1)-N(2) 2.052(4) N(2)-Ru(1)-C(23) 93.11(18)
C(23)-Ru(1) 1.990(6) N(1)-Ru(1)-C(23) 89.19(19)
C(23)-N(3) 1.165(7) Ru(1)-C(23)-N(3) 173.0(5)
C(24)-N(3) 1.385(7) C(24)-N(3)-C(23) 169.4(6)
C(24-28)-Fe(1)
(average)
2.0418(6) C(29-33)-Fe(1) (average) 2.0432(6)
Complex 2
Ru(1)-N(1) 2.054(3) N(1)-Ru(1)-N(2) 90.10(13)
Ru(1)-N(2) 2.048(3) N(2)-Ru(1)-C(23) 88.12(15)
C(23)-Ru(1) 1.970(5) N(1)-Ru(1)-C(23) 86.64(15)
C(23)-N(3) 1.173(5) Ru(1)-C(23)-N(3) 173.6(4)
C(24)-N(3) 1.385(6) C(24)-N(3)-C(23) 177.6(4)
C(24-28)-Fe(1)
(average)
2.0342(5
)
C(29-33)-Fe(1) (average) 2.0314(6)
C(29)-N(4) 1.390(7) C(34)-N(4) 1.153(8)
C(34)-C(7) 3.541(5) C(34)-H(51) 3.768(8)
C(34)-H(221) 3.846(7) C(34)-C(3) 3.967(6)
19
Figure 3. Molecular structures of complexes 1 (left) and 2 (right) at 50% thermal ellipsoids. The toluene
solvent molecule of crystallization observed in the structure of complex 2 is omitted. All hydrogen atoms are
omitted for clarity.
Both complexes 1 and 2 crystallize in monoclinic unit cells with the ruthenium(II) atom
located at the molecular center of symmetry. For complexes 1 and 2, the central ruthenium
atom features a distorted octahedral C2N4 coordination with the Ru-N bonds being 0.06 –
0.08 Å longer than the Ru-C bonds. The Ru-CN bond lengths in complexes 1 and 2 are
very close to thosed documented for similar ruthenium porphyrin complexes axially
coordinated with 4-cyano-1-isocyano-2,6-diisopropylbenzene11 reflecting similar -
donor/-acceptor ratios10 of the FcNC and (CNC5H4)2Fe ligands to that of the former aryl
isocyanide. However, the Ru-CN bond distance in reference compound 4 is slightly longer
than that observed for aryl isocyanides coordinated to the same macrocyclic platform. The
isocyanide C≡N bond distances in both complexes 1 and 2 are in a typical range for the
20
transition-metal isocyanide-containing complexes exhibiting modest extend of
backbonding.10 It is instructive to point out that the C≡N bond length of the non-
coordinated isocyanide group in complex 2 is 0.02 Å shorter than the C≡N bond distance
of the coordinated isocyanide substituent, which echoes the difference in their (CN)
vibrations. The RuCN and isocyanide CNC angles are close to linear and the
porphyrin core is planar in both complexes. The ferrocenyl groups in both complexes
assume nearly eclipsed conformations and no significant disorder of the Cp rings was
encountered in the Fourier density map. The Fe-C bond distances are within the typical
range expected for the isocyanoferrocene ligands.10 The torsion angle between coordinated
and non-coordinated isocyanide groups in the crystal structure of complex 2 is ~58o. As a
result, the terminal carbon atom of the non-coordinated isonitrile group forms a close
contact (~2.64 Å) with the hydrogen atom of the phenyl group of the porphyrin. Moreover,
this isocyanide group is located at only ~4 Å above the pyrrole ring of porphyrin core,
which is only slightly larger than the sum of Van der Waals radii of two carbon atoms.
Again, such arrangement may facilitate the dissociation of one axial ligand and the
transformation of the remaining ligand into a bidentate platform. The Fe···Fe distances in
1 and 2 were found between 11.62 and 11.70 Å, while Fe-Ru distances were found between
5.81 and 5.85 Å. The intermolecular contacts in the solid state structure of both complexes
1 and 2 are rather weak (Supporting Information Figures S4 and S5).
21
Figure 4. DFT-predicted orbital energy diagrams for (RNC)2RuTPP complexes.
22
Figure 5. DFT-predicted frontier molecular orbitals for (RNC)2RuTPP complexes.
Table 2. DFT-predicted molecular orbital compositions for (RNC)2RuTPP complexes.
% Composition
Complex 4
MO Energy(eV) Symmetry Ru Porphyrin Ph t-BuNC
210 -4.779 au 0.48 96.33 3.17 0.02
211 -4.53 ag 68.03 30.06 0.46 1.45
212 -4.466 ag 53.85 35.9 0.43 9.83
213 -4.456 ag 53.18 37.98 0.45 8.4
214 -4.366 au 0.46 76.44 19.62 3.48
215 -1.976 ag 6.49 81.33 11.02 1.16
HOMO-4 HOMO-1 HOMO LUMO LUMO+1
(t-BuNC)2RuTPP
(FcNC)2RuTPP
([C5H4NC]2Fe)2RuTPP
23
216 -1.974 ag 4.91 83.34 10.82 0.94
Complex 1
MO Energy(eV) Symmetry Ru Porphyrin Ph FcNC
262 -4.892 au 0.62 96.19 3.09 0.1
263 -4.722 ag 69.58 29.53 0.38 0.51
264 -4.665 ag 48.2 39.93 0.47 11.39
265 -4.579 ag 53.59 26.1 0.26 20.04
266 -4.477 au 0.57 74.1 18.29 7.05
267 -2.096 ag 7.14 80.54 10.24 2.08
268 -2.09 ag 4.41 84.33 10.24 1.02
Complex 2
MO Energy(eV) Symmetry Ru Porphyrin Ph CNFcCN
274 -4.954 au 0.57 96.11 3.05 0.26
275 -4.845 ag 70.42 28.81 0.47 0.3
276 -4.791 ag 47.43 41.61 0.6 10.36
277 -4.736 ag 53.55 30.19 0.3 15.96
278 -4.539 au 0.54 74.12 18.59 6.74
279 -2.174 ag 5.16 82.6 9.67 2.57
280 -2.134 ag 5.12 83.98 9.49 1.41
a. HOMO and LUMO are in bold.
DFT and TDDFT calculations. DFT and TDDFT calculations were employed to correlate
spectroscopic and redox properties of the (RNC)2RuTPP complexes with their electronic
24
structures. A DFT-predicted energy diagram for all three complexes reported herein is
shown in Figure 4, while the corresponding frontier molecular orbital drawings are given
in Figures 5 and S5 – S8. In addition, molecular orbital compositions for these complexes
are listed in Tables 2 and S2. For all complexes, the classic Gouterman's30 "a2u" and "a1u"
type (in D4h point group notation) porphyrin-centered orbitals were predicted to be the
HOMO and HOMO-4 (Table 2, Figures 4 and 5). In all cases, DFT predicted that the "a2u"
MO has higher energy compared to the porphyrin-centered "a1u" MO, which is a typical
case for tetraarylporphyrins.31 These two porphyrin-centered -orbitals are closely spaced
with three predominantly ruthenium-centered HOMO-1 to HOMO-3 orbitals. For instance,
the HOMO to HOMO-1 gap is only 0.09 - 0.2 eV for the porphyrins considered herein.
