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ll

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PHOTOPHYSICS AND PHOTOCHEMISTRY
OF MULTIPLY BONDED METAL-METAL DIMERS

by

[-1 y Chang

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the requirements

for the degree of

DOCTOR OF PHILOSOPHY

Department of Chemistry

1988

ABSTRACT

PHOTOPHYSICS AND PHOTOCHEMISTRY
OF MULTIPLY BONDED METAL-METAL DIMERS

b y
I-Jy Chang

The excited state dynamics and excited state chemistry of
multiply bonded metal-metal complexes (M-LM) have been
investigated. The lowest energy excited state, 1(56*), of
dimolybdenum tetrahalo(tetrakis)phosphine (M02X4P4) has been
studied by using emission and time-resolved picosecond laser
spectroscopy. Emission quantum yields and lifetimes are significantly
affected by the electronic configuration of the individual molecule

and are only slightly affected by the eclipsed-to-staggcred

conformational changes. Molecules with D2d symmetry possess high
emission quantum yields and long lifetimes, complexes with DZh
symmetry possess lower emission quantum yields and shorter
lifetimes. Transient absorption spectra of M02X4P4 complexes
display decays corresponding to prompt relaxation of the 1(66*)
excited state to ground state. In addition, complexes possessing
phosphines with sterically bulky constituents (e.g. monodentate or
bidentate phenyl phosphines) exhibit a long-lived nonemissive
excited state. It is believed that this nonemissive excited state is due

to the dissociation of the phosphine ligands within a solvent cage.

A new class of multiply bonded dimolybdenum phosphate
dimers, M02(HPO4)44' and M02(HPO4)43', has been prepared and

spectrosc0pically and electrochemically characterized. The electronic
absorption spectrum of M02(HPO4)44' in acidic solution is typical
MAM species with the 62 -* 56* transition occurring at 516 nm.
Cyclic voltammograms of phosphoric acid solutions of M02(HPO4)44'
are characterized by two reversible waves at -0.67 and -0.25 V vs.
SCE that have been assigned to the M02(HPO4)43’/4' and M02-
(HPO4)42'/3' couples, respectively. Oxidation of M02(HPO4)44'
produces the mixed-valence dimer M02(HPO4)43‘, which exhibits an

intense near-infrared absorption band which is assigned to the 5 ->

5* transition. Irradiation (x > 355 nm) of phosphoric acid solutions

of the MA-M dimer leads to facile production of M02(HPO4)42' and

hydrogen. The photochemical reaction mechanism is consistent with
sequential one-electron oxidation of the M02 core [i.e., M02(II,II) ->
M02(II,III) -9 M02(III,III)]. Electronic absorption spectra of the
M02(HPO4)4n' (n = 2, 3, 4) dimers in the ultraviolet spectral region,
wavelength-dependent quantum yield measurements, and
photochemical studies of phosphate dimers under N20 atmospheres
confirm the assignment of a n 11* photoactive state of M02 phosphato

complexes.
Chemistry from the 1(55") excited state of the M02(HPO4)44'

complex is circumvented by proton quenching. To this end, a novel

photoreceptor, Moz(02P(OC6H5)2)4, was prepared to promote the

low energy multielectron reactions. This diphenyl phosphate
dimolybdenum species exhibits 52 -9 55* absorption at 515 nm in

CH2C12 solution. Cyclic voltammograms show two reversible waves

at +0.06 and +1.00 V vs. SCE which correspond to
Mo2(02P(oc6H5)2)4+/0 and Mo2(02P(oc6H5)2)42+/+ couples,
respectively. One electron oxidation of M02(02P(OC6H5)2)4 yields
mixed-valence Moz(02P(OC6H5)2)4+ species which is a direct
analogue to the M02(HPO4)43'. In spite of the similarity of
M02(02P(OC6H5)2)4 to M02(HPO4)44’, M02(02P(0C6H5)2)4 emits
directly from 1(655*) excited state in nonaqueous solutions. The
preserved 1(66*) lifetime is long enough to permit direct reactions
of the excited state in nonaqueous solution. Visible irradiation (X >
530 nm) of solutions containing M02(02P(OC6H5)2)4 and ClCHzCHzCl
results in the facile oxidation of the metal core and the production of

ethylene.

To My Parents,
Shiow-Ing and Jong-Way

ACKNOWLEDGMENTS

I would like to thank Dr. Daniel G. Nocera for his
encouragement, guidance and friendship during the course of this
study. His scientific insight has opened my eye and directed me into
a new field.

I also like to express my gratitude to the present members of
the group for their friendship, encouragement and many helpful
discussions. More importantly, they have created a great working
atmosphere to make my graduate life enjoyable.

I must express my thanks to Dr. Jay R. Winkler at Brookhaven
National Laboratory. I was fortunate to have the opportunity to work
with him. From his guidance I learned how to use the picosecond
laser instrument. Moreover, I learned how to review results in
different way from him.

Finally, I like to thank my parents and family, without their

support and love I could not finish this study.

vi

TABLE OF CONTENTS

Page
LIST OF TABLES ......................................................................................... x
LIST OF FIGURES ........................................................................................ xii
CHAPTER I INTRODUCTION .......................................................... 1
CHAPTER II EXPERIMENTAL ........................................................ 3 2
A. Syntheses ................................................................................ 3 2
1. General Procedures ................................................ 32
2. Syntheses of Dimolybdenum
Complexes ................................................................... 3 2
B. Solvent Purification ............................................................ 41
1. Solvents Used for Syntheses ................................ 41
2. High Purity Solvents ............................................... 42
C Instrumentation and Methods ....................................... 43
1. Absorption Spectroscopy ...................................... 43
2. Steady-State Luminescence Spectroscopy 43
3. Time-Resolved Laser Spectroscopy ................ 44
4. Magnetic Measurements ....................................... 44
5. Resonance Raman ................................................... 44
6. Electrochemical Measurements ........................ 4 5
7. Photochemistry ........................................................ 45
8. Gas Analysis .............................................................. 51

vii

D. Crystal Structure Determinations ................................. 51

1. General Procedures .............................................. 5 1
2. M02(02P(OC6H5)2)4'2THF ................................... 5 1
3 . M02(02P(0C6H5)2)4BF4 ...................................... 5 6

CHAPTER 111 SPECI'ROSCOPIC STUDIES OF
M02X4(PBu3)4 AND M02Cl4(LL)2

COMPLEXES ................................................................. 5 9

A. Background ............................................................................. 5 9
B. Results and Discussion ....................................................... 6 5
1. Absorption Spectroscopy ...................................... 6 5

2. Emission Spectroscopy .......................................... 6 9

3. Transient Absorption Spectroscopy ................ 9 2

CHAPTER IV PHOTOCHEMISTRY OF NOVEL
DIMOLYBDENUM PHOSPHATE

COMPLEXES IN ACIDIC SOLUTION ................ 1 15
A. Background ............................................................................. 115
B. Results and Discussion ....................................................... 117

1. Magnetic and Spectroscopic
Characterization of M02(HPO4)4“‘,

(n = 2, 3, 4) Species ................................................. 1 17
2. Oxidation-Reduction Chemistry ......................... 133
3. Photochemistry ......................................................... l3 8

CHAPTER V PHOTOCHEMISTRY OF A NOVEL
DIMOLYBDENUM DIPHENYL PHOSPHATE

IN NONAQUEOUS SOLUTION .............................. 161
A. Background ............................................................................ 161
B. Results and Discussion ....................................................... 1 6 7

viii

1. Structures of M02(02P(0C6H5)2)4'2THF
and M02(02P(OC6H5)2)4BF4 ............................. 167

2. Oxidation-Reduction Chemistry ......................... 1 8 0

3. Magnetic and Spectroscopic
Characterization of M02(02P(OC6H5)2)4

and Moz(02P(OC6H5)2)4BF4 ............................. 1 85

4. Photochemistry ........................................................ 193

CHAPTER VI FINAL REMARKS ..................................................... 209
REFERENCES ......................................................................................... 21 3

ix

LIST OF TABLES

Table Page
1 State Orderings in d4-d4 Complexes: [Re2C18]2' ................ 27
2 Quantum Yield Data for Ferrioxalate Photoreaction ............. 49

3 Torsional Angles for Structurally Characterized
M02X4(PBu3)4 and M02C14(LL)2 Complexes ....................... 64

4 Magnetic Susceptibility of K3M02(HPO4)4 ............................... 1 19

5 Electronic Absorption Maxima of Dimolybdenum

Ions in Aqueous Solution ................................................................ 146

6 Crystallographic Data of M02(02P(OC6H5)2)4'2THF
and M02(02P(OC6H5)2)4BF4 ......................................................... 168

7 Selected Atomic Positional Coordinates of
M02(02P(OC6H5 )2)4'2THF .............................................................. l 7 4

8 Selected Atomic Positional Coordinates of
M02(02P(0C6H5)2)4BF4 .................................................................. l 7 5

10

11

12

13

14

Selected Bond Distances of Moz(02P(OC6H5)2)4°2THF 176

Selected Bond Distances of M02(02P(OC6H5)2)4BF4 ........ 177
Selected Bond Angles of M02(02P(OC6H5)2)4°2THF ......... 178
Selected Bond Angles of M02(02P(OC6H5)2)4BF4 ............ 179

The Formal Reduction Potentials of the
M02(II,III)/(II,II) and M02(III,III)/(II,III)

Couples of M02 Sulfato, Phosphato, and
Diphenyl Phosphate Complexes ................................................... 183

Quantum Yield Data for the Photoreaction of

M02(02P(0C6H5)2)4 and CICI‘I2CH2CI in

Various Nonaqueous Solutions ...................................................... 211

xi

Figure

LIST OF FIGURES

Latimer diagram for a transition metal complex M.

The relationship among the 0, 0 transition energy

(BO-0) and the ground and excited state redox

potentials is described by equations (1) and (2) .............

Schematic illustration of a water-splitting cycle

involving photosensitizer, electron relay, and

redox catalysts ................................................................................

Water-splitting photochemistry of a bifunctional
catalyst involving modified semiconductor
particles. CB, conduction band; EF, energy of Fermi

level; R, electron relay (e.g. MV2+); and S, sensitizer

(e.g. Ru(bpy)32+) (ref 30) ..........................................................

Reaction cycle for the conversion of isopropanol

to acetone and hydrogen by using Pt2(POP)44'

as a photocatalyst ..........................................................................

Photoinduced bimolecular reaction of

Ir2(p -pz)2(COD)2 with 1,2-dichloroethane .......................

xii

Page

..... 3

..... 6

..... 8

..... l3

..... 16

10

ll

Bimolecular photoreaction pathways from a long-
lived triplet excited state of an inorganic
complex M. kr, radiative decay rate; km,

nonradiative decay rate; and kisc, intersystem

crossing rate ........................................................................................

Proposed bimolecular photoreaction pathways from

a long-lived singlet excited state of an inorganic

v . . .
complex M . The intersystem crossrng rate, kisc’ IS

18

negligible for a large singlet-triplet energy gap .................. 21

Relative energies of the d-derived molecular
orbitals in Dooh M2 and in a D4,, M2L3 complex.

The ground state of the d4-d4 M2L3 complex is

1A1g(02n452) .................................................................................. 24

Experimental apparatus for typical photolysis ...................... 47

Experimental apparatus for photoreaction quantum

yield measurements .......................................................................... 5 2

(a) 02d structure of quadruply bonded
chlorophosphino dimolybdenum M02X4L4;
(b) Newman projection of M02X4(LL)2 complexes

where X = Cl, Br, I; L and LL = mono- and bi-dentate

phosphine ligands, respectively ................................................... 62

xiii

12

13

14

15

16

17

18

Electronic absorption spectra of M02X4(PBu3)4

where X = Cl (——-), Br (---), and I (- - -) ...................................

Energies of the lowest electronic states of

M02C14(PMe3)4 and M02C14(dmpe)2 (ref 79) .................

Emission of M02C14(LL)2 complexes in CHZCIZ

solution at room temperature: (a) M02C14(dmpm)2;

(b) M02C14(dppe)2; and (c) M02C14(dmpe)2 ......................

Luminescence of M02C14(LL)2 complexes at 77 K
in a CH2C12/toluene glass: (a) M02C14(dppm)2; (b)
M02C14(dmpm)2; (c) M02C14(dppe)2; and (d)

M02C14(dmpe)2 ...............................................................................

Absorption and emission spectra of 62 :2 65*

transitions of staggered (a) M02C14(dppe)2 and (b)

M02C14(dmpe)2 complexes .........................................................

Absorption and emission spectra of 62 t‘-’ 65*

transitions of eclipsed (a) M02C14(dppm)2 and (b)

M02C14(dmpm)2 complexes ........................................................

Relative energies of the d-derived molecular
orbitals in 02d (i.e. M02C14(PBu3)4) and 02h

(i.e. M02C14(dppm)2) symmetries ............................................

xiv

66

7 0

7 3

7 7

82

87

90

19

20

21

22

Transient difference spectra of (a) MozBr4(PBu3)4

in CH2C12 and (b) M0214(PBu3)4 in CH2C12. The

spectra were recorded 50 ps and 1 ns, respectively,

after the 532-nm pulse of a Nd:YAG laser

(FWHM = 25 ps) ................................................................................... 94

Transient difference spectra of the following

M02C14(LL)2 complexes: (a) M02C14(dPPm)2.

recorded 75 ps after a 355-nm laser excitation

pulse; (b) M02C14(dmpm)2, recorded 25 ps after a
532-nm laser excitation pulse; (c) M02C14(dppe)2,

recorded 100 ps after a 532-nm laser excitation

pulse; and (d) M02C14(dmpe)2, recorded 100 ps

after a 355-nm laser excitation pulse ....................................... 98

Transient kinetics for dimethylphosphine complexes:

(a) M02C14(dmpm)2 recorded at 600 nm (excitation by

2nd harmonic of a NszAG, FWHM = 25 ps) (b)
M02Cl4(dmpe)2, recorded at 450 nm (excitation

by 3rd harmonic of a NszAG, FWHM = 25 ps) ...................... 105

Transient kinetics for M02Cl4(dppm)2 in CH2C12

solution. The decay kinetics were recorded at
(a) 400 nm and (b) 620 nm (excitation by 3rd
harmonic of a NszAG, FWHM = 25 ps) ..................................... 109

XV

23

24

25

26

27

28

29

Transient kinetics of M02C14(dppe)2 in CH2C12

solution. The decay kinetics were recorded at
420 nm (excitation by 2nd harmonic of a NszAG,
FWHM = 25 ps) .................................................................................... 1 1 1

Temperature dependence of the corrected magnetic
susceptibility of K3M02(HPO4)4 .................................................. 121

X-band (9.460 GHz) EPR spectrum of a frozen
solution of K3M02(HPO4)4 in 7.5 M H3PO4 at 5 K ............... 123

Electronic absorption spectrum of M02(HPO4)44'

ion in 2 M H3PO4 at room temperature ................................... 126

Electronic absorption spectrum of K3M02(HPO4)4
in 2 M H3PO4 at 25 0C .................................................................... 128

Near-infrared absorption band of K3M02(HPO4)4

in a frozen phosphoric acid solution at 77 K ........................... 131
Cyclic voltammogram of a 2.5 mM solution of the

pyridinium salt of M02(HPO4)44' in 2 M H3PO4.

The scan rate was 2 mV s'1 ........................................................... 134

xvi

30

31

32

33

34

Electronic absorption spectral changes during
irradiation (at > 335 nm) of M02(HPO4)44' in

2 M D3PO4. Due to spectral congestion in the
visibile spectral region, the 45-min trace is not
illustrated between 350 and 800 nm. The

absorbance sensitivity in the visible spectral region

is twice that of the infrared spectral region ........................... 139

Absorption changes resulting from irradiating
(x > 335 nm) 2 M D3PO4 solutions of M02(HPO4)43‘.

The visible absorbance scale is 5 times greater than

the near-infrared absorbance scale ............................................ 141

Electronic absorption spectral changes during
irradiation (x > 335 nm) of M02(HPO4)42' in 2 M

D3PO4. No absorption bands appear in the

near-infrared spectral region during the

photolysis reaction ............................................................................. 1 4 4

Terminating electronic absorption spectrum of

photolyzed (X > 335 nm) phosphoric acid solutions

of M02(HPO4)4“' (n = 2, 3,4) ....................................................... 149

Action spectrum of the photolysis reaction:
M02(HPO4)43' + H+ _. M02(HPO4)42' + 1/2 H2.

Quantum yields were measured by monitoring the

5 -» 5* absorption band of M02(HPO4)43’ ......................... 153

xvii

35

36

37

38

39

40

Ultraviolet absorption bands of M02(HPO4)44‘ (---),
M02(HPO4)43' (—), and M02(HPO4)42' (~ - -)

in 2 M H3P04 at 25 0C ...................................................................... 155

Photoreaction pathway of "M0208" complexes

in acidic solutions, where "M0208" corresponds to

M02(aq)x4+, M02(SO4)44', and M02(HPO4)4n' .................... 162

4

Proton quenching of M—M excited states .............................. 165

Structure and labeling scheme for the quadruply
bonded complex M02(02P(OC6H5)2)4°2THF. Atoms

are represented by their 50% probability ellipsoids.

Due to structural congestion, the two tetrahydrofuran

molecule are not illustrated ........................................................... 170

Structure and labeling scheme for the

mixed-valence complex M02(02P(OC6H5)2)4BF4.

Atoms are represented by their

40% probability ellipsoids ............................................................... 172

Cyclic voltammograms of (a) M02(02P(OC6H5)2)4
and (b) M02(02P(OC6H5)2)4BF4 in CH2C12 solution
at 23 °C. NBu4PF6 was used as the supporting

electrolyte, scan rate = 20 mV s'1 .............................................. 181

xviii

41

42

43

44

45

46

X-band (9.434 GHz) EPR spectrum of a frozen
solution of M02(02P(OC6H5)2)4BF4 in CH2C12

solution at 17 K .................................................................................... 186

Electronic absorption and emission spectra of

M02(02P(0C6H5)2)4 in CH2C12 solution at 25 0C ................. 189

Electronic absorption spectrum of

M02(02P(OC6H5)2)4BF4 in CH2C12 solution at 25 0C ......... 191

Transient absorption kinetics of Moz(02P(OC6H5)2)4

in THF solution. The decay kinetics were recorded

at 460 nm and excitation was with a 532-nm

Nd:YAG laser pulse (FWHM = 25 ps) .......................................... 194

Electronic absorption spectral changes during
irradiation ()‘exc > 530 nm) of M02(02P(OC6H5)2)4
in CICHZCHZCI solution at 25 °C. The visible

absorbance scale is 2 times greater than the

near-infrared absorbance scale .................................................... 197

FABMS spectrum of a photolyzed ClCHzCHZCl

solution of M02(02P(OC6H5)2)4 ................................................... 199

xix

47

48

Electronic absorption spectrum of the product of
the reaction between M02(02P(OC6H5)2)4 and

0.5 equivalent of C6H51C12 in CH2C12 solution at room

temperature .......................................................................................... 201
Electronic absorption spectral changes during

irradiation ()‘exc > 530 nm) of M02(02P(OC6H5)2)4
in CH2C12 solution at 25 °C ............................................................. 206

XX

CHAPTER I

INTRODUCTION

A central theme of inorganic chemistry is the activation of
small molecules by transition metal complexes. The main difficulty
of small molecule activation chemistry lies in the large kinetic and/
or thermodynamic barriers which typically confront these activation
reactions. To this end, inorganic photochemistry can play an
important role in the activation of small molecules, because the
energy of the electronically excited transition metal complex can be
utilized to surmount these reaction barriers. Owing to the fact that
small molecule activation reactions involve oxidation-reduction
chemistry, the development of the photoredox reaction pathways of
transition metal complexes is crucial to the ultimate design of
photochemical activation schemes.

