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31293 01563 5216

This is to certify that the

dissertation entitled

METAL LOCALIZED PHOTOCHEMISTRY OF QUADRUPLY
BIMETALLIC COMPLEXES

presented by

SARA ANNE HELVOIGT

has been accepted towards fulfillment
of the requirements for

Ph.D. Chemistry

degree in

 

 

   

Major pro essor

Date \1 June. F131

MSU is an Affirmative A (lion/Equal Opportunity Institution 0- 12771

 

 

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WIMMHJ

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

METAL LOCALIZED PHOTOCHEMISTRY OF QUADRUPLY BONDED
BIMETALLIC COMPLEXES

By

Sara Anne Helvoi gt

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Chemistry

1997

ABSTRACT

METAL LOCALIZED PHOTOCHEMISTRY OF QUADRUPLY BONDED
BIMETALLIC COMPLEXES

By

Sara Anne Helvoigt

Multielectron reactions are fundamental to energy conversion, which is
illustrated in the biological systems, photosynthesis and nitrogenase. Many
synthetic systems developed use metal complexes capable of one electron
reactions. This requires the coupling of successive reactions to effect the net multi-
electron reaction. The goal of our research is to perform important discrete
multielectron photochemistry.

Our general approach has been to use quadruply bonded bimetallic
complexes, M—4—M. These complexes possess the zwitterionic excited state, 155*,
accessible with low energy excitation. Calculation of the electronic coupling
between the metal centers by Hush theory supports the tenet that the metals are
weakly coupled indicating that the valence electrons are contained in essentially
atomic orbitals as opposed to molecular orbitals. Therefore, the ground state, 28,
corresponds to one electron localized on each metal center and the 185* excited
state is a metal-to-metal charge transfer transition, which due to localization of two
electrons on one metal center is ionic. For symmetric M—4—M complexes, the

electrons have an equal probability of being localized on either metal center due to

the symmetry that exists for the molecule. This creates an excited state that is,
overall, nonpolar and inaccessable for photochemistry. Intramolecular distortions
are efficient at removing this symmetry and trapping the zwitterion. Studies of the
temperature dependence of the emission lifetimes of M2C14(PR3)4 (M2 2 M02,
MoW, W2; PR3 = PMe3, PMeZPh, PMeth) show that the 155* excited state is in
thermal equilibruim with a higher energy state, providing an additional
nonradiative decay pathway.

This excited state is predisposed to two electron reactions, such as oxidative
addition, as shown by the photoreaction of W2Cl4(dppm)2 and PhSSPh and EtSSEt.
However, the homonuclear complexes are limited to oxidative addition, typically
exhibited by substrates with low activation barriers, due to the loss of energy in the
excited state upon formation of the intermediate. This is illustrated in the reaction
of W2C14(dppm)2 with N20, which has a high activation barrier (59 kcal/mol) and
typically displays atom transfer chemisty. The inherent assymetry of
MoWCl4(PMe2Ph)4 avoids the need for distortions and atom transfer

photochemistry is observed with the substrate Ph3PS.

To Mom, Dad, Heidi and Joe

ACKNOWLEDGMENTS

I would like to thank Dan N ocera his encouragement and guidance over the
years. I am also grateful to everyone in the Nocera group, both present: J .P., Al,
Wanda, Dan, Eric, Jim, Jude, Deng and past members: Ann, Carolyn, Janice,
Claudia, Jeff, Janice, Doug, Mark, Zoe for all the stimulating conversations and
making the work fun. I would especially like to thank Carolyn Hsu for teaching me
so much and being so patient with me. The other members of the “quadruple bond
club” Dan Engebretson, Ann Macintosh, and Claudia Turro, were always willing
to discuss chemistry and help with experiments. J .P. Kirby and Al Barney were
always ready to goof-off with me whenever I got tired of writing or just being in
the building.

Kerry and Rob Cedergren, Susan Baker, Greg Noonan, Mike Thelen,
Charles N gowe, Carl Iverson made graduate that much more bearable and even
fun. I will miss the trips to Dag’s with Michelle Mac, Chris Powell, Per Askeland
and Matt Gardner and Friday happy hours. Michelle is always so goofy (“I didn’t
say she was crazy...”) that she can make anything fun. Chris has been, among
other things, my bestest buddy and drinking partner at MSU. These are the people
that I will miss the most when I am gone. Luckily, I am moving to the greater
Chicago area, where half of MSU already resides, so it won’t really be like

leaving.

Of course, I never would have made it this far without the glass shop or the
electronic shop. They were very understanding when everything had to be done
right away. Linda Krause, Lisa Dillingham, Beth Townsend and Long Le have
always been especially helpful and always with a smile. I don’t know how they do
it. I also appreciate all the help the Mass Spec facility has given to me over the

years, particularly Bev Chamberlin and Dr. Huang.

vi

 

 

TABLE OF CONTENTS
Pa

LIST OF TABLES ...................................................................................................
LIST OF FIGURES ................................................................................................ 1
LIST OF ABBREVIATIONS .............................................................................. x‘

CHAPTER 1 ............................................................................................................

INTRODUCTION ...................................................................................................

A- Photosynthesis6 ..........................................................................................................
B- Nitrogenase8

C- Multielectron Transfer ................................................................................................
D- Bimetallic Systems ....................................................................................................
E- Quadruply Bonded Bimetallic Complexes ................................................................ i
F- Photochemistry .......................................................................................................... i
G- Thesis Outline ........................................................................................................... i
H- References ................................................................................................................. 1

CHAPTER 2 ........................................................................................................... .

OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO

EXPERIMENTAL .................................................................................................. :

A. General Procedures ....................................................................................................

B. Synthesis .....................................................................................................................
1. Homonuclear Molydenum and Tungsten Complexes

a. M02Cl4(PR3)4 (PR3 = PMeg, PMezPh, PMePh2)3 .................................................
b. W2Cl4(PR3)4 (PR3 = PR3 = PMe3, PMezPh, PMeth, PBu3)4 ............................. :

C. W2Cl4(dppm)25 .................................................................................................... .
d. W2Cl6(dppm)26
e. W2Cl4(dppm)2(SPh)2 ........................................................................................... i
f. W2C15(dppm)2(SEt) .............................................................................................. 1
2. Heteronuclear Molybdenum-Tungsten Complexes ................................................ 1
a. Mo(6n-PhPMePh)(PMePh2)37 ............................................................................. :
b. Mo(6n-PhPMe2)(PMe2Ph)38 ................................................................................ .
c. WCl4(PPh3)29 ....................................................................................................... .
d. MoWCl4(PMePh2)4‘° .......................................................................................... .
e. MoWCl4(PMe2Ph)4“ ........................................................................................... .

vii

f. MoWCl4(PMe3)4l° ............................................................................................... 4

3. W(S)Cl4(PMe2Ph) ................................................................................................... 4
4. Reactions of W2Cl4(dppm)2 .................................................................................... 4
a. Photoreaction of W2Cl4(dppm)2 with PhSSPh, EtSSEt ...................................... 4
b. Thermal reactions of W2Cl4(dppm)2 with PhSSPh, EtSSEt ............................... 4
c. Photoreaction of W2Cl4(dppm)2 with N20 .......................................................... 4
5. Reactions of MoWCl4(PMe2Ph)4 ........................................................................... 4
a. Photoreaction of MoWCl4(PMe2Ph)4 with Ph3PS ............................................... 4
b. Thermal reaction MoWCl4(PMe2Ph)4 with Ph3PS .............................................. 4
6. Reactions of M2Cl4(PMe2Ph)4 with Ph3PS (M = M0, W) ...................................... 4
C- Instrumentation and Methods .................................................................................... 4
1. Photolysis ................................................................................................................ 4
2. Absorption Spectroscopy ........................................................................................ 4
3. Solution Infrared Spectroscopy .............................................................................. 4
4. Nuclear Magnetic Resonance Spectroscopy (NMR) .............................................. 4
5. Electron Paramagnetic Resonance Spectroscopy (EPR) ........................................ 4
6. Electron Spin Echo Envelope Modulation (ESEEM) ............................................. 4
7. Steady-State Luminescence Spectroscopy .............................................................. 4
8. Time-Resolved Laser Spectroscopy ....................................................................... 4
a. Nanosecond Lifetimes and Transient Absorption ............................................... 4
b. Picosecond Transient Absorption ........................................................................ 4
9. Variable-Temperature Emission, Absorption and Nanosecond Lifetimes ............. 4
l O. Electrochemistry ................................................................................................... 4
l 1. Mass Spectrometry ............................................................................................... 5
a. FAB/MS .............................................................................................................. 5
b. GC/MS ................................................................................................................ 5
D. References ................................................................................................................. 5
CHAPTER 3 ........................................................................................................... 5
PHOTOREDUCTION OF DIARYL/DIALKYL DISULFIDES BY
QUADRUPLY BONDED DITUNGSTEN COMPLEXES ................................... 5
A. Background ............................................................................................................... 5
B - Photochemistry of W2C14(dppm)2 and PhSSPh ........................................................ 5
1. Quantum Yields ...................................................................................................... 6
C. Transient Absorption ................................................................................................. 6
D. Photochemistry of W2Cl4(dppm)2 and EtSSEt .......................................................... 7
E. Conclusions ............................................................................................................... 7
F. References .................................................................................................................. 7
CHAPTER 4 ........................................................................................................... 7
ATOM TRANSFER PHOTOCHEMISTRY OF W2C14(dppm)2 AND
MOWCI4(PMe2Ph)4 ................................................................................................. 7
A. Background ............................................................................................................... 7
viii

1. Atom Transfer ......................................................................................................
B. Photoreaction of W2Cl4(dppm)2 and N20 ..............................................................
1. Characterization of Inorganic Products ...............................................................
2. Characterization of Organic Products ..................................................................
3. Reaction Pathway ................................................................................................
C. Photoinitiated Sulfur Atom Transfer to MoWCl4(PMe2Ph)4 .................................
1. Photophysics ........................................................................................................

4. Quantum Yields ...................................................................................................

5. Electron Paramagnetic Resonance Spectroscopy ................................................
6. Electron Spin Echo Envelope Modulation72 ........................................................

a. Nuclear Modulation Effect72 ............................................................................
D. Summary ................................................................................................................
E- Future Directions ....................................................................................................
F- References ...............................................................................................................

CHAPTER 5 .........................................................................................................

A STUDY OF THE NONRADLATIVE DECAY OF M2C14(PR3)4 .....................

A- Background ............................................................................................................
B . Temperature Dependence of Emissive Lifetimes ...................................................

C- N onradiative Decay Theoryll .................................................................................
1 . Calculation of the Huang-Rhys Factor ................................................................

2. Weak Coupling Limit“ .......................................................................................
a. Low-Temperature Limit of Weak Coupling .....................................................

b. Temperature-Dependent Limit of Weak Coupling ............................................

3 . Intervalence Charge Transfer ................................................................................
D- Results and Discussion ............................................................................................
E. References ...............................................................................................................

ix

 

LIST OF TABLES

Page
Table 1.1 8 —)8* Electronic Transition Energies For Selected Quadruply Bonded
Bimetallic Complexes ...................................................................................... 22
Table 3.1 Wavelength Dependent Quantum Yields for the Photoreaction of
M2Cl4(dppm)2 and PhSSPh ............................................................................... 61
Table 4.1 Bond Dissociation Energies for Atom Transfer and Oxidative Addition
Substrates ........................................................................................................ 100
Table 4.2 Physical PrOperties of M2C14(PMe2Ph)4 .............................................. 107
Table 4.3 Wavelength Dependence of Quantum Yields for Photoreaction of
MoWCl4(PMe2Ph)4 with Ph3PS ..................................................................... 115
Table 4.4 Representative g-values of MoV Octahedral Complexes ..................... 118

Table5.1 Vibrational Frequencies of the Excited and Ground State for Several M--
4—-M Complexes and Their Corresponding Change in the M—M Bond Length in the
Excited State ............................................................................... 143.

Table 5.2 Temperature Dependence of AE, k1 and k2 of M2C14(PR3)4 ................ 153
Table 5.3 Temperature Dependence of (be, 13, kr and km of M02C14(PMe3)4 ......... 155
Table 5.4 Temperature Dependence of (be, 1:, kr and km of M02C14(PMe2Ph)4 ..... 156
Table 5.5 Temperature Dependence of (be, I, kr and km of M02C14(PMePh2)4 ..... 157

Table 5.6 Fractional of Decay Through the Upper Excited State at 290K and 40K

. ....................................................................................................................... 158
Table 5.7 kr and krlr for a Series of M02X4(PR3)4 Complexes .............................. 160
Table 5.8 Calculated Excited and Ground state M—M Bond Lengths, Experimental

M—M Bond Lengths and the Calculated Huang-Rhys Factor. ....................... 162
Table 5.9 Temperature Dependence of (be, t, kr and km of W2C14(PMe3)4 ........... 168

Table 5.10 Temperature Dependence of (be, 1', k, and km of W2C14(PMe2Ph)4 169

Table 5.11 Temperature dependence of (be, t, kr and km of W2Cl4(PMePh2)4 ..... 170

Table 5.12 Temperature Dependence of (be, 1:, kr and km. of MoWCl4(PMe3)4 1’
Table 5.13 Temperature Dependence of (be, 1:, RI and km of MoWCl4(PMe2Ph)4 1'
Table 5.14 Temperature Dependence of (be, 13, kIr and km of MoWCl4(PMePh2)4 1'

Table 5.15 emax, vmax, AT/V2 and Calculated H AB for M2Cl4(PR3)4 ....................... 1:

xi

LIST OF FIGURES
Pag

Figure 1.1 Zigzag (Z) scheme of the electrons and protons in the thylako:
membrane .................................................................................

Figure 1.2 The schematic representation of N 2 fixation by N itrogenase .............
Figure 1.3 Strategy for the development of multielectron photochemistry ..........

Figure 1.4 Mechanism of photoinduced charge separation in layered zirconiu:
Viologen phosphonate compounds .....................................................

Figure 1.5 Assembly of thin film chromophores. The first step involw
attachment of the Ru chromophore (1%). The second step is the parti
hydoloysis of the remaining 802 groups (<5%) .................................... 1

Figure 1.6 Schematic representation of the events occuring within the polymer
film following excitation (reference 20) ..................................... _ ........ 1

Figure 1.7 Mechanism for the formation of associative biradicals (a) ar
dissociative biradicals (b) ............................................................... 1

Figure 1.8 Upon irradiation of Cp'2M02(CO)5L the mixed valence carbon;
bridged intermediate forms. This complex disproportionates upon coordinatic
Of a second phosphine ligand ......................................................... 1

Figure 1.9 Qualitative molecular orbital diagram for M2L3 quadruply bonde
bimetallic complexes ................................................................... 2

Fi gure 1.10 State diagram for the 88* manifold of M02C14(PMe3)4 ................. 2

IFigure 1.11 Proposed mechanism for photochemical reduction of 1,2
dihalocarbons by M02[02P(0Ph)2]4 .................................................. 2

IFigure 1.12 A comparison of the photochemistry of W2C14(dppm)2 and t1
thermal chemistry of Vaska’s complex for the oxidative addition of alkj
iodides .................................................................................... 2'

xii

Figure 3.1 The two electron oxidation of [Ph4P]2[Moz(OzCPh)4C12] by CC14. Two
benzoate ligands rearrange from u—nz to u—nl bonding modes .................. 54

Figure 3.2 The formation of an edge-sharing bioctahedron (ESBO) upon two
electron oxidation of a quadruply bonded bimetallic complex ................... 55

F‘igure 3.3 Electronic absorption spectral changes during photolysis of toluene
solutions of W2Cl4(dppm)2 with excess PhSSPh at —10 °C ........................ 58

Figure 3.4 FAB/MS spectrum of the photoproducts isolated from the irradiation of
toluene solutions of W2Cl4(dppm)2 and excess PhSSPh ........................... 59

Figure 3.5 Quantum yields for the photolysis of M2Cl4(dppm)2 and excess
PhSSPh. Panel (a) is W2Cl4(dppm)2 in toluene at 0 °C; (b) is MozCl4(dppm)2 in
CH2C12 at 20 0C ......................................................................... 62

Figure 3.6 Qualitative correlation diagram for D2h and D2d symmetry quadruply
bonded bimetallic complexes. (reference 17) ....................................... 64

Figure 3.7 Picosecond transient absorption of M02C14(dmpm)2 in CH2C12
following a 3 ps excitation pulse at 600 nm. Spectra were recorded at 2, 20 and
50 ps following excitation. The inset shows the ln plot of the recovery of the
bleach at 630 nm ...................................................... . .................. 66

Figure 3.8 Comparison of the transient absorption, following a 532 nm excitation
pulse, of W2Cl4(dppm)2 (o) to the ground state absorption of W2Cl6(dppm)2 (—
), both are in benzene .................................................................. 67

Figure 3.9 Formation of the chemically distorted ESBO upon excitation of the
71:8* absorption band of M2Cl4(LL)2 complexes .................................... 69

Figure 3.10 Electronic absorption spectral changes during photolysis of toluene
Solutions of W2Cl4(dppm)2 with excess EtSSEt at —20 °C ........................ 71

IFigure 3.11 (a) Spectral absorption changes associated with the irradiation of
W2Cl4(dppm)2 and EtSSEt in toluene at —20 °C. The irradiation was stopped
after 1 hour and 45 minutes. (b) FAB/MS spectrum of the photoproducts
isolated from the final spectrum above .............................................. 72

E lgllre 4.1 General reaction mechanism for atom transfer reactions ................ 79

IFigure 4.2 Comparison of the coordination spheres of MC12P4 and M2C14P4.
Dissociation of a phosphine ligand is the first step in the reactions of MC12P4
complexes. As a result, atom transfer occurs at a square pyramidal
coordination sphere in both complexes .............................................. 82

xiii

Figure 4.3 The photoreactivity of szTa(C2H4)CH3. The coordinative
unsaturated "szTaCH3" is generated upon photolysis

szTa(C2H4)CH3 ........................................................................ E

Figure 4.4 (a) Spectral changes observed upon irradiation of W2Cl4(dppm)2 a1
N20 in THF. (b) AbSorption spectrum of independently synthesize

W2C16(dppm)2 ........................................................................... E
Figure 4.5 1H NMR of photoreaction of W2Cl4(dppm)2 and N20. The * indicat
resonances due to W2Cl6(dppm)2 ..................................................... 9

Figure 4.6 Products from the photolysis of THF (kexc > 185 nm). Quantum yiell
are shown in parenthesis ............................................................... 9

Figure 4.7 FI‘IR spectrum of organic side products from the photolysis
W2Cl4(dppm)2 and N20 in a THF solution .......................................... 9

Figure 4.8 The formation of the Ni(II) alkoxide by reaction of bpyNi(C4H8) wi
N20. A variety of functionalized hydrocarbons can be generated und
different reaction conditions .......................................................... 9

Figure 4.9 Proposed mechanism for the formation of the mixed valence excite
state of heteronuclear and homonuclear quadruply bonded bimetall
complexes .............................................................................. 1(

Figure 4.10 Formation of the mixed-metal dimers from MC12P4 at
M(S)C12P3 ............................................................................. 1(

Figure 4.11 Absorbance spectra of M02C14(PMe2Ph)4 (— -), MoWCl4(PMe2Ph
(——), and W2Cl4(PMe2Ph)4 (- - -) in benzene ...................................... 10

Figure 4.12 Electronic absorption and emission spectra of the 86* transition «
MoWCl4(PMe2Ph)4 in benzene ...................................................... 10

Figure 4.13 Spectral changes associated with the photolysis (it > 375 nm) .
MoWCl4(PMe2Ph)4 and Ph3PS in benzene ........................................ 10

Figure 4.14 1H NMR of the photoreaction upon photolysis of MoWCl4(PMe2Ph
and Ph3PS. * denotes resonances due to MoW starting material ............... 11

Figure 4.15 31P{1H} N MR of the photoreaction of MoWCl4(PMe2Ph)4 and Ph3P.
........................................................................................... 11

xiv

Figure 4.16 FTIR of benzene solutions of (a) the neat photoreaction and 4
W(S)Cl4(PMe2Ph). The absorbances between 550 and 400 cm in (a) are d
to Ph3PS and Ph3P ..................................................................... 11

Figure 4.17 The X-band EPR spectrum of the photoreaction in a 2—MeTl
glass ..................................................................................... 1

Figure 4.18 (a) The inhomogeneously broadened EPR line of a paramagne
center. (b) Each spin packet can be considered independently from each oth
each having its own Larmor frequency .................. . .......................... 1.

Figure 4.19 Magnetization of spin packets i and j during a two pulse experime
(a) during a 115/2 pulse; (b) after time t; (c) after the 7: pulse; (d) time t after 1
TE pulse .................................................................................. 12

Figure 4.20 (a) Energy level diagram for a S = 1/2 and I = 1/2 system. Ti1
behavior for the magnetization of "forbidden" spin packet A and "allowe
spin packet B. (b) at time t after the 7t/2 pulse; (c) after the 1t pulse; ((1) at t
time of the echo ........................................................................ 12

Figure 4. 21 Fourier transforms of the photoproduct spin- echo envelope. (a) 11»
measured at 3260 G and ‘C— - 300 ns and (b) was measured at 4375 G and I

250 ns .................................................................................... 12
Figure 5.1 Electronic absorption spectra of M02C14(PMe2Ph)4 (— —) a
W2Cl4(PMe2Ph)4 (- - -) 1n benzene .................................................. 1.
Figure 5.2 Potential energy diagram for nonradiative excited state decay ........ 1‘

Figure 5.3 Potential energy wells in the weak coupling limit (a) and the stro
coupling limit (b) of nonradiative decay ........................................... 1‘

Figure 5.4 Fit of the variation of the observed emission decay constant
M02C14(PMe3)4 to equation 5.4 in the 40-290 K temperature range ............ 11

Figure 5.5 Fit of the variation of the observed emission decay constant
M02C14(PMe2Ph)4 to equation 5.4 in the 40-290 K temperature range ......... 1.‘

Figure 5.6 Fit of the variation of the observed emission decay constant
M02C14(PMePh2)4 to equation 5.4 in the 40-290 K temperature range ......... 1:

Figure 5.7 Energy gap plots for M02C14(PR3)4 at 290 K (0) and 40 K (0) ........ It

Figure 5.8 Fit of the variation of the observed emission decay constant
W2Cl4(PMe3)4 to equation 5.4 in the 40-290 K temperature range ............. 11

XV

Figure 5.9 Pit of the variation of the observed emission decay constant 1
W2C14(PMe2Ph)4 to equation 5.4 in the 130-290 K temperature range. . . . . 16

Figure 5.10 Fit of the variation of the observed emission decay constant 1
W2C14(PMePh2)4 to equation 5.4 in the 40-300 K temperature range .......... 16

Figure 5.11 Energy gap plots for MoWCl4(PR3)4 (o) and W2Cl4(PR3)4 (c) at t
and 40 K, respectively ................................................................ 17

Figure 5.12 Fit of the variation of the observed emission decay constant 1
MoWCl4(PMe3)4 to equation 5.4 in the 60-280 K temperature range .......... 17

Figure 5.13 Fit of the variation of the observed emission decay constant 1
MoWCl4(PMe2Ph)4 to equation 5.4 in the 60-290 K temperature range ...... 17

Figure 5.14 Fit of the variation of the observed emission decay constant 1
MoWCl4(PMePh2)4 to equation 5.4 in the 60-290 K temperature range ...... 17

Figure 5.15 Proposed energy diagram for MozCl4(PMe2Ph)4 ...................... 18

xvi

 

ATP
bpy
tBu-LNS
3-Clpyr

DMA
dmpm
dppm
edta
ESBO
IT
LI-
LMCT
4~Mepyr
MLCT
MMCT
M~3.5—M

LIST OF ABBREVIATIONS
adenosine diphosphate
adenosine triphosphate
bipyridine
bis(p—tert-butylphenyl)-2-pyridylmethanethiolate1’
3—chloropyridine
methlycyclopentane
p-cyano-N,N-dimethylaniline
bis(dimethylphosphino)methane
bis(diphenylphosphino)methane
ethylenediaminetetracetate
edge-sharing bioctahedron
intervalence charge transfer
bidentate ligand
ligand-to-metal charge transfer
4-methylpyridine
metal-to-ligand charge transfer

metal-to-metal charge transfer

one electron oxidized quadruple bond

xvii

 

M—4—M
N ADP+
NADPH
NO
OEC
OEP
PED
phen
P0P

PQ+
pr-salen
PR3

pyr

PYZ
tetraphos
TPP
T’I‘P

quadruple bond

nicotinamide adenine dinucleotide phosphate (oxidized form)
nicotinamide adenine dinucleotide phosphate (reduced form)
p-cyano—N,N-dimethy1aniline N -oxide

oxygen evolving complex

octaethylporphyrinatoz'

1-phenyl-l,2-ethanediol

1 , 10-phenanthroline

PZOSHZ'

paraquat

N ,N’-ethylenebis(2,2’-dipropylsalicylideneiminato)2’
monodentate phosphine

pyridine

pyrazine

thPCHZCH2P(Ph)CHzCH2P(Ph)CH2CH2PPh2
meso-tetraphenylporphyrinatoz'

meso—tetrakis(p-tolyl)porphyrinatoz'

xviii

 

CHAPTER 1
INTRODUCTION

The importance of electron transfer reactions is reflected in the sheer

volume of literature that has been generated on the subject. Multiple electron
oxidation-reduction reactions are typical of many biological and chemical
transformations; these reactions, such as small molecule activation, include H20
——) H2 + 1/2 02 (photosynthesis),l N2 + 3H2 —> 2NH3 (nitrogenase),2 SO32’ —> HS—
(sulfite reductase),3 and N03" —> N02" (nitrate reductase).4 The success of these
chemical transformations relies on the ability to overcome large kinetic and/or
thermodynamic barriers. Electronically excited transition metal complexes are
useful in this endeavor since the increased chemical potential provides the driving
force necessary to surmount the barriers that exist for the ground state partner.
EXcitation of a molecule promotes an electron to a high energy orbital generating
an electron hole. In this new excited state configuration, the molecule is both
easier to oxidize and reduce with the availability of the low energy hole and the
high energy electron, respectively. Most attempts at multielectron photochemistry
have relied on the one electron hole/charge separation/storage strategy that is
inspired by biological systems, such as photosynthesis and nitrogenase.

Although the driving force of photosynthesis and nitrogen fixation differs,

fundamentally both involve systems that utilize a series of 1 or 2 electron transfer

 

reactions designed to prevent back electron transfer and store electrons in orde1
achieve the net reaction. Enzymes and membranes are basically la
supramolecular arrays where the main purpose of the protein and lipids is
impose long range order for the specialized donor and acceptor molecules.5 ']
efficiency of energy and electron transfer is a function of the donor-accep
distances, their orientation and environment, thus requiring long raI

organization.
A. Photosynthesis6

The photosynthetic process in green plants takes place in the thylakt
membrane of chloroplasts and consists of 2 reaction centers, Photosystem I am.
(PSI and PSII), where the reduction of NADP+ and the oxidation of water occ
respectively. P680, a specialized chlorophyll, is central to NADP+ reduction. '1
subscript, 680, refers to the Xmax of the excitation wavelength for this chloroph
dimer. Excitation of P630 generates the excited state, P680*, which is a both a strc

reductant and oxidant. P630* reduces pheophytin generating P6802 the pheophy
in turn reduces plastoquinone QA and the electron is finally transferred
Plastoquinone QB. Upon reduction by 2 electrons, QB is protonated by 2 prot<
and leaves the binding site as QBHZ. This is then replaced by another plastoquinc
from the quinone pool. This process is shown schematically in Figure 1.1. QBH;
1‘ eoxidized by the cytochrome b6/f complex to result in transmembrane pI‘Ol
transfer6 and the electrons are transferred to plastocyanin and finally to PS1. H:
the absorption of a photon by the P700 reaction center of PSI initiates elect1

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ferrodoxin NADP“ reductase, where NADP+ is reduced to NADPH. To complete
the cycle, P6362 a strong oxidant, is reduced by a tyrosine residue, Y2, which is in
turn reduced by the Mn; oxygen evolving complex (OEC). The OEC goes through
5 oxidation states, S0—>S4, and in the S4 oxidation water is oxidized to release H“,

which is used in the production of NADPH, and Oz.
B. Nitrogenase8

The enzyme nitrogenase and the Haber-Bosch process each account for the
conversion of approximately 108 tons of N2 to NH3 per year.9 The Haber-Bosch
process uses an Fe catalyst and temperature and pressure as high as 450 °C and
250 atm to overcome a large kinetic barrier. Biology’s counterpart, nitrogenase, is
able to perform this reaction at ambient temperature and 1 atm. This is possible
due to the sophisticated mechanism of the enzyme, shown schematically in Figure
1.2. Nitrogenase consists of two proteins, an Fe protein and MoFe protein.

The Fe protein has two subunits and contains one redox active [Fe4S4]'+/2+
cluster. It is the site of MgATP hydrolysis and has two binding sites for MgATP,
one on each subunit. The protein is reduced in a series of 2-electron steps by a low

1+] 2+ cluster.8 Upon

potential ferredoxin or flavodoxin, probably via the [Fe4S4]
hydrolysis of 2 MgATP molecules two electrons are transferred by this cluster to
the MoFe protein using dithionite as an electron donor. To complete the Fe protein
cycle, the protein is reduced by S02", two MgADP molecules are released and
replaced by two MgATP to be hydrolyzed again.