The HOMO energy decreases by 0.17 eV upon going from complex 4 to complex 2, which
reflects the electron-withdrawing capabilities of the axial ligands. The (dxy)2, (dxz)
2, (dyz)2
electronic configuration is typically expected for low-spin distorted octahedral
ruthenium(II) porphyrins.11,18,32 Because of the axial compression and electronic
characteristics of the isocyanide ligands, one might expect that the ruthenium-centered dxz,
dyz MOs should be less stable compared to the dxy orbital. In agreement with this prediction,
our DFT calculations suggest that the energies of the predominantly ruthenium-centered
dxz (HOMO-1) and dyz (HOMO-2) orbitals in all (RNC)2RuTPP complexes are slightly
higher (0.05 - 0.06 eV) compared to the predominantly ruthenium-centered dxy (HOMO-3)
orbital. It is interesting to note that the DFT predicted energies of dxz, dyz, and dxy MOs are
close to each other with the largest difference being only 0.14 eV. Such near- degeneracy
probably arises from two competing phenomena: an axial compression and the -accepting
capability of the axial isocyanides. A very small energy gap between the DFT-predicted
25
porphyrin-centered HOMO and the predominantly ruthenium-centered HOMO-1 makes it
ambiguous to pinpoint the centricity of the first oxidation process as the ground state
calculations cannot predict spin-polarization in a single-electron oxidized species. Indeed,
previously published EPR, electrochemical, and chemical oxidation data for the reference
complex 418 and a few other ruthenium porphyrins with axial isocyanide ligands11, as well
as our spectroelectrochemical and chemical oxidation data discussed below, suggest that
the first oxidation process in (RNC)2RuTPP complexes is ruthenium-centered. A subtle
(0.09 – 0.2 eV) stabilization of the ruthenium-centered HOMO-1 compared to the
porphyrin-centered HOMO can be explained by the presence of a small (10%) amount of
Hartree-Fock exchange in the hybrid TPSSh exchange-correlation functional. Indeed, it
has been pointed out that energies of the metal-centered and n-type MOs tend to decrease
with the increase of the extent of Hartree-Fock exchange in the exchange-correlation
functional, while relative energies of the -orbitals in the same systems are much less
sensitive to this phenomenon.34a We also tested several “pure” LDA, GGA, and meta-GGA
exchange-correlation functionals in order to improve agreement between theory and
experiment for the (RNC)2RuTPP systems. Although several exchange-correlation
functionals predicted ruthenium-centered HOMO closely followed by the porphyrin-
centered HOMO-1 (Supporting Information Table S3), when these functionals were used
in TDDFT calculations, no reasonable agreement between predicted and experimental
spectra of complexes 1 – 3 was documented (Figure S9). In contrast, TDDFT calculations
with TPSSh exchange-correlation functional resulted in similar UV-vis spectra for
complexes 1 – 3 and predicted energies of the Ru TPP charge-transfer transitions to be
at lower energies than the porphyrin-centered Q-band, which is in excellent agreement with
26
the experimental data. Thus, the TPSSh exchange-correlation functional offers a reasonable
compromise between the ground state electronic structures and excited state properties of
complexes 1 - 3. DFT predicts several ferrocenyl-centered MOs below the porphyrin-
centered "a1u"-type orbital. These ferrocenyl-centered orbitals are ~0.5 eV more stable in
the case of complex 2 compared to complex 1, which reflects the electron-withdrawing
influence of an additional isocyanide substituent (Figure 4, Table 2). Due to their
delocalized nature, ferrocenyl-centered MOs can potentially support an electron-transfer
pathway for the metal-metal electronic coupling documented for complex 1 on the basis of
electrochemical experiments. The LUMO and LUMO+1 in all complexes were predicted
to constitute nearly degenerate porphyrin-centered * MOs, similar to the "eg" set of
Gouterman's four-orbital model,30 and are well-separated in energy from the other
porphyrin-centered * orbitals. In the case of the ferrocenyl-containing compounds,
LUMO and LUMO+1 are followed by a set of higher energy unoccupied ferrocenyl-
centered MOs. These are well-separated (~0.8 eV) in energy in compound 1 but are
significantly closer (~0.4 eV) in complex 2 (Figure 4).
In simplified approach,29,33 DFT-predicted electronic structures suggest that in addition to
classic porphyrin-centered * transitions, the electronic absorption spectra of
(RNC)2RuTPP complexes should be further enriched with a number of low-energy Ru →
* (TPP) charge-transfer bands and (in the case of ferrocenyl-containing axial ligands)
with Fc → * (TPP) and Ru → Fc charge-transfer transitions. Since the TDDFT method
was shown to provide a reasonable accuracy for energies of * and charge-transfer
transitions in porphyrins and their analogues, we used this approach to assign the observed
bands for the (RNC)2RuTPP systems.34 The TDDFT-predicted UV-Vis spectra of all
27
(RNC)2RuTPP complexes along with the calculated intensities for major transitions are
shown in Figure 6. Symmetry requirements for Ci point group restrict that only 1Au excited
states would contribute into TDDFT predicted intensities for the (RNC)2RuTPP
complexes. Thus, although from the electronic structure standpoint, numerous low-energy
MLCT bands originating from the predominantly ruthenium-centered dxy, dxz, and dyz
orbitals to TPP * MOs could be expected, they are symmetry forbidden as the LUMO and
LUMO+1 have ag symmetry. In the case of reference compound 4, the Q-band region
formed by the excited states 6 and 7, which are dominated by the classic Gouterman's
"a2u"(HOMO), "a1u" (HOMO-4) → "eg" (LUMO, LUMO+1, au → ag in Ci point group)
single electron transfers.30 The Soret band region for this complex is dominated by two
nearly degenerate excited states (excited states 9 and 10, Supporting Information Table
S4). These, again, originate from admixture of two Gouterman's classic "a2u", "a1u" → "eg"
(au → ag in Ci point group) single electron transfers. According to TDDFT calculations,
the first two intense MLCT bands originating from HOMO-1 - HOMO-3 (Ru) → LUMO+2
(TPP, *, excited states 13 and 14) should appear at 357 and 356 nm, respectively, which
reflects the LUMO to LUMO+2 energy gap. Thus, both Q- and Soret band regions in the
reference complex 4 can be described by the porphyrin-centered * transitions, while
the low-energy MLCT (Ru → TPP) bands should have negligible intensity because of the
symmetry considerations. In the case of complexes 1 and 2, the low-energy Q-band region
can be described by a set of symmetry forbidden MLCT (Ru → TPP) transitions followed
by two porphyrin-centered * transitions dominated by the HOMO, HOMO-4 →
LUMO, LUMO+1 excitations, which is similar to the reference complex 4. Again, in both
complexes, the Soret band region is dominated by the classic porphyrin-centered *
28
transitions. Unlike the predominantly ruthenium-centered d-orbitals, combinations of two
sets of individual ferrocenyl-centered occupied and unoccupied orbitals results in
delocalization over both ferrocenyl-based orbitals of au and ag symmetries. Because of the
availability of low-energy unoccupied ferrocene-centered orbitals of au symmetry,
predominantly Ru → Fc transitions would have non-zero intensities. Indeed, TDDFT
predicts that the lowest energies for such bands would be observed at ~407 nm for complex
1 (excited states 26 and 27, Supporting Information Table S4) and at ~447 nm for complex
2 (excited states 17 and 18, Supporting Information Table S4). The low-energy shift of
these predominantly Ru → Fc transitions for complex 2 is a result of the electron-
withdrawing effect of the unbound isocyanide group, which is associated with ~0.4 eV
stabilization of the ferrocenyl-centered unoccupied MOs in this complex. It should be
noted, however, that TDDFT predicted intensities for these bands are about an order of
magnitude lower compared to the Soret band intensity. In addition, symmetry allowed low-
intensity predominantly Fc → Fc (or substituted ferrocene → substituted ferrocene)
transitions between the Q- and Soret bands were also predicted by the TDDFT method in
~495 and ~425 nm regions in both ferrocenyl-containing porphyrins. Overall, the Q-band
region in all three compounds described herein is dominated by the classic Goutermans'
* transitions,30 while the low-energy MLCT (Ru → TPP) bands are symmetry
forbidden. Similarly, the Soret band region for all three complexes is dominated by the
porphyrin-centered * transitions, while TDDFT predicted intensities of the
predominantly Fc → Fc (or substituted ferrocene → substituted ferrocene) and Ru → Fc
(or Ru → substituted ferrocene) bands in ferocene-containing porphyrins are significantly
smaller. Not surprisingly, such dominance of the porphyrin-centered * transitions in
29
the Q- and Soret band regions for complexes 1, 2, and 4 explains similarities in the UV-
Vis and MCD spectra of these compounds.