1

Since Adamson's initial studies in which the electron transfer

reactivity of the LMCT excited state of Co(NH3)5I was demonstrated,
the energetics and mechanisms of photoinduced electron transfer of
a large number of transition metal complexes have been
delineated.2'5 A feature of paramount importance is that the metal

complex in its excited state is a considerably stronger oxidant and

1

2

reductant than in its ground state. The reduction potentials of an
excited state can be estimated from its spectroscopic energy (0-0

transition defined as E0_0 (M/M*)) and the oxidation and reduction

potentials of the ground state species as follows,

E*,cd (M*/M') = Bored (M/M') + E 0_0 (M/M*) (1)

13’“ox (M+/M*) = 500x (M+/M) - E 0_0 (M/M*) (2)

The Latimer diagram shown in Figure 1 illustrates the simple
thermodynamic relations described by equations (1) and (2). This
enhanced redox reactivity of electronically excited complexes has
been exploited in the experimental testing of electron transfer

6-11

theories, the elucidation of photoinduced electron transfer in

12'” and the activation of small molecules for

biological systems,
the design of solar energy conversion schemes.l6'21 In the case of
the latter, because most small molecule activation processes involve
two or more electrons, present photocatalytic schemes have relied on
coupling the one-electron chemistry of electronically excited
complexes to drive the overall multielectron process. This

photochemistry is exemplified in the water-splitting reactions of

mononuclear polypyridyl complexes. The tris(bipyridyl)ruthenium

(11) ion, Ru(bpy)32+, provides the cornerstone example of this
photochemistry. Electronically excited Ru(bpy)32+ ion, [Ru(bpy)32+]*,
produced by visible irradiation can readily transfer one electron to
acceptors, such as methylviologen (i.e. MV2"’).22’23 The reduced

viologen reacts with protons in the presence of colloidal platinum to

Figure 1. Latimer diagram for a transition metal complex M. The

relationship among the 0, 0 transition energy (130-0) and the ground

and excited state redox potentials is described by equations (1) and

(2).

 

B 0-0

 

 

Figure 1

5

produce hydrogen.24 This chemistry is described in the following

scheme,
Rutbpm2+ ——’E- Ruibpy>32+* (3)
Ru(bpy)32+* + MV2+ —. Ru(bpy)32 + MV+ (4)
MV+ + H+ ——PL- MV2+ + 1/2H2 (5)

Scheme I

The platinum catalyst couples the one electron chemistry of MV+ to
the two-electron hydrogen production chemistry by effectively

storing the reducing equivalents of the viologen. The photogenerated

oxidant Ru(bpy)33+ produced in Scheme I is a strong oxidant. It has
been shown that Ru(bpy)33+ can react with water or hydroxide in
the presence of a Rqu catalyst under suitable conditions to produce
oxygen.25 In principle, the hydrogen and oxygen photochemistry of
the Ru(bpy)32+ system can be combined to construct the
photocatalytic water-splitting cycle schematically represented in
Figure 2.26'28 This photocatalytic chemistry has been realized to a
limited extent with bifunctional photosystems employing colloidal

29'33 A schematic illustration of one such

semiconductors.
bifunctional photosystem is displayed in Figure 3.30 An
electronically excited sensitizer donates an electron to the conduction
band of the colloidal Ti02 particle. Protons are reduced to hydrogen
at platinum dispersed on the surface of the semiconductor particle.

The oxidized sensitizer takes an electron from the valence band of

the Ti02/Ru02 particle and ensuing oxidation of water is mediated

Figure 2. Schematic illustration of a water-splitting cycle involving

photosensitizer, electron relay, and redox catalysts.

N oSwE

 

fbfibmt e s\
r M, w

L
E D
ME<3 >mummzm

m0 Sac/m HZDSQAmHm;
m0 .257:

 

 

 

 

 

Figure 3. Water-splitting photochemistry of a bifunctional catalyst
involving modified semiconductor particles. CB, conduction band; EF,

energy of Fermi level; R, electron relay (e.g. MV2+); and S, sensitizer
(e.g. Ru(bpy)32+) (ref 30).

 

V 1 8 9 em
0: . n\
\ -mo + E m:
\ \\ \. x r A+ENIV om

mm
+m WW mismd s\\\\\\\\ \\ can
a

 

 

 

ooooooooooooo

10

by RuOZ. The efficiency of the cycles such as those shown in Figures

2 and 3 are extremely low for several reasons. The reaction yield for
H20 oxidation by RuOZ is negligibly small. A series of studies by

2034 and others35 have shown that a more

Sutin and coworkers,
efficient system for water oxidation involves cobalt(II) complexes as

the water oxidation catalysts. The multistep mechanism which

accounts for 02 production is shown in Scheme II34b where the

cobalt catalyst and Ru(bpy)33+ is represented by [C02+] and [Ru(III)].

[Co2+] + 2H20 :2 Co(OH)2 + 211+ (6)
Co(OH)2 + [Ru(III)] 2 Co(OH)2+ + [Ru(II)] (7)
Co(OH)2+ + [Ru(III)] -> Co02+ + [Ru(II)] + H20 (8)

Co02+ + H20 -+ [Co2+] + H202 (9)

Coo2+ + [Co2+] .. CoOCo4+ -> "C0203"s (10)
2[Ru(III)] + H202 _. 2[Ru(II)] + 02 + 2H+ (11)

4Ru(bpy)33+ + 21120 .. 4Ru(bpy)32+ + 411+ + 02 (12)

Scheme 11

Despite the success of replacing Ru02 with other catalysts, overall

low efficiencies are retained owing to the limited diffusion of
reactants, efficient back reaction of primary photoproducts, and
difficulties with channelling reactivity selectively along the desired

routes in these complicated multifunctional schemes. Even if these

11

problems are overcome, there remains one major problem

confronting these systems: Ru(bpy)32+ multielectron chemistry is
limited to water-splitting and is not applicable to other small
molecule (e.g. N2, C02, CO) activation reactions due principally to the

lack of suitable catalysts, such as Pt and RuOZ, to couple the single

electron transfer steps to the desired multielectron transformation.
An alternative approach to the ultimate design of multielectron
photochemical schemes is to develop photochemical excited state
reaction pathways in which an electronically excited complex can
participate directly in multielectron processes. With such systems,
the problems inherent with coupling one electron steps to drive
overall multielectron processes are circumvented.

A promising approach to photoinduced multielectron reactivity
relies on coupling formal oxidation state changes of metal centers
into a polynuclear core to drive the multielectron transformation. An
issue of paramount importance to this approach is the necessity to
maintain the structural integrity of the polynuclear core upon light
absorption. For instance, a representative metal-metal bonded
polynuclear system is dimanganese decacarbonyl Mn2(CO)10. From
molecular orbital theory, the 12 electrons of this d7-d7 dimer reside
in predominantly metal orbitals of TI and 6 symmetries. The

remaining two electrons populate the metal-metal O bonding orbital

formed from the overlap of the 4d22 orbitals. The lowest energy

transition in Mn2(CO)10 arises from the promotion of an electron

from the O bonding orbital to 0* antibonding orbital. Consequently,
photoexcitation of these complexes significantly weakens the metal-

metal bond, and photofragmentation to give two manganese

12

pentacarbonyl free radicals is the dominant excited state reaction
pathway.36’37 As is typical of most singly bonded metal-metal
systems,38'41 the photon is consumed for bond breaking and thus
the polynuclear complex in its excited state is not preserved for
redox chemistry.

This problem of photofragmentation can be avoided by a
variety of approaches. The structural integrity of metal-metal cores
of dinuclear species can be preserved by bridging the metal-metal
core with bidentate ligands. This approach has been most
successfully exploited with d8-d8 complexes where bridging ligands
such as pyrophosphite in Pt2(POP)44' (POP : (HOZP)ZO),42 pyrazolyl
in Ir2(p-pz)2(COD)2 (pz = pyrazolyl, COD = cyclooctadiene)43 and
diisocyanoalkanes in Rh2((CN)CnH2n(NC))444 chelate a dinuclear

metal center possessing no formal metal-metal bond. Simple
molecular orbital arguments suggest that the respective ground and
excited state configurations of these binuclear d8-d8 complexes are
represented as (d22)2(d22"‘)2 and (dzz)2(d22*)l(pz)l. Promotion of
an electron from the metal-metal antibonding framework to the
bonding orbital results in an increased metal-metal interaction in the

45-47

excited state. Electronic absorption and emission spectra and

”'50 of the ground and excited states of these

Raman spectroscopy
dimers substantiate this prediction.

With the molecular structure preserved in the excited state, a
rich photooxidation chemistry of d8-d8 complexes has been
observed.51’54 For instance, Pt2(POP)44' photocatalytically converts
isopropyl alcohol into acetone and hydrogen.51 The reaction

pathway for this transformation is shown in Figure 4. In this

13

Figure 4. Reaction cycle for the conversion of isopropanol to acetone

and hydrogen by using Pt2(POP)44' as a photocatalyst.

14

Pt2(POP)44‘ Cf“
CH3CCH3
ll Pt2(POP)4H4'
CH3CCH3
on

CH CCH

Pt2(POP)4H4‘ ill 3
Pt2(Pop)44-*
Pt2(POP)4H24‘
Pt2(POP)44‘ “V
H2

Figure 4

relic
me:
(Cl
equ

The

(11
llll
of

ch
st
ex
ex
ex
co

COi

F161
3101
Con

ltdu

15

reaction scheme, electronically excited Pt2(POP)44' abstracts the
methine hydrogen of isopropanol to give Pt2(POP)4H4' and
(CH3)2COH radical. Subsequent reaction of this radical with another
equivalent of Pt2(POP)44' results in the production of Pt2(POP)4H4'.
The Pt(II)Pt(III) mixed-valent Pt2(POP)4H4' intermediate undergoes
disproportionation to give Pt2(POP)44' and Pt2(POP)4H24’ which
has recently been observed.51b Reductive elimination of H2 from

Pt2(POP)4H24' completes the photocatalytic cycle. The overall two-

electron reaction is comprised of a photoinduced one-electron step

followed by an ensuing radical reaction. A similar reaction pathway,
observed in the reaction of Ir2(p -pz)2(COD)2 with 1,2-

dichloroethane,52

is shown in Figure 5. In this scheme, the ensuing
thermal reaction of the photoinduced radical is the terminating step
of the reduction of 1,2-dichloroethane to ethylene.

Careful examination of the above systems reveal a general
characteristic of all photosystems studied to date : a triplet excited

state yields photoproducts of triplet spin.2'5

More generally,
excitation of a transition metal complex usually produces a singlet
excited state which intersystem crosses to the lowest energy triplet
excited state. Relatively small energy gaps between the singlet and
corresponding triplet state in conjunction with strong spin orbital
coupling results in extremely efficient conversion of the singlet state
to the triplet state. The triplet excited state reacts to exclusively
yield primary photoproducts arising from electron transfer (8 = l) or
atom abstraction (s = 1). This chemistry is summarized in Figure 6.

Conversely, multielectron processes such as oxidative-addition,

reductive-elimination, and atom transfer inevitably yield singlet

16

Figure 5. Photoinduced bimolecular reaction of Ir2(p-pz)2(COD)2

with 1,2-dichloroethane.

17

 

 

 

h V ‘
06’ngch r ...... Egg/Ecx‘cc

l/ l I
0’ re
ClCHzCH2Cl

_ 'CH2CH2C l

+
Cl
fl /
4171 P 1‘13

 

OVA/1

47 “Nb

+

C2H4

Figure 5

 

fl

‘‘‘‘‘ Ir—Ir~.

l/ \“'

ClCH2CH2C l

 

.l

18

Figure 6. Bimolecular photoreaction pathways from a long-lived

triplet excited state of an inorganic complex M. kr, radiative decay
rate; km, nonradiative decay rate; and kisc, intersystem crossing rate
Bimolecular photoreaction pathways from long-lived triplet excited
state of an inorganic complexes M. kr, radiative decay rate; km, non-

radiative decay rate; kiso intersystem crossing rate.

l9

 

 

 

 

 

 

 

 

1(M*)
ll \
kisc
kabs
kct HX
hv i
kl' knr kr km
A/D
M-H + X-
L atom abstraction
M+/' + A'lD+
electron transfer
(l
M

Figure 6

20

products (Figure 7). Accordingly, spin considerations suggest that
successful multielectron photoreagents will derive their reactivity
from singlet electronic excited states. The successful development of
singlet excited state chemistry, however, will require that
intersystem crossing from the singlet state to triplet state be
inhibited. In addition a problem of equal significance in the
development of singlet excited state chemistry is the short lifetimes
of the photoactive state. Unlike tn'plet states which possess lifetimes
from microseconds to milliseconds owing to the spin-forbiddeness of
the relaxation process back to ground state, singlet state decay
(fluorescence) is usually only a few nanoseconds or even shorter.
These intrinsically short lifetimes do not permit the singlet excited
state to easily participate in bimolecular reactions. It is undoubtedly
these two features of singlet excited states that has inhibited their
reactivity. Thus, a critical issue to this thesis is whether inorganic
photosystems possessing lowest energy excited states of pure singlet
character exist with lifetime long enough to permit bimolecular
reactivity.

This question is addressed by beginning with multiply bonded
metal-metal (M-n-M) dimers. Since their discovery in 1964,
numerous M-A-M complexes, comprised primarily of rhenium,
chromium, molybdenum and tungsten metal cores with a variegated
array of ligating systems, have been discovered primarily due to the

55

efforts of Cotton and coworkers. The prevalence of quadruply

bonded metal complexes has stimulated numerous theoretica156'63

164-78

and experimenta investigations directed toward elucidating

their electronic structures.

21

Figure 7. Proposed bimolecular photoreaction pathways from a long-
lived singlet excited state of an inorganic complex M'. The

intersystem crossing rate, kisc’ is negligible for a large singlet-triplet

energy gap.

22

 

 

 

 

 

 

 

 

1(M'*>
ll
km
kox
kisc
hv HX BO
kr knr
1(X-M'-H)
oxidative-addition
1(M'=O) + B
oxygen transfer
Ml

Figure 7

23

The general molecular orbital diagram depicted in Figure 865

has evolved from these studies. The metal-metal bonding in
L4M ML 4 quadruply bonded dimers is most easily derived from a
MM core (Dock symmetry) in which the z axis of a right-handed

Cartesian coordinate system is chosen to lie along the metal-metal
axis. Linear combinations of the dz29 (dxz, dyz), and (dx2-y2,dxy)
atomic orbitals on each metal produce bonding and antibonding
molecular orbitals of 0, TI, and 5 symmetries, respectively. The n
and 5 molecular orbitals are each doubly degenerate in aDooh point
group. From the nodal theorem and atomic orbital overlap
considerations, the molecular orbital energy ordering of the diatomic

MM core shown in Figure 8 is obtained. The equidistant positioning

of eight ligands in aL4MML4 complex perturbs the bonding of the

diatomic metal core by lowering the symmetry of the system to D 4),,

thus, splitting the 5 orbital degeneracy. By definition of the
coordinate system in Figure 8, the ligands will lie along the x and y
axes and their orbitals will overlap with dx2-y2 as well as s, px, py
(which are not shown in Figure 8) to form its four metal-ligand
bonds. The linear combination of the ligand and metal dx2-y2 orbitals
produces 5(blg) and 5*(b2u) molecular orbitals which are
essentially metal-ligand O antibonding in character. This
antibonding interaction causes the energies of these orbitals to
increase dramatically in energy. The energies of the O, n, and

5(dxy) molecular orbitals, for the most part, are unperturbed as the

ligands approach the metal core and the L4MML4 molecular orbital

diagram for D4}, symmetry shown in Figure 8 is obtained.

Additionally, the characteristic properties of a diamagnetic ground

24

Figure 8. Relative energies of the d-derived molecular orbitals in
Dooh M2 and in a D4;1 M2L8 complex. The ground state of the d4-d4

M2L8 complex is lAlg (0211452).

25

 

 

 

 

 

 

__________ -— 821,03“)
0* .— ........
b2u(dx2-y2)
’o::—— blg(dx2-y2)
‘ e egmi‘)
11* """ .
> x, x
i: __. b1u(5*>
:: 5* ’ .’
m bzgas)
5 "
...... _ll_ _.l.L_ eu(fl)
n .......
_______________ Ji— alg(c)
O ________
y y {y {Y
L /L /
/ / I L I I-
/ /

 

 

x/

/ /
x/ l X/L '5” i

 

Figure 8

26

state, eclipsed ligand geometry (which is necessary for dxy overlap),

and exceptionally short metal-metal bond exhibited by quadruply
bonded complexes are also explained by this bonding scheme.
According to this simple molecular orbital picture, the lowest
energy transition arises from the promotion of an electron from the
5 to 5* molecular orbital. Polarized electronic absorption
spectroscopic investigations of various quadruply bonded metal

complexes verify that the lowest energy absorption corresponds to
the 52 -> 55* transition thereby confirming molecular orbital

predictions,65.68a,o9

Though conceptually quite appealing, the molecular orbital
model does not provide an accurate quantitative picture of the
electronic structure of quadruply bonded metal dimers, especially in
terms of 5 bonding. The poor overlap of the dxy orbitals involved
in 5 bonding suggests that a valence bond description of the
electrons in these orbitals would be more appropriate. Such an

approach was used in an ab initio calculation of the RezClgz'

electronic structure.58 Some of the results of which are outlined in
Table 1. In the valence bond model, the 1A1 g ground state has one
electron in each dxy orbital. At slightly higher energy (0.4 eV) is the
3A2u excited state, which still has one electron in each dxy orbital,
though now triplet paired. This state correlates with the 3(55*)
excited state of the molecular orbital model. Lying much higher in
energy are two ionic singlet excited states that arise from
antisymmetric (lAzu, 1.8 eV) and symmetric (lAlg, 2.2 eV)
combinations of configurations with both dxy electrons on one metal

center. These two states correlate with the 1(55*) and l(5"'2)

27

Table 1. State Orderings in d4-d4 Complexes: [Re2C18]2'

 

 

Ea/eV States

2.6 3A2.,(n n *)

2.6 1Egmfi‘“)

2.2 1A1g(5*5*)31[(xyA)(XyA) + (xyaxyan
1.3 1A2u(55*): lltxyAXxyA) - (xyBxyBH
1.3 3Eg(n5*)

0.4 3A2u(66*): 3[(xyA)(xyB)l

0.0 1A1g(55)3 11(XYA)(XYB)1

 

a Reference 58.

28

molecular orbital configurations, respectively. An important feature

of this model is that it predicts a large 1A2u 4 3A2u energy gap and
a relatively small separation between the lAlg ground state and the

3A2u level. This prediction is consistent with energy gaps estimated

from magnetic susceptibility measurements of MAM complexes.79

Within this manifold of four states, the only spin and dipole allowed

transition from the lAlg ground state is to the 1A2u excited state.

This effectively corresponds to a metal-metal charge transfer
(MMCT) transition and correlates with 1(52) -> 1(55*) excitation. On
the basis of this valence bond model, the large 1(55"')-3(55"')
energy gap should inhibit intersystem crossing and consequently the
55* excited state produced upon excitation should be of pure singlet
character. This prediction has been verified by several experimental
studies. The good 0-0 overlap between the absorption and emission
spectra of M-Z-i-M dimers at low temperature shows that the long-
lived excited states (I ~ 100 ns)8o’82 of quadruply bonded metal
dimers directly correspond to the 1(55*) excited state.65’81
Moreover, the long lived excited state is not quenched by typical

triplet energy transfer acceptor molecules,83

thereby further
verifying the singlet character of the 55*emissive state

Besides these unique long-lived singlet excited states, the
quadruply bonded metal dimers possess many other features that
presage their utility as polynuclear multielectron photoreagents.
First, the longevity of the 55 * excited state should permit the
electronically excited metal complex to participate in bimolecular
reactions. Secondly, MAM complexes should be capable of

maintaining their structural integrity upon irradiation. Theoretical

29

estimates predict the overall energetic contribution of the 5 bond to

58

the metal-metal bond energy to be less than 10%. Irradiation of

the 52 .. 55* transition will, therefore, only slightly weaken the
metal bond and photoinduced metal-metal bond cleavage reactions
will be excluded in these complexes. Third, the multiple metal-metal
bond may act as an electron source or sink in multielectron transfer
reactions while maintaining a strong metal-metal interaction. Walton

and coworkers have prepared a series of L4MML4 triply bonded

metal complexes with O 2 n 4 5 25 * 2 ground state electronic

84'86 In all cases investigated, these compounds

configurations.
exhibit facile one electron oxidation. The absence of the 5 bond in

02114525’“2 triply bonded complexes does not greatly perturb the
L4MML 4 structural unit as evidenced by the crystal structures of

Re2Cl4(PhPMe2)4n+ (n = 0, 1 ,2) complexes which show only slightly
shortened Re-Re bond distances (r(Re-Re) of Re2Cl4(PhPMe2)4n+, n =
o, 1, 2 is 2.241(1)A, 2.218(1)A and 2.215(2)A, respectively).87 The
ability of the multiple metal-metal bond to undergo only minor
structural reorganization upon changes in 5 and 5* occupancies may
greatly facilitate the photoredox chemistry of quadruply bonded
metal complexes. Fourth, the metal-metal core is coordinatively
unsaturated and substrates may readily add to the redox-active
metal center.88'90 Finally, the lowest energy 52 -* 55* transition
falls in the visible spectral region, and low energy photochemistry is
therefore possible.