The MoFe protein is made up of four subunits in an 01202 arrangement and it

contains two P clusters ([Fe4S4]2(S)2) and two FeMo cofactors (MoFe7SB_9), the

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structures of which were recently solved.10 The proposed function of the P clusters
is to transfer electrons from the Fe protein to the FeMo cofactor. N 2 is most likely
reduced at the FeMo cofactor, although the mode of binding has not been
established. While a total of 8 electrons are transferred between the proteins, N2 is
released before the full complement of electrons has been transferred. It is believed
that the coupling of MgATP hydrolysis to the electron transfer step is necessary
either to overcome the unfavorable activation energy or to prevent back electron

transfer.2
C. Multielectron Transfer

This thesis is concerned with effecting multielectron reactions with a
photon. The strategy for multielectron processes is derived from biological
systems, such as the ones above. Figure 1.3 summarizes the strategy from a
photochemical perspective. A receptor molecule acts as a light harvester to initiate
charge separation. An electron is promoted to the excited state, generating a hole,
and is moved spatially away through a series of electron transfer steps. The design
goal of molecular systems is to ensure forward electron transfer that is more
efficient than the recombination of the electron with the hole. This is difficult in
photochemically driven charge separation schemes because inevitably back
recombination is a thermodynamically favorable process. However, the
recombination becomes less favorable as the electron and the hole become more
spatially separated due to the inverse exponential distance dependence of the

electron transfer rate.ll

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Successful schemes, therefore, are ones that can achieve charge separation
and impose some sort of long range order on the system. Some strategies have
utilized sol-gel glasses,12 clays,13 colloids,l4 polymer films15 , hydrogen bonding16
and zeolitesl7 to achieve organization of the photosensitizers and electron donors
and acceptors. The importance of spatial organization can be observed in the
photoinduced charge separation in layered zirconium Viologen phosphonate
compounds. The irradiation (7t = 300 nm) of thin films of
Zr(O3PCH2CH2(bipyridinium)CH2CHZPO3)X2 (X = Cl, Br), produces the
characteristic blue color of a Viologen radical with a photochemical quantum yield
greater than 0.03.18 When the UV light is removed the blue color of the reduced
Viologen gradually fades. The photochemistry involves a Viologen centered
transition, probably a n—>7c* transition, creating a strong oxidant which oxidizes

the X— contained between the Viologen appendages, equations 1.1 and 1.2.

hv *
WVIOL2+W ——> M’{VIOLZ+}W~A (1.1)
>1:
VV\’{VIOL2+}WV + X- ———'> WVIOL.+W ‘l‘ X . (1.2)

The halide radical then abstracts a hydrogen from the a-methylene on an adjacent
Viologen group. The incipient methylene-based radical can be stabilized by
delocalization onto the pyridine, Figure 1.4, resulting in charge separation that is
stable for several hours under vacuum or under Ar.

Charge separation, while a major research area on its own, is only the first

step to multielectron photochemistry. The charge separation step needs to be

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coupled to a catalyst, such as M(bpy)3"+, which can either oxidize or reduce a
substrate. The metal-to—ligand charge transfer (MLCT) excited state of polypyridyl
complexes of Ru(II), Os(II), and Re(I) typically undergo efficient oxidative or
reductive electron transfer quenching in solution generating separate redox

products,19 equations 1.3 and 1.4.

hv *
Ru<bpy)32* —> Ru<bpy)32* 0-3)
Ru(bpy)32+'+ Po2+ ——> Ru(bpy)33++PQ+ (1.4)
Ru(bpy)33++ 130* ——> Ru<bpy)32++ P02+ (1.5)

bpy = 2,2'-bipyridine; PQ2+ = Me—ND—CN—Me

The ability to harvest the stored redox equivalents is hindered by the fast
recombination of Ru(bpy)33+ and PQ”, equation 1.5. This quenching can be
controlled by mounting the Ru2+ on a polymer film.20 For instance, (bpy)2Ru(5-
NHzphen)2+ can be attached to a chlorosulfonated polystyrene ([—CH2CH(p-
(1‘6H4SOZCl)—]n = PS—SOZCI) on 21 Pt electrode at a 1% loading. The chromophore
is added to the polymer by soaking the PS-SOzCl film in a Ruz” solution; the
concentration of Ru2+ is therefore greatest at the top of the film and lowest near the
electrode. Subsequent partial (< 5%) hydrolysis of the remaining sites creates ion

channels in the film (Figure 1.5). The hydrolysis of PS-SOgCl also provides an ion-

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attachment of the Ru chromophore (1%). The second step is the partial
hydroloysis of the remaining S02 groups (<5 %).

12

The partially hydrolyzed films with the chromophore and PQ2+ generate a
current when irradiated in the presence of an acetonitrile solution of
tetraethanolamine (TEOA) as an oxidative scavenger, Figure 1.6. The absorption
of light by Ru2+ generates RUN, which is oxidatively quenched by PQ2+ to give
PQ“. The reduction equivalent (an electron) is transferred through PQ+ migration
and electron transfer to the Pt electrode. Ru?” is reduced by TEOA that has
diffused through the surface of the film and upon one electron oxidation TEOA
decomposes. A photocurrent is generated with a quantum yield of about 0.14. The
photocurrent comes from the diffusional quenching of Ru2+ sites that are near SO3‘
ion channels created in the hydrolysis step. This system has combined spatial
organization and the mobilization of electron transport species to create a current.

As demonstrated above, whereas systems developed for ‘multielectron
photochemistry10 are diverse and sophisticated, they all have a common feature of
single metal centers capable of only single electron reactions.21 An approach
pursued in the Nocera group is to tie two metal centers together, such as a
bimetallic complex, in order to couple the reactivity of the redox centers to achieve

multielectron reactions.

D. Bimetallic Systems

Most bimetallic complexes are of the three basic structure types shown
below.” Type I dimers have bridging atoms, ions or groups holding the metals
together. There is no direct metal interaction in this structure type. Dimers of type
II are held in close contact by bridging ligands. While there is intermetal

interaction, there is no M—M bond. In dimers of type 111, there is a direct M—M

13

 

 

/ OA+
/ R”2+ \
/ dec
Pt electrode film solution

Figure 1.6 Schematic representation of the events occuring within
the polymeric film following excitation (reference 20).

 

bond that, for this discussion, is a single bond. X, in this case, may be either a

monodentate or bidentate ligand.

 

 

i i
M—X—M M M M—M

I |

L L

In linearly bridged dimers, structure I, the two metals are typically in
different oxidation states and are bridged by either an atom or a bridging ligand,
such as Cl‘ and CN’, respectively.” These complexes exhibit low energy metal-to-
metal charge transfer (MMCT) transitions from the reducing metal to the oxidizing
metal via the bridging group. The photochemistry of these complexes is dominated
by rapid back electron transfer. For example, upon excitation, the binuclear
complex (NH3)SOsm—NC—M"(CN)5’ (M = Fe, Ru, Os) undergoes MMCT to form
the redox isomer, (NH3)_<,Os"—NC—Mm(CN)5".21 This isomer is substitutionally
inert and through thermal back electron transfer forms the original complex. For
complexes that are substitutionally labile, decomposition can result before back
electron transfer occurs. This is observed for (PPh3)3CuI(u-C1)FemCl3, which
undergoes photolysis in low polarity solvents.22 Upon excitation of low energy
MMCT transitions, the complex decomposes to Cu2+ and Fe2+.

In structure II, there is no formal M—M bond in the ground state. Upon

excitation, a M—M bond is formed, giving rise to an associative diradical excited

15

state,21 Figure 1.7. This excited state is characteristic of d8—d8 systems, such as
Pt2(PzOsH2)44‘ (= Pt2(pop)44’)23 and Rh2(1,3--diisocyanopropane)42+,24 dlo—d10
systems, such as Au2(dppm)22+ (dppm = bis(diphenylphospino)methane)25 and dm—
(18 systems, AuPt(dppm)2(CN)2+.26 The excited states of the above molecules are
quenched by halogen atom transfer agents, such as alkyl and aryl halides, and
hydrogen atom donors, such as (CH3)2CHOH, PhCH(OH)CH3, Bu3SnH, Et3SiH
and H3PO3. In the two electron reaction of the triplet excited state of Pt2(pop)44',
halogen atom abstraction is the first step (equation 1.6) followed by
comproportionation of two Pt2(II,III)(pop)4X4' complexes to give
Pt2(III,IH)(pop)4X24' and Pt2(II,II)(pop)44' (equation 1.7).23 This net two electron

reaction is observed through consecutive one electron reactions.

Ptz(pop)44"* + RX —> Pt2(pop)4X4_ + R . (1.6)

2 Pt2(pop).x4‘ —-> Ptztpop)4x24‘+ Pt2<pop)44" (1.7)

The lowest energy excited states of singly bonded bimetallic d7—d7 and d9—
(19 systems, structure 111, correspond to o—>o* and rtd—>o* transitions.27 Excitation
of these dimers results in dissociative diradicals formed from the homolytic

cleavage of the M—M bond to yield 17 electron radicals as shown in equation 1.8.

M2(CO)10 —h—V—» 2 oM(CO)5 (1.8)
alkane

The formation of a dissociative diradical can only give rise to one electron

products (Figure 1.7). The radical produced can be trapped by halogen donors,

Associative Biradical

 

 

 

 

 

 

 

 

Dissociative Biradical

 

 

 

 

 

 

 

 

Figure 1.7 Mechanism for the formation of associative
biradicals (a) and dissociative biradicals (b).

 

l7

polar solvents or other metal radicals. Re(CO)5Cl is formed with a quantum
efficiency of 0.6 from the irradiation of CCl4 solutions of ReQ(CO)10,
demonstrating that this is a highly efficient process.28 The rate of photolytic M—M
cleavage is >1010 3’1 and competes effectively with the cleavage of M—CO bonds;
however, irradiation of M2(CO)10 in the presence of good ligating substrates, such
as PPh3, results in products that are due to CO substitution.29

While the typical reaction of M—M bonded carbonyl dimers, M2(CO)10, is

2137129 this structure exhibits

M—M homolytic cleavage or M—CO bond dissociation,
another reaction type30 that is important to the work described in this thesis. In
addition to homolytic cleavage, internal charge transfer can result in heterolytic
cleavage to afford ionic products. The lowest energy bands of metal carbonyl
dimers are generally assumed to be rtd——>o* and o——>o* transitions“ and

accordingly, they have been assigned as such in the dimer Cp'2M02(CO)6.

Irradiation of Cp'2M02(CO)6 with L (= PR3) proceeds as shown below.32

Cp’2M02(CO)6+L M» Cp’Mo(CO)2L2++Cp'Mo(CO)3— (1.9)

The primary photoproduct is Cp'2M02(CO)5L. Excitation of the 100* transition of
Cp’2M02(CO)5L causes the formation of a carbonyl bridged intermediate.
Coordination of a second ligand forms a mixed valence intermediate and polarizes
the M—CO-M unit. This induces inner sphere electron transfer, which leads to
disproportionation to form Cp’Mo(CO)3" and Cp’Mo(CO)2L2+. This reaction
mechanism is summarized in Figure 1.8. Disproportionation of Cp’2M02(CO)5L is
only observed upon irradiation with it > 290 nm. This wavelength dependence

implies

18

 

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that the M—M bond homolysis, through the 300* state, is not sufficient to induce
the disproportionation.

In the system above, two electrons are paired on one metal center which
leads to the formation of ionic products. Is it possible to perform similar chemistry
in an excited state where two electrons could be localized on one metal center of a
bimetallic dimer? This type of excited state, known as a zwitterion, was proposed
to exist in systems where two electrons reside in weakly coupled orbitals33 and has
been the subject of theoretical studies for over 50 years. It has roots in organic
chemistry with twisted ethylene,34 and in physical chemistry with stretched
hydrogen.” For the latter, there is significant overlap between the atomic orbitals
of hydrogen in its equilibrium state; when the bond is stretched from its
equilibrium position, however, the overlap decreases. The situation is also similar
in twisted ethylene. As one half of the molecule twists relative to the other, the
overlap of the it orbitals decreases and at 90° the overlap is zero. As the o orbitals
in hydrogen and 7t orbitals in ethylene become more weakly coupled, a molecular
orbital description of the atomic orbitals is no longer appropriate and the electronic
structure is better described by a valence bond description.

Four states arise from two electrons in two weakly coupled orbitals. Using
H2 as the example, these states are '02, 300*, 1oo*, 10*2. The two lower energy
states, 102 and 366*, are diradical, while the two higher energy states, 100* and
10*2, are zwitterionic in nature. In the diradical states each orbital is half filled,
while in the zwitterionic state the electrons are spin paired. The lower energy
diradical, '02, corresponds to two electrons spin paired in bonding orbitals and

366* has one electron each in a bonding and antibonding orbital with parallel

20

spins. The lower energy zwitterion, 100*, has paired electrons in a bonding and
antibonding orbital, while 10*2 has a doubly occupied antibonding orbital.
Zwitterions are by definition singlet excited states and are localized on one center.
The problem with identifying zwitterionic excited states in o and 1t manifolds, is
that the states are not stable. It is difficult to stretch a 0 bond to a stable
configuration for spectroscopic investigation. Inevitably the excitation places the
system on a dissociative surface and the molecule dissociates.36 Similarly, it is
difficult to twist an olefin and trap it in its 90° configuration. This has led the
Nocera group to consider 8 bonds as the place to find the zwitterionic excited
states. Ironically, the weakly coupled orbitals necessary for the formation of this
excited state are found in complexes with some of the shortest M—M bond

distances, quadruple bond complexes.37
E. Quadruply Bonded Bimetallic Complexes

The qualitative molecular orbital diagram for the D”, symmetry M2L8
complexes is shown in Figure 1.9. Two ML4 fragments are brought together to
form a o (dzz), two TC (dxz, dyz), and a 5 (dxy) bond. Each metal is (14 leading to four
M~M bonds. The lowest energy transition is between the highest occupied

mOlecular orbital, 5, and the lowest unoccupied molecular orbital, 5*, and
COI‘responds to 8—>5*. The next highest energy transitions are 5—>1I* and rt——>6*.
While the molecular orbital scheme is conceptually appealing, several
Observations imply that it may not be an accurate description. The spectroscopy of
M~4—M and M—3.5—M, the one electron oxidized quadruple bond, is not consistent

With the MO scheme. Consider the energy of the 5 to 6* transitions of the

21

 

 

*

 

 

 

 

5*

 

 

 

 

 

 

 

 

 

Figure 1.9 Qualitative molecular orbital diagram for M2L8 quadruply
bonded bimetallic complexes.

 

22

compounds in Table 1.1.38 1(23—)5’“) and 2(5——>8*) denote transitions for M—4—ly
and M—3.5—M, respectively. The latter transition is consistently red shifted, by a:
much as 12,280 cm“1 for the M02(SO4)44‘/3’ couple. To a first approximation, by a1
MO description, these transitions should be equivalent. Additionally, SCF-Xor-SVV
calculations of the 8/8* splitting (AW(calc) in Table 1.1) are approximately equa
to the experimental 2(ES—)8“) values.38 These results indicate that the one electrOl
energy, AW (ES-8* splitting), contributes less significantly than the two e1ectror

energy (Coulomb and exchange) in the 5 manifold.

Table 1.1 8 —>5* Electronic Transition Energies For Selected Quadruply Bonded

Bimetallic Complexes

 

 

compound 1(8 —>5*) / cm "1 2(8 —>5*) / cm ’1 AW(calc) / cm ‘1]
Mo2(02Cl>r")°’+ 22,700 13,300 12,200
M02(SO4)44"3" 19,420 7140 —
M02Cl4(PMe3)4 17,090 — 8390
W2C14(PMe3)40’+ 15,150 7350 7340

Moreover, the d(W—W) is, on average, only 0.12 A longer than that for the

M02 complexes.37 The larger radial extension of the W dxy orbitals shoulc
COInpensate for the marginal increase in the distance and a blue shift in the 1(55*:
tl’ansition might be expected. However, a significant red shift is observed in going
from Mo to W. Additionally, the absorption intensity of the 1(88*) transition i:

low, with an extinction coefficient (8) of about 1 x 103 M’lcm“l, while other meta

23

localized transitions are an order of magnitude higher (Mn2(CO)10 sum ~104 M—
Icm‘l).39 The oscillator strength, which is proportional to the extinction coefficient
for that transition, can be related to the square of the overlap of the orbitals.40
Finally, the stabilization due to the 8 bond has been calculated to be worth about
10 kcal/mol.38 The red shift of the W2 88* as compared to the M02 88* transition,
the low intensity of the absorbance of the 88* transition, and the small amount of
stabilization due to the 8 bond all point to poor overlap of the d,y orbitals.

Owing to this weak overlap of the dxy orbitals and the failure of MO theory
to predict the energy of 1(8—98*), valence bond theory gives a better description of
the transitions involving the 8 and 8* orbitals. Figure 1.10 shows the
experimentally determined state diagram for M02C14(PMe3)4. In the ground state,
one electron is localized on each metal, giving an 1A 1 g state. This correlates with

l. 31P NMR spectroscopy41 and magnetic

the 82 state of the molecular orbital mode
susceptibility measurements42 show that the 3A2u state lies 4840 cm ’1 above the
lAlg state. In a valence bond model these states can be considered diradicals. Thus
their splitting is small when the electrons are in orbitals that are weakly coupled, as
is the case for the situation here. At higher energy are the 1A2u and lAlg states
corresponding to the l(88*) and l(8*8*) states, respectively. The lA2u and IA“;
states lie high in energy because two electrons are paired in the confined volume of
an “atomic-like” orbital. Thus the population of the 1(88*) excited state
corresponds to charge transfer from one metal center to another. The energy

separation of the lA2u and 'Alg excited states has recently been measured by two-

photon spectroscopy43 to be 4800 cm’1 apart thereby confirming the zwitterionic

Energy/ 1000 cm '1

24

:: :- xx— 1:—

5 _ — 3A2u(58*) g

 

o _ _1A.,.s g

Figure 1.10 State diagram for the 88* manifold of M02C14(PMe3)4.

 

25

nature. The transition can be formally written as the linear combination shown

below:

MEM ——’ +M—M2_ + — IM-'-M+

As a result of the linear combination, there should be no electric dipole
moment because each ion pair will contribute equally. If, however, the molecule
loses its inversion center by a low symmetry distortion in the coordination
environment, then the contribution from each pair will not be the same. The
transient absorption spectrum of W2Cl4(PBu3)4 in CHZCIZ suggests that
intramolecular distortion may trap the zwitterionic state.44 The excited state
absorption (it,xc = 355 nm) of W2Cl4(PBu3)4 is remarkably similar to the ground
state absorbance of W2C16(PBu3)4. A high energy intermediate based on this is

proposed below:45

P P P P
CI\V|V/C|\V|V . C|\V|V/CI\V|V/Cl
_ . / ___/

Cl/ [\Cl/ | Cl |\Cl |\CI
P P P P
intermediate W2C|5(PBU3)4

Upon MMCT excitation, there is an increase in charge at one metal center and a
decrease at the other. The C1“ ligands shift towards the decreased electron density
in order to balance the charge shift. This chemical distortion opens up a
coordination site on the metal center where a pair of electrons are proposed to

reside. The low symmetry distortion, which traps the zwitterion, is attractive

 

26

because it offers a site for multielectron activation of substrate via the 188* excited
state for multielectron photochemistry. Aside from the zwitterionic nature of the
excited state, the M—4—M complexes are coordinatively unsaturated allowing
substrate activation without dissociation of a ligand, the metal centers are electron
rich to reduce substrates, and metal-metal cleavage is less likely due to the
quadruple bond. All of these features suggest that the M—4—M complexes are
attractive multielectron reagents. This is the basic tenet that will be investigated in

this thesis.
F. Photochemistry

Photochemical studies of quadruply bonded bimetallic complexes began in
our group with a M—4—M core strapped by four bridging ligands. Visible
irradiation (it > 546 nm) of M02[OZP(OPh)2]4 leads to the two electron reduction of
saturated dihalocarbons to olefins.46 This reaction has also been generalized to
unsaturated dihalocarbons.47 As summarized in Figure 1.11, the reaction proceeds
through one electron, chlorine abstraction intermediates, producing the mixed-
valence photoproduct, M02(II,III)[OZP(OPh)2]4Cl. The two electron oxidized
species, M02(III,III)[OZP(OPh)2]4C12, is not observed because the rigid
coordination sphere can not accommodate the increased charge at the metal center.
Thus, the zwitterionic excited state can not be trapped due to the bulky bidentate
ligand.

By utilizing a more flexible coordination sphere, the first example of
photoinduced concerted two electron oxidation of a quadruply bonded bimetallic

complex was observed.48 Irradiation (it > 435 nm) of Mel solutions of

 

27

 

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W2Cl4(dppm)2 reacts cleanly 'to the oxidative addition product,
W2Cl4(dppm)2(Me)(I). This reaction progresses with wavelengths coincident with
the n8* state,49 which lies just to lower energy of 88*. In order to reach the
distorted intermediate, excess energy must be used to weaken the M—M 1: bonds so
they are available for bonding with the bridging Cl‘ ligands. Figure 1.12
summarizes the similarities between the W2Cl4(dppm)2 photochemistry and the
thermal chemistry of Vaska’s complex, Ir(CO)(PR3)2Cl.5° Both are d4 metals in a
square planar coordination sphere with open coordination sites, which form
complexes with octahedral geometries upon reaction. Both react with MeI to form
the two electron oxidative addition products. Vaska’s complex reacts with longer
chain alkyl groups, such as Ed, to form the one electron product,
Ir(CO)(PR3)2(Cl)(I)2. Likewise, W2Cl4(dppm)2(I)2 and W2C15(dppm)2(l) are

observed with Ed, which are indicative of a radical mechanism.
G. Thesis Outline

The discovery of the two electron mixed valence excited state in the d4—d4
quadruply bonded bimetallic systems presents the possibility of discrete
multielectron transformations. The 1(88*) excited state of M—4—M complexes is, in
general, sufficiently long lived (I ~100 ns) for bimolecular photochemistry.
Chapter 2 gives the experimental details of all the reactions and Chapter 3
generalizes the photochemistry of W2Cl4(dppm)2 to dialkyl/diaryl disulfides.
Photoreaction with dialkyl/diaryl disulfides proceeds upon excitation with visible

light to two electron products.

 

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29

 

 

 

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W =
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CI CI
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i T
III", ”\\\Cl \ /C:
H3C/V|V\Cl/V|VC'
P P

—— Oxidative-Addition Activation Chemistry —

Mel —) 8N2
Etl —> radical

Mel —> 8N2 or concerted
Etl —> radical

Figure 1.12 A comparison of the photochemistry of W2Cl4(dppm)2 and the
thermal chemistry of Vaska's complex for the oxidative addition of alkyl

iodides.

30

Chapter 4 presents work on atom transfer photochemistry of the M—4—M
complexes, W2Cl4(dppm)2 and MoWCl4(PMe2Ph)4. The first half of the Chapter
focuses on the photochemistry of W2Cl4(dppm)2 and N 20. As shown in Chapter 3,
upon excitation, this complex forms a high energy intermediate with two electrons
localized on one W metal center. In order to access this excited state of
W2Cl4(dppm)2, the complex must undergo intramolecular distortion to form the
structure proposed previously. Energy is wasted through the process of rearranging
the chloride ligands and therefore the excited, state is not as energetic as might be
expected. As a result, the excited state of this complex is limited to the oxidative
addition chemistry that is typical of Mel and RSSR.51 Furthermore, atom transfer
photochemistry with N20 is not observed. A more efficient way to trap the excited
state is to build asymmetry into the ground state by using mixed metal quadruply
bonded complexes. The second half of Chapter 4 presents the sulfur atom transfer
photochemistry of MoWCl4(PMe2Ph)4. This system uses lower excitation energy
to create MoWCl4(PMe2Ph)4*, which is used to cleave the Ph3P—S bond. This bond
is substantially stronger than previous substrates (Ph3P—S: 92 kcal/mol;52 Me—I: 56
kcal/mol;53 PhS—SPh: 55 kcal/mol;54 EtS—SEt: 74 kcal/mol;54 N20: 34 kcal/molss).

In Chapter 5, a systematic study of the photophysics of M2C14P4 (M = M0,
W, MoW; P = PMezPh, PMe3) will be discussed. Temperature dependent
absorption, emission and lifetime measurements have been undertaken. With this,
the nonradiative decay processes can be studied to gain a better understanding of
the deactivation of the excited state. It may be possible to predict the reactivity of
these complexes and better design a system capable of a catalytic energy cycle

with enough understanding of the excited states.

31

H. References

1. Stryer, L. Biochemistry; W. H. Freeman: New York, 1988; Ch 22.

2. Orme-Johnson, W. H. Science 1992, 257, 1639.

3. (a) Lui, S. M.; Soriano, A.; Cowan, J. A. J. Am. Chem. Soc. 1993, 115, 10483.
(b) Lui, S. M.; Liang, W; Soriano, A.; Cowan, J. A. J. Am. Chem. Soc. 1994, 116,
4531.

4. Adams, M. W. W.; Mortenson, L. E. In Molybdenum Enzymes; Spiro, T. G.,
Ed.; Wiley—Interscience: New York, 1985; pp 519-593.

5. Balzani, V.; Moggi, L.; Scandola, F. In Supramolecular Photochemistry;
Balzani, V., Ed.; D. Reidel: Boston, 1987; pp 1-28.

6. Hoganson, C. W.; Babcock, G. T. In Metal Ions in Biological Systems; Sigel,

H.; Sigel, A., Eds; Dekker: New York, 1994; Vol. 30, pp 77-107.

7. Graige, M. S.; Paddock, M. L.; Bruce, J. M.; Feher, G.; Okamura, M. Y. J. Am.
Chem. Soc. 1996, 118, 9005.

8. Burgess, B. K.; Lowe, D. J. Chem. Rev. 1996, 96, 2983.

9. Jolly, J. L. The Inorganic Chemistry of Nitrogen; Benjamin: New York, 1964.
10. (a) Georgiadis, M. M.; Komiya, H.; Chakrabarti, P.; Woo, D.; Komuc, J. J.;
Rees, D. C. Science 1992, 25 7, 1653. (b) Bolin, J. T.; Ronco, A. E.; Morgan, T. V.;
Mortenson, L. E.; Xuong, N.-H. Proc. Natl. Acad. Sci. U. S. A. 1993, 90, 1078.

11. (a) Marcus, R. A.; Sutin, N. Biochem. Biophys. Acta. 1985, 811, 265. (b)
Connelly, J. S.; Bolton, J. R. In Photoinduced Electron Transfer; Fox, M. A.,
Chanon, M. Eds; Elsevier: Amsterdam, 1988; Part D, pp 303-393.

12. Slama-Schwok, A.; Ottolenghi, M.; Avnir, D. Nature 1992, 335, 240.

13. Villemure, G.; Detellier, C.; Szabo, A. G. J. Am. Chem. Soc. 1986, 108, 4658.

32

14. Diirr, H.; Trierweiler, H.—P.; Willner, 1.; Maidan, R. New. J. Chem 1990, 14,
317.

15. Ohtani, B.; Ye, M.; Miyadzu, H.; Nishimoto, S.; Kagiya, T. J. Photochem.
Photobiol. A: Chem. 1991, 56, 359.

16. Cusack, L.; Rao, S. N .; Fitzmaurice, D. Chem. Eur. J. 1997, 3, 202.

17. Krueger, J. S.; Mayer, J. E.; Mallouk, T. E. J. Am. Chem. Soc. 1988, 110, 8232.
18. Vermeulin, L. A. In Progress in Inorganic Chemistry; Meyer, G. J. Ed.; John
Wiley and Sons: New York, 1997; Vol 44, pp 143-166.

19. (a) Meyer, T. J. Acc. Chem. Res. 1978, 11, 94. (b) Adamson, A. W.; Gafney, H.
D. J. Am. Chem. Soc. 1972, 94, 8238.

20. (a) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163. (b) Surridge, N. A.;
McClanahan, S. F.; Hupp, J. T.; Danielson, E.; Gould, S.; Meyer, T. J. J. Phys.
Chem. 1989, 93, 294. (c) Surridge, N. A.; Hupp, J. T.; McClanahan, S. F.; Gould,
S.; Meyer, T. J. J. Phys. Chem. 1989, 93, 304.

21. Roundhill, D. M. Photochemistry and Photophysics of Metal Complexes;
Plenum: New York, 1994.

22. Kunkely, H.; Vogler, A. Inorg. Chem. 1995, 34, 2468.

23. Roundhill, D. M.; Gray, H. B.; Che, C.-M. Acc. Chem. Res. 1989, 22, 55.

24. (a) Sigal, I. 3.; Mann, K. R.; Gray, H. B.; J. Am. Chem. Soc. 1980, 102, 7252.
(b) Miskowski, V. M.; Sigal, I. S.; Mann, K. R.; Gray, H. B.; Midler, S. J.;
Hammond, G. S.; Ryanson, P. R. J. Am. Chem. Soc. 1979, 101, 4383.

25. (a) Che, C.-M.; Kwong, H.-L.; Yam, V. W.-W.; Cho, K.-C J. Chem. Soc.
Chem. Comm. 1989, 885. (b) Che, C.-M.; Kwong, H.-L.; Poon, C.-K.; Yam, V.
W.-W. J. Chem. Soc. Dalton Trans. 1989, 3215.

 

33

26. Yip, H.-K; Lin, H.-M. Cheung, K.-K., Che, C.-M.; Wang, Y. Inorg. Chem.
1994, 33, 1644.

27. Wrighton, M. S.; Graff, J. L.; Luong, J. C.; Rechel, C. L.; Robbins, J. L. In
Reactivity of Metal-Metal Bonds; Chisholm, M. H., Ed.; ACS Symposium Series
155; American Chemical Society: Washington DC, 1981; pp 85-110.

28. Wrighton, M.; Bredesen, D. J. Organomet. Chem. 1973, 50, C35.

29. (a) Balzani, V.; Carassiti, V. Photochemistry of Coordination Compounds,
Academic: New York, 1970. (b)Geoffroy, G. L.; Wrighton, M. S. Organometallic
Photochemistry; Academic: New York, 1979. (c) Wrighton, M. S.; Ginley, D. S. J.
Am. Chem. Soc. 1975, 95, 2065.

