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This is to certify that the

dissertation entitled

MULTI ELECTRON PHOTOCHEMI STRY OF

QUADRUPLY BONDED BINUCLEAR COMPLEXES

presented by

Colleen Marie Partigianoni

has been accepted towards fulfillment

of the requirements for

PhoDo degreein ChemiStry

)000 [ADC/‘—Major professor

Date July 30, 1991

MSU is an Affirmative Action/Equal Opportunity Institution 0-12771 i

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PLACE IN RETURN BOX to remove this checkout from your record.

TO AVOID FINES return on or before date due.

DATE DUE DATE DUE DATE DUE

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Liv—'10:]MSU Is An Affirmative Action/Equal Opportunity Institution

chnS-nt

MULTIEIECTRONPHOTOCHEMISTRYOF QUADRUPLYBONDED

BINUCLEARCOMPLEXES

It

Colleen Marie Partigianoni

ADISSERTATION

submittedto

Michigan State University

in partial fulfillment ofthe requirement

for the degree of

DOCTOROFPHILOSOPHY

DepartmentofChemistry

1991

ABSTRACT

MULTIELECTRONPHOTOCHEMISTRYOF QUADRUPLYBONDED

BINUCLEARCOMPLEXES

by

ColleenMariePartigianoni

Guidelines for the development of multielectron excited state

chemistry of quadruply bonded binuclear complexes (M-LM) are rendered

from the study of a specific class of these complexes comprised of Mo(II)

and W(II) centers ligated by four chloride donors and either monodentate

(PR3) or bidentate (PP) ligands: M2014(PP)2 and M2C14(PR3)4. The mixed

valence character of the metal-to-metal charge transfer excited states of

these complexes, coupled with the structural flexibility of their ligation

sphere gives way to rich photophysics and photochemistry. Transient

absorption studies indicate that these complexes undergo structural

rearrangements upon metal—to—metal charge transfer excitation.

Specifically a bioctahedral distortion which ensures an octahedral

geometry about the oxidized metal center of the excited state is observed.

This rearrangement further provides cooperative stabilization and

coordinative unsaturation of the reduced metal center. These latter

features may be the crucial factors that enable the photochemical

multielectron transformations of the M-LM cores. Particularly intrigueing

is the direct addition of CH31 to the bimetallic core of electronically excited

ColleenMarie Partigianoni

W2014(dppm)2. This photochemistry is unique because photoproducts that

are a signature of overall multielectron transformations proceeding by

sequential one—electron transfer reactions are not observed for this system.

In short, the framework for the design of multielectron

photochemical schemes of quadruply bonded complexes to arise from these

studies directly parallels that established for the ground state reactivity of

square planar ML4 monomers containing d8 08(0), Ir(I) and Pt(II) metal

centers. Namely, low valent coordinatively unsaturated redox active metal

centers best facilitate oxidative addition of substrates. The subtle interplay

of chemical and electronic structure that appears to be required for

formation of photoinduced transients with these ideal features is presented.

To my family

forbeingwithmeeverystep oftheway

ACKNOWIEDGEMENTS

First and foremost I'd like to thank Dr. Daniel G. Nocera (how's that

for respect) for his support, guidance, and for taking the time to foster my

scientific growth during each and every of the step of my graduate career.

Dan's scientific training has taught me to look for the big picture, and his

support, enthusiasm and concern for students as individuals are qualities

to be emulated during my career. Most of all, I'd like thank him for being

one of the few research advisors who could tolerate me for five years.

I'd like to thank the present Nocera group for the memorable

celebrations and their support, especially during those stressful times,

when I needed it the most. Best wishes to all of you; and believe or not,

there is some truth to Dan's claim that "your graduate school years are the

best times of your life." I'd like to leave a special regard for my fellow

psychotic excited state chemist, Janice, but I can't find the appropriate

words to express my feelings (besides they wouldn't pass the censor.) I'm

so fortunate to have her little marks permanently documented in my lab

notebooks as a memoir. All who know her will agree, she's certainly one of

a kind; (could the world handle two??) I haven't forgotten the former

endeared "assholes" of the group, Bob Mussel], Randy King, I-Jy Chang,

Mark Newsham and Joel Dulebohn, who provided inspiration, training

and lots of life to the lab during my early years. The lab just wasn't the

same without them.

I am especially grateful to Claudia Turro for spending innumerable

days collecting that ”just one last" transient absorption spectrum over one

hundred times. I am greatly indebted to Claudia and the rest of the "rescue

squad", SuHane Chen, Jeong—a Yu, and Yeung Shin, for their help

during those last minute crunches.

The MSU College of Natural Science and Dow Chemicals are

recognized for their financial support. I was most impressed with the

outstanding moral support I have felt from the MSU Department of

Chemistry as a whole. In particular, I appreciate the support of Dr.

Dunbar, who not only served as my second reader, but shared numerous

helpful and insightful scientific discussions throughout my graduate

years. Of course, I can't forget the finest glassblowers in the US, Manfred

Langer, Scott Bankroff, and Keki Mistry, who definitely aim to please.

Nobody does it better.

Then there are those who have helped me maintain my sanity

through it all (or tried anyway). I'm grateful for the special and lasting

friendships I've made during the graduate study, especially those with Sue-

Jane, Janice, Brenda, Yeung, the Noceras, Claudia and all of my St.

John's family, especially Fr. Mark, Patrick Patterson, Dan Boyer, Chuck

Graff, Anne Curie, and Cindy Novak, who provided me with a home away

from home and many memories that will last forever.

Finally thanks a million to those back at home, whose contribution

extends beyond the past five years, esmcially Liz Kopp and the Radmores.

Needless to say, the once mentors and soon to be colleagues at good ol‘

Ithaca College Department of Chemistry have made a large contribution to

my career, (greater than I could have hoped for.) Thanks for having me

back on board, '63 a dream come true.

Most of all I want to thank Kelly, Jamie, Kathy, Pam, Mom, Dad and

Gram, for never expecting and wanting anything more from me than my

happiness. With your steadfast love support and love.............. WE DID IT!

TABLE OFCONTENTS

LIST OF TABLES ...................................................................

LIST OF FIGURES .................................................................

CHAPTER I INTRODUCTION ...........................................

CHAPTER II EXPERIMENTAL ...........................................

A. Solvent Purification ...................................

1. Solvents used for Synthesis ....................

2. Solvents used for Spectroscopy and

Photochemistry ....................................

B. Synthesis ..................................................

1. General Procedures ..............................

2. Synthesis of M2014(PR3)4 Complexes ...........

a. Precursors ......................................

i. W014 ............................................

ii. Mo(na-PhPMePhXPMePh2)3 ...........

iii. WCl4(PPh3)2 .................................

b. Dimolybedum Complexes ..................

i. D2d Isomer ....................................

ii. Green Isomer of M02014(PMePh2)4

c. Ditungsten Complexes .......................

d. MoWCl4(PMePh2)4 ............................

3. Synthesis of M2014(PP)2 (Dzh) Complexes

a. M02Cl4(dppm)2 and MegCl4(dmpm)2 34

b. W2Cl4(dppm)2 .................................. 34

c. MoWCl4(dppm)2 .............................. 35

4. Synthesis ofMZCI4(PR3)4X2 and

MzCl4(PP)2X2 Complexes ....................... 35

a. W2015(PR3)4 (PR3 = PEt3, PBu3) .......... 35

b. W2014(dppm)212 ............................... 35

c. W2015(dppm)2 .................................. 37

. Photochemistry ......................................... 37

1. General Procedures .............................. 37

2. Isolation and Photoproducts ................... 38

a. Photolysis of W2014(dppm)2 with CH3I 38

b. Photolysis ofW2014(PBu3)4 with PhSSPh 40

c . Photolysis ofW2Cl4(PBu3)4 with CH2012 40

. Electrochemistry ....................................... 40

1. General Procedures .............................. 4O

2. Preparation and Purification of Electrolytes 41

3. Bulk Electrolysis ofW204(PBu3)4 ........... 41

. Spectroscopic Instrumentation and Methods 42

1. Electronic Absorption Spectroscopy ............. 42

2. Steady-State Luminescence Experiments 43

3. Transient Absorption Spectroscopy .......... 44

4. Electron Paramagnetic Resonance .......... 44

5. Nuclear Magnetic Resonance ................. 44

6. Mass Spectrometry ................................ 44

CHAPTER III TRANSIENT ABSORPTION SPECTROSCOPY ..... 53

A. Background ............................................... 53

B. Results and Discussion ............................... 63

1. M2014(PP)2 (D211) Complexes ................... 63

a. Photochemically Inert Solutions ......... 63

b. Photochemically Active Solutions ........ 82

2. M2014(PR3)4 (ng) Complexes .................. 92

3. MozCl4(PMePh2)4 (D21, / Dzd) Isomers ...... 122

CHAPTER N PHOTOINDUCED REDOX CHEMISTRY ........... 127

A. Background ............................................... 128

B. Results ..................................................... 13)

1. Photo-oxidation Chemistry Accompanied

by Phosphine Displacement .................... 130

a. Photoreaction ofWzCl4(PBu3)4

with CH2012 ..................................... 130

b. Photoreaction of M02014(PBu3)4

with PhSSPh .................................... 142

2. Photo-oxidation Accompanied

by Disproportionation ............................. 149

3. On'dative Addition Reaction .................... 170

C. Discussion ................................................ 185

D. Conclusion ................................................ 188

CHAPTER V. FINAL REMARKS ........................................... 192

REFERENCES ........................................................................ 201

LIST OFTABLES

Table 1. Properties of the Luminescent 1(66*) State of M2X4(PR3)4

Table II. Comparison of Structural Properties of M2X4(PR3)4

Complexes and Lifetime of Nonluminescent Transient

LIST OF FIGURES

Latimer diagram for a transition metal complexes M, depicting

the relationship among the 0—0 transition energy (EM) and the

ground state (E0) and excited state (E') redox potentials described

in equations 1.1 and 1.2. ....................................................

Schematic diagram of a water splitting cycle utilizing

electronically excited Ru(bpy)32+ and relay molecules

methylviologen (MV)2"' and EDTA to transfer one—electron

equivalents to the Pt and Rqu catalysts. .............................

Reaction cycle for the conversion of isopropanol to acetone and

hydrogen with electronically excited Pt2(POP)4“ as a photo-

catalyst (Reference 62c). ....................................................

Relative energies of the d-derived molecular orbitals in a D4,,

binuclear M2L3 complex constructed from linear combination of

two ML4 fragments. ...........................................................

Relative energies of the lowest electronic states of M-LM

complexes as a function of torsional angle (t) or dxy orbital

overlap. The pictorial representation of the valence bond

description of these states as well as the corresponding

molecular orbital formalism is shown (Reference 137). ..........

xi

10.

Newman projection of M02X4(PP)2 complexes depicting the

torsional angle ¢. ...............................................................

(a) Electronic absorption spectrum of a finely ground state

sample of the edge—sharing bioctahedral complex W2016(PEt3)4.

(b) Electronic absorption spectral changes associated with the

conversion of W2016(PEt3)3 to the confacial bioctahedral complex

W2016(PEt3)3 in toluene solution at ambient temperature, In,”

nm (e M-1 cm-l) (a) W2016(PEt3)4: Am = 470 nm, (e = 1992 M-1

cm'l); 380 nm (1280 M-1 cm’l) (b) W2016(PEt3)3: Am = 510 nm,

(e = 2077 M-1 curl); 328 run (2078 M-1 curl). ........................

Electronic absorption spectrum of dichloromethane solutions of

the confacial bioctahedral complex W2016(PBu3)3, km“ = 500

nm, (e = 1970 M-1 cm-l); 315 run (2268 M-1 cm-l). .................

Positive fast atom bombardment mass spectrum of a product

from the thermal reaction of W2014(dppm)2 with 12. The

clusters centered at 1531, 1497 and 1405 amu are consistent with

(a) WzCl4(dppm)2lzt, (b) W2 C l 3( d p p m ) I 2 + and (c)

WZCl4(dppm)zl+, respectively. ............................................

Electronic absorption spectrum W2(II,III)C14(PBu3)4+ prepared

by bulk electrolysis in CH2012 in the (a) visible region and (b)

xii

11.

12.

13.

14.

15.

16.

NIR region. km, = 388 nm, (e = 2559 M-1 curl); 485 nm (949 M-1

cm’l); 1510 nm (2243 m-1 cm‘l). .........................................

(a) D26 structure of MZCl4(PR3)4 complexes; (1)) D21. structure of

MzCl4(PP)2 complexes where PP = bidentate phosphines. ......

Relative energies of the d—derived molecular orbitals in

M2014(PR3)4 complexes as a function of torsional angle (1)

(Reference 161). .................................................................

Electronic absorption spectra of dichloromethane solutions of

M2C14(dppm)2, where M = M0 ( ) and W (------) (a)

MozCl4(dppm)2: In,“ = 634 nm, (e = 2490 M‘1 cm‘l); 462 nm (900

M-1 cm-l); 364 run (2040 M-1 cm-l); 325 am (5600 M-1 cm-l) (b)

W1;Cl4(dppm)2:l.mIn = 710 nm, (e = 2585 M-1 cm-l); 500 nm (598

M’1 cm'l); 405 run (1923 M-1 curl); 325 run (2948 M‘1 cm'l). .....

Transient difference spectrum of M02014(dppm)2 in CH2012

recorded after 355 nm laser excitation. ...............................

Transient kinetics for W2014(dppm)2 in CsHs (1.95 x 10" M)

recorded at 440 nm following laser excitation at 532 nm. .......

Transient difference spectra of WZCI4(dppm)2 in CGHB recorded

recorded 100 ns and 4 us after 532 nm laser excitation (see

legend). ............................................................................

P889

17.

18.

19.

21.

Electronic absorption spectrum of dichloromethane solutions of

W2016(dppm)2, km“ = 822 nm, (e = 740 M‘1 cm‘l); 468 run (4800

m-1 curl); 387 an (3200 M-1 cm'l) (Reference 166). ..............

Electronic absorption spectrum of dichloromethane solutions of

M02016(dppm)2. .................................................................

Proposed edge—sharing bioctahedral distortion of the 1(n5“‘) (or

1(81t"')) excited state of the M2014(PP)2 complexes. Although the

former is designated in the diagram, the high energy metal

localized transitions of these complexes have not been

definitively assigned (see text). Transients are not observed from

the 1(86") excited state on the nanosecond time scale. The metal

centers which are oxidized and reduced in the transient species,

relative to the ground state, are denoted with + and —,

respectively. (See footnote 180 regarding M-«M notation). .......

Transient kinetics recorded at 440 am following 532 nm laser

excitation of (a) W2Cl4(dppm)2 (1.5 x 10" M) in 01131 (5.0

M)/C,H6. (b) W2014(dppm)2 (2.75 x 10“ M) in 011301121 (5.65

M)/CGH¢ (c) W2C14(dppm)g (3.0 x 10" M) in 011,012. ...............

Transient difl'erence spectrum of W2014(dppm)2 (1.5 x 10" M) in

CH31(5.0 MYCQHG in Cefie recorded 100 ns and 4 us after 532 nm

laser excitation (see legend). ...............................................

xiv

P88

. Transient difference spectrum of dichloromethane solutions of

W2014(dppm)2 (3.0 x 10‘11 M) collected 100 us after 532 nm laser

excitation. ........................................................................

. Transient difference spectrum upon 532 nm laser excitation of

W2C14(dppm)2 (2.75 x 10" M) in CH30H21 (5.65 M)/C,;H3 recorded

(a) 100 us after excitation (b) 10 us afier excitation. ................

. Emission of W2014(PMePh2)4 in toluene solution at ambient

temperature upon 691.5 nm excitation. ...............................

. Transient difference spectrum of W201‘(PHPh2)4 in CH2012 (~1.0

x 10'2 M) recorded 2 ps after 590 nm laser excitation. ...........

. Transient difi‘erence spectrum of W2014(PMePh2)4 in THF (~ 6.0

mmol) recorded 70 us after 683 nm laser excitation. ............

. Transient difference spectrum of WZCIJPBua)‘ in hexane (~ 6.0

mmol) recorded 70 us after 580 nm laser excitation. ............

. Transient difi'erence spectrum of W201‘(PEt3)4 in toluene (~ 6.0

mmol) recorded 70 us after 683 nm laser excitation. .............

the confacial bioctahedral complex WzCls(PEt3)3, In“ = 510 nm,

(e = 2077 M“ coo-1); 328 nm (2077 M-1 an“). ........................

XV

P820

101

KB

. Electronic absorption spectrum of a dichloromethane solution of _

109

30.

31.

Electronic absorption spectrum of the product obtained from

thermal reaction of one half molar equivalent of dichloroiodo—

benzene to angel, solutions 0fw2Cl4(P3113)4, An,“ = 500 nm, (e =

969 M-1 arr-1); 400 nm (588 M-1 coo-1); 330 on (3322 M-1 cm'l).

Proposed confacial bioctahedral distortion of the 1(88") excited

state for WZCl4(PR3)4 complexes. Transients are not observed

from the LMCT state on the nanosecond time scale. The metal

centers which are oxidized and reduced in the transient species,

relative to the ground state, are denoted with + and —,

respectively. (See footnote 180 regarding the M-«M notation).

Electronic absorption spectral changes accompanying addition

of dichloroiodobenzene to CH2C12 solutions of W2014(PBu3)4,

1...... =665 nm, (e = 3813 M-1 crn-l) in the (a) visible region and (b)

NIR region. The solid lines represent net addition of 0.5

equivalent and the dashed lines represent further addition.

Plot of phosphine v(CO) stetch vs cone angle of phosphines (a)

PBus, (b) PEta, (c) PMes, (d) PMCzPh, (e) PMOPhg, (0 PHth.

Photoinduced chemical intermediates are not observed from

M02014(PR3)4 where PR9 = PMe3 and PHth , which are marked

with *. ............................................................................

xvi

111

114

116

37.

Electronic absorption spectrum of THF solutions of D2 d

M02Cl4(PMePh2)4 (—) and green isomer of M02014(PMePh2)4 (--

--) that is proposed to have D2,, configuation of phosphines.

Transient difference spectrum of the green isomer of

M02014(PMePh2)4 in THF collected 1 us after 355 nm laser

excitation. .......................................................................

Electronic absorption spectral changes during photolysis (km >

375 nm) of deoxygenated dichloromethane solutions of

W2014(PBu3)‘ at 22°C in the (a) visible region and (b) NIR region.

The absorbance range in the NIR region at I > 800 nm is

expanded by a factor of two. The total time of photolysis was 20

minutes. ..........................................................................

X—band (9.598 GHz) EPR spectrum of a 2—MeTHF/CH2012 glass

ofphotolyzed CH2012 solutions ofW2C14(PBu3)4 at 77°K. ..........

Cyclic voltammograms of 0112012 solutions with 0.1M TBAPFG

of(a) Wzm4(PBu3)4 (b) w.,(:14(1>13113)4 and 3.2 x 10-2 M THACl (c)

photolyzed solutions of WZCI4(PBu3)4. ..................................

(a)1H and (b) 31P NMR smctra of products from photoreactions of

W204(PBu3)4 with 011,012. The spectra were recorded in CD202

solutions at -80 °C. ...........................................................

xvii

40.

41.

Electronic absorption spectral changes during thermal

reactions of benzene solutions of M02014(PBu3)4 containing a ten

fold excess of PhSSPh. The total reaction time at ambient

temperature was four hours. .............................................

(a) Electronic absorption spectral changes during photolysis

(1.,“ > 570 nm) of benzene solutions of M02014(PBu3)4 containing

a tenfold excess of PhSSPh. The total reaction time at ambient

temperature was 20 minutes. (b) Electronic absorption

spectrum of a photoproduct isolated from photolyzed solutions by

column chromography. The FABMS of this product is provided

in Figure 42. ....................................................................

Fast atom bombardment mass spectrum of a photoproduct

isolated from photolyzed solutions of M02014(PBu3)4 containing

PhSSPh. Selected assignments of the clusters in the spectrum

are: (a) M02014(PBu3)2(SPh)4t (b) M0201‘(PBu3)2(SPh)3+ (c)

M02C14(PBu3)(SPh)4+ (d) M02014(PBu3)2(SPh)4t. ...................

Electronic absorption spectral changes during photolysis (2.,“c >

570 nm) of dichloromethane solutions of M02014(PMe2Ph)‘

containing a ten fold excess of PhSSPh at 22°C. The total

reaction time at ambient temperature was 24 hours. ............

xviii

Pace

144

146

148

47.

49.

Fast atom bombardment mass spectrum of a photoproduct

isolated from photoreaction of MmCMPMezPh)‘ with PhSSPh.

The cluster is consistent with Mo,Clg(PMe2Ph)‘(SPh). ..........

Electronic absorption spectral changes during photolysis (1.“ >

435 nm) of acetone solution of Mogcl4(dppm)2 containing a forty

fold excess of TolSSTol at 22°C. ..........................................

Fast atom bombardment mass spectrum of photoproducts

isolated from photoreactions of Mogcl4(dppm)2 with TolSSTol.

Selected assignments of the clusters in the spectrum are (a)

[Mo]Cl4(STol)2t; (b) [MolCla(STol)2+; (e) [MolCls(STol)t; (d)

[Mo]Cl,(s'ro1)+; (e) [Mo]le where [Mo] = M02(dppm)2. .......

Electronic absorption spectral changes during photolysis (1.“ >

435 nm) of ethyl iodide solutions of W2014(dppm)2 at 0°C. The

total reaction time was 1.5 hours. .......................................

Fast atom bombardment mass spectrum of products isolated

fiom photolyzed ethyl iodide solutions of W2014(dppm)2. Selected

assignments of the clusters in the spectrum are (a) [WJCl4lzt;

(b) [W]Clsla"’; (c) [W]Cl;It; (d) may; where [W] =

W2(dppm)2. .......................................................................

Molecular ion cluster region of the fast atom bombardment mass

spectra of photolyzed (Ln. > 435 nm) solutions of

xix

161

163

51.

52.