It is rather surprising that the photochemistry of these systems
has virtually been ignored in view of the attractive photochemical

properties of quadruply bonded metal complexes. Most

30

photochemical studies to date have been confined to high energy

reaction pathways. The Re2C182' 91 and M02Cl4(PBu3)492 dimers

exhibit solvent assisted metal-metal bond cleavage chemistry upon

UV irradiation. More generally, however, the metal—metal bond is

retained and UV irradiation of M-4-M dimers in acidic solutions

results in photooxidation as shown by the following equations,93'95

M02(SO4)44' + H+ -—-}l]-)—- M02(SO4)43- + 1/2 H2 (13)

M02(aq)4+ + 2H+ _11_1_{_. M02(aq)(}J-OH)24+ + H2 (14)

 

M02C184' + W “V 0‘ A... M02C13H3' (15a)

M02C13H3‘ + W + Cl' —- M02C193' + H2 (15b)

Photochemistry of M-4—M dimers promoted by visible light has only

recently been observed. Dichloromethane or acetonitrile solutions of

Re2C182' can be photooxiditized (xexc > 600 nm) to the confacial

bioctahedral, RezClg', in the presence of electron acceptors TCNE

(tetracyanoethylene) or DDQ (2,3-dichloro-5,6-dicyano-1,4-benzo-

quinone) and chloride ion.96 However, the photochemistry basically

2

entails simple one-electron quenching of the Re2C13 ' excited state

to yield Re2C19' followed by trapping by chloride ion.

In view of the potentially rich multielectron photochemistry of
MA-M dimers, studies aimed at the systematic development of
1(55*) excited state chemistry were undertaken. A prerequisite for

elucidating the photochemistry of MAM dimers is a clear

31

understanding of their excited state dynamics. Accordingly, time-

resolved picosecond laser spectroscopic studies of M02X4L4 and

M02Cl4(LL)2 (where X = Cl, Br, I; L and LL = mono- and bi-dentate

phosphine ligand) dimers were performed at Brookhaven National
Laboratory. Transient absorption and kinetics results of these
studies will presented in Chapter 111, along with a discussion of the
excited state properties. Chapter IV describes our initial efforts to
develop multielectron transformations by quenching reactive
hydrides from MAM dimers. Investigations of the spectroscopy,
electrochemistry and the photoinduced two-electron transfer
chemistry of M02(HPO4)4"' (n = 2, 3, 4) in acidic solution are

presented. An important result of this work was the discovery that

protons efficiently quench 1(55*) excited states of MAM dimers
thereby circumventing multielectron reactivity. Accordingly,

Chapter V describes studies focussed on the photochemistry of
quadruply bonded M02(II,II) diphenyl phosphate in non-aqueous

solutions. The first observations of multielectron chemistry of M-4-

dimers promoted by visible light are presented. Specifically, two-

electron photoreductions of organic substrates are effected upon

irradiation of nonaqueous solutions containing M02(OZP(OC6H5)2)4

with wavelengths greater than 500 nm.

CHAPTER II

EXPERIMENTAL

A. Syntheses

1. General Procedures. Synthesis of all complexes were
performed by using standard Schlenk-line techniques. All chemicals
were reagent grade and were used as received unless otherwise
noted. Solvents were rigorously deoxygenated and dried prior to use.
For previously prepared compounds, electronic absorption
spectroscopy, cyclic voltammetry and NMR (when previously
reported) were employed for characterization. The details for
characterization of new compounds are presented with the discussion

on the preparative method.

2. Syntheses of Dimolybdenum Complexes.
a. M02(02CCH3)4.97 Molybdenum acetate, the starting

compound for most quadruply bonded dimolybdenum species, was
prepared by refluxing 10 g of Mo(CO)6 (37.9 mmole) (Aldrich

Chemical Co.) in 250 ml of o-dichlorobenzene which contained 30 ml
of CH3COOH and 10 ml of (CH3CO)20. The solution was refluxed for 5

32

33

h. The needle-shaped, yellow crystals which formed with cooling on

the sides of the round bottom flask were filtered and washed by

EtzO three times, dried, and stored under vacuum. Molybdenum

acetate is fairly air-stable but will decompose if exposed to air longer

than one week.

b. K4M02C18.98 Potassium chloride (6 g, 80 mmole) was
added to 200 ml of 12 M HCl(aq) with stirring. The solution

immediately changed from yellow to red upon the addition of 5 g of
M02(02CCH3)4 (11.7 mmole). After 1 h of stirring, a bright red

precipitate was collected, washed with EtOH (0 0C) and EtzO, and
then dried under vacuum. Similar to M02(02CCH3)4, K4M02C18 is

fairly air-stable.

c. (NH4)5M02C19°H20.99 The ammonium salt of
M02C184‘ was prepared by the same procedures as used for the

preparation of K4M02C18, except 4 g of NH4C1 (76 mmole) was used
instead of KCl. A reddish-purple solid precipitated after 1 h of

stirring. The solid was collected, washed three times with EtZO and

dried under vacuum.

d. CS3M02Br3.100 A 100 ml 48% HBr(aq) solution
containing 1 g of M02(OZCCH3)4 (2.3 mmole) was heated to 60 °C for

l h, after which 2.5 g of CsBr in 50 ml of 48% HBr(aq) was added

with stirring. The yellow-brown precipitate was collected by suction

filtration, washed in succession with EtOH and EtZO, and dried under

vacuum. The solid is air-stable.

34
101,81 -
e. M02C14(PMe3)4. Dimolybdenum chlorophos-
phine was prepared by either of two methods: (1) 0.5 g of

(NH4)5M02C19°H20 (0.807 mmole) was added to 50 ml of a pre-
mixed MeOH solution which contained 0.32 ml (3.23 mmole) of PMe3

(Strem chemicals). The final solution was stirred for 3 days, during
which the color of the solution slowly changed from red to purple.

The purple solid, which formed on the sides of the flask over this

period, was collected by suction filtration, washed with 5 ml of H20

to dissolve unreacted (NH4)5M02C19'H20, and finally washed three

times with 10 ml aliquots of MeOH. The compound was dried and

stored under vacuum. (2) The second method employed
Moz(02CCH3)4 as a starting material. To 40 ml of THF, 0.5 g of

M02(02CCH3)4 (1.17 mmole) was dissolved. Under an argon
counterflow, 0.59 ml (4.65 mmole) of Me3SiCl (Aldrich Chemical Co.)
and 0.47 ml of PMe3 (4.74 mmole) were added separately by

syringe. The blue solution was refluxed for 10 h. The blue residue
collected from removing solvent from the reacted solution was
column chromatographed on Florasil by using CH2C12 as the eluting
solvent. The first blue band was collected, and the solvent was
removed under vacuum to produce a blue solid. Compound produced
by each of these methods exhibit identical properties. Absorption
show strong absorption bands at 583 and 325 nm (E = 3110, 3350
M'1 cm'l, respectively) and a weak band at 440 nm (E = 200 M'1
cm'l). M02C14(PMe3)4 is air-sensitive and decomposes in l h when
solid compound is exposed to air. Solutions of the dimer decompose

within minutes.

35

f. M02C14(PBu3)4.102 This dimer is most easily prepared
by Method 1 of Section A.2.e : 0.5 g of (NH4)5M02C19°H20 (0.807

mmole) was added to 40 ml of a pre-mixed MeOH solution containing
0.81 ml of PBu3 (3.25 mmole) (Aldrich Chemical Co.). The purple

solution was refluxed for 3 h to yield purple crystals which were
collected by suction filtration, and washed with 5 ml of H20 to
remove unreacted (NH4)5M02C19'H20. The purified solid was
subsequently washed with MeOH and EtzO, dried, and stored under
vacuum. M02C14(PBu3)4 decomposes in air within few minutes. In

CH2C12 solution, M02C14(PBu3)4 exhibits three strong absorption

bands at 588, 330 and 287 nm (e: 3150, 3820, 3050 M-1 cm-l,
respectiveIY). and a weak band at 454 nm (E = 260 M'1 cm'l).

g. MozBr4(PBu3)4.100 A 30 ml MeOH solution containing
0.5 g of CS3M02Br8 (0.407 mmole) and 0.405 ml of PBu3 (1.63
mmole) was stirred for 3 h. The deep blue crystals, which formed
during stirring, were collected, washed with H20, EtOH and Et20 and
then dried and stored under vacuum. This air-sensitive complex
exhibits two strong absorption bands at 600 and 352 nm and two
weak bands at 485 and 460 nm (e = 3520, 5130, 210, and 162 M-1

cm'l, respectively).

h. M0214(PBu3)4. Procedures analogous to those
described for the synthesis of M02C14(PMe3)4 by Method 2 were
employed. A 40 ml THF solution charged with 0.5 g of
M02(02CCH3)4 (1.17 mmole), 0.67 ml (4.7 mmole) of Me3SiI (Aldrich
Chemical Co.) and 1.17 ml of PBu3 (4.7 mmole) was refluxed for 16 h.

36

The residue, collected by pumping the reacted solution to dryness,
was purified with chromatographic procedures identical to those
previously described (Section A.2.e). This air-sensitive compound

strongly absorbs at 590 nm.

i. M02C14(dppm)2.103 0.77 g (2.0 mmole) of dppm

(dppm = Bis(diphenylphosphino)methane, Aldrich Chemical Co.) was
added to 40 ml of MeOH which contained 0.5 g of (NH4)5M02C19°H20
(0.807 mmole) to yield a blue-green solution. This solution was

refluxed for 3 h. The blue crystals, formed during refluxing, were

collected by suction filtration, and consecutively washed with H20,
EtOH and EtzO. The blue solid is air-stable but in solution, it is air-
sensitive, decomposing within an hour after exposure to air.
Deoxygenated CH2C12 solutions of M02C14(dppm)2 exhibit two strong
absorption bands at 634 nm (5 = 2490 M-1 cm-l) and 325 nm (e =
5600 M'1 cm'l) and a weak band at 462 nm (E = 900 M'1 cm'l).

j. M02Cl4(dmpm)2.107 0.5 g of (NH4)5Mo2c19-H20
(0.807 mmole) and 0.26 ml (1.63 mmole) of dmpm (dmpm =

bis(dimethylphosphino)methane, Strem Chemicals) were reacted in
40 ml of MeOH solution following a procedure analogous to that used
for the preparation of M02C14(dppm)2. M02C14(dmpm)2 is an air-
sensitive compound, and in CH2C12 solutions, exhibits two absorption

bands at 604 nm (e = 1730 M'1 cm'l) and 426 nm (e = 270 M'1
cm'l). 1H NMR: 1.51(s), 3.08(p) (1H,}, = 5.1 Hz).

37

k. M02Cl4(dppe)2.103:105 This dimer is prepared by
refluxing a 40 ml THF solution charged with 0.5 g of M02(OZCCH3)4
(0.17 mmole), 0.59 ml of Me3SiCl (4.65 mmole) and 0.96 g (2.4
mmole) of dppe (dppe = bis(diphenylphosphino)ethane, Aldrich
Chemical Co.) for a minimum of 12 h. The solution changed from
deep red to blue within 15 min, and then to mauve after 5 h of
refluxing. The mauve precipitate was collected by suction filtration,

and washed consecutively with H20, EtOH and EtZO. M02C14(dppc)2

is a very air-stable compound in both solid and solution forms. In

CHzClz solutions, the compound exhibits four electronic absorption

bands at 762, 548, 469, and 345 nm (E = 1030, 210, 780, and 8050

M '1 cm'l , respectively)

I. M02Cl4(dmpe)2.106 This complex is prepared
according to the previously described procedure for the synthesis of
Mo2C14(PMe3)4 by Method 2. A 40 ml THF solution which contained
0.5 g of M02(02CCH3)4 (1.17 mmole), 0.59 ml of Me3SiCl (4.65

mmole) and 0.4 ml (2.4 mmole) of dmpe (dmpe = bis(dimethyl-
phosphino)ethane, Aldrich Chemical Co.) was refluxed for 8 h. The
volume of the final reddish-brown solution was slowly reduced to
about 10 ml over 2 h by vacuum distillation. The reddish-brown
crystals which formed during solvent evaporation were collected by
suction filtration under a heavy argon flow. M02C14(dmpe)2 is an
air-sensitive compound which decomposes within an hour in air. The
absorption spectrum of this dimer exhibits three weak bands at 803,
565, and 445 nm and a strong absorption at 345 nm (E = 210, 190,
300, and 3200 M'1 cm'l, respectively).

38
m. M02(HPO4)44' Anion. The M02(II,II) phosphate
complex was synthesized by ligand exchange reactions. Solutions of
the dimer were prepared by three methods: (1) K4Mo2C18 (0.1 g)
was added to 10 ml of a 1 M H3PO4 solution and stirred under
purified argon for 4 h; (2) 0.1 g of K4M02(SO4)4107 was added with
stirring to 2 M H3PO4 and allowed to react for 2 h; and (3) 1 ml of

concentrated H3PO4 was added to 10 mM solution of M024+ 108

aquo dimer blanketed with argon. For each of these methods, a pink
solution was obtained with a characteristic absorption band at 516
nm. Aqueous solutions of the dimer are extremely sensitive to
oxygen decomposing within minutes to dark brown uncharacterized
solutions. Even in the absence of oxygen, the quadruply bonded
dimer slowly reacts over days to produce blue-gray solutions which
exhibit a near-infrared absorption profile diagnostic of
M02(HPO4)43' (vide infra). Solutions of the quadruple-bond dimer
prepared by each of the three methods are contaminated inevitably
with M02(HPO4)43', and accordingly, addition of K2HPO4 to any of
the above solution yields solid K4M02(HPO4)4 contaminated with
K3M02(HPO4)4. However, addition of zinc amalgam to these
solutions smoothly converts M02(HPO4)43' to the quadruply bonded
M02(HPO4)44' species, as evidenced by the disappearance of the
near-infrared absorption band of M02(HPO4)43'. In fact, stirring 2
M H3PO4 solutions of K3M02(HPO4)4 over zinc amalgam provides a

convenient route to the preparation of pure M02(HPO4)44' solutions.

11. K3M02(HPO4)4. The potassium salt of M02(HPO4)43'
was prepared from solutions of M02(HPO4)44'. Addition of Pt foil (1

39

cm2) to 25 ml of 1 M H3PO4 containing M02(HPO4)44’ (0.4 mmole)
quantitatively produces M02(HPO4)43' in 2 h. Dropwise addition of
a saturated solution of KZHPO4 leads to the immediate precipitation
of a fine blue-gray powder. The product was filtered under vacuum
and washed with three 5 m1 portions of 1 M H3PO4 at 0 oc. Aliquots
for all washings were delivered by cannula. The solid was dried and

stored under vacuum. Alternatively, a more facile route to the

mixed-valence dimer involved the addition of 0.505 g of K4M02C13
(0.800 mmole) to 15 ml of degassed 1 M H3PO4. After the solution

was stirred at room temperature for 1 h under argon, 0.108 g of

K28203 (0.400 mmole) was added to the solution via a sample

charger. The red solution quickly turned blue, and a blue-gray
powder precipitated from the solution. The solid was collected by

using Schlenk-line techniques. Anal. Calcd (Found) for
K3M02H4P4016: Mo, 27.68 (27.60). Solids and solutions of the

mixed-valence dimer are sensitive to oxygen, but the compound is

indefinitely stable under inert atmospheres.

o. Py3M02(HPO4)4Cl.109 A solution of 2.5 m1 2 M
H3PO4, charged with 0.1 g of K4M02C13 (0.16 mmole), was slowly

mixed with 0.3 g of pyridinium chloride (2.62 mmole) dissolved in

2.5 ml 2 M H3PO4. The mixture was allowed to stand in air and after

2-3 days, deep purple crystals had formed. A more facile synthesis
of M02(III,III) phosphate involved the use of K28208 (0.135 g, 0.5

mmole) as an oxidant which was added with stirring to 10 ml of a 1
M H3PO4 solution containing 0.316 g of K4M02C18 (0.5 mmole). The

solution turned from red to colorless to violet within minutes, and a

40

violet solid precipitated immediately after the addition of 0.202 g of
CsCl (1.2 mmole) in 10 ml of 1 M H3PO4. The solid was collected by

suction filtration under argon.

p. M02(02P(OC6H5)2)4. The M02(II,II) diphenyl
phosphate was prepared by a simple ligand substitution reaction.
The anion (C6H50)2P02' was generated by stirring 3.25 g of
(C6H50)2P(O)OH (l3 mmole) with 0.702 g of NaOMe (13 mole) in
60 ml of MeOH. After 10 min, 0.5 g of (NH4)5M02C19°H20 (0.807
mmole) was added to the above solution which was then heated to
reflux for 3 h. A pink precipitate, formed during refluxing, was
collected by suction filtration and washed with three 15 ml portions
of MeOH. Aliquots for all washings were delivered by cannula. The
solid was dried and stored under vacuum. Yield: 76%. Suitable
crystals of M02(02P(OC6H5)2)4-2THF for X-ray structural
determination were obtained by layering a tetrahydrofuran solution
of M02(02P(OC6H5)2)4 with cyclohexane in a Schlenk tube. Anal.
Calcd (Found) for M02C43H4016P4'2C2H30: Mo; 14.4 (14.87); P, 9.29

(9.17); C, 50.46 (49.93); H, 4.24 (4.15). This newly synthesized
dimolybdenum diphenyl phosphate complex is extremely air-
sensitive. It decomposes within seconds upon exposure to air. This

dimer is not soluble in most organic solvents, but it exhibits fairly
good solubilities in halogenated hydrocarbons and THF. A CHZCIZ

solution of M02(02P(OC6H5)2)4 exhibits a moderate intensity band

at 515 nm , which is very similar to M02(HPO4)44' in acidic solution.

41

q. M02(02P(OC6H5)2)4BF4. The mixed-valent
M02(II,III) diphenyl phosphate complex can be prepared by direct
oxidation of the M02(II,II) complex. 1.001 g (0.85 mmole) of
M02(02P(OC6H5)2)4 and 0.099 g (0.85 mmole) of NOBF4 (Aldrich
Chemical Co.) were rigorously stirred in 30 ml of CH2C12. After 45

min the solution changed from pink to green. The volume of solution
was reduced to 10 ml by vacuum distillation, whereupon 50 ml of
cyclohexane was added. A green precipitate formed immediately.
The solid was collected by suction filtration. Anal. Calcd (Found) for
M02C43H40016P4BF4 : C, 45.20 (41.23); H, 3.16 (3.15); P, 9.71
(9.33); F, 5.96 (6.13). Yield 85%. X-ray quality crystals were

obtained by layering a dichloromethane solution of
M02(OZP(OC6H5)2)4BF4 with cyclohexane in a Schlenk tube. The

Mo2(II,III) diphenyl phosphate complex is not as air-sensitive as the
corresponding M02(II,II) dimer. Solid exposed to air is stable but not
longer than 30 minutes. However, solutions of M02(02P(OC6H5)2)4-

BF4 is very air-sensitive; both the solid and solution are thermally

unstable, decomposing when heated higher than 80 0C.

B. Solvent Purification

l. Solvents Used for Syntheses. Hexane, cyclohexane and
diethyl ether were refluxed over Na for no less than 6 h and freshly
distilled prior to use. MeOH was also refluxed with Na. The amount of
Na added to MeOH was 20% more than the amount required to react
with the water contained in a freshly opened bottle (which can be

calculated from the water content shown on the label of the bottle).