30. (a) Nieckarz, G. F.; Weakley, T. J. R.; Miller, W. K.; Miller, B. E.; Lyon, D.
K.; Tyler, D. R. Inorg. Chem. 1996, 35, 1721. (b) Male, J. L.; Davis, H. B.;
Pomeroy, R. K.; Tyler, D. R. J. Am. Chem. Soc. 1994, 116, 9353. (c) Avey, A.;
Weakley, T. J. R.; Tyler, D. R. J. Am. Chem. Soc. 1993, 115, 7706. (d) Mao, F.;
Tyler, D. R.; Bruce, M. R. M.; Bruce, A. E.; Rieger, A. L.; Rieger, P. H. J. Am.
Chem. Soc. 1992, 114, 6418.

31. Levenson, R. A.; Gray, H. B. J. Am. Chem. Soc. 1975, 97, 6042.

32. (a) Steigman, A. B.; Tyler, D. R. Acc. Chem. Res. 1984, I7, 61. (b) Steigman,
A. E.; Tyler, D. R. J. Am. Chem. Soc. 1982, 104, 2944.

33. Turro, N. J. Modern Molecular Photochemistry; University Science Books:
Sausalito, California, 1991; pp 215-221.

34. Mulliken, R. S. Phys. Rev. 1932, 41, 75.

35. (a) Mulliken, R. S. Phys. Rev. 1936, 50, 1017. (b) Coulson, C. A.; Fischer, 1.
Philos. Mag. 1949, 40, 386.

34

36. (a) Zewail, A. H. Science 1988, 242, 1645. (b) Rosker, M. J.; Dantus, M.;
Zewail, A. H. Science 1988, 241, 1200.

37. Cotton, F. A.; Walton, R. A. Multiple Bonds Between Metal Atoms; Clarendon:
Oxford, 1993; p 632.

38. Hopkins, M. D.; Gray, H. B.; Miskowski, V. M. Polyhedron 1987, 6, 705.

39. Zietlow, T. C.; Hopkins, M. D.; Gray, H. B. J. Solid St. Chem. 1985, 59, 112.
40. Mulliken, R. S. J. Chem. Phys. 1939, 7, 20.

41. (a) Cotton, F. A.; Eglin, J. L.; Hong, B.; James, C. A. Inorg. Chem. 1993, 32,
2104. (b) Cotton, F. A.; Eglin, J. L.; Hong, B.; James, C. A. J. Am. Chem. Soc.
1992, 114, 4915.

42. Hopkins, M. D.; Zietlow, T. C.; Miskowski, V. M.; Gray, H. B. J. Am. Chem.
Soc. 1985, 107, 510.

43. Engebretson, D. S.; Zaleski, J. M.; Leroi, G. E.; Nocera, D. G. Science 1994,
265, 759.

44. Partigianoni, C. M.; Turré, C; Shin, Y.-g. K.; Motry, D. H.; Kadis, J.;
Dulebohn, J. 1.; Nocera, D. G. In Mixed Valency Systems: Applications in
Chemistry, Physics and Biology; Prassides, K., Ed.; NATO ASI Series C:
Mathematical & Physical Sciences; Kluwer Academic: Dordrecht, 1991; Vol. 343,
pp 91-106.

45. Winkler, J. R.; Nocera, D. G.; Netzel, T. L. J. Am. Chem. Soc. 1986, 108,
4451.

46. Chang, I-J.; Nocera, D. G. Inorg. Chem. 1989, 28, 4309

47. Hsu, T.-L. C.; Chang, I-J.; Nocera, D. G. Inorg. Chem 1994, 33, 2932.

35

48. (a) Partigianoni, C. M.; Nocera, D. G. Inorg. Chem. 1990, 29, 2033. (b)
Partigianoni, C. M.; Turro, C.; Hsu, T.-L. C.; Chang, I-J.; Nocera, D. G. In
Photosensitive Metal-Organic Systems; Kutal, C., Serpone, N ., Eds.; Advances in
Chemistry 238; American Chemical Society: Washington, DC, 1993; pp 147-163.
49. Hay, P. J. J. Am. Chem. Soc. 1982, 104, 7007.

50. (a) Nocera, D. G. Acc. Chem. Res. 1995, 28, 209. (b) Nocera, D. G. J. Clust.
Sci. 1994, 5, 185.

51. Collman, J. P.; Hegedus, L. 8.; Norton, J. R.; Finke, R. G. Principles and
Applications of Organotransition Metal Complexes; University Science Books:
Mill Valley, California, 1987; Ch 5.

52. (a) Chernick, C. L.; Pedley, J. B.; Skinner, H. A. J. Am. Chem. Soc. 1957, 79,
1851. (b) Hall, K. A.; Critchlow, S. C.; Mayer, J. M. Inorg. Chem. 1994, 33, 3593.
53. Streitweiser, A. Jr; Heathcock, C. H. Introduction to Organic Chemistry, 3rd
ed.; Macmillan: New York, 1985.

54. Benson, S. W. Chem. Rev. 1978, 78, 23.

55. Holm, R. H.; Donahue, J. P. Polyhedron 1993, 12, 571.

CHAPTER 2

EXPERIMENTAL

A. General Procedures

All manipulations were carried out under an argon atmosphere of a Vacuum
Atmosphere drybox or the manifold of a Schlenk line. Molybdenum (V) chloride,
(MozCllo, Aldrich Chemicals), tungsten (VI) chloride (WCl6, Aldrich Chemicals),
and tungsten (IV) chloridel (WC14) were purified by sublimation. Monodentate
phosphines (trimethyl phosphine (PMe3), dimethylphenyl phosphine (PMezPh),
methyldiphenyl phosphine (PMeth), tributyl phosphine (PBu3)), triphenyl
phosphine sulfide (Ph3PS) and antimony sulfide (Sb2S3) were purchased from
Strem Chemicals, triphenyl phosphine (Ph3P) was purchased from Baker, diphenyl
disulfide (PhSSPh), diethyl disulfide (EtSSEt), chlorotrimethyl silane (Me3SiCl),
and bis(diphenylphosphino)methane (dppm) were purchased from Aldrich
Chemicals. All chemicals were reagent grade and used as received, except PhSSPh
and EtSSEt. PhSSPh was recrystallized from benzene and dried in vacuo and
EtSSEt was dried over activated 4 A sieves. Nitrous oxide (N20, Purity Cylinder
Gases) was breathing quality and oxygen was removed by freezing the N20 in
liquid nitrogen and removing 02 in vacuo.

Solvents used for synthesis were dried by general procedures.2 Benzene

(C6H6), toluene and tetrahydrofuran (THF) were refluxed over Na/benzophenone

36

37

ketyl. Hexanes, dichloromethane (CHzClZ) and acetonitrile (CH3CN) were refluxed
over CaHz. PMe3, PMezPh, PMePh2 and PBu3 were stored in flasks with Kontes
quick-release valves. Benzene, toluene, THF, 2-methyltetrahydrofuran, 2-MeTHF,
and 2-methylpentane for transfer on the high vacuum manifold were stored over

NaK/benzophenone ketyl in flask with Kontes quick—release valves.

B. Synthesis

1. Homonuclear Molydenum and Tungsten Complexes

a. M02Cl4(PR3)4 (PR3 = PMe3, P M821) 1], PMePh2)3

K4M02C13 or (NH4)5M02C19 (0.5 g, 0.807 mmol) and four equilavents of
PR3 were refluxed in methanol for 5 hours until a blue precipitate formed. The
blue crystals were filtered in the air and washed with water, methanol, and diethyl
ether and dried in vacuo. M02C14(PR3)4 was then purified on a Florisil colunm

under argon with CH2C12 as the eluent. This was characterized by UV-vis, 1H and

31P{ 1H} NMR.
b. WZCI4(PR3)4 (PR3 = PR3 = PMe3, PMezPh, P Meth, P Bll3)4

WC14 (2.6 g, 8 mmol) was added to 0.5% Na/Hg amalgam in THF in a dry
ice/acetone bath. This was followed by 4 equivalents of PR3. The solution was
allowed to warm slowly by replacing the dry ice/acetone bath with a dry
ice/acetonitrile bath, an ice bath and finally allowed to warm to room temperature.
The dark blue-green solution was filtered through Celite and the THF removed.
The oily residue was extracted with hexanes in the air. The solution was filtered

and the volume reduced. MeOH was added and a blue-green precipitate formed.

38

This was dried overnight under vacuum and characterized by UV-vis, 1H and

31P{1H} NMR.
c. W2Cl4(d10pm)25

W2Cl4(PBu3)4 (2.6 g, 2.0 mmol) and dppm (1.5 g, 4.0 mmol) were refluxed
in a 40/60 mixture of hexanes/toluene for 4 hours. The solution was allowed to
cool and a brown crystalline precipitate formed. This was filtered and washed with
hexanes. The solid was dried overnight under a flow of argon and brought into the
drybox. UV—vis (C6H6, nm): am, = 740, 508, 370. 1H NMR (C6D6, ppm): 7.62 (s),
6.83 (t), 5.15 (t).

d- W2C16(dppm)26

W2C14(dppm)2 was dissolved in CHZCIZ and stirred at room temperature for
several days. The solvent was removed to give W2C16(dppm)2. 1H NMR (CDZCIZ,
ppm): 5 8.0-7.0 (m, 40H); 3.85 (t, 4H). 31P{1H} NMR (CD202, ppm): 5 —106 (hr).
UV-vis (CHZCIZ, nm): 822, 465, 384.

e. W2Cl4(dPPm)2(SPh)2

W2C14(dppm)2 (0.1 g, 78 umol) and PhSSPh (0.02 g, 92 umol) were
dissolved in 25 m1 of toluene and gently heated for a half hour. The solution
changed from brown to dark maroon. W2Cl4(dppm)2(SPh)2 was precipitated by the
addition of hexanes and filtered. UV-vis (CH2C12, nm): 730, 504. FAB/MS (amu):
1496 (W2Cl4(dppm)2(SPh)2), 1387 (W2Cl4(dppm)2(SPh)).

f. W2C15(dppm)2(SEt)

W2Cl4(dppm)2 (0.048 g, 37.6 umol) and EtSSEt (0.011 g, 89 umol) were

dissolved in toluene and heated to produce a red-brown solution. A mixture of

39

W2C15(dppm)2(SEt) and W2Cl4(dppm)2 was isolated as above. FAB/MS (amu):
1373 (W 2C15(dppm)2(SEt)), 1347 (W2C16(dPPm)2), 1312 (W2C15(dPPm)2).

2. Heteronuclear Molybdenum-Tungsten Complexes

3. Mo(6n-PhPMePh)(PMePh2)37

A THF/ PMePh2 (6.7 g, 33 mmol) solution was bubbled with Ar in a three
neck round bottom flask with activated Mg (3.5 g, 0.15 mol) turnings in a side
arm. The Mg was activated proir to use with 12. This was placed in an ice bath and
MozCllo (2.0 g, 7.3 mmol) was added via a Shlenk flask. Mg was added over the
course of an hour. The solution was allowed to warm slowly to room temperature
producing an orange-red solution over the course of 2 hours. The solution was
filtered through Celite and concentrated. MeOH was added and an orange solid
precipitated. This was washed with MeOH to remove any excess phosphines and
dried under a flow of Ar. Mo(6n-PhPMePh)(PMePh2) was recrystallized by
dissolving in benzene, filtering through Celite and precipitating with MeOH. This
compound is extremely air and moisture sensitive and better yields can be obtained
by avoiding the use of a vacuum when filtering. (Note: Ar is necessary since
Mo(6n-PhPMePh)(PMePh2)3 reacts with N2.) 1H NMR (C6D6, ppm): 5 7.3-7.0 (m,
30H, thMeP); 4.4 (s, 1H, 6n-Ph-PMePh); 4.2 (s, 2H, 6n-Ph-PMePh); 3.7 (s, 1H,
6n-Ph-PMePh); 3.6 (s, 1H, 6n-Ph—PMePh); 1.76 (s, 9H, thMeP); 1.2 (d, 3H, 6:1—
Ph-PMePh); 31P{1H} NMR (C6D6, ppm): 8 +361 (3, 3P, thMeP); —29.1 (3, IF,
6n-Ph-PMePh).

40

b. Mo(‘n-PhPMe2)(PMe2Ph)38

The experimental set up was the same as above, except that heat is required.
MOZCIIO (0.7 g, 13 mmol) was added to a THF solution of PMezPh (1.8 g, 13
mmol) in an ice bath to give a green solution. The ice bath was removed and
activated Mg (2.0 g, 82 mmol) added. The solution was heated at 70 °C for two
hours. After 15 minutes the solution turned orange-brown. The solution was
cooled, filtered through Celite and concentrated. MeOH was added and the
solution stored at —20 °C for several hours to maximize precipitation. This was
filtered and dried under a flow of Ar. As with the above compound, Mo(6n-
PhPMe2)(PMe/2Ph)3 is extremely air and moisture sensitive and it is best to avoid
using the vacuum when filtering. Better yields can be obtained if glassware is
flame-dried. 1H NMR (C6D6, ppm): 8 7.5 (m, 6H, o-PhMezP); 7.3-7.2 (m, 9H,
m,p—PhMe2P); 4.0 (m, 2H, 6n-PhPMe2); 3.4 (m, 1H, 6n-PhPMe2); 3.2 (m, 2H, 6n-
PhPMez); 1.5 (hr, 18H, PhMezP); 1.0 (d, 6H, 6n-PhPMe2). 311>{‘H} NMR ((361),,
ppm): 6 + 15.8 (s, 3P, PhMezP); —46.7 (3, IF, 6n-PhPMe2).

c. WCl4(PPh3)29

The complex was prepared by the reduction of WCl6 by amalgated mossy
Zn. The Zn was prepared prior to use by reaction with HgO (0.2 g, 0.9 mmol)
dissolved in hot 12M HCl (~2 ml). The Zn was filtered, washed with water and
acetone and dried in the oven. Amalgated Zn (5.5 g, 84 mmol) was added to WC16
(6.6 g, 17 mmol) under argon. WC16 was dissolved in CH2C12 to produce a dark
orange-red solution. Ph3P (8.5 g, 32 mmol) was added slowly (~3 min) and the
solution started to gently reflux and turn green producing an orange precipitate.

The precipitate was filtered and the remaining Zn picked out under a flow of Ar.

41

The solid was washed with CHZCIZ and dried under vacuum. 1H NMR (C6D6,
ppm): 11.2 (d, 2H), 8.2 (t, 2H), 7.8 (t, 1H); 3"1>{1H} NMR (C6D6, ppm): 7.2 (s).

d. MoWCl4(PMePh2)41°

A benzene solution of Mo(6n-PhPMePh)(PMePh2)3 (0.1 g, 0.11 mmol) was
added dropwise to a benzene solution of WC14(PPh3)2 (0.25 g, 0.3 mmol) in a
glove bag inside of the dry box. This step is very sensitve to water and all
glassware was flame-dried on a high vacuum manifold (10"5 torr). The benzene
was predried over Na/benzophenone ketyl and stored over activated 4A sieves.
The bright green solution was filtered through Celite and the benzene removed in
vacuo. This green residue was dissolved in toluene and filtered. The solution was
concentrated and any additional WC14(PPh3)2 was removed by filtration. MeOH
was added to precipitate a blue-green solid. This was filtered, washed with MeOH
and dried in vacuo. UV-vis (C6H6, nm): 650, 460, 320. 1H NMR (C6D6, ppm): 5
7.174 (m, 20H, thMeP); 1.79 (s, 6H, thMeP—W); 1.62 (s, 6H, thMeP—Mo).
3'P{ ‘H} NMR (C6D6, ppm): 5 +22.6 (t, ‘pr = 267 Hz); —12.4 (t, 2pr = 95 Hz).

e. MoWCl4(PMe2Ph)4"

The procedure was the same as that for MoWCl4(PMePh2)4. A benzene
solution of Mo(6n-PhPMe/))(PMe2Ph)3 (120 mg, 0.185 mmol) was added dropwise
to WC14(PPh3)2 (180 mg, 0.213 mmol) in benzene to yield MoWC14(PMe2Ph)4.
UV-vis (C6H6, nm): 645 (a = 1597 M" cm"); 460 (a = 205 M’1 cm“); 320 (e =
2663 M”1 cm"). IH NMR (C6D6, ppm): 8 7.61 (m, p—PhMezP—W); 7.16 (m, p-
PhMeQP—Mo); 7.03 (s, o, m-PhMezP—W); 6.91 (s, o, m-PhMezP—Mo); 1.90 (t,

42

PhMezP—W); 1.82 (t, PhMezP—Mo). 311>{‘H} NMR ((361),, ppm): 5 +18.0 (t, ‘pr =
271 Hz); —19.4 (t, 2pr = 98 Hz).

r. MoWCl4(PMe3)410

PMe3 (73.5 mg, 0.97 mmol) was added to a benzene solution of
MoWCl4(PMePh2)4 (146 mg, 0.120 mmol) on a high vacuum manifold. This was
heated at 80 °C for 8 hours. PMezPh, excess PMe3 and the solvent were removed
by heating at 55 °C under vacuum. The blue residue was dissolved in benzene and
MeOH added. The resulting blue precipitate was filtered, washed with MeOH and
dried under vacuum. UV-vis (C6H6, nm): 634. 1H NMR (C6D6, ppm): 5 1.45 (q,
36H). 31P{‘H} NMR (C6D6, ppm): 5 +109 (t, ‘pr = 269 Hz); — 27.3 (t, 2pr = 97
Hz).

3. W(S)Cl4(PMe2Ph)

W(S)Cl4(PMe2Ph) was synthesized following a modified literature
procedure.12 A mixture of WCl6 (6g, 15 mmol) and Sb2S3 (1.7g, 5.1 mmol) was
heated at 60 °C until a red molten liquid formed. This was dissolved in C82 (20
ml), S8 (0.36g, 1.4 mmol) added and the mixture stirred. After about 10 minutes
the red solution was filtered, washed with C82 and the solution concentrated in
vacuo. Hexanes were added and the solution heated until most of the solids were
dissolved. Any residual solids were filtered away from the solution and the
solution cooled to room temperature. PMezPh (3.1 ml, 22.4 mmol) was added
slowly and an orange solid crashed out immediately leaving a clear solution. This
was dissolved in benzene, filtered and the solvent removed. W(S)Cl4(PMe2Ph) was

recrystallized by dissolving in benzene and inducing precipitation by the addition

43

of hexanes. 1H NMR (c6136, ppm): 5 7.65 (m); 7.03 (m); 1.36 (d, 11,.“ = 13 Hz);
31P{‘H} NMR (C6D6, ppm): 5 +30 (8). IR (C6H6, cm’l): 594 (m); 578 (m); 482
(w); 394 (w). GC (direct inlet, benzene, amu): 493. FAB/MS (C6H6, amu): 751
(W(S)Cl4(PMe2Ph)-Sg); 494 (W(S)Cl4(PMe2Ph)).

4. Reactions of W2Cl4(dppm)2

a. Photoreaction of W2Cl4(dppm)2 with PhSSPh, EtSSEt

Toluene solutions of W2Cl4(dppm)2 (0.035 g, 0.027 mmol) and a 20-fold
excess of PhSSPh were photolyzed (7t > 495 nm, —10 °C) to completion in 45
minutes (for EtSSEt, 3 days at —20 °C), as determined by UV-vis spectroscopy.
The solution was concentrated and the photoproduct precipitated by the addition of
hexanes. The compound was further purified by chromatography on a Florisil
packed column with CHzClz/CH3CN as the eluent. The product was isolated in a
47% yield. In the case of PhSSPh, the FAB/MS and UV-vis were identicle to those
of W2Cl4(dppm)p_(SPh)2.13 FAB/MS (amu): 1496 ([M]+), 1419 ([M—C6H5]+), 1387
([M—C6H5—S]+). UV-vis (toluene, nm): 730, 504. In the case of EtSSEt, the
FAB/MS was identicle to that of W2C16(dppm)2 and W2C15(dppm)2(SEt). FAB/MS
(amu): 1378 (W2C15(dppm)2(SEt)), 1348 (W2Cl6(dppm)2), 1314 (W2C15(dppm)2).

No photoreaction is observed in the absence of substrate.
b. Thermal reactions of W2Cl4(dppm)2 with PhSSPh, EtSSEt

W2C14(dppm)2 (0.05g, 0.04 mmol) and excess PhSSPh were dissolved in
toluene, and the solution shielded from the light at -20 °C. The reaction was
complete in 24 hours (for EtSSEt, 1 month), as monitored by UV-vis. The isolated

product was shown to be W2Cl4(dppm)2(SPh)2 by FAB/MS and UV-vis. The

44

products for EtSSEt were shown to be W2Cl6(dppm)2 and W2C15(dppm)2(SEt) by
FAB/MS.

c. Photoreaction of W2Cl4(dppm)2 with N20

W2Cl4(dppm)2 (8.7 mg, 6.8 umol) was added to the absorption cell of the
evacuable cell described in the photolysis section. THF was vacuum distilled to the
bulb side on a high-vacuum manifold and 02 rigorously removed. W2Cl4(dppm)2
was dissolved in THF and isolated from the bulb by a Kontes valve. 1 atm of N20
was transferred to the bulb on a high-vacuum manifold and 02 removed. N20 and
the THF solution of W2C12(dppm)2 were allowed to mix. Irradiation (2. > 375 nm,
—80 °C) of this solution was complete after 1 week, as determined by UV-vis
spectroscopy. The inorganic product was identified by UV-vis, 1H and 31P{1H}
NMR and FAB/MS and is consistent with W2C16(dppm)2. W2C16(dppm)2: 1H NMR
(CD202, ppm): 5 8.0-7.0 (m, 40H); 3.85 (t, 4H). 311>{‘H} NMR (CD2C12, ppm): 5
—109 (br). UV-vis (THF, nm): 822, 468, 387. FAB/MS (CHZCIZ, amu): 1348.

5. Reactions of MoWCl4(PMe2Ph)4

a. Photoreaction of MoWCl4(PMe2Ph)4 with Ph3PS

MoWCl4(PMe2Ph)4 (2.1 mg, 21 umol) was dissolved in benzene containing
a two-fold excess of triphenyl phosphine sulfide and irradiated with 2» > 375 nm
(20 15°C). After 16 days the reaction is near completion, as monitored by UV-vis,
producing W(S)Cl4(PMe2Ph); 26% yield (by NMR). NMR of photoreaction: lH
(C6D6, ppm): 5 = 7.8 (m, Ph3PS); 7.7 (m, W(S)Cl4(PMe2Ph)); 7.6 and 7.3 (m,
MoWCl4(PMe2Ph)4); 7.4 (m, Ph3P); 7.0 (m, Ph3PS, Ph3P, MOW,
W(S)Cl4(PMe2Ph)), 1.86 (d of t, MoWCl4(PMe2Ph)4); 1.36 (d, 'JPH = 13 Hz,

45

W(S)CI4(PMe2Ph)). 3‘P(C6D6, ppm): 5 = +43.1 (s, Ph3PS); +31.6 (s,
W(S)CI4(PMe2Ph)); +18.0 (t, ‘pr = 271 Hz, MoW); —4.7 (s, Ph3P); —19.4 (t, 2pr =
98 Hz, MoW). IR (C6H6, cm“): 594 (m, W(S)CI4(PMe2Ph)); 570 (m,
W(S)Cl4(PMQPh) ); 541 (s, Ph3P); 517 (vs, Ph3P + Ph3PS); 480 (m, Ph3PS); 440
(m, Ph3PS). GC (direct inlet, MeOH, amu): 355 (W(S)Cl4); 294 (Ph3PS); 262
(Ph3P). EPR (2-methy1tetrahydofuran,—150 C): g = 1.9757; 1.9345.

b. Thermal reaction MoWCl4(PMe2Ph)4 with Ph3PS

Benzene solutions of MoWCl4(PMe2Ph)4 and Ph3PS, prepared as above, are
indefinitely stable in the absence of light. This was determined by UV—vis

spectroscopy.
6. Reactions of M2C14(PMe2Ph)4 with Ph3PS (M = M0, W)

No reactions of benzene solutions of M2Cl4(PMe2Ph)4 and Ph3PS upon
irradiation (M = M0, Km > 335 nm after 24 hrs; M = W, Km > 375 nm after 36
hrs) or at room temperature in the dark was observed, as determined by UV-vis

spectroscopy.

C. Instrumentation and Methods

1. Photolysis

Sample irradiations were performed by using a collimated beam from a
Hanovia 1000W Hg/Xe high pressure lamp. Excitation wavelengths were selected
with a Schott glass, high-energy cutoff filters purchased from the Oriel
Corporation. Photolysis experiments were performed on solutions at constant

temperatures contained in two-arm evacuable cells consisting of a solvent bulb

46

isolated from an absorption cell by a Kontes quick-release Teflon valve. Solution
were prepared by bulb-to-bulb distillation of solvent on high vacuum manifold
For photochemical quantum yields, the excitation wavelength was isolated bj
using an interference filter purchased from the Oriel Corporation with a half-widtl
of less than 10 nm at a given mercury line. Quantum yields for W2Cl4(dppm)
photochemistry were determined on toluene solutions containing 1 x 10‘4 Iv
W2C14(dppm)2 and a 20-fold excess of PhSSPh. Quantum yields fo.
MoWCl4(PMe/2Ph)4 photochemistry were determined on benzene solution:
containing 1 x 10'3 M of MoWC14(PMe2Ph)4 and a 2-fold excess of Ph3PS. Botl
quantum yields were determined by monitoring the disappearance of the 182—9185=I
transition (W2Cl4(dppm)2: 8740 = 2585 M‘lcm“, MoWCl4(PMe2Ph)4: 8645 = 2169

M’lcm’l) and were standardized by using a ferrioxalate actinometer.l4
2. Absorption Spectroscopy

Absorption spectra were measured on a CARY-17D spectrometer modifiec
by OLIS. Extinction coefficients were determined in the high vacuum cells
described previously, by successive dilutions and were calculated by a Beer-

Lambert plot using 7 points.
3. Solution Infrared Spectroscopy

Solution IR spectra were recorded on benzene or THF solutions on 2
Nicolet 750 IR spectrometer in a Perkin-Elmer KBr sealed liquid IR cell. The

appropriate solvent was subtracted out as a reference.

47

4. Nuclear Magnetic Resonance Spectroscopy (NMR)

1H and 31P{1H} NMR spectra were obtained on Varian [nova-300 and
Varian VXR—500 MHz spectrometers, with the 31P{1H} experiment recorded at
121.4 MHz and referenced externally to 85% H3PO4. C6D6 (Aldrich Chemicals),
THF—dg (Cambridge Isotope Laboratories), CD2C12 (Matheson) were used as

received.
5. Electron Paramagnetic Resonance Spectroscopy (EPR)

EPR spectra were recorded on a Varian E—4 spectrometer by using a X-band
TB 102 cavity with 100 KHz modulation, a 2.5 G modulation amplitude, and a
microwave power of 45.7 mW at —151 °C. G-Values were measured directly from
the chart paper. 2-methyl tetrahydrofuran was distilled from CaHz and stored over

N aK.
6. Electron Spin Echo Envelope Modulation (ESEEM)

ESEEM data were collected on a home built spectrometer15 using a three
pulse stimulated echo (90°-r-90°-T-90°) pulse sequence at 4K. Prior to Fourier
transform, dead time was reconstructed, as described before.16 The experimental
data was analyzed utilizing software written by Matlab (Mathworks, Nantick,

MA).
7. Steady-State Luminescence Spectroscopy

Emission spectra were recorded on a spectrometer constructed at Michigan
State University.17 The original experimental set up was modified by replacing the
EG&G Princeton Appled Research Model 5209 lock-in amplifier with a Stanford

Research Systems SR4000 two channel gated photon counter and using a

48

Hamamatsu R943 PMT. Absolute emission quantum yields on optically dilute
samples (A < 0.2) were measured using M02C14(PMe3)4 ((1)em = 0.256 at 3.6,“, = 583
nm in 2—methylpentane18) as standard for M02C14(PR3)4 and M0214(PMe3)4 ((1)em =
0.12 at 3.6m = 636 nm in 2-methylpentane18) as a standard for W2Cl4(PR3)4 and

MoWCl4(PR3)4. The quantum yields were calculated using the following

_ AM.) 77..2 _l?_.._
(D... — (D‘iA.(/1u)]x[mzix[05] (21)

where u and s represent the unknown and the standard, respectively, n is the

equation”:

 

average refractive index of the solution, D is the integrated area under the
corrected emission spectrum, and A0») is the absorbance per unit length (cm) of

the solution at the excitation wavelength.