M02014(dppm)2 in the presence of phenyl/tolyl disulfide

mixtures. Assignments of the clusters in the spectrum are (a)

MogCl4(dppm)2(SPh)2 (b) M02014(dppm)g(SPh)(STol) and (c)

Mogcl4(dppm)2(STol)g. ........................................................

Cyclic voltammograms of 1:1 CH3Utoluene solutions containing

0.1 M THAPF, of(a) wzolgdpprn)2 (h) waolgdmnn)2 (1.0 x 104)

and THAI (5.0 x 10-3 M) (c) WgCl4(dppm)2(I)2. .....................

Electronic absorption spectral changes during photolysis (km. >

435 nm) of methyl iodide solutions of WzCl4(dppm)2 at 0°C. The

wavelength scale in the near infrared region (3. = 900—1000 nm)

is twice that of the visible region (I < 900 nm). ......................

Fast atom bombardment mass spectra of (A) photolyzed (21,xc >

436 nm) solutions of W2014(dppm)2 and methyl iodide and of (B)

solutions of WZCI4(dppm)2 and methyl iodide refluxed in the

absence of light. Selected assignments of the clusters in the

spectrum are: (a) [W]Cl4CH3It; (b) [WIClsCH31+; (c)

(17101201135; ((1) [WlCltCH3+; (e) [W101412t; (t) [W1C1312+; (g)

[W]C151t; and (h) [WIC14I+ where [W] = W2(dppm)3. ...........

The relative isotopic distribution of the molecular ion cluster for

chl‘(dppm)2(CH3XI). The simulated relative abundances,

designated with solid lines, are superimposed on the observed

peaks. .............................................................................

XX

165

1%

172

174

57.

13C NMR of photoproduct from photolyzed solutions .of 1/1

“CI-1311126131 solutions ofW2C14(dppm)2, in 0112012 at -60°C.

Electronic absorption spectra of 01131 solutions of W2014(dppm)2

before (—) and alter (----) photolysis at A > 335 nm at 0°C. ......

Electronic absorption spectral changes observed upon refluxing

CHsl solutions of W2014(dppm)2. .........................................

Electronic absorption spectral changes during photolysis (1,“ >

405 nm) of CHal solutions of MoWCl4(dppm)2 at 0°C. No further

change in the spectra were noted alter an additional 0.5 hour of

irradiation. .....................................................................

Transient absorption kinetics recorded at 390 nm following 532

nm laser excitation of hexane solutions of displaying (a) the 120

ns transient (b) the initial rise and relative intensity of an

additional transient absorption (c) the rise and decay of the

additional long lived transient. ..........................................

Electronic absorption ofM02014(PBu3)‘ (—). M02C1¢(dppm)2 (~-

-) and irradiated solutions ofMo,Ch(PBu3)4 containing a 100 fold

excess of CH3N(PF2)2 (----- ). ................................................

xxi

179

CHAPTER I

INTRODUCTION

Both biological and chemical energy conversion processes typically

involve multiple oxidation-reduction transformations. Beyond an emcient

means of electron transport, the success of carrying out these

transformations is predicated on the ability to overcome the large kinetic

and/or thermodynamic barriers which confront these reactions. To this

end, electronically excited transition metal complexes are useful catalysts

in these transformations [1-3] because the increased driving force

garnered from an electronically excited state provides the impetus for

surmounting the large barriers confronting the corresponding ground

state species. Momover, electronically excited states are particularly useful

in redox reactions because the excited state is both a stronger oxidant and

redth than the ground state species.

The enhanced oxidation potential results from the promotion of an

electron to a higher energy orbital upon excitation. Likewise, owing to the

resultant hole produced in the orbital from which the electron was

promoted, the excited state is easier to reduce as well. Quantitative

2

comparision of the relative reduction potentials of the excited state and

ground state species are given by equations 1 and 2, where Eo_o(M—M*) is

defined as the spectroscopic energy of the 0—0 transition. The Latimer

diagram shown in Figure 1 illustrates the simple thermodynamic

relationship described by equations 1 and 2.

E*,ed(M*/M') = E°nd(M/M‘) + Eo-o(M/M*) (1.1)

13*” (M+/M*) = E°o,(M*/M) — Eo_o(M/M") (1.2)

Although numerous investigations during the past two decades have

proven single electron transfer to be an ubiquitous pathway of electronically

excited transition metal complexes [4—7], the capacity for multielectron

transfer which is ultimately required for energy conversion schemes is far

less common. Ingenious schemes have been designed to effect an overall

multielectron transformations by coupling successive excited state one-

electron transfers via relay molecules and catalysts [8-13], This is

exemplified by the classic water-splitting cycle of

tris(bipyridyl)ruthenium(II). Ru(bpy)32+, shown in Figure 2 [ll-13]. In this

scheme, electronically excited Ru(bpy)32"' ion transfers an electron to W“

which relays the electron to the platinum catalyst. The platinum catalyst

then couples the one-electron chemistry of MV"' to the two-electron

hydrogen production chemistry by effectively storing the reducing

equivalents of the viologen. Oxygen production is achieved from the

reducing equivalent of the photogenerated oxidant Ru(bpy)33+, which is

reduced back to mutiny);2+ by EDTA. The oxidized EDT/1+ reacts with H20

in the presence ofRu02 catalyst to produce oxygen.

Figure 1. Latimer diagram for a transition metal complexes M,

depicting the relationship among the 0—0 transition energy (EM) and the

ground state (E0) and excited state (E') redox potentials described in

equations 1.1 and 1.2.

Figune2. Schematic diagram of a water splitting cycle utilizing

electronically excited Ru(bpy)32"' and relay molecules methylviologen

(MV)2+ and EDTA to transfer one—electron equivalents to the Pt and Ruo2

catalysts.

hv

450nm

/\

Ru

2’

R2"

H20

EDTA

(130103

“(Willa

MV'

"2H2

114()2

EDTA.

Ru(bpy)33’

MV2+

H+

“@192

7

An alternative approach to multielectron chemistry predicated on

one—electron transfer is to mimic biological assemblies such as the

photosynthetic reaction center. The overall mechanism of the

photosynthetic assembly is not unlike that described for the Ru(bpy)32+

system in that electrons and holes resulting from the single electron

transfer reactions of the light harvester (i.e. porphyrin) are sequentially

transferred to and stored in catalytic centers which are capable of effecting

the overall multielectron transformation upon accumulation of sufficient

charge equivalents [14—22]. The primary difference in biological systems is

that the transport of electrons from the light harvester to the catalytic

center occurs through covalently bonded networks. Intramolecular

electron transport offers the potential for directional electron transport

which is difiicult to achieve in the intermolecular reactions of the relay

molecules used in the Ru(bpy)32+ scheme.

The required intricacy of designing covalently bonded networks

capable of efficient charge transport and storage is formidable. Research is

focused on successfully mimicking components of the overall system

including single electron transfer reactions of porphyrins [23-24], proteins

[25—27] and more generally covalently bonded organic networks [28—32].

Even the primary task of emcient separation of a single electron and hole is

challenging the imagination of several research groups. The inherent

difficulty in the seemingly simple charge separation process is that

recombination reaction of the hole and electron is thermodynamically

favored with respect to their continued evolution along the charge

separating network. In general, a highly exergonic electron transfer

between a donor (D) and acceptor (A) which is driven by the excess energy of

an excited state will yield D+ and A" which are unstable with respect to

8

back electron transfer owing to the predictions of Marcus inverted region

electron transfer [33]. The recombination of the hole and electron is less

favorable as they become spatially separated further due to the inverse

exponential distance dependence of the rate of electron transfer [33—34].

Thus a successful approach to efficient charge separation, which mimics

the photoreaction center [35—38], is the design of covalently bonded networks

which can transport electrons and holes away from the primary reaction

center to independent and far removed sites. ‘

This approach is exemplified by the cofacial diporphyrins [39], and

porphyrin based diads [40—43] and triads [44—45]. The latter consists of a

light harvesting porphyrin (P) and a covalently juxtaposed acceptor (A) and

donor (D). In this case charge separation is achieved by the primary

electron transfer from the electronically excited porphyrins to the acceptor

followed by subsequent electron transfer from the donor to the porphyrin, as

follows.

kfl kfzDP“A"

DP’A D+PA’ (1.3)

The photogenerated charge separated state D+PA’ may persist into the

microsecond range; the quantum yield however is usually relatively low

[46]. Efficient production of charge separated states requires forward

electron transfer rates (kfl, kfz) which are faster than the back electron

transfer rates. Clever ways to control these relative rates through efi'ects of

energy, distance and molecular structure are currently being explored [47-

48].

9

Yet the successful design of charge separating networks represents

only the first step towards the ultimate goal of multielectron

photochemistry. The initial electron]hole pair must then be stored at the

terminus of the network, and the overall process must be repeawd to build

up the necessary multielectron hole and electron equivalents. Finally, the

charge equivalents must then be coupled to a catalytic center capable of

promoting the overall multielectron process. Indeed, the synthesis of such

centers comprises a major research area in its own right with the design of

biomimetic models for nitrogenase [49], the oxygen-evolving complex [50—

51], copper-based proteins [52], cytochrome P—450 [53—55], hemerythrin [56—

57], hydrogenase enzymes [58], and a host of other metal based proteins and

enzymes [59-60].

An alternative approach to multielectron reactivity that does not rely

on independent catalytic centers employs electronically excited polynuclear

metal compounds (where polynuclear defines two or more metals in a

discrete complex). In these systems, transformations can be effected

directly at the electronically excited metal centers. Moreover, multielectron

capacity is offered by the combination of the one—electron chemistry of the

individual metal centers in the polynuclear core. Thus the polynuclear

core represents a self contained multielectron reaction center and therefore

the need to transfer, store and accumulate charge in independent catalytic

centers is obviated. Success with this approach is exemplified by the

plethora of exciwd state reactions of binuclear ds—d8 binuclear complexes

such as Pt2(P205H2)4" (P205H2 a POP) [61—63], Ir2(2,5-diisocyano-2,5-

dimethyl-hexaneh2+ [64] and [Ir(u-pyrazanole)-1,5-cyclooctadiene]2 [65],

which all ultimately effect the two-electron reduction of substrates. For

example, Pt2(POP)4“ photocatalytically converts isopropyl alcohol into

1 0

acetone and hydrogen by a pathway shown in Figure 3 [62s]. The primary

step in the photoreaction involves hydrogen atom abstraction by

electronically excited Pt2(POP)4" to give a Pt(II)Pt(II) mixed valence

intermediate Pt2(POP)4H" and the corresponding reactive (CH3)ZCOH

radical. Subsequent reaction of this reactive radical with another

equivalent of Pt2(POP)4" results in production of acetone as well as

Pt2(POP)4H" which can disproportionate to give Pt2(POP)4" and

Pt2(POP)4H24‘. Reductive elimination of H2 from the latter complex

completes the cycle.

The primary process of this reaction, namely hydrogen atom

abstraction, is exemplary of numerous other photoreactions of Pt2(POP)4"

with a wide variety of organic substrates R3EH for E = Sn, Ge, Si, C [62 b,c,

63]. Halide atom abstraction is an additional common reaction of the

electronically excited Pt2(POP)4"'. Recent transient absorption studies have

shown that this is the primary photoprocess in the reactions with

xcnzcnzx (X = Br, Cl) and aryl halides [62 b,d].

Each of the reactions of the electronically excited Pt2(POP)“‘

complexes described above involves an initial one-electron transformation

yielding a reactive organic or organometallic radical which can

subsequently be trapped by either the mixed valence intermediate or in

some cases the Pt2(POP)4"' complex to yield the two-electron reduced

substrate. This appears to be completely general for the photochemistry of

the binuclear da—d8 complexes [66]. Thus, even with the multiple redox

capacitance of the polynuclear core, multielectron transformations are still

confined to coupling one-electron reactions.

Our research is focused on a novel basic reaction type for electronic

excited states that difi‘ers from coupled one—electron schemes, namely a

11

Figure 3. Reaction cycle for the conversion of isopropanol to acetone and

hydrogen with electronically excited Pt2(POP)4" as a photo—catalyst

(Reference 62c).

12

Pr2(POP)44- 9”

' CH3C3CH3

i’ .CH3CCH3 PtthOP)4H

on

CH CCHPt2(POP)4H4‘

3) 3

P12(POP)44"'

Pt2(POP)4l-i24'

Pt2(P0p)44- hv

H2

13

discrete multielectron event. Concerted multielectron chemistry from an

excited state molecule is appealing from a practical standpoint because the

approach avoids competitive side reactions of one-electron intermediates,

including energy—wasting, back electron transfer. More fundamentally,

the ability to initiate a discrete multielectron process with a pulse of laser

light can contribute significantly to the understanding of the mechanisms

of multielectron processes, which are the least understood of any in

chemistry or biology. In particular, discrimination between a concerted

multielectron process and one that proceeds by sequential very rapid one-

electron transfers within a solvent cage is often problematic. This issue

can be addressed by opening the temporal window of multielectron

reactions from their heretofore conventional arena of study in the

millisecond range of stopped-flow kinetics to the picosecond and even

femtosecond range of laser kinetics. Thus the development of excited state

multielectron processes ofl‘ers the opportunity to explore and discover new

fundamental and practical aspects of oxidation—reduction chemistry.

Our initial approach to the development of discrete multielectron

transformations of electronically excited transition metal complexes uses a

photon to promote charge separation in a binuclear core to produce excited

states of the type M‘”—M"°". These excited states retain the two

important features contained in the binuclear ds—d8 complexes, multiple

redox equivalents of the polynuclear core and metal centers that can serve

as catalytic sites, but also includes charge transfer within the polynuclear

core. The latter feature may provide the final impetus required for the

novel discrete multielectron photoreactivity because the two electron mixed

valence character derived from charge separation should promote

multielectron redox reactivity; the M“ and M"+1 metal centers have the

1 4

capacity to serve as two electron donors and two electron acceptors,

respectively.

Two approaches to production of M‘m—M"l electronically excited

species are currently being pursued in our laboratory. The most

conceptually obvious one is the simple excitation of ground state species of

M““—M“'l character:

11 eM‘tl-M“ —"——— MN—Mx-1 (1.4)

To this end, the recently prepared Rh2(0,II) fluorophosphine

complexes are promising candidates [67]. Photophysical studies show that

they have long lived excited states which retain the mixed Rh2(0,II)

character and photochemical studies are currently being initiated. The

Rh2(0,II) complexes however represent a rare example of a

thermodynamically stable two-electron mixed valence species. A more

general approach to production of (MM—M“). excited states via MMCT

charge transfer in M—Mcomplexes:

M-M hv > w+1-w-1‘MMCT (1.5)

This latter approach has an advantage in that it allows for production of

less stable and more reactive M‘*l—M“’l transient species.

The preparation of localized charge-separation (M‘*1—M"1)‘ excited

states by optical excitation requires the transfer of electrons between weakly

coupled orbitals localized on the independent metal centers. Numerous

fundamental studies of MMCT excited states [68-72] have contributed

15

greatly to an understanding of the factors that govern the rate of electron

transfer such as the interconnecting bonding network [73—83], distance

[84—90], solvent [91-98], temperature [99-103] and free energy driving force

[104—110]. However few studies have centered on the photochemistry of

such excited states. The activation of 02 by the mixed valence anions,

[(CN)5M"(p—CN)Com(CN),J‘-, represents a relatively rare example [111].

Excitation of the metal-to-metal charge transfer (MMCT) induces an

intramolecular photoredox event to produce the corresponding

[(NC)5Mm(u-CN)Con(CN)5]6' (M = Ru, Os, Fe) mired valence species,

which undergoes subsequent dissociation to Mm(CN)33" and Con(CN)53'.

The coordinatively unsaturated intermediate is efficiently trapped by

oxygen to generate the peroxo dimer [(CN);Com(022') -Com(CN)5]6', which

submquently decomposes in acidic media to H202 and Com(CN)5(H20)2".

Whereas this is multielectron photochemistry, it reveals a

fundamental problem confi-onting MMCT photochemistry. Owing to the

weak coupling of the metal centers, population of the MMCT state results in

dissociation to monomeric species. Photodissociation of electronically

excited (M'M—M‘dr species can be circumvented by incorporating

bridging bidentate ligands. Alternatively multiply bonded M-LM binuclear

complexes have sufficiently strong metal—metal interactions to prevent

dissociation. Moreover, in the case of the quadruply bonded metal-metal

binuclear complexes (MA-M), the lowest energy transitions of are between

weakly coupled orbitals and hence the lowest energy excited states of these

species exhibit significant MMCT character. To this end, we became

interesmd in exploring the potential multielectron photochemistry ofMJ-M

binuclear complexes.

l 6

Since the initial discovery of 11620132' in 1964 [112], numerous other M

LM complexes comprised primarily of d‘ rhenium(III), chromium(II),

molybdenum(II) and tungsten(II) metal cores with an array of ligands

have been discovered, owing primarily to the efforts of Cotton and

coworkers [113]. The formulation of a quadruple bond was proposed to

account for the unusually short metal-metal bond distance, the

diamagnetic behavior and the eclipsed conformation of the two ML4 units

observed in these complexes. This proposal stimulated numerous

theoretical [114—120] and experimental [121-135] investigations and a self-

consistent description of their electronic structure has emerged in recent

years. The general molecular orbital diagram for M-LM species depicted in

Figure 4 has evolved from these studies. It is constructed by taking the

linear combination of orbitals for two ML4 fragments. Each ML4 fragment

contains one highly destabilized M—L 0‘ molecular orbital, resulting from

interaction of the d,2.y2 orbital with the four ligands. Linear combination of

the M—L 6* orbital in the two M114 fragments gives rise to blg and b2u

molecular orbitals of 8 and 5* symmetry, respectively, which are nearly

degenerate. The M—M bonding interactions in the M-L-M binuclear

complexes arises from the linear combination of d.2, (du, dyg), and d,,

orbitals to form bonding and antibonding o, x, and 8 molecular orbitals,

respectively.

Numerous electronic absorption spectroscopic studies of various

M-‘-M complexes verify that the lowest energy absorption corresponds to the

spin and dipoled allowed 52a1(58’)(1A1‘-)1A2u) transition which is

predimd from this general molecular orbital scheme [136]. The energies of

these transitions are typically in the range of ~15,500 to 25,000 cm“.

Interestingly the 8—95’ transition typically red shifts by 10,000 cm"1 for the

l7

Figure4. Relative energies of the d—derived molecular orbitals in a D4},

binuclear M2L8 complex constructed from Linear combination of two ML4

fragments.

l8

0* am

e

\

M—LO’"

\

\

\be.

I

I

I

“no.

I.008:

M—LO’"

(dB-.1)(Gui-,2)

I'll-.4

l9

one—electron oxidized or reduced species [115]. The significant red shift

suggests that the 8/8‘ orbital splitting in Figure 4 is small owing to poor

overlap of the d” orbitals. This contention is further supporwd by estimates

of 10 kcal for 8 bond strength [115]. Owing to this relatively weak interaction

molecular orbital theory does not provide an accurate physical

representation of the states derived from population of the 8/8‘ orbital

manifold, and two electron exchange energies must be a significant

contributor to the 824'(85‘) transition ofM-LM species. Accordingly, states

derived from the population and depopulation of the 8 orbitals are more

accurately described within a valence bond framework [114—116].

The relative energies and pictorial representation of the valence bond

description of the four states derived from the purely atomic noninteracting

dry orbitals are shown in the right hand site of Figure 5 [137]. The lowest

energy degenerate states (1A13,3A2u) have one electron on each metal

center. These two states correspond to the 3(88’) and ‘(82) states in

molecular orbital formalism. At significantly higher energy are two

degenerate 1A“, 1A2“ states representing the positive and negative linear

combinations of electronic configurations where both electrons are on the

same metal center. These two states are far removed from the lA13,3A2u

states owing to the two-electron exchange energy associated with pairing of

two electrons in the (IU orbital [115]. The energies of these states are

perturbed as the overlap (S) of the d,y increases fiom 0 to the typical value of

0.1 for the M-LM species in the manner also shown in Figure 5. The ‘A13

state corresponding to 1(82) is stabilized and the A13 state corresponding to

1(8“) is destabilized. The energies of the 3A2“ (3(88‘))and 1A2“ (1(88'» states

are roughly invariant over this range.

20

Figure 5. Relative energies of the lowest electronic states of M-i-M

complexes as a function of torsional angle ((0) or dxy orbital overlap. The

pictorial representation of the valence bond description of these states as

well as the corresponding molecular orbital formalism is shown (Reference

137).

(l_UJO) Afiieua

dxyOrbitalOverlap

L4M-ML4TorsionAngle———>

VB

MO

W5

21

22

The valence bond model is distinguished from simple molecular

prbital theory by the following predictions: a small energy gap between the

3A2“ and the ‘AIS ground state, a large energy gap between the 3A2u and the

1A“ state, and the lowest energy spin-dipoled electronic 1A lg—ilAzu

transition (correlating with the 8241(88") transition) involving conversion

from a ground state with a single electron on each metal center to an

excited state with both electrons on a single metal center is MMCT in

character. Each of these predictions of the valence bond model have been

experimentally verified in recent years. Evidence for the relative energy

ordering of states is provided by magnetic susceptibility measurements of

the torsionally strained M02Cl4(dmpe)2 complex [134]. Owing to the steric

contraints of the bridging bidentate dmpe, the two ML4 fragments in this

complex are nearly staggered (o= 40°) with respect to each other as depicted

in Figure 6, and thus the interaction of the d,, orbitals is very near the limit

of zero for the completely staggered configuration (¢= 45"). Indeed, as

predicted from Figure 5, magnetic measurements places the energy of the

thermally populated 3A2“ state only 400-500 cm“1 above the ground state,

whereas the lAlg—i1A2u transition is observed to be at 12,500 cm'l. From

these data and the observed 4700 cm'1 blue shift in the lAlg—ilAzu transition

of M02014(PMe3)4 complex, the 3142u state is predicted to be 5,200 cm'1 above

ground state. The increased energy for M02014(PMe3)4 is predicted owing to

maxium overlap of the dxy orbitals in an eclipsed conformation. Perhaps

the most compelling evidence supporting a valence bond model comes from

the studies of Winkler, Sutin, and their co-workers. Solvent dependence

1(88") luminescence reveal that the dipole moment of the 1A2“ (1(88'»

luminescent electronically excited state of M02014(PMe3)4 is 4.0 Debye (as

compared to the zero dipole moment of the ground state) [138]. Moreover,

23

Figure6. Newman projection of M02X4(PP)2 complexes depicting the

torsional angle (0.