42

Benzene, toluene and tetrahydrofuran were refluxed with sodium

and small amount of benzophenone to form the blue or purple ketyl
radical. Halogenated hydrocarbons such as CH2C12, ClCHzCHZCl, and

CHzBrz were refluxed over P205 for at least 6 h prior to use.
Acetonitrile and acetone was purified by refluxing over CaH2 and

anhydrous CaSO4, respectively.

2. High Purity Solvents. All solvents used for
electrochemical, spectroscopic and photochemical experiments were
purchased from Burdick and Jackson Laboratories (spectroscopic
grade). Purified solvents were stored in glass containers which
consisted of a 1 liter flask equipped with a Kontes high-vacuum
valve. Dichloromethane, 1,2-dichloroethane and isopropanol were
degassed by seven freeze-pump-thaw cycles and then vacuum
distilled onto activated (heated to 250 0C under dynamic vacuum
(10"6 torr) for 12 h) Linde 4 A molecular sieve contained in the
storage flask. Hexane, cyclohexane, and diethyl ether were also
degassed by seven freeze-pump-thaw cycles and vacuum distilled
onto sodium-potassium alloy. Tetrahydrofuran, benzene, and toluene,
again after seven freeze-pump-thaw cycles were distilled into a flask
containing sodium-potassium alloy with a small amount of
benzophenone. The purple ketyl form of benzophenone formed over
a two-day period. Acetonitrile was degassed by seven freeze-pump-
thaw cycles and vacuum distilled onto activated Linde 3 A molecular

sieve.

43

C. Instrumentation and Methods

1. Absorption Spectroscopy. Absorption spectra were
recorded with Cary 17D or Cary 2300 spectrophotometers. Extinction
coefficients were determined by using high-vacuum cells consisting
of a l-cm quartz cuvette and a 10-ml side arm. These two chambers
were separated by two Kontes high-vacuum quick-release teflon
stopcocks. For measurements of molar absorptivity coefficients in
nonaqueous solution, weighed samples were placed in the cuvette
and isolated by the Kontes valve. The appropriate high purity solvent
was transfered to the 10-ml side arm by bulb-to-bulb vacuum
distillation, and three subsequent freeze-pump-thaw cycles were
performed before mixing with the sample. After the absorption
spectra was recorded, the solution were pipetted out of the cell and
the volume was measured. For measurements in aqueous solutions,
known volumes of phosphoric acid were pipetted into the side arm

and subject to five freeze-pump-thaw cycles before mixing with

weighed samples of solid K3M02(HPO4)4 contained in the cuvette
cell. For Moz(HPO4)44', known concentrations of M02(HPO4)44'
were prepared by zinc amalgam reduction of M02(HPO4)43'

solutions. Extinction coefficients were calculated from Beer-Lambert

plots composed of at least seven points.

2. Steady-State Luminescence Spectroscopy. Emission
spectra were recorded on an emission spectrometer constructed at
Michigan State University. A detail description of the instrument is

described in the doctoral thesis of Mark D. Newsham. Emission

44

quantum yields, were measured by using Ru(bpy)3(ClO4)2 as the

quantum yield standard ($6 = 0.042 in H20 at 23 0C). The emission

quantum yield was calculated with the following equation,110

<I>x=<brx [Ar()\r)/Ax(>\x)l x [Tle/l'lrzl x [Dx/Drl (16)

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 AOc)

is the absorbance/cm of the solution at the exciting wavelength X.

3. Time-Resolved Laser Spectroscopy. Picosecond laser
spectroscopic experiments were undertaken in Brookhaven National

Laboratory. The instrumentation has previously been described.111

4. Magnetic Measurements. EPR spectra were recorded by
using a Bruker ER 200D X-band spectrometer equipped with an
Oxford ESR-9 liquid-helium cryostat. Magnetic fields were measured
with a Bruker ER035M gaussmeter, and frequencies were measured
with a Hewlett-Packard 5245L frequency counter. Magnetic
susceptibility measurements were made with a SHE 800 series

variable-temperature SQUID magnetometer.

5. Resonance Raman. Raman spectra were recorded with a
Spex 1401 double monochromator and associated Ramalog
electronics. A Spectra Physics 165 argon ion laser was the excitation

source, and incident powers were 20-40 mW. All spectra were

45

collected at a 900 scattering geometry from solid samples at room

temperature.

6. Electrochemical Measurements. The oxidation-reduction
potentials of dimolybdenum complexes were determined by cyclic
voltammetry 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. Cyclic
voltammetry measurements of aqueous solutions were performed at
room temperature by using a conventional H-cell design and a three-
electrode system consisting of a polished glassy carbon working
electrode (A = 0.08 cmz), a Pt wire auxiliary electrode, and a
saturated calomel reference electrode. For nonaqueous solutions, a
three-electrode system was again used with a standard H-cell
configuration. The working electrode was a Pt button, the auxiliary
electrode was a Pt gauze, and a Ag wire served as an adequate
reference potential. Ferrocene was used as an internal standard.
Potentials were related to the SCE reference scale by using a

ferrocenium-ferrocene couple of 0.307 V vs. SCE”:2

7. Photochemistry. Sample irradiations were executed by
using a Hanovia 1000-W Hg/Xe high-pressure lamp. The source for
ultraviolet light at 254 nm was an Oriel 14W Hg/Ar low-pressure
lamp. The beam was collimated and passed through a 10-cm
circulating water filter. Photolysis experiments were performed in

two-arm evacuable cells equipped with Kontes quick-release teflon

46

valves. Sample temperatures were thermostated at 15.0 i 0.5 0C in
all photoreactions. The photolysis experimental set-up is shown in
Figure 9. Photolyses were accomplished by using Schott color glass
high-energy cutoff filters to eliminate the unwanted higher energy
light. For quantum yield measurements, excitation wavelengths were
isolated by using interference filters with half-band widths of less
than 10 nm at the given mercury line ( e.g. 254, 313, 367, 405, 436,
546, 577 nm ). All interference filters were purchased from Oriel
Corporation. Quantum yields were determined with the following

expression

or = [np/tl /lIo(M<1-10'AM/tl (17)

where np is the number of product molecules formed during
photolysis, 10(X) is the intensity of exciting light at wavelength 7t,
A)‘ is the absorbance of reactants at the exciting wavelength X, and

t is the time of irradiation.113 100.), determined by using the

ferrioxalate actinometer,

[Fe(CzO4)3]3' —h—V—. [Fe(CZO4)2]2' + 2c02 (18)

follows directly from,

Iom=npea+ / til-10‘“) x «>1. x tl (19)

where AX is the absorbance of ferrioxalate at exciting wavelength X,

(bx is the actinometer's quantum yield at X (listed in Table 2), t is

47

Figure 9. Experimental apparatus for typical photolysis.

48

a oSmE
:8 29:3 3853
~33:

53 533 l \
macs—zoto/t/ \

5:: 5.8

      

 

 

 

3:25
3.22:6

 

 

 

om:o__aoou

 

 

 

\
53 co~m3.\.\

mace—:85

 

 

49

Table 2. Quantum Yield Dataa for Ferrioxalate Photoreaction.

 

 

7t Inm [K3Fe(C204)3]/M (D173 2+
577 0.15 0.013
546 0.15 0.15
436 0.15 1.01
405 0.006 1.14
367 0.006 1.21
313 0.006 1.24
254 0.006 1.25

 

a Reference 1 13.

50

2+

the time for irradiation, and “Fe2+ is the number of Fe ions formed.

2+ ion is most easily accomplished by complexing the

2+

Analysis of Fe

ion with 1,10-phenanthroline. The complexed Fe species absorbs

strongly at 510 nm (E = 1.11><10'4 M'1 cm'l) in water. “Fe2+ is

calculated directly from,

nF.2+ =l6.023x1020xV1xV3xAFe2HIIV2 x EFe2+l (20)

where V1 is the volume of the actinometer (ml), V2 is the volume of
the aliquot withdrawn for analysis (ml), V3 is the final volume to
which aliquot V2 is diluted (ml), Apez+ is the absorbance of ferrous

phenanthroline at 510 nm, and EFe2+ is its molar extinction

114

coefficient. Quantum yields of multiply bonded molybdenum

dimers were computed by reexpressing eq. (17) as follows,

ox =[AA xVx 6.023XI020]/[E XIOX (1-10'A7k) x t] (21)

where AA is the change of absorbance at the monitoring wavelength
(1438 nm for Mo2(HPO4)43' and 516 nm for M02(02P(OC6H5)2)4),
V is the volume of irradiated solution in ml, 8 is the molar
extinction coefficient of the monitored absorption band (180 and 156
M'1 cm'1 for M02(HPO4)43' and M02(02P(OC6H5)2)4, respectively),
A)‘ is the absorbance of reactants at excitation wavelength X, and t
is the time employed for photolysis. In quantum yield
measurements, the excitation procedures for standard and unknown

have to be identical. Therefore, the special photolysis train, shown in

51

Figure 10, was designed to ensure that the required reproducibility

was attained.

8. Gas Analysis. Gas produced from photoreactions was
qualitatively identified by gas chromatographic analyses. A Hewlett-
Packard 5710A gas chromatograph equipped with a 100/200
carbosieve S-II column and FID detector was employed for CC
measurements. Quantitative determination of the gas evolved was
made by Toepler pumping the gas above reacted solutions through
two traps (liquid-nitrogen traps for hydrogen measurements, and

pentane/N2(1) traps for ethylene measurements) into a calibrated

volume and manometrically measuring the pressure.
D. Crystal Structure Determinations

1. General Procedures. Crystal structure determinations were
performed by Dr. Donald L. Ward of the X-ray Crystal Structure
Facility at Michigan State University. Preliminary examination and
data collection were performed with Mo K01 radiation (A =
0.71073A) on a Nicolet P3F diffractometer. All calculations were
performed on a VAX 11/750 computer by using SDP/VAX.114

2. M02(02P(0C6H5)2)4°2THF.

a. Data Collection. A dark purple prismatic crystal of
tetrakis(diphenyl phosphate)dimolybdenum, M02(OZP(OC6H5)2)4,

having approximate dimensions of 0.40 x 0.40 x 0.68 mm, was

52

Figure 10. Experimental apparatus for photoreaction quantum yield

measurements.

53

2 255

 

 

noses _ao:ao

 

 

 

 

 

.oxoa_
553 an r 1111
. .
_
.tt _
530; 3
_
:oo . _
29:3 (1\ n _
b p

5::
55555::

 

 

 

l0. 0 ‘I
"0‘.

 

 

 

 

 

 

 

 

 

 

 

 

342;. 114.
c5225

53 553
3:22:85

 

 

 

 

 

 

N

5.5 533
act—2:35

 

 

 

 

biawcfih
32.53
55:3
.1 953
-1 m:
525. :58
5:25
52.55

54

mounted in a glass capillary with its long axis roughly parallel to the
(p axis of the goiniometer.

Cell constants and an orientation matrix for data collection
were obtained from least-squares refinement, by using the setting
angles of 17 reflections in the range 15 < 26 <20°. The monoclinic
cell parameters and calculated volume are: a = 13.359(3), b =
29.64l(15), c = 14.719(3)A, [3 = 93.43(2)°, V = 5818(6)A3. For Z = 4
the calculated density is 1.522 g/cm3. As a check on crystal quality,
omega scans of several intense reflections were measured; the width
at half-height was 0.260 with a take-off angle of 6.00, indicating good

crystal quality. From the systematic absences of

OkO k = 2n (22)
h01 h+l = 2n (23)

and from subsequent least-squares refinement, the space group was
determined to be P21/n.

The data were collected at a temperature of 23(1) 0C by using
the u) scan technique. The scan rate was 4°/min (in U) ). Data were
collected to a maximum 28 of 45°. The ratio of peak counting time
to background counting time was 1:1. The diameter of the incident
beam collimator was 1.5 mm and the crystal to detector distance was

19 cm.

b. Data Reduction. A total of 7973 reflections were
collected, of which 7597 were unique and not systematically absent.

As a check on crystal and electronic stability three representative

55

reflections were measured every 93 reflections. The slope of the
least-squares line through a plot of intensity versus time was
-325(6) counts/h which corresponds to a total loss in intensity of
68.5%. A nonlinear decay correction was applied. The correction
factors on F ranged from 1.0 to 2.71 with an average value of 1.41.
Lorentz and polarization corrections were applied to the data.
The linear absorption coefficient is 5.964 cm'1 for Mo K01 radiation.
A numerical absorption correction was made. Relative transmission
coefficients ranged from 0.801 to 0.863 with an average value of
0.837. A secondary extinction correction was applied.116 Intensities
of equivalent reflections were averaged. The agreement factors for

the averaging of the 752 observed and accepted reflections was 2.5%
based on intensity and 2.2% based on F0.

c. Structure Solution and Refinement. The structure
was solved by using the direct methods which revealed the positions
of the Mo, 0, and P atoms. The remaining atoms were located in
succeeding difference Fourier syntheses. Hydrogen atoms were
included in the refinement but restrained to ride on the atom to
which they are bonded. The structure was refined in full-matrix
least-squares where the function minimized was £w(|Fol - IFCI)2 and
the weight w is defined as 1.0 for all observed reflections.

Scattering factors were taken from Cromer and Waber.117
Fe; 118

Anomalous dispersion effects were included in the values for

Af' and Af" were those of Cromer.119 Only the 2363 reflections
having intensities greater than 3.0 times their standard deviation

were used in the refinements. The final cycle of refinement, which

56

included 351 variable parameters, have not yet converged (largest
parameter shift was 2.23 times is esd) with unweighted and

weighted agreement factors of,

R1 = 2 11:0 - Fcl / 2 F0 = 0.091 (24)
R2 = [(2 w(|Fo| - chhz/z wF02)]2 = 0.103 (25)

The standard deviation of an observation of unit weight was 10.72.
The highest peak in the final difference Fourier had a height of 1.30

F120 of 0.15; the minimum

e/A3 with an estimated error based on 0
negative peak had a height of -1.03 e/A3 with an estimated error

based on OF of 0.15.

3. M02(02P(0C6H5)2)4BF4.
a. Data Collection. A dark green prismatic crystal of
tetrakis(diphenyl phosphate)dimolybdenum tetrafluoroborate, M02-

(02P(OC6H5)2)4BF4, having approximate dimensions of 0.40 x 0.40 x

0.8 mm, was mounted in a glass capillary with its long axis roughly
parallel to the (9 axis of the goiniometer.

Cell constants and an orientation matrix for data collection
were obtained from least-squares refinement, by using the setting
angles of 20 reflections in the range 20 < 26 <24°. The triclinic cell
parameters and calculated volume are: a = 10.917(8), b = 11.793(4), c
= l2.430(4)A, or = 6345(2) p = 7038(4), y = 70.64(5)° V =
5818(6)A3. For 2 = 1 the calculated density is 1.608 g/cm3. As a
check on crystal quality, omega scans of several intense reflections

were measured; the width at half-height was 0.260 with a take-off

57

angle of 6.00, indicating good crystal quality. From subsequent least-
squares refinement, the space group was determined to be PT.

The data were collected at a temperature of 29.0 0C by using
the 6/26 scan technique. The scan rate was 4°/min (in 26). Data
were collected to a maximum 26 of 55°. The ratio of peak counting
time to background counting time was 1:1. The diameter of the
incident beam collimator was 1.5 mm and the crystal to detector

distance was 19 cm.

b. Data Reduction. A total of 12192 reflections were
collected, of which 6096 were unique and not systematically absent.
As a check on crystal and electronic stability three representative
reflections were measured every 93 reflections. The slope of the
least-squares line through a plot of intensity versus time was —26(5)
counts/h which corresponds to a total loss in intensity of 1%. A
linear decay correction was applied. The correction factors on F
ranged from 1.000 to 1.005 with an average value of 1.002.

Lorentz and polarization corrections were applied to the data.
The linear absorption coefficient is 6.625 cm'1 for Mo K01 radiation.
A numerical absorption correction was made. Relative transmission
coefficients ranged from 0.779 to 0.828 with an average value of
0.808. A secondary extinction correction was applied.116
Intensities of equivalent reflections were averaged. The agreement

factors for the averaging of the 12192 observed and accepted

reflections was 1.2% based on intensity and 1.0% based on F0.

58

c. Structure Solution and Refinement. The structure
was solved by using the Patterson heavy-atom method which
revealed the positions of the Mo and O atoms. The remaining atoms
were located in succeeding difference Fourier syntheses. Hydrogen

atoms were refined isotropically. The structure was refined in full-

matrix least-squares where the function minimized was Zw(|Fol -

IFcl)2 and the weight w is defined as 1.0 for all observed reflections.

Scattering factors were taken from Cromer and Waber.11‘7

Anomalous dispersion effects were included in PC;118 the values for

Af' and Af" were those of Cromer.119 Only the 5107 reflections
having intensities greater than 3.0 times their standard deviation
were used in the refinements. The final cycle of refinement, which
included 436 variable parameters, has not yet converged (largest
parameter shift was 3.31 times is esd) with unweighted and
weighted agreement factors described by equations (24) and (25)

with R1 = 0.027 and R2 = 0.027. The standard deviation of an

observation of unit weight was 0.654.

CHAPTER III

SPECTROSCOPIC STUDIES OF
M02X4(PBu3)4 AND M02C14(LL)2 COMPLEXES

A. Background

A prerequisite to the elucidation of the photochemistry of
quadruply bonded metal dimers is a clear understanding of their
excited state properties. As mentioned in Chapter I, the
spectroscopic properties of quadruply bonded dimers have been
intensely investigated64'78 since Cotton and coworkers first
identified this class of complexes in 1964. A general conclusion of
the absorption studies is that the lowest electronic transition, which
typically falls in the visible region, is 52 -’ 55*.65 In contrast, the
emissive properties of quadruply bonded metal dimer complexes
vary significantly and a consistent description of 55* luminescence
has not yet been achieved.

Promotion of an electron from the 5 bonding orbital to the 5*
antibonding orbital results in an excited state with an electronic

configuration of 02n455* thereby reducing the overall bond order

from 4 to 3. Simple group theory shows that the d22 orbital is totally

59

60

symmetric and the linear combinations of dxz and dyz orbitals are

also totally symmetric; therefore, for complexes with an electronic

configuration of 02n455*, there is no energy barrier for rotation

about the metal-metal bond. Thus, the eclipsed geometry (D 4h) of
the quadruply bonded complex in the ground state is lowered to D4d

or D4 by torsional rotation of the ML4 fragments in the 1(55*)

excited state.80 This free rotation provides an effective nonradiative
decay pathway from the excited state thereby resulting in low
emission quantum yields and short emission lifetimes. The torsional
model accounts for the lack of a mirror image between the
absorption and emission spectra of many quadruply bonded
complexes. Presumably, sterically small ligands of M2X8 or M2X4L4
complexes permit free rotation about the metal-metal bond. The fact
that the apparent origins in low temperature emission spectra for

such complexes do not overlap results from distortions along two

coordinates (valg(M-M) and the torsional mode); this commonly is

121

referred to as a Duschinsky effect. On the other hand, large

bulky ligands of quadruply bonded metal dimers such as the
phosphines in M02C14(PMe3)4 and M02Cl4(PBu3)4 are interlocked in

a Dz d geometry and create a large steric barrier to rotation about

the metal-metal bond. In these cases, the 02d symmetry is

preserved in the 1(55*) excited state and a mirror image of
absorption and emission profiles is observed.80’81

The above observations imply that the excited state dynamics
of quadruply bonded metal dimers are closely related to the
molecular structures in the ground and the 1(55*) excited state.

Our interest in developing the chemistry of electronically excited

61

M-4—M dimers has prompted us to assess the generality of the D4},

4* 04d torsional model, and more generally to define the factors

governing the nonradiative processes of MAM dimers. These
studies have become possible with Cotton and coworkers synthesis of

a series of complexes with a general formula M02X4(LL)2, where X is

a halide, and LL is a bridging bidentate phosphine

(1.106’107’122'124 In these complexes, steric restrictions of the

ligan
LL ligands introduce various angles of internal rotation away from
the perfectly eclipsed conformation. The rotation is defined by a
torsional angle, X, shown in Figure 11. The torsional angles of a
series of M02X4(LL)2 complexes as determined by X-ray
crystallography are shown in Table 3. When it is equal to zero, there
is no distortion and the molecule is in an eclipsed geometry which

maximizes the dxy overlap. The overlap of dxy orbitals decreases

with increasing x, and becomes nonbonding when a completely
staggered conformation X = 45° is achieved.12421

In this chapter, we report and discuss the absorption, emission,
and transient absorption spectroscopic studies of a series of
M02X4(LL)2 complexes possessing various torsional angles. All the
lifetime measurements and transient absorption spectra displayed in
this chapter were collected at the picosecond laser facility, under the

administration and directorship of Dr. Jay Richmond Winkler, at

Brookhaven National Laboratory.