8. Time-Resolved Laser Spectroscopy

. a. Nanosecond Lifetimes and Transient Absorption

Nanosecond lifetimes were measured using an experimental set-up
described elsewhere.20 The original set—up was modified by the use of the third
harmonic (2.5 ns pulse width) of a Coherent Infinity Nszag laser pumping a
Lambda Physik scanmate OPPO using the corresponding dye to obtain the correct
wavelengths to seed the optical parametric oscillator. Rhodamine 6G (Exciton)
was used to obtain 2» = 580 nm and sulforhodamine B (Lambda Physik) for X =
630 nm; the corresponding Idler wavelengths were filtered out using a KG3 heat
absorbing Schott filter. 355 nm light was removed with a CG-385 nm Schott high-
energy glass filter. Nanosecond transient absorption measurements were made

with the pulse-probe technique with instrumentation20 housed in the LASER

49

(Laser Applications in Science and Engineering Research) Laboratory at Michigan

State University. The same excitation source described above was used.
b. Picosecond Transient Absorption

The transient absorption signals on the picosecond timescale were obtained
using a pump-probe technique described in detail elsewhere21 and were measured

by Dr. Turro.
9. Variable-Temperature Emission, Absorption and N anosecond Lifetimes

Variable-temperature emission, lifetime and absorption measurements were
recorded on samples cooled with an Air Products closed cycle cryogenic system by
methods described elsewhere.20 Samples, contained in a sealed Pyrex square
emission cell, were held in contact with copper block by a copper cell holder and
wrapped in indium foil. Temperatures were also measured near the sample by a K-
type thermocouple using a Digi-Sense thermocouple thermometer from the Cole-

Parmer Instrument Company and were found to vary by 12K.
10. Electrochemistry

Electrochemical measurements were made with a PAR Model 173
potentiostat, 175 universal programmer, 179 digital coulometer, and a Housten
Instruments Model 2000 X-Y chart recorder. Cyclic voltametry was performed at
room temperature in THF by using a conventional H—cell design in an inert dry-
box atmosphere and a three—electrode system consisting of a Pt button electrode (A
= 0.08 cmz), a Pt wire auxiliary electrode and a Ag wire provided a reference
potential with ferrocene used as the internal standard. Potentials were related to

SCE reference scale by using a ferrocinium-ferrocene couple of 0.307 V vs SCE.22

50

11. Mass Spectrometry

a. FAB/MS

Fast atom bombardment mass spectrometry (FAB/MS) analyses were
performed on a JEOL HX-110 double focusing mass spectrometer housed in the
National Institute of Health/Michigan State University Mass Spectrometry facility.
Samples were dissolved in 3-nitrobenzyl alcohol matrices and the instrument run

in the positive ion detection mode.
b. GC/MS

Analyses by GC/MS were carried out on a JEOL JMS-AX505H double
focusing MS coupled to a Hewlett Packard 5890] GC via a heated interface by

direct probe injection at 30 °C for 2 minutes followed by 10 °C / minute.

D. References

1. Santure, D. J.; Sattelberger, A. P. Inorg. Synth. 1989, 26, 221.

2. Gordon, A. J.; Ford, R. A. The Chemist’s Companion: A Handbook of Practical
Data, Techniques and References; Wiley-Interscience: New York, 1972; p 429.

3. SanFilippo, J. Jr. Inorg. Chem. 1972, 11, 3140.

4. Schrock, R. R.; Stugeoff, L. G.; Sharp, P. R. Inorg. Chem. 1983,22, 2801.

5. Canich, J. M.; Cotton, F. A. Inorg. Chim. Acta. 1988, 142, 69.

6. Fanwick, P. B.; Harwood, W. S.; Walton, R. A. Inorg. Chem. 1987, 26, 242.

7. Luck, R. L.; Morris, R. H.; Sawyer, J. F. Organomet. 1984, 3, 247.

8. Cotton, F. A.; Luck, R. L.; Morris, R. H. Organomet. 1989, 8, 1287.

51

9. (a) Butcher, A. V.; Chatt, J.; Leigh, G. H.; Richards, R. L. J. Chem. Soc. Dalton
Trans. 1972, 1064. (b) Dilworth, J. R.; Richards, R. L. Inorg. Synth. 1980, 20, 124.
10. (a) Luck, R. L.; Morris, R. H.; Sawyer, J. F. Inorg. Chem. 1987, 26, 2422. (b)
Morris, R. H. Polyhedron 1987, 76, 793.

11. Cotton, F. A.; Falvello, L. R.; James, C. A.; Luck, R. L. Inorg. Chem. 1990,
29, 4759.

12. Baratta, W.; Calderazzo, F.; Daniels, L.M. Inorg. Chem, 1994, 33, 3842.

13. Canich, J. M.; Cotton, F. A.; Dunbar, K. R.; Falvello, L. R. Inorg. Chem. 1988,
27, 804.

14. Calvert, J. G.; Pitts, J. N. Photochemistry; Wiley-Intersceince: New York,
1966.

15. McCracken, J .; Shin, D.-H; Dye, J. L. Appl. Magn. Reson. 1992, 3, 30.

16. Mims, W. B. J. Magn. Reson. 1984, 59, 291.

17. Mussel], R. D.; Nocera, D. G. J. Am. Chem. Soc. 1988, 110, 2764.

18. Zietlow, T. C.; Hopkins, M. D.; Gray, H. B. J. Solid St. Chem. 1985, 57, 112.
19. Demas, N. J.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991.

20. Newsham, M. D.; Giannelis, E. P.; Pinnavaia, T. J.; Nocera, D. G. J. Am.
Chem. Soc. 1988, 110, 3885.

21. Turro, C. Ph.D. Michigan State University, 1992.

22. Gagne, R. R; Koval, C. A.; Lisensky, G. C. Inorg. Chem. 1980, 19, 2854.

CHAPTER 3
PHOTOREDUCTION OF DIARYL/DIALKYL DISULFIDES BY QUADRUPLY

BONDED DITUNGSTEN COMPLEXES

A. Background

Without a flexible coordination sphere the redox reactions of M—4—M
complexes often proceed no further than the one electron oxidation product. This
is evident in the thermal and photochemical redox chemistry of quadruply bonded
bimetallic complexes with four rigid bridging ligands, M2(LL)4 (M = M0, W; LL =
bidentate ligand).l

A typical one electron oxidation is the reaction of M02(OZCR)4 (R = C2H5,
CMe3, Ph) with 12 to give [Mo(OzCR)4]I3.2 This is not sursprising since an
electrochemical study of Mo2(OzCC3H7)4 in acetonitrile, dichloromethane and
ethanol revealed only one electron quasi-reversible oxidations at + 0.39, + 0.45,
and + 0.30 V vs. SCE, respectively.3 Similar one electron oxidations are observed
for the oxidation of W2(OZCCMe3)4 by 12 to yield [W2_(OZCCMe3)4]I.4 This
difficulty in accessing the two electron oxidation product is preserved in the
photochemisry of M02(LL)4 complexes.

Photolysis (k > 254 nm) of M02(SO4)44" in 5M HZSO4 results in the
reduction of H‘”; for each mole of the one electron oxidized product, Moz(SO4)43’,

a 1/2 mole H2 is produced.5 Likewise, the product of irradiations of

52

53

Moz[OzP(OPh)2]4 with dichlorohydrocarbons is the one electron product
M02[OZP(OPh)2]4Cl, as discussed in Chapter 1. Replacing sof— or OZP(OPh)2‘
with HPO42‘, increases the reduction potential of the M02 core by almost 1V and
two electron chemistry is observed.6 The reason for this increase in reduction
potentials is unclear. HPOaz", S0427 and OZP(OPh)2‘ are structurally and
electronically similar, so it is unreasonable to account for the ~1V increase of
Mo2(HPO4)42’ on this basis. The chemical properties of the three differ, however.
It is possible to protonate the phosphate ligand, while this is unlikely with the
sulfate and the diphenyl phosphate. This may be the reason for the disparities in
the reduction potentials.7 Irradiation of 2M H3PO4 solutions of M02(HP04)42' with
k > 335 nm results in the formation of 1 mol each of H2 and the two electron
oxidized product, Moz(HPO4)42". Detailed mechanistic studies revealed that the
photochemistry proceeds through two consecutive one—electron reactions. Another
example of a two electron oxidation, is the thermal reaction of
(PhaP)2[Moz(OZCPh)4C12] with CC14 in CHzClz to give
(PhaP)2[M02(OZCPh)4C14]°CH2C12,8 where two of the 11412 benzoate ligands have
rearranged to u-nl ligands, Figure 3.1. The rearrangement of the benzoate ligands
in the last system raises the issue of a flexible coordination sphere. If two of the
bidentate ligands are replaced by ligands that are capable of rearranging, two
electron oxidations of quadruply bonded bimetallic complexes are conceivable.
Such multielectron chemistry is observed in the thermal reactions of the
complexes M2X4L4 (M = M0, W; X = anionic ligand; L = neutral ligand). The
formation of edge-sharing bioctahedrals (ESBO) upon two electron oxidation is a

common structural motif for these M—4—M complexes,9 Figure 3.2. The bridging

54

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Figure 3.2 The formation of an edge-sharing bioctahedron (ESBO) upon
two electron oxidation of a quaduply bonded bimetallic complex.

56

ligands make a flexible coordination sphere crucial in forming the octahedral
geometry of the ESBO; without this flexibility, one electron oxidations are often
the result. These complexes undergo oxidative addition with a variety of
substrates, such as X2,10 disulfides,ll diselenides,12 and HCl;13 all of which form an
ESBO structure.

A similar trend is observed in the photochemistry of this structure type.
Single photon concerted two electron photochemistry is observed upon replacing
two of the bridging ligands with chlorides, as opposed to the two photon two
electron chemisz observed with M02(HPO4)44". When CH3I (Mel) solutions of
W2Cl4(dppm)2 are irradiated with A. > 435 nm, the observed photoproduct is

consistent with the oxidative addition of Mel,14 as shown below:

 

P01 P01 P P
l/ |/ Mel Cl\ I /C|\ I /|
/| /| 7c>435 nm Cl | Cl | Me
Cl CI
P P P P

While the coordination geometry about the M—M core has not been unequivocally
established, the absorption profile is similar to that of reported ESBO structures.13
Additionally, the Me is proposed to be in a terminal position based on 13 C NMR.
This is consistent with the proposed open equatorial coordination site of the
intermediate, as discussed in Chapter 1. The observed photchemistry is in contrast
to the thermal chemistry of the system. The thermal reaction of W2C14(dppm)2 with

alkyl iodides results in W2C15(dppm)2l and W2Cl4(dppm)212, the formation of

57

which can be explained by invoking a radical mechanism. W112C14(dppm)2 can
react with R1 to produce the mixed valence complex, W2Cl4(dppm)2l. This can
disproportionate by either chloride or iodide atom abstraction to yield
W2C15(dppm)21 and W2C14(dppm)212, respectively. The formation of these products
is indicative of a radical mechanism. Since neither are observed in the mass
analysis of the photoreaction, this indicates that the oxidative addition of Mel is
concerted. This is further supported by the absence of any ethane as a product of
the photoreaction. We wished to broaden the scope of this chemistry to

diaryl/dialkyl disulfides.

B. Photochemistry of W2Cl4(dppm)2 and PhSSPh

The spectral changes associated with the irradiation of toluene solutions of
W2Cl4(dppm)2 and PhSSPh (k > 495 nm, —10 °C) are shown in Figure 3.3. The
isosbestic point indicates the formation of only one photoproduct. The absorption
maxima at 730 and 504 nm compare well with independently prepared
W2Cl.i,(dppm)2(SPh)2.12 Additionally, the FAB/MS spectrum of the purified
photoproduct shows a molecular ion peak at 1496 amu and fragments at 1419 and
1387 amu corresponding to W2C14(dppm)2(SPh)2+ (= [M]+), [M—C6H5]+ and [M—
C6H5—S]+, Figure 3.4. This has also been confirmed by ES/MS analysis.
W2C15(dppm)2(SPh) is not seen in the FAB/MS, indicating that the reaction does

not proceed through the radical mechanism shown below.

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hv *
W2CI4(dppm)2 —" W2Cl4(dppm)2

W2Cl4(dppm)2*+PhSSPh —> W2C|4(dppm)2(SPh)+-SPh
Clatom
abst'ad' " W2C'5(dppm)2(SPh) + "wacladppma"
2 WzCI4(dppm)2(SPh)

SPh W2CI4(dppm)2(SPh)2 + WzCl4(dppm)2
abstraction

While toluene solutions of W2Cl4(dppm)2 and PhSSPh do react in the dark at —10
°C, there is virtually no reaction on the time scale of the photoreaction. Under the
above conditions the thermal reaction is complete in 24 hours as opposed to 45

minutes for the photoreaction.
1. Quantum Yields

The action spectrum of W2Cl4(dppm)2/PhSSPh photochemistry is consistent
with the reaction originating from the metal complex. W2C14(dppm)2 reacts with
PhSSPh when irradiated with wavelengths as low as 546 nm with a quantum yield
((1)) of 0.025. The quantum yields increase to the blue, as shown in Table 3.1. The
onset of photochemistry is coincident with the absorption band immediately to
higher energy of the 52—>'58* transition (1mm, = 710 nm). This coincidence
between the position of the absorption band immediately to higher energy of
529153” and the action spectrum of W2Cl4(dppm)2 is preserved in the analogous

photoreaction of M02C14(dppm)2/PhSSPh,15 Table 3.1.

61

Table 3.1 Wavelength Dependent Quantum Yields for the Photoreaction of
M2Cl4(dppm)2 and PhSSPh.

 

 

raw/um (Dp1W2C14(dPPm)2la 9.1M02c14(dppm>21"
546 0.025 <10‘5
405 0.048 0.010
435 0.075 0.1 1
365 0.15 0.23
313 0.39 0.27

 

a. T = 0 °C in toluene; b. T = 16 °C in CHzClz.

The ~50 nm blue shift of the MozCl4(dppm)2 action spectrum, as compared to that
of the W2C14(dppm)2 homologue, is consistent with the ~50 nm blue shift of . the
absorption spectrum of the dimolybdenum complex, Figure 3.5. This wavelength
dependence of the photochemistry demonstrates that the photoreaction is metal-
based and excludes the possibility of the photochemistry being derived from
PhSSPh, by direct homolysis of the disulfide bond to produce RS- radicals.
Comparison of the action spectrum of W2Cl4(dppm)2 to M02C14(dppm)2
reveals the nature of the excited state responsible for the photochemistry. For
quadruply bonded bimetallic complexes, ligand-to-metal charge transfer (LMCT)
transitions blue shift upon exchange of W for Mo, due to greater difficulty in
reducing W".l6 Metal localized transitions, however, red shift when the metal is
changed from Mo to W, as demonstrated by the 652—9165? Therefore, even the

transitions in the higher energy range (450—300 nm) which are typically ligand-to-

62

 

 

 

 

 

'2
.‘2 >
>' 3
E o
a a
g 3
3 o
0 CD
0.4 (b)
E
9 e
E 0 8
3 <— 0'
3 8
O
O I I L J l I I l I

 

300 400 500 600 700 800

Wavelength/nm

Figure 3.5 Quantum yields for the photolysis of M2Cl4(dppm)2 and excess
PhSSPh. Panel (a) is W2C14(dppm)2 in toluene at 0 °C; (b) is M02C14(dppm)2
in CH2C12 at 20 °C.

63

metal or metal-to-ligand charge transfer transitions, are still metal-localized
transitions.l6 As a result, the Mo—)W red shift of the action spectra implies the
M2C14(dppm)2 photochemistry is metal localized.

The red shift of the transitions upon substitution of Mo with W of the
M2C14(dppm)2 (Dzh) complexes in the near—UV is in sharp contrast to the blue shift
of the near-UV transitions of the M2C14P4 (DZd) complexes, indicating a LMCT
transition of the latter.16 As a result, the photochemisty of the D2d complexes is
characterized by LMCT reactivity.l7 The differences in parentage of the high
energy transitions of the D21, complexes as compared to the D2d complexes is due
to the nondegeneracy of the 1: dyZ and 1t dxz orbitals in the former,18 Figure 3.6. The
presence of metal localized transitions in the near-UV is likely manifested in the

photochemistry of these complexes.
C. Transient Absorption

The photochemistry was complemented by transient absorption
spectroscopy. The dmpm derivative, M02C14(dmpm)p19 (dmpm =
bis(dimethylphosphino) methane), was used due to stimulated emission of the
dppm ligand (1: ~ 1 ns)20 upon high energy excitation (Km = 355 nm). Stimulated
emission over the wavelength range 450—520 nm, although weak, was problematic
due to the weak changes in optical density (AODs) of the transient absorption band
of M02C14(dppm)2 in this region. Replacement of dppm with dmpm eliminated the
stimulated emission and allowed sufficient signal to noise ratio. The absorption of
M02C14(dppm)2 is nearly identical to that of M02C14(dmpm)2 but is red shifted by
about ~40 nm. Also, the irradiation of M02C14(dmpm)2 and PhSSPh, under

 

A: 8553: .85388 223085 common b95336 c3658? 8Q was fin— H8 88me coca—oboe oZEstO Rm 253

 

c m. c on

 

 

64

 

 

 

 

 

5ND ONO ND 2ND LNG
d x n n a
d x x d _ x x a. _ x _ x n
_\ _\ \ \ /\ \ \ _ x
o o o 6.2 cs. 6.2 cs. \
_2 _2 _2 0.2
\_ \_ \_ \_ s/ \_ x\_ \
x a. x a n x x n a x x n n x”.

 

65

identical conditions as M02C14(dppm)2, produces M02C15(dmpm)2(SPh). Based on
this, the electronic structure of M02C14(dmpm)2 is expected to be similar to that of
M02C14(dppm)2. The problem of stimulated emission in the transient absorption of
W2C14(dppm)2 did not arise due to the red shift of the absorption spectrum, as
compared to M02, which allows the use of lower energy excitation. Picosecond
transient absorption of W2Cl4(dppm)2 and MozCl4(dmpm)2 affords a short lived
intermediate with lifetimes of < 1 ns and 40 ps, respectively. The transient
absorption profile of M02C14(dmpm)2 following a 3 ps excitation pulse at 600 nm
at 2, 20 and 50 ps after the pulse is shown in Figure 3.7. Concurrent with this
absorption is the recovery of the bleach of the 55* transition at 630 nm on the
same time scale. The short lifetimes of the 58* excited states preclude bimolecular
photochemistry and explain the absence of any M2C14(dppm)2/PhSSPh
photochemistry upon 55* excitation.

If the absorption profile immediately to higher energy of 88* is excited, a
long-lived transient is observed. Benzene solutions of W2C14(dppm)2, excited at
Aexc = 532 nm, display the transient absorption spectrum (Am, = 500 nm, ‘17 = 46 11s)
in Figure 3.8. The transient absorption profile is similar to the ground state
absorption spectrum of the edge-sharing bioctahedron, W2Cl6(dppm)2.21 A similar
result is observed for M02C14(dmpm)2.15 Dichloromethane solutions of
M02C14(dmpm)2 (km 2 355 nm) have a transient absorption profile (7+.max = 520
nm) that is similar to the ground state absorption of M02C16(dmpm)2.2| The
correlation of the spectra of the transients and the edge-sharing bioctahedra
suggest that the long-lived nonluminescent transient is derived from a chemically

distorted ESBO, the formation of which is consistent with the electronic structure

66

 

 

 

 

 

 

 

 

004-—
2 ps
0'02 ' 0 10 20 30
20 PS t/ps
o
O 0.00 I 450 p5
< 411111.
— “WM
-0.02 — 5° '03 ‘
20 ps
004 ~— 2 ps
1 I 1 | 1 I 1

 

400 500 600 700 800
k/nm

Figure 3.7 Picosecond transient absorption of M02C14(dmpm)2 in CHZCIZ
following a 3 ps excitation pulse at 600 nm. Spectra were recorded at 2,
20 and 50 ps following excitation. The inset shows the In plot of the
recovery of the bleach at 630 nm.

0J0

100D

005

000

67

 

 

eoueqrosqv

 

 

400

5d)

K/nm

6m)

Figure 3.8 Comparison of the transient absorption, following a 532 nm excitation
pulse, of W2Cl4(dppm)2 (O) to the ground state absorption of W2Cl6(dppm)2 (—), both
are in benzene.

68

of quadruply bonded bimetallic complexes. Although the electron count of the
chemically distorted ESBO (MIMIII = d5—d3) differs from that of M2X6(LL)2
complexes (Mmz = d3—d3) the absorption profiles of the two species should be
similar. The ordering of the molecular orbitals of the M2X6(LL)2 complexes are c
<<1c < 8*~5 < 71:* << 0*, with a small 8*/8 splitting.22 As a result, the Tr—->8*, 7r—>8,
8*—>1t* transitions in the d3—d3 ESBO should be energetically similar to the 8—m
and 5*-—>71:* transitions of the dS—d3 chemically distorted intermediate.
Additionally, if a lone pair of electrons is contained in a 1: orbital in the equatorial
plane, the electron count of the intermediate would be d3—d3(1tin_plane)2. The metal
localized transitions from this electronic configuration would even more closely
resemble the metal localized transitions of the native d3—d3 ESBO complex. The
metal localized excited states associated with 8 and 8* have charge transfer
character, M2* = MIMI".23 The chemical distortion to the ESBO can provide
cooperative stabilization with an octahedral geometry about the partially oxidized
metal center and a diminished electron density about the partially reduced metal
center. Furthermore, the existence of an activation barrier may prevent the
formation of the ESBO upon 55* excitation. The formation of the long-lived
nonluminescent intermediate is summarized in Figure 3.9. Excitation into the it
manifold is predicted to be necessary in order to diminish the metal-metal Tl:
bonding relative to the ground state. This is expected to enhance the formation of
the ESBO because the bridging M—L 7r dyz or 1: dxz bonds occur at the expense of

the M—M 1! bonds.

69

 

 

 

FI)/X
M———4 M
| {I
P P

p
I)‘

/
X

Figure 3.9 Formation of the chemically distorted ESBO upon excitation
of the n8* absorption band of M2Cl4(LL)2 complexes.

70

D. Photochemistry of W2Cl4(dppm)2 and EtSSEt

The spectral changes associated with the irradiation (7» > 495 nm, —20 °C)
of toluene solutions of W2C14(dppm)2 and EtSSEt, over the course of three days,
are shown in Figure 3.10. The lack of isosbestic point indicates the formation of
more than one product. These products are consistent with the formation of
W2C15(dppm)2(SEt) and W2C16(dppm)2, as determined by FAB/MS (1374 and
1394 amu, respectively). Some insight into the mechanism can be gained if the
photoreaction is stopped prior to completion. Figure 3.11 shows the absorption
spectra and FAB/MS after 1 hour and 45 minutes of irradiation. A significant
amount of W2C14(dppm)2(SEt)2 (1400 amu) has formed along with
W2C15(dppm)2(SEt) and W2Cl6(dppm)2. The UV-vis spectrum shows an
absorbance at 570 nm, which is believed to be due to W2C14(dppm)2(SEt)2, as well
as those due to the final absorbances. The formation of W2Cl4(dppm)2(SEt)2 early
in the photoreaction implies that it is the primary photoproduct but decomposes to
W2C15(dppm)2(SEt) and W2C16(dppm)2. The isosbestic point also indicates that, at
this point, the reaction is clean. A similar result was observed for
M02C14(dppm)2/RSSR photochemistry in toluene.15 When the photoproducts are
isolated prior to completion, M02C14(dppm)2(SR)2 is observed, which then
decomposes to M02C15(dppm)2(SR). No M02Cl6(dppm)2 is observed.

While no crystal structure of W2C15(dppm)2(SEt) has been obtained, 13C
NMR can reveal some information about the structure. In the 13C NMR of
W2C15(dppm)2(SEt) resonances at 5 +1236 and +88] ppm can be assigned to the
dppm ligands, based on the 13C NMR of W2Cl4(dppm)2, 5 +123.2 and +87.6 ppm.

71

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T 1 (a)
(D
O
C
. I
E
O
(D
.Q
< ,\
| l | | | {—1
400 500 600 700 800 900
Wavelength / nm
100-
I (b)
80 '[W]Cl [W]C|2
7 I WCISEt 1
60, 1 1 [ l ( ) 1
4O ‘ » 11 1
1 [W] i , , 1 [WKSEDZ
20' ”I!" 1'” '
1260 1280 1300 1320 1340 1360 1380 1400

am U
Figure 3.11 (a) Spectral absorption changes associated with the irradiation of
W2Cl4(dppm)2 and EtSSEt in toluene at —20 °C. The irradiation was stopped

after 1 hour and 45 minutes. (b) FAB/MS spectrum of the photoproducts
isolated from the final spectrum above.

73

Resonances at 5 +30.1 and —50.5 ppm have been assigned to a terminal SEt group.
The diarnagnetic anisotropy of the multiple metal-metal bond typically induces a
upfield shift in terminal ligands and an downfield shift in ligands bridging the two
metals.24 Based on the large upfield shift of the resonance at —50.5 ppm, +14.7
ppm in uncoordinated EtSSEt, this has been assigned to the —CH2- group of the
EtS ligand. The resonance at +30.l ppm is shifted from +33.3 ppm in
uncoordinated EtSSEt. The relatively small upfield shift indicates that this is less
reduced than the methylene group. The position of the SEt group, although not the
primary photoproduct, is consistent with the proposed chemically distorted
intermediate. Similar to the photoreaction of W2C14(dppm)2 and PhSSPh, no
appreciable reaction occurs at —20 0C for the duration of the photoreaction. The

dark reaction is complete in a month, as opposed to 3 days for the photoreaction.
E. Conclusions

The reaction of RSSR with W2Cl4(dppm)2 shows oxidative addition to be a
general photoreaction pathway for this complex. Additionally, the transient
absorption studies show that the 188* is not efficiently trapped upon excitation into
this absorption band and therefore gives rise to no photochemistry. If higher
excitation energy is used, the excited state can be adequately trapped via a
chemically distorted ESBO and used for substrate reduction. That
W2C14(dppm)2(SEt)2 is unstable is not easily explained. However, since it is seen
in the early stages of the photoreaction, the photochemical mechanism for EtSSEt

activation appears to be oxidative addition.

74

F. References

1. Cotton, F. A.; Walton, R. A. Multiple Bonds Between Metal Atoms; Clarendon:
Oxford, 1993; pp 161-162, 233.

2. McCarley, R. E.; Templeton, J. L.; Colburn, T. J .; Kotavic, V.; Hoxmeier, R. J.
Adv. Chem. Ser. 1976, 150, 318.

3. Cotton, F. A.; Pederson, E. Inorg. Chem. 1975, 14, 399.

4. Santure, D. J .; Huffman, J. C.; Sattelberger, A. P. Inorg. Chem. 1985, 24, 371.

5. Erwin, D. K.; Geoffroy, G. L.; Gray, H. B.; Hammond, G. S.; Solomon, E. I.;
Trogler, W. C.; Zagars, A. A. J. Am. Chem. Soc. 1977, 99, 3620.

6. Chang, I—J.; Nocera, D. G. J. Am. Chem. Soc. 1987, 109, 4901.

7. Nocera, D. G. J. Cluster Sci. 1994, 5, 185.

8. Jansen, K.; Dehnicke, K.; Fenske, D. Z. Natmforsch 1987 , 426, 1097.

9. (a) Cotton, F. A. Polyhedron 1987, 6, 677. (b) Cotton, F. A.; Hong, B.; Shang,
M.; Stanley, G. G. Inorg. Chem 1993, 32, 3620. (c) Cotton, F. A.; Maloney, D. J.;
Su, J. Inorg. Chim. Acta 1995, 236, 21. ((1) P011, R.; Torralba, R. C. Inorg. Chim.
Acta. 1993, 212, 123.

10. (a) Cotton, F. A.; Daniels, L. M.; Dunbar, K. R.; Falvello, L. R.; O’Connor, C.
J.; Price, A. C. Inorg. Chem. 1991, 30, 2509. (b) Canich, J. M.; Cotton, F. A.;
Daniels, L. M.; Lewis, D. B. Inorg. Chem. 1987, 26, 4046. (c) Agaskar, P. A.;
Cotton, F. A.; Dunbar, K. R.; Falvello, L. R.; O’Connor, C. J. Inorg. Chem. 1987,
26,4051.

11. Cotton, F. A.; Diebold, M. P.; O’Connor, C. J .; Powell, G. L. J. Am. Chem.
Soc. 1981, 107, 7438.

75

12. Canich, J. M.; Cotton, F. A.; Dunbar, K. R.; Falvello, L. R. Inorg. Chem. 1988,
27, 804.

13. (a) Cotton, F. A.; James, C. A.; Luck, R. L. Inorg. Chem. 1991, 30, 4370. (b)
Fanwick, P. B.; Harwood, W. S.; Walton, R. A. Inorg. Chem. 1987, 26, 242.

14. (a) Partigianoni, C. M.; Nocera, D. G. Inorg. Chem. 1990, 29, 2033. (b)
Partigianoni, C. M.; Turro, C.; Hsu, T .-L. C.; Chang, I-J.; Nocera, D. G. In
Photosensitive Metal-Organic Systems; Kutal, C., Serpone, N ., Eds.; Advances in
Chemistry 238; American Chemical Society: Washington, DC, 1993; pp 147-163.
15. Hsu, T.-L. C.; Helvoigt, S. A.; Partigianoni, C. M.; Turro, C.; Nocera, D. G.
Inorg. Chem. 1995, 34, 6186.

16. Hopkins, M. D.; Miskowski, V. M.; Gray, H. B. J. Am. Chem. Soc. 1988, 110,
1787.

17. Hsu, T.-L. C.; Engebretson, D. S.; Helvoigt, S. A.; Nocera, D. G. Inorg. Chim.
Acta. 1995, 240, 515.

18. Agaskar, P. A.; Cotton, F. A.; Fraser, 1. F.; Manojlovic-Muir, L.; Muir, K. W.;
Peacock, R. D. Inorg. Chem. 1986, 25, 2511.

19. Cotton, F. A.; Falvello, L. R.; Harwood, W. 8.; Powell, G. L.; Walton, R. A.
Inorg. Chem. 1986, 25, 3949.

20. Fife, D. J .; Morse, K. W.; Moore, W. M. J. Photochem. 1984, 24, 249.

21. Cotton, F. A.; Eglin, J. L.; James, C. A. Inorg. Chem. 1992, 31, 5308.

22. (a) Shaik, S.; Hoffman, R.; Fisel, R.; Summerville, R. H. J. Am. Chem. Soc.
1980, 102, 4555. (b) Chakravarty, A. R.; Cotton, F. A.; Diebold, M. R; Lewis, D.
B.; Roth, W. J. J. Am. Chem. Soc. 1986, 108, 971. (c) Cotton, F. A.; Diebold, M.
P.; O’Connor, C. J.; Powell, G. L. J. Am. Chem. Soc. 1985, 107, 7438.

76

23. (a) Engebretson, D. S.; Zaleski, J. M.; Leroi, G. E.; Nocera, D. G. Science
1994, 265, 759. (b) Hopkins, M. D.; Gray, H. B.; Miskowski, V. M. Polyhedron
1987, 6, 705.