25

time—resolved picosecond emission spectroscopy show the striking result

that the temporal evolution of 1(88”) luminescence occurs on the time-scale

of the microscopic solvent relaxation time [138]. These results closely show

that solvent is an important controlling factor in the dynamics of 1(88")

luminescence, and strongly corroborate a valence bond description of an

ionic excited state in which one metal center is on'dized and the other metal

center is reduced relative to the ground state.

The ionic character of 1(88*) state deems these complexes as logical

candidates for multielectron derived from charge separated excited states.

Beyond the mixed—valence character, additional assets of the 1(88") also

arises from the weak coupling of the dxy orbitals. Unlike the singlet excited

states of most transition metal complexes, the 1(88*) excited state of M-LM

binuclear complexes are sufficiently long-lived (1: ~ 100 ns) in some cases

[118d] to permit bimolecular reactivity [121d, 139—141]. The longevity of the

MMCT state most probably arises from the large 1(88"')-—3(88"') energy gap

which inhibits intersystem crossing. Furthermore, photodissociation of the

metal-metal bond is avoided upon population of 1(88$), since the 8 bond

contributes less than 10% to the overall M—M bond energy. The multiple

bonding in the M-LM complexes provides an array of excited states, and

although few studies have focused on excited states other than the 1(88"),

the valence bond description predicts mixed valence character in additional

states such as 1(1:8'") and 1(8x"') states [114]. These excited states formally

represent a one-electron charge separated state as compared with the

formal two—electron charge separation in the 1(88") excited state.

Irrespective of the formal electron counts, each of the MMCT transitions

yields partial reduction of one metal center and oxidation of the other metal

center relative to the ground state.

26

Studies described herein are focused on investigation of the

multielectron photochemistry of M-LM binuclear complexes M201‘(PR3)4

and M2014(PP)2, whose ligating spheres are comprised of chloride and

either monodentate (PR3) or bidentate (PP) phosphines in D2,, and D2}I

molecular geometries, respectively. A prerequisite for elucidating the

photochemistry of these binuclear complexes is a clear elucidation of the

physical and chemical properties in the excited states. Accordingly,

nanosecond transient absorption spectroscopic studies of the M2014(PR3)4

and M02Cl4(PP)2 complexes are described in Chapter III. These studies

reveal that the photophysics of MMCT states of these complexes is much

richer and more extensive than was heretofore realized. Efforts to develop

the multielectron photochemistry of these transients has centered on two-

electron oxidative addition reactions of mixed-valence excited states. The

observed photoreactions of the D2,! and D2}, complexes are described in

Chapter IV. Mechanisms of multielectron photoreactions of a M-LM

species with a variety of substrates are presented based on transient

absorption and electrochemical studies. Particularly intriguing is the

photoinduced oxidative addition of CH3I to electronically excited

W2Cl4(dppm)2. A framework for the rational design of variable discrete

multielectron schemes of M-LM complexes was formulated from these

studies and is briefly summarized in Chapter V.

CHAPTERII

EXPERIMENTAL

A. SolventPurification

LSolventsusedforSyntheses

All solvents were refluxed under N2 for no less than 8 h and freshly

distilled prior to use. Hexane, cyclohexane, diethyl ether, benzene, toluene

and tetrahydrofuran were refluxed over Na. In the case of the latter three

solvents, small amounts of benzophenone were added to form the blue or

purple ketyl radical anion or dianion, respectively, as an indicator. All

halogenated solvents were refluxed over P205. Methanol was refluxed over

Mg(OMe)2 prepared by initially refluxing 50 mL MeOH containing 5.0 g of

Mg turnings and 0.5 g of I2. Once the color of 12 disappeared, an additional

1 liter ofMeOH was added.

2. SolventsusedbrSpecuoecopyandPhotochemishy

With the exception of CH3I, all solvents used for electrochemical,

spectroscopic and photochemical experiments were spectroscopic grade

27

28

(Burdick and Jackson Laboratories). Each of the solvents was subject to

seven freeze—pump—thaw cycles (10'6 torr) and then vacuum distillamd into

storage flasks equipped with Kontes high-vacuum valves containing

various drying agents. Dichloromethane was stored over Linde 4A

molecular sieves, which had been activated under dynamic vacuum (10'6

torr) at 250 °C for 12 h. Hexane was stored over mixtures of sodium-

potassium alloy. Benzene, toluene, and THF was also stored over sodium-

potassium alloy except in these cases small amounts of benzophene were

used. Owing to the increased water content of THF, it was first vacuum

distilled and predried for two days over Na/benzophenone and finally

transferred into a storage flask containing the Na/K alloy and

benzophenone.

Methyl iodide (Baker Reagent Grade) used for photolysis was

refluxed over P205 and Cu for no less than 12 h. A Vigereux column was

used to further induce condensation of the very low boiling solvent (43 °C).

The doubly distilled solvent was collected in a round bottom flask containing

P205 and Cu, which was subsequently subject to seven freeze-pump—thaw

cycles (10"6 torr). Only freshly distilled solvent was used to avoid 12

impurties which will sublime at these pressures.

BSynthesis

LGeneralProcedures

Synthesis of all complexes were performed using standard Schlenk-

line techniques with rigorously deoxygenated and dried solvents; All

chemicals were reagent grade and were used as received unless otherwise

29

noted. The dppm (dppm = bis(diphenylphosphino)methane) and PBu3

phosphines were purchased from Aldrich Chemical Company, and the

remainder of the phosphines were obtained from Strem Chemicals. The

phosphines were stored under vacuum in flasks equipped with Kontes

high—vacuum valves. The highly reactive dmpm (dmpm a

bis(dimethylphosphino)methane) was dissolved in toluene (1.0 M in dmpm)

for storage and further use. Previously prepared compounds were

characterized by electronic absorption spectroscopy, cyclic voltammetry and

1H NMR spectroscopy.

2. SynthedsofMgCLa’Rggalmplems

aPrecur-sors

1. WCl4 [142]. A mixture ofWClg (9.9 g, 0.025 mol) (Aldrich Chemical

Co.) and W(CO)g (4.4 g, 0.0125 mol) (Aldrich Chemical Co.) in 50 ml of dry

chlorobenzene was refluxed until evolution of CO ceased (~ 12 h). The grey

product was filtered by suction filtration, washed with chlorobenzene, and

subsequently washed with dichloromethane to remove all traces of the

aromatic solvent. The solid was dried and stored under vacuum as

prolonged exposure (~ 1 h) to air results in decomposition.

ii. Mo(n'-PhPMePh)(PMePh,), [143]. This exceedingly moisture

sensitive complex was prepared by literature methods with slight

modifications to minimize the previously reported oligomerization. As

expected, formation of the brown—black oligomers was found to be

concentration and temperature dependent, and therefore reactions were

run under dilute conditions at 0 °C. To a solution of 50 mL of THF

containing 2.3 mL of PMePh2 (12 mmol), 0.65 g of MoC15 (2.38 mmol)

30

(Aldrich Chemical Co.) was dissolved. Untreated Grinard Mg turnings

(1.05 g, 43 mmol) (Mallinckrodt) were slowly added to the solution over a

period of 0.5 h. Numerous intermediates were observed during the

multiple reduction reaction. An initial orange precipitate observed after

the addition of Mg was complete, eventually redissolved with an ensuing

reaction to yield an olive green solution which finally turned brownish

orange. The last step was very exothermic and the heat induced

oligomerization when the temperature was not carefully controlled. Upon

completion of the reaction, the solution was allowed to slowly warm to room

temperature, stirred for an additional 0.5 h, and filtered to remove the Mg.

Complete removal of THF from the filtrate, as suggested in the literature,

was found to result in considerable oligomerization. Alternatively,

precipitation of the orange product from dilute solutions was achieved by

partial removal of the THF, addition of MeOH, and further removal of the

more volatile THF. Oligomer, if present, was removed by redissolving the

product in benzene, filtering and precipitating the pure product with

MeOH.

iii. WCI4(PPh,), [144]. This complex was prepared by reduction of

WClg (Aldrich Chemical Co.) with amalgamated mossy Zn (Baker

Chemical Co.). Mossy Zn (7.5g) was amalgamated by dissolving 0.2 g of red

HgO (Matheson, Coleman and Bell) in 2 mL of 12 M HCl (aq). This solution

was added dr0pwise to 80 mL of H20 containing the mossy Zn. The

amalgamated Zn was filtered, washed with H20 and acetone and dried in

the oven. The addition of 7.5 g of PPh3 (0.0325 mol) (Baker Chemical Co.) to

a saturated 60 mL solution of WCIG (3.75 g, 0.0095 mol) (Aldrich Chemical

Co.) containing 7.5 g Zn mossy (0.115 mol) resulted in immediate

31

precipitation of the yellowish-orange WCl4(PPh3)2. Afier shaking the

reaction mixture periodically for 15 min, the precipitate was filtered and

washed with small aliquots of CH2012. The mossy Zn was separated out

with forceps. Although the complex is quite stable in dry air, a blue

decomposition product is observed when moisture is present. Two triplets

at 8.0 and 8.4 ppm are observed in the 1H NMR of the WCMPPth complex

and the UV-visible spectrum exhibits a single absorption maximum at 420

nm.

h. DimolybedumComplemes

i. D“ learners. The entire series of M02014(PR3)4[145] complexes for

PR3 = PEt3, PBu3, PMezPh, PMeth, and PHth, were prepared by a

standard method from (NH‘)5M02C19-H20 [146]. A complete description of

the synthesis of (NH4)5M02C19-H20 is provided in the doctoral dissertation of

I—J. Chang [147]. Phosphine (4.5 mmol) was added to a suspension of 1

mmol of (NH4)5M02019-H20 in 50 mL of MeOH. The purple solution was

refluxed for 4 h to yield a blue microcrystalline product that was filtered by

suction filtration and washed with H20 to remove unreacted

(NH4)5M02019-H20, MeOH and diethyl ether (except for the soluble PEts and

PBu3 complexes). The complexes were purified by column chromatography

using Florisil as a solid support and CH2012 as the eluant. A blue band was

collected from the column, which retained yellow impurities at its top. The

purified complexes were precipitated by addition of MeOH. The complexes

exhibit very similar absorption profiles with intense absorptions at ~ 600 nm

(8 ~ 3200 M‘lcm‘l) and 330 nm (6 ~ 3000 M'lcm‘l) and a very weak

absorption at ~ 450 nm (8 ~ 250 M'lcm'l) [121 a].

32

11. Green Isomer of Mo,Cl4(PMePh,)4 [148]. The green isomer of

MozCl4(PMePh2)4 was prepared by the reaction of the very moisture

sensitive precursors MoCl,('l'HF)2 and Mo(n‘-PhPMePh)(PMePh2)3. A 30

mL THF slurry containing 400 mg of Mo(ne-PhPMePh)(PMePh2)3 (0.4

mmol) and 170 mg of McCl4(THF)2 (0.4 mmol) was stirred at ambient

temperature for a period of 10 h. The solution was concentrated to ~ 5 mL to

induce precipitation of unreacted MoCl4(THF)2, which was subsequently

filtered 011'. Complete removal of THF was found to result in production of

the blue isomer. The green isomer may be precipitated by addition of

cyclohexane to the filtrate followed by partial removal of the more volatile

THF. Residual of Mool,('rHr)2 and Mo(n“-PhPMePh)(PMePh2)3 can be

removed by column chromatography with Florisil and THF as an eluant.

The first green band was collected and precipitated with cyclohexane. The

UV-visible spectra of both the blue Dad isomer and the green isomer are

provided in Figure 34. The green isomer has a characteristic absorption at

450 run that is not observed in the blue D2,; isomer. Furthermore the ratio of

intensities of the absorption maxima at 600 and 330 nm vary for the two

isomers. It should be noted that the spectra of samples from different

preparations show different ratios of intensities of the absorption maxima

at 330 nm and 600 nm, as expected for varying mixtures of the green and

blue isomer. Furthermore, this ratio for a given sample decreased over a

period of hours indicating that isomerization from the green to blue isomer

might well occur in THF solution.

c. Ditungsten Complexes. The series ofW204(PR.3)4 complexes [142]

for PR3 = PEta, PBu3, PMe3, and PMeth, was prepared by Na reduction of

W014 [143]. Numerous attempts to prepare these complexes with

3 3

commercially available WC], (Aldrich Chemical Co.) were unsuccessful.

The synthesis described here is for the case of PR3 = PBu3. The PEta,

PMePh2 and PMes analogues were prepared in a similar manner, with the

slight variations previously reported in the literature: 2.5 mL of PBus (19

mmol) and 2.6 g ofW01. (8 mmol) were sequentially added to a 60 mL ofcold

THF (—78°C) containing a stoichiometric amount of 0.41% Na amalgam

(0.368 g Na (16 mmol)/ 90g Hg). Approximately 15 min after the reactants

were mixed, the reaction flask was put in an ice bath and vigorously stirred

and shaken periodically as it was allowed to slowly (~1.0 h) warm to room

temperature. The resultant blue green solution, obtained about 0.5 h after

the solution reached room temperature, was carefully decanted from the

amalgam. THF was then completely removed and the product was

extracted with hexane, and filtered to remove grey sideproduct. Complete

removal of hexane from the filtrate yielded a dark green-black oily residue.

The blue powder obtained upon addition of MeOH, was collected by suction

filtration and washed extensively with MeOH. The complex is stable

enough to be worked up in air, however prolonged exposure to air will

result in decomposition.

The complex was purified by column chromatography with Kieselgel

60 (EM Science) as a solid support and CH2012 as an eluant. The absorption

spectrum exhibits two intense maxima at 665 (e = 3813 M'lcm‘l) and 303

nm (e = 8933 M‘lcm’l) as well as a very weak absorption at 495 nm (e = 290

M‘lcm'l)

d. MoWCl,(PMePh,), [148]. A 15 mL 06H, solution of Mo(n“-

PhPMePh)(PMePh2)3 (100 mg, 0.1 mmol) was added dropwise over a 15 min

period to a suspension of WCl4(PPh3)2 (250 mg, 0.3 mmol) in 20 mL of CgHg.

34

After ~ 45 min, the green solution was concentrated to ~ 5 mL to induce

precipitation ofWCl‘(PPh3)2, and filtered to remove the latter.‘ The benzene

was entirely removed and a minimal amount (~ 3 mL) ofTHF was added to

yield a saturated solution followed by addition of 15 mL of MeOH. A

turquoise precipitate formed after 1 h at 0 °C was filtered and washed with

MeOH. The absorption spectrum exhibits maxima at 650 nm (e = 2609 M“

1cum-1), 460 nm (e = 394 M-lcm-l) and 320 nm (e a 7133 M-lcm-l). *(It should

be noted that the final olive green precipitate collected from the filtrate of

more dilute solutions contained significant amounts of WCl4(PPh3)2, as

determined by 1H NMR).

3. Synmsshormmmplms

a. MogCI..(dppm), and MogCLklmpm), [149-150]. The dimolybdenum

complexes, M02014(dppm)2 and M02014(dmpm)2 were prepared by the

method described for the M02014(PR3)4 complexes. Dichloromethane

solutions of M02014(dppm)2 exhibit two intense maxima at 634 nm (e = 2490

M‘lcm'l) and 325 nm (e = 5600 M'lcm'l) as well as a weak band at 462 nm

(e = 900 M'lcm‘l) . Two absorptions at 604 nm (e = 1730 M'lcm‘l) and 426

nm (e = 270 M’lcm'l) are present in the UV-visible spectrum of

M02014(dmpm)2.

b. W,Cl4(dppm), [151]. W2C14(PBu3)4 (0.5 g, 0.38 mmol) and dppm

(0.30 g, 0.788 mmol) were refluxed in a 1:3 mixture of toluene (10 mL) /

hexane (30 mL) for a period of 6 h. The resulting green crystalline solid (or

sometimes brown) was washed with hexane to remove traces of

W2014(PBu3)4. Benzene solutions of both the green and brown precipitates

dissolved in benzene are light brown, with identical absorption spectra with

3 5

maxima at 710 nm (e = 2585 M-lcm'l), 500 nm (e = 598 M-lcm-1)405 nm

(sh) (e = 1923 M-lcm-l) and 364 nm (e = 2948 M’lcm‘l) (Figure 14). The

change in color is attributed to the previous reported difi‘erences in the UV-

visible spectrum of W2014(dppm)2 in the solid state and solution [151]. The

W2014(dppm)2 complex is very reactive and exceedingly sensitive to oxygen,

and will decompose within seconds to yield green, blue, and purple

products. The complex will react thermally with CH2012 at ambient

temperature, as previously reported [152], as well as acetonitrile. These

reactions however do not result in major color changes but can be detected

by an increase in the 500 nm absorption band ofWZCI4(dppm)2.

c. MoWCleppm), Benzene solution of 0.15 g MoWCl4(PMePh2)4

(0.123 mmol) and 0.25 g (0.62 mmol) of dppm were stirred for 12 h at room

temperature (heating was found to result in decomposition). The green

precipitate was collected, redissolved in CH2C12 and filtered to remove

yellow-green insoluble material. The absorption spectrum exhibits

features at 665 nm (A = 0.5), 370 (sh) (A = 0.5), and 320 nm (A = 1.0).

4 aynmdmmAandma’PhxoCo-nflm

a. W,Cl.(PR.)4 (PR, I PEt., PBup) [142]. These complexes were

prepared by a method reported for the PM63 derivative, except that the

rather unstable precursor, chlg(THF)4, was prepared in situ by adding 2.6

g of WCl4 to 70 mL of THF containing 0.41% sodium amalgam (0.368 g Na

(16 mmol)/ 98 g Hg) at ambient temperature. After 1 h of vigorous shaking

and stirring, the resultant yellow-green solution of W2016(THF)4 was

decanted from the amalgam and filtered through Celite to remove a grey

sideproduct. The filtrate immediately turned red upon addition of

3 6

stoichiometric amounts of phosphine. The solution was stirred for 0.5

h,and then THF was completely removed in vacuo. Addition of MeOH

prompted the precipitation of a brown powder precipitate, which was

collected. Although reasonably good yields were obtained with the PEta

complex (61%), only 10% yield was obtained for the PBus complex. A UV—

visible spectrum of a finely ground powder of W2016(PEt3)4 is provided in

Figure 7a (1,“, = 770, A = 0.06; km, = 470, A = 0.53; In“ a 380 nm, A =

0.45). This complex has been shown to convert to confacial WzClg(PEt3)3 in

solution at ambient temperature [153]. The complete conversion requiring

~ 3 min was monitored by UV-visible spectroscopy (Figure 7b). The final

spectrum, with maxima at 510 nm (e = 2077 M-lcm-l) and 328 nm (e = 2080

M‘lcm’l), corresponds to W2016(PEt3)3. As reported for the PEta complex,

only signals attributable to W2016(PBu3)3 were observed in the ”P NMR

spectrum of a sample of W2013(PBu3)‘ dissolved in toluene at ambient

temperature: 8 = 421.17 (t), —33.9 (d), Jpp a 35 Hz. The UV—visible smctrum

of CH2012 solutions of W2013(PBu3)3, with maxima at 500 nm (e = 1970 M"

1cm‘l) and 315 (sh) (e = 2269 M'lcm’l), is similar to that of W2016(PEt3)3

(Figure 8). The FABMS ofWZClg(PBu3)3 exhibits a parent ion cluster at 1188

amu, as well as a fragment at 1150 amu corresponding to loss of Cl.

b. W,Cl4(dppm).lg. Equimolar amounts of 12 (0.03 g, 0.117 mmol) and

W2C14(dppm)2 (0.15 g, 0.117 mmol) in 20 mL ofbenzene were stirred for 0.5 h

at ambient temperature. The rose solution was passed through a Florisil

column with THF as the eluant and yielded a rose precipitate upon addition

of hexane. The solid was filtered, washed with benzene and hexane, and

dried under vacuo (10"6 torr) at 50 °C to remove excess I2. A parent ion

cluster at 1532 amu as well as two fragments at 1405 and 1497 amu,

3 7

corresponding to loss of I and Cl, respectively, are present in the FABMS

shown in Figure 9. The U'V-visible spectrum exhibits an intense

maximum at 500 nm (e = 6954 M’lcm'l) and a less intense shoulder at 390

nm (e = 3477 M’lcm‘l).

c. W3C1.(dppm),[152]. A CHzclz solution of W201‘(dppm)2 was

stirred for a period of two days. A precipitate of the air sensitive

W2C16(dppm)2 formed upon addition of hexane to the solution. The solid

was washed sequentially with benzene and pentane, and stored under

vacuum. The absorption spectrum exhibits two intense maxima at 468 nm

(e = 4800 M‘lcm'l) and 387 nm (e = 3200 M‘lcm‘l), as well as very weak

absorbances at 822 nm (e = 740 M‘lcm'l) and 641 nm (e = 285 M'lcm‘l).

GPhotochanish-y

LGeneralProcedures

Monitored photoreactions were carried out in a specially adapted

high vacuum UV-visible cell described in section E1. Bulk solutions were

also prepared under high vacuum conditions in quartz tubes. Sample

irradiations were performed by using a Hanovia 1000—W Hg/Xe high

pressure lamp. The beam was collimated and passed through a 10 cm

circulating water filter. The irradiation wavelength was selected with

Schott color glass high-energy cutoff filters which were placed in a glass

water circulating bath to avoid their heating. The collimated and filtered

beam was finally passed into the sample isolated in a separate water

circulating bath. Isolation of the sample in a separate compartment is

necessary to prevent previously encountered problems with reflected

3 8

unfiltered light reaching the sample. The sample temperatures were

thermostated with the water bath. An identical setup was used for

quantum yield determinations except that Oriel interference filters were

used in place of the cutofi‘ filters. Ferrioxalate actinometer [154, 155] was

used as the reference and described in detail in the dissertation of Dr. I-J.