62

Figure 11. (a) 02d structure of quadruply bonded chlorophosphino
dimolybdenum M02X4L4; (b) Newman projection of M02X4(LL)2
complexes where X = Cl, Br, I; L and LL = mono- and bi-dentate

phosphine ligands, respectively.

63

: 255E

 

 

 

 

 

 

 

 

 

 

64

Table 3. Torsional angles of structurally characterized

M02X4(PBu3)4 and M02C14(LL)2 complexes.

 

 

MoiMo complexes XIde g ref.
M02C14(PBu3)4 0 a
MozBr4(PBu3)4 0 a
M0214(PBu3)4 0 a
M02C14(dppm)2 0 1 22
M02C14(dmpm)2 0 107
M02C14(dppe)2 30.5 123
M02C14(dmpe)2 4 0 106

 

a X-ray crystal structure determinations of M02X 4(PBu3 )4

complexes, have not been performed. However, the assumed eclipsed

geometry for these dimers is reasonable by analogy to the data
provided for M02X4(PMe3)4 complexes (ref. 78c).

65

B. Results and Discussion

1. Absorption Spectroscopy. The electronic absorption of
M02X4(PR3)4 (R = alkyl) complexes has been studied in detail by

68,78,79

Gray and Trogler and their coworkers. Spectra of

M02X4(PBu3)4 complexes, shown in Figure 12, exhibit a distinct
band at ~600 nm possessing an large molar extinction coefficient
(E~3x103 M'1 cm’l) attributable to the 52 -> 55* transition. The
shift of this transition to lower energy relative to M02C184‘ (515 nm)

and "M0203" complexes (510-520 nm) may be explained within a

simple valence bond framework. As discussed by Trogler et al.,68
good donor ligands (such as phosphine) place significant electron
density on the metal centers, thereby expanding the valence shell
and increasing the 5 overlap. Increased dxy orbital overlap is
manifested in the 1(55*) excited state acquiring more covalent
character and, as a result, the spin pairing term contribution to the
overall transition energy is reduced. Therefore the 1(55*) excited
state is shifted to lower energy. The molar extinction coefficients for

the 52 -’ 55* transition of quadruply bonded metal complexes, in

general, are small for a transition which is fully allowed. However,

125

from the work of Mulliken, it is known that the oscillator

strength of a one-electron N -’ V transition is related to the square of

the orbital overlap. The relatively low intensity of this transition is

therefore a consequence of the small orbital overlap (~01) of the dxy

orbitals that comprise the 5 bond. To this end, the unusually large

1(55*) intensities of the M02X4(PR3)4 complexes can be attributed

to the enhanced overlap of the 5 bond in phosphine ligated

66

Figure 12. Electronic absorption spectra of M02X4(PBu3)4 where X =
Cl (—), Br (---), and I (- - -).

67

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68

complexes. However as recently pointed out by Hopkins et al.,78 the
intensities are much too large to be exclusively accommodated by
increased orbital overlap resulting from secondary donor effects by
the phosphine ligands. This has led these authors to suggest that the
strong 52 -* 55* absorption of M02X4(PBu3)4 complexes is due to
the stealing of intensity from the lowest energy ligand-to-metal

charge transfer (LMCT) transition which occurs at ~330 nm (£~104

M’1 cm'l) in the M02X4(PBu3)4 complexes.78d

The absorption spectra of eclipsed M02C14(LL)2 complexes,
M02C14(dppm)2 and M02C14(dmpm)2, are similar to that of
M02C14(PBu3)4. The 52 -+ 55* transition and the molar extinction
coefficients are 634 nm (e = 2490 M’1 cm'l) and 604 nm (e = 1730
M'1 cm'l) for M02C14(dppm)2 and M02C14(dmpm)2, respectively.
In contrast, the staggered M02C14(LL)2 complexes exhibit 1(52 -r
55*) absorptions of much lower intensity and energy. The 1(52 -’
55*) transition of M02C14(dppe)2 maximizes at 762 nm (E = 1030
M'1 cm'l) whereas M02Cl4(dmpe)2 maximizes at 803 nm (E = 210
M"1 cm'l). The lower energy of the 1(52 -’ 55*) transition is due

to the decreased dxy orbital overlap and consequently reduced

energy splitting of the 5 and 5* levels. The energetic differences
between the eclipsed 1(55*) and staggered 1(55"')excited states can
readily be attributed to the dependence of the one- and two-electron
interactions of the dxy orbital overlaps. It has been shown
previously that the 5 interaction decreases as the distortion angle
increases. A 40° distortion angle, such as that found in

M02C14(dmpe)2, reduces the 5 interaction to 0.17 times that found

at 0°.123 Inasmuch as the 5 overlap of the eclipsed molecules are

69

on the order of S ~ 0.1,65 the energetic splitting of the one-electron 5
and 5* levels of the M02C14(dmpe)2 is clearly quite small. As shown
in Figure 13,79 the energetic splitting between the covalent l(52)
ground state and 3(55*) excited state decreases with increasing
distortion angle to a nearly degenerate pair (for x = 45°). In contrast,
the energy of the ionic 1(55*) state is dominated by an electron

repulsion term that is roughly invariant over the range of overlaps
(0.01< S < 0.1). Thus, even near the zero-overlap limit (400 torsional
angle), M02C14(dmpe)2 exhibits a 1(52 -» 55*) transition of 12500

-1

cm owing to the significant two-electron spin pairing term.

The poor overlap of the dxy orbitals in the torsionally distorted
molecules will be manifested in extremely small oscillator strengths.
Hence, the question is not why the molar absorption of these
complexes is small, but rather by what mechanism do these
transitions acquire their intensity. Similar to the M02X4(PBu3)4
complexes, intensity stealing from low lying LMCT transitions most
reasonably accounts for the high oscillator strengths of the 1(52 -’

55*) transitions of these staggered M02C14(LL)2 complexes.

2. Emission Spectroscopy. The M02X4(PBu3)4 complexes

provide the benchmark for the interpretation of the luminescence of
quadruply bonded phosphine complexes. Solids and solutions of the
M02X4(PBu3)4 complexes exhibit intense emission at room and low
temperatures. The luminescence spectra mirror the absorption
spectra and indeed these complexes were the models for which the

torsional model was initially proposed.80 On the basis of this model,

M02Cl4(LL)2 complexes in which the bidentate phosphine ligands

70

Figure 13. Energies of the lowest electronic states of M02C14(PMe3)4
and M02C14(dmpe)2 (ref 79).

71

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72

strap the metal-metal bond in an eclipsed conformation would be
expected to exhibit strong emission directly from 1(55*) excited
state. On the other hand, staggered quadruply bonded M02C14(LL)2
dimers would display weak emissions. But this is not the case. The
staggered M02C14(dppe)2 and M02Cl4(dmpe)2 complexes emit with
appreciable intensities in CH2C12 solution at room temperature
whereas M02C14(dmpm)2 emits only weakly, and M02C14(dppm)2 is
non-luminescent in CH2C12 solution at 25 °C. The emission spectra of
the M02C14(dppe)2, M02C14(dmpe)2, and M02C14(dmpm)2 complexes
are show in Figure 14. All four M02C14(LL)2 complexes emit in low
temperature glasses (Figure 15). The intensities of the emission
parallel that observed at high temperatures. Namely, the staggered
complexes luminesce much more intensely than the eclipsed dimers.
Typical of binuclear metal-metal emission, the luminescence bands
sharpen considerably upon cooling.126

Because the complexes do not exhibit vibrational fine structure

even at low temperature, the excited state dynamics can not be

analyzed by conventional spectroscopic techniques. However, Stokes

shifts of the complexes do provide some information. The 1(52 ->
55*) absorption and emission bands of M02Cl4(dppe)2 and
M02C14(dmpe)2 are shown in Figure 16. It is obvious that both

complexes exhibit large Stokes shifts. It should be noted that these
Stokes shifts represent a lower limit because the photomultiplier
tube of our emission instrument is attenuated significantly for A >
1000 nm. From the valence bond model of quadruply bonded metal

dimers, as discussed previously, the 52 ground state of

M02Cl4(dmpe)2 and M02C14(dppe)2 is nearly energetically

73

Figure 14. Emission of M02C14(LL)2 complexes in CH2C12 solution at
room temperature: (a) M02C14(dmpm)2; (b) M02C14(dppe)2; and (c)
M02C14(dmpe)2.

74

 

 

 

 

 

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A/nm

Figure 14

75

 

(b)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

A/nm

Figure 14

 

76

 

 

 

 

 

 

800 9 00 l 000

Figure 14

77

Figure 15. Luminescence of M02C14(LL)2 complexes at 77 K in a
CH2C12/toluene glass: (a) M02C14(dppm)2; (b) M02C14(dmpm)2; (c)
M02C14(dppe)2; and (d) M02C14(dmpe)2.

78

 

 

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700

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Figure 15

 

82

Figure 16. Absorption and emission spectra of 52 2 55* transitions

of staggered (a) M02C14(dppe)2 and (b) M02C14(dmpe)2 complexes.

83

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degenerate to the 3(55*) excited state.79 Therefore, the emission
clearly does not originate from 3(55*), and we assign the emission to
the 1(55*) excited state. To explain the Stokes shifts of the emission
spectra of M02C14(dmpe)2 and M02C14(dppe)2 complexes, we adopt
the explanation for the large red-shift from absorption to emission
for the Re2C182’ dimer.80 Namely, emission is observed from an
equilibrium series of complexes possessing torsional angles between
a maximized dxy orbital overlap (x = 0°) and the configuration with a
steric minimum (X = 45°). For the M02Cl4(dppe)2 and
M02Cl4(dmpe)2 complexes the equilibrium geometry is with X:
30.50 and X = 40°. Rotation to smaller angles will increase the metal-
metal distance owing to steric constraints of the LL backbone. Hence
rotation away from the equilibrium torsional angles will result in
emission from a spectrum of complexes with metal-metal distances
greater than that occurring from simple population of the 5 *

antibonding orbital near equilibrium geometry. This will result in

luminescence from significantly distorted M02C14(dppe)2 and

M02C14(dmpe)2 excited states, and hence large Stokes shifts will be

observed.

This explanation is supported by the emission properties of

M02Cl4(dppe)2 dimer in poly(methy1 methacrylate) (PMM) films at
room temperature. The solution spectrum of the MozCl4(dppe)2
complex is frozen by incorporation into the rigid matrix of the PMM
film. The emission maximum of M02C14(dppe)2 shifts from 964 nm
in CH2C12 to 936 nm in the PMM film. The decrease in the Stokes

shifts upon incorporation into PMM indicates that emission is from a

complex geometrically more similar to the ground state structure.

86

This result is significant because it shows that the distorted
M02Cl4(LL)2 dimers emit directly from staggered l(55*) excited

states.

Emission of M02C14(dppm)2 and M02C14(dmpm)2 complexes
exhibit smaller Stokes shifts (Figure 17) than that for complexes with
staggered structures. This indicates that emission arises from an
excited state complex structurally similar to the ground state. This is
not unreasonable because the methylene bridged LL complexes can
maintain the geometry of the M-4-M complex in the excited state
better than that possessing bidentate phosphines with an ethylene
backbone. The important point is that the ethylene complexes are
distinguished from the methylene complexes by significantly lower

emission intensities of the latter (~102 less). Of equal significance is

that the emission of the eclipsed M02 C14 (d p p m ) 2 and
M o 2Cl4 ( d m p m) 2 complexes is 104 less than the eclipsed
M02C14(PBu3)4 complexes ((156 = 0.013).80 This discrepancy can not

be explained by the simple D 4h 41-) 04d torsional model which

predicts nearly equivalent intensities for the two classes of eclipsed
complexes.

Some insight into the issue of emission intensities is provided
upon closer examination of the molecular and electronic structures of
the eclipsed and staggered complexes. The electronic structure of MA-
M dimers is significantly affected by the symmetry of the molecule.
As shown in Figure 18, the (dxz, dyz) orbitals are no longer
energetically degenerate when the symmetry of the chlorophosphino
dimolybdenum complexes changes from 02d (e.g. M02X4(PBu3)4) to
D2}, (e.g. M02C14(dmpm)2), whereas the dxy 5-bonding and 5-

87

Figure 17. Absorption and emission spectra of 52 :2 55* transitions

of eclipsed (a) M02C14(dppm)2 and (b) M02C14(dmpm)2 complexes.

88

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Figure 18. Relative energies of the d-derived molecular orbitals in

D2d (i.e. M02Cl4(PBu3)4) and D2}: (i.e. M02C14(dppm)2) symmetries.

91

 

b2(0*) -— ----------------------- —— b2u(o*)
b2(M'L 0*) __ _____________________ __ b1u(M-L 0*)
a1(M-L 0*) — —-‘- ag(M-L 0*)
........... —_ b3g(n*)
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92

antibonding interactions are not greatly perturbed. The energy gaps
between 1'! and 5 states are smaller for complexes with D2 h
symmetry and hence contributions (i.e. decreased energy gap) of
lower lying states proximate to the excited state will increase the

nonradiative decay rate, and hence result in lower emission yields
for 02h complexes (i.e. M02C14(dmpm)2). Staggered M02C14(LL)2
complexes possess symmetries between the D2d and 02 h structures
and therefore, to a first approximation, exhibit higher luminescence
intensities than D2 h molecules but lower than those of 02d
complexes. A more quantitative assignment of the nonradiative
processes of these molecules will only be achieved when good
theoretical calculations of the energy levels of these MA—

complexes are in place.

3. Transient Absorption Spectroscopy. Kinetic studies of
M02X4(PBu3)4 and M02C14(LL)2 complexes were investigated by

time-resolved transient absorption spectroscopy. Owing to the
relatively weak luminescence intensity of M02C14(dmpm)2 and
related complexes, transient species important to nonradiative decay
can remain undetected with time-resolved luminescence
spectroscopy. This issue is best illustrated by picosecond absorption
measurements of M02C14(PBu3)4. Time-resolved emission spectro-
scopy identifies a single excited state species with a lifetime of 16
ns.80 Transient absorption spectroscopy, however, reveals two
kinetic decays of lifetimes 9 and 90 us.82 The transient species with
the 9 ns decay is consistent with the emission lifetime; thus, this

species has been assigned to the 1(55*) eclipsed excited state. The

93

nature of the long-lived 90-ns nonemissive transient species is not
yet clear, but three possibilities for the transient are: (l) the low-
lying 3(55") state (~5000 cm'l); (ii) an electronic excited state of
M02C14(PBu3)4 that lies only slightly lower in energy than 1(55*)

state but carries very little oscillator strength to the ground state;

and (iii) a highly distorted chemical intermediate of M02C14(PBu3)4.

The decay has been attributed to most likely arise from a distorted

chemical intermediate.82

Because the transient spectroscopy of M02C14(PBu3)4 is well
defined, studies were initiated by recording the spectra of its halide
analogues, MozBr4(PBu3)4 and M0214(PBu3)4. Figure 19 displays
the transient absorption spectra of M02C14(PBu3)4 (X = Br, I) at 50
ps and 1 ns, respectively, after excitation with a ~25 ps pulse of 532-
nm laser light. The spectra, concordant with that of M02Cl4(PBu3)4,
exhibit a moderately intense absorption maximum at 440 nm, an
absorption rising into the near UV, and a bleaching of the 1(55"')
transition at ~600 nm. That the moderately intense absorption
maxima of M02C14(PBu3)4, MozBr4(PBu3)4, and M0214(PBu3)4 are
energetically similar (440, 460, and 440 nm for the chloro, bromo,
and iodo, respectively) distinguishes this transition as a metal
localized transition or an ligand-to-metal charge transfer (LMCT).
Because the energy of the transition is invariant across the halide
series, an LMCT involving the phosphine ligands is the preferred
assignment.82 The extinction coefficient of this band is ~2000 M'l
cm'l. This value clearly indicates a spin allowed transition but can

not distinguish between the LMCT and metal-localized excitations.

For the latter, assuming D 4h selection rules, five dipole-allowed one-

94

Figure 19. Transient difference spectra of (a) MozBr4(PBu3)4 in
CH2C12 and (b) M0214(PBu3)4 in CH2C12. The spectra were recorded

50 ps and 1 ns, respectively, after the 532-nm pulse of a Nd:YAG
laser (FWHM = 25 ps).

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electron metal-localized excitations can arise from a 1(55*) excited
state: 5 .. 5*; n .. 11*;11 -» 5'(dx2_y2); 5* .. 11*; and n -» 5. These
transitions will produce, respectively, 1A1 g(5*2), 1A1 g(11355*rt"‘),
1l.=.g(n355*5'), lr~:€.;(5r1"‘), and 1Eg(n5*) states. The level arising

1

from the 440 nm absorption must lie ~39000 cm“ above the ground

state, and calculations suggest that58 of the five metal localized
transitions, the 5* -’ TI* excitation is the most plausible assignment
for this feature. However, the data of Mo2X4(PBu3)4 series do not
provide an unequivocal assignment to either 5* -+ 11* or LMCT.

The kinetics of M02C14(PBu3)4 excited state decay can readily
be ascertained from these transient absorption spectra. For the
chloro, bromo, and iodo complexes, the ~440 nm absorption and
1(55*) bleaching signals decay biexponentially. The shorter lifetime
component in each case is nearly identical to the measured

luminescence lifetimes (I = 9, 2, 6 us for chloro, bromo, and iodo

M02X4(PBu3)4 complexes, respectively). Because the luminescence
spectrum of each complex is a mirror image of the 1(52 4 55*)

transition, the short-lived transient is assign to the eclipsed 1(55*)
excited state. The longer lived species (I = 90, 34, 108 ns for chloro,
bromo, and iodo M02X4(PBu3)4 complexes, respectively) is due to
one of the three possibilities mentioned previously. In an effort to
provide further insight into the nature of this longer lived non-

luminescent intermediate, the transient absorption spectra of

M02C14(LL)2 complexes were investigated
The transient absorption spectra of the M02C14(LL)2 complexes
is similar to that of the M02Cl4(PBu3)4 dimer. Spectra, collected 25-

100 ps after the excitation pulse (shown in Figure 20), exhibit a

98

Figure 20. Transient difference spectra of the following M02C14(LL)2
complexes: (a) M02C14(dppm)2, recorded 75 ps after a 355-nm laser
excitation pulse; (6) M02C14(dmpm)2, recorded 25 ps after a 532-nm
laser excitation pulse; (c) M02C14(dppe)2, recorded 100 ps after a
532-nm laser excitation pulse; and (d) M02C14(dmpe)2, recorded 100

ps after a 355-nm laser excitation pulse.