24. Cotton, F. A.; Walton, R. A. Multiple Bonds Between Metal Atoms; Clarendon:
Oxford, 1993; p 648.

CHAPTER 4

ATOM TRANSFER PHOTOCHEMISTRY OF W2CI4(dppm)2 AND MoWCl4(PMe2Ph)4

A. Background

1. Atom Transfer

Atom transfer reactions have been the subject of research for many years
owing to the importance in oxidation-reduction chemistry."2’3’4’S The basic reaction
is ubiquitous in chemistry and biology.6 Molybdenum enzymes,7 for example, are
known to catalyze oxygen atom transfer for a variety of substrates, such as
xanthine (to uric acid),8 sulfite (to sulfate),9 nitrate (to nitrite),10 dimethyl sulfoxide
(to dimethyl sulfide)11 and aldehydes (to carboxylic acids):8 The area of atom
transfer chemistry was pioneered by Henry Taube’s studies of the late 19508. With
the use of isotopic labelling and product appearance studies, it was determined that
some atom transfer occurs by what Taube coined an inner sphere electron transfer

mechanism.12 This reaction type was first demonstrated with the reaction below.
H20

 

or(H20).,2+ + Co(NH3)5C|2+ Cr(H20)5C|2+ + CO(H20)52+ (4.1)

 

From the appearance of CI’ in the substitutionally inert CrIII product, it was
reasoned that when the electron transfer occured both metal centers were bound by

the Cl', a tenet that was later verified by C1 isotope studies. After the electron

77

78

transfer, the substitutionally labile CoII loses the Cl" ligand to the inert CrIII

complex. The simple fact that the electron transfer occurs by an inner sphere
mechanism introduces an element of structural definition. The electron transfer
distance is defined by the bridging ligand and the relative positions of the reducing
agent and oxidizing agent are determined by the structures of the complexes.13 The
atom transfer event can accordingly be reduced to a series of steps, which are
outlined in Figure 4.1, where any of the steps can be rate limiting in the reaction.
Diffusion-controlled formation of the precursor complex, equation 4.2,13 is
followed by formation of the activated complex, equation 4.3.13 As formulated by
Taube, the activated complex is formed with an atom bridging both the oxidizing
and reducing center.14 The electron transfer step occurs in equation 4.4, leading to
the formation of the successor complex, which subsequently dissociates to
products CI'(H20)5C12+ and Co(NH3)5(HZO)2+, equation 4.5. Due to the lability of
Co", the ammonia ligands dissociate to yield Co(HzO)62+, equation 4.6, in a
kinetically unimportant step.

The electron equivalents transferred in an atom transfer are typically
determined by the atom that is transferred. For instance, the transfer of univalent
atoms will usually mediate one electron reactions.l4 Divalent atoms, such as sulfur
and oxygen, are known to mediate two electron redox reactions,14 and as such are
quintessential in the study of multielectron chemistry. For this reason, such atom
transfer reactions form the foundation upon which multielectron photochemistry
may be built.

A major focus of oxygen atom transfer studies has been the study of model

enzymatic reactions. For example, the work of Holm has focused, in part, on

79

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80

synthetic analogues for molybdenum oxotransferases.15 One system that has
developed from this large body of research is the oxygen transfer to/from a

MONO/MOVIOZ complex, as shown below, equation 4.7.16

MoV'Oz(tBuL-NS)2+X ——————- Mo'VO(tBuL-NS)2+NO (4.7)

tBuL-NS =

 

Whereas the work of Holm has focused mostly on atom transfer between metal
complexes and organic substrates, the research efforts of Wool7 and, separately,
West,18 have concentrated on gaining a better understanding of intermetal atom

transfer. Two examples of typical reactions studied are shown below.

(TTP)Ti=O + (OEP)TiC|

II

(TTP)TiCI + (OEP)Ti=O (4.8)

(TPP)Cr + O=Cr(TPP) (TPP)Cr—O—Cr(TPP) (4.9)

l

The atom transfer reaction between titanium porphyrines, equation 4.8, is referred
to as complete atom transfer since the transferred atom is no longer bound by the
donor molecule,4 while equation 4.9 is an example of incomplete atom transfer

since the atom maintains bonds to the donor and acceptor molecules.4

81

Oxygen and sulfur atom transfer reactions that are of relevance to this thesis
occur with MC12P4 (M = M0, w; P = PMe3, PMeth).19’20’21’22 The reactions
involve the dissociation of the PR3 ligand from the octahedral MIIC12P4 complex
and ensuing oxidation to MIV(E)C12P3 (E = OZ— or S2”) octahedral complexes upon
atom transfer from the substrate. A typical sulfur atom transfer reaction of WC12P4

is shown below.19

E
II P
C|—/-W—P + X + P
P |
Cl C'
| /P
P7W—P + EX —-> (4.10)
P |
Cl E
II /P
S C|P—;V|V—X + 2P
EX=SPR3,Z_\. Cl

In these reactions, atom transfer does not control the reaction kinetics; phosphine
dissociation is the rate limiting step in these reactions. Correspondingly, PMe3
dissociates from MoC12(PMe3)4 and WC12(PMe3)4 with a half-life of 18 minutes at
ambient temperature and 7 minutes at 69 °C, respectively, thereby determining the
reaction rate of the system.19 Likewise, the bulky labile PMeth is substituted by
PMe3 in WC12(PMePh2)4, and the subsitution is complete within 24 hours.19
Despite that the atom transfer is inconsequential to the overall kinetics, the
reactions are important to studies discussed herein. Dissociation of a phosphine

ligand creates an intermediate where the d4 metal is in a square pyramidal

82

 

MCI2P4 Complexes M2CI4P4 Complexes
CI CI P
P P CI
l/ I/ I/
p/l ,/| e./[
Cl Cl P
11
-P
Cl P
I/
P7MH—P
P I
CI

Figure 4.2 Comparison of the coordination spheres of MC12P4 and M2C14P4.
Dissociation of a phosphine ligand is the first step in the reactions of MC12P4
complexes. As a result, atom transfer occurs at a square pyramidal
coordination sphere in both complexes.

83

coordination sphere. This is analogous to the reactive site of M—4—M complexes,
as summarized in Figure 4.2. It is, therefore, conceivable that similar products
would result from sulfur atom transfer reactions with quadruply bonded bimetallic
complexes. Moreover, because phosphine dissociation is not relevant to the
M—4—M systems, the direct atom transfer should be kinetically resolved thus
allowing us, in principle, to study the details of two electron atom transfer.

While there are many examples of thermal atom transfer reactions,23
photoinitiated atom transfer is less well known. An atom can be transferred to a
photochemically generated transition metal intermediate or through direct
interaction with the transtion metal excited state. The first type of reactivity is well
established. This is typified by the creation of a reactive intermediate by M—M or
M—L bond homolysis in the excited state followed by atom abstraction.24 For
example, excitation of [CpM(CO)3]2 (M = Mo, W)25 and MM’(CNMe).,2+ (M = M'
= Pd, Pt; M = Pd, M' = P026 results in the formation of the 17 electron species
CpM(CO)3 and the 15 electron species M(CNMe)°+, both of which are
coordinatively unsaturated and very reactive. These intermediates can abstract
halogen atoms from alkyl/aryl halides to yield CpM(CO)3X and M(CNMe)X+,

respectively. Examples of these reactions are shown in equations 4.11 and 4.12,

 

 

below.
hv
C W CO 2 C W CO
[PI 1312 P ( )3 (4.11)
CpW(CO) 3 + Ph3CBr ——> CpW(CO) 3Br + Ph 3C.
[(CNMe)3Pd]2+ ~h—V— 2 (CNMe)3Pd+
(4.12)

(CNMe)3Pd++CCI4 —> (CNMe)3PdCI+

84

Another recent example of atom transfer via a photochemically generated
intermediate involves the complex, szTa(CH3)(C2H4) (l).27 The photoreactivity
of this complex is summarized in Figure 4.3. The photoreaction of (1) is driven by
the dissociation of ethylene from the excited state Ta complex to form of the
coordinatively unsaturated intermediate, szTa(CH3) (2). This reacts with
stoichiometric amounts of thiirane or SPMe3 to generate szTa(=S)(CH3) (3).
Irradiation of (1) with 0.5 equivalents of thiirane results in the formation of
Cp2(CH3)Ta(u-S)Ta(CH3)Cp2 dimer(4), which can also be prepared by irradiation
of (1) in the presence of (3). Complex (3) undergoes migratory insertion of the Ta—
CH3 bond upon irradiation to form “szTa—SCH3”, which then reacts with thiirane
to yield szTa(=S)(SCH3) (5). This is in contrast to the thermal chemistry of (3),
which reacts with thiirane to give the persulfido complex, szTa(u—S2)(CH3) (6)
and can be regenerated upon reaction of (6) with PR3. Irradiation of (6) yields the
atom abstraction products (3), (5), SPR3 and other products, in low yields.

The above systems are all examples of atom transfer to/from a reactive
intermediate generated from an excited state. Examples of direct atom transfer to
an excited state are not as pervasive. Certainly, the most developed chemistry in
this front is that of the dg—d8 dimers.28 The 3A2u (do*po) excited state of
Pt2(pop)44‘ abstracts hydrogen atoms from (CH3)2CHOH, PhCH(OH)CH3,
Bu3SnH, Et3SiH and H3PO3. The triplet state of Pt2(pop)44" can also abstract
halogens from alkyl and aryl halides. Transient absorption spectroscopy shows that
the initial product is a Pt"PtIII mixed valence species, Pt2(pop)4X4',29’3O which then

111

undergoes thermal X atom transfer to give a Ptht final product. Likewise, the

d8—d8 system Rh2L2(dppm)22+ (L = 2,5-di-isocyano-2,5-dimethylhexane) undergoes

85

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photoinitiated atom abstraction with alkyl and aryl halides directly from the excited
state,31 along the same mechanistic lines described for PtHPtH system. The Rh—Rh
system has an advantage over the Pt2(pop)44" system in that it can be excited in the
visible region (595 nm), as opposed to excitation at 367 nm for Pt2(pop)44’.
Therefore, the Rh—Rh complex has some potential for use in solar energy
conversion.

Because the transferring atom is uniValent, the above examples are confined
to one electron redox processes. Can two electron processes be photochemically
induced? The place to look is 02— transfer. There are some examples in the
literature, but these are all photoinitiated; atom transfer does not directly occur
with the excited state species. The photochemical oxygen atom transfer from N-
oxides to 1-pheny1-1,2-ethanediol (PED) is catalyzed by Cr'l‘PP(Cl).32 Irradiation
of (TPP)CrHICl in the presence of the oxygen atom transfer reagent, p-cyano-N,N-
dimethylaniline N-oxide (NO) forms the strong oxidant (TPP)CrVO and p-cyano-
N,N-dimethylaniline (DMA). (TPP)Cer can then either disproportionate with the
starting material, (TPP)CrmCl, to form (TPP)CrIVO or transfer on oxygen to PED
to form benzaldehyde, formaldehyde, water and (TPP)CrmCl. This reaction is

summarized in equations 4.13 - 4.15.
(TPP)Cr"'C| + NO 4‘1» —> (TPP)(CI)CrVO + DMA (4.13)

(TPP)(CI)CrVO + (TPP)Cr'"C| 339» 2(TPP)Cr'VO +2HC| (4.14)

(TPP)(CI)CrVO + PED —>

(TPP)Cr'”CI + PhCHO + HCHO + H20 (4.15)

87

Likewise, irradiation of MnTPP(OAc) in the presence of 104‘ and various
hydrocarbons (R3CH) results in the catalytic production of R3COH.33 Upon
dissolution of MnTPP(OAc) and R4NIO4, the complex MnTPP(IOa) is formed.
Irradiation of MnTPP(IO4) creates the strong oxidant O=MnTPP+ by oxygen atom

abstraction from 104’. This oxo then oxidizes R3CH to R3COH, as shown below.

MnTPP(OAc) + 104‘

 

MnTPP(IO4) + OAc" (4.16)

 

h
MnTPP(lO4) —-Y—> O=MnTPP+ + 103‘ (4.17)

O=MnTPP+ + R3CH —> MnTPP+ + R3COH (4.18)

B. Photoreaction of W2Cl4(dppm)2 and N20

We have shown that quadruply bonded bimetallic complexes form a long-
lived high energy intermediate with two electrons localized on one W metal center
upon excitation of the 1(115*) transition.34 The localization of two electrons on one
metal of the bimetallic core ideally sets the stage for atom transfer reactions with
two electron substrates such as oxygen or sulfur. In an effort to demonstrate the
principle of a two electron reaction, the most energetically favorable structure for
atom transfer was crafted. The W2 quadruple bond systems are the most easily
oxidized. Thus, W2C14(dppm)2 was chosen as the M—4—M photoreagent. With
regard to the atom transfer reagent, nitrous oxide (N20) is ideal. There is a large
driving force for the loss of N2 (AGf° = 25 kcal/mol),35 which upon atom transfer is
the only side product. Due to its high activation barrier (59 kcal/mol), it has

potential as a selective reagent.36

88

There are many examples of reactions between transition metals and N20
that involve the formation of M—O bonds.37 A characteristic reaction is the
formation of metal-oxo clusters; for example the reaction of szM (M = V, Cr)
with N20 affords the cluster compounds, (115 -Cp)5V5(1,13 -O)6 and (nS-Cp)4Cr4(u3-
O)4.38 In contrast, a terminal M=O bond is less commonly observed owing to the
propensity of the oxygen to bridge metals to form higher nuclearity products.
Cluster formation has been observed to be circumvented when 0' donating ligands
are also present.39 For example, the reaction of *szTi(C2H4) and N20 in
pyridine/1‘ HF solutions results in *szTi(=O)(py) (*Cp = Me5C5; py = pyridine).

Of course, oxygen atom transfer reactivity is not the only reaction available
to NZO. There is one known complex with NZO as a ligand, [Ru(NH3)5(NZO)]2+.40
From X—ray powder and IR results, the N20 is bound in an ln—N fashion, Ru-
N=NO. Moreover, (ArRN)3Mo=N and (ArRN)3Mo—-NO were recently formed by
the cleavage of the N=N bond of nitrous oxide by (ArRN)3Mo (Ar = 3,5-C6H3Me2;
R = C(CD3)2CH3).41 However, examples of this type of reactivity for N20 have
rarely been observed in the literature,42 and N20 typically reacts to deliver an
oxygen atom to a metal center. On the basis of this literature, the photochemistry

of W2Cl4(dppm)2 with NZO was studied.
1. Characterization of Inorganic Products

Irradiated solutions of W2C14(dppm)2 and THF are stable. However,
irradiation (km > 37.5 nm, —80 0C) in the presence of N20 results in the spectral
changes in Figure 4.4. The reaction requires one week to reach completion and the
lack of isosbestic points indicates that more than one product is formed. The final

absorption spectrum is characteristic of edge-sharing bioctahedrals and indeed

89

 

1 ‘l (a)

Absorbance
/ I
\\
l l
—->

 

(b)

Absorbance

_/\

l l I l l

400 500 600 700 800

 

 

 

Wavelength / nm

Figure 4.4 (a) Spectral changes observed upon irradiation of W2Cl4(dppm)2
and N20 in THF. (b) Absorption spectrum of independently synthesized
W2C16(dPPm)2

90

compares well to that of independently synthesized W2Cl6(dppm)2;43 which is also
shown in Figure 4.4. Additional evidence for the formation of W2Cl6(dppm)2 is
provided by NMR spectroscopy. 1H NMR resonances due to W2C16(dppm)2,
marked with an asterisk in Figure 4.5, at 5 +7.4 (m; Ph—P), 7.2 (m; Ph—P) and 3.5
(s; P—CHz—P) ppm correspond to the dppm ligands of W2Cl6(dppm)2. The 31P{1H}
NMR spectrum shows a broad absorption at 8 —106 ppm. Both the 1H and 31F
values compare well to the reported literature values.43 The upfield shift and
broadening of the 31P signal as compared to that of free dppm (—22.7 ppm in
CHzClz) is due to the thermal population of a low lying paramagnetic state. The
nuclear relaxation effects under these conditions are expected to quench the 31P—lH
coupling.44 A similar result has been observed with Re2C16(dppm)2.45 The NMR
spectrum reveals, however, that W2C16(dppm)2 is not the sole inorganic product.
Based on integration of the 31P{1H} NMR, the formation of W2Cl6(dppm)2
accounts for only ~12% of the total inorganic product. Another ~10% accounts for
free dppm and resonances at 8 +26 ((1; Ph2P(O)CH2PPh2) and —26 (br;
Ph2P(O)CH2PPh2) have been assigned to dppm oxide (33 %).46 Concordant with
these observations is the formation of an insoluble blue film. The possibility that
this is the slightly soluble blue W2Cl5(u-H)(dppm)2 complex has been dismissed
due to the absence of the characteristic AB pattern at 5 +6.14 ((1) and +4.55 ((1)
ppm (ZJAB = 12 Hz; in CDzClz), of the methylene protons in the 1H NMR.43 The
blue film is most likely due to the formation of tungsten oxides, which

charateristically form blue, insoluble films.4L7

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2. Characterization of Organic Products

Most of the 1H NMR resonances (Figure 4.5) of the photolyzed solution are
due to organic side products from reaction of THF. While none of the organic side
products have been definitively characterized, the nature of the products should be
similar to those characterized previously from the direct photolysis of ethers and
alcohols.“49 It has been shown that the photolysis (km > 185 nm) of THF in the
liquid or gas phase results in decomposition to form the products in Figure 4.6.48’49
For cyclic ethers, the primary reaction of the no* excited state involves C—O bond
homolysis. The diradical that is produced undergoes various reactions involving
fragmentation and rearrangement to aldehydes, olefins and alcohols. Resonances
due to these species are observed in the 1H NMR spectrum of the photoreacted
solutions. Absorptions due to these functional groups are also observed on the IR
spectrum of the organic products, as shown in Figure 4.7. In addition, an
absorption at 729 cm’1 in the region of C—Cl bonds (800 - 600 cm‘l)SO is observed.
These results suggest that the products of the photoreaction of THF are similar to

those formed from the direct photolysis but also include some chlorinated

products.
3. Reaction Pathway

It is clear from the above results that oxygen atom transfer from N20 to
W2C14(dppm)2 is not observed. A radical mechanism is most likely responsible for
the photochemistry observed with W2C14(dppm)2 and N20, as indicated by the
formation of W2C16(dppm)2 (see Chapter 3). T wo processes appear to be ocurring

in this reaction; the coordination of THF by W2C14(dppm)2 and the insertion of an

93

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oxygen atom into the bound THF and its ensuing cleavage. Literature precedence
for both of these processes exist.

Ligation of THF in the axial position of W2Cl4(dppm)2 could weaken the
C—0 bond, facilitating the insertion of an oxygen atom and the ring opening
reaction of the newly formed metal-ester. Precedence for axial coordination of
THF comes in the photochemistry of M02[02P(0Ph)2]4. Determination of the
structure of a single crystal of M02[02P(0Ph)2]4 grown in THF solvent shows two
molecules of THF coordinated in this position on either end of the complex.51
Consequently, the photoreaction of M02[02P(0Ph)2]4 with 1,2-dichloroethane is
significantly hindered by coordinating solvents such as THF and CH3CN.

Moreover, W—THF bonds are very strong. For instance, THF can cleave the
W—Clb—W bonds of [WC14(NC6H4Me—p)]2 to form WC14(NC6H4Me—p)(THF).52
The crystal structure of the product shows that the W—O (T HF) bond is elongated
relative to normal WIV—O oxo bonds (2.237 A as compared to 1.7 A for
W(O)C12L3).53 Illustrating that the M-ester bond is still not as strong as the M oxo
bond.

Once coordinated, the C—0 bond of a THF axially ligated W2C14(dppm)2
species is susceptible to insertion and subsequent cleavage. The ability of N20 to
insert an oxygen atom into M—L bonds is well established. Recently, it has been
shown that oxygen atom transfer to organometallic complexes can introduce
functionality into hydrocarbon substrates.54 Nitrous oxide reacts with the
metallocyclopentane complex, (bpy)Ni(C4Hg), to effect oxygen atom insertion into
the Ni—C bond and the elimination of N2.55 The product of this reaction can

eliminate various hydrocarbons under different reaction conditions, as shown in

 

96

Figure 4.8. Similar reactivity has been shown to be general for the NiII alkoxides.56
Nitrous oxide will react with both cyclic and acyclic nickel alkyls to produce a
variety of nickel alkoxides, which can then be used to generate functionalized
hydrocarbons.

Precedence for the cleavage of THF by transition metal complexes57 is
shown in the reaction of VC13(THF)3 and the octaethylporphyrinogen macrocyclic
ligand, Et3N4Li4(THF)4. This reaction results in the evolution of ethane and the

formation of the vanadium ynolate, below.58

H
THF\ O/ALi/THF
R

Li/ /
p \4‘
Et8N4Li4(THF)4+VC|3(THF)3 —» R' “RV/N? + 0sz
, ‘N/ \ R
/ N \
R \
R=CEt2 I «Am

 

The hydrocarbon fragments are formed from the cleavage of coordinated THF,
initiated by the d2 V center and assisted by the Lewis acidic Li cations on the
macrocycle. This occurs via the intermediate complex, (Et3N4)VLiCl(THF), and
the cleavage of the C—0 bond of THF is facilitated by Li—O coordination.

On the basis of the above results, it appears reasonable that the following

process is occuring.

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Full characterization of the liquid and gaseous organic photoproducts is neccesary
to fully understand the mechanism of this photoreaction. In addition, using a
noncoordinating solvent, such as toluene, could prevent the formation of organic
side products and make it possible to observe oxygen atom transfer products of

W2Cl4(dppm)2.
C. Photoinitiated Sulfur Atom Transfer to MoWCl4(PMe2Ph)4

The problem confronting the W2Cl4(dppm)2/NZO photoreactivity appears to
be of kinetic origins. In general, atom transfer involves reagents with high bond
dissociation energy. Thus, even though the overall energetics of the

W2Cl4(dppm)2/N20 system may be favorable for atom transfer, there is still the

99

issue of the strong N=O bond. More generally, P—O, N—O, and S—O bonds are
more stable, therefore a high activation barrier can be expected for photoinduced
atom transfer reactions. Table 4.1 compares the bond energies for substrates that
react by atom transfer or oxidative addition. On average, the bond strengths for
atom transfer substrates are much higher than those for oxidative addition
substrates. Consequently, the excited state inducing atom transfer cleavage must be
energetic enough to overcome the kinetic barrier imposed by the substrate
transferring the atom. The problem with quadruply bonded bimetallic complexes is
that the two electron character of the homonuclear bimetallic complexes arises
from an intramolecular distortion from the zwitterionic excited state. Due to this
distortion, much of the energy originally put into the M2Cl4(PR3)4 excited state is
lost through ligand rearrangement. As a result, their utility for multielectron
photochemistry is limited to substrates with a low activation energy and therefore,
oxidative addition reactions. The overall problem is schematically shown in Figure
4.9. Are there other ways to chemically trap the zwitterionic character without
inducing an energy wasting intramolecular distortion? One potential solution is to
build the asymmetry directly into the ground state by using a mixed-metal '
quadruply bonded bimetallic complex, such as MoWCl4(PMe2Ph)4. This approach
avoids the need for trapping the mixed valence excited state via intramolecular
distortions, Figure 4.9, since the inversion center of the complex is removed. Thus
electron density in these complexes may localize on a metal without distortion. In
the case of Mo—W species, the electron pair is expected to reside on the Mo, since

111

it is easier to reduce than W. Consequently, the W—MoI state should have a

100

Table 4.1 Bond Dissociation Energies for Atom Transfer and Oxidative Addition

 

 

 

Substrates.
Atom Transfera Oxidative Addition
substrate BDE!”c (kcal) substrate BDEb (kcal)
03 21 I—I 36(1
N 20 34 Br—Br 46d
C12SeO 58 PhS—SPh 55e
PhCH=N(O)Ph 63 CH3—I 56f
C5H5NO 72 C1—C1 58d
Pr"N=N(O)Prn 59 HS—SH 61g
503 83 MeHg—Me 61g
MeZSO 87 CH3—Br 70e
MoOC14 101 H—1 7 1e
Ph3AsO 103 PhS—H 83g
SOC12 104 CH3—Cl 84e
Me2802 1 18 H—Br 88e
C13PO 124 R3Si—H ~90g
WOCl4 127 H—H 104°
Ph3PO 133
(EtO)3PO 148

 

a. reference 35; b. BDE = bond dissociation energy; c. for reaction X0 = X + 0; d.
Morrison, R. T.; Boyd, R. N. Organic Chemistry; Prentice Hallenglewood Cliffs,
NJ, 1992, 6th edition. e. reference 78; f. reference 77; g. Handbook of Chemistry
and Physics, 69th ed.; CRC: Boca Raton, Florida, 1988; p F-183.

101

n5*

Clme/Ck

Cl/ \Cl/

 

 

 

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Figure 4.9 Proposed mechanism for the formation of the mixed valence
excited state of heteronuclear and homonuclear quadruply bonded bimetallic
complexes.

102

greater contribution to the linear combination of the 55* excited state, as opposed
to the homonuclear complexes where each resonance form contributes equally.
Along these lines, the excited state of MoWCl4(PMe/2Ph)4 can be expected to be
more reactive than homobimetallic M—4—M complexes.

The atom transfer reactions of MC12(PMe3)4 (M = M0, W) with a variety of
substrates are well characterized and provide a good reference for developing
MoWCl4(PMe2Ph)4 atom transfer photochemistry. MoClz(PMe3)4 abstracts a sulfur
atom from SPMe3 to yield Mo(S)C12(PMe3)3, which conproportionates with
MoC12(PMe3)4 to form the dimer, Moz(u—S)(u—Cl)C13(PMe3)5.20 If the complexes
M(S)C12(PMe3)3 and M’C12(PMe3)4 (M, M’ = M0 or W) are heated at 80 °C, they
undergo incomplete atom transfer to form the dimer MM’(u—S)(u—
C1)Cl3(PMe3)4,59 as shown in Figure 4.10. These products are similar to the two-
electron oxidized products of M—4—M complexes,60 and could also reseasonably be
a product of sulfur atom transfer to MoWCl4(PMe2Ph)4. With this in mind, the
sulfur atom transfer photochemistry of the mixed-metal quadruply bonded
complex, MoWCl4(PMe2Ph)4, with the sulfur atom transfer reagent, Ph3PS, was

studied.
1. Photophysics

Before the photochemistry of MoWCl4(PMe2Ph)4 was undertaken, an
understanding of the photophysics of the complex was desired. The visible
absorption spectrum of MoWCl4(PMe2Ph)4 is shown in Figure 4.11, along with
those of MozCl4(PMe2Ph)4 and W2Cl4(PMe2Ph)4. The assignment of the electronic
transitions is consistent with the M02 and W2 homologs.“ Correspondingly, the

two lowest energy transitions can be assigned to l(6—)8*) and l(1r—)5*), as

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105

discussed in Chapter 1. These transitions for MoW fall between those for M02 and
W2. This is consistent with there MMCT character since a decrease in
contributions of the two electron energy to the overall transition energy is expected
on moving from Mo to W. The higher energy transitions have been assigned to
LMCT in the homobimetallics. These transitions exhibit blue shifts upon
substitution of W for Mo due to the larger radial extension of the W orbitals. We
see that this transition for the MoW complex does indeed shift to the blue of W2
but lies to lower energy than M02.

MoW dimer has emission and lifetime characteristics that also fall in line
with its homonuclear congeners. MoWCl4(PMe2Ph)4 exhibits red luminescence
upon excitation of the l(5—>5*) transition. The electronic absorption and emission
spectra are essentially mirror images, Figure 4.12, indicating a small Stokes shift
and an excited state with a similar geometry as the ground state. Table 4.2 lists the
lifetimes, luminescent quantum yields, and the luminescence maxima for
MozCl4(PMe2Ph)4, MoWC14(PMe2Ph)4 and W2Cl4(PMe2Ph)4. The lifetime of
MoW (44 ns) is longer than the lifetime of M02 (11 ns) and very similar to W2 (45
ns). The lifetime of MoW is long enough for a bimolecular reaction at high
concentrations.62 The luminescent quantum yield of MoWCl4(PMe2Ph)4 is 0.069
and, as expected based on the lifetimes, similar to the quantum yield for
W2C14(PMe2Ph)4 (4)., = 0.051). The luminescent maximum of MoW (9‘. = 810 nm)
is red shifted from both M02 and W2 0» = 690 and 790 nm, respectively). Since the
185* absorption transition falls between those of M02 and W2, it is suprising that

the 156* emission energy does not.

106

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Table 4.2 Physical Properties of M2C14(PMe2Ph)4

 

M2 To/IIS (be A'em,max /nm 8ox N 8red I V

 

Mo2 11a 0.011a 690 030° —1.63°
MoW 44 0.069b 810 0.430 —1.73°
w2 45a 0.057b 790 —0.106 —2.14

 

a. Hsu, C. T.-L. Ph.D. Dissertation, Michigan State University, 1995.

b. Emission quantum yield was measured on dilute benzene solution of
M2Cl4(PMe2Ph)4 (absorbance < 0.1) and determined by using a 2-methylpentane
solution of M0214(PMe3)4 at km = 636 nm at 300K.

c. reference 65

108

The zwitterionic nature of the 155* excited state of MoW has been
confirmed by the two-photon excitation of the fluorescence of MoWCl4(PMe3)4.63
The 15*8*-185* energy gap (AE) was calculated from the difference between the
km of the 15*8* excitation band and the absorption band of 168* and is equal to
6003 cm“1. The small AB is consistent with the valence bond model that has been
used for the M—4—M homonuclear complexes?”64 This establishes that the 155*
excited state of MoWCl4(PR3)4 is zwitterionic and capable of two electron
chemistry.