Chang [147]. Measurements were made under optically dense conditions

(A > 2) at the exciting wavelengths. Photoproduct concentrations were

limimd to less than 10% to avoid inner filter effects. Conversions were

determined by monitoring the disappearance of the 8241(88‘) absorption of

the quadruply bonded binuclear complex.

2. IsolationofPhotoploduct:

a. Photolysis ofW,Cl4(dppm), with CHJ. The photoproduct ofthe

reaction of WzClg(dppm)2 with CH31 was precipitated upon addition of

hexane. There was no evidence of unprecipitated sideproducts in the

colorless mother liquor. The absence of sideproduct was further confirmed

by the fact the the UV-visible spectrum of the precipitated solid was

identical to the final spectrum of the photolysis. Elemental analysis of the

"crude" (i.e. nonrecrystallized) precipitate was performed at Galbraith

Laboratories. The photoproduct is not stable at ambient temperature and

photolyzed solutions were kept at S 0 °C at all times. Failure to do so

resulted in formation of W2014(dppm)2I2, as evidenced by a relative increase

in the absorption at 500 nm, and appearance of a parent ion cluster of the

diiodide complex in FABMS. Additionally, a decomposition product with

an absorption maximum at 470 nm was observed when the slightest trace of

moisture was present. Attempts to grow crystals layered from

3 9

CHzclzlhexane solutions at -20 °C were unsuccessful; only decomposition

products which were completely insoluble in CH2012 were obtained.

A 13C NMR spectrum of the photoproduct was obtained by

photolyzing W2014(dppm)2 in a 1:1 mixture of 12C and 13C enriched (99%,

Aldrich Chemical Co.) CHal. Upon completion of the photolysis, the CH3I

was completely removed under vacuum, and the photoproduct was further

subjected to dynamic vacuum (10“ torr) for 8 h. The product was

redissolved in CDzClz (Aldrich 99.6+%) and the 13C NMR spectrum was

recorded on a Bruker WM-250 NMR at -60 °C.

Analysis of ethane in the atmosphere above photolyzed CH31

solutions of W2C14(dppm)2 was performed by Toepler pumping a

quantitative volume V of the photolyzed solution containing a calculated

quantity (n) of the W2014(dppm)2(CH3)I photoproduct. The solutions were

kept at 0 °C to avoid decomposition of the photoproduct, but were not subject

to freeze-pump—thaw cycles at liquid N2 temperature.* The CH31 was

partially condensed in three successive traps at temperatures above the

boiling point of ethane (-78 °C). The collected gas was vacuum transferred

into a 0.5 ml tube equipped with a Kontes stopcock and an Ace high vacuum

septum. Identical procedures were carried out with two control solutions,

one was a blank sample of CH31 with volume V, and the other a sample of

CH3I with volume V containing n moles of ethane (control 2) These three

gas samples were analyzed with a Hewlett Packard 5985 GS/MS with an

open tubular 18" column at 50 °C. The relative intensities of the CHal peak

at 142.1 amu and the ethane peak at 30.1 amu were measured: (1142.1, 130:

Blank, 4824/5, control 2: 5000/105, sample 5464/3). *(Ethane was barely

detectable from control 2 solutions subject to freeze—pump—thaw cycles at

liquid N2 temperature.)

40

b. Photolysis ofMo,Clg(PBu3)4 with PhSSPh. A photoproduct Ohm,=

540 nm) from irradiated solutions of M02014(PBu3)4 with PhSSPh was

separated from a product (Inn: 460 nm) by column chromatography on

Florasil. The 540 nm product was selectively eluated from the column with

a 5% CH3CN and 95% CH2012 solvent mixture and the secondary product

was then removed by eluting with pure CHscN.

c. Photolysis ofW,CI.(PBu3)4 with CH,CI,. The photoproduct fiom

irradiated CH2C12 solution of W2014(PBu3)4 displayed absorption man'ma at

1430, 430, 404 and 335 nm. Attempts to isolate the photoproduct were

hindered by thermal decomposition at ambient temperature as evidenced by

the appearance of an absorption at 500 nm and disappearance of the NIR

absorption. The decomposition reaction is concentration dependent and

consequently forms immediately upon complete removal of solvent from

photolyzed solutions.

D.Electrochemish'y

LGeneralProeedures

Cyclic voltammograms were recorded with a Princeton Applied

Research (PAR) Model 173 potentiostat, Model 175 programmer, and a

Model 179 digital coulometer. The output of the digital coulometer was fed

directly into a Houston Instrument Model 2000 X-Y recorder. A Pt wire

gauze, a Pt button and a Ag wire were used as the counter, working and

reference electrodes, respectively. Potentials were referred to the SCE

reference scale by using the szFe+le2Fe couple of +0.31 V vs SCE as an

4 1

internal standard [156]. Typical concentrations were ~ 5 x 10" M in

quadruply bonded binuclear complexes with 0.1 M electrolyte. A two

compartment standard H cell was used in most cases except for

experiments involving the addition of halide anion to the sample

compartment. In this case, the reference electrode was separated from the

sample compartment by using a four compartment cell to avoid differences

in potential before and afizer the addition of halide. Bulk electrolysis

experiments were performed with the above apparatus in conjunction with

a PAR 179 coulometer. A standard three compartment H cell was used

with a platinum mesh electrode as the working electrode.

2. PreparationandPurificationofElecholytes

Tetrahexylammonium hexafluorophosphate (THAPFS) was prepared

by metathesis of tetrahexylammonium iodide (THAI, Fluka) and

ammonium hexafluorophosphate (NH4PF6 Aldrich) in 95 % EtOH [157].

The THAPFg precipitated immediately upon slow dropwise addition of a

saturated solution ofTHAI to a saturated to NH4PF6 solution. The THAPFG

was filtered and recrystallized from 95 % EtOH. Tetrabutylammonium

hexafluorophosphate (TBAPFG) was dissolved in ethyl acetate containing

MgSOb filtered, then recystallized by addition of ether. Both electrolytes

were dried under vacuum (10‘6 torr) at 90 °C for 10 h.

3. aulhalectrolysisorwpwsup,

Bulk electrolysis of a 4 mL CH2012 solution of W2Cl4(PBu3)4, (6.96 x

10" M) was performed in the N2 atmosphere of a glove box at an applied

potential of +0.1 V. The coulombs generated, 26.4 x 10’2, were consistent

with a one—electron oxidation process (theoretical = 26.8 x 10"2 coulombs).

42

Cyclic voltammograms obtained before and alter bulk electrolysis were

identical and showed no indication of side reactions accompanying the

electrolysis process. The UV—visible spectrum of the W2(II,III)C14(PBu3)4+

(1m, = 1510 nm, e = 2243 M-lcm-1,i.,,,,,x = 380 nm, e = 2240 M-‘cm-1.1m =

480 nm, e = 852 M‘lcm'l) species is provided in Figure 10. Although the

W2(II,III)Cl4(PBu3)4+ complex may be prepared chemically with NOBF4 as

an oxidant, the spectral changes indicate additional byproducts are also

formed.

E. Spectroscopic Instrumentation andMethods

1. ElectronicAbsorption Spectroscopy

Absorption spectra were recorded with Cary 17D or Cary 2300

spectrophotometers. In most cases solutions were prepared under high

vacuum conditions in a cell consisting of a 1 cm quartz cuvette and a 10 ml

side arm. The two chambers were separated by two Kontes high-vacuum

quick release teflon stopcocks. The samples were placed in the cuvette and

the appropriate solvent was transferred to the 10 ml side arm by bulb-to-

bulb vacuum distillation. After these subsequent freeze-pump—thaw cycles

were performed, the solvent was mixed with the sample. Extinction

coefficients were determined by standard procedures from solutions

prepared in a glove box. Three stock solutions with varying amounts of

weighed sample were first prepared. Appropriate dilutions of these stock

solutions provided seven solutions of known concentrations, with

absorbances varing from 0.20 to 0.90. Extinction coefficients were

calculated from Beer-Lambert plots.

43

asteady-StateLuminescenceExperiments

The emission spectra were recorded on an emission spectrometer

constructed at Michigan State University which is fully described in the

doctoral thesis of Dr. M. D. Newsham [158]. The R1104 Hamamatsu PMT

was used in the case of the dimolybdenum complexes and the R316

Hamamatsu PMT in the case of the ditungsten complexes. Absolute

emission quantum yields of optical dilute samples (A < 0.2) were measured

using M02014(PBu3)4 as a quantum yield standard (item = 0.013 in 2-Me-

pentane at 300 K) [121d]. The quantum yield was calculated from the

following equation [159]:

2Arm.) n_,_ g

4‘1.“ X [AthQ] X [111:2] X [Dr] (2.1)

where x and r designate the unknown and standard solutions, respectively,

n is the average refractive index of the solution, D is the integrated area

under the corrected emission spectrum, and A (2.) is the absorbance per

unit length (cm) of the solution at the exciting wavelength 2..

3. IramieritAbeorptionSpectroscopy

The nanosecond time resolved absorption spectroscopy of the

dimolybedum complexes was performed on an instrument described

elsewhere [160]. The same instrument was used for the ditungsten

transient spectra except that the signals were digitized on a Tetronix DSA

digitizing signal analyzer with a model 11A72 two-channel amplifier. The

solutions with absorptions of ~2 at the exciting wavelengths, except where

4 4

otherwise noted, were continually circulated through a flow cell with a 1

mm pathlength throughout the experiment. The solutions were prepared

in a glove box with freshly vacuum distilled high purity solvents.

4. Electron Paramagnetic Resonance

X band EPR spectra were obtained using a Bruker ER 200D

spectrometer. The magnetic field strength was measured with a Bruker

ER035M NMR Gaussmeter and a Hewlett-Packard 5245L frequency counter

equipped with a 3-12 GHz adapter was used to measure the microwave

frequencies.

5. Nuclear Magnetic Resonance

The phosphorus and proton NMR were recorded on a Varian VXR-

500 spectrometer. The 31PilH} chemical shifts were measured relative to

85% H3PO4. Deuterated methylene chloride (Aldrich, 99.6+%) and

deuterated chloroform (99.8%, Cambridge Isotope Laboratories) were dried

under high vacuum conditions as described in Section A2.

6. Mass Spectrometry

The fast atom bombardment mass spectra (FABMS) were performed

on a JEOL HX 110 double focusing mass spectrometer housed in National

Institute of Health/Michigan State University Mass Spectrometry Facility.

Samples were dissolved in 2-(octyloxy)nitrobenzene matrices.

45

Figure 7. (a) Electronic absorption spectrum of a finely ground state

sample of the edge—sharing bioctahedral complex W2016(PEt3)4. (b)

Electronic absorption spectral changes associated with the conversion of

W2C16(PEt3)3 to the confacial bioctahedral complex W2016(PEt3)3 in toluene

solution at ambient temperature, In,” nm (e M‘1 cm'l) (a) W2C16(PEt3)4:

4...... = 470 nm, (e = 1992 M’1 cm-l); 380 nm (1280 M-1 cm-l) (b) W2016(PEt3)3

: 1...... = 510 nm, (e = 2077 M-1 cm‘l); 328 nm (2078 M-1 cm-l).

Absorbance

4 6

600700

WavelengthI nm

Figure'la.

47

Figure 8. Electronic absorption spectrum of dichloromethane solutions

of the confacial bioctahedral complex W2C16(PBu3)3, In,“ = 500 nm, (e =

1970 M‘1 cm‘l); 315 nm (2268 M-1 cm'l).

48

can

opus...—

E:

\56:235.;

com

00?

q

com

dd

eoueqiosqv

49

Figure 9. Positive fast atom bombardment mass spectrum of a product

from the thermal reaction of W2014(dppm)2 with 12. The clusters centered

at 1531, 1497 and 1405 amu are consistent with (a) W2014(dppm)212, (b)

W2013(dppm)I2 and (c)WZCI4(dppm)21, respectively.

50

eouepunqv ennslea

M/Z

“m9

51

Figure 10. Electronic absorption spectrum W2(II,III)Cl4(PBu3)4+

prepared by bulk electrolysis in CH2C12. km” = 388 nm, (e = 2559 M‘1 cm‘l);

485 nm (949 M‘1 cm'l); 1510 nm (2243 M-1 cm-l).

a.

52

3

CHAPTERIII

TRANSIENTABSORPTIONSPECTRIBCOPICSTUD-

A. BACKGROUND

The electronically excited molecule is the crucial reactant in a

photochemical transformation. Thus a clear understanding of the

photochemistry must begin with the elucidation of the electronic structure

of the photoreactant. The quadruply-bonded complexes whose electronic

structure is most intensively investigated are the M2014(PR3)4 complexes.

The lowest energy transition in the uv-visible absorption spectra of these

complexes, which adopt the D24 geometry depicted in Figure 11, is the

typical 8241(88") transition. Higher energy transitions in the electronic

absorption spectrum of M2Cl4(PMe3)4 have been assigned as well [121a]. A

weak transition (6 = 210 M’lcm‘l) centered at 441 nm is attributed to the

dipole-allowed 10:48") transition and a ligand—to—metal charge transfer

(LMCT) assignment has been made for the intense transition (6 = 3720 M"

1cm’l) centered at 324 nm based on its characteristic blue shift (11,,“ = 290

nm) for the ditungsten analogue.

53

54

Figure 11. (a) D2d structure of MZCI4(PR3)4 complexes; 0)) D2}1 structure of

MzCl4(PP)2 complexes where PP = bidentate phosphines.

55

.0

£2

D2ci M20'4(PRa)4

/'\

P P

”on

(Cl\

e“

02h M20'4(PP)2

Cl I CI l

P PV

Figurell

56

In contrast to the excited state properties of the majority of

quadruply-bonded dimers, the 1(88”) state of these complexes is long-lived

and the luminescence is intense. For example, the 1(88") luminescent

lifetime of MozCl4(PBu3)4 in 2-methylpentane at ambient temperature is 16

ns and the emission quantum yield is 0.013 [121d]. The enhanced lifetime of

the 1(88*) excited state of M02014(PR3)4 complexes relative to other M02C14L4

complexes has been ascribed to the steric bulk of the coordinating

phosphines [139]. For complexes with sterically unhindered ligands such

as CH3CN and Cl’, free rotation about the metal-metal bond plays a

prominent role in the nonradiative decay of the 1(88") excited state due to the

annihilation of the 8 bond upon 82 -)1(88*) excitation. Indeed the broad and

weak emission and notable lack of a mirror image between absorption and

emission spectra of M02018“ [129] is consistent with the poor Franck—

Condon factors between the molecule in its staggered D“ excited state and

its eclipsed D41, ground state [139]. The lifetimes of the staggered excited

states are typically in the picowcond range [139]. This is not the case for the

M02014(PR3)4 complexes. The mirror image absorption and emission of the

l(88") excited state is explained by the steric constraint of the phosphines

which prevent rotation about the metal-metal bond. In effect, the excited

state is locked into the eclipsed geometry of the ground state. Nevertheless,

despite this simple excited state model, transition absorption studies of

M0201‘(PBu3)4 have identified multiple decay processes of the 1(88") excited

states. In addition to prompt decay from the luminescent 1(88") excited

state, a nonluminescent transient with a lifetime of 90 as is also observed

[139]. An even longer lived (1: = 46 us) nonluminescent transient is observed

with the M0201‘(CH30N)4 complex, though no analogous transient exists

for electronically excited M02013“. While definitive assignment of the

5 7

transients has not been achieved, the possibilities of a 3(88*) excited state,

3(1t8"‘) excited state or a distorted chemical intermediate have been

suggested. The longevity of this nonluminescent suggests that it is a

promising candidate for bimolecular photochemistry.

In principle, an eclipsed conformation about the quadruply bonded

core can be imposed not only by a bulky phosphine, but by a bidentate

phosphine spanning the bimetallic core as well. Despite the basic

similarities in the ligation spheres of the M2Cl4(PR3)4 and M2014(PP)2

complexes, a D2,, symmetry of the latter complexes is imposed by the trans

arrangement of the bridging bidentate phosphines depicted in Figure 11.

Although the electronic excited state properties of the MzCl4(PP)2 complexes

have not been investigated as extensively as those of MzCl4(PR3)4, there are

data that suggest this change in symmetry is not without consequences.

Indeed differences are predicted from analysis of the simple molecular

orbital diagram provided in Figure 12 [161]. The relative energetics of the

metal-metal based orbitals are displayed as a function of rotation of a

MClsz fragment about the metal-metal bond from 0° (D211) to 90° (Dzd): with

D2 symmetry occuring for all angles in between. The relative energies of

the 8 orbitals are a result of the variation of the d" overlap with the

torsional angle 0. Maximum overlap and hence the largest 8/8’ splitting is

achieved at o = 0° (Dzh) or 90° (D24) where the MClsz fragments are

eclipsed. The overlap and resultant 8/8' splitting decreases monotonically

with rotation away from the eclipsed conformation, until it reaches a

minimum of 0 at the completely staggered conformation (t = 45°). The

relative energetics of the x and xt orbitals vary with ¢~ In D24 symmetry, the

M—M it" and icy, orbitals are degenerate. However this degeneracy is

removed in D2,, symmetry. The (1,, metal orbitals, which are strongly

58

Figure 12. Relative energies of the d—derived molecular orbitals in

MZCl4(PR3)4 complexes as a function of torsional angle 0 (Reference 161).

59

60

metal-chlorine (M—Cl) x antibonding, are destabilized, and the d"

orbitals, which are strictly nonbonding with phosphine ligands, are

stabilized. The splitting of the it" and my, orbitals increases monotonically

with o. The in orbitals are perturbed in the same manner. Since theory

predicts 27% chlorine character in M—M x orbitals of M020134' [129], the

relative energetics of these orbitals are expected to be significantly affected

by this perturbation.

Experimental study of the torsional perurbation of the electronic

structure has become accessible with Cotton and coworkers synthesis of a

series of torsionally distorted D2 B—M2014(PP)2 complexes where a variation

in (t is induced by the steric restrictions of the bridging PP ligands. This

series of compounds reveal that the 8241(88') transition energies are

correlated with 00st where x = 0 or «I2 - 4) when t < 45 or t > 45, resmctively

[162, 163]. The electronic spectra of torsionally distorted complexes are far

more complex than those of their D24 cogeners. The optical activity of the

disymmetric D2 B-M2X4(S,S,—dppb)2 (S,S,-dppb = (28,38)-

bis(diphenylphosphino)butane) complexes provides a benchmark for the

assignment of these transitions based on their resultant circular dichroism

(CD) spectra [161]. As with the M,CI,(PR,), l)2d complexes, a ‘(x—is“)

transition was assigned to an electronic absorption band located

immediately to higher energy of 8241(88") absorption. However, the

assignment of the electric-dipole forbidden, magnetic-dipole allowed

1(rt-+8") transition of the B-M2X4(S,S,-dppb)2 complex was made for a

transition at considerably higher energy (1,“, = 362 nm) and much greater

intensity (2 > 3000 M‘lcm'l) than the similarly assigned electric-dipole

allowed transition centered at 440 nm (e = 200 M‘lcm’l) in the spectrum of

M02014(PMe3)4 [121a]. Also, as predicted from the model in Figure 12, the

61

nondegeneracy of the it orbitals potentially leads to two lbw—>8") transitions

in the case of the D2 dimers. In support of this contention, an absorption at

362 nm has been ascribed to the Tun—>8” component and a 340 nm feature

in the CD spectrum has been assigned to l(rim-+8"). The blue shift of the

l(rt—>8") transition upon a D2 to D2,, perturbation is opposite to that expected

from the model in Figure 12. As there was insuficient data in either case

for unequivocal assignment, it is not clear if either of these assignments is

correct. In general, the l(rt-48'") transition is difficult to distinguish from

the 1(8—-)rt"') which is predicted by theory to be energetically proximate. An

additional assignment of a 848x212 transition, where the 8x212 orbital is

primarily metal-ligand antibonding in character, was ascribed to an

absorption centered at 470 nm with approximately one-half the intensity of

the 8241(88") transition.

The monotonic increase in the run" splitting with increasing ¢ may

provide a basis for interpreting the luminescence intensity of M2014(PP)2

complexes. Previous studies described in the dissertation of Dr. I-J. Chang

show that the emission quantum yields of D2 complexes like M02014(dppe)2

(dppe = bis(diphenylphosphino)ethane) and M02014(dmpe)2 (dmpm =

bis(dimethylphosphino)ethane) are ~ 102 less than those of their respective

D25 analogues, M02014(PMePh2)4 and M02014(PMe3]4 [147]. An additional

102 attenuation is observed from the D2,, cogeners with dppm and dmpm

ligands. These relative emission quantum yields fit nicely with the model

in Figure 12. The energy gap between low lying 3(81'), 3(r:8"') states and the

1(88") state is smaller in the D21, complexes, accounting for the increased

nonradiative decay of their 1(88") states. The intermittent quantum yields of

emission from the D2 complexes are consistent with the predicted relative

energy gaps.

62

Despite these differences in their electronic excited state properties, a

recurrent theme in the redox chemistry common to both the M2X4(PP)2 and

M2014(PR3)4 complexes is that oxidation of the metal core is accompanied by

a major rearrangement of the ligation sphere. Two electron oxidized

M2X4(LL)2 species are stabilized by an edge-sharing bioctahedral

configuration of ligands [152, 164—167]. In the case of the more flexible

M2X4L4 complexes (M = Mo(II), W(II), Re(III); X = halide or pseudohalide;

and L = donor ligand including halide), which do not contain bridging

bidentate phosphines, the alternative confacial bioctahedral geometry is

sometimes observed upon oxidation [168-173] as well as the edge-sharing

bioctahedral structure [174—177]. Both the edge and confacial bioctahedral

arrangements ensure a stabilizing octahedral coordination geometry about

the on’dized metal center.