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prominent bleaching signal corresponding to the 1(55*) transition.
It should be noted that the bleaching of M02C14(dmpe)2 lies well into
the red and is weak owing to low response of the photomultiplier
tube. However the leading edge of the bleaching signal is clearly
distinguishable in Figure 20(d). The spectra of the four complexes

are dominated by pronounced absorption in the 430-450 nm; the
spectra of M02Cl4(dmpe)2 displays an additional absorption at 525

nm. The extinction coefficient of the ~440 nm bands of M02C14-
(dppm)2, Mo2Cl4(dmpm)2, and M02C14(dppe)2 are 1000-3000 M'1
cm’l. On this basis, the intensity of the 525 nm band of
M02C14(dmpe)2 identifies it as the corresponding transition, and thus

the 435-nm absorption of the M02C14(dmpe)2 complex is unique.
This is further suggested by the fact that the band halfwidth of the

435 nm is much smaller than that of the other observed absorptions
in M02Cl4(LL)2 complexes. The obvious question becomes what
transition is responsible for the respective 430, 450, 470, and 525
nm absorptions of the M02C14(dppm)2, M02C14(dmpm)2,
M02C14(dppe)2, and MozCl4(dmpe)2 complexes. The energy and

intensities of these absorptions suggest them to be analogous to those

observed in the spectra of the M02X4(PBu3)4 complexes. We have
previously suggested that the ~440-nm absorption for the
electronically excited M02X4(PBu3)4 complexes is attributed to
either a 5* -' 11* transition or a charge-transfer transition involving
the phosphine ligands. Consideration of the electronic structure of

MAM complexes imply that the red-shift in the absorption along the
series M02C14(dppm)2, M02Cl4(dmpm)2, M02C14(dppe)2, and

M02C14(dmpe)2 is inconsistent with a 5* -’ 11* transition. The n and

104

n * orbitals of the M-4—M complexes are cylindrically symmetric

about the metal-metal axis and hence the energy of these orbitals is
invariant to rotation about the metal-metal bond. The 5-5 * splitting
however will decrease dramatically with rotation owing to
stabilization of the 5 * orbital and destabilization of the 5 orbital
with a decreasing 5 interaction. To this end, the 5 * -> n * transition
should blue-shift with increasing 7:. On the other hand, charge
transfer transitions to the 5* orbital will red-shift as 71 increases.
Therefore, the most likely assignment of the transition leading to
transient absorption in M02Cl4(LL)2 and M02C14(PBu3)4 complexes
is not metal-localized but ligand(phosphine)-to-metal(5*)~charge-
transfer. The assignment of the 435-nm band in the spectrum of

M02C14(dmpe)2 has not been elucidated at this time.

Although the transient absorption spectra of the four
M02C14(LL)2 complexes suggest that the complexes are similar,

analysis of the lifetime decay of the transient show that the
diphenylphosphine complexes (i.e. M02C14(dppm)2 and M02C14-
(dppe)2) are distinctly different from their dimethylphosphine
counterparts (i.e. M02C14(dmpm)2 and M02Cl4(dmpe)2). Figure 21
presents the decay kinetics of the transient species of the M02C14-
(dmpm)2 and M02C14(dmpe)2 complexes. In both cases the
transients decay by monoexponential kinetics with lifetimes of 43
and 960 ps for the M02Cl4(dmpm)2 and Mo2C14(dmpe)2,
respectively. The decay kinetics of M02C14(dmpm)2 were obtained
from the 1(55*) bleaching signal and thus the 43 ps represents the

1(55*) lifetime of the excited state. Because the bleaching signal of
the M02Cl4(dmpe)2 complex is not easily observed, the direct

105

Figure 21. Transient kinetics for dimethylphosphine complexes: (a)

M02C14(dmpm)2 recorded at 600 nm (excitation by 2nd harmonic of
a Nd:YAG, FWHM = 25 ps) (b) M02C14(dmpe)2, recorded at 450 nm

(excitation by 3rd harmonic of a Nd:YAG, FWHM = 25 ps).

 

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analysis of 1(55*) is not feasible. However, it is reasonable that the
prompt decay of the transient absorption is also the lifetime of the
1(5 5 I") excited state. These excited state properties of the bidentate
methylphosphine complexes are parallel to those of the

monoexponential 1(55*) decay of the unbridged methylphosphine
dimer, Mo2Cl4(PMe3)4. Analogously, the phenyl substituted bridged

phosphine complexes, M02C14(dppm)2 and Mo2C14(dppe)2, exhibit
decay kinetics similar to unbridged phosphine complexes possessing
bulky substituents. The transient absorption at 400 nm and 1(55*)
bleaching at 620 nm decay by the same kinetics indicating that the
transient spectrum arises from a single species (exemplary decays

for M02C14(dppm)2 are shown in Figure 22). These data are fit

extremely well with two decay lifetimes of 130 ps and 2.1 ns for

M02C14(dppm)2. As shown in Figure 23, a similar biexponential
decay process was observed from the 1(55*) excited state of
M02C14(dppe)2 (lifetimes of 410 ps and 2.4 ns). By analogy to
M02Cl4(PBu3)4, the shorter lifetime is assigned to 1(55"') excited

state and the longer lifetime corresponds to a non-luminescent
transient intermediate. Although the data does not allow

unequivocal assignment of the long-lived intermediate, it is

interesting that the lifetime decay kinetics of M02X4(PR3)4 and
M02Cl4(LL)2 are related to the cone angle and basicity of the
phosphine ligands. For M02X4(PMe3)4 and the bidentate analogues
Mo2Cl4(dmpm)2 and M02C14(dmpe)2, the coordinated phosphines
possess the smallest cone angle (PMe3= 118°, dmpe: 107°) and

127

strongest Lewis basicity. These complexes decay directly from

the 1(55*) and their excited state dynamics are not mediated by any

109

Figure 22. Transient kinetics for M02C14(dppm)2 in CH2C12 solution.

The decay kinetics were recorded at (a) 400 nm and (b) 620 nm
(excitation by 3rd harmonic of a Nd:YAG, FWHM = 25 ps).

110

 

 

 

 

 

 

 

 

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111

Figure 23. Transient kinetics of M02C14(dppe)2 in CH2C12 solution.

The decay kinetics were recorded at 420 nm (excitation by 2nd

harmonic of a Nd:YAG, FWHM = 25 ps).

 

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113

other intermediates. Conversely, Mo2Cl4(PBu3)4, M02C14(dppm)2,
and M02C14(dppe)2 possess phosphines with much larger cone angles
(PBu3 = 132°, dppm = 121°, dppe = 12501127 The fast bimodel

decays and lower quantum yields of these species are consistent with
the formation of a chemical intermediate correlated with the cone
angle of the phosphine. In this regard, numerous studies have
shown that the dissociation of phosphines from metal centers is
related intimately to the phosphine's cone angle.127’128 An obvious
explanation of the bimodel decay kinetics is that a chemical
intermediate is formed by dissociation of phosphine from the metal
core in the 1(55"') excited state followed by recombination of the
phosphine with the coordinatively unsaturated intermediate. The
observed lifetimes for the chemical intermediate are well above the
diffusion controlled limit for a bimolecular process. Thus if
dissociation does occur, then it must do so within the solvent cage.
This model also explains the similarity between the lifetimes of the
chemical intermediate derived from bridged and unbridged
complexes. For the latter, although the rate for recombination of the
phosphine on the metal core should be much larger owing to a
chelate effect, this effect will be attenuated for processes occurring
within a solvent cage.

In summary, subtle factors appear to be governing the excited
state dynamics of MA-M complexes. Excited state lifetimes and

emission quantum yields are not explained fully by a simple

torsional model. Eclipsed M02C14(LL)2 (02h) complexes are weakly
luminescent while torsionally distorted M02C14(LL)2 (D2) complexes

possess much higher emission quantum yields. Rather, the excited

114

state dynamics are dictated by the symmetry of the M-A-M complex.
The trend of decreasing quantum yields along the series of eclipsed
M02X4(PMe3)4 (02d) > staggered M02Cl4(LL)2 (D2) > eclipsed
M02C14(LL)2 (D 2h) suggests that it is the presence of states
proximate to the 1(5 5 *) singlet that are important to nonradiative
decay and not the torsional angle about the metal-metal bond.

A single excited state following excitation is observed for
halophosphine M-4-M complexes when the substituents on the
phosphines are sterically small. However, with increasing steric
bulk, the presence of two transient intermediates are observed, one
of which is attributable to the 1(55*) excited state and the other to a
long-lived chemical intermediate. Although a definitive assignment
of this intermediate is not possible, the data are consistent with

formation of a coordinatively unsaturated transient produced by

simple excited state dissociation of phosphine within a solvent cage.

CHAPTER IV

PHOTOCHEMISTRY OF NOVEL
DIMOLYBDENUM PHOSPHATE COMPLEXES
IN ACIDIC SOLUTION

A. Background

Conversion of a M-n-M dimer with a metal-metal bond order of
4 to a species of bond order of 3 conceptually represents the simplest
multielectron transformation available to a MAM system. The
overall reaction constitutes a two-electron oxidation of the metal-
metal core. Multielectron studies were initiated by exploring the
oxidation photochemistry of M-4-M dimers in acidic solution with the

goal of promoting the following reaction sequence

Mfr-M __fll’_... MA—M“ (26)
MA-M* + HB —- 114-11w + 13' (27)

M—3—M + HB —- M-3-M2+ + H2 + 13' (28)

Scheme III

115

116

In this scheme, electronically excited MA-M reacts with HB in a two-
electron oxidation step to produce a hydride intermediate and
Bronsted base B'. Trapping the hydride with another equivalent of
HB gives oxidized dimer M-3-M and hydrogen. It should be noted
that the hydride production reaction is fundamentally important to
excited state multielectron chemistry because it represents one of
the simplest multielectron reaction pathways available to the excited
quadruply bonded dimer. Practically, the reaction is important
because metal hydrides are crucial intermediates in small molecule
activation schemes such as those for water splitting chemistry. The
hydrogen donor is not limited to a Brensted acid but in principle can
involve any small molecule reactant. Thus, the reaction chemistry
described in Scheme III is significant in regard to our efforts in
designing new excited state reactivity patterns.

Examination of Scheme III reveals that hydrogen production
relies on a quadruple bond (=2 triple bond conversion. Although,
conceptually quite appealing, photoreaction to triply bonded species
is an exceptional reaction pathway and the excited state reaction
chemistry of M-A-M dimers has been limited primarily to single-

electron photoreactions. For instance, the photochemistry of

M02(SO4)44’ is exemplary of typical MAM species: under relatively

strong oxidizing conditions (Xexc > 254 nm), irradiation of
Mo2(SO4)44' in H2804 is confined to the production of the one
electron product M02(SO4)43'.93 Presumably, the chemical

instability of the triply bonded sulfate dimer precludes multielectron
photochemistry. Accordingly, we set out to synthetically design M-n

M complexes which could undergo triple/quadruple

117

photoconversions. In this context, the discovery of the triply bonded

109

phosphate complex M02(HPO4)42' stimulated our interest

because the M02 phosphate system could possibly provide insight

into the mechanism of multielectron photoredox chemistry of M-n-M
dimers in acidic solution with the successful preparation of

quadruply bonded molybdenum phosphate dimer. The existence of

the quadruply bonded M02(II,II) sulfate complex suggested that the
M02(II,II) phosphate could be prepared in view of the electronic and
structural similarities of HPO42' and 8042’. This chapter describes
the spectroscopy and electrochemistry of M02(HPO4)44‘ and
M02(HPO4)43' dimers as well as the photoinduced two-electron

oxidation of M02(HPO4)44’ in acidic solution.

B. Results and Discussion

1. Magnetic and Spectroscopic Characterization of
M02(HPO4)4“' (n = 2, 3, 4) Species. A central result to emerge
from exhaustive spectroscopic studies of multiply bonded metal-
metal dimers is a consistent description of the electronic structure of
these complexes in terms of a 0, n, and 5 molecular orbital
framework.55“"65 One consequence of this model is the prediction
that dimers of bond orders 4 (0211452) and 3 (02114) will possess a
diamagnetic ground state whereas the mixed-valence dimer of bond
order 3.5 (0211451) will be paramagnetic. Investigation of the

magnetic properties of M02(HPO4)43' and M02(HPO4)44' were of

interest because these studies in connection with a previous

susceptibility measurement on the pyridinium salt of M02(HPO4)42'

118

which was shown to be diamagnetic,129

would provide magnetic
data on the first homologous M-n-M series possessing a 021145" (11 =
0, 1, 2) ground state configuration. The predicted paramagnetic
behavior of M02(HPO4)43' is confirmed by magnetic susceptibility
measurements of solids from 5 to 300K at a field of 5500 G.
Correction for the diamagnetic contribution to magnetic moment was
provided by the room temperature magnetic susceptibility of the
pyridinium salt of M02(HPO4)42’. The susceptibility data is shown in
Table 4. The temperature dependence of the magnetic susceptibility
follows Curie law behavior (Figure 24) and the magnetic moment of

K3M02(HPO4)4 is 1.58 BM. In accordance with susceptibility results,

EPR spectra of the pyridinium salt of frozen solution of
K3Mo2(HPO4)4 at 5 K (Figure 25) show a paramagnetic species in an
axial environment, g1 = 1.894, g" = 1.886. As described in detailed

studies of the EPR spectra of M02(SO4)43',130 and other M-z'LM

dimers, 1 31

the EPR signal is consistent with a species in which the
unpaired electron is coupled between two equivalent molybdenum
nuclei.

Reduction of M02(HPO4)43‘ by one electron will yield a
quadruply bonded complex which should exhibit rigorous
diamagnetism. Unfortunately, the accurate determination of the
magnetic susceptibility of M02(HPO4)44' is precluded by our
inability to cleanly isolate salts of the quadruple bond complex from

the mixed-valence dimer. Indeed, EPR spectra of solid samples of

K4M02(HPO4)4 at 5 K show signals identical to that of
K3M02(HPO4)4. That the intensity of the signal varies with sample

preparation indicates that the observed paramagnetism arises from

119

Table 4. Magnetic Susceptibility of K3M02(HPO4)4.

 

 

Tl K XMa/emu mol'1 1/XM/emu'lmol
5 0.066219 15.1014
6 0.053052 18.8494
8 0.038362 26.0675
10 0.030215 33.0961
12 0.024878 40.1962
14 0.021185 47.2032
15 0.019661 50.8621
16 0.018452 54.1947
18 0.016427 60.8754
20 0.014713 67.9671
22 0.013333 75.0188
25 0.011682 85.6018
27 0.010850 92.1149
30 0.009801 102.0346
35 0.008420 118.7719
40 0.007454 134,1562
45 0.006638 150.6478
50 0.006053 165.2073
55 0.005517 181.2744
60 0.005124 195.1715
65 0.004741 210.9260
70 0.004455 224.4669
75 0.004171 239.7794
80 0.003725 268.4924
85 0.003725 295.6743
90 0.003561 280.8279
95 0.003382 295.6743
100 0.003241 308.5087
110 0.003004 332.8562
120 0.002763 361.9910
129 0.002595 385.4307

140

0.002545

393.0045

Table 4 (cont'd.).

120

 

148 0.002321 430.9230
163 0.002301 434.5937
166 0.002071 482.8352
184 0.002086 479.2715
191 0.001923 520.0479
205 0.001913 522.6572
221 0.001723 580.4167
223 0.001754 570.0604
254 0.001565 638.8144
255 0.001612 620.3859
279 0.001436 696.5729
281 0.001470 679.7634
310 0.001415 706.5141

 

a Data were corrected from the diamagnetic contribution by using
the magnetic susceptibility of Py3M02(HPO4)4Cl at room

temperature (XM = -100x10'6 emu mol'l).129

121

Figure 24. Temperature dependence of the corrected magnetic

susceptibility of K3M02(HPO4)4.

122

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Figure 25. X-band (9.460 GHz) EPR spectrum of a frozen solution of
K3M02(HP04)4 in 7.5 M H3PO4 at 5 K.

124

 

 

 

 

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125

M02(HPO4)43' impurities and no evidence for a distinct EPR signal
attributable to M02(HPO4)44’ can be discerned. Thus, to the best of
our knowledge, the magnetic properties of M02 phosphate series are
accommodated by the electronic structure model of M-n-M (D 4h)

dimers.

Solids and solutions of M02(HPO4)44’ are pink in color. Figure

26 displays the electronic absorption spectrum of the dimer in 2 M
H3PO4. The lowest energy absorption band (A max = 516 nm, E =

196 M'1 cm’l) is comparable in energy and intensity to that of

M02(SO4)44‘ and by analogy is assigned to the 52 -’ 55* transition.

As expected for a dipole allowed transition, the band sharpens upon
cooling solutions to 77 K, but the integrated intensity of the band
remains constant; the absorption profile remains vibrationally

featureless at low temperature. In addition to the 52 -’ 5 5 *

transition, visible and ultraviolet absorption systems in
M02(HPO4)44' have direct analogues in M02(SO4)44' spectra. The

absorption (x = 408 nm, e = 27 M'1 cm'l) immediately to higher
energy of the 52 -’ 55* transition is especially noteworthy; a

similar positioned band in M02(SO4)44' has tentatively been

assigned to the n -' 5* transition.129

The electronic absorption spectrum of M02(HPO4)43' (Figure

27) is dominated by a prominent absorption profile in the near-

infrared (71max = 1438 nm; e = 180 M'1 cm'l) and two weak visible
bands at 595 nm (6 =10 M‘1 cm'l) and 420 nm (E = 8 M’1 cm’l).
Similar to the quadruply bonded dimers, the spectra of M02(II,III)

sulfate and phosphate are nearly identical. The results of previous

spectroscopic studies of M02(SO4)43' are consistent with the

126

Figure 26. Electronic absorption spectrum of M02(HPO4)44' ion in 2

M H3PO4 at room temperature.

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H3PO4 at 25 °C.

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130

assignment of the near-infrared absorption band to the 5 -' 5*
(2B1u ‘- 2B23) transition.93’132 This significant red shift of the 5
-t 5* transition of mixed-valence M02 dimers is explained by the
absence of two-electron term contributions to the transition energy
for 0211451 configured ground state species. Similar to M02(SO4)43'
ion, the 5 -> 5* absorption band of M02(HPO4)43' exhibits a

vibrational progression in solution at room temperature. As shown in
Figure 28, the vibrational peaks sharpen considerably upon cooling

the phosphoric acid solution to 77 K. The 334 cm'1

spacing is
consistent with a progression in the symmetric metal-metal

stretching vibration.
With regard to the visible absorption profile of M02(HPO4)43‘,

the analogous absorption bands at 417 and 595 nm in M02(SO4)43'
have been assigned by Hopkins et al. to the n -* 5* and n -r 5

transitions, respectively.129 These assignments, which resulted from

a comparative analysis of M02(II,II) and M02(II,III) sulfate and M02
(III,III) phosphate spectra, are predicated on the assumption that
bridging so42' and HPO42' ligands will perturb the electronic
structure of a M02 core in a similar fashion. In this context, the
absorption spectra of the homologous M02 phosphate series reported
herein confirm this contention and accordingly support the n -* 5
and n -' 5* assignments for the visible absorption bands of
Mo2(HPO4)43‘ and M02(SO4)43'.

Raman spectra of solid samples of M02(HPO4)4“' (n = 2, 3, 4)
at room temperature were recorded with 4880 A excitation light. The
excitation frequency falls within the contour of the metal localized

absorption profiles and significant enhancement of Raman peaks

131

Figure 28. Near-infrared absorption band of K3M02(HPO4)4 in a

frozen phosphoric acid solution at 77 K.

132

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133

associated with metal-metal vibrations can be expected.133

Prominent bands are observed at 345, 352, and 356 cm’1 in Raman

spectra of M02(II,II), M02(II,III), and M02(III,III) phosphate,

respectively; this latter frequency is in excellent agreement with a

previous study of M02(HPO4)42' in which a band at 358 cm'1

was
attributed to valg(Mo-Mo).129 Assignment of the observed bands

in the M02 phosphate series to valg(Mo-Mo) is consistent with their

frequencies which will fall squarely within the 350-400 cm‘1 range

which characterizes metal-metal vibrations in M-fl-M dimers and is

also supported by the presence of energetically similar peaks in M02

sulfate dimers.134 Given the valg(Mo-Mo) assignment, a striking

result of M02(HPO4)4n' (n = 2, 3, 4) Raman spectra is the observed

increase in metal-metal stretching frequency with decreasing bond

order. Precedent for this puzzling trend has heretofore existed as an
anomaly in the Raman spectra of M02(II,II) (valg(Mo—Mo) = 371

cm'l) and M02(II,III) (valg(Mo-Mo) = 373, 386 cm'l) sulfate. An

explanation for this apparent inconsistency is the ability of the
bridging ligand to modulate the metal-metal frequency,135
although, the origin of this effect has not clearly been established.
Nevertheless, the Raman results of the phosphate and sulfate series
clearly demonstrate the ability of bridging ligands to vitiate

predictions using simple bond order arguments.

2. Oxidation-Reduction Chemistry. The cyclic voltammo-
gram of M02(HPO4)42' in 1 M H3PO4, shown in Figure 29, exhibits

two oxidation processes. Reversible electrochemical behavior is

suggested by linear plots of the cathodic and anodic currents vs.