The redox potentials for MoWCl4‘(PMe2Ph)465 are listed in Table 4.2, along
with those for MozCl4(PMe2Ph)4 and W2C14(PMe2Ph)4. While, the potentials for
MoW fall close to the average for M0265 and W2, they are closer to M02. This is
especially apparent in the reduction potentials where the differences are A(M02 —-
MoW) = 0.10 and A( MoW — W2) = 0.41 V. Therefore, the ground state reactivity

might be expected to be similar to M02C14(PMe2Ph)4.

2. Photochemistry

Whereas benzene solutions of MoWCl4(PMe2Ph)4 and Ph3PS are
indefinetly stable in the absence of light at room temperature, irradiation
MoWCl4(PMe2Ph)4 in the presence of excess Ph3PS produces the spectral changes
in Figure 4.13. The lack of isosbestic points indicates that there is not a clean
conversion from reactants to products. The 1H and 31P{1H} NMR and solution IR
spectra compare well to those of independently synthesized W(S)Cl4(PMe2Ph).
Resonances from W(S)Cl4(PMe2Ph) show up at 8 +7.7 ppm (m), +7.03 (m), +1.36
((1, 1JPH = 13 Hz) in the 1H NMR and at 5 = +31.6 ppm (s) in the 31P{1H} NMR,

Figures 4.14 and 4.15. Additionally, absorbances in the IR spectrum of the

109

 

 

 

Absorbance

 

 

 

 

300 400 500 600 700 800

Wavelength / nm

Figure 4.13 Spectral changes associated with the photolysis (2» > 375 nm)
of MoWCl4(PMe2Ph)4 and Ph3PS in benzene.

110

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305388 88:30 * .mmMME Moan vflnmmcgmvfioaoz Mo mMmboMoMMm com: coMMomoMoMoMMm 05 Mo MEZ EM 3.9 oustm

 

 

 

 

 

_taa M N m 4 m o s m
_ _ _ _ _ _ _ 1C
13.1133 3... .11.... .3

: 1. ..
__. _
L n
3 amca
M
_4
.
6306293
05293

 

am;¢.wam;m.

 

manna

111

.manME ea eMMManozevMoBoz Mo 888888 35 Mo mzz EM EMm m3. 9:.me

Ema O_N- O— ..- O O_N O_m 0? 0m
M _

3 III-3113433331131} .33 - I, .333...
JMMMfl/I MM .M

M >>O2M _. .

>>o_>_ _

 

 

 

 

 

 

 

 

63303293

acn—

mn_ cn_

 

 

112

photoreaction at 594 cm‘1 (m), 570 (m) are due to W(S)Cl4(PMe2Ph). Additional
resonances in the 1H and the 31P{1H} NMR and absorbances in the IR spectra of
the photoreaction have also been assigned (Figures 4.14 - 4.16). A molecular ion
peak corresponding to W(S)Cl4 (355 amu, MeOH) is observed in the GC/MS of
the photoreaction due to the displacement of PMeZPh from W(S)Cl4(PMe2Ph) by
MeOH. This is observed in the 31P{1H} NMR of independently synthesized
W(S)Cl4(PMe2Ph) in MeOH, which shows resonances due to uncoordinated
PMezPh. If the incomplete photoreaction is removed from the lamp and stored in
the dark at about 28 °C, the reaction proceeds to completion in an additional 21
days. Attempts to isolate the photoproducts on alumina or silica columns failed due
to decomposition.

The reaction of Ph3PS and MoWCl4(PR3)4 was investigated with other
phosphine ligands. The photoreaction of MoWCl4(PMe3)4 and Ph3PS (km > 375
nm) in benzene was slower MoWCl4(PMe2Ph)4 (about 5% reacted in 6 days). The
reaction was not taken to completion and no products were isolated.

The atom transfer photoreaction is unique to MoWCl4(PMe2Ph)4. As
expected, the excited state reactivity of the heteronuclear complex is markedly
different than that of the homometallic complexes. Benzene solutions of the
homonuclear complexes, M2Cl4(PMe2Ph)4 (M = Mo, W), do not react with Ph3PS
upon irradiation (M = M0, km > 335 nm after 48 hrs; M = W, km > 375 nm after
36 hrs) or at room temperature in the dark.

Changing the R groups on the phosphine ligand has a large effect on the
reactivity of both the homonuclear and heteronuclear complexes. For example, no

thermal reaction of W2Cl4(PMe3)4 or MoWCl4(PMe3)4 with Ph3PS is observed and

113

 

0.08 - fi

 

0.06 -

0.04 U

0.02 -

 

 

 

 

Absorbance

p (b)
0.60 '

 

 

0.40 -

0.20 F‘

 

 

l l l I

600 500 400
1

 

Wavenumbers / cm ‘

Figure 4.16 FTIR spectra of benzene solutions of (a) the neat
photoreaction and (b) W(S)Cl4(PMe2Ph). The absorbances between 550
and 400 cm’1 in (a) are due to Ph3PS and Ph3P.

114

W2C14(PMe3)4/Ph3PS solutions exhibit no photochemistry. Howeve1
W2Cl4(PMePh2)4 and MoWCl4(PMePh2)4, both react thermally with Ph3PS to yiell
PhZMePS, which was identified by 1H and 31P{1H} NMR. Consistent with th
formation of thMePS, the 1H and 31P{1H} NMR of both reaction mixtures have
doublet at 8 +1.93 ppm66 and a singlet at + 36 ppm, respectively. The reaction i
probably metal mediated and not due to dissociation of thMeP, since thl
equilibrium constant for sulfur exchange between Ph3PS and thMeP is only 11
M'1 at 130 °C.66 The MozCl4(PR3)4 (PR3 = PMe3 and PMeth) complexes show 113

reactivity, photochemically or thermally, with Ph3PS.

4. Quantum Yields

Table 4.3 summarizes the wavelength dependence of th:
MoWCl4(PMe2Ph)4/Ph3PS photoreaction quantum yields in benzene, (hp. There i
considerable reaction in the visible spectral region (¢546 = 5.0 x 104), with th1
quantum yields rising to the UV spectral region ((1)365 = 1.8 x 103). The fact tha
neither M02C14(PMe2Ph)4 nor W2C14(PMe2Ph)4 react with Ph3PS when irradiate-
with excitation wavlengths up to 335 nm, excludes the possibility of the reactio:
being derived from R3P—S bond homolysis. The photochemistry does not aris
from the 1(823*) state, as indicated by the absence of quenching of the 1636*
fluorescence until benzene solutions of MoWCl4(PMe2Ph)4 are saturated wit
Ph3PS. Furthermore, no reaction is observed for km = 577 nm, which is in th
high energy tail of the ](65*) transition. The quantum yields track well with th
l(7:5"‘) transition, which lies just to higher energy of 1(88*), and suggest that th

photochemistry is derived from this state. This may also imply that there is still

Table 4.3 Wavelength Dependence
MoWCl4(PMe2Ph)4 with Ph3PS

115

of Quantum Yields for Photoreaction of

 

 

km / nm ¢p[MoWCl4(PMe2Ph)4]
577 NR.
546 5.0 x 10*1
436 1.2 x 10‘3
365 1.8 x 10‘3

 

116

need for a slight intramolecular distortion in order to trap the excited state and

therefore slightly higher excitation energies are necessary.
5. Electron Paramagnetic Resonance Spectroscopy

While the W photoproduct is partially characterized, the Mo species is not
characterized. Thus EPR studies were undertaken to determine if there were any
paramagnetic products. And as shown by the 77K EPR spectrum of the
photoreaction mixture in a 2-MeTHF glass, in Figure 4.17, there are. The EPR
spectrum exhibits a strong central line and surrounding hyperfine lines
characteristic of MoV (95Mo: 15.72 %, I : 5/2).67 Table 4.4 lists some
representative g-values for MoV nuclei with a variety of ligands.68’69’70’71 The g-
values of the paramagnetic photoproduct (g1 = 1.960; g2 = 1.948) compare well to
the range reported. The only example of a monomeric complex containing a
[Mo=S]3+ center, [HB(Me2pz)3]MoVSC12, has trigonal symmetry with gyy = 1.941,
gZZ = 1.971, gxx = 1.934.69 It is difficult to draw conclusions about the coordination

sphere from this result, only that it is a Mov nucleus.
6. Electron Spin Echo Envelope Modulation72

In an effort to acquire more information as to the nature of the paramagnetic
photoproduct pulsed EPR, Electron Spin Echo Envelope Modulation (ESEEM)
studies were undertaken. The description of a spin echo experiment is based on the
presence of inhomogeneously broadened EPR absorption, which obscures useful
information regarding the hyperfine coupling of the nuclei to the paramagnet. This
inhomogeously broadened line is composed of populations of spins, or “spin

packets”, whose magnetic moments have the same precessional frequency but

117

 

g = 1.960

g = 1.948

 

 

 

 

ll

1 l l l | l
31 50 3250 3350 3450

 

 

 

H / Gauss

Figure 4.17 The X-band EPR spectrum of the photoreaction in a 2-MeTHF
glass.

 

118

Table 4.4 Representative g-values of MoV Octahedral Complexes

 

 

complex gxx gyy gzz references
LMo(S)C12a 1.921 1.941 1.919 68
LMo(O)C12a 1.941 1.971 1.934 68
LMo(O)(OMe)2a 1 .960 1 .942 1.904 69
LMo(O)(OSet)2a 2.01 1 1.952 1.931 69
LMo(O)(OH)C1a 1.966 1.946 1.914 70
LMo(O)(OH)NCSa 1.966 1 .944 1.922 70
[Ph4As][MoOC14] 1 .952 — - 7 1
[EN] [MoOC14(OPPh3)] 1 .943 - — 71

 

a. L = hydrotris(3,5-dimethyl-1-pyrazloyl)borate

119

different phases (Figure 4.18a). Application of an external magnetic field, H0,
results in net magnetization, M0, both of which are parallel to Z. This
magnetization has a characteristic precessional frequency, a), that is known as the
Larmor frequency and is equal to 6), = 7H0; where y is the gyromagnetic ratio. To
understand the build. up and decay of spin echoes, a rotating coordinate system
(Figure 4.18b), with rotational frequency 030 and axes X’, Y’ and Z’ is used.
Following the first n/2 pulse applied along X’, the magnetization will flip into the
XY plane, Figure 4.19a. Since the spin packets have frequencies different from (no
(a), and (oj in Figure 4.18a) they quickly begin to dephase, Figure 4.1%. After a
time I, a second microwave pulse that is sufficient to rotate the spin packets by
180° is applied, Figure 4.19c. The precessional frequency and the phasing are
unaltered by the pulse and after a time 1:, the spins packets realign along —Y’. This
creates a build up of magnetization that is known as a spin echo, Figure 4.19d. The
amplitude of the spin echo as a function of interpulse time 1' is measured in the
experiment. A three-pulse stimulated echo experiment is similar to the two-pulse
experiment except the n-pulse has been broken into two 1t/2 pulses and the
amplitude of the stimulated echo is measured as a function of 1:+T, where r is the
delay between the first two 1t/2pulses and T is the delay between the second and

the third pulses.
a. Nuclear Modulation Effect72

The decay of the spin echo amplitude is not monotonic but is modulated by
anisotropic hyperfine and nuclear quadrupole interactions of surrounding magnetic
nuclei. These modulations are the direct result of the fact that microwaves can

induce transitions that start at one pair of levels and end at another pair of levels

 

120

311133

(Di (00 (0]

(b)

2'
H, C

 

 

XI of = yHi - (00
Hi = I"Io + I‘Ilocal

Figure 4.18 (a) The inhomogeneously broadened EPR line of a
paramagnetic center. (b) Each spin packet can be considered independently
from each other, each having its own Larmor frequency.

 

121

 

 

 

 

 

 

 

 

715/2
3 b
ZI
Ho
Yl
X' 000
2'
(b)
(DJ
M ‘\ Y.
J.
XI ml
(d)
_TM,

 

 

Figure 4.19 Magnetization of spin packets i and j during a two pulse
experiment. (a) during a 1r/2 pulse; (b) after time t; (c) after the TC pulse; (d)

time t after the it pulse.

122

upon the application of a second pulse. This is called branching of transitions and
as a consequence, the probability for each transition is non-zero. To illustrate the
effect of nuclear modulation, it is useful to follow the “allowed” spin packet “A”
and the “forbidden” spin packet “B” during the pulse sequence, Figures 4.20b-d. If
spin packet “A” is excited with a n/2 pulse, “A” is rotated into the XY plane and
starts to dephase immediately, Figure 4.20b. Due to transition branching, transition
“B” is excited along with the transition “A” during the 13: pulse, although the
transition probabilities for “A” and “B” are not equal (Figure 4.200). The echo
amplitude at time t is the projection of the spin packet “vectors” onto —Y’. At time
’C after the 1: pulse, “A” is aligned along —Y’ and contributes fully to the spin echo.
Spin packet “B” does not fully align along —Y’ because its precessional frequency
is different than the rotating frame and may add or subtract to the spin echo
amplitude, depending on the projection of “B”. The observed experimental echo
amplitude is a sum of the spin packets that are excited in the inhomogeneously
broadened EPR line and the sum is the source of the modulation pattern observed
in the spin echo experiment. The frequencies of the modulation yield the hyperfine
couplings. Fourier transformations of the time domain data make the frequencies
easier to observe and hyperfine peaks can be found centered around the Larmor
frequency, v,,, for that nuclei and split by Aiso, the hyperfine coupling, or centered
around A/2 and split by V".

Figure 4.21 shows the Fourier transforms of the photoproduct electron spin-
echo envelopes obtained at 3260 and 4375 G with a 'c of 300 and 250 ns,

respectively. In Figure 4.21a, a peak at 13.8 MHz corresponds closely to the

123

 

 

 

 

 

 

 

 

 

 

 

 

 

(a) j A I1 >
I A l2 >
Bi C? D
V I3 >
V 31 I4 >
(b) (C) /
time I . 7: pulse
__.> Y M,
after (03 (DA
7: /2 pulse
Cl)B
X' ‘61) x'
(d) ,A
time I
——-—-> Y1
CDB
XI

Figure 4.20 (a) Energy level diagram for a S = 1/2 and I = 1/2 system. Time
behavior for the magnetization of "forbidden" spin packet A and "allowed"
spin packet B. (b) at time 't after the TC /2 pulse; (c) after the 1: pulse; ((1) at
the time of the echo.

124

 

13.8 (a)

2.4
5.2

 

 

 

Amplitude

(b)
1.9

5.9

 

l l l

0 4 8 12 16 20 24
Frequency / MHz

 

 

 

Figure 4.21 Fourier transforms of the photoproduct spin-echo envelope.
(a) was measured at 3260 G and 1: = 300 ns and (b) was measured at 4375 G
and 1: = 250 ns. Both were measured at 4K.

 

125

Larmor frequency of 1H (yn = 4.26)73 at 3260 G and is due to protons in the

solvent. The Larmor frequency at a given field is calculated from equation 4.1974
hvn = gnflnH (4.19)

At lower frequency, in Figure 4.21a, two peaks at 2.4 and 5.2 MHz are
centered at A/2 (3.8 MHz) and split by twice the Larmor frequency for 35C1 at
3260 G (vn(3260) = 1.4 MHz). At higher field, the hyperfine coupling should
remain the same, while the peaks will move further apart depending on the Larmor
frequency. At 4375 G, Figure 4.21b, the peaks shift to about 1.9 and 5.9 MHz,
which corresponds nicely to the expected shift of 0.4 MHz for a 35Cl nucleus. yn
for 35C1 and 37C1 are similar and vn will differ by only about 1 MHz. Additionally,
the natural abundancies of the 35Cl and 37C1 isotopes are 75 and 24%, respectively.

Therefore, the lines due to chlorine can be expected to be broad.

Nuclei with a nuclear spin quantum number I > 1/2 have a quadrupole
moment that results from the nonspherical distribution of the positive charge in the
nucleus.75 Fluctuating electric fields coming from the interaction of dipolar solvent
and solute molecules can efficiently relax quadrupolar nuclei.76 The mechanism of
relaxation depends on the interaction of the quadrupole with the electric field
gradient of the nucleus. The gradient arises when a quadrupolar nucleus is in a
molecule in which it is surrounded by a nonspherical distribution of electrons, such
as a terminal chloride ligand. This rapid relaxation can broaden the lines due to the

quadrupolar nucleus. This effect can be observed in Figure 4.21a upon comparing

 

126

the width of the peaks due to 1H and those assigned to 35Cl. Based on the magnetic
resonance studies, the molybdenum photoproduct is a MoV nucleus with C1—

ligated in a terminal position.
D. Summary

The observation of atom transfer products for the photoreaction of
MoWCl4(PMe2Ph)4 and Ph3PS represents a rare example of photoiniated sulfur
atom transfer to a transition metal complex. The atom transfer product,
W(S)C14(PMe2Ph) is formed in approximately 25% yield, with Ph3P formed in
greater than 50% yield. The Mo product has not been fully characterized, except

that it is paramagnetic. The photoreaction is summarized below.

hv
MoWCI4(PMe2Ph)4+Ph3PS W W(S)Cl4(PMe2Ph)+MoV
6 6

The photoreaction is unique to the heterobimetallic complex, as MozCl4(PMe2Ph)4
and W2Cl4(PMe2Ph)4 do not react photochemically with Ph3PS. The reactivity of
the MoW complex with Ph3PS illustrates that the atom transfer reaction is
thermodynamically inaccessible to these complexes.

The photoreaction of the mixed-metal complex MoWCl4(PMe2Ph)4 with
Ph3PS is unparalleled in the reactivity of the homonuclear quadruply bonded
bimetallic complexes with Ph3PS. Since a large intramolecular distortion is
neccesary, in the case with the homometallic complexes, the excited state is
deactivated and is confined to low energy substrates, such as Me—I (57 kcal77) and
PhS—SPh (55 kcal78). The asymmetry of the MoW complex leads to a less distorted

excited state and therefore more energetic. Now we are able to utilize substrates

127

that typically react by atom transfer and have a higher activation energy; i. e. Ph3P—
S: 92 kcal.79 Moreover, neither the homonuclear complexes nor the MoW

complexes react thermally with Ph3PS.
E. Future Directions

It might be of interest to develop photocatalytic systems that could react
with environmentally interesting molecules, such as C02 or SOZ. Several problems
exist that must be overcome inorder to do so. First, MoWCl4(PMe2Ph)4
decomposes to two monomeric complexes, which is detrimental to the
development of a catalytic system and, second, the bond dissociation energies of
C02 and S02 are quite high (12780 and 13280 kcal, respectively). The complex
MoWCl4(tetraphos),81 where tetraphos is a tetradentate phosphine, could
potentially solve both problems. A tetradentate ligand would be less likely to
dissociate from the metal than a mono or bidentate ligand and would keep the
basic framework of the M—4—M complex intact. Also, tetraphos binds

asymmetrically to the quadruple bond core, as shown below.

9,, c:
EP&-M.:§CI

PM"
on‘ ”’CI
P/ \P
\——/

This, combined with the asymmetry of the Mo—4—W core, may introduce enough
asymmetry to preclude any need for ligand distortion giving an excited state that is

even more energetic that MoWCl4(PMe2Ph)4.

 

128

F. References

1. Taube, H. Angew. Chem. Int. Ed. Engl. 1984, 23, 329.

2. Nugent, W. A.; Mayer, J. M. Metal-Ligand Multiple Bonds; John Wiley & Sons:
New York, 1988.

3. Holm. R. H. Chem. Rev. 1987, 87, 1401.

4. Woo, L. K. Chem. Rev. 1993, 93, 1125.

5. Schwarz, C. L.; Bullock, R. M.; Creutz, C. J. Am. Chem. Soc. 1991, 113, 1225.
6. Steifel, E. 1. Prog. Inorg. Chem. 1977, 22, 1.

7. Holm, R. H. Coord. Chem. Rev. 1990, 100, 183.

8. Couglan, M. P. In Molybdenum and Molybdenum Containing Enzymes;
Coughlan, M. P., Ed.; Pergamon: New York, 1980; pp 119-185.

9 Rajagopalan, K. V. In Molybdenum and Molybdenum Containing Enzymes;
Coughlan, M. P., Ed.; Pergamon: New York, 1980; pp 241-272.

10. Adams, M. W. W.; Mortenson, L. E. In Molybdenum Enzymes; Spiro, T. G.,
Ed.; John Wiley & Sons: New York, 1985; p 519.

ll. Satoh, K.; Kurihara, F. N. J. Biochem Tokyo 1987, 102, 191.

12. (a) Taube, H.; Myers, H.; Rich, R. L. J. Am. Chem. Soc. 1953, 75, 4118. (b)
Taube, H.; Myers, H. J. Am. Chem. Soc. 1954, 76, 2103.

13. Meyer, T. J .; Taube, H. In Comperhensive Coordination Chemistry, Wilkinson,
G., Ed.; Pergamon Press: Oxford; vol 1, pp 331-384.

14. Taube, H. In Mechanistic Aspects of Inorganic Reactions; Rorabacher, D. B.;
Endicott, J. F., Eds.; ACS Symposium Series 198; American Chemical Society:
Washington, DC, 1982; p 151.

129

15. (a) Schultz, B. E.; Holm, R. H. Inorg. Chem. 1993, 32, 4244. (b) Harlan, E.
W.; Berg, J. M.; Holm, R. H. J. Am. Chem. Soc. 1986, 108, 6992.

16. Gheller, S. F.; Schultz, B. E.; Scott, M. J.; Holm, R. H. J. Am. Chem. Soc.
1992, 114, 6934.

17. Woo, L. K.; Hays, J. A.; Goll, J. G. Inorg. Chem. 1990, 29, 3916.

18. Liston, D. J.; West, B. W. Inorg. Chem. 1985, 24, 1568.

19. Hall, K. A.; Mayer, J. M. J. Am. Chem. Soc. 1992, 114, 10402.

20. Hall, K. A.; Critchlow, S. C.; Mayer, J. M. Inorg. Chem. 1991, 30, 3593.

21. Jang, S.; Atagi, L. M.; Mayer, J. M. J. Am. Chem. Soc. 1990, 112, 6413.

22. Su, F.-M.; Bryan, J. C.; Jang, S.; Mayer, J. M. Polyhedron 1989, 8, 1261.

23. see for example: (a) Zhang, Y.; Holm, R. H. Inorg. Chem. 1990, 29, 911. (b)
Oku, H.; Ueyama, N.; Kondo, M.; Nakamura, A. Inorg. Chem. 1994, 33, 209. (c)
Arzoumanian, H.; Lopez, R.; Agrifoglio, G. Inorg. Chem. 1994, 33, 3177. (d)
Templeton, J. L.; Ward, B. C.; Chen, G. J .-J .; McDonald, J. W.; Newton, W. E.
Inorg. Chem. 1981, 20, 1248. (e) Bhattacharjee, S.; Bhattacharyya, R. J. Chem.
Soc. Dalton Trans. 1993, 1151.

24. (a) Roundhill, D. M. Photochemistry and Photophysics of Metal Complexes;
Plenum: New York, 1994. (b) Suslick, K. 8.; Watson, R. A. New J. Chem 1992,
16, 633.

25. Scott, S. L.; Espenson, J. H.; Zhu, Z. J. Am. Chem. Soc. 1993, 115, 1789.

26. (a) Lemke, F.; Kubiak, C. P. J. Organomet. Chem. 1989, 373, 391. (b)
Reinking, M. K.; Kullberg, M. L.; Cutler, A. R.; Kubiak, C. P. J. Am. Chem. Soc.
1985, 107, 3517. (c) Miller, T. D.; St. Clair, M. A.;-Reinking, M. K.; Kubiak, C. P.
Organomet. 1983, 2, 767.

130

27. Proulx, G.; Bergman, R. G. J. Am. Chem. Soc. 1994, 116, 7953.

28. Roundhill, D. M.; Gray, H. B.; Che, C.-M. Acc. Chem. Res. 1989, 22, 55.

29. (a) Roundhill, D. M.; Atherton, S. J. Inorg. Chem. 1986, 25, 4071. (b)
Roundhill, D. M.; Shen, Z.-P.; King, L.; Atherton, S. J. J. Phys. Chem. 1988, 92,
4088.

30. (a) Roundhill, D. M.; Atherton, S. J. J. Am. Chem. Soc. 1986, 108, 6829. (b)
Che, C.-M.; Gray, H. B.; Atherton, S. J.; Lee, W.-M. J. Phys. Chem. 1986, 90,
6747. (c) Cho, K. C.; Che, C.-M. Chem. Phys. Lett. 1986, 124, 313.

31. Che, C.-M.; Lee, W.-M.; Kwong, H.-L.; Yam, V. W.-W.; Cho, K.-C. . J.
Chem. Soc. Dalton Trans. 1990, 1717.

32. Yuan, L.-C.; Calderwood, T. S.; Bruice, T. C. J. Am. Chem. Soc. 1985, 107,
8273.

33. Suslick, K. S.; Acholla, F. V.; Cook, B. R. J. Am. Chem. Soc. 1987, 109, 2128.
34. Engebretson, D. S.; Zaleski, J. M.; Leroi, G. E.; Nocera, D. G. Science 1994,
265, 759.

35. Holm, R. H.; Donahue, J. P. Polyhedron 1993, 12, 571.

36. Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.; John
Wiley & Sons: New York, 1988; p 808.

37. (a) Berg, D. J .; Burns, C. J .; Andersen, R. A.; Zalkin, A. Organomet. 1989, 8,
1865. (b) Bottomley, F.; Egharevba, G. 0.; Lin, 1. J. B.; White, P. S. Organomet.
1985, 4, 550. (c) Liu, H.-F.; Liu, R.-S.; Liew, K. Y.; Johnson, R. E.; Lunsford, J.
H. J. Am. Chem. Soc. 1984, 106, 4117.

38. Bottomley, F.; Paez, D. E.; White, P. S. J. Am. Chem. Soc. 1982, 104, 5651.

131

39. Smith 1H, M. R.; Matsunaga, P. T.; Andersen, R. A. J. Am. Chem. Soc. 1993,
115, 7049.

40. Bottomley, R; Brooks, W. V. F. Inorg. Chem. 1977, 16, 501.

41. LaPlaza, C. B.; Odom, A. L.; Davis, W. M.; Cummins, C. C.; Protasiewicz, J.
D. J. Am. Chem. Soc. 1995, 117, 4999.

42. Hintz, P. A.; Sowa, M. B.; Ruatta, S. A.; Anderson, S. L. J. Chem. Phys. 1991,
94, 6446.

43. Fanwick, P. E.; Harwood, W. S.; Walton, R. A. Inorg. Chem. 1987, 26, 242.
44. LaMar, G. N.; Horrocks, W. D., Jr.; Holm, R. H., Eds. NMR of Paramagnetic
Molecules; Academic: New York, 1973; p 566.

45. Barder, T. J .; Cotton, F. A.; Lewis, D.; Schwotzer, W.; Tetrick, S. M.; Walton,
R. A. J. Am. Chem. Soc. 1984, 106, 2882.

46. Grim, S. O.; Walton, E. D. Inorg. Chem. 1980, 19, 1982.

47. Greenwood, N. N.; Eamshaw, A. Chemistry of the Elements; Pergamon: New
York, 1984; p 1175.

48. Gilbert, A.; Baggott, J. Essentials of Molecular Photochemistry; CRC: Boston,
1991; pp 468-475.

49. (a) von Sonntag, C.; Schuchmann, H.—P. In Advances in Photochemistry; Pitts,
Jr., J. N.; Hammond, G. S; Gollnick, K., Eds., John Wiley & Sons: New York,
1977; , vol. 10, pp 59-145. (b) Roquitte, B. C. J. Am. Chem. Soc. 1969, 91, 7664.
(c) Fleming, G.; Anderson, M. M.; Harrison, A. J .; Pickett, L. W. J. Chem. Phys.
1959, 30, 351.

50. Williams, D. H.; Fleming, 1. Spectroscopic Methods in Organic Chemistry, 4th
ed.; McGraw-Hill: New York, 1987.

 

132

51. Hsu, T.-L. C.; Chang, I-J.; Ward, D. L.; Nocera, D. G. Inorg. Chem. 1994, 33,
2932.

52. (a) Baratta, W.; Calderazzo, F.; Daniels, L. M. Inorg. Chem. 1994, 33, 3842.
(b) Bradley, D. C.; Errington, R. J .; Hursthouse, M. B.; Short, R. L.; Ashcroft, B.
R.; Clark, G. R.; Nielson, A. J .; Rickard, C. E. F. J. Chem. Soc. Dalton Trans.
1987, 2067.

53. (a) Bryan, J. C.; Mayer, J. M. J. Am. Chem. Soc. 1990, 112, 2298. (b) Su, F.-
M.; Bryan, J. C.; Jang, S.; Mayer, J. M. Polyhedron 1989, 8, 1261. (c) Bates, J. R.;
Taylor, H. S. J. Am. Chem. Soc. 1927, 49, 2438.

54. (a) Vaughan, G. A.; Hillhouse, G. L.; Rheingold, A. L. J. Am. Chem. Soc.
1990, 112, 7994. (b) Vaughan, G. A.; Sofield, C. D.; Hillhouse, G. L.; Rheingold,
A. L. J. Am. Chem. Soc. 1989, 111, 5491. (c) Vaughan, G. A.; Rupert, P. B.;
Hillhouse, G. L. J. Am. Chem. Soc. 1987, 109, 5538.

55. Matsunaga, P. T.; Hillhouse, G. L. J. Am. Chem. Soc. 1993, 115, 2075.

56. Matsunaga, P. T.; Mavropoulos, J. C.; Hillhouse, G. L. Polyhedron 1995, 14,
175.

57. (a) Jubb, J .; Gambarotta, S. Inorg. Chem. 1994, 33, 2.503. (b) Gambarotta, S.;
Edema, J. J. H.; Minhas, R. K. J. Chem. Soc. Chem. Commun. 1993, 1503.