30
Figure 6. Experimental UV-Vis (top) and TDDFT-predicted (bottom) spectra of complexes 4 (A), 1 (B), and 2 (C).
A
B
C
31
Figure 7. DPV (red) and CV (blue) electrochemical data for complexes 4 (top) and 1 (bottom) recorded in
DCM/0.05 M TFAB solutions. In all cases, the CV data were recorded at 100 mV/s scan rate.
Redox properties. The redox characteristics of the (RNC)2RuTPP complexes were assessed
through electrochemical (CV and DPV, Figures 7 and S10), spectroelectrochemical
(Figures 8 - 10), and chemical oxidation (Figures S11 - S13) experiments. Table 3
summarizes the half-wave potential data relevant to the redox processes of (RNC)2RuTPP
compounds. In the case of the reference complex 4, two reversible oxidation processes in
CV and DPV experiments were observed for solutions in DCM/TFAB, which correlate
well with the earlier reports on this compound (Figure 7).18 Spectroelectrochemical and
32
chemical oxidation data presented below also correlate well with the earlier published
chemical oxidation data on the same complex and allow to assign the first oxidation couple
to the RuII/RuIII process. The second oxidation process is assigned to the porphyrin ring
oxidation.14 The CV and DPV data for the complex 1 were recorded using the non-
coordinating TFAB electrolyte because of its well-known ability to increase resolution
between redox waves in mixed-valence compounds.35 The electrochemical profile of
complex 1 in a DCM/TFAB system consists of four reversible oxidation waves, with the
second and third processes being in close proximity (E1/2 100 mV) to each other. The
first oxidation potential is ~60 mV more positive than the similar process documented for
reference compound 4 and is assigned to the RuII/RuIII couple on the basis of the
spectroelectrochemical and chemical oxidation data. The closely spaced second and third
redox waves are assigned to stepwise oxidations of two axial ferrocenyl moieties as these
potentials are close to the FeII/FeIII redox couple of the free ligand.13,16 The fourth redox
process corresponds to the oxidation of the porphyrin macrocycles based on the similarity
of its potential to that of the earlier reported (RNC)2RuTPP complexes.11,18 The ~100mV
separation between the stepwise oxidation of two ferrocenyl fragments (Kc = 49; see,
however, a cautionary warning on the use of Kc values obtained from the electrochemical
data for analysis of the mixed-valence compounds)36 is similar to the 80 mV separation
observed earlier for the (FcNC)2FePc complex13 and suggests electronic coupling between
the axial isocyanoferrocene ligands separated by ca. 11.62 Å in complex 1. Collecting CV
and DPV data on the complex 2 turned out to be a very complicated task due to several
reasons. First, as clearly evident from the 1H NMR data discussed above, complex 2 is only
stable in the presence of the excess axial ligand. The mobility of the axial ligand makes it
33
difficult to collect full range electrochemical data as an excess of the axial ligand, which is
necessary to use in order to stabilize the complex 2, would mask redox processes associated
with the axial ferrocenyl fragment oxidations in 2. Second, a problem arises due to a high
affinity of the isocyanide group toward metal coordination and even glassy carbon
electrode surfaces. Indeed, interaction of 1,1'-diisocyanoferrocene with a gold surface as
well as formation of polymeric structures with gold(I) salts are well documented.37 In all
cases, it was proposed that the 1,1'-diisocyanoferrocene coordinates with the metal centers
in the μ-1:1-motif.37 Thus, it could be expected that both the free ligand and the non-
coordinated isocyanide group in complex 2 can be immobilized on the electrode surface
thereby complicating analysis of the electrochemical experiments. Indeed, the first
oxidation event for complex 2 is fully reversible during the initial electrochemical scan
involving a freshly polished electrode. The E1/2 value for the first oxidation of 2 is close to
that documented for the the RuII/RuIII couple in the oxidation of other (RNC)2RuTPP
complexes11,18 and this assignment was further confirmed by the spectroelectrochemical
and chemical oxidation experiments. The RuII/RuIII half-wave potential for the oxidation
of complex 2 is ~150 mV more positive compared to the corresponding potential for
complex 1, which reflects the electron-withdrawing influence of the additional isocyanide
substituent in the former complex. Having the central metal-centered first oxidation
process is important for prospective usage of complex 2 and polymer 3 as molecular wires
because such electronic structure would better facilitate electron transfer along the axial
direction. Expanding the scanning range to involve the axial isocyanide ligands' potentials
resulted in a gradual shift of the oxidation potentials and a decrease of reversibility of the
first oxidation process. This oxidation event (Figure S10) occurred almost instantaneously
34
with the use of a platinum working electrode, while it was slower when a glassy carbon
electrode was employed (we did not explore using a gold working electrode because of its
known high affinity toward adsorbing 1,1'-diisocyanoferrocene37). The reversibility of the
first redox event can be easily restored after polishing the working electrode and such
behavior is suggestive of the interaction of the non-coordinated isocyanide group with the
electrode surface. Due to the adsorption of complex 2 on the electrode surface as well as
the high mobility of the axial ligand and the continuous formation of the insoluble polymer
3 during the electrochemical experiments, we were unable to observe well-separated
oxidation waves for the second and third oxidation events for complex 2. Despite these
complications, the high coordination affinity of the uncoordinated isocyanide group in
complex 2 may be considered a desirable characteristic as using polymer 3 would require
anchoring these molecular wires to an electroactive surface.
Table 3. Half-wave potentials (V) for (RNC)2RuTPP in DCM/0.05M TFAB solution at room temperature.a
Complex\ Couple RuII/RuIII FeII/FeIII TPP(2-)/TPP(1-)
4 -0.023 0.713
1 0.033 0.437, 0.533 0.997
2 0.187
a All potentials are referenced to the FcH/FcH+ couple and are ± 5 mV.
35
Figure 8. Spectroelectochemical oxidation of the reference complex 4 in DCM/0.15M TFAB solution at
room temperature.
36
Figure 9. Spectroelectochemical oxidation of the complex 1 in DCM/0.15M TFAB solution at room
temperature. (A) First oxidation; (B) combined second and third oxidations.
37
Figure 10. Spectroelectochemical oxidation of complex 2 in DCM/0.15M TFAB solution at room temperature.