134

Figure 29. Cyclic voltammogram of a 2.5 mM solution of the
pyridinium salt of M02(HPO4)44’ in 2 M H3PO4. The scan rate was 2

mV s'l.

135

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136

121/2 (scan rate, 12: 5-100 mV sec'l), and values of 0.98 i 0.04 for
ratios of the anodic and cathodic peaks currents.112 The identities
of these two electrode processes are revealed by electrochemical
experiments on solutions of the quadruply bonded and mixed-valent
phosphate dimers. Cyclic voltammograms identical to the one
displayed in Figure 29 are observed upon scanning th electrode

potential from the appropriate resting potentials of solutions of
M02(II,II) and M02 (11,111) phosphates. These results lead us to

assign the waves at -0.25 V and -0.67 V vs. SCE to the M02(HPO4)42'
B” and M02(HPO4)43'/4' couples, respectively. The syntheses of
M02 phosphate dimers and their reaction chemistry are easily
understood in the context of the above electrochemcial results. Our
observation that solids of M02(HPO4)44' precipitated from acidic
solutions are inevitably contaminated with M02(HPO4)43’ is

consistent with the negative potential of the M02(HPO4)43‘/4'

couple with respect to the standard hydrogen electrode. Indeed
M02(HPO4)44' in 2 M H3PO4 solution slowly converts to
M02(HPO4)43’ as evidenced by the disappearance of the 1(52 -+

5 5*) absorption band at 516 nm and the concomitant growth of the
2(5 -* 5*) absorption of Moz-(HPO4)43'. Closer analysis of the

reaction reveals the quantitative conversion of quadruple bond

dimer to mixed-valence species (I ”2 = 27.5 h) and chromatographic

analysis identifies hydrogen as a reaction product. Toepler pumping

the gas above completely reacted solutions reveals that 0.39 moles of
112/mole of M02(HPO4)44' is evolved thereby establishing the

overall reaction stoichiometry as

137
M02(HPO4)44' + 11* -» M02(HPO4)43‘ + 1/2 H2 (29)
Reduction of protons to hydrogen radicals is a highly energetic
process [E(H+/H) = 2.6 V vs. NHE],136 and hence, direct reaction of
the quadruple bond dimer with H+ to M02(HPO4)42' produce the

mixed-valence dimer is not thermodynamically feasible. We propose
that M02(HPO4)44’ reacts in acidic solution to M02(HPO4)42' which
then reacts in a comproportionation reaction to produce mixed-

valence dimer according to the following,

M02(HPO4)44' + 211* -» M02(HPO4)42‘ + H2 (30)

Mo2(HPO4)44' + M02(HPO4)42' _. 2M02(HPO4)43‘ (31)

Consistent with reactions (30) and (31) is our observation that

addition of a phosphoric acid solution of M02(HPO4)42‘ to one of
M02(HPO4)44' leads to the immediate and quantitative production of

Moz-(HPO4)43’. In view of previous studies of M-n—M dimers which

have demonstrated large rate constants for electron exchange

83

reactions involving 5 orbitals, the facility of reaction (27) is not

surprising since the comproportionation simply involves the transfer

of an electron between the 5 orbitals of the M02 phosphato dimers.

We therefore believe that the slow thermal conversion of
M02(HPO4)44' to M02(HPO4)43' in phosphoric acid solutions, in the

context of the above scheme, is a consequence of large kinetic

barriers associated with reaction (30).

138

3. Photochemistry. In striking contrast to the slow thermal
chemistry, reaction of M02(HPO4)44’ in acidic solution is markedly

accelerated by ultraviolet irradiation. Spectral changes for the room
temperature irradiation (A > 335 nm) of M02(HPO4)44' in 2 M
D 3P0 4 are shown in Figure 30. An initial decrease in the intensity of
the 52 -* 55* absorption band of M02(HPO4)44' is accompanied by
an increase in the 5 4 5* absorption of M02(HPO4)43‘. Ensuing
reaction of the mixed-valence dimer is revealed by the
disappearance of the near-infrared 5 -’ 5* absorption. A series of
weak changes in the visible region is observed with continued
irradiation and the photolysis reaction terminates with an absorption
profile distinguished by a strong absorption band at 385 nm and a
weaker band at 685 nm. The absence of an isobestic point during
the photolysis reaction, and the growth and decay of the
characteristic near-infrared band of M02(HPO4)43' are consistent
with a multistep photooxidation pathway with subsequent reaction
of the M02 primary photoproduct.

Further analysis of the M02(HPO4)44' photoreaction was
pursued with investigations of the photochemistry of M02(II,III) and
M02(III,III) phosphate dimers. Irradiation (A > 335 nm) of M02-
(HPO4)43' in 2M D3PO4 produces the spectral changes illustrated in
Figure 31. The intensity of the near-infrared band of M02(HPO4)43'

monotonically decreases and bands at 420 and 540 nm appear
during the initial stages of irradiation. With continued irradiation,
the 540-nm band disappears and absorptions grow in at 385 and

690 nm. This latter spectrum is identical to the terminating

spectrum of M02(HPO4)44' photolysis. The quantum yield for the

139

Figure 30. Electronic absorption spectral changes during irradiation
(x > 335 nm) of M02(HPO4)44’ in 2 M D3PO4. Due to spectral
congestion in the visibile spectral region, the 45-min trace is not
illustrated between 350 and 800 nm. The absorbance sensitivity in
the visible spectral region is twice that of the infrared spectral

region.

140

 

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Figure 31. Absorption changes resulting from irradiating (A > 335
nm) 2 M D3PO4 solutions of M02(HPO4)43'. The visible absorbance

scale is 5 times greater than the near-infrared absorbance scale.

142

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disappearance of Mo2(HPO4)43' is 0.046 (Xexc = 313 nm) whereas

no reaction is observed for solutions of the mixed-valence dimer
stored in the dark at room temperature.

The appearance of the 540-nm band during the early stages of
Mo2(HPO4)43' photolysis is consistent with the formation of

M02(HPO4)42’ as a primary photoproduct. Support for this
contention is provided by the photochemistry of M02(HPO4)42'.
Figure 32 displays the absorption changes accompanying irradiation
(A > 335 nm) of 2 M D3PO4 solutions of M02(HPO4)42‘. Smooth
conversion of M02(HPO4)42' to the photoproduct, characterized by
the 385- and 685-nm absorptions, is observed and in contrast to
M02(II,II) and Mo2(II,III) photochemistry, isobestic points at 500
and 608 nm attest to a stoichiometric photoreaction. While the
identity of this ubiquitous photoproduct has not yet been revealed, a
clue to its identity is provided by comparison to the absorption
spectra of other dimolybdenum oxo and hydroxo cores.l37'146
Table 5 displays absorption maxima and molar absorptivity
coefficients of polynuclear molybdenum complexes. Examination of
these data reveals that the absorption spectrum of the primary
photoproduct of M02(HPO4)42' photolysis (Amax(€) : 385 nm (1800
M'1 cm'l); 690 nm (350 M'1 cm'1)) closely matches that of the
M02(III,III)-u -dihydroxo complex.131 These results suggest that
irradiation of M02(HPO4)42' triple bond dimer promotes ligand

substitution of the phosphate to yield the p-hydroxo species. The

slight wavelength discrepencies between the primary photoproduct
and the M02(III,III)(u-OH)2(aq) complex may be due to the

differences of the ancillary ligands (H20 vs. HPO42') on the M0204-

144

Figure 32. Electronic absorption spectral changes during irradiation
(X > 335 nm) of M02(HPO4)42' in 2 M D3PO4. No absorption bands

appear in the near-infrared spectral region during the photolysis

reaction.

 

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148

01-024+ core. This hypothesis can be tested by investigating the
substitution chemistry of Moz(}.1-OH)2(aq)4+ in phosphate containing
solutions. Interestingly continued ultraviolet photolysis of the
proposed Moz(u-OH)24+ phosphate photoproduct yields a species
with the absorption spectrum shown in Figure 33. Detection of

hydrogen above reacted solutions by gas chromatography suggests
that the M02(p-OH)24+ core is photooxidized. This observation is
confirmed by the fact that addition of K28208 to phosphoric acid

4-

solutions of M02C13 yields a product with an absorption spectrum

identical to the one shown in Figure 33. These results are consistent

with photooxidation M02(}J-OH)24+ phosphate produce a high valent
M02 oxo or hydroxo core (e.g. M02042+).

The above photochemical results of M02(HPO4)4n‘ (n = 2, 3, 4,)

dimers substantiate a sequential oxidation pathway. The relatively

complicated series of spectral changes induced by irradiation of

M02(HPO4)44' solutions are explained by a reaction scheme in which
the final photoproduct (A max = 385 and 685 nm) is generated
directly from M02(HPO4)42', which in turn is a primary
photoproduct of M02(HPO4)43' which is the primary photoproduct of
M02(HPO4)44'. Gas chromatography reveals a concomitant

production of hydrogen with each of these discrete dimer photo-
processes. A reaction sequence accommodating these observations is
shown by equation (32). As previously noted, ensuing M02(HPO4)42'
photo-oxidation leads to the generation of a presently undetermined
higher valent molybdenum species and hydrogen. Thus the overall
photochemical reaction corresponds to a multielectron process which

involves the exchange of at least two electrons.

149

Figure 33. Terminating electronic absorption spectrum of photolyzed
(7x > 335 nm) phosphoric acid solutions of M02(HPO4)4“’ (n = 2, 3, 4).

 

 

 

 

150

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151

+
H

4_ th. > 335 nm)
M02(HPO4)4 \ :

 

l/2H2
H

3_ S lit/(7x > 335 nm) 2_
M02(HPO4)4 \; M02(HPO4)4 (32)

l/2H2

 

In an effort to ascertain the nature of the intermediates leading

to hydrogen production, we photolyzed solutions of M02(HPO4)44'
and M02(HPO4)43' under atmospheres of N20, which is an effective
trap of highly energetic radical intermediates. Ultraviolet irradiation
(xcxc > 335 nm) of M02(HPO4)44' and M02(HPO4)43' in N20
saturated 2 M H3PO4 solutions yields the one-electron photo-
oxidized M02 phosphato complex and N2. These results are consistent

with the generation of H atoms as primary photoproduct of M02

phosphato dimers because nitrogen, which is generated at the

expense of hydrogen formation, is a product expected from a reaction

0.147

of hydrated hydrogen atoms with N2 Annihilation of two

hydrogen atoms or subsequent oxidation of the starting complex by a
hydrogen atom to yield H' followed by a facile proton trapping
reaction will result in hydrogen production.

Insight into the nature of the photoactive state of the M02
phosphate systems is provided by the wavelength dependence of
M02(HPO4)4“' (n = 2, 3, 4) photoprocesses. The photochemistry of

each of the M02 phosphate system is inhibited as the irradiation

152

wavelength is increased. Illustrative of this reactivity trend is the
photooxidation reaction M02(HPO4)43' which proceeds promptly
with ultraviolet irradiation but is suppressed upon shifting the
excitation wavelength into the visible spectral region; Figure 34
summarizes the wavelength dependence of the reaction quantum
yield. No photoreaction can be detected when M02(HPO4)43'
solutions are irradiated at wavelengths longer than 475 nm. Between
300 and 400 nm, the quantum yield increases monotonically with
decreasing irradiation wavelengths. And for wavelengths less than
300 nm, the quantum yield asymptotically approaches a limiting
value of 0.05. Similarly, facile conversion of M02(HPO4)44' is
observed for excitation wavelengths less than 367 nm, whereas
solutions of M02(HPO4)44' do not photoreact with irradiations into
the 1( 52 -> 65*) absorption band. These results imply that the
photoreactivity of the M02 phosphato dimers is not associated with

the lowest energy metal localized transition of the M02 core but

rather with high energy excited states that lie in the ultraviolet
spectral region.

In an effort to identify these high energy photoactive excited

states, the ultraviolet absorption spectra of the M02 phosphato

complexes in 2 M H3PO4 was recorded. As illustrated in Figure 35, a

prominent absorption band dominates the ultraviolet spectral region
of the three phosphato dimers (X max, M02(HPO4)44' = 206 nm;
xmax, M02(HPO4)43' = 210 nm; xmax, M02(HPO4)42' = 220 nm).
The relative insensitivity of the band maximum to the oxidation state

of the M02 core suggests that the transition is metal localized.

Theoretical calculations predict that the allowed Tl -> n * transition of

153

Figure 34. Action spectrum of the photolysis reaction: M02(HPO4)43'
+ H+ -* M02(HPO4)42’ + 1/2 H2. Quantum yields were measured by
monitoring the 6 -9 5* absorption band of M02(HPO4)43'.

 

 

 

 

154

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(p 60'

155

Figure 35. Ultraviolet absorption bands of M02(HPO4)44' (---),
M02(HPO4)43' (—), and Moz(HPO4)42‘ (- - -) in 2 M H3PO4 at 25 °C.

 

 

 

156

 

 

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300
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200

157

MA-M dimers should be observed in ultraviolet energy region,148

149

and polarized absorption spectroscopic studies support this

contention. Accordingly, we assign the 206 nm absorption of
M02(HPO4)44' to the TI -* 11* transition. The modest red-shift of the

band across the M02(II,II), M02(II,III), and M02(III,III) series is

consistent with a concomitant decrease in the metal-metal

interaction. An additional absorption band is observed at 237 nm of
M02(III,III) phosphate. That this absorption is not present at lower

energies in the spectra of M02(II,III) and M02(II,II) phosphato

dimers precludes its assignment to a transition corresponding to
oxidation of the metal core, such as charge-transfer-to-solvent
(CTTS), because absorption bands attributable to transitions of this

type would exhibit a marked red-shift with reduction of the metal
core. Conversely, the absence of this band in M02(II,II) and

M02(II,III) phosphate spectra is presumably due to its blue-shift

into the hard-ultraviolet thereby suggesting that the transition
exhibits significant LMCT character. Thus, although the TH!“ and
LMCT excited states of the M02(II,III) dimer are potentially

photoreactive, our inability to detect charge transfer excited states in
the ZOO-250 nm region for the M02(II,II) and M02(II,III) phosphato

complexes implies that the photoreactivity of these species is derived
exclusively from TI -’ TI* excitation. From molecular orbital point of
view, this assignment is consistent with the observed chemistry
because the metal centered TI * antibonding orbital exhibits
significant electron density away from the metal-metal core along
the z direction. Thus promotion of one electron from TI bonding to n *

antibonding orbitals increases the probability of electron transfer

158

from the metal core to substrates occupying the open axial
coordination sites. It is at these axial coordination sites which protons
are most likely to reside.

The smooth conversion of M02(II,II) phosphate to M02(III,III)
phosphate dimer with the concomitant production of hydrogen
clearly demonstrates the capacity of the M-ILM core to engender
multielectron photochemical transformations by coupling the
oxidation-reduction chemistry of the two metal centers along a
controlled reaction pathway. It is evident from the reduction
potentials of the M02 phosphato complexes that the overall reaction
is thermodynamically favored and hence the stabilities of the
M02(HPO4)44' and M02(HPO4)43' ions in H3PO4 must necessarily
arise form large kinetic barriers associated with oxidation of the M02
cores. Our results demonstrate that these barriers are easily
surmounted along photochemical reaction pathways. The 11 -’ 11*

assignment of the photoreactive state of M02 phosphato complexes

conforms with the general reactivity pattern which has developed
for M-n-M dimers in acidic solution. Namely, low-energy metal-
localized excited states are not responsible for the photooxidation
chemistry of M-n-M dimers in protic environments. We believe that
the proposed photochemical mechanism is not specific to M02
phosphato complexes but may be extended to accommodate the
photochemistry of other M0208 complexes in acidic solution.94 For
instance, the electronic absorption spectra of M02(SO4)43’ and
M02(SO4)44' are completely analogous to the spectra of the

corresponding M02 phosphato complexes in that an intense

absorption band whose energy is independent of the oxidation state

159

of the M02 core (X max = 239 and 235 nm of M02(SO4)43' and
M02(SO4)44', respectively) dominates the ultraviolet region. These

data support assignment of the band to a 1'! -t 1'!" transition, and it is

reasonable to propose that the photoreactivity of the M02(SO4)44'

system, similar to the phosphato series, originates from TI -* 11*

excitation. The two-electron photochemistry of the M02 phosphate

system, as opposed to the one-electron transformations of the sulfate

system, reflects the increased susceptibility of the M02 core toward

oxidation when ligated by phosphate. This difference between the

oxidation-reduction properties of M02 phosphato and sulfato

complexes may derive from the fact that the ligands of the former
complex possess a dissociable proton. In this connection, phosphoric
acid and its ions exhibit enhanced proton ionization upon

coordination to a metal; and ionization is enhanced further when the

150,151

ions chelate a metal center. In view of the relatively high

charge of the M02 core and the bridging coordination geometry of

phosphate, significantly reduced interactions of a proton with the

3' may be expected. Weak or complete dissociation of

2..

oxygens of P04

a proton from one or more of the HPO4 ligands will increase the

overall negative charge of the complex and facilitate oxidation of the

M02 core. If this model is correct then the reported electrochemistry

and photochemistry should exhibit a large pH dependence.
Unfortunately, the insolubility and instability of M02 phosphate

dimers in H3PO4 solution at concentrations less than 2 M precludes

pH dependence studies. The ability of the phosphate ion to

significantly shift the oxidation potential of metal cores to more

160

positive values may be responsible for the unique photooxidation

chemistry of the dimolybdenum phosphate system in acidic solution.

CHAPTER V

PHOTOCHEMISTRY OF A NOVEL
DIMOLYBDENUM DIPHENYL PHOSPHATE
IN NONAQUEOUS SOLUTION

A. Background

The multielectron photochemistry of quadruply bonded
M02(HPO4)44' is derived clearly from sequential one-electron
oxidation steps. The desired two-electron oxidative-addition
pathway described in Scheme III is not observed. However, the
photochemistry of the M02(HPO4)4n' system is similar to other
MA-M systems in acidic solution and establishes a general reactivity
pattern: M-n-M dimers do not exhibit appreciable lifetimes in acidic
solution and consequently photoreactivity is confined to high energy,
pseudo unimolecular one—electron processes. For the specific case of
the M02(HP04)44' system, the 66* excited state is short lived and
the photoreactivity is derived from the one-electron reduction of H+
by the highly energetic TI TI * state (Figure 36). The issue of
paramount importance to multielectron reactivity is by what

mechanism the 66* excited state is deactivated in acidic solution.

161

162

Figure 36. Photoreaction pathway of "M0208" complexes in acidic
solutions, where "M0208" corresponds to M02(aq)x4+, M02(SO4)44’,
and M02(HPO4)44-.

Energy/av

 

6.0T-

2_4,_

 

163

+
H

1 a:
A1g( nn )

1/2 H2 + M—“iLZM

1 at:
_ A2u(6 6 )

3 at:
—) A2u(55 >

 

1 2
’— Alg(5 )

Figure 36

164

X-ray crystallography has identified stable hydride M—M products
from the reaction of protons with the quadruple bond core.152 In
these complexes, the metal in the M-M core has been formally

oxidized by one electron with subsequent two-electron reduction of

H+ to H'. Alternatively, a resonant structure for an M—M core is

one where the core is simply protonated,

/"\ __
MEM — M
(a) (b) (33)

\
[Ill/a

We believe that for most M-‘l-M systems the major contributing
structure is (b). Indeed, we have shown the 66* excited states are
efficiently quenched by protons and a quenching mechanism
consistent with the above discussion is shown in Figure 37. In this
scheme protons quench the 66* excited state to give a "hydride-like"
intermediate which expels the proton from the metal core to yield
ground state M-4-M dimer and H4". Therefore, the photon energy is
wasted in the proton quenching process and reactivity derived from
the 66* excited state is circumvented.

These results suggest to us that if 66* multielectron
photochemistry is to be observed, the proton quenching of 66*
excited state and the energy consuming back proton transfer step

must be inhibited. Owing to our knowledge of M02 phosphate

systems coupled with the demonstrated photoredox chemistry of

165

Figure 37. Proton quenching of MAM excited states.

 

166

“V M-4-M*

Figure 37

167

M02(HPO4)44', we initiated studies on the synthesis of M02

phosphates which could be introduced into aprotic environments. To
this end, we have prepared the quadruply bonded tetrakis(diphenyl
phosphate)dimolybdenum M02(02P(OC6H5)2)4 complex which
exhibits good solubility in nonaqueous solvents.