58. Jubb, J .; Gambarotta, S. J. Am. Chem. Soc. 1993, 115, 10410.

59. (a) Hall, K. A.; Mayer, J. M. Inorg. Chem. 1995, 34, 1145. (b) Hall, K. A.;
Mayer, J. M. Inorg. Chem. 1994, 33, 3289.

60. (a) Cotton, F. A. Polyhedron 1987, 6, 677. (b) Cotton, F. A.; Hong, B.; Shang,
M.; Stanley, G. G. Inorg. Chem 1993, 32, 3620. (c) Cotton, F. A.; Maloney, D. J .;

 

133

Su, J. Inorg. Chim. Acta 1995, 236, 21. (d) Poli, R.; Torralba, R. C. Inorg. Chim.
Acta. 1993, 212, 123.

61. Miskowski, V. M.; Gray, H. B.; Hopkins, M. D. Inorg. Chem. 1992, 32, 2085.
62. Turro, N. J. Modern Molecular Photochemistry; University Science Books:
Sausalito, CA, 1991; pp 316-317.

63. Engebretson, D. S. Ph.D. Dissertation, Michigan State University; 1997.

64. Hopkins, M. D.; Gray, H. B.; Miskowski, V. M. Polyhedron 1987, 6, 705.

65. Luck, R. L.; Morris, R. H.; Sawyer, J. F. Inorg. Chem. 1987, 26, 2422.

66. Baechler, R. D.; Stack, M.; Stevenson, K.; Vanvalkenburgh, V. Phosphorus,
Sulfur, and Silicon 1990, 48, 49.

67. Chang, C.-S. J.; Pecci, T. J .; Carducci, M. D.; Enemark, J. H. Inorg. Chem.
1993, 32, 4106.

68. Young, C. G.; Enemark, J. H.; Collison, D.; Mabbs, F. E. Inorg. Chem. 1987,
26, 2927.

69. (a) Cleland, W. E., Jr.; Barnhart, K. M.; Yamanouchi, K.; Collison, D.; Mabbs,
F. E.; Ortega, R. B.; Enemark, J. H. Inorg. Chem. 1987, 26, 1017. (b) Enemark, J.
H.; Cleland, W. E., Jr.; Collison, D.; Mabbs, F. E. In Frontiers in Bioinorganic
Chemistry; Xavier, A. V., Ed.; VCH Verlagsgesellschaft:Weinhem, FRG, 1986; p
47.

70. Xiao, Z.; Bruck, M. A.; Doyle, C.; Enemark, J. H.; Grittini, C.; Gable, R. W.;
Wedd, A. G.; Young, C. G. Inorg. Chem. 1995, 34, 5950.

71. Boorman, P. M.; Garner, C. D.; Mabbs, F. E. J. Chem. Soc. Dalton Trans.
1975, 1299.

134

72. Keijers, C. P.; Reijers, E. J .; Schmidt, J. Pulsed EPR: A new field of
applications; Koninkijke Nederlandse Akademie van Wetenschappen: New York,
1989; pp 11-27.

73. Handbook of Chemistry and Physics, 69th ed.; CRC: Boca Raton, Florida,
1988; p E80.

74. Wertz, J. E.; Bolton, J. R. Electron Spin Resonance; McGraw-Hill: New York,
1972.

75. West, A. R. Solid State Chemistry and Its Applications; John Wiley & Sons:
New York, 1984; p 98.

76. Drago, R. S. Physical Methods for Chemists, 2nd Ed.; Saunders College
Publishing: New York, 1992; pp. 269-270.

77. Streitweiser, A. Jr.; Heathcock, C. H. Introduction to Organic Chemistry, 3rd
Ed.; Macmillan Publishing: New York, 1985.

78. Benson, 8. W. Chem. Rev. 1978, 78, 23.

79. Chernick, C. L.; Pedley, J. B.; Skinner, H. A. J. Chem. Soc. 1957, 1851.

80. Handbook of Chemistry and Physics, 69th ed.; CRC: Boca Raton, Florida,
1988; p F-l83.

81. (a) Cotton, F. A.; Eglin, J. L.; James, C. A. Inorg. Chem. 1993, 32, 687. (b)
Chen, J .-D.; Cotton, F. A.; Hong, B. Inorg. Chem. 1993, 32, 2343. (c) James, C. A.
Ph.D. Dissertation, Texas A & M University, 1992.

 

CHAPTER 5

A STUDY OF THE NONRADIATIVE DECAY OF M2C14(PR3)4

A. Background

Since the discovery1 of the quadruple bond in 1964 and its recognitionz,
computations3 and experiments4 have provided a comprehensive and coherent
description of the electronic structure of this class of compounds. Figure 5.1 shows
the absorption spectra of M02C14(PMe3)4 and W2Cl4(PMe3)4 with the electronic
transitions as they have been assigned. The lowest energy 1(593*) transition is
flanked by 1(t:—)8*) and to higher energy lies 1(M(Cl)—>d*) LMCT transitions.4b’5’6
The ng M2Cl4(PR3)4 complexes (M2 = M02, MoW, W2; PR3 = PMe3, PMezPh,
PMeth) luminesce from the 155* excited state with quantum efficiencies of about
0.01 to 0.37 and emission lifetimes between 15-50 ns in nonpolar solvents;8
MozCl4(PMe3)4 is an exception with lifetimes of 135 ns in 2—methy1pentane.8
Notwithstanding, not much is known about the excited state decay processes of M—
4—M complexes to date. The goal of this study was to better understand the
nonradiative decay of the 158* excited state and to quantify the electronic coupling

between the two metal centers.

135

136

68.28 a M- - - -C w.MMMnMaManMM0N3 Meg M I Iv 458234682 Mo 266% 8:963“ 28:85 M.m 23E

 

E: M 595.68;
cow con com com oov oom
M M _ M
I}, /. Xiii-IIVHIVDIIHHIVDGII.
/_ M M V J. .
._ . . Nx . ..

M. M’ \ r I I \ I M I —.

o. __ M _ I I I /\\t~ I\.\ I It! _ o.

M. M“ M \ I I

J
‘
A
'K
00
ti
v
‘—
all—‘—

1
N
L.uJo L.w 90L /3

 

 

 

137

B. Temperature Dependence of Emissive Lifetimes

The intramolecular decay processes of an electronic excited state are
defined by a radiative, k,, and nonradiative, km, rate constants. These two kinetic
rate constants describe the fundamental properties of an electronic excited state,
namely the lifetime, 1:0, and emission quantum yields, the. At a given temperature,

these quantities are defined by equations 5.1 and 5.2, respectively.9

——1—:k1:kr+knr (5.1)
T0

¢e : Tokr (5.2)

The nonradiative rate constant, km, can therefore be calculated from a
measurement of the lifetime and emission quantum yield. The temperature
dependence of the emission quantum yield at temperatures other than 298 'K, can ‘
be estimated by equation 5.310

¢LT = ¢298(1LL] (53)

1298

where In and 1298 are the integrated emission intensity at low temperature and 298
K, respectively. The temperature dependence of emission lifetimes varies
according to Equation 5.4, an expression based on the excited state decay constant
for a two-state Boltzmann distribution,10

kl + k2 exp£ 2A?)

B

obs = [_ AE']
l + exp
k 3T

 

(5.4)

 

 

138

where k, and k2 are the decay constants for two nondegenerate states in thermal
equilibrium separated by an energy gap, AE’. The denominator is necessary when
AE’ S 3kBT.lo The fraction of decay that occurs through the upper excited state at a

given temperature is given by equation 5.5.10

_ kzz'exp(—AE'/kBT)
77‘ 7 k, + k, exp(—AE'/k,T)

 

(5.5)

The overall form of equation 5.4 yields plots with a temperature independent
region, the plateau region, followed by a temperature dependent region. An ill-
defined plateau region is a major source of error in fits of equation 5.4 to
experimental lifetime data and must be experimentally well-defined in order to

determine a reasonable value for k1.lo
C. N onradiative Decay Theory11

The overall treatment of kr and km in section B is largely
phenomenological. The radiative rate constant is quantitatively defined by the
transition moment integral,as formulated by the Einstein equation for spontaneous

emission, equation 5 .6.12

A

4E”,3
- I1

k_

' 3h4c3 (5.6)

 

 

 

gs

 

2
‘1’... )

 

(‘1’

‘I’gs and ‘I’es are the ground and excited state electronic wavefunctions,
respectively, and u is the transition dipole moment operator. For a series of
compounds with a common chromophore, the transition dipole and the emission
energy do not change significantly, especially with regard to temperature.
Consequently the dominating parameter with regard to the basic photophysical

properties of lifetime and intensity is the nonradiative decay constant.

 

139

Nonradiative decay occurs via an isoenergetic electronic transition from a
thermally equilibrated vibronic excited state, v.80, to a vibronically excited ground
state, vgs". This is followed by vibrational relaxation to the molecule’s ground state
configuration, vgso. The overall process is schematically represented in Figure 5.2.
The efficiency of the nonradiative decay is directly related to the vibrational
overlap of V.,,0 and ng". An estimate of the wavefunction overlap is given by

equation 5.7,

 

 

ha)

oz El.- coth[’6h2w’":l (5.7)
G is the coupling strength, Eml is related to the Stokes shift, hwm is the mean
vibrational frequency of the accepting mode and B = 1/kBT; k3 is the Boltzmann
constant. The extent of the vibrational overlap is greatly dependent on the
magnitude of the vibrational wavefunction, which is greatest at the edges of the
potential energy well. As the vibrational quantum number, 11, increases the
magnitude of the wavefunction at the classical turning point also increases. The
vibrational overlap, and therefore km, can be increased by large distortions between
the ground and excited states, AQf, high frequency ground state accepting

vibrations, (cm, or a small energy gap, AE.

The nuclear distortion (equation 5.8) between the equilibrium

0(6)

0(2) ' . '
1 , and ground states, Q j. , defines two coupling

configurations of the excited, Q

limits, strong and weak, shown schematically in Figure 5.3.

AQJO = jSte) _ Q?(x) (58)

The weak coupling limit occurs when the displacement between the energy wells

is small. Because vibrational overlap is poor, Figure 5.3a, coupling is weak, as

 

 

140

 

 

 

 

 

 

Nuclear Coordinate *

Figure 5.2 Potential energy diagram for nonradiative excited state decay.

141

58% 033638: M0 M8
MMMMMMM wMMMMnMMMoo wMMOMMm 05 use ME ME: wMMMMMMMMoo M83 2: MMM £63 385 MMMMMMMoMoMM Wm 95me

 

 

44 144—3

 

142

compared to the strong coupling limit, which is defined by the interaction of the

wells.
1. Calculation of the Huang-Rhys Factor

The important quantity, AQj.’ can be approximated by a Franck-Condom
analysis. Experimentally, this distortion is provided by the Huang-Rhys factor,

S l3
5,, = éZAsz (5.9)

which is related directly to the Franck-Condom factor. Sm can be approximated

from equation 5. 1014

"I

2
s (Lg—)1? (5.16)

where Mm is the reduced mass, vm (=hcom) is the accepting frequency and R is the
difference between the equilibrium bond distances in the ground and excited states

For the case of M—4—M species, the major accepting mode may logically be
assumed to be the M-M bond stretch. Detailed Raman studies of M02X34—ions (X

Cl, Br) demonstrates that these complexes exhibit a strong resonance

enhancement of the totally symmetric A1(Moz) vibration along with excitation of
55*.4b’8’15 Therefore, the majority of the distortion in the excited state will be along
this axis. The extent of nuclear distortion along the metal-metal axis will largely
determine whether these systems are in the weak or strong coupling limits.

The change in M—M bond lengths in the excited state of various quadruple

bonds are listed in Table 5.1. These are calculated either by a Franck-Condon

143

Table 5.1 Vibrational Frequencies of the Excited and Ground State for several M—
4—M Complexes and Their Corresponding Change in the M—M Bond Length in the
Excited State.

 

 

complex v / cm‘l v* / cm’1 R / A
M62c184‘ .- 346 336 0.03g
MozCl4(6—mhp)2(PEt3)2 b 389 370 0.048g
M02C14(PEt3)4 C 342 320 0.07g
1262082— d 272 248 0.079g
Moz[(CH2)2P(CH3)2]4 e 388 345 0.092h
M02[OZCCH3]4 f 406 —— 0.11h

 

3. Clark, R. J. H.; Franks, M. L. J. Am. Chem. Soc. 1975, 97, 2691. Fanwick, P. E.;
Martin, D. 8.; Cotton, F. A.; Webb, T. R. Inorg. Chem.1977, 16, 2103.

b. Macintosh, A. M.; Nocera, D. G. Inorg. Chem. 1996, 35, 7134.

c. Filippo, J. S.; Sniadoch, H. J. Inorg. Chem. 1974, 12, 2326. Filippo, J. S.;
Sniadoch, H. J .; Grayson, R. L. Inorg. Chem. 1974, 13, 2121.

d. Cowman, C. D.; Gray, H. B. J. Am. Chem. Soc. 1973, 95, 8177. Clark, R. J. H.;
Franks, M. L. J. Am. Chem. Soc. 1976, 98, 2763. Bratton, W. K.; Cotton, F. A.;
Debeau, M.; Walton, R. A. J. Coord. Chem. 1971, 1, 121.

e. reference 14b. Martin, D. S.; Newman, R. A.; Fanwick, P. E. Inorg. Chem.
1979, 18, 2511.

f. Cotton, F. A.; Fanwick, P. E. J. Am. Chem. Soc. 1979, 101, 5252.

g. Calculated using the method in reference 25.

h. Calculated by a Franck-Condom analysis of the low temperature absorption.

144

analysis of the low temperature absorption spectrum or from the M—M vibrational
frequencies of the excited and ground states, as discussed above. In general the
increase in the M—M bond length in the excited state is < 0.11 A. From the small
nuclear displacement in the excited state, it can be concluded that M—4—M
complexes are in the weak coupling limit of nonradiative decay theory. Therefore,

only the weak coupling limit will be discussed.
2. Weak Coupling Limit“

Within the weak coupling limit of nonradiative decay, there are two limits;
these are called the low-temperature and temperature-dependent limits. In the low-
temperature limit the quantum spacings for the acceptor modes are large compared
to kBT, hwm >> k BT and the nonradiative decay takes place from the lowest
vibrational component of the higher electronic state. The expression for
nonradiative decay in this limit has been developed by Meyer for the d6 monomers,
the M(bpy)3 complexes and their derivatives (M = Ru, Os, Re),16 and is applicable
to other transition metal systems. In the temperature-dependent limit the quantum
spacings for the acceptor modes are small compared to kBT, ha)", 3 kBT.ll The

thermal population of higher vibrational levels must be accounted for in this case.
a. Low-Temperature Limit of Weak Coupling

The nonradiative decay constant in the low temperature, weak coupling limit is

given by equation 5.11.

so,—

 

C2 '— 27r [—7AE
=— - ex —S ex —— 5.11
h [MAE] p( m) pL 7160,. ] ( )

where, C is the electronic coupling matrix element, AB is the energy gap between

the excited state and the ground state. 7 is defined below by equation 5.12.

145

 

 

y = ln ME 2 —1 (5.12a)
ZhwmAQj’
AE
zl —l 5.12b
7 nlSmhcom] ( )

If there is one primary deactivating accepting mode then equation 5.12a can be

reduced to equation 5.12b and Equation 5.11 reduces to

 

 

yAE
lk =1 —S — 5.13
n", (na m) hwm ( )
l
2 2
a=-C—[ 2” ] (5.14)
h 72me

For a series of compounds, changes in C and Sm are expected to be small and 'y and
In 0t should be invariant with AE. Therefore, if y and In or are considered constants,
a linear relationship between 1n km and AE can be expected, with the rate of

excited state deactivation inceasing as the energy gap decreases.
b. Temperature-Dependent Limit of Weak Coupling

As developed previously, a reasonable acceptor mode for M—4—M
complexes is the M—M bond stretching mode, which is ~26O — 350 cm'l.l7
Because the frequency of the primary accepting mode, (0m, small with respect to
kBT, the temperature dependent limit of the weak coupling must be used at high

temperatures. This can be described by equation 5.15.ll

__c: 27:

2 — -}/'AE
kn, h [hwmAE] exp( Sm(2nm +1))exp[ ha) :| (5.15)

m

7': In 2M”; —1 (5.16)
ZhijQj.’ (nm +1)

 

 

 

146

n +1: (5.17)

 

Equation 5.17 is the thermally averaged population number for mode In and
incorporates the temperature dependence of the coupling parameter G. Equation
5.15 can be treated in a similar manner as equation 5.11, to give equation 5.18.
Based on this equation, a linear dependence of In knr and AB is also predicted in
the temperature dependent limit of weak coupling.

7'AE
ha)

"I

(5.18)

 

ln kn, = [in a — Sm(2nm + 1)] +

3. Intervalence Charge Transfer

Typically, MMCT absorption bands of mixed-valence ground state
complexes are observed at low energy in the absorption spectra. This type of
transition has been termed intervalence charge transfer (IT) by Hush.18 The band
shape, absorption energy, extinction coefficient and the electron transfer distance
can used to determine the electronic coupling between the electron donor and
acceptor. This can be derived from the following relationships. Equation 5.19

gives the expression for the oscillator strength, f, of an electronic transition.19

f = (1.085x10”)GT/ M2 (5.19)

max

where G is the degeneracies of the states concerned, 9m is the transition energy

ax
in cm’1 and M is the transition dipole moment. The oscillator strength can be

calculated from the following experimental properties. 19

f = 4.61x10‘9 email/2 (5.20)

147

where smax is the extinction coefficient and AV],2 the bandwidth at half-height. For
weakly coupled systems, the transition dipole moment can be approximated by the

following expression. ‘9

MzeaR (5.21)

where e is the electronic charge, a is the mixing coefficient of the donor and
acceptor wavefunctions and R is the transition dipole length. The mixing

coefficient, on, is given by equation 5.22.20

a = 541 (5.22)

vmax

HAB is the electronic coupling. From these relations, it can be seen that the
electronic coupling can be defined by equation 5.23. This approximation is only

valid at room temperature.

2.05x10: . _ 1

HM, R (e v Av,/,)2 (5.23)

”I

max max

Mixed-valence complexes have been grouped in three classes.” In class I,
the interaction between the two metal centers is very weak and the properties of
the complex are essentially those of the individual components of the complex. In
class 11., there is a greater interaction between the metals and the properties of the
complex may or may not be those of the constituent parts. In class HI, the
interaction between the two metal centers is large and the properties of the
complex are unique from those of the fragments. The two metals are connected by
a bridge, which may range from a long organic molecule to a single atom or to
nothing (M—M bond). The nature of the bridge and distance between the electron

donor and acceptor greatly affects the extent of the electronic coupling between the

 

 

148

1

metal centers.2 The effect of the distance between the electron donor and

acceptors can be seen with the electronic coupling of complexes ( 1) and (2),

 

 

below.22
6.8 A
n ’ _ ‘ ||| 5+ -1
[(NHslsRU —N.\ /N—RU (NH3)5] HA3 ~ 3300 cm (1)
[(NH3)5Ru”-NC/>'—<\3N-Ru"'(NH3)5]5+ HA3 ~ 400 om‘1 (2)
L J
Y
11.3 A

The distance between Run/Rulll is 6.8 A and 11.3 A in complexes (1) and (2),
respectively. This has a dramatic effect on HA3, which drops by an order of

magnitude for complex (2), as compared to complex (1).
D. Results and Discussion

In order to understand the nonradiative decay of the 155* excited state of
M—4—M complexes fully, a study of the temperature dependence of the emission
lifetimes of MozCl4(PR3)4, MoWC14(PR3)4 and W2Cl4(PR3)4, where PR3 = PMe3,
PMezPh, PMeth, was initiated.

1. M02Cl4(PR3)4

Absorption and emission spectra of glasses of 2—MeTHF solutions of

M02C14(PR3)4 exhibit vibrational fine structure at 40 K. The half-width of the

149

emission band is temperature dependent, increasing by about 300 cm“1 between 40
and 290 K.

Figures 5.4 - 5.6 display the temperature dependence of the excited state
decay constants of M02C14(PR3)4, the experimental fits to equation 5.4 are shown
as solid lines in each Figure. The lifetimes could be fit to a monoexponential decay
expression for all three complexes at all temperatures. Table 5.2 tabulates the rate
constants, k1 and kg, and energy gaps for MozCl4(PR3)4 as calculated from equation
5.4. k] is ~5 x 106 and k2 is about 2 x10”; with the exception of M02C14(PMePh2)4
for which k2 is an order of magnitude faster. The energy gap between 155* and the
higher energy state is about 1750 cm‘1 for M02C14(PR3)4.

Figure 5.5 displays a sharp discontinuity between the temperatures 130 and
180 K. The expression for km, equations 5.14 and 5.20, contain the Franck-Condon
factor in the term exp(—Sm (2"m+1)) exp[—y'AE /ha)m], which accounts for both
intramolecular and solvent vibrational modes. This discontinuity is consistent with
solvent modes coupled to the intramolecular vibrations. In a rigid glass, the solvent
dipoles are immobile in the time scale of the excited state lifetime and do not have
time to reorient themseleves to the electronic structure of the excited state. As the
glass begins to soften, the reorientation time becomes comparable to the lifetimes
and AE decreases due to an enhanced stabilization of the excited state by partial
dipole reorientation. At higher temperatures, as the glass becomes fully fluid, the
solvent reorientation time is fast compared to the time scale for the excited state
decay and the solvent has time to fully reorient itself in response to the change in
electronic structure. This has been observed in the temperature dependence of the

lifetimes of [Os(bpy)3]2+, which shows a relatively sharp discontinuity between the

kobS/ 106 s'1

8.0

7.5

7.0

6.5

6.0

5.5

5.0

150

 

I I I l I I I I I I I I I l

I I l I I I

 

1 JL l l l l L l l l I l l l l l l L I l l l L l l

 

 

50 100 150 200 250 300
T / K

Figure 5.4 Fit of the variation of the observed emission decay constant
of M02C14(PMe3)4 to equation 5.4 in the 40-290 K temperature range.

 

kobs/ 107 s;1

151

 

4.0 _
3.5 f

3.0

IfTI

I I I I

2.0

1.5

I I I I I rm I

1.0}

 

0.5}

 

 

lllll lllllLJlllLLlllll 11 l

0 50 100 150 200 250 300 350

 

T/K

Figure 5.5. Fit of the variation of the observed emission decay constant of
M02Cl4(PMe2Ph)4 to equation 5.4 in the 40-290 K temperature range.

kobsl 107 s:1

152

 

6.0

5.09

4.0j

2.0 —

 

ll 1 l l 1L1 l 1 ll 11 l J 1L1 _l_L l l l lLJl

0 50 100 150 200 250 300 350
T/K

 

 

Figure 5.6 Pit of the variation of the observed emission decay constant of
Mo2C14(PMePh2)4 to equation 5.4 in the 40290 K temperature range.

 

153

Table 5.2 Temperature Dependence of AE, k1 and k2 of M2Cl4(PR3)4

 

 

 

 

 

 

 

 

 

M02C14(PR3)4
PR3 AE/ cm ‘1 kll 106 k2/ 10“
PMe3 1400 5.0 0.16
PMezPh 1730 5.5 0.28
PMeth 2155 5.0 15
MOWCl4(PR3)4
PR3 AE/cm"1 k1/ 106 k2/ 1010
PMo3 291 8.9 0.0027
PMezPh 1980 15 8.9
PMeth 1480 5.4 3.7
W2C14(PR3)4
PR3 AE/ om“1 lo/ 106 k2/ 106
PMo3 625 1.8 8.2
PMezPh 496 1.9 1 1
PMeth 679 0.93 6.5

 

154

temperatures 100 to 150 K.23 The change in dipole in the excited state for
[Os(bpy)3]2+, ~13 D, is significant in terms of solvent interactions, therefore
producing a large effect at the glass—to—fluid transition temperature when the
solvent dipoles are able to reorient themselves again. As a consequence of the
polar nature of the M—4-M excited state, 4 D,24 this phenomenon at the glass—to—
fluid temperature is also observed for these complexes.

Tables 5. 3 - 5.5 list (1),, 'c, kr and km for M02C14(PR3)4 over the temperature
range of 40 - 290 K. In order to correct for the change in the absorption with
temperature, the absorption spectrum of W2C14(PMe3)4 was measured as a function
of temperature. A correction file was generated from this and used to correct the
¢e(T) for all complexes. The rates measured, Table 5.3, at 290 K for
MozCl4(PMe3)4 are in good agreement with those previously reported (kr = 1.96 x
106 s"1; knr = 5.5 x 106 s71)8. The nonradiative decay constants for M02C14(PR3)4

1, with the exception of M02C14(PMe3)4, which is an order of

are about 3 x 107 s—
magnitude slower at 6 x 106 8"]. The smaller rate for M02C14(PMe3)4 is consistent
with its much longer lifetime, 127 ns at 290 K, as compared to 26 and 34 us for
M02C14(PMe2Ph)4 and MozCl4(PMePh2)4, respectively. Clearly, there is a thermal
decay pathway for the other complexes that is not available to M02C14(PMe3)4.

The fraction of decay that occurs through the upper excited state can be
calculated from equation 5.5. This has been calculated for 290 and 40 K, Table
5 .6. At 290 K there is significant deactivation of the 155* excited state through this
higher” energy excited state, while at 40 K there is considerably less. The fraction

of decay, n 1, and knr are consistent with an activationless decay at low temperature,

indicative of direct internal conversion to the ground state, while at high

155

Table 5.3 Temperature Dependence of (be, 12, kr and km of M02C14(PMe3)4

 

 

T/K (be/104 r / ns k,/106 km/ 106
39 66 196 3.3 1.8
51 67 194 3.5 1.7
61 68 197 3.5 1.6
70 68 198 3.4 1.6
80 67 200 3.4 1.6
90 65 201 3.3 1.7
99 64 201 3.2 1.8
110 65 202 3.2 1.8
121 67 205 3.3 1.6
130 67 203 3.3 1.6
141 66 203 3.3 1.7
150 66 205 3.2 1.7
161 64 203 3.2 . 1.8
170 63 204 3.1 1.8
181 62 202 3.1 1.9
190 61 201 3.0 1.9
201 59 197 3.0 2.0
210 57 196 2.9 2.1
220 56 194 2.9 2.3
230 53 191 2.8 2.5
241 51 184 2.8 2.7
250 47 177 2.7 3.0
261 43 164 2.6 3.4
270 38 150 2.5 4.1
281 33 132 2.5 5.0

290 29 127 2.3 5.6

156

Table 5.4 Temperature Dependence of (be, 1:, kr and km of M02C14(PMe2Ph)4

 

 

 

T/K 98/10’2 I / ns 1<,/106 l<,,,/106
40 39 202 2.0 3.0
51 40 208 1.9 2.9
60 39 206 1.9 2.9
71 36 199 1.8 3.2
80 36 198 1.8 3.2
90 39 196 2.0 3.1
100 43 194 2.2 2.9
110 44 190 2.3 3.0
121 44 189 2.3 3.0
131 44 187 2.3 3.0
141 43 155 2.8 3.7
151 42 154 2.7 3.8
161 40 154 2.6 3.9
170 39 153 2.5 4.0
181 36 149 2.4 4.3
191 34 146 2.3 4.5
202 31 140 2.2 4.9
211 28 132 2.1 5.5
221 24 118 2.1 6.4
230 20 108 1.9 7.4
241 16 89.9 1.8 9.3
250 14 73.7 1.9 12
260 11 57.8 1.9 15
270 7.9 45.9 1.7 20
281 5.8 35.6 1.6 26

300 3.5 25.9 1.3 37

 

157

Table 5.5 Temperature Dependence of (be, I, kr and km of MozCl4(PMePh2)4

 

 

T/K 6.,/10‘2 r / ns k,/105 km/ 106
40 8.5 187 4.6 4.9
60 9.0 195 1 4.6 4.7
80 8.7 189 4.6 4.8
100 8.2 186 4.4 4.9
120 7.4 187 3.9 4.9
140 6.8 187 3.6 5.0
161 6.2 199 3.1 4.7
181 5.8 196 3.0 4.8
201 4.8 193 2.5 4.9
221 4.2 179 2.4 5.4
241 3.0 126 2.3 7.7
261 1.9 72 2.7 14

280 1.2 34 3.6 29

 

158

Table 5.6 Fractional of Decay Through the Upper Excited State at 290K and 40K

 

 

 

complex ‘11 (290 K) 111 (40 K)a
M02C14(PM€3)4 0.75 4.3 x 10 ’24
M02C14(PM62Ph)4 0.49 4.8 x 10 ‘24
M02C14(PMePh2)4 0.87 6.5 x 10 "29
MoWCl4(PMe2Ph)4 0.24 6.9x 10 ‘23
M9WC14(PMePh2)4 0.81 5.2x 10 ‘20
W2C14(PMe3)4 0.67 7.8x 10 '9
W02C14(PM62Ph)4 0.83 7.5 x 10 "7
M02C14(PMCPh2)4 0.71 1.1 x 10 "’9

 

a. MoWCl4(PR3)4 are calculated at 60 K.

159

temperature they are consistent with thermally activated decay through a potential
surface crossing between 155* and some higher energy excited state.

Table 5.7 lists kr and km. for a series of M02X4(PR3)4 complexes. kr and knr
are very similar, with the exception of PMe3 for which the nonradiative decay is
almost an order of magnitude smaller. This indicates that the nature of the
phosphine ligand does not have a large effect on the nonradiative decay. When the
halide is changed to Br and I, the nonradiative decay increases from 5.5 x 106 s—1
for C1 to 9.2 x 106 and 3.0 x 107 871, respectively. This indicates that the mixing of
the halidem’j’15 with the 155* excited state contributes significantly to the
nonradiative decay.