As discussed in Chapter I, the metal-to—metal charge transfer in

82—3 1(88*), 1(1:48") and l(8-nt"‘) transitions yield mixed valence

electronically excited states. Owing to the stabilization offered by confacial

or edge-bridging geometries about oxidized metal cores, the preparation of a

mixed valence excited states by light excitation should not be without

chemical consequences. To this end, these octahedrally based

rearrangements may presage a rich transient spectroscopy of D2,, and D24

complexes. Indeed the previous picosecond transient absorption studies of

M0201‘(PBu3)4 and M02Cl‘(CH3CN)4 indicate that nonluminescent

transients are generated upon 82-91(88"') absorption. These initial studies of

MozCl4L4 complexes (L= CH30N, PBu3, 01") are now elaborated by

nanosecond transient absorption studies of a complete series of

Mozcl‘(PR3)4 with phosphines of varying the basicity and lability. The

properties of 1(88") excited state of the ditungsten analogues are also

63

presented. Moreover, the electronic absorption spectroscopic results of the

D2 M2X4(PP)2 complexes suggest that studies should not be limited to the

1(88*) excited state, but rather extended to include the plethora of metal

localized excited states at higher energy. Accordingly, the deactivation

processes of the high energy states of both the M2014(PR3)4 D2,, and

M2X4(PP)2 D211 and complexes were probed. Transient absorption studies

reveal varying decay processes of the high energ states of the D2,, and Dzh

complexes. The M02014(PMePh2)4 complex provides an ideal test for the

model emerging from the studies of the MzCl4(PR3)4 D2d and M2X4(PP)2 D2,1

complexes because both the D2,, and D2d isomers can be prepared [148]. The

absorption spectra of the transient species generated in the photochemically

inert solutions are compared with those of photochemical intermediates

observed in halocarbon solutions.

B. RESULTSAND DISCUSSION

1. MgCMPP); (1);.)Complenes

a. PhotochemicallylnertSolutiom

Figure 13 displays the electronic absorption spectrum of

M02014(dppm)2, which is typical of D2}l complexes. The lowest energy

8241(88’) transition maximized at 634 nm (e = 2490 M’lcm‘l) is clearly

identified and a weaker transition at 462 nm (e = 900 M’lcm‘l) is

comparable in energy and intensity to the assigned 82—)1(88x2_y2) transition

in the spectrum of B—M2014(S,S,-dppb)2 [162]. At even higher energy (Inn =

325 nm) is a more intense transition (6 = 5600 M‘lcm'l) that displays a

shoulder at 364 nm. A similar spectral profile for the ditungsten analogue,

64

Figure 13. Electronic absorption spectra of dichloromethane solutions of

M2014(dppm)2, where M = M0 (——) and W (------ ) (a) MogCl4(dppm)2: km”

= 634 nm, (e = 2490 M"1 cm‘l); 462 nm (900 M’1 cm'l); 364 run (2040 M’1 cm’

1); 325 nm (5600 M'1 cm-l) (b) WzCl4(dppm)2: 1m = 710 nm, (e = 2585 M-1

cm-l); 500 nm (598 W1 cm“); 405 nm (1923 M-1 cm-l); 325 nm (2948 M-1

cm‘l).

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shown in Figure 13, is red-shifted. This red shift of the higher energy

transitions is indicative of metal-localized transitions and is in clear

contrast to the expected blue shift of an LMCT transition, as observed for the

D2,, dimers (Imu(LMCT) = 324 nm and 290 nm for M02014(PMe3)4 and

W2Cl4(PMe3)4 complexes, respectively) [121a]. By analogy to the model of

Figure 12 for the torsionally distorted complexes, the energetically

proximate features at 325 and 364 nm absorptions of M02C14(dppm)2 are

consistent with the lacy—)5“) or 1(8--)rt,.,"') and 1(rim-98'") or l(8—)rt,,,"')

transitions, respectively. Irrespective of their specific assignment, the

intense high energy transitions are clearly metal-localized and are not of

LMCT character.

On the nanosecond time scale, no transients are observed with

excitation into 1(88*). Yet as reported in the doctoral dissertation of Dr. I—J.

Chang, short lived absorptions are observed for the 1(88*) excited state on

the picosecond time scale [147]. A more elaborate transient spectroscopy is

observed with high energy excitation. A long-lived (1 ~ 5 us)

nonluminescent transient, whose decay back to ground state is

monoexponential, is observed upon excitation of the higher energy metal

localized transitions (1.“ = 355 nm) of M02014(dmpm)2 in CHzclz. A

transient absorption (Figure 14) collected 1 us after excitation exhibits a

maximum at 520 nm. Unfortunately emission from an impurity in the free

ligand has prevented us from measuring the transient absorption

smctrum at wavelengths less than 460 nm [178].

High energy excitation (Am a 355 nm) of M2014(dppm)2 complexes (M

= M0, W) yields ligand based emission which exhibits a maximum at 460 ,

nm. The fluorescent lifetime of t = 2 us for free dppm [179] agrees with that

for the transient observed upon 355 nm excitation of M02014(dppm)2 as

67

Figure 14. Transient difference spectrum of MozCl4(dppm)2 in CH2012

recorded after 355 nm laser excitation.

A0.0.

68

0010— l

:1 iiiiif

0.....— E if}

' 450 500 550 M[1:100 700 750 '

69

reported in the dissertation of Dr. I-J. Chang. This ligand emission

precludes detection of nonluminescent transients of M02C14(dppm)2

complex arising from high energy metal localized excited states. However,

owing to the red-shifted spectral profile of the ditungsten analogue, ligand

based emission can be avoided with 532 nm excitation of W2014(dppm)2.

Accordingly the laser excitation of benzene solutions at 532 nm was

performed. A transient absorption which decays back to ground state

within 100 as is observed at 440 nm (Figure 15). The transient spectra

recorded 100 ns, 1 us and 4 us after excitation are provided in Figure 16.

Although slight variations are noted in these spectra, each of the profiles

exhibits common features at 390, 420 and 480 nm.

Both the ditungsten and dimolybdenum transient spectra exhibit

features which are comparable to those in the electronic absorption spectra

of the edge-sharing bioctahedral complexes. The feature centered at 480

nm in the ditungsten transient spectrum, and the 520 nm absorption of the

dimolybdenum transient spectrum, agree well with those at 465 and 500 nm

in the electronic absorption spectra of chlg(dppm)2 (Figure 17) [166] and

M02014dmpm)2 (Figure 18), respectively. Prominent absorption features in

the spectral range of 450 to 550 nm are ubiquitous to the spectra of

numerous dimolybdenum and ditungsten edge-sharing bioctahedral

complexes with M(III)—M(III) (d6) bimetallic cores. These include:

MozClg(dppm)2, In,“ = 530 nm [164b]; M02014(dppm)2Br2, km“ a 540 nm

[164a]; and M02015(dppm)2(SPh), In,“ a 540 nm. Similar features are

observed in the ditungsten complexes, although they are generally slightly

blue shifted relative to the dimolydenum complexes, (W2013(dppm)2, 1,,“ =

468 nm [163]; chl4(dppm)2I2, km“ = 500 nm; WZCl4(dppm)2(SPh)2, Am“ =

504 nm [164b]; W2015(dppm)2H, In“ = 464 nm [166]). Accordingly, the

70

Figure 15. Transient kinetics for WzCl4(dppm)2 in CSHG (1.95 x 10‘4 M)

recorded at 440 nm following laser excitation at 532 nm.

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Figure 16. Transient difference spectra of WZCI4(dppm)2 in C6H6 recorded

recorded 100 ns and 4 us after 532 nm laser excitation (see legend).

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74

Figure 17. Electronic absorption spectrum of dichloromethane solutions

of W2016(dppm)2, km“ = 822 nm, (e = 740 M”1 cm'l); 468 run (4800 M‘1 cm'

1); 387 nm (3200 M‘1 cm'l) (Reference 166).

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Figure 18. Electronic absorption spectrum of dichloromethane solutions

0f M02016(dppm)2.

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transient spectra of W2014(dppm)2 in benzene exhibits a distinct maxima at

480 nm, which is slightly blue shifted from that of the dimolybdenum

transient. Moreover, the ditungsten transient spectrum exhibits an

additional feature ~ 390 am that is typically observed in the spectra of

M(III)—M(III) edge-sharing bioctahedral complexes [166, 167a].

(Unfortunately, the dimolybdenum transient spectra could not be collected

in this region.) These data support the transient assignment of an edge-

sharing bioctahedral intermediate formed by the foldover of two chlorides to

the edge-bridging positions as depicted in Figure 19.

The relative intensities of the 390 and 480 absorptions of the proposed

edge—bridging bioctahedral transients differ formed from M-LM excitation

differ from those of the stable and known edge-sharing bioctahedral

complexes. This is not unexpecmd on the basis of the variation in electron

counts of the binuclear cores in the transient of MzCl4(PP)2 (ds) and the

M2016(PP)2 edge-sharing bioctahedral complexes (d6) [180]. Furthermore,

there are differences in the coordination numbers of the two species, with

the bioctahedral transient of M2014(PP)2 featuring two vacant coordination

sites (see Figure 19). It is known that the relative intensities of the

absorptions at ~ 390 and ~ 465 nm in the spectra of edge-sharing

bioctahedral complexes vary significantly with coordination sphere [166,

167a], as exemplified the relative intensities of these maxima in the

spectrum of W2015(dppm)2H (6464 = 460 M‘lcm‘l, 6336 = 3300 M'1cm‘1)

compared with those observed in the spectrum of W2013(dppm)2, (8463 = 4800

M’lcm’l 6336 = 3200 M‘1cm’l). Based on this dramatic difference observed in

replacing a chloride with a hydride, it is not surprising to find that the

relative intensities in the spectrum of the coordinatively unsaturated

79

\

Figure 19. Proposed edge—sharing bioctahedral distortion of 1(81t"‘) (or

1(rc8"‘)) excited state of the MzCl4(PP)2 complexes. Although the only the

former is designated in the diagram, the high energy metal localized

transitions of these complexes have not been definitively assigned (see text).

Transients are not observed from the 1(88*) excited state on the nanosecond

time scale. The metal centers which are oxidized and reduced in the

transient species, relative to the ground state, are denoted with + and -,

respectively. (See footnote 180 regarding the M-«M notation).

80

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81

transient do not directly concur with those in the spectrum of

W2016(dppm)2.

Nonetheless, despite the differences in coordination number and

electronic configurations, the spectral profiles do exhibit common features.

The similarities in the spectral profiles of the d8 transient species and the d°

complexes can be explained by the near degeneracy of their highest

occupied molecular orbitals (HOMO), as indicated by theoretical [181, 167s]

and experimental [167d,e, 175a] studies. The small energy gap between the

HOMO's of the occupied manifold arises from a weak interaction of metal

orbitals of the 8 symmetry which are primarily M—L antibonding in

character. Transitions with this manifold are predicted to lie at very low

energy. Thus the high energy absorptions at ~ 470 and 390 nm of edge-

sharing bioctahedra, in the (16 complexes are likely to lie outside the HOMO

manifold. Thus the visible transitions (likely to be from electron promotion

from o and 1c orbitals, in the (1° and d8 edge—sharing bioctahedra might be

expected to be similar.

An edge-sharing ligand rearrangement of the mixed valence excited

state of M-LM complexes provides stabilization of the oxidized metal center

by achieving an octahedral coordination geometry and a depleted

coordination geometry of halides about the reduced metal center.

Moreover, owing to the assymmetry of the chemically distorted

intermediaite, the mixed valence character is enhanced by this

rearrangement. It should be noted that this structural distortion of

electronically exciwd MzCl4(PP)2 complexes does not occur from the lowest

energy MMCT 1(88") state, but rather from the higher energy l(rt8"') or

1(816") MMCT states. That edge-sharing bioctahedral distortions are not a

photophysical pathway of the 1(88*) excited state could simply be due to

82

insufficient energy. However, an additional feature of the l(8--)rt"') and

l(rt-48'“) excitation that may be important to the formation of the edge-

sharing bioctahedral intermediate is that the metal-metal x bonding

relative to the ground state is significantly weakened. This feature is

expected to enhance formation of a bioctahedral intermediate because

interactions of the metal dyx orbital with the bridging ligands are at the

expense of M—M x interactions. This may well explain why no transients

are formed from 82 -) 1(88") excitation since the M—M x interactions are

unperturbed in the 1(88") excited state relative to the ground state.

Thus a model for the proposed transient absorption studies of the D211

M2014(PP)2 complexes is summarized in Figure 19. Whereas long-lived

transients are not observed on the nanosecond time range upon 82 -) l(88$)

excitation, transients assigned to edge-sharing bioctahedral intermediates

are formed upon excitation of the higher energy 1(8—-)7c*) and l(rt—98'”)

transitions. In the absence of reactive substrates, this intermediate

converts to ground state.

hPhotochemicalb'ActiveSolutiom

Transient species with difi‘erent properties are observed upon 532 nm

excitation of W2014(dppm)2 in the presence of halocarbon solvent substrates.

(The photochemistry observed with these various substrates is described in

Chapter IV). The spectra of solutions containing CH3I and CchHzl were

only measured at wavelengths longer than those absorbed by the substrates

(1. > 430 nm). The initially observed transient spectra with each of the

various substrates are not that of the photoproduct. Moreover the transient

signal decreases with increasing photochemical conversion of the

quadruply bonded species, thereby establishing that the transient is from

83

an intermediate proceeding photochemical reaction. The transient

absorption decay profiles of WzCl4(dppm)2 in CHal (5.0 M)/ CsHs and

011301121 (5.65 M)/ CsHs and neat CH2012 solutions, provided in Figures 20a,

20b, and 20c, respectively, were each collected at 440 nm. In each case a

transient is observed which decays within approximately 1 us to a nonzero

absorption. In the case of CH3I substrate, the transient absorption is

similar to that observed in benzene. Namely, transient spectra recorded 100

ns and 4 us after excitation of the CH31/ CgHg solutions (Figure 21) each

display a dominant feature at 480 nm which is characterisitic of the edge-

sharing bioctahedral complexes. This is not true for the case of CH3CH2I

and CH2012 substrates. The transient spectrum in CH2C12 collected 100 ns

after excitation (Figure 22) exhibits an intense absorption at 380 nm with a

discrete shoulder at 420 nm. The transient spectra with CH2012 and CH31

substrates are most clearly difi'erentiable in the region between 450 and 500

nm. While a sharp increase in absorbance is observed in CHsl, the

transient absorption in CH2012 significantly decreases in this region to

nearly zero at 500 nm (Figure 22). The transient spectra in CH3CH21

solutions collected 100 us after excitation (Figure 23a) do not show any

notable features. At 10 us after excitation (Figure 23b) the absorption shows

a marked decrease between 450 and 500 nm, as observed in CH2012

solutions. Although present data is insufficient for assignment of the

transients in 011,012 and CHscHzl solutions, the spectra clearly do not

exhibit an absorption maximum at ~ 480 nm, which is characteristic of

edge-sharing bioctahedral complexes and they are not consistent with the

photoproduct. We believe that these transients in (311,012 and CH30H2I are

formed with one—electron oxidation of the bimetallic core (vide infra

Chapter IV).

84

Figure 20. Transient kinetics recorded at 440 nm following 532 nm laser

excitation of (a) W2Cl4(dppm)2 (1.5 x 10-4 M) in CH31 (5.0 M)/C,H6. (b)

WzCl4(dppm)2 (2.75 x 10-4 M) in CH30H21 (5.65 M)/C,;H6 (c) W2Cl4(dppm)2

(3.0 x 10-4 M) in CH2012 .

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Figure 21. Transient difference spectrum of W2Cl4(dppm)2 (1.5 x 10" M)

in CH3I (5.0 M)/CGH6 in CSHG recorded 100 ns and 4 us after 532 nm laser

excitation (see legend).

A0D.

87

0.12

. I 100 ns

0.10 I o 4115

0.08 . I '

l I

I

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400 450 500 550 600 650 700

88

Figure 22. Transient difference spectrum of dichloromethane solutions of

W2Cl4(dppm)2 (3.0 x 10’11 M) collected 100 us after 532 nm laser excitation.

89

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Figure23. Transient difference spectrum upon 532 nm laser excitation of

W2014(dppm)2 (2.75 X 10_4 M) in CH3CH21 (5.65 M)/C6H6 recorded (a) 100 us

after excitation (b) 10 us after excitation.

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The transient spectrum of the 1(88"') excited state of M02014(PBu3)‘

has previously been obtained. As expected, the absorption maximum at 450

nm decays with a lifetime that is identical to that of the luminescent

lifetime of the complex. Table I lists the emission quantum yields and

lifetimes for the 1(88,") excited state previously reported for M02014(PR3)4

complexes PR3 = PMe3,PBu3,PEt3, PMezPh, PMePh2 and PHth. The

quantum yields for the series are quite similar except for the case where

PR3 = PMe3, which is an order of magnitude larger. This enhancement of

the luminescent properties of the PMe3 complex is also evidenced in the

lifetime of the 1(88") state. While the lifetime for the PMe3 complex is 135

ns, those of the remainder of the series are typically ~ 10 ns. We have found

that the 1(88") excited state of ditungsten analogues exhibit luminescence as

well. Consistent with the red shift in the 8241(88") absorption, the

ditungsten analogue exhibits an emission band centered at approximately

800 nm, as exemplified by the spectrum of W2014(PMePh2)4 provided in

Figure 24. The 1(88") lifetime and emissive quantum yield of the ditungsten

complex is slightly greater than those of the dimolybedum analogue;

W204(PMePh2)4 (t = 44.6 ns, 4),”In = 0.0437), M02014(PMePh2)4 (‘t = 11.4 ns, item

= 0.0114).

In addition to the luminescent 1(88") excited state, a nonluminescent

transient species has previously been observed upon 8241(88") absorption of

the MozCl4(PBu3)4 complex. We now extend these studies to include series

of dimolybedum and ditungsten MzCl4(PR3)4 complexes with varying with

PR3. The discussion described henceforth is restricted to the transient

93

Table I. Properties ofthe Luminescent 1(88") State ofM2X4(PR3)4‘

PR$ 1018, 300 K) 9cm

PMe3 135 0259

PEts 14 0.013

PBu3 21 0.013

PMePh2 11.4 0011

PHth 0.18 - (a) Data for PR3 = PMe3, PEta, and PBu3 taken fiom reference 118d.

Table 11. Comparison of Structural Properties ofM2X4(PR3), Complexes

and Lifetime of Nonluminescent Transient.

p33 vco (curl-1)ll 6 (deg)‘ M-M-Cl (deg)b M-M-P (deg)b t (ns)

PBu3 2060.3 132 12)

W3 2061.7 132 104.8(2) 1052 (2) 12)

”hp; 20641 118 1122(2) 1023 (1) '

PMegPh 20653 122 108.0(7) 1052(2) 12)

PMePhg 2067.0 136 so

PHPhg 20733 1% 111.0(1) 93.0(1) '

(a) From reference 169. (b) From reference 170. " transient not observed.

94

Figure24. Emission of W2Cl4(PMePh2)4 in toluene solution at ambient

temperature upon 691.5 nm excitation.

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species other than the 1(88") excited state, which we will refer to as the

nonluminescent transient detected by nanosecond transient absorption.

Similar to the M02C14(PBu3)4 result, excitation within the 8241(88’)

absorption (Inc: 532 nm) of dichloromethane solutions of M02Cl4(PR3)4

complexes with PR3 = PMOPhg, PMezPh and PEta gives rise to a transient

which exhibits a monoexponential nonradiative decay back to ground state.

With the exception of M02014(PMePh2)4. (t = 80 ns) a lifetime of 120 us is

observed for the transient as indicated in Table II (pg 93) [182]. Each

transient, whose spectrum was recorded 60 as after excitation, exhibits a

maximum at ~ 400 am that is comparable to the reported 390 nm

maximum for M02014(PBu3)4 in CH30N (e = 1080 M'lcm’l). Although we

recorded spectra between 390 and 460 run, an additional very broad feature

(6 a 640 M'lcm‘l) centered at 740 nm and weak absorption at 470 nm (e = 341

M'lcm’l) have also been reported in the full spectrum of M02Cl‘(PBu3)4 in

CHscN. Of the complexes investigated, (i.e. M02C14(PMePh2)4 (4.3 x 10" M)

and M02014(PBu3)4 (3.8 x 10‘3 M), the lifetimes of the transient generated

from dichloromethane solutions do not change upon addition of excess

phosphine to concentrations as high as 0.50 M. Previous picosecond

transient absorption studies have shown that the 1(88") excited state of

M02014(PM63)4 directly decays back to ground state [139]. A nonluminescent

transient species is not observed. Similar results are observed with

picosecond transient absorption spectroscopic studies of M02014(PHPh2)4

upon 580 nm absorption. The only transient species detected is one that

monoexponentially decays back to ground state in 150 ps, whose absorption

spectrum (Figure 25) is consistent with the 1(88") excited state [139].

The W2014(PR3)4 (PR3 = PEta, PBua, PMeth, PM83) series also shows

a transient with properties comparable to that of the dimolybdenum series

97

Figure 25. Transient difference spectrum of WzCl4(PHPh2)4 in CH2012

(~1.0 x 10'2 M) recorded 2 ps after 590 nm laser excitation.

A0.0.

0.09

0.06

0.03

0.00

0.03

0.06

98

8r

99

upon excitations energetically coincident (Ina = 683 nm) with the 8241(88")

absorption. As was the case for dimolybdenum series, the transient decays

nonradiatively and monoexponentially back to the ground state with the

lifetimes that are virtually invariant to the nature of the phosphine.