Spectroelectrochemical and chemical oxidation data pertaining to the first oxidation
process for various (RNC)2FePc complexes are very close to each other (Figures 8 - 10 and
Figures S11 - S13). During the first oxidation event, the Soret band decreases in intensity
and undergoes blue shift from ~420 nm to ~400 nm, while the Q-band at 530 nm transforms
into three new bands at ~510, ~560, and ~640 nm. All of these changes are very similar to
those observed earlier for one-electron oxidation of reference complex 418 for which
formation of the [4]+ complex was confirmed by a variety of spectroscopic methods. Thus,
the first oxidation event for all complexes was attributed to the RuII/RuIII couple. Changes
in the UV-Vis spectra of [1]+ upon its transformation into the mixed-valence [1]2+and [1]3+
species during the second and the third oxidation processes are shown in Figures 9, 11,
S12, and S14. In the case of the chemical and spectroelectrochemical oxidations, the
38
appearance of several new bands in the 600 - 800 nm region was observed in the
corresponding UV-Vis spectra. The overall intensities of these bands are close to what
might be expected for porphyrin-centered * transitions although the presence of the
lower intensity, lower energy FeII → RuIII, RuIII → FeIII, and TPP → FeIII charge-transfer
transitions cannot be excluded either. The appearance of an inter-valence charge-transfer
(IVCT) band in the NIR region of the electronic spectrum of [1]2+ (FeII-RuIII-FeIII core) can
be considered a marker for the formation of the mixed-valence complex in solution.38 The
NIR spectral transformations during the spectroelectrochemical and chemical oxidation
experiments are shown in Figures 11 and S14. These changes are indicative of a rather
complex redox picture. Indeed, during oxidation of the initial [1]+ complex with the FeII-
RuIII-FeII core to form the mixed-valence [1]2+ dication with FeII-RuIII-FeIII core, we
observed growth of a broad NIR band between ~1000 and 2600 nm with a poorly defined
maximum at ~1620nm. Upon further transformation of the mixed-valence [1]2+ to [1]3+
(FeIII-RuIII-FeIII core), this broad diffuse NIR band evolved into a more defined and intense
NIR peak with max ~1465 nm. A very similar growth in intensity and high-energy shift of
the most intensive NIR transition were observed earlier for several organometallic
complexes of general formula Fc-C≡C-M-C≡C-Fc,39 which is close to an axial Fc-N≡C-
Ru-C≡N-Fc group in complex 1. Similar to the previous reports,39 one of the possible
assignments of this band could be LMCT, but the other CT transitions cannot be excluded.
It is interesting to note, however, that no such behavior was documented for the reported
earlier mixed-valence [(FcII/IIICN)2FeIIPc(1-)]2+ complex and fully oxidized
[(FcIIICN)2FeIIPc(1-)]3+ compound.13 The overlap of the IVCT with other NIR transitions
is quite common for mixed-valence compounds and often complicates data analysis.38-40
39
In order to locate the IVCT band in [1]2+, we attempted band deconvolution analysis for
its NIR envelope (Figure 11). Although NIR region of the spectrum of [1]2+ cannot be
described by a single LMCT Gaussian function, position and width of the possible IVCT
band cannot be precisely determined through fitting algorithm as many possible fitting
solutions could be found. Because of the above complication, accurate analysis of the IVCT
band is impossible to perform and, therefore, it is difficult to estimate the extent of metal-
metal coupling in the mixed-valence [1]2+ complex.
Figure 11. NIR portion of the electronic spectra for transformation of [1]+ (black line) into the mixed-valence
[1]2+ (red line) and [1]3+ (blue line) in DCM/0.15M TFAB solution at room temperature (left). One of several
possible band deconvolutions of the NIR transitions envelope (right).
Conclusions
New heterotrinuclear Fe-Ru-Fe complexes 1 and 2 involving the ruthenium(II)
tetraphenylporphyrin axially coordinated with two isocyanoferrocene - or 1,1'-
diisocyanoferrocene ligands, respectively, - were characterized by UV-Vis, MCD, NMR,
and IR spectroscopies, as well as by ESI-MS spectrometry and X-ray crystallography. The
preparation and isolation of insoluble polymeric molecular wire 3 was also achieved for
40
the first time. The 1H NMR titration experiments suggested significantly more pronounced
lability of the 1,1'-diisocyanoferrocene ligand compared to that of isocyanoferrocene in
these complexes. The redox properties of 1 and 2 were probed by employing
electrochemical (CV and DPV), spectroelectrochemical, and chemical oxidation methods
and were correlated with those of the reference compound 4. In all cases, the first oxidation
process was attributed to a reversible oxidation of the RuII center. The second and third
reversible oxidation events for 1 are separated by ~100 mV and were assigned to two
single-electron FeII/FeIII couples suggesting a weak long-range metal-metal coupling in this
complex despite a fairly large distance (11.5 Å) between the two iron centers.
Electrochemical data obtained for 2 are complicated by the interaction between the
uncoordinated isocyanide substituent of the axial 1,1'-diisocyanoferrocene ligand and
electrode surface as well as by the axial ligand dissociation processes.
Spectroelectrochemical and chemical oxidation methods were used to elucidate the
spectroscopic signatures of the [1, 2, and 4]n+ species in solution. A very broad absorption
band was observed in the near-infrared region for the mixed-valence [1]2+ with FeII-RuIII-
FeIII core that transforms into a more defined band upon oxidation of the dicationic
complex to form [1]3+ with FeIII-RuIII-FeIII core. The DFT and TDDFT calculations were
emloyed to correlate spectroscopic and redox properties of complexes 1, 2, and 4 with their
electronic structures.
41
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52
CHAPTER 2
Introduction
Organic isocyanides are well-known σ-donor and acceptors1, which can create very
stable coordination bonds with transition metals of d6 electronic configuration. While the
coordination of organic isonitriles to a variety of porphyrins2 and phthalocyanines3 has
been intensely studied, the redox active organic and organometallic isonitriles provide
additional opportunities for a creation of the molecular wires.4 Indeed, the axial
coordination of the ferroceneisonitrile and ferrocenediisonitrile5 to the porphyrins and
phthalocyanines have been recently studied and their redox properties were found to be
promising for applications in molecular electronics6 and in particular in molecular wires.7
The isocyanoazulenes8 belongs to non-traditional isonitrile ligands as the azulene core has
non-classic electron density distribution and dipole moments which can easily affect the
electronic structure of the transition metal complexes. In order to investigate the potential
use of the isocyanoazulenes for preparation of molecular wires, in this paper we have
investigated two general systems which consists of the ruthenium tetraphenylporphyrin
axially coordinated to the 2- and 6- positional isomers of isocyanoazulene8. The choice of
these positional isomers was dictated by the striking difference in optical and redox
properties of the 2-isocyanoazulene in which isocyano group is located at five membered
ring and 6-isocyanoazuelen in which the isocyano group is located on seven membered
ring. The properties of the new L2RuTPP9 with isocyanoazulenes will be evaluated and
compared with reference (t-BuNC)2RuTPP10 compound.
53
2.2. Experimental Section
1.2.1 Materials
All commercial reagents were ACS grade and were used without further purification. All
reactions were performed under a dry argon atmosphere with flame-dried glassware.
Toluene was distilled over sodium metal. Dichloromethane (DCM) and hexanes were
distilled over CaH2. Tetrabutylammonium tetrakis(pentafluorophenyl)borate (TBAF,
(NBu4)[B(C6F5)4]),11 2-isocyanoazulene (2-CNAz),8 6-isocyanoazulene (6-CNAz),8 and
(t-BuNC)2RuTPP (3)10 were prepared according to the literature procedure.
1.2.2 Synthetic work
Synthesis of (2-CNAz)2RuTPP (1). A total of 0.16 g (0.21 mmol) of the commercially
available (OC)RuTPP was added to a solution of 0.20 g (1.3 mmol) of the 2-CNAz ligand
in 20 mL of the mixture of toluene: DCM (1/1, v/v) under an argon atmosphere. The
reaction mixture was stirred at room temperature for 2h and solvent was removed under
the reduced pressure. The solid was washed several times with hexanes, dried under
reduced pressure. Yield: 0.129 g (58.6%). Elemental analysis: calculated for
C66H42N6Ru·0.36CH2Cl2·0.1C6H14: C, 77.70; H, 4.15; N, 8.24. Found: C, 76.01; H, 4.22;
N, 7.90. 1H NMR (500 MHz, C6D6): δ 9.10 (s, 8H, β- pyrrole), 8.34 (m, 8H, m-Ph), 7.43
(m, 12H, o-Ph, p-Ph), 6.86 (d, 4H, H4,8, C10H7NC, J = 10 Hz), 6.64 (t, 2H, H6, C10H7NC, J
= 10 Hz), 6.21 (t, H5,7, C10H7NC, J = 10 Hz), 4.85 (s, 4H, H1,3, C10H7NC). 13C NMR (125
MHz, C6D6): δ 144.12 (α-pyrrole), 144.06 (Cipso, Ph), 133.3 (Cortho, Ph), 130.62 (β-pyrrole),
125.9 (Cmeta, Ph), 125.3 (Cpara, Ph), 120.3 (Cmeso), 68.9 (C5H5), 64.3 (β-C, C5H4NC), 64.2
(α-C, C5H4NC), 52.4 (ipso-C, C5H4NC). Because of the often11 broad nature of the 13C
NMR signal for the isocyano carbon atom, it was not clearly observed in the spectrum of
54
(2-CNAz)2RuTPP. UV-vis [DCM; λ, nm (log ε, M-1cm-1)]: 417 (5.69), 297 (5.27). IR
(KBr), cm-1: 2067 (NC). HR ESI MS: Calculated for C66H42N6Ru: 1020.2527; Found:
1020.1956 [M]+.