As discussed previously, the change in the equilibrium M—M bond distance
upon excitation can describe the amount of distortion in the excited state. This
change in bond lengths is defined by equation 5.24,14 where re and r: are the
equilibrium bond distances in the ground state and the excited state, b corresponds

to the total number of bonds that are changing.

R = [20; — (YT (5.24)

The bond distances in the ground and excited states can be estimated from
Woodruff’s modification of Badger’s rule.25 The equilibrium bond distances for
elements Rb - Xe and for elements Cs - Rn are given by equations 5.25 and 5.26,

respectively.

-—-k
= 1.83 1.51 {—1 5.25
r + exp 2.48 ( )

 

160

Table 5.7 kr and knr for a Series of M02X4(PR3)4 Complexes.

 

 

complex k,/ 106 km/ 107
M02C14(PM63)4 1.92 0.55
M02C14(PMe2Ph)4 1.3 3.7
M02C14(PMePh2)4 0.36 2.9
M02C14(PEt3)4 0.929 7.1
M02C14(PPr"3)4 0.762 4.7
M02C14(PBu"3)4 0.929 7.1
MozBr4(PMe3)4 1.92 0.92

 

M0214(PMe3)4 4.24 3.0

161

k
= 2.01 1.31 {—j 5.26
r + exp 2.36 ( )

k is the force constant and can be calculated from equation 5.27.25

k = 355x10'7M v 2 (5.27).

m ”I

The ground state and excited state stretching frequencies for the complex
M02C14(PMe3)4 are 3545’26 and 3357 cm’l, respectively. re was calculated from
equation 5 .13 and corresponds well with the experimentally determined bond
distance,27 Table 5.8. Therefore, this equation should afford a good approximation
of the excited state equilibrium bond distance, r5, Table 5 .8. From the change in
bond distance in the excited state, R = 0.08 A, S,m was determined (equation 5 .10)
to be 1.5, thereby confirming that the M02 complexes are in the weak coupling
limit of nonradiative decay. Plots of In knr vs the emission energy, AE, for T = 40
and 290 K for M02C14(PR3)4, Figure 5.7. The largest error in the calculation of km
is due to (be and is estimated at 20%. The linear correlation indicates that this series
of complexes follows energy gap theory. The nature of the chromophore for M02,
MoW and W2 changes with the metal and therefore the three series of complexes

should not be expected to fall on the same line, with respect to the energy gap law.

2. W2Cl4(PR3)4

Vibrational fine structure is observed at 40K in the absorption and emission
spectra of 2—MeTHF solutions of W2C14(PR3)4. The band width of the emission
exhibits a temperature dependence, increasing by about 700 cm"1 between 40 and

290 K.

 

162

Table 5.8 Calculated Excited and GrOund state M—M Bond Lengths, Experimental
M—M Bond Lengths and the Calculated Huang-Rhys Factor.

 

re(calc) IA r(exptl) IA re*(calc) IA Ar Sma

 

M2
MozCl4(PMe3)4 2.17 2.1325 2.25 0.08 1.51
MoWCl4(PMe3)4 2.21 2.20927 2.25 0.04 0.50

W2C14(PBU3)4 2.29 2.26726 2.35 0.06 1.21

 

a. calculated from equation 5.10.

 

163

 

17.5 E

PMezPh

17.0
16.5 3
16.0 1

15.5 "— I PMe3

 

 

In knr

L
15'5 ~ PMezPh

* PMePh2
15.0 _—

14.5 —
I PMe3

 

 

p—

lLllllJlJllJLllllLlllll

1.45 1.47 1.49 1.51
AE / 104 cm"1

 

Figure 5.7 Energy gap plots for M02C14(PR3)4 at 290 K ( ° ) and 40 K ( 0 ).

164

The experimental fits to the temperature dependent lifetimes are shown as a
solid line in Figures 5.8 - 5.10. The lifetimes of all complexes could be fit to a
monoexponential decay expression at all temperatures. kobs below 130 K for
W2Cl4(PMe2Ph)4, Figure 5.9, could not be fit along with the high temperature data.
This is possibly due the phase transition from a glass to solution at this
temperature, as discussed above.

The rate constants, k1 and kg, and energy gaps for W2Cl4(PR3)4 as calculated
from equation 5.4 are listed in Table 5.2. k1 for all three complexes is ~2 x 106 and
k2 is about 7 x106; with the exception of W2C14(PMe2Ph)4, which has rate constant
k2 of 11 x 106. In general, the decay rate constants for the upper excited state, k2,
are 4 to 5 orders of magnitude slower than the M02C14(PR3)4 and MoWC14(PR3)
(see below). AE, the energy gap between 158* and the higher energy excited state
is calculated to be about 600 cm"1.

The luminescent quantum yields, (lie, and lifetimes, ‘C, and the radiative, k,,
and nonradiative, km, decay constants for W2Cl4(PMe3)4, W2Cl4(PMe2Ph)4 and
W2C14(PMePh2)4, respectively, are listed in Tables 5.9 - 5.11. kIr and kn, are similar
for all three complexes, 1.3 x 106 s’1 and 1.8 x 107 s". The lifetimes start to
converge to 100 us at 40 K.

The fraction of decay through the upper excited state at 290 and 40 K are
listed in Table 5.6. The amount of deactivation of 165* through the upper excited
state at 290 K is substantial. Due the small energy gap, the amount of deactivation
at 40 K is still quite significant. This is consistent with deactivation through
thermal population of some higher energy excited state and due to the small energy

gap, there is still considerable deactivation at 290 K. This can be observed in the

 

165

2.15

 

I

2.10_
2.05}

2.00"-

 

1.95:

kobsl 107 s'1

1.90}

1.85}

 

 

1.80}

 

 

lllllillLJlLllllllllllll

0 50 100 150 200 250 300
T / K

 

Figure 5.8 Fit of the variation of the observed emission decay constant of
W2Cl4(PMe3)4 to equation 5.4 in the 40-290 K temperature range.

kobsl 107 s:1

166

 

2.6

2.5

2.4

2.3

2.2

2.1

2.0

 

1.9

IITTIII'IIIIIIIUIUrlllllllIIr'lljl'llll

 

 

 

100 150 200 250 300
T / K

Figure 5.9 Fit of the variation of the observed emission decay constant of
W2Cl4(PMe2Ph)4 to equation 5.4 in the 130-290 K temperature range.

 

167

 

1.20

1.15

 

1.00

0.95

 

 

_\
I I I I I I I I I I I I I l I I I I I I I I I l I I I I

 

 

lllll!llLlllllJllllIlllllllllILlll

0 50 100 150 200 250 300 350
T/K

 

Figure 5.10 Fit of the variation of the observed emission decay constant of
W2Cl4(PMePh2)4 to equation 5.4 in the 40-3 00 K temperature range.

168

Table 5.9 Temperature Dependence of (be, t, kr and km of W2Cl4(PMe3)4

 

 

 

 

T / K (lye/10‘2 'c / ns 1c,/106 km/ 107
40 14 56 2.6 1.5
49 14 55 2.6 1.5
60 15 56 2.6 1.5
69 15 55 2.7 1.6
81 15 55 2.7 1.5
90 15 54 2.7 1.6
99 14 55 2.6 1.6
109 14 54 2.5 1.6
120 14 54 2.5 1.6
131 13 54 2.4 1.6
140 13 54 2.3 1.6
151 12 54 2.3 1.6
161 12 54 2.2 1.6
169 12 53 2.2 1.7
180 11 53 2.1 1.7
190 11 54 2.1 1.7
200 11 53 2.0 1.7
211 10 52 2.0 1.7
220 10 52 1.9 1.7
230 9.7 51 1.9 1.8
240 9.4 51 1.8 1.8
250 9.0 51 1.8 1.8
260 8.6 50 1.7 1.8
270 8.3 49 1.7 1.9
280 8.0 48 1.6 1.9

290 7.8 47 1.7 2.0

 

169

Table 5.10 Temperature Dependence of (be, 1:, kr and km of W2Cl4(PMe2Ph)4

 

 

 

 

T/K (be/10‘2 t/ns 1<,/106 ‘ km/106
40 17 111 1.6 7.4
49 26 111 2.3 6.7
59 24 104 2.3 7.3
70 21 96 2.2 8.2
80 15 68 2.2 12
90 12 55 2.6 16
100 14 55 2.2 16
109 12 56 2.2 16
121 12 55 2.2 16
131 12 54 2.2 17
140 11 52 2.2 17
149 11 52 2.1 17
160 11 51 2.1 17
171 10 50 2.0 18
180 9.7 49 2.0 18
190 9.3 49 1.9 19
200 8.9 48 1.9 19
211 8.5 47 1.8 19
220 8.1 46 1.8 20
230 7.6 45 1.7 20
240 7.3 44 1.7 20
250 7.1 43 1.7 2.2
260 6.9 42 1.6 22
270 6.6 41 1.6 23
280 6.0 40 1.5 23

290 5.5 39 1.4 24

 

170

Table 5.11 Temperature dependence of (be, 1:, kr and km of W2Cl4(PMePh2)4

 

 

 

T / K 111,410"2 t/ns k,/106 km/ 106
39 20 108 19 7.4
60 10 111 9.4 8.1
80 10 108 9.6 8.3
100 10 106 9.5 8.5
121 9.9 107 9.3 8.4
140 9.6 104 9.3 8.6
160 9.4 103 9.1 8.7
181 9.1 105 8.7 8.6
201 8.7 102 8.6 8.8
210 8.4 101 8.3 8.9
221 8.2 100 8.2 9.1
230 8.0 99 8.1 9.2
240 7.8 98 7.9 9.3
250 7.5 98 7.7 9.2
260 7.1 95 7.4 9.6
270 6.9 92 7.4 . 9.9
281 6.5 92 7.1 10

290 6.2 89 7.0 10

 

171

lifetimes at 40 K, which start to converge to 100 us, but are not quite as long as
expected relative to the M02 homologs.

The ground state stretching frequencies for W2C14(PBu3)4 is 260 cm"1 and
was measured by Raman spectroscopy.28 The excited state M—M stretching
frequency for M02C14(PMe3)4 changes by ~5.6% relative to the ground state. This
was used to estimate the excited state M—M stretching vibration for W2 to be 246
cm’l. Using these values and equation 5.26, re and r; were calculated to be 2.29
and 2.39 A, respectively. The experimentally determined M—M bond distance is
2.267 A27, showing that the assumptions made are valid. The change in the M—M
bond length, calculated from equation 5.24, is 0.06A. There Sm is calculated to be
1.21, establishing that the W2 complexes are in the weak coupling limit of km. A
plot of In knr vs the emission energy, AB, is shown in Figure 5.11. The error in km
of W2Cl4(PMe2Ph)4 is about 30%, while the error in the others is about 20%. There
is no clear linear correlation, which is expected based on equation 5.18. This is
probably due to the limited range that is spanned by the data set, which is only 15 -
17 for In km and 1.29 - 1.35 x 104 cm'1 for AB. A larger data set, i.e. more
complexes, are required to clearly establish that the W—4—W complexes follow the

energy gap law.
3. MoWC|4(PR3)4

In contrast to the homobimetallic complexes, vibrational structure is absent
for the heterobimetallc MoWC14(PR3)4 in 2—methy1pentane down to 60 K. An
explanation for this difference may lie in the greater dipole moment of the
heteronuclear complex resulting in a greater coupling of the dipole with the

solvent. This may lead to greater inhomogeneous line broadening. 2—

 

172

 

17.0

16.5 — . W2PM63

 

MoWPMezPh
W2PMePh2 ‘j

 

 

 

 

 

 

 

 

 

E _
x -_ O
E 16.0 - J
3 +w2PMe2Ph
<> MoWPMe3
15.5 —
o MoWPMePh2
1
lllllll:l_lLlllllllllllllllll
1.29 1.31 1.33 1.35

AE / 103 cm"1

Figure 5.11 Energy gap plots for MoWCl4(PR3)4 ( 0 ) and W2Cl4(PR3)4
( 0 ) at 60 and 40K, respectively.

173

methylpentane was used instead of 2—MeTHF for MoWC14(PR3)4 complexes due
to decomposition of MoWCl4(PR3)4 in the latter solvent. The formation of fissures
in the optical glass below 60 K for 2—methy1pentane precluded measurements
below this temperature, which may be necessary in order to observe the vibrational
fine structure of these complexes. In addition, the emission band width exhibits a
temperature dependence and increases by 400 cm '1 from 60 to 290 K.

The temperature dependence of the luminescence lifetimes of each complex
is shown in Figures 5.12 - 5 .14. The solid line in each Figure is the fit of equation
5.4 to the experimental data. A monoexponential decay expression could be used
to fit the luminescent lifetimes of all three MoW complexes at all temperatures
studied. The fit to the experimental rates of MoWCl4(PMe3)4, Figure 5.12, is very
poor due to the scatter in the data.

Table 5.2 lists the calculated rate constants, k1 and kg, and the energy gap.
AE’. The decay rate constants, k1, for MoWCl4(PMe3)4 and MoWCl4(PMePh2)4 are
the same order of magnitude, ~6-8 x 106, while k] for MoWCl4(PMe2Ph)4 is an
order of magnitude faster (15 x 106). The decay rate constants, kg, for ‘
MoWCl4(PMezPh)4 and MoWCl4(PMePh2)4 are both 101°, while MoWCl4(PMe3)4
is three orders of magnitude slower (2.7 x 107). This large discrepency for the
latter complex is due to the poor data. Due to the large error in the data for this
complex the calculated energy gap for MoWCl4(PMe3)4 (291 cm"l) is much lower
than either MoWCl4(PMe2Ph)4 or MoWCl4(PMePh2)4, which is about 1730 cm".
Therefore, the energy gap for MoWCl4(PMe3)4 must be discounted.

Tables 5 .12 - 5.14 list the luminescent quantum yields, (be, lifetimes, ‘L‘, and

the radiative, k,, and nonradiative, km, decay constants for MoWCl4(PMe3)4,

 

174

 

‘L20

1.15

1.10

1.05

kobsl 107 s'1

I I I I I I F T I I I I l I I I I l I I I I I

(195

I I I I I I

(180

 

 

 

IJLI l l I ILI l I l l Illll l l I 1 LI

50 100 150 200 250 300
T/I(

 

Figure 5.12 Fit of the variation of the observed emission decay constant of
MoWC14(PMe3)4 to equation 5.4 in the 60-280 K temperature range.

 

 

 

 

 

 

175

 

2.1

2.0

1.9

1.8

kobs I 107 8-1

1.7

1.6

I T I I I I I I I I I I I I I I l I I I I I I I I I I I I I

 

 

1.5 .11....1
50 100

l I I l L l I I I l l l

150 200 250 300
T / K

I

 

Figure 5.13 Fit of the variation of the observed emission decay constant
of MoWCl4(PMe2Ph)4 to equation 5.4 in the 60-290 K temperature range.

 

176

 

31)

215

2f)

 

kobsl 107 s."1
01

‘L0

I I I I I I I I I I I I I I I I I I I

 

 

(15

T I I I I

 

 

I L l J l l l l l I I l I l L l I I l J

50 100 150 200 250 300
T/I(

 

Figure 5.14 Fit of the variation of the observed emission decay constant
of MoWCl4(PMePh2)4 to equation 5.4 in the 60-290 K temperature range.

 

177

Table 5.12 Temperature Dependence of (be, r, kr and km of MoWC14(PMe3)4

 

 

 

T/K (be/104 e / ns k,/106 km/ 106
60 29 113 2.6 6.3
70 29 106 2.7 6.7
81 29 109 2.6 6.6
91 28 110 2.6 6.5
101 28 109 2.5 6.6
110 27 112 2.5 6.5
121 25 113 2.3 6.6
129 26 113 2.3 6.6
139 27 105 2.6 7.0
151 27 85 3.2 8.5
160 27 87 3.1 8.4
170 26 89 3.0 8.3
180 25 88 2.9 8.5
190 24 90 2.7 8.4
200 22 89 2.5 8.7
210 21 89 2.4 8.8
220 19 87 2.2 9.3
230 18 88 2.1 9.3
240 17 86 2.0 9.6
250 16 85 1.9 9.9
260 15 84 1.8 10
271 14 85 1.7 10
280 14 87 1.6 9.9

291 13 88 1.5 9.9

 

 

178

Table 5.13 Temperature Dependence of (be, 1:, k, and km of MoWCl4(PMe2Ph)4

 

 

 

T/K 111,410-2 t/ns 1o/106 km/ 106
60 22 77 3.8 9.2
70 19 75 3.9 9.4
79 19 73 3.9 9.8
90 18 66 4.2 11
99 18 63 4.4 11
110 18 65 4.3 11
121 18 64 4.0 12
131 17 65 4.0 11
141 17 64 4.2 11
150 17 66 4.2 11
161 16 65 4.1 11
171 16 66 4.0 11
181 15 66 3.8 11
190 15 65 3.7 12
201 15 65 3.4 12
210 14 65 3.3 12
221 13 65 2.9 12
230 13 63 2.9 13
241 12 63 2.8 13
250 11 62 2.7 14
261 10 59 2.6 14
270 9.3 57 2.5 15
281 8.3 53 2.6 16

290 7.6 50 2.6 18

 

179

Table 5.14 Temperature Dependence of (1)6, 15, kr and km of MoWCl4(PMePh2)4

 

 

 

T/K q).,/10‘2 e / ns k,/105 km/ 106
59 13 214 6.3 4.1
70 12 174 7.1 5.0
80 12 172 7.0 5.1
90 11 172 6.5 5.1
100 11 175 6.4 5.1
111 11 178 6.3 5.0
121 11 180 6.0 4.9
130 10 180 5.8 5.0
140 9.0 181 5.0 5.0
150 9.2 181 5.1 5.0
161 8.7 182 4.8 5.0
1.71 7.8 183 4.3 5.0
190 8.2 176 4.6 5.2
210 8.1 167 4.9 5.5
230 7.7 132 5.8 7.0
241 7.4 104 7.1 8.9
251 7.0 80 8.8 12
261 6.9 60 12 16
270 6.4 46 14 20
281 6.7 39 16 24

291 6.0 36 17 26

 

 

 

 

180

MoWCl4(PMe2Ph)4 and MoWCl4(PMePh2)4, respectively. km, at 290 K, increases
from 9.9 x 106 s", 18 x 106 s"1 and 26 x 106 s"1 on going from MOWCI4<PMC3)4,
MoWC14(PMe2Ph)4 to MoWCl4(PMePh2)4, displaying a similar trend that is
observed for the MozCl4(PR3)4 complexes. Interestingly, this trend is not observed
for W2Cl4(PR3)4.

The amount of decay, as calculated from equation 5.5, through the upper
excited state at 290 K is remarkable. The convergence of the lifetimes to 200 ns is
consistent with a much smaller fraction of decay through this higher energy excited
state at 60 K, indicating a small contribution from this state at this temperature.

The ground and excited state M-M stretching frequencies are 32226 and 305
cm", respectively, for MoWCl4(PMe3)4. The ground state vibration was measured
by Raman spectroscopy, while the excited state vibration was estimated in a
similar manner as that for W2. The ground state equilibruim bond length was
calculated by averaging the results from equations 5.25 and 5.26, which gives a
value that compares well to the experimentally determined length, 2.21 A
(calculated) and 2.209 A (experimental).29 The excited state equilibrium bond
distance that was calculated for MoW is the same as that calculated for M02, Table
5 .8. This indicates a slightly less distorted excited state for MoW, relative to M02
and W2, and is reflected in Sm (0.5) and R (0.04 A). This also defines a weak
coupling limit of km for the MoW series. Figure 5.11 shows the energy gap plots
for MoWCl4(PR3)4 at 60 K. The error in MoWCl4(PMe3)4 was estimated at 30%,
while the error of the other two complexes is estimated at 20%. As discussed in the
case of W2Cl4(PR3)4, a greater number of complexes need to be studied to

definitively determine if Mo—4—W complexes follow the energy gap law.

 

 

181

4. Electronic Coupling

The calculated electronic couplings for M2Cl4(PR3)4 (M2 = M02, MoW, W2)
are listed in Table 5.15, along with AT/Vz, emu, Tim. HA3 for all nine complexes is
about 2300 cm’l. While this number seems large for valence electrons localized in
parallel dxy orbitals, considering the short distance for charge transfer (2.13-2.26
A), it is entirely reasonable. When this number is compared to a coupling of 3300
cm‘1 for [(NH3)5RuH]2pyzs+, which is over a distance of 6.8 A,22 it is relatively
small. If all the numbers remain the same, except to increase the M—M bond
distance to 6.8A, H AB drops by an order of magnitude.

Based on this the electronic coupling between the metal centers can be
considered small and the electrons localized on separate metal centers. For
comparison, L(NH3)4Ru(NC)Fe(CN)5’ (L = NH3, 4-Mepy, 3-Clpy) was calculated
to have an electronic coupling of ~3200 cm” over an electron transfer distance of
3.5 A and is considered to be localized.30 It is possible to calculate the electronic
coupling from the rate expression for km. However, the electronic coupling, as
calculated from Hush theory, while not considered to be in the strong coupling
limit of nonradiative decay theory, is too strongly coupled to use equation 5 .1 1 or
5.20. The electronic coupling usually observed is ~600 cm"l .3 1

In. summary, the excited state dynamics of M2Cl4(PR3)4 clearly show the
existence of efficient deactivation pathways of l(88*). A proposed energy diagram
for M02C14(PMe2Ph)4 is shown in Figure 5.15. This excited state is in thermal

equilibrium with a higher energy excited state, S), that is ~2000 cm '1

away for
M02 and MoW and ~600 cm"l away for W2. Photochemistry is observed with

quadruply bonded bimetallic complexes with 71m Z 1(715*). This absortion band

 

 

182

Table 5.15 emax, vmax, AV,” and Calculated HA3 for M2Cl4(PR3)4.

 

 

 

 

 

 

complex 8mx / M"1 cm"l vmax / cm"l AT/l/2 / cm’l HA3 / cm“l

MozCl4(PMe3)4 3110a 17229 1408 p 2644
M02Cl4(PMe2Ph)4 2943 16909 1307 2455
MozCl4(PMePh2)4 2810 16782 1535 2589
MoWC14(PMe3)4 ~2200" 16045 1723 2290
MoWC14(PMe2Ph)4 2169 15590 1475 2074
MoWCl4(PMePh2)4 2609 15590 1478 2276
W2Cl4(PMe3)4 4170a 15234 1154 2454
W2Cl4(PMe2Ph)4 3750 14902 . 1201 2348
W2C14(PMePh2)4 4012 15228 1437 2685

 

a. from reference 7; in 2-methy1pentane; b. approximated

 

 

 

183

 

 

 

 

 

22 —
111:6 *1

‘T 20 ‘—
E —
0 . ESBQ
8 Intermediate
2
T:
a; 18 *—
C
u.1

16

\\
0

11

Nuclear Coordinate

Figure 5.15 Proposed energy diagram for M02C14(PMe2Ph)4.

 

184

maximum is typically 5400 - 6300 cm‘1 away from the absorption maximum of
1(88’“) in M2C14(PR3)4 complexes. The fact that photochemistry has not been
observed upon excitation of 1(65*), indicates that there is an activation barrier to
reaching the high energy intermediate believed to be necessary to trap the
zwitterion, as described before. It appears that the asymmetry of MoW is not
enough to circumvent the need for the intramolecular rearrangement, since
photochemistry is still not observed with wavelengths coincident with 1(85*), as
discussed in Chapter 4. Additionally, the deactivation of S2 cannot occur through
this distorted intermediate. As calculated from equation 5.5, the population of this
state at room temperature is significant. If this state, S2, underwent molecular
rearrangement to the distorted ESBO, we would expect to observe photochemistry

upon the population of 1(58*).
E. References

1. (a) Cotton, F. A.; Harris, C. B. Inorg. Chem. 1965, 4, 330. (b) Cotton, F. A.;
Curtis, N. E; Harris, C. B.; Johnson, B. F. G.; Lippard, S. J.; Mague, J. H.;
Robinson, W. R.; Wood, J. S. Science 1964, 145, 1305.

2. Cowman, C. D.; Gray, H. B. J. Am. Chem. Soc. 1973, 95, 8177.

3. Hay, J. P. J. Am. Chem. Soc. 1982, 104, 7007.

4. (a) Trogler, W. C.; Cowman, C. D.; Gray, H. B.; Cotton, F. A. J. Am. Chem.
Soc. 1977, 99, 2993. (b) Hopkins, M. D.; Schaefer, W. P.; Bronikowski, M. J.;
Woodruff, W. H.; Miskowski, V. M.; Dallinger, R. B; Gray, H. B. J. Am. Chem.
Soc. 1987, 109, 408.

 

185

5. Hopkins, M. D.; Miskowski, V. M.; Gray, H. B. J. Am. Chem. Soc. 1988, 110,
1787.

6. Miskowski, V. M.; Gray, H. B.; Hopkins, M. D. Inorg. Chem. 1992, 31, 2085.
7. Hopkins, M. D.; Gray, H. B. J. Am. Chem. Soc. 1984, 106, 2468.

8. Zietlow, T. C.; Hopkins, M. D.; Gray, H. B. J. Solid St. Chem. 1985, 57, 112.

9. Turro, N. J. Modern Molecular Photochemistry; University Science Books:
Sausalito, CA, 1991; pp 176-180.

10. (a) Lumpkin, R. S.; Kober, E. M.; Worl, L. A.; Murtaza, 2.; Meyer, T. J. J.
Phys. Chem. 1990, 94, 239. (b) Van Houten, J .; Watts, R. J. J. Am. Chem. Soc.
1976, 98, 4853. (c) Durham, B.; Caspar, J. V.; Nagle, J. K.; Meyer, T. J. J. Am.
Chem. Soc. 1982, 104, 4803.

11. Engleman, R.; Jortner, J. Mol. Physics 1970, 18, 145.

12. (a) Ballhausen, C. J. Molecular Electronic Structures of Transition Metal
Complexes; McGraw-Hill: New York, 1978; Ch 4. (b) Hertzger, G. Molecular
Spectra and Molecular Structure; Van Norstrand: New York, 1950; Vol. 1, Ch 4.
13. Barbara, P. E; Meyer, T. J .; Ratner, M. A. J. Phys. Chem. 1996, 100, 13148.
14. (a) Caspar, J. V.; Westmoreland, T. D.; Allen, G. H.;- Bradley, P. G.; Meyer, T.
J.; Woodruff, W. H. J. Am. Chem. Soc. 1984, 106, 3492. (b) Trogler, W. C.;
Solomon, E. I.; Trajberg, [B.; Ballhausen, C. J .; Gray, H. B. Inorg. Chem. 1977,
I6, 828.

15. Hopkins, M. D.; Gray, H. B.; Miskowski, V. M. Polyhedron 1987, 6, 705.

16. See for example: (a) Kober, E. M.; Casper, J. V.; Lumpkin, R. S.; Meyer, T. J.
J. Phys. Chem. 1986, 90 3722. (b) Coe, B. J .; Thompson, D. W.; Culbertson, C. T.;
Schoonover, J. R.; Meyer, T. J. Inorg. Chem. 1995, 34, 3385. (c) Claude, J. P.;

 

186

Meyer, T. J. J. Phys. Chem. 1995, 99, 51. (d) Strouse, G. F.; Schoonover, R.;
Duesing, R.; Boyde, S.; Jones Jr., W. E.; Meyer, T. J. Inorg. Chem. 1995, 34, 473.
17. Trogler, W. C.; Gray, H. B. Acc. Chem. Res. 1978, 11, 232.

18. (a) Allen, G. C.; Hush, N. S. Prog. Inorg. Chem. 1967, 8, 357. (b) Hush, N. S.
Prog. Inorg. Chem. 1967, 8, 391.

19. Rezvani, A. R.; Evans, C. E. B.; Crutchley, R. J. Inorg. Chem. 1995, 34, 4600.
20. Meyer, T. J .; Taube, H. In Comprehensive Coordination Chemistry, Wilkinson,
G. Ed.; Pergamon Press: Oxford, 1987; Vol 1, p. 361.

21. Creutz, C. Prog. Inorg. Chem. 1983, 30, 1.

22. (a) Beattie, J. K.; Hush, N. S.; Taylor, P. R. Inorg. Chem. 1976, 15, 992. (b)

 

Creutz, C. Inorg. Chem. 1978, 17, 3723.

23. Lumpkin, R. S.; Meyer, T. J. J. Phys. Chem. 1986, 90, 5307.

24. Zhang, X.; Kozik, M.; Sutin, N.; Winkler, J. R. In Electron Transfer in
Inorganic, Organic, and Biological Systems, Bolton, J. R.; Mataga, N.; McLendon,
G., Eds; Advances in Chemistry No. 228, American Chemical Society:
Washington, DC, 1991; p 247.

25. (a) Tait, C. D.; Garner, J. M.; Collman, J. P.; Sattelberger, A. P.; Woodruff, W.
H. J. Am. Chem. Soc. 1989, 111, 9072. (b) Miskowski, V. M.; Dallinger, R. F.;
Christoph, G. G.; Morris, D. B.; Spies, G. H.; Woodruff, W. H. Inorg. Chem. 1987,
26, 2127.

26. Carlin, R. T.; McCarley, R. E. Inorg. Chem. 1989, 28, 280.

27. Cotton, F. A.; Extine, M. W.; Felthouse, T. R.; Kolthammer, B. W. 8.; Lay, D.
G. J. Am. Chem. Soc. 1981, 103, 4040.

28. Sharp, P.; Schrock, R. R. J. Am. Chem. Soc. 1980, 102, 1430.

 

 

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29. Luck, R. L.; Morris, R. H.; Sawyer, J. F. Inorg. Chem. 1987,26, 2422.

30. Doug, Y.; Hupp, J. T. Inorg. Chem. 1992, 31, 3170. .

31. Worl, L. A.; Duesing, R.; Chen, P.; Ciana, L. D.; Meyer, T. J. J. Chem. Soc.
Dalt. Trans. 1991, 849.

 

 

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