However, a notable difference between the dimolybdenum and ditungsten

series is that a transient of the latter is observed when PR3 = PMes. The

transient spectra for PR3 = PMeth, PBus and PEts collected 70 ns after

excitation are provided in Figures 26—28, respectively. The spectra are of

varying quality, but all are similar in as much as an absorption in the UV

region is preeminent. Additionally, weaker absorption features in the

visible region of the spectra are noted. These are most clearly seen in the

spectrum of MozCl4(PMePh2)4 in THF solutions, which shows two weak

absorptions at 400 and ~ 475 nm. The latter may resolve into a double

maximum at 460 and 500 nm, but these features are within experimental

error limits, and in the cases of PEt3 and PBu3, are not resolved. A distinct

shoulder at 345 nm is observed on the rising UV absorption in all of the

spectra. This shoulder is most clearly evident in the spectrum of the PEta

complex, which was collected far into the UV region. For clarity, the

spectrum is expanded in an inset. Higher energy excitation of the D2,,

dimolybdenum or ditungsten complexes in hexane does lead to the

production of transients on the nanosecond time scale.

In the case of the dimolybdenum complexes, the occurrence of a

transient appears to be related to the nature of the phosphine. An

important result, and one that was previously observed, is the anomalous

behavior of the PMe3 complex. Excitation of Mo,Cl,(PMe,), produces l(2)15“)

luminescence which directly decays back to ground state. Only the 1(88")

excited state is observed with lifetime of 135 ns, and no additional transient

100

Figure26. Transient difference spectrum of WZCI4(PMePh2)4 in THF (~

6.0 mmol) recorded 70 ns after 683 nm laser excitation.

101

0.20

0.15-{i

o: l00.10- I

<1

0.05P !.

ii iii iii!

ii . t ,l l L l

0'00“ 350 400 450 500 550

I/nm

102

Figure 27. Transient difference spectrum of W2014(PBu3)4 in hexane (~ 6.0

mmol) recorded 70 ns after 580 nm laser excitation.

6.0

A0.0.

0.05

0.04

0.03

0.02

0.01

0.00

-0.01

103

104

Figure 28. Transient difference spectrum of WZCI4(PEt3)4 in toluene (~ 6.0

mmol) recorded 70 ns after 683 nm laser excitation.

10 5

0.10

I

0.03 '-

O:

O 0.3 P

- 6.0 <

0.02 r ' ' - '

d u

d 0 -- . ' 4' ILL-.1

4 300 350 400

0.01 M"m

I

I I I

ul".u . u.. l

0,00 '4Ia.._.

. l 1 I 1 I .

300 400 500 600

1 0 6

is detected. The absence of a nonluminescent transient from M02C14(PMe3)4

is informative. First, it can account for the fact that the quantum yield of

emission (0.26) and lifetime (140 ns) of the 1(88*) excited state of

M02Cl4(PMe3)4 are each an order of magnitude larger than M02014(PR3)4

complexes from which nonluminescent transients are observed (see Table

I). Moreover, the anamolous behavior of M02014(PMe3)4 and

M02014(PHPh2)4 provides some insight into the electronic and chemical

nature of the transient species generated from the remainder of the series.

The previously proposed assignments of 3(88*) and 3(it8"') seem unlikely

since the slight perturbation upon replacing ethyl in M02014(PEt3)4 with

methyl in MozCl4(PMe3)4, or in replacing a methyl in M02Cl4(PMePh2)4 with

a hydride in M02C14(PHPh2)4 seem insufiicient to significantly alter the

intersystem crossing rate between the 1(88‘) state and these triplet states.

As was the case for the D21, complexes, the transient of dimolybedum

D2,, does exhibit spectral features which are similar to the cl6 edge—sharing

bioctahedral complexes. Most noted is the intense feature at 390 nm

reported of the dimolydenum transient does correlate with that reported for

d° edge-sharing bioctahedral complex M02016(PMePh2)4 at 400 nm (sh) (e =-

2400 M’lcm‘l). However, differences are noted in the lower energy

absorptions of the transient at 470 (e = 341 M‘lcm'l) and 740 nm (e = 640 M‘

lcm'l) relative to those of the d° bioctahedral complex at 526 nm (e = 800 M‘

lcm"1) and 650 nm (e = 640 M'lcm‘l). An alternative bioctahedral

geometry that is typically observed observed upon oxidation of the more

structurally flexible M2X4L4 complexes that do not contain bridging ligands

is a confacial bioctahedron. In fact the edge-sharing MOzCls(PR3)4

complexes have been shown to slowly disproportionate to confacial

M02016(PR3)3 complexes and monomers, indicating that the confacial

107

bioctahedral geometry is thermodynamically favored over the edge-sharing

bioctahedral geometry [175a]. This is also true with the“ ditungsten

analogues. While the edge-sharing W2016(PEt3)4 complex has been isolated

and characterized by X-ray crystallography, “P NMR spectra of crystalline

samples of this complex dissolved in toluene at ambient temperature

exhibit signals attributable to free phosphine and the confacial bioctahedral

WzClg(PEt3)3 complex [153].

Although the distinction between a confacial and edge-sharing

bioctahedral intermediate is difficult for the dimolybdenum complexes,

owing to the similarities in the spectral profiles of M(III)—M(III) confacial

and edge-sharing bioctahedral complexes [183],the spectra of edge-sharing

and confacial bioctahedral ditungsten complexes are distinguishable. The

W2(III,III)Clg(PEt3)4 (D2,!) edge-sharing bioctahedral complex exhibits an

absorption at 390 nm (Figure 7), as do the D211 W2(III,III)ClG(PP)2

analogues. But the d6 confacial bioctahedral complex, W2(III,III)C16(PEt3)3

has a distinct absorption at ~ 330 nm (Figure 29). The prominent feature at

~ 345 in the the d8 ditungsten transient spectrum is in good agreement with

the UV absorption of the d° confacial complex, and is distinguished from

the 390 absorption of the edge-sharing bioctahedral complexes. Moreover

the very weak absorptions of the transient at ~ 475 and 400 nm in the latter

are energetically similar to those of the confacial bioctahedral complex.

However discrepancies are noted in the relative intensities of the bands

which we believe may arise from the difi'erences in electron counts of the d8

transient as compared to the (1‘5 species. For instance the electronic

spectrum of a complex formed by addition of dichloroiodobenzene to

W2C14(PBu3)4 is shown in Figure 30. Although this complex has not been

definitively characterized, its EPR spectrum is consistent with a one-

108

Figure 29. Electronic absorption spectrum of a dichloromethane solution

of the confacial bioctahedral complex W2C16(PEt3)3, km” = 510 nm, (e = 2077

M-1 cm-l); 328 pm (2077 M"1 cm"1).

1 solution

(E = 2077

109

X8

1

600

700

l

500

Wavelength

/nm

300

eoueqiosqv

Figure29

110

Figure 30. Electronic absorption spectrum of the product obtained from

thermal reaction of one half molar equivalent of dichloroiodobenzene to

CH2012 solutions of WzCl4(PBu3)4, 1...... = 500 nm, (e = 969 M-1 cm‘l); 400

nm (588 M-1 cm-l); 330 nm (3322 M‘1 cm-l).

lll

Absorbance

1 l l l l

300 400 500 600

Wavelength / nm

Flames!)

112

electron oxidized (17 species (vide infra, Chapter IV). The absorption

features at 500, 390 and 330 nm, which are energetically similar to those of

the confacial bioctahedral W2(III,III)Cls(PEt3)3 complex, indicate that it has

the typical confacial bioctahedral geometry observed upon oxidation of the

M2Cl4(PR3)4 complexes. The relative intensities of these absorptions of the

d7 complexes are markedly similar to those of the d8 transient species.

These data support the assignment of the transient species as a

confacial bioctahedrally distorted intermediate depicted in Figure 31. The

three chlorides assuming bridging positions and provide the stabilizing

octahedral coordination geometry about the oxidized metal center in the

MMCT excited state.

The similarities in the spectral features of the d8 transient with those

of the d7 and (16 complexes, despite the obvious differences differences in

electronic configuration are consistent with our observations that the

energies of the absorptions of confacial bioctahedral complexes appear to be

somewhat independent of oxidation state. This contention is based on the

spectral changes accompanying the addition of dichloroiodobenzene to

W2014(PBu3)4 shown in Figure 32. A decrease in the 82->1(88’) absorption of

W2Cl4(PBu3)4 and increases in NIR (I. = 1420 nm) and visible absorptions

with maxima at 335, 404, and 430 and 500 nm is observed. Four isosbestic

points are maintained during these changes until the 8241(88') absorption

maxima of the quadruply bonded W2(II,II) species have completely

disappeared. The solid line spectra correspond to the net addition of one

half molar equivalent of the oxidant. The presence of the isosbestic points is

consistent with smooth conversion of the quadruply bonded binuclear

complex to a W2(II,III) intermediate. Subsequent spectral changes upon

further addition of oxidant are denoted with dashed lines. Reaction of the

113

Figure 31. Proposed confacial bioctahedral distortion of the 1(88*) excited

state of WzCl4(PR3)4 complexes. See footnote 180 regarding M---M

designation.

114

8*

“V Cl/\/\P’PZS

8

PR3 Cl

o0! “P123

M_______ M"

Cl/ I PR3

PR3 Cl

Figlne31

115

Figure32. Electronic absorption spectral changes accompanying addition

of dichloroiodobenzene to CH2C12 solutions of WzCl4(PBu3)4, km” =665 nm,

(e = 3813 M"1 cm’l). The solid lines represent net addition of 0.5 equivalent

and the dashed lines represent further addition.

116

89

32.5..—

E:

\255.263

08

pcon

'il

‘

o8

I‘-

I-

int

-‘DII

I'l-

.

eoueqrosqv

117

W2(II,III) intermediate is evidenced by a decrease in the NIR absorption

and loss of all four isosbestic points. The final spectrum shows no NIR

absorption and the visible spectrum is similar to that of the independently

prepared W2(III,III) complex, W2(III,III)Clg(PBu3)3 (Figure 8). Despite the

change in the relative intensities of the visible transitions in the spectra, the

energies of the absorptions remain relatively unperturbed for the

conversion of the W2(II,III) to a W2(III,III) species.

Like the edge-bridging bioctahedra, the visible and UV transitions

are not predicted to involve orbitals within the HOMO—LUMO manifold

[184, 185]. Although the M—M bond in the (16 complexes may be formally

considered a triple bond, the extensive metal-ligand interactions in

complexes with bioctahedral geometries limits the metal-metal

interactions. Thus the energy gap between the HOMOs of (l6 and (18 species

is exceedingly small and transitions within this manifold are predicted to

lie in the NIR. As a point of reference, an absorption at 683 nm in the

spectrum of the M02013“ complex represents a transition from the al' (M—

M o) orbital to a metal-ligand antibonding orbital that lies well above the

LUMO level [185]. Thus the transitions between 330 and 550 nm of the d‘

and (17 complexes, which are comparable to those of the d8 transient species,

should lie well outside the manifold, thereby accounting for the similarities

of the spectra of confacial bioctahedra of difi'erent electron counts.

The transient absorption studies of the Dzd M2014(PR3)4 are

summarized in Figure 31. A bioctahedral intermediate is formed upon

excitation of the lowest energy MMCT 8241(88") transition. The spectra of

the ditungsten transient is consistent with a confacial bioctahedral

intermediate. The spectral similarities of the d6 edge-sharing and

confacial bioctahedral dimolybedum complexes precludes definitive

1 1 8

assignment of the dimolybedum transients, however the dependence of the

transient formation on the nature of the phosphine can be reconciled with

the assignment of either an edge-sharing or confacial bioctahedral

intermediate. Table II lists the cone angles of the phosphines and the CO

frequencies of Ni(CO)3(PR3) complexes which provides a relative measure of

the basicity of the phosphine [186]. Inspection of these data reveal that the

basicity of the phosphine is not an important controlling factor. Namely,

the complexes from which no nonluminescent transient is observed

M02014(PMe3)4 and MOZCI4(PHPh2)4, are ligated by one the most basic

phosphines, PMe3, and least basic phosphines, PHth. Similarily there is

no obvious correlation with cone angle of the phosphine. Although the cone

angles of PMe3, is the smallest in the series, the cone angle of PHPh2 is

larger that that of PMezPh. Moreover there is no interrelation between

these properties and transient formation. As discussed by Tolman, the

lability of the phosphines dependence can be correlated to both steric and

electronic effects on the basis of this plot [186]. Figure 33 which shows a plot

of cone angle vs CO stretching frequency of the Ni(CO)3(PR3) complexes.

The least labile phosphines located at the lower left hand corner of the plot

are strongly basic and sterically unhindered. The positions of PMe3 and

PHPh2 ligands on this plot indicate that phosphine dissociation is not a

controlling factor governing from formation of the transient. There is

however, a correlation of the transient with Mo—Mo—P and Mo—Mo—Cl

bond angles of the quadruply bonded dimers. Inspection of Table II reveals

that nonluminescent transients are not observed for complexes that have

the smallest Mo—Mo—P angle and the largest Mo—Mo—Cl angle. This

correlation may be rationalized in the context of a bioctahedral distortion

which involves the bend-over of chlorides to the bridging positions (is a

119

Figure33. Plot of phosphine v(CO) stetch vs cone angle of phosphines (a)

PBu3, (b) PEt3, (c) PMe3, (d) PMezPh, (e) PMeth, (0 PHth. Photoinduced

chemical intermediates are not observed from MOZCI4(PR3)4 where PR3 =

PMe3 and PHth , which are marked with *.

0(deg)

120

140

Be

an Elb

130i

Elf'

Eld

120 -

:10"

110 l a l x l a l

2055 2060 2065 2070 2075

V00 (cm")

W33

2080

121

decrease in the Mo—Mo—Cl angle) and concomitant increase in the Mo—

Mo—P angle. Namely, transients are not observed from complexes with

the greatest bond M—M—Cl angles and smallest M—M—P bond angles,

where a greater degree of structural distortion is predicted. No additional

transients species are observed on the nanosecond time scale with either

the dimolydenum nor the ditungsten D2,, M2014(PR3)4 complexes upon

higher energy excitation.

These results are in contrast to the transient absorption studies of the

MzCl4(PP)2 (D2).) complexes summarized in Figure 19. The differences in

the excited state properties of the D24 and D211 complexes can be accounted

for by the variation of their chemical and electronic structure. In the case

of the M2Cl4(PP)2 complexes, a confacial geometry is precluded by the

bridging bidentate phosphines. Thus the absence of transient species upon

8241(88’) absorption is consistent the assignment of a confacial

bioctahedral intermediate in the case of the D2,, M2014(PR3)4 complexes.

However, chemical structure cannot account for the differences observed in

the high energy states. Both edge-sharing MZCI4(PP)2X2 (D2).) and

MzCl4(PR3)4X2 (Dzd) complexes exist and thus the edge-sharing bioctahedral

intermediate could be achieved by simple foldover of chlorides to the

bridging position in both the electronically excited D2,, and D21, complexes.

That this distortion occurs upon high energy absorption of the the latter

complexes and not the D2,, complexes, can be explained by difl'erences in the

electronic nature of the excited states. Ligand-to-metal charge transfer

transitions are present in the high energy spectral region of the Dzd

complexes, while transitions at this energy in the spectra of the 132}.

complexes are clearly metal localized and are energetically coincident with

1(rt-48'") and 1(8-->x"') transitions. The charge transfer between metal

122

centers in the metal localized transitions of the D211 complexes yields a

mixed valence metal core. The observed edge-sharing bioctahedral

distortion provides cooperative stabilization ofboth the oxidized and reduced

metal centers in the mixed valence bimetallic core. In contrast, the net

one-electron reduction of the metal core which results from the LMCT

transition of the Dzd complexes does not facilitate formation of biOctahedral

type geometries that stabilize Mo(III) and W(III) centers. This model is

supported by the studies of the D2,, and D21, isomers of M02014(PMePh2)4

described below.

3. MW),(DalDu) learners

The green isomer of MozCl4(PMePh2)4 has been proposed to have a D2,,

configuration of phosphine ligands [148, 188]. This hypothesis is supported

by Figure 34 which displays the absorption spectrum ofTHF solutions of the

structurally characterized blue Dzd isomer M02C14(PMePh2)4 superimposed

over the spectrum of THF solutions containing the green isomer. While the

lowest energy 82 -) l(88") transition of both isomers is centered at 600 nm,

there are several differences in the spectra. First, the spectrum of the

solution containing the green isomer displays an absorption band at 450 nm

that is not observed in the spectrum of the Dad isomer. This distinguishing

absorption is comparable to those centered at 462 and 470 nm in the spectra

of M2014(dppm)2 and B—M2014(S,S,-dppb)2 respectively. This transition

appears to be characteristic of D2,, and D2 complexes and, as discussed, has

been tentatively assigned to 8 —) 8x2_y2. Second the relative intensities of the

82 —) l(88"') absorption band and the transition centered at 325 nm clearly

vary for the two solutions. As discussed, this transition has been assigned

as a LMCT in the case of the D24 complexes based on the characteristic blue

123

Figure 34. Electronic absorption spectrum of THF solutions of D2,,

MozCl4(PMePh2)4 (—) and green isomer of MozCl4(PMePh2)4 (-----) that is

proposed to have D21, configuation of phosphines.

124

39:53

E:

\589935

com

b------’

eoueqiosqv

125

shift in the spectrum of the ditungsten analogue. The relative intensifies of

the absorption maximized at 330 nm (A = 2.1) and the 82 —) 1(88*) transition

(A = 1.0) in the spectrum of the green isomer are comparable to those of

analogous transitions at 325 nm (e = 5600 M'lcm“1) and 634 nm (e = 2490 M”

1cm“) in the spectrum of MozCl4(dppm)2 (Figure 15). The lower energy

transition in the spectrum of Mo2Cl4(dppm)2 is clearly the 82 —) 1(88*)

absorption band and, as discussed, the higher energy transition is metal

localized as well.

Excitation of a THF solution containing only the blue isomer, gives

rise to an emissive transient observed upon 355 nm whose energy (km, =

460 nm) is consistent with the reported emissive localized excited state of

the ligand [179]. No additional transient is observed. As expected, high

energy ligand-based emission is also observed upon excitation at 355 nm of

a THF solution containing the green isomer of M02014(PMePh2)4, however,

in contrast to solutions containing only the Dzd isomer, an additional non-

luminescent transient is also observed that decays monoexponentially back

to the ground state with a lifetime (1 = 3 us) and absorption (11,,“ = 520 nm)

provided in Figure 35 which is virtually identical to that of M02014(dmpm)2

transient. These results directly concur with those observed upon high

energy excitation of the D24 M201‘(PR3)4 complexes and the D2,, M2014(PP)2

complexes; namely, nonluminescent transients are only observed on the

nanosecond time scale with the D2,, complex and the transient spectrum is

consistent with an edge-sharing bioctahedral intermediate.

126

Figure35. Transient difference spectrum of green isomer of

MozCl4(PMePh2)4 in THF collected 1 us after 355 nm laser excitation

A0.0.

0.006

0.004

0.002

0.000

127

l l I - 550 600 650 700

Mnm

750

CHAPT‘ERIV

PHOTOINDUCEDREDOXCHEMISTRY

A. BACKGROUND

The photochemistry of M-LM complexes is generally characterized by

one—electron reactions promoted by ultraviolet irradiation. For instance,

despite long lived 1(88”) excited states for the M2Cl4(PR3)4 complexes, the

single photochemical reaction of these complexes reported to date requires

high energy excitation. Photolysis of CH2012 solutions of MozCl4(PEt3)4 at

254 nm yields products that are believed to be trichlorobridged binuclear

species [189]. Until very recently, the requirement for ultraviolet

irradiation in photochemistry of quadruply bonded metal-metal binuclear

complexes was completely general. Under these conditions, electronically

excited M02018" [1901,Mo2(so,),4- [191], M02(I-IP04)4" [192a], and Mo,(eq)‘*

(aq = H20) [190], in acidic media are capable of the two electron reduction of

protons yielding dihydrogen. The photochemistry of "M0203" has been

generalized with a study of M02(HP04)4“. The mechanism for H2

production involves two one-electron oxidations of independent bimetallic

cores. Detailed electronic absorption spectroscopy and action spectra

128

129

cores. Detailed electronic absorption spectroscopy and action spectra

identify the photoactivated state in this class of molecules'to be "(1trt*)"

[192a]. In this regard, the two electron reduction of dichloroethane to

ethylene effected upon 1(88*) irradiation of M02[02P(OCGH5)2]4 marks an

important development in the design of photochemical schemes of M-LM

complexes, because it represents the first example of multielectron

photochemistry accessible in the visible spectral region [191b].

Nevertheless, the overall transformation is again achieved by coupling the

redox function of independent metal cores. The final products are the

M02(II,III) mixed valence species, M02[(OCsH5)2]4Cl, and ethylene.

The plethora of thermal two electron oxidation M2014(PP)2 complexes

with strong oxidants such as halogens (Xz) [167] suggests that two electron

chemistry can be promoted at a discrete bimetallic core. The

thermodynamic accessibility of two—electron oxidation products may in

part from the structural rearrangement of the ligation sphere to a

bioctahedral geometry. Every oxidation reaction of the M2014(PP)2

complexes reported to date gives rise to an edge-sharing bioctahedron,

although in some cases products are obtained that do not correspond to

simple addition of substrate. For example, the edge-sharing bioctahedral

complex MozClg(dppm)2(SPh) has been structurally characterized from

reactions of MozCl4(dppm)2 with PhSSPh [167b]. The prevalence of

alternative pathways not involving simple addition of substrate is even

more prominent in the thermal oxidation chemistry of the M2014(PR3)4

complexes. Reaction of toluene solutions of M02I4(PMe3)4 with 12 results in

formation of the confacial bioctahedral complex M0217(PMe3)2‘, which is

deficient in phosphine [170b]. A similar product M02017(PMe3)2' is obtained

from reactions of MozCl4(PMe3)4 with PhIClz in CHzClz. In the case of the

1 3 0

thermal reaction of M02014(PEt3)4 with C014, all of phosphines are displaced

to yield M020193' [193]. Nonetheless, these reactions indicate that a

confacial bioctahedral geometry is important for stabilization of the metal

core upon its oxidation. This is most clearly demonstrated by a

comprehensive study of electrochemistry and photochemistry of Rezclaz'

[173]. One-electron oxidation of quadruply bonded Re2(III,III)Clsz' in the

presence of excess Cl‘ yields the confacial bioctahedral R02(III,N)0192-

complex. Similar chemistry may be affected photochemically upon

82-)1(88‘) absorption of Re20182'. One electron quenching of the 1(88‘)

excited state by acceptors TCNE (tetracyanoethylene) or DDQ (2,3-dichloro-

5,6-dicyano-1,4-benzoquinone) followed by trapping with a chloride anion

results in production of R82C192- [173].