Synthesis of (6-CNAz)2RuTPP (2). A total of 0.098 g (0.13 mmol) of the commercially
available (OC)RuTPP was added to a solution of 0.15 g (0.99 mmol) of the 6-CNAz ligand
in 50 mL of the mixture of toluene: DCM (1/1, v/v) under an argon atmosphere. The
reaction mixture was stirred at room temperature for 2h and solvent was removed under
the reduced pressure. The solid was washed several times with hexanes, dried under
reduced pressure. Yield: 0.129 g (65.2%). Elemental analysis: calculated for
C66H42N6Ru·0.68C6H5CH3: C, 77.70; H, 4.15; N, 8.24. Found: C, 78.49; H, 4.41; N, 7.76.
1H NMR (500 MHz, C6D6): δ 9.10 (s, 8H, β- pyrrole), 8.34 (m, 8H, m-Ph), 7.43 (m, 12H,
o-Ph, p-Ph), 7.35 (t, H2, C10H7NC, J = 10 Hz), 6.69 (m, 8H, H4,8, H1,3, C10H7NC), 4.02 (d,
4H, H5,7, C10H7NC, J = 10 Hz). 13C NMR (125 MHz, C6D6): δ 143.7, 143.3, 132.1, 130.8,
125.6, 125.0, 123.8, 121.3, 113.3, 113.2, 112.4, 111.8.
UV-vis [DCM; λ, nm (log ε, M-1cm-1)]: 417 (5.35), 297 (4.93). IR (KBr), cm-1: 2061 (NC).
HR ESI MS: Calculated for C66H42N6Ru: 1020.2527; Found: 1020.1862 [M]+.
1.2.3 Instrumentation
A Varian Unity INOVA NMR instrument was used to evaluate spectra taken at 500 MHz
frequency for protons and 125 MHz for carbon atoms. Each was referenced to TMS as an
internal standard and chemical shifts were recorded in parts per million. All UV-Vis data
was obtained on a JASCO-720 spectrophotometer at room temperature. An OLIS DCM 17
CD spectropolarimeter with a 1.4 T DeSa magnet was used to obtain all Magnetic Circular
55
Dichroism (MCD) data. IR-data was obtained on a Perkin Elmer FT-IR-Spectrometer
Spectrum 100 at room temperatures in KBr pellets Electrochemical measurements were
conducted using a CHI-620C electrochemical analyzer utilizing the three-electrode
scheme. Carbon or platinum working, platinum auxiliary and Ag/AgCl pseudo-reference
electrodes were used in 0.05 M solution of TBAF in DCM. In all cases, the redox potentials
are referenced to the FcH/FcH+ couple using decamethylferrocene as an internal standard.
Spectroelectrochemical data were collected using a custom-made 1 mm cell, a working
electrode made of platinum mesh, and a 0.15M solution of TBAF in DCM. HR ESI mass
spectra were recorded using a Bruker MicrOTOF-III system in THF as a solvent. Elemental
analysis was performed by Atlantic Microlab, Inc. in Atlanta, Georgia.
1.2.4 Computational Details
All computations were performed using the Gaussian 09 software package running under
Windows or UNIX OS13 Molecular orbital contributions were compiled from single point
calculations using the QMForge program.14 In all single-point calculations, the CAM-
B3LYP exchange-correlation functional15 was used. The double-zeta quality effective core
potential LANL2DZ basis set16 was used for all atoms in all calculations. Frequencies were
calculated for all optimized geometries in order to ensure that final geometries represent a
minima on potential energy surface. TDDFT calculations were conducted for the first 80
((2-CNAz)2RuTPP, (6-CNAz)2RuTPP), and (t-BuNC)2RuTPP) excited states in order to
ensure that all charge transfer (CT) and * transitions of interest were calculated.
56
2.3. Results and Discussion
Synthesis, Spectroscopy, and X-ray structures. The complexes 1 and 2 were synthesized
using the following synthetic strategy (Scheme 1).
Scheme 1. Synthetic strategy for preparation of the (RNC)2RuTPP complexes.
The “reference” complex 3 was synthesized and purified according to the literature.10
Similar to the other axially coordinated diamagnetic ruthenium and iron porphyrins and
phthalocyanines,17 1HNMR signals for axial ligands in complexes 1 and 2 with bidentate
isonitriles are shifted upfield compared to the corresponding free ligands. Both species
show the same number of resonance with 4 signals for porphyrin macrocycle indicating
there is a pseudo D4h symmetry with free rotation on the Ru-C bond; and 4 signals
consistent with the azulene ligand signifying free rotation on the Ru-C bond. The 1HNMR
57
signals of tetraphenylporphyrin in complexes 1 and 2 are very similar to the previously
reported proton signals of RuTPP complexes.7
The 1HNMR signals of axial ligands illustrate the different isomers. In complex 1 H1,3
signal is shifted -2.43 ppm compared to the free ligand, and H5,7 signal is shifted -1.1 ppm
to the upfield, the large shift of H1,3 signal is consistent to the closer proximity of the
hydrogen atom to the porphyrin ring. In complex 2 H1,3 signal is shifted -0.82 ppm
compared to the free ligand, and H5,7 signal is shifted -3.19 ppm. In agreement with the
previous statement, the H5,7 signal is shifted –3.19 ppm to the upfield, the large shift of H1,3
signal is consistent to the closer proximity of the hydrogen atoms to the porphyrin ring.
H5,7 signal in complex 2 is shifted -3.19 ppm compared to the free ligand, while H1,3 signal
in complex 1 is shifted -2.43 ppm. This is consistent with the H5,7 in complex 2 being closer
to the porphyrin ring as compared to H1,3 in complex 1; which can be interpreted by internal
C-C-C angels of 5 and 7 membered rings for complexes 1 and 2 respectively and it is
supported by the X-ray crystal structure which will be discussed later. 1HNMR has been
very informative to analyze the geometry of these species. The two isocyanoazulene
ligands have very similar electronic properties which can be confirmed by IR spectroscopy.
The isonitrile peak in complex 1 and 2 has been detected at 2067 cm-1 and 2061 cm-1,
respectively. The shifts from the free ligands 2-CNAz and 6-CNAz (2118 for 2-CNAz and
2111 for 6-CNAz)8 are nearly identical at 51 cm-1 and 50 cm-1 to lower energy values
compared to the free ligands respectively.
ESI mass spectra of (2-CNAz)2RuTPP and (6-CNAz)2RuTPP complexes are shown in
Figure 1 and confirm formation of the isocyanoazulene adducts to the ruthenium porphyrin
core.
58
Figure 1. ESI MS spectra of the (2-CNAz)2RuTPP (A) and (6-CNAz)2RuTPP (B) complexes. The
experimental and theoretical isotope distributions for molecular ions are given as insets.
The UV-vis and MCD spectra of the (2-CNAz)2RuTPP, (6-CNAz)2RuTPP, and (t-
BuNC)2RuTPP complexes are shown in Figure 2. Similar to our previously reported
(RNC)2FePc complexes,18 MCD spectra of all target compounds presented in this paper
are very similar to each other but the UV-Vis spectra for (2-CNAz)2RuTPP and (6-
CNAz)2RuTPP are slightly different in the Q-band region due to the absorption of azulene
ligand. The Q-band region of the UV-vis spectra of all three complexes consists of a single
band observed between 529 and 537 nm and a prominent shoulder located around 580 nm.
These two features in the UV-vis spectra of complexes 1 and 2 are associated with two
Faraday A-terms in their MCD spectra centered around 535 and 580 nm. The Soret band
region, in the UV-vis spectra of the (CNAz)2RuTPP complexes is dominated by a single
Soret band observed around 419 nm, which is associated with a Faraday A-term in the
59
respective MCD spectra. As expected for the effective four-fold symmetry of the porphyrin
core in (CNAz)2RuTPP complexes, ΔHOMO > ΔLUMO, which is reflected in negative-to-
positive (in ascending energy) sequence of MCD signals.19 Intense UV-vis band and MCD
A-term signal of the Soret band can be easily assigned to the porphyrin-centered *
transition.20 Assignment of the Q-band region in UV-vis and MCD spectra of complexes 1
and 2 is less straightforward. Indeed, MCD A-terms around 582 (stronger) and 538 nm
(weaker) can be assigned as a vibronic Q0-1 component and as a Q0-0 transition,
respectively20 or, alternatively, they can be assigned as Q0-0 transition and MLCT (Ru →
*, TPP) band. These two alternative assignments will be discussed below on a basis of
DFT and TDDFT calculations.
60
Figure 2. Experimental UV-vis (top) and MCD (bottom) spectra of (2-CNAz)2RuTPP (A) and (6-
CNAz)2RuTPP (B) complexes.
The molecular structures of compounds 1 and 2 were determined by X-ray crystallography.
Slow evaporation of a toluene solution of 1 or 2 at room temperature provided X-ray quality
crystals for analysis. Complexes 1 and 2 co-crystalized with one toluene molecule in the
asymmetric unit; Platon Squeeze21 was used to remove the badly disordered toluene in the
structure of 2. The size of the crystals were small and resulted in a resolution of 0.90 Å
and 0.84 Å, respectively and are displayed in Figure 3. Despite the low resolution of 1, it
can clearly be seen that the 2- and 6- isomers were isolated.
Complex 1 crystalized in the space group P21/c and complex 2 crystallized in space group
C2/c. The ruthenium atom in complex 2 is located on an inversion center resulting in one-
61
half of the molecule unique. In both complex 1 and 2, the ruthenium atom features a
tetragonal environment with an indistinguishable C2N4 coordination environment. The Ru
– N bond distances for 1 and 2 are within two s.u. and the Ru – C bond distances for 1 and
2 are within one s.u. The Ru atom sits in the planar porphyrin ring in both complexes. Due
to the low resolution of the structure of 1, only the bond distances of 2 will be compared to
similar complexes. Consistent with earlier reported Ru-porphyrin-isonitrile complexes, the
Ru-N bond distance is about 0.08 Å longer than the Ru-C distances.22 Additionally, the
Ru-C-N and C-N-C bond angles are approximately linear at about 170°. The Ru-C distance
is 1.986 (6) and is very similar to previously reported Ru-porphyrin complexes with aryl
and ferrocene based isocyanide axial ligands.22 The C≡N bond distance of 1.164 (7) Å
observed in 2 suggests modest back-bonding with the Ru atom and -donor:-acceptor
ratios.22
Figure 3. Molecular structures of complex 1 (left) and 2 (right) as 50% thermal ellipsoids. The toluene
solvent molecule of crystallization observed in the structure of both complex 1. All hydrogen atoms are
omitted for clarity.
62
Table 4. Selected bond distances (Å) and angles (deg) for (2-CNAz)2RuTPP and (6-CNAz)2RuTPP
complexes.
(2-Az)2RuTPP i (6-Az)2RuTPP
Empirical formula C66 H44 N6 Ru C66 H44 N6 Ru
Formula weight 1020.17 1020.17
Space Group, Z P 21/c C 2/c
a (Å) 23.4732(17) 23.4759(13)
b (Å) 12.9276(2) 12.6316(5)
c (Å) 19.052(4) 19.1332(11)
(deg) 90 90
(deg) 112.963(8) 111.110(2)
(deg) 90 90
volume (Å3) 5323.2(10) 5293.0(3)
calc (g/cm3) 1.388 1.280
Radiation Type Cu K Mo K
(mm-1) 2.790 0.343
max (deg) 68.2 25.067
Rint 0.123 0.134
GOF (F2) 0.9352 1.0494
R1a (F2> 2(F2) 0.1090 0.0729
wR2a (all data) 0.2541 0.1648
i atoms were refined with isotropic displacement parameters
63
DFT and TDDFT Calculations. The DFT predicted frontier orbitals energy diagram and
molecular orbital compositions for both isocyanoazuelene complexes as well as reference
compound shown in Figures 4, 5 and Table 2. The choice of CAM-B3LYP exchange-
correlation functional was dictated by the best agreement between the theory and
experiment obtained in TDDFT calculations. The large contribution of Hartree-Fock
exchange part in this exchange correlation functional however, resulted in strong
stabilization of the metal-centered orbitals compared to the porphyrin orbitals. Indeed, in
the case of the reference (t-BuNC)2RuTPP the HOMO and HOMO-1 were predicted to be
porphyrin centered followed by the predominantly Ru-centered molecular orbitals. The
difference between TPP-centered HOMO and Ru-centered HOMO-2 was predicted to be
about 0.6 eV in energy. The CAM-B3LYP correctly predicts the LUMO and LUMO+1
orbitals in the reference (t-BuNC)2RuTPP complex are porphyrin-centered and were found
to be degenarate in energy. Similarly both 2-CNAz and 6-CNAz coordinated compounds
have porphyrin-centered orbitals as HOMO and HOMO-1 followed by pridominanetly
Ru-centered orbitals. Again the difference between porphyrin centered HOMO and Ru-
centered HOMO-2 in (2-CNAz)2RuTPP and (6-CNAz)2RuTPP complexes were found to
be about 0.6 eV. The presence of the isocyanoazuelene ligands in the Ru compounds (2-
CNAz)2RuTPP and (6-CNAz)2RuTPP can be clearly revealed from the electronic structure
calculations as DFT predicts the LUMO and LUMO+1 will be azulene-centered. The
classic unoccupied porphyrin-centered molecular orbitals were predicted to be LUMO+2
and LUMO+3 in both azuelene contaning systems. In addition, the azuelene-centered
occupied molecular orbitals were found to be HOMO-4 and HOMO-5 in both azuelene
contaning complexes. Although electrochemical experiments discussed below suggestive
64
of the first Ru-centered oxidation, the CAM-B3LYP exchange-correlation functional still
predicts the HOMO to be porphyrin-centered. In order to overcome such a discrepancy
between theory and experiment, we explored a large number of exchange correlation
functionals in calculation of electronic struicture of (2-CNAz)2RuTPP complex. Although
in the case of the TPSSh exchange-correlation functional the energy gap between the
highest occupied porphyrin-centered and highest occupied Ru-centered orbitals was found
to be smaller, the HOMO was predicted to be porphyrin-centered orbital in all tested
exchange correlation functionals. In order to overcome such a discrepancy we also
conducted CASSCF calculations on our systems, which will be discussed elsewhere.
Figure 4. DFT-predicted orbital energy diagrams for (RNC)2RuTPP complexes.
65
Figure 5. DFT-predicted frontier orbitals for (RNC)2RuTPP complexes.