The importance of bioctahedral geometries to the design of

multielectron pathways is presaged by the transient absorption

spectroscopy of MZCl4(PR3)4 and M2C14(PP)2 complexes presented in Chapter

III. Photoreactions of these structurally distorted bioctahedral

intermediates with various substrates is described in this chapter.

Independent electrochemical studies provide some insight into the

mechanisms of the photochemistry.

BRIBULTS

1. Photo-oxidationChemistryAccompeniedbyPhosphineDisplacement

a. “EDWINd W4CI4M4With@2013

Although dichloromethane solutions of W2014(PBu3)4 irradiated with

low energy light (A > 610 nm) are stable indefinitely, photolysis does proceed

131

when the LMCT transition is excited (I > 375 nm). The spectral changes

associated with the photochemistry are shown in Figure 36. A decrease in

the 8241(88‘) absorption maximum (8 = 3813 M‘1cm‘1) of the quadruply

bonded complex WZCl4(PBu3)4 is accompanied by the appearance of

absorptions exhibiting maxima at 1420 nm (e = 1628 M‘lcm‘l), 335 nm (e =

3164 M-‘cm-l), 404 nm (e = 744 M-1cm'1)and 430 nm (e = 725 M-‘cm-l).

Four isosbestic points are maintained throughout the course of the reaction

indicating that the reaction involves smooth conversion to a single product.

Although ensuing thermal decomposition of the photoproduct has

prevented definitive characterization of the photoproduct, its EPR,

electronic absorption, and NMR spectrum is informative in this regard.

The EPR spectrum of the photoproduct in a 2-MeTHF/CH2C12 glass at

77K (Figure 37) is consistent with a paramagnetic species in an axial

environment, g" = 2.020, g1: 1.835. The paramagnetism of the photoproduct

is indicative of a W2(II,III) species having one unpaired electron, since a

two-electron oxidized complex is diamagnetic. Whereas the photoproduct is

a W2(II,III) core, its electronic absorption spectrum varies significantly

from that of the mixed-valence species, W2(II,III)C14(PBu3)+, which retains

the D2,, structure of its quadruply bonded counterpart. The NIR transition

of the photoproduct (21,," = 1420 nm, e = 1628 M‘lcm‘l) is red-shifted and

less intense than that of W2(II,III)C14(PBu3)4'*' (Am, = 1510 nm, e = 2243 M‘

lcm“) whose spectrum is shown in Figure 10. Conversely, the spectral

features of the photoproduct are very similar to the tentatively assigned

confacial bioctahedral W2(II,III) intermediate formed by addition of

dichloroiodobenzene to WzCl4(PBu3)4 presented in Chapter III (Figure 29).

Further support for the production of a confacial bioctahedron comes

from electrochemical trapping studies. The cyclic voltammogram of

132

Figure36. Electronic absorption spectral changes during photolysis (Inc

> 375 nm) of deoxygenated dichloromethane solutions of WZCI4(PBu3)4 at

22°C. The total time of photolysis was 20 minutes.

133

82.6...—

E:

\5929:;

8m—

83

eoueqiosqveoueqiosqv

134

Figure 37. X-band (9.598 GHz) EPR spectrum of a 2-MeTHF/CH2012 glass

of photolyzed CH2012 solutions ofW2014(PBu3)4 at 77°K.

135

5958—:—

mmamo

\Romegages.

comm

Gown

—1

8mm

1

800

Ausueiui

136

photolyzed solution exhibits a reduction wave at -1.15 V and irreversible

oxidation waves at +0.14 V and +0.36 V (Figure 38c). This cyclic

voltammogram is similar to the that of a dichloromethane solution of

W2Cl4(PBu3)4 containing tetrabutylammonium chloride shown in Figure

38b. In the presence of C1‘ the cathodic component of the W2CI4(PBu3)4*’°

redox wave at —0.35 V (figure 38a) significantly decreases and new anodic

waves at +0.14 V and +0.36 V along with a reversible cathodic wave at -1.15

V are generated. Identical changes are observed in the cyclic

voltammogram when chloride anion is added to bulk electrolyzed solutions

of W2(II,III)C14(PBu3)‘+. Moreover, the UV-visible spectral profile of these

solutions show the 1420, 403 and 430 nm features of the photoproduct. The

observed electrochemistry is consistent with the thermal reaction studies of

W2014(PBu3)‘ in Chapter III. One electron oxidation of quadruply bonded

species in the presence of chloride typically yields confacial bioctahedra

species. For instance electrochemical oxidation of Re2(III,III)C182‘ in the

presence of 01-, yields the Re2(III,IV)0192- complex [173]. In the case of

M2Cl4(PR3)4 complexes, oxidation of the complex with subsequent trapping

by halide is usually accompanied by a concommitant displacement of

phosphine. For instance, the reaction of M02014(PMe3)4 with PhIC12 and

M021‘(PMe3)4 with 12 yield M02Cl1(PMe3)2' and M0217(PMe3)2', respectively

[170]. In these reactions simple halide addition to yield M2X5(PR3)4

confacial structures is supplanted by the addition of excess halide to the

core. Phosphine dissociation occurs to maintain the confacial bioctahedron

geometry. This also appears to be the case in the photooxidation reaction.

The spectral similarities of the electrochemical oxidation product

photoproduct and the product of the oxidation of W2014(PBu3)4 by P111012

suggest the photoproduct to be W2(II,III)C13(PBu3)3‘ or

137

Figure 38. Cyclic voltammograms of CH2C12 solutions with 0.1M TBAPF6

of (a) wzcmpsug)4 (b) W2Cl4(PBu3)4 and 3.2 x 10-2 M THACl (c) photolyzed

solutions of W2Cl4(PBu3)4.

138

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W2(II,III)C17(PBu3)2’. In this case free phosphine should be observed in

the photoreaction mixture. Accordingly, NMR studies were undertaken.

Figure 39a shows the 1H NMR specrum of photolymd solutions. Very

weak and broad resonances of the protons of the phosphine ligands between

2.8 and 4.2 ppm are further evidence of a paramagnetic W2(II,III) center.

Similarly, very weak and broad 31P signals are observed for coordinated

phosphine (Figure 39b). Of greater interest is an intense peak at +34.4 ppm

(vs 85% H3PO4). This signal is comparable to that observed for the

chloromethyltrimethylphosphonium cation (5 = 433.3 ppm) [194].

Unfortunately, the tributyl derivative has not been reported, however it

appears that the chemical shift does not vary significantly with the

substituents on the phosphine, as exemplified by the resonances of various

diphenyl derivatives [thRPCHzClP which are all in a region ~30 ppm

comparable to that of the trimethyl analogue [195]. The intensity ofthe peak

relative to those of the other signals indicates that [Bu3P(3I-12Cl]+ exists free

in solution, and thus provides evidence for phosphine displacement during

the photoreaction.

Although it remains for the photoproduct to be definitively

characterized by X-ray analysis, the issue of primary interest to us is that

the photolysis involves one-electron oxidation of the bimetallic core. Indeed

the one-electron reduction of the core upon ligand-to-metal charge

transfer would facilitate such reactivity. This one-electron photochemistry

arising from LMCT absorption is in contrast to photoreactions of

M02014(PR3)4 complexes with PhSSPh which proceed from MMCT 5241(88’)

absorption.

140

Figure 39. (a) 1H and (b) 31P NMR spectra of products from photoreactions

of W2Cl4(PBu3)4 with CH2C12. The spectra were recorded in CD2012

solutions at —80 °C.

141

(a)

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142

b. PhotmeacfionofMozChTBug4wifl1PhSSPh

Benzene solutions of M02C14(PR3)4 (PR3 = PBu3 and PMeth)

containing a ten-fold excess of phenyl disulfide (PhSSPh) at ambient

temperature are not indefinitely stable in the absence of light. Thermal

reaction is evidenced by the spectral changes shown in Figure 40 for the

case of PR3 = PBu3, where an absorption at 480 nm appears as the 82->1(56')

transition of M02C14(PBu3)4 decreases. Similar spectral changes are

observed with the MozCl4(PMePh2)4 complex. Reactions are accelerated by

irradiation of the 5241(88‘) transition (2. > 530 nm). More importantly, the

spectral changes associated with the photolysis differ from those of the

thermal reactions. An exemplary spectrum for the photolysis of

M02C14(PBu3)4 is provided in Figure 41a. The photolyzed solutions exhibit

absorption in the region between 500 and 550 nm which is not observed in

the spectra of the thermal reactions. Consistent with the lack of isosbestic

points, two products have been separated from photolyzed solutions of

M02Cl4(PBu3)4 by column chromatography. One product exhibits an

absorption maximum at 480 nm which is consistent with that of the

thermal product. Insuficient data precludes assignment of this product at

this time. Figure 41b shows the spectrum of an additional product which is

unique to the photoreaction with an absorption maximum at 540 nm. The

FABMS of this photoproduct reveals a parent ion cluster centered at 1173

amu, consistent with M02C14(PBu3)2(SPh)4, along with three fragments

corresponding respectively to loss of one SPh unit, one PBu3 and one of each

(Figure 42).

The M02014(PBu3)2(SPh)4 product corresponds to addition of two

equivalents of diphenylsulfide with accompanying displacement of two

143

Figure 40. Electronic absorption spectral changes during thermal

reactions of benzene solutions of MozCl4(PBu3)4 containing a ten fold excess

of PhSSPh. The total reaction time at ambient temperature was four hours.

144

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Figure 41. (a) Electronic absorption spectral changes during photolysis

(A.exc > 570 nm) of benzene solutions of M02C14(PBu3)4 containing a tenfold

excess of PhSSPh. The total reaction time at ambient temperature was 20

minutes. (b) Electronic absorption spectrum of a photoproduct isolated

from photolyzed solutions by column chromography. The FABMS of this

product is provided in Figure 42.

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Figure42. Fast atom bombardment mass spectrum of a photoproduct

isolated from photolyzed solutions of M02C14(PBu3)4 containing PhSSPh.

Selected assignments of the clusters in the spectrum are: (a)

M02014“)BU3)2(SPh)4 (b) M02014(PB113)2(SP}1)3 (C) M02014(PBI13XSPh)4.

RelativeAbundance

800 900

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149

phosphine ligands. The observed phosphine displacement is not entirely

surprising based on independent studies which ' show that

thermodynamically unfavorable phosphine substitution reactions are

promoted by 8241(58') absorption of M02C14(PBu3)4 [196]. Nonetheless,

although quantitative oxidative addition has not been achieved, this

reaction with PhSSPh exemplifies the ability of MzCl4(PR3)4 complexes to

effect multielectron transformations upon 8241(85‘) absorption. However

these studies suggest that further pursuit of such photochemical schemes

should be restricted to M2Cl4(PR3)4 complexes with less labile phosphines

which are less easily displaced such as PMe3, and PMe2Ph and bidentate

phosphine ligands.

2. PhotooxidafionAmompaniedbyDiqropa-fionafion

On the basis of the results of Section 1b, the photolysis ofPhSSPh with

Mo2X4(PR3)‘ possessing less labile phosphines were undertaken.

Photoreactions are not observed within 10 h of irradiation (7L > 530 nm) of

M02C14(PMe3)4 complex in the presence of a tenfold excess of PhSSPh.

Photoreactions do proceed under the same conditions with the

M02C14(PMe2Ph)4 complex, however the photooxidation chemistry is

markedly different from that of the MozCl4(PBu3)4 complex. Furthermore

unlike the reactivity of the M02Cl4(PBu3)4 complex, thermal reactions of

PhSSPh with M02C14(PMe2Ph)4 do not proceed at ambient temperature.

The prompt reaction upon irradiation of the 82a1(86') absorption

M02C14(PMe2Ph)4 containing a ten-fold excess PhSSPh at A > 530 nm is

evidenced by the spectral changes shown in Figure 43. An isosbestic point

is maintained at 553 nm during the photolysis and two discrete maxima at

150

Figure43. Electronic absorption spectral changes during photolysis (Km.

> 570 nm) of dichloromethane solutions of MozCl4(PMe2Ph)4 containing a

ten fold excess of PhSSPh at 22°C. The total reaction time at ambient

temperature was 24 hours.

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495 and 390 nm appear with the concommitant decrease of the 62—)‘(883

absorption. No NIR absorption is observed during the photolysis. The

FABMS of the photoproduct (Figure 44) isolated by addition of hexane,

reveals a parent ion cluster at 1128.8 amu consistent with

M02C15(PMe2Ph)4(SPh). Thus phosphine substitution is not observed with

the less labile PMezPh, but rather one observes addition of one Cl‘ and one

SPh‘ group to the bimetallic core.

Unlike the M02014(PMe2Ph)4 (Dzd) complex, acetone solutions of

M02C14(dppm)2 (D211) containing a forty-fold excess of PhSSPh are

photochemically inert upon irradiation at wavelengths coincident with the

82-41(88') absorption. However, photolysis does proceed upon higher energy

excitation (A. > 436 nm). The requirement for high energy irradiation is

completely general to all of the observed photoreactions of the D211

complexes, both bimolecular and pseudo first—order.

While solutions of M02C14(dppm)2 in the presence of a twenty fold

excess of tolyl disulfide (TolSSPhTol) are stable in the complete absence of

light, irradiation at x > 436 nm results in a concomitant decrease in the

8241(88’) absorption maximum at 634 nm with increases in absorption at

525 and 450 nm (Figure 45). No absorptions were observed in the NIR

region. Although isosbestic points are maintained throughout the reaction,

FABMS analysis of the solid precipitated with hexane with reveals the

presence of two products (Figure 46). One product, exhibiting a parent ion

cluster at 1260 amu, is the pentachloro product M02C15(dppm)2(STol), as is

the case for the photoreactions of M02014(PMe2Ph)4. However a second

product observed at 1348 amu is consistent with the oxidative addition

product, M02C14(dppm)2(STol)2. Similar reactions are observed with the

phenyl disulfide. The absorption spectrum of the major product is that of

153

Figure 44. Fast atom bombardment mass spectrum of a photoproduct

isolated from photoreaction of M02C14(PMe2Ph)4 with PhSSPh. The cluster

is consistent with MogCls(PMe2Ph)4(SPh).

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Figure 45. Electronic absorption spectral changes during photolysis (2.exc

> 435 nm) of acetone solution of M02C14(dppm)2 containing a forty fold

excess of TolSSTol at 22°C.

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Figure46. Fast atom bombardment mass spectrum of photoproducts

isolated from photoreactions of MozCl4(dppm)2 with TolSSTol. Selected

assignments of the clusters in the spectrum are (a) [Mo]Cl4(STol)2t; (b)

[MoJCl3(STol)2+; (c) [Mo]015(STol)-:; (d) [Mo]Cl4(STol)"'; (e) [Mo]C15"'

where [Mo] = M02C14.

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the pentachloro product M02015(dppm)2(SPh), (kmam = 525 and 450 nm),

whose identity has been confirmed by X-ray crystallography, and the minor

product, MozCl4(dppm)2(SPh)2, absorbs at 460 nm.

The pentachloroproduct is not restricted to reactions with RSSR but is

also observed from photoreactions with other substrates such as ethyl

iodide. Photolysis of CH3CH2I (EtI) solutions of WzCl4(dppm)2 at 0 °C

results in the spectral changes shown in Figure 47. The decrease in the

8241(55') transition of the quadruply-bonded binuclear complex is

accompanied by an increase in an absorption maximum at 500 nm (e = 6718

M’lcm’l). This absorption is consistent with that of independently

prepared W2Cl4(dppm)212 (6504 = 6954 M'lcm’l), as is the parent ion cluster

centered at 1531 amu in the mass spectrum of the solid sample isolated by

addition of hexane (Figure 48). However, an additional cluster centered at

1441 amu, not observed in the FABMS of WzCl4(dppm)212 (Figure 9), is also

present. The isotopic distribution of this cluster agrees well with that

theoretically calculated for the pentachloroproduct, W2015(dppm)2I,

comparable to that obtained from reactions of PhSSPh with the

dimolybdenum complexes.

The fact that direct addition of neither phenyl disulfide nor ethyl

iodide is effected in these photoreaction pathways is suggestive of a radical

mechanism. Compelling evidence for a radical mechanism in the disulfide

system is provided by performing photolysis of M02C14(dppm)2 in the

presence of equimolar mixtures of phenyl and tolyl disulfides. As Figure 49

shows, the molecular ion region reveals the presence of the the crossover

product MozCl4(dppm)2(STol)(SPh) in addition to the oxidative addition

products M02Cl4(dppm)2(SPh)2 and MozCl4(dppm)2(STol)2. Photolysis of

160

Figure 47. Electronic absorption spectral changes during photolysis (ken

> 435 nm) of ethyl iodide solutions of W2014(dppm)2 at 0°C. The total

reaction time was 1.5 hours.

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Figure48. Fast atom bombardment mass spectrum of products isolated

from photolyzed ethyl iodide solutions of WZCI4(dppm)2. Selected

assignments of the clusters in the spectrum are (a) [W]Cl412t ; (b)

[W101312+; (c) [W]Cl51‘; (d) [W]CI4I"' where [W]: W2(dppm)2.

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Figure 49. Molecular ion cluster region of the fast atom bombardment

mass spectra of photolyzed 0.,“ > 435 nm) solutions of M02C14(dppm)2 in the

presence of phenyl/tolyl disulfide mixtures. Assignments of the clusters in

the spectrum are (a) MozCl4(dppm)2(SPh)2 (b) MozCl4(dppm)2(SPh)(STol)

and (c) MozCl4(dppm)2(STol)2 .

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mixtures of independently prepared M02014(dppm)2(SPh)2 and

M02014dppm)2(STol)2 show no evidence of exchange.

The photochemistry of the M2Cl4(dppm)2 complexes with phenyl

disulfide and ethyl iodide can be summarized by the following overall

reaction.

XY

M2Cl4(dppm)2 ——-> M2015(dppm)2x + M2Cl4(dppm2)Xg (4.1)

(X=I, SR; Y = Et, SR)

The production of M2C15(dppm)2(X) is suggestive of a free radical

mechanism. A radical mechanism that accounts for the formation of both

products, M2(III,III)C15(dppm)2(X) and M2(III,III)C14(dppm)2(X)2 is as

follows,

XY

M201, II)Cl4(dppm)2.T M2(II, III)Cl4(dppm)2X (4.2)

M2(II, III)C14(dppm)2X —>

étMsaII. III)CI.<dppm>sx + Meat. II)C13(dppm)2X] (4.3)

M201, III)Cl4(dppm)2X —->

{1504'er III)CI4(dppm)2X2 + M201. II)Clc(dppm)2] (4.4)

The primary step (4.2) involves one-electron oxidation of electronically

excited M2(II,II)Cl4dppm)2 by the substrate XY to yield the mixed-valence

M2(II,III)CI4(dppm)2(X) intermediate. Disproportionation upon chlorine

atom abstraction (eq 4.3) yields M2(III,III)Cls(dppm)2(X) and

M2(II,II)C13(dppm)2(X). Alternatively, disproportionation by X atom

transfer (eq 4.4) generates M2(III,III)C14(dppm)2(X)2 and

l 6 7

M2(II,II)CI4(dppm)2. A similar disproportionation reaction involving

halogen atom transfer has previously been proposed for reactions of

dinuclear platinum complexes with aryl halides [197].

This mechanism was investigated electrochemically. The

W2(II,III)CI4(dppm)2X mixed-valence intermediate can be produced by one-

electron oxidation of W2(II,II)CI4(dppm)2 in the presence of X‘. The cyclic

voltammogram of toluene/CH31 solutions of W2Cl4(dppm)2 reveal a one-

electron redox wave at +0.1 V (vs Ag wire reference electrode) (Figure 50a).

The cathodic component of this wave is not observed in the presence of I“

(Figure 50b) although a new wave at —0.82 V is present on the return scan.

This reduction wave is consistent with that of the independently prepared

W2(III,III)Cl4(dppm)2I2 shown in Figure 50c. These data are consistent

with the disproportionation mechanism shown in reactions 4.3 and 4.4.

The loss of the cathodic component of the WzCl4(dppm)2"'/° redox wave is

indicative of a trapping of W2(II,III)C14(dppm)2+ with I" to yield

W2(II,III)C14(dppm)2I. Disproportionation of the W2(II,III)C14(dppm)2I

intermediate is evidenced by the appearance of the reduction wave of

W2(III,III)Cl4(dppm)2I2. Unfortunately, we have not been able to prepare

and cleanly isolate the W2(III,III)C15(dppm)2I complex and thus its cyclic

voltammogram is not available for comparison. However the potential for

W 2(III,III)Clg(dppm)2 is only 0.13 V greater than that of

W2(III,III)CI4(dppm)212 and thus the wave at ~0.85 V could well be

reduction of both W2(III,III)C15(dppm)2I and W2(III,III)CI4(dppm)212.

168

Figure 50. Cyclic voltammograms of 1:1 CH3I/toluene solutions

containing 0.1 M THAPF6 of (a) WZCl4(dppm)2 (b) WzCl4(dppm)2 (1.0 x 10‘s)

and THAI (5.0 x 10-3 M) (c) W2Cl4(dppm)2(1)2

Current

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3. OxidafiveAddifionReacflon

The spectral changes upon irradiation at 71. > 436 nm of CH3I (MeI)

solutions of W2Cl4(dppm)2 clearly differ from those of the reactions with

ethyl iodide. In the case of CH3I (Figure 51) two absorption maxima at 490

(e = 2580 M‘lcm’l) and 582 nm (e = 1916 M‘lcm“) appear in the visible

region, as compared with the single absorption maximum at 500 nm in the

case of the reaction with ethyl iodide. Additionally, a distinguishing NIR

transition at 1090 nm (e = 646 M‘lcm'l) is observed from photolyzed CH3I

solutions. Two isosbestic points are maintained throughout the course of

the reaction with CH31. Consistent with the presence of these isosbestic

points, a single product is quantitatively obtained by addition of hexane to

photolyzed solutions. Elemental analysis of the isolated purple solid

corresponds to addition of CH31 to the tungsten-tungsten bond; Cald.