The TDDFT calculations on the reference (t-BuNC)2RuTPP complex suggest that the
whole UV-Vis spectrum will be dominated by the four classic Gouterman’s transitions23
originating from the single electron excitation originated from the HOMO, HOMO-1 →
LUMO, LUMO+1. Indeed, TDDFT predicts that the first and second excited states at 520
nm would have relatively low intensity and would pridominantely originated from the
HOMO → LUMO, LUMO+1 single electron excitations while the most intense band
predicted at 394 nm would be dominated by the HOMO-1→ LUMO, LUMO+1 single
electron transitions.
The presence of the azulene-centered molecular orbitals in the HOMO and LUMO regions
in the case of azulene-contaning porphyrin will result in the presence of the additional
bands in their UV-Vis spectra. Indeed, TDDFT calculations on both isomers is suggestive
of two excited states which originate from the azuelene centered * transtions in the low
energy region, which is in agreement with experimental data.
In the case of the (6-CNAz)2RuTPP complex, TDDFT predicts that the first and second
66
excited states would be dominated by azulene-centered * single electron excitations.
The excited state three and four in this isomer were predicted at slightly higher energy and
higher intensity, and are dominated by classic porphyrin centered * single electron
transitions originated from the HOMO, HOMO-1 → LUMO+2, LUMO+3 orbitals. In the
case of the (2-CNAz)2RuTPP complex in which isocyano group is located at five
membered ring, the azulene-centered * transtions were predicted to slightly higher
energy than the classic Gouterman * excitation.23 In both isocyanoazuelene systems
Ru-to-porphyrin metal to ligand charge transfer (MLCT) transitions should be symmetry
forbidden and the lowest energy for them was predicted at around 460-470 nm. The most
intense bands predicted by TDDFT calculations in azuelene-containing porphyrins were
between 375 and 395 nm and are dominated by classic Gouterman’s porphyrin-centered,
HOMO, HOMO-1 → LUMO, LUMO+3 single electron excitation. Thus, TDDFT
predictions on the UV-Vis spectra on the azuelene containing pophyrins are in a reasonable
agreement with the experimental data and explain the broadening of the Q-band region in
these systems because of the presence of the azuelene centered * transtions.
67
Table 5. DFT-predicted molecular orbital compositions in (RNC)2RuTPP complexes.
%Composition
(t-BuNC)2RuTPP (3)
MO Energy(eV) Symm. Ru Porphyrin Phenyl t-BuNC
195 -9.74 au 5.39 86.15 0.87 7.58
196 -9.222 ag 0.17 75.71 24.08 0.04
197 -9.202 ag 0.11 78.84 21 0.05
198 -8.899 ag 20.42 57.7 6.36 15.52
199 -8.897 ag 20.4 58.21 5.98 15.41
200 -8.57 au 0.06 18.56 81.36 0.02
201 -8.539 au 0.36 26.22 72.26 1.16
202 -8.536 au 0.05 2.47 97.47 0.01
203 -8.497 ag 1.15 4.58 93.68 0.59
204 -8.497 ag 0.97 5.06 93.48 0.49
205 -8.369 au 0.06 11.64 88.28 0.01
206 -8.336 au 1.03 72.52 16.65 9.8
207 -8.303 ag 0.55 22.31 76.99 0.15
208 -8.245 ag 0.46 23.94 75.47 0.13
209 -8.216 au 0 84.35 15.64 0.02
210 -7.063 ag 68.65 30.93 0.4 0.02
211 -6.547 ag 55.82 36.03 0.47 7.67
212 -6.547 ag 55.72 36.14 0.49 7.65
213 -6.351 au 0.04 97.38 2.56 0.02
214 -5.928 au 0.44 82.12 15.17 2.27
215 -1.311 ag 4.42 86.3 8.92 0.35
216 -1.311 ag 4.48 86.26 8.89 0.37
217 0.424 au 0.16 83.03 16.78 0.02
218 0.87 ag 0.94 14.37 84.41 0.27
219 0.9 au 7.62 12.21 79.67 0.5
220 0.908 au 7.61 10.2 81.59 0.6
221 0.917 ag 0 10.12 89.84 0.04
222 0.922 au 1.15 12.91 85.62 0.32
223 0.926 au 0.25 21.64 77.92 0.19
224 0.981 ag 3.4 6.54 88.99 1.07
225 1.003 ag 0.09 9.32 90.49 0.1
226 1.487 au 27.99 19.32 6.26 46.42
227 1.489 au 27.23 18.98 6.61 47.18
228 1.72 au 68.71 2.42 3.07 25.8
68
229 2.095 au 45.01 26.84 15.98 12.18
230 2.096 au 45.03 27.08 15.67 12.23
231 2.157 ag 46.22 22.9 9.38 21.51
232 2.42 au 0.75 94.54 4.45 0.26
233 2.775 ag 6.05 4.28 4.42 85.25
234 2.776 ag 6.19 4.46 4.15 85.2
(2-CNAz)2RuTPP (1)
MO Energy(eV) Symm. Ru Porphyrin Phenyl 2-CNAz
229 -9.023 ag 24.67 57 4.37 13.96
230 -8.575 au 0.05 22.5 77.42 0.03
231 -8.544 au 0.47 30.91 66.96 1.66
232 -8.533 au 0.05 2.51 97.43 0.02
233 -8.506 ag 0.48 1.97 96.71 0.83
234 -8.496 ag 0.71 3.91 95.09 0.29
235 -8.376 au 1.01 66.56 23.46 8.97
236 -8.372 au 0.1 13.54 86.01 0.34
237 -8.317 ag 0.3 20.65 78.5 0.55
238 -8.255 ag 0.39 22.88 76.63 0.1
239 -8.248 au 0 80.65 19.28 0.06
240 -7.972 au 0.05 2.34 0.05 97.56
241 -7.925 ag 0.19 24.79 1.05 73.97
242 -7.235 ag 69.21 30.33 0.44 0.02
243 -7.076 au 0 0 0.04 99.96
244 -7.076 ag 0 0.04 0.04 99.91
245 -6.72 ag 53.21 38.47 0.56 7.75
246 -6.579 ag 51.4 30.83 0.42 17.36
247 -6.382 au 0.04 97.39 2.55 0.03
248 -5.97 au 0.29 81.69 15.06 2.96
249 -1.571 au 0.05 1.39 0.08 98.48
250 -1.457 ag 0.01 32.48 2.99 64.52
251 -1.356 ag 3.92 86.87 8.76 0.45
252 -1.311 ag 6.14 56.91 5.94 31.01
253 -0.393 au 0 0.02 0.16 99.82
254 -0.393 ag 0 0.21 0.16 99.63
255 0.407 au 0.15 84.04 15.79 0.03
256 0.861 au 16.7 15.74 41.91 25.66
257 0.871 ag 0.87 14.71 84.18 0.24
258 0.898 au 8.58 12.54 76.12 2.76
259 0.919 ag 0.02 10.35 89.48 0.15
69
260 0.923 au 0.72 17.76 81.27 0.25
261 0.924 au 0.84 15.59 83.27 0.3
262 0.982 ag 3.8 6.16 87.8 2.24
263 0.986 au 5.08 9.56 44.3 41.07
264 1.004 ag 0.09 8.92 90.94 0.06
265 1.205 au 14.47 15.82 6 63.71
266 1.792 ag 5.71 0.41 2.25 91.63
267 1.804 au 79.29 2.77 3.63 14.32
268 2.033 au 46.1 24.96 14.87 14.07
(6-CNAz)2RuTPP (2)
MO Energy(eV) Symm. Ru Porphyrin Phenyl 6-CNAz
229 -9.149 ag 27.3 56.62 3.52 12.56
230 -8.62 au 0.08 27.59 72.17 0.16
231 -8.595 au 0.56 36.59 60.61 2.23
232 -8.568 au 0.04 2.48 97.48 0.01
233 -8.538 ag 0.5 2.97 95.99 0.54
234 -8.536 ag 0.75 2.81 96.15 0.29
235 -8.445 au 0.91 63.62 27.37 8.1
236 -8.411 au 0.07 11.44 88.47 0.02
237 -8.355 ag 0.42 20.56 78.94 0.08
238 -8.314 au 0 76.41 23.38 0.21
239 -8.299 ag 0.29 22.17 77.04 0.51
240 -7.986 au 0.12 1.3 0.02 98.56
241 -7.968 ag 0.36 17.73 0.79 81.12
242 -7.375 ag 69.51 29.99 0.42 0.08
243 -7.005 au 0 0.01 0.04 99.96
244 -7.005 ag 0 0.04 0.05 99.91
245 -6.862 ag 52.11 39.76 0.59 7.54
246 -6.75 ag 50.43 33.61 0.49 15.47
247 -6.457 au 0.03 97.39 2.57 0.01
248 -6.045 au 0.32 82.11 15.12 2.44
249 -1.768 au 0.4 1.98 0.15 97.47
250 -1.57 ag 0.21 29.4 2.63 67.77
251 -1.437 ag 3.85 87.09 8.57 0.49
252 -1.387 ag 6.84 60.48 6.32 26.36
253 -0.47 ag 0 0.62 0.39 98.98
254 -0.47 au 0 0.05 0.46 99.49
255 0.341 au 0.14 84.63 15.22 0.01
256 0.832 ag 1.1 14.87 83.07 0.96
70
257 0.851 au 12.66 13.4 65.69 8.24
258 0.862 au 8.87 13.3 76.57 1.27
259 0.881 ag 0.01 10.67 89.27 0.05
260 0.885 au 0.29 20.34 79.24 0.13
261 0.887 au 1.9 13.15 84.65 0.3
262 0.946 ag 3.93 6.19 87.88 2
263 0.966 ag 0.09 8.95 90.93 0.03
264 1.039 au 13.18 16.34 15.5 54.98
265 1.282 au 26.07 19.7 8.58 45.65
266 1.633 au 77.99 2.79 3.45 15.77
267 1.974 au 45.93 24.79 13.59 15.69
268 2.057 ag 44.21 20.29 7.71 27.79
a. HOMO and LUMO are in bold.
Figure 6. Experimental UV-vis (top) and TDDFT predicted (bottom) spectra of (t-BuNC)2RuTPP (A), (2-
CNAz)2RuTPP (B), and (6-CNAz)2RuTPP (C).
71
Redox Properties. The redox properties of the azulene containing porphyrins (2-
CNAz)2RuTPP and (6-CNAz)2RuTPP complexes were investigated by the electrochemical
and spectroelectrochemical approaches. The typical cyclic voltammetry (CV) and
differential pulse voltammetry (DPV) of two isomers are shown in Figure 7 and
numerically in Table 3. In all cases the first oxidation has been assigned to RuII/RuIII redox
couple based on the potential and spectroelectrochemical data. Similar to the
(FcNC)2RuTPP systems the RuII/RuIII oxidation potential depends on the nature of the axial
ligand. For instance, the (6-CNAz)2RuTPP complex has about 150 mV lower oxidation
potential compared to the (2-CNAz)2RuTPP complex. The second reversible oxidation
wave in (CNAz)2RuTPP was assigned to the oxidation of the porphyrin ligand. Finally, in
both isocyanoazulene compounds we have observed a broad irreversible oxidation peak at
higher potential, which was assigned to the oxidation of the axial ligands. The broad nature
of these oxidation waves allows us to suspect that the small separation between oxidation
potentials of two axial ligands can be seen in DCM/0.05 M TBAF system. However, since
the ion-pairing effects plays significant role in the observed electrochemical potentials and
the oxidation of the axial ligands is irreversible, we are not able to confirm the degree of
electronic coupling between the axial ligands. Finally, we were not able to observe any
reduction couples within the electrochemical window, which is similar to early discussed
RuTPP axially coordinated with isonitrile ligands. Overall, the sequence of the redox
events in isocyanoazulene containing ruthenium porphyrins is slightly different from that
observed in the case of earlier reported ruthenium tetraphenyl porphyrin axially
coordinated with isonitrileferrocenes. Because in isocyanoazulene compounds the axial
ligands oxidation waves are irreversible and were observed at the higher potentials
72
compared to the ruthenium and porphyrin oxidations.
In order to confirm the tentative assignments of oxidation waves observed in
electrochemical experiments we have conducted spectroelectrochemical measurements on
both isocyanoazulene systems. In the case of the (6-CNAz)2RuTPP the two clear
transformations were observed during spectroelectochemical experiments; during the first
oxidation the most intense Soret band undergoes reduction in intensity and higher energy
shift while Q-band undergoes red shift with the formation of three bands at 531, 569, and
641 nm. Similar transformation has already been observed in the case of the Ru porphyrins
axially coordinated with ferroceneisonitrile and (t-BuNC) and is very characteristic of the
formation of RuIII porphyrins. During the second oxidation step, the intensity of the Soret
band decreases and the formation of six new bands at 583, 632, 733, 971, and 1580 nm has
been observed in UV-Vis NIR spectra. The formation of broad, low intensity bands around
1000 nm is a very characteristic indicator of the formation of diffused porphyrin cation-
radical. Similar transformations have been observed in the case of (2-CNAz)2RuTPP
although the formation of rather broad and porphyrin cation-radical band around 1000 nm.
Overall the spectroelectrochemical data confirms our tentative electrochemical
assignments and correlate well with the previous reports on ruthenium(II)
73
tetraphenylporphyrin complexes axially coordinated with isocyanoazulene ligands.
Figure 7. DPV (red) and CV (blue) electrochemical data for (2-CNAz)2RuTPP
complex in DCM/0.05 M TBAF solution. In this case CV data was recorded at 100 mV/s rate.
Table 6. Oxidation potentials (V) for (RNC)2RuTPP complexes determined by electrochemical experiments
in DCM/0.05M TBAF system at room temperature.a
Complex RuII/RuIII TPP(2-)/TPP(1-) L/L+1
(2-CNAz)2RuTPP 0.033 0.631 1.051
(6-CNAz)2RuTPP -0.125 0.514 0.974
(t-BuNC)2RuTPP -0.023 0.713 -
a All potentials are referenced to the FcH/FcH+ couple and are ± 5 mV.
74
Figure 8. Spectroelectochemical oxidation of the (2-CNAz)2RuTPP complex in DCM/0.15M TBAF system
at room temperature.
Figure 9. Spectroelectochemical oxidation of the (6-CNAz)2RuTPP complex in DCM/0.15M TBAF system
at room temperature.
75
2.4. Conclusions
Two new complexes of the ruthenium(II) tetraphenylporphyrin axially coordinated with
two isocyanoazuelene ((2-CNAz)2RuTPP, (1) and ((6-CNAz)2RuTPP, (2) ligands [Az =
Azulene, TPP = 5,10,15,20-tetraphenylporphyrinato(2-) anion], were characterized using
UV-vis, MCD, NMR, IR, and ESI-MS spectroscopy as well as X-ray crystallography. The
redox properties of the new complexes 1 and 2 were probed using electrochemical (CV
and DPV), spectroelectrochemical, and correlated to those in the (t-BuNC)2RuTPP (3)
reference compound. In all cases, the first and second oxidation processes were attributed
to the reversible oxidation of the RuII center, and TPP(2-)/TPP(1-) respectively.
Spectroelectrochemical and chemical oxidation methods were used to elucidate
spectroscopic signature of the [1 and 2]n+ species in solution. DFT and TDDFT calculations
were used to correlate spectroscopic and redox properties of complexes 1, 2, and 3 with
their electronic structure.
76
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