(Found) for WzCl4(dppm)2(CH3)(I): C, 43.1 (43.0); H, 3.32 (3.37); I, 8.93 (8.43);

P, 8.72 (8.54).

Further evidence that photolysis cleanly yields the simple oxidative-

addition product is provided by fast atom bombardment mass spectrometry

(FABMS). The molecular ion cluster in the FABMS spectrum of the

photoproduct (Figure 52), centered at 1418 amu, represents the molecular

ion, W2014(dppm)2(CH3)(I). Selective fragmentation of the parent ion

cluster at 1418 amu gives rise to the two major fragments centered at 1383

and 1291 amu also appearing in Figure 52, which corresponding to loss of

Cl and I, respectively. A simulation of the natural isotope distribution for

W2Cl4(dppm)2(CH3)(I) agrees well with the observed spectrum; the

simulated and observed relative abundances of the individual isotopic peaks

corresponding to the molecular ion are shown in Figure 53. The FABMS

171

Figure 51. Electronic absorption spectral changes during photolysis (Am,c

> 435 nm) of methyl iodide solutions of W2Cl4(dppm)2 at 0°C. The

wavelength scale in the near infrared region 0. = 900-1000 nm) is twice that

of the visible region (7. < 900 nm).

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Figure 52. Fast atom bombardment mass spectra of (A) photolyzed O.exc >

436 nm) solutions of WzCl4(dppm)2 and methyl iodide and of (B) solutions of

WzCl4(dppm)2 and methyl iodide refluxed in the absence of light. Selected

+

assignments of the clusters in the spectrum are: (a) [W]Cl4CH3I'; (b)

++

[WJCI3CH31+; (c) [WIClzCH31'; (d) [W]CI4CH3+; (e) [W]Cl412'; (f)

+

[W]Cl312+; (g) [W]ClsI’ ; and (h) [W]Cl4I+ where [W] = W2(dppm)2.

174.

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Figure 53. The relative isotopic distribution of the molecular ion cluster

for WzCl4(dppm)2(CH3)(I) . The simulated relative abundances, designated

with solid lines, are superimposed on the observed peaks.

176

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shows no evidence of the one-electron crossover products, W2014(dppm)212

and WzCl5(dppm)2I that arise from free radical pathways of Section 2. The

absence of free radical pathways is further supported by GC/MS analysis of

samples obtained by Toepler pumping photochemical reaction mixtures,

which gave no evidence of the production of ethane.

The coordination geometry about the metal-metal bond has not

unequivocally been established because we have not yet been able to obtain

single crystals suitable for X-ray diffraction. However, the absorption

spectrum is typical of the edge-sharing geometry which is observed from

every heretofore reported addition reaction of the MzCl4(PP)2 complexes. In

particular, close similarities are noted in the features of the photoproduct at

490, 582 and 1090 nm and a NIR transition with those of the edge-sharing

bioctahedral complex WzCl4(dppm)2(H)(I) at 464 nm, (e = 460 M'lcm‘l), 602

nm, (e = 450 M-lcm-l) and 1012 nm, (e = 340 M-‘cm-l), respectively. Insight

into the coordination position of the methyl group is provided by 13C nmr

spectroscopy. A single 13C resonance shifted 21 ppm upfield from TMS

(Figure 54) indicates that the methyl group is more likely to be in a terminal

rather than bridging coordination position, as the diamagnetic anisotropy

of the metal-metal quadruple bond would induce a downfield shift of the

resonance of a bridging ligand [198].

The quantum yield of the photoreaction is wavelength dependent @405

= 0.029(1), 4’436 = 0.011(2). ¢5lo = 0.001(3). t = 0 at A > 570 nm). These results

show that the photoactive state is not directly accessed by the 8241(55')

transition nor the absorption centered at 500 nm which has been assigned

as 8—95‘2_y2. The photoreaction is coincident with the 1(lt-i6')/1(6-m')

transition of M-LM species. Maximum quantum yields could not be

determined with increasing wavelength owing to absorption of the

178

Figure54. 13C NMR of photoproduct from photolyzed solutions of 1/1

13CH3I/12CH31 solutions of WZCI4(dppm)2, in CD2C12 at —60°C.

Intensity

179

CDzClz

CH3l

180

substrate at l. < 390 nm. The photochemistry is clearly not derived from

excitation of the substrate. Excitation at l. > 335 nm result in the spectral

changes shown in Figure 55. The characteristic absorption of

W2C14(dppm)2(CH3)(I) at 582 nm does not appear but rather a maxima at 500

nm, comparable to that observed from photoreactions with ethyl iodide,

appears. This result is not surprising, because it is well known that

photoexcitation of CH31 results in cleavage of the C—I bond yielding -CH3

and -I. Thus for this case radical mechanisms are initiated which would

result in production of ethane, W2Cl4(dppm)212 and W2015(dppm)21.

Furthermore photolytic cleavage of CH3I results in production of I2, which

we have shown independently will react with W2Cl4(dppm)2 to yield

WzCl4(dppm)2I2.

The photoproduct is unique. Thermal reactions occur at elevated

temperatures but the thermal reaction clearly differs from the

photochemical reaction as evidenced by the spectral changes observed upon

refluxing CH3I solutions ofW2Cl4(dppm)2 displayed in Figure 56. While the

shift in the 8241(88') transition is not understood, the absorption at 500 nm

is comparable to that of photolyzed EtI solutions. Indeed FABMS (Figure

52b) of these solutions reveal parent ion clusters at 1531 and 1441 amu,

consistent with WzCl4(dppm)2I2 and WzCl5(dppm)2I, respectively. Although

FABMS analysis indicates that W2014(dppm)2I2 is obtained in the thermal

decomposition of W2014(dppm)2(CH3XI) which proceeds even at ambient

temperature, W2015(dppm)2l is not observed. Thus the photochemical

pathway appears to be unique and corresponds to addition at a discrete

bimetallic core.

181

Figure 55. Electronic absorption spectra of CH3I solutions of

WZCl4(dppm)2 before ( ) and after(----- ) photolysis at A > 335 nm at 0°C.

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C. DISCUSSION

Transient absorption results of the Dzd M2C14(PR3)4 and D211

MzCl4(PP)2 complexes of Chapter III provide the underpinning to the

observed MMCT photochemistry of the M-LM complexes. First, a distinct

correlation is noted between the photochemical reactivity of the

dimolybdenum complexes with PhSSPh and the formation of the

bioctahedral intermediates. As summarized in Figure 19, structurally

distorted intermediates are not observed for 5241(85') excitation of the D2},

complexes, wheras an edge-sharing bioctahedral intermediate is formed

from the higher energy MMCT states. Accordingly, photoreactions of the

D211 complexes only proceed upon higher energy irradiation. Photoreactions

are however accessible upon 82* 1(551") absorption of the D24 complexes and

studies of Chapter III show that a bioctahedral intermediate is formed

under these conditions (Figure 31). A crucial role of the bioctahedral

intermediate is further implied by the relative photoreactivity within the

series of M02C14(PR3)4 complexes. Namely while photoreactivity is observed

for the cases of PR3 = PMeth, PBua, PMezPh, all of which form

bioctahedral intermediates, no photoreactivity is observed from

electronically excited M02C14(PMe3)4. The latter complex displays a

transient absorption for the 1(861") excited state with no distortion to a

bioctahedral intermediate.

Thus the emerging trend for the M-LM complexes is that light

initiated photoredox reactions predominates when bioctahedral

intermediates are formed. Indeed, results from Chapter III indicate that a

bioctahedral intermediate is responsible for the quantitative addition of

CH3I to the electronically excited bimetallic core of W2Cl4(dppm)2.

l 8 6

Specifically, the photochemical intermediate exhibits an absorption at 480

run that is characterisitic of an edge-sharing bioctahedral intermediate. A

photopathway of this reaction consistent with this data and with terminal

coordination of the methyl group is addition of the substrate at the open

axial position in the reduced metal center in the edge-sharing bioctahedral

intermediate as depimd in below

n A /\

P P P P

CI ClC1,, ,. Cl, , CL, 1 Cl,M'____ M‘ llv : /"M "M CH3! : /"M "M

Cl/ CI/ CI \Cl/ CI ’

P P P P P PV V \_/

This photoreaction with CH31 represents the first multielectron

transformation efl‘ected at a discrete electronically excited M-LM core. We

believe that the formation of the the edge-sharing bioctahedral intermediate

is central to this novel photoreactivity. The distortion enhances the mixed-

valence character and simultaneously provides two open coordination sites

at the reduced metal center. Indeed low valent, coordinatively unsaturated

redox active metal centers exhibit a propensity for oxidative addition of

substrates.

This pathway proposed for the photoreaction with methyl iodide is

directly analogous to that proposed for the concerted thermal oxidative

addition of H2 to (18 square planar complexes [199,200]. In these systems an

octahedral distortion of the square planar complex upon approach of the

substrate to yield a transition state that can undergo concertively addition

via a three center bond is proposed as depicted below.

CH3

187

””0... ..o\\\“\ L A

L/ M \L H2 i L/

An additional parallel between the photoreactions of the quadruply bonded

complexes and the thermal reactions of the d8 square square planar

complexes is noted in the varying mechanisms with ethyl iodide and

methyl iodide substrates. Whereas photoreactions of W2Cl4(dppm)2 with

MeI yield quantitative oxidative addition, the production of crossover

products W2Cl5(dppm)2I and W2Cl4(dppm)212 in the photoreactions with EtI

are consistent with pathways involving one-electron intermediates outside

a solvent cage. Similarly, a variation in the mechanisms of oxidative

addition reactions of the Vaska's complex, trans-IrCl(CO)(PR3)2 with EtI

and Mel has been noted [201]. Radical chain mechanisms have been

proposed for the the reactions with ethyl iodide, based on the attenuation of

the rate upon addition of radical inhibitors. The presence of these

inhibitors show no effect on the rate of reactions with CH3I. The solvent

dependence and large negative activation entropy in the reactions with

methyl iodide are consistent with a nucleophilic 8N2 mechanism, entailing

addition of the electropositive +CI-I3 to the metal center, followed by rapid

addition of the displaced iodide. However, a very short lived radical cage

mechanism cannot be ruled out.

Herein lies a common problem with definitive elucidation of

mechanistic details of thermal reactions, which do not offer temporal

resolution beyond the conventional arena of study in the millisecond range

of stopped-flow kinetics. We are now in a position, however, to investigate

188

the intimate mechanistic details of the multielectron transformation of

CHsl at the bimetach core of W2014(dppm)2. Smcifically, with' the ability to

initiate this reaction with a pulse of light, we can precisely monitor the

disappearance of photoreactant and formation of products with transient

absorption spectroscopy. The preliminary transient absorption studies of

Chapter III are promising in this regard. Whereas the photochemical

intermediate observed from methyl iodide solutions exhibits an absorption

at 480 nm, which is characteristic of an edge-sharing bioctahedral

intermediate, the transient generated in ethyl iodide solutions does not,

indicating that the one-electron intermediate in this photoreaction has a

distinct absorption profile. Thus the one-electron or two-electron

photochemical pathway of the CH3I system can be elucidated by further

defining the kinetics of both the ethyl iodide and methyl iodide systems on

the picosecond time scale.

D. CONCLUSION

The structural rearrangement to bioctahedral geometries has

important ramifications in the development of discrete multielectron

transformations of electronically excited quadruply bonded metal-metal

binuclear complexes. First it is noted that regardless of the overall

mechanism, each of the photoreactions of the MMCT states of the D2,,

M2014(PR3)4 and D2,, M2014(PP)2 complexes , results in two-electron

oxidation of the binuclear core. This is in contrast to the previously

observed redox reaction of M-‘-M complexes accessible by visible irradiation.

For instance, although a two electron reduction of 1,2 dichloroethane to

yield ethylene has been affected by the 1(88") excited state of

1 8 9

M02[02P(OC6H5)2]4, this reaction is achieved by coupling one-electron

changes at individual metal cores as follows,

Meiosmoccnchl. —h"—> Monomeric»: <45)

MoiOleOCgI-IM‘ + 01011,ch —>

Mo,[O,P(OC¢I-Ig)2LCl + - CHZCHZCI (4.6)

MOiOflOCM+ . CHchzcl ‘_’

MoiOngOCgthhCl + CH2CH2 (4.7)

The primary step involves one-electron transfer from electronically excited

M02[02P(OC6H5)2]4 to DCE yielding M02(II,III)[02P(OCGH5)2]4Cl and

°CHCH2C1. Ensueing thermal reaction of the reactive -CHCH2C1 with

M02(II,II)[02P(OCsH5)2]‘ results in the final formation of ethylene [192b].

Although the intimate mechanistic details of the primary step have not yet

been elucidated, the quantum yield was found to be severely attenuated in

solvents that coordinate in the axial positions, indicating that the primary

transformation is effected at this site. Indeed this is certainly expected,

owing to the encumbering ligation sphere that completely encases the

equatorial plane of the bimetallic core, as confirmed by computer generated

space filling molecular models.

We postulate these structural constraints as well as the structural

rigidity of the Mog[02P(OcsH5)2]4 complex govern the observed one-electron

oxidation of the binuclear complexes. First, two-electron oxidized M-LM

cores with structurally rigid ligation spheres are typically unstable. This is

evident in the case of the M02[02P(OCGH 5)2]4 complex by the

comproportionation of M02(III,III) and Moz(II,II) to yield M02(II,III)

190

smcies. Secondly, although axial attack of the substrate at a single metal

center is dictated by the steric congestion of the ligation sphere, oxidative

addition of the substrate at a single metal center is inhibited in these

structurally rigid "lantern" complexes containing four bidentate ligands

spanning the binuclear core. To date there is not a single report of a

lantern structure with two additional ligands in the equatorial positions at

a single metal center. Rather, oxidative addition typically results in

addition of a single ligand in the axial position at each metal center. Thus

two-electron reactions will rely on coupling one-electron changes at

individual metal centers. Because the steric congestion of the ligation

sphere of M02[02P(OCGH5)2]4 inhibits the ability of the substrate to

simaltaneously access both metal centers, a discrete two-electron

transformation at a single binuclear core will be difficult to achieve.

Rather, as discussed, a primary step involving a one-electron

transformation in the most easily accessed axial position of a single metal

center is postulated. For this case a net two-electron reaction, requiring

diffusion of the reactive organic radical intermediate to an additional metal

center, is likely to involve competitive reactions with other binuclear metal

cores in solution [202].

Thus the ramifications of a bioctahedral rearrangement upon

oxidation of the more structurally flexible M-LM complexes in multielectron

photochemical schemes is two fold. This rearrangement which ensures an

ocathedral coordination sphere about M(III) centers allows for two—

electron oxidation of the binuclear core. Moreover, the bioctahedral

distortion of the MMCT excited state provides open coordination sites at the

reduced metal center, which facilitates direct addition of substrate. Beyond

its apparent role in the quantitative oxidative addition of CH31 to

191

electronically excited WzCl4(dppm)2, the coordinative unsaturation of the

reduced metal in the bioctahedral intermediate may account for its

enhanced reactivity relative to the structurally undistorted l(65') excited

state. These studies suggest that oxidative addition reactions may be

accessible from the bioctahedral intermediates generated upon 1(88‘)

absorption of the Dad complexes are logical candidates for the future

development of multielectron photochemical schemes. However, future

pursuit of the multielectron photoreactivity of these complexes should be

limited to complexes from which nonluminscent transients are observed,

yet are ligated by "nonlabile" phosphines such as PMezPh and PEta [203].

CHAPTERV

FINALREMARKS

The aforementioned studies show that charge separation within a

polynuclear metal complex is a promising approach to discrete

multielectron transformations. However an additional hypothesis to arise

from these studies is that such reactivity in the case of the electronically

excited quadruply bonded dimers is facilitated by an additional feature

beyond the formal change in oxidation state of the metal centers induced by

MMCT. Transient absorption studies indicate that an edge-sharing

bioctahedral intermediate is responsible for the photoinduced quantitative

oxidative addition of CH3I to the bimetallic core of WzCl4(dppm)2. The

proposed pathway for this transformation based on transient absorption

studies is addition of the substrate at the two open coordination sites of the

reduced metal center in the edge-sharing bioctahedral intermediate.

Future investigations of the photoreaction of MoWCl4(dppm)2 with CH3I

provide an indirect test of this photopathway. Namely, based on the relative

redox properties of Mo and W, such a mechanism is expected to involve

production of Mo(I)——W(III) species, and addition of Mel should therefore

192

193

occur at the Mo center. Preliminary results are promising in this regard.

The spectral changes associated with the photolysis (l. > 405 nm)

proceeding from CH3I solutions of MoWC14(dppm)2 at 0°C are shown in

Figure 57. Two distinct absorptions at 520 and 655 nm. The latter is only

slightly shifted from the 8241(88‘) absorption of MoWCl4(dppm)2. No

further spectral changes were noted after an additional 45 minutes of

irradiation, indicating that the absorption corresponds to a photoproduct.

This absorption profile with two main features is comparable to that of

W2Cl4(dppm)2(CH3)(I), and is distinguished from the spectrum obtained

from the thermal reaction of MoWCl4(dppm)2 with I2, which exhibits a

single feature centered at 525 nm. The FABMS of a solid sample

precipitated by addition of hexane, reveals the presence of both

MoWCl4(dppm)2(CH3)(I) and MoWCl4(dppm)2I2 at 1330 and 1446,

respectively. The fact that a cluster corresponding to MoWC15(dppm)2I is

not observed suggests that the reaction may involve quantitative oxidative

addition to yield MoWCl4(dppm)2(CH3)(I), with ensuing thermal

decomposition to yield the observed MoWCl4(dppm)212. Indeed although the

W2Cl4(dppm)2(CH3XI) complex is stable at 0 °C, it will decompose at

ambient temperature to yield W2Cl4(dppm)212. That the

MoWCl4(dppm)2(CH3)(I) complex is apparently less stable than

W2014(dppm)2(CH3XI) is promising because Mo—CH3 bonds are less stable

than W—CHa bonds [204]. Future studies at lower temperatures which

may prevent decomposition of the MoWCl4(dppm)2(CH3)(I) photoproduct,

will be enlightening.

Because the reaction with MeI can be initiated with a pulse of light,

the intimate mechanistic details of the addition of substrate can be directly

addressed with picosecond laser spectroscopy. Thus investigations of this

194

Figm‘e 57. Electronic absorption spectral changes during photolysis (7texc

> 405 nm) of CH3I solutions of MoWC14(dppm)2 at 0°C. No further change

in the spectra were noted after an additional 0.5 hour of irradiation.

195

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reaction can fundamentally lead to a better understanding of multielectron

reactivity and practically enable construction of a framework for design of

multielectron catalysts not only for excited state processes but for ground

state processes as well.

More generally, transient absorption spectroscopy opens up new

avenues in the deisgn of bimolecular photochemical schemes of M-LM

dimers. Although bimolecular photochemistry from numerous quadruply

bonded metal-metal dimers has heretofore remained unexplored owing to

the limited lifetime of the 1(85') state, studies presented here suggest that

long-lived l(88‘) excited states are as important for bimolecular

photochemistry as nonemissive structurally distorted intermediates. Thus

further pursuit of bimolecular multielectron photochemistry of quadruply

bonded metal-metal dimers may be significantly broadened to include the

majority of these complexes with shortlived nonemissive l(88’) excited

states.

In summary, the framework for the future design of photoinduced

multielectron small molecule activation schemes of MJ-M dimers to arise

from the studies described herein directly parallels that established for the

ground state reactions of square planar d8 Pd(II), Pt(II), and Ir(I)

mononuclear complexes. Namely, low valent, coordinately unsaturated

metal centers are ideal candidates for oxidative addition of substrates.

Moreover, the bioctahedral intermediate of the electronically excited

quadruply bonded dimers is directly analogous to an octahedral transition

that is believed to be important for concerted thermal oxidative addition

reactions of the mononuclear complexes. Electronically excited binuclear

complexes have the intiguing asset of mixed-valence character which

drives the formation of this ideal intermediate in the absence of substrate.

Appendix

197

Figure 58. Transient absorption kinetics recorded at 390 nm following 532

nm laser excitation of hexane solutions of displaying (a) the 120 ns

transient (b) the initial rise and relative intensity of an additional transient

absorption (c) the rise and decay of the additional long lived transient.

198

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199

Figure 59. Electronic absorption of M02C14(PBu3)4 (—), MozCl4(dppm)2

(-----) and irradiated solutions of M02Cl4(PBu3)4 containing a 100 fold excess

0f CH3N<PF2)2 (°°°°° ).

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181.

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Interestingly, an additional transient of much weaker intensity has

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Mo¥C13(PMePh2)4 at 400 nm (sh) (c r 2400 M'lcm'l), 526 nm (e = 300

M“ cm“) and 650 nm (e = 640 M‘lcm‘l) are very similar to those of

the confacial bioctahedral analogue, MozCls(PMePh2)3 at 390 nm (sh)

(s = 1800 M-lcm-I), 515 nm (sh) (c = 540 M'lcm'l) and 590 (c = 800

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to MozCl4(dppm)2. The thermodynamic unfavorability of this reaction

is evidenced by the fact that M02C14(PBu3)4 is reformed within

minutes after the irradiation is discontinued, as confirmed by

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has prevented us from isolating this product, which is believed to be

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As previously noted these complexes exhibit an additional transient

species that has not yet been characterized, which may prove to be a

promising candidate for multielectron photoreactivity. Namely, the

equilibrium between confacial and edge-sharing bioctahedral

complexes may account for the presence of two transient species.

nICHIan STATE UNIV. LIBRARIES

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