The compound reacts with oxidants such as NOBF4 to yield the
one-electron oxidized mixed-valence species
M02(02P(OC6H5)2)4B F4. Both the quadruple-bond and mixed-
valence species have been characterized by X-ray crystallography,

electronic spectroscopic, and electrochemical methods. In contrast to

the high energy photochemistry of M02(HPO4)44' in acidic solution,
the photochemistry of M02(02P(OC6H5)2)4 in halogenated
hydrocarbon solvent is promoted by visible irradiation. The
chemistry and photochemistry of this new complex is presented in

this chapter.

B. Results and Discussion

1. Structures of M02(02P(OC6H5)2)4-2THF and
M02(02P(OC6H5)2)4BF4. The molecular structures of
M02(02P(OC6H5)2)4-2THF and M02(02P(OC6H5)2)4BF4 are typical
structures of multiply bonded dimolybdenum complexes.55
Molecules of the quadruply bonded dimer occupy general positions
of the space group P21/n whereas the mixed-valent species is
situated on inversion sites of the P1 space group. Crystallographic
data for the two complexes are listed in Table 6. Molecular

structures of the two dimers and atom numbering schemes are

168

Table 6. Crystallographic Data of Moz(02P(OC6H5)2)4°2TI-IF and

M02(02P(OC6H5 )2)4BF4

 

 

M02(02P(OC6H5)2)4'2THF M02(02P(OC6H5)2)4BF4
space group P21/n P1
cryst syst monoclinic triclinic
a, 13.359(3) 10.917(8)
b, A 29.641(15) 11.793(4)
c, A 14.719(3) 12.430(4)
(2, deg 63.45(2)
6, deg 93.43 70.38(4)
y, deg 70.64(5)
V, A3 5818(6) 1316.8(12)
d, g/gm 1.522 1.608
Z 4 1
radiation Mo Kc Mo KG
abs coeff(}.1(Mo 1(a)), cm-1 5.964 6.625
26 limits, deg 45 55
no. of reflens collcd 7973 12192
no. of unique reflens 7597 6096
no. of unique reflens
used with F0 >30 2363 5107
R(F) .091 .027
Rw(F) .103 .027
GOF 10.7 .654

169

illustrated in Figures 38 and 39. The positional parameters for
heavy atoms (e.g. Mo, P, and O) are listed in Tables 7 and 8 for
M02(02P(OC6H5)2)4'2THF and M02(02P(OC6H5)2)4BF4, respectively.
Tables 9 and 10 list selected bond distances; and the selected bond
angles are listed in Tables 11 and 12 for quadruply—bonded and
mixed-valent species, respectively.

Even though the molecular structures of both compounds
appear similar, closer examination reveals some significant
differences between the two molecules. The most noticeable
difference is the Mo-Mo bond distance which increases from 2.154 A

in M02(02P(OC6H5)2)4-2THF to 2.191 A in M02(02P(OC6H5)2)4BF4.
This increase of 0.04 A resulting from the oxidation of the dimer by
one electron is comparable to that observed for the M02 sulfates
which exhibit a lengthening of the Mo-Mo bond by 0.05 A upon
oxidation (2.111 A and 2.164 A for quadruple bond and mixed-
valence complexes, respectively)13°. One—electron oxidation of the
quadruple bond reduces the formal 6 bond order from 1 to 1/2.
Besides a decreased bond order, the increased formal charge on each
metal (from +2 to +2.5) causes a constriction of the valence orbitals of
the metal atoms and corresponding decreased orbital overlap.
Because of the longer metal-metal bond distance in the mixed—
valent species, there is less steric strain of phosphate ligands
bridging the one-electron oxidized metal core. The five membered
M02(02P) ring possesses trans O-Mo-O angles close to 180° in the
mixed-valent species as compared to the more strained angles in the
quadruple-bond compound (e.g. 1710 and 1670 respectively). This is

manifested in the mixed-valent species to possess a higher

170

Figure 38. Structure and labeling scheme for the quadruply bonded
complex M02(02P(OC6H5)2)4'2THF. Atoms are represented by their

50% probability ellipsoids. Due to structural congestion, the two

tetrahydrofuran molecule are not illustrated.

 

 

 

 

171

 

Figure 38

172

Figure 39. Structure and labeling scheme for the mixed-valence

complex M02(02P(OC6H5)2)4BF4. Atoms are represented by their
40% probability ellipsoids.

 

 

 

Figure 39

174

Table 7. Selected Atomic Positional Coordinates of
M02(02P(0C6H5)2)4°2THF

 

 

atom x y z

Mo(l) 0.82506 0.11940 0.26235
Mo(2) 0.66364 0.11702 0.25292
P(l) 0.74433 0.10992 0.45245
P(2) 0.73685 0.21593 0.28329
P(3) 0.74340 0.13519 0.06147
P(4) 0.75817 0.02147 0.24665
0(1) 0.64447 0.10769 0.39247
0(2) 0.83149 0.10697 0.40465
0(3) 0.83597 0.19041 0.28522
0(4) 0.64912 0.18690 0.27072
0(5) 0.65419 0.13151 0.10958
0(6) 0.84402 0.13110 0.11602
0(7) 0.84850 0.05124 0.23834
0(8) 0.66053 0.04395 0.23834
0(9) 0.73567 0.06964 0.52455
0(10) 0.75655 0.15712 0.50616
0(11) 0.72432 0.24283 0.37444
0(12) 0.74407 0.25134 0.20039
0(13) 0.74688 0.09620 -0.01629
0(14) 0.73748 0.18258 0.01391
0(15) 0.76189 -0.00184 0.34847
0(16) 0.75853 -0.02398 0.18216

Table 8. Selected Atomic Positional Coordinates of

175

 

 

M02(02P(0C6H5 )2)4BF4

atom x y z

Mo(l) 0.00738 -0.09327 ~0.00349
P(1) 0.25799 0.01683 -0.l6557
P(2) 0.13565 -0.13277 0.19945
0(1) 0.19858 -0.10096 -0.11524
0(2) 0.08782 ~0.20395 0.15108
0(3) -0.l7874 -0.11616 0.10870
0(4) -0.07255 -0.01050 -0.15827
0(5) 0.26835 0.07531 -0.30863
0(6) 0.40088 -0.01817 -0.l4397
O(7) 0.10424 -0.20480 0.34273
0(8) 0.28892 -0.14036 0.15318

l 7 6
Table 9. Selected Bond Distances of M02(02P(0C6H5)2)4°2THF

 

Bond Distances/A Bond Distances/A
Mo(l) -Mo(2) 2.154
Mo(l) -O(2) 2.123 Mo(2) -O(1) 2.103
-O(3) 2.135 -O(4) 2.098
-O(6) 2.211 -O(5) 2.149
-O(7) 2.078 -O(8) 2.205
P(1)-O(1) 1.556 P(2) -O(3) 1.524
-0(2) 1.399 -O(4) 1.457
-O(9) 1.606 -O(12) 1.617
P(3) -0(5) 1.426 P(4) -O(7) 1.506
-O(6) 1.529 -O(8) 1.482
-O(13) 1.629 -O(15) 1.648
-O(14) 1.569 -O(16) 1.648

177
Table 10. Selected Bond Distances for Moz(02P(0C6H5)2)4BF4

 

Bond Distances / A
Mo(l) -MO(1) 2.191
Mo(l) -O(1) 2.082

-O(2) 2.073
-O(3) 2.067
-O(4) 2.074
P(1)-0(1) 1.513
-O(3) 1.509
-O(5) 1.572
-O(6) 1.559
P(2) -O(2) 1.514
-O(4) 1.515
-0(7) 1.563
-O(8) 1.562

1 7 8
Table 11. Selected Bond Angles of M02(02P(0C6H5)2)4-2THF

 

Bond Angle Angles/o Bond Angle Angles/o
Mo(l)-Mo(2) -O(1) 97.0 Mo(2)-Mo(l) -O(2) 92.2
-O(4) 93.4 -0(3) 95.8
-O(5) 93.2 -O(6) 96.6
-O(8) 93.0 -O(7) 96.8
0(2)-MOO) -O(3) 91.0 0(1)-Mo(2) -O(4) 89.9
-O(6) 171.1 -O(5) 168.9
-O(7) 90.1 -O(8) 92.9
0(3)—Mo(1) -0(6) 89.3 0(4)-Mo(2) -O(5) 85.7
-O(7) 167.3 -O(8) 172.9
0(6)-Mo(l) -O(7) 87.7 0(5)-Mo(2) -O(8) 90.8
0(1)-P(1) 00) 115.0
-O(9) 104.5 0(2)-P(1) -O(9) 112.4
-O(10) 111.9 -O(10) 103.8
0(3)—P(2) -0(4) 113.6
-O(11) 111.8 0(4)-P(2) -O(11) 106.3
-O(12) 104.2 -O(12) 111.8
0(5)-P(3) -0(6) 117.9
-O(13) 110.8 0(6)-P(3) -0(13) 104.6
-O(14) 105.6 -O(14) 108.9
0(7)-P(4) -O(8) 114.7
-O(15) 109.8 0(8)-P(4) -O(15) 111.3
-O(16) 113.7 -0(16) 106.0
Mo(l) -0(2)-P(1) 120.0 Mo(l) -O(1)-P(1) 113.5
-O(3)-P(2) 115.8 -O(4)-P(2) 121.3
-O(6)-P(3) 112.1 -O(5)-P(3) 120.1
-O(7)-P(4) 115.2 -O(8)-P(4) 113.3

179
Table 12. Selected Bond Angles of M02(02P(0C6H5)2)4BF4

 

Bond Angle Angles/o
MO(1)-MO(1) -O(1) 94.6
-O(2) 94.8
-O(3) 94.9
-O(4) 94.5
0(1)-M00) -O(2) 89.7
-0(3) 170.5
-O(4) 90.1
0(2)-M00) -O(3) 88.6
-O(4) 170.8
0(3)-Mo(l) -O(4) 90.1
Mo(l) -O(1)-P(1) 117.6
-0(2)-P(2) 116.9
-O(3)-P(1) 118.2

-0(4)-P(2) 117.2

180

symmetry structure, which includes a center of inversion, despite

identical virtual symmetries (D 4 h) for both complexes.

2. Oxidation-Reduction Chemistry. The cyclic
voltammogram of M02(02P(0C6H5)2)4 in CH2C12 is shown in Figure
40a. Two reversible oxidation processes are observed upon
anodically scanning solutions containing the quadruple bond
complex. Reversible electrochemical behavior is indicated by linear
plots of the cathodic and anodic currents vs. 121/2 (scan rate, 12: 20-
200 mV seC‘l), and values of 0.99 :1: 0.02 for ratios of the anodic and

112

cathodic peak currents. Figure 40b illustrates the cyclic

voltammogram of the mixed-valence species M02(02P -
(0C6H5)2)4BF4. The two complexes exhibit identical E1/2 values
(+0.067 and +0.997 V vs. SCE), with the quadruple bond having two
reversible oxidations, whereas the mixed-valence complex possesses
one reversible oxidation and one reversible reduction. These results
clearly show that the wave at 0.067 V vs. SCE corresponds to the
M02(02P(0C6H5)2)4°/+ couple. Owing to the reversibility of the
wave at 0.997 V vs. SCE at all scan rates, we assign this wave to the
M02(02P(0C6H5)2)4+/2+ couple.

Table 13 collects the reduction potentials of the "M0203"
complexes M02(SO4)44’,13° Moz(HP04)44', and M02(02P-
(0C6H5)2)4. Inspection of these data obviously reveal that the
ligands perturb the redox properties of the metal core in a different

manner. Not surprisingly, proton association with the ligands of the

M02 sulfato and diphenyl phosphate complexes is unlikely and the
M02(II,III)/M02(II,II) reduction potentials are similar. It is

181

Figure 40. Cyclic voltammograms of (a) M02(02P(0C6H5)2)4 and (b)
M02(02P(0C6H5)2)4BF4 in CH2C12 solution at 23 °C. NBu4PF6 was

used as the supporting electrolyte, scan rate = 20 mV s'l.

182

 

 

 

 

 

(a)

(b)

 

O . 5
EN vs. SCE

Figure 40

 

183

Table 13. The Formal Reduction Potentials of the M02(II,III)/(II,II)
and M02(III,III)/(II,III) Couples of M02 Sulfato, Phosphato, and

Diphenyl Phosphate Complexes

 

 

Complexes E1/2/V vs. SCE E1 [ZN vs. SCE
M02(II,III)/(II,II) M02(III,III)/(II,III)
Sulfato +0.25 a
Phosphato -0.67 -0.24
Diphenyl phosphate +0.06 +1.00

 

a Not available.

184

noteworthy that the M02(III,III)/M02(II,III) couple for the sulfato
complex lies in the background of solvent (in this case H20) and
heretofore has not been determined. 0n the basis of the energy
separation between the M02(II,III)/M02(II,II) and
M02(III,III)/M02(II,III) couples of the diphenyl phosphate complex,

we predict the latter couple of the sulfato complex to lie at 1.19 V vs.

SCE. As originally inferred in Chapter IV, the very negative
potentials of M02(HP04)44’ oxidation processes is attributed to the

dissociation of a proton from one or more of the HP042' ligands. The
electronic and structural similarities of the HP042' and 029mm-
ligands are virtually identical and hence the only logical explanation
for the ~l.0 V shift to negative potentials for M02(HP04)44' is that
the negative charge of the M02(HP04)44' is increased by proton
dissociation, thereby facilitating oxidation of the M02 core.

Similar to the M02(HP04)44' system, it is easy to understand

the syntheses of the molybdenum diphenyl phosphates and their
reactions on the basis of the oxidation-reduction potentials.
Although oxidation of the quadruple bond complex by N0BF4
proceeds smoothly to the pure mixed-valence compound
Moz(02P(0C6H5)2)4BF4, oxidation to M02(02P(0C6H5)2)42+ by using
NOBF4 can not be achieved. From the redox potentials, it is
understandable that the N0+INO couple (estimated to be ~0.85 to 1.0
V vs. SCE in nonaqueous solution)153 is insufficient to bring about
the second oxidation. Even stronger oxidants, such as magic green

(tris(4-bromophenyl)aminium hexachloroantimonate (E1 ,2: 1.04 V

vs. SCE) or dichloroiodobenzene (estimated 0.9 to 1.2 V vs. SCE) with
just enough potential to generate the dication M02(02P(0C6H5)2)42+,

1 8 5
yield only the mixed-valence M02(02P(0C6H 5)2)4+ product. This

may be a result of significant kinetic barriers for generating the
dication. Unfortunately, solutions of the M02(02P(0C6H5)2)4BF4
dimer decompose upon heating, and hence the oxidations must be
run at or below room temperature. An alternate method to the
synthesis of the dication species is by using bulk-electrolysis. The
dication complex can be generated in solution at electrodes whose
potentials are as high as 1.7 V vs. SCE. Unfortunately, this dication
species failed to recrystallize. Although small concentrations of
impurities (mostly higher-valent molybdenum complexes) from the
bulk-electrolysis obscure the visible absorption region (from our
experience on the M02 phosphate system, we believe that the
dication should exhibit weak absorption in visible region), the
absence of near-infrared absorption suggests that the dication

M o 2( 0 2 P ( 0 C 6 H 5 )2 )4 2 + species is generated in solution and that

electrochemical preparation of this triply bonded species is plausible.

3. Magnetic and Spectroscopic Characterization of
M02(02P(0C6H5)2)4 and M02(02P(0C6H5)2)4BF4. Typical of
M-‘LM complexes, M02(II,II) diphenyl phosphate has a 0211462
ground state electronic configuration and will possess a diamagnetic
ground state. Oxidation by one electron to yield the paramagnetic
mixed-valence complex whose EPR spectrum recorded on frozen
solutions at 5 K is shown in Figure 41. An axial doublet signal with
g1 = 1.909 and g" = 1.863 is observed at low temperatures and is
consistent with a species in which the unpaired electron is coupled

between two equivalent molybdenum nuclei.

186

Figure 41. X—band (9.434 GHz) EPR spectrum of a frozen solution of
M02(02P(0C6H5)2)4BF4 in CHZCIZ solution at 17 K.

 

 

187

 

 

 

 

 

 

3400 3500 3600 3700 3800
H / G

Figure 41

 

188
The absorption profiles of the M02 diphenyl phosphates in the

visible and near infrared are directly analogous to the phosphates.

Figure 42 illustrates the electron absorption spectrum of the

M02(02P(0C6H5)2)4 complex in CH2C12 solution. The lowest energy
absorption band ()‘max = 515 nm, E = 156 M'1 cm'l) is comparable

in energy and intensity to that of M02(HP04)44'. The next higher

energy absorption band at 404 nm (E = 38 M’1 cm'l) is assigned to

the TI -’ 6* transition.129

The electronic absorption spectrum of
M02(02P(0C6H5)2)4BF4, shown in Figure 43, is dominated by a
prominent absorption profile in the near-infrared (Xmax = 1469 nm
(2:142 M'l cm'1)) and a weak visible band at 600 nm (e: 27 M-1
cm'l). From the previous spectroscopic studies on M02(SO4)43', we
are able to assign the near-infrared absorption band to the 6 4 6*
(23m .— 213 2g) transition.93’132 As with the M02 phosphates, the

large red-shift of the 6 -’ 6* transition in mixed-valence M02 dimer

is explained by the absence of two electron spin pairing term
contributions of the 0211461 configured ground state species.
Similar to M02(S04)43‘ and M02(HP04)43‘ ions, the 6 -' 6*
absorption band of M02(02P(0C6H5)2)4BF4 exhibits a vibrational

progression in solution at room temperature. The 308 cm'1 energy
spacing is consistent with the symmetric metal-metal stretching
vibration. The lower energy of the progression as compared with the
M02(S04)43' and M02(HP04)43' (352 and 334 cm’l, respectively) is
probably due to the longer M-M bond distance of this mixed-valent
M02 diphenyl phosphate. The 600-nm band in M02(02P—
(0C6H5)2)4BF4 is a direct analogue to the lower energy absorption in
M02(II,III) sulfato and phosphato complexes and is by analogy,

189

Figure 42. Electronic absorption and emission spectra of
M02(02P(OC6H5)2)4 in CH2C12 solution at 25 0C.

190

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191

Figure 43. Electronic absorption spectrum of M02(02P(0C6H5)2)4BF4
in CHZCIZ solution at 25 OC.

 

 

 

192

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193

assigned to the 11 4 6 transition. The lack of higher energy
absorptions in the visible spectral region could be explained by the
strong ultraviolet absorption from the 1'! -’ 11 * transition of the

aromatic ring of the ligands (A max = 260 nm) which tails into the

visible and presumably shields the weak absorption of the other
metal-localized transitions.
In spite of the similarities of the absorption properties of

M02(02P(0C6H5)2)4 to M02(HP04)44', there is one major

spectroscopic difference between these two compounds. The new

M02(II,II) diphenyl phosphate species emits in nonaqueous solution

as shown in Figure 42. This observation supports our original
hypothesis that M0208 quadruple-bond dimers do not emit in
aqueous solution because of proton quenching of the 66* state.
Because the emission quantum yield for this complex is quite low,
the lifetime of the excited state was measured by transient
absorption spectroscopy. The transient absorption band at 460 nm
decays exponentially after the 532-nm laser excitation pulse (FWHM
= 25 ps). By using a monoexponential fitting method, the decay
lifetime is 68 ns (Figure 44). This excited state lifetime is long

enough to permit bimolecular reactivity.

4. Photochemistry. In striking contrast to the high energy
photoreactions of M02(HP04)4“' in acidic solution,

M02(02P(0C6H5)2)4 reacts with halogenated hydrocarbon solvents

upon low energy irradiation. This low energy photoreaction directly

originates from the long-lived 1(66*) excited state. Spectral changes
for the photoreaction of M02(02P(0C6H5)2)4 in 1,2-dichloroethane

194

Figure 44. Transient absorption kinetics of M02(02P(0C6H5)2)4 in

THF solution. The decay kinetics were recorded at 460 nm and

excitation was with a 532 nm Nd:YAG laser pulse (FWHM = 25 ps).

 

195

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196
upon irradiation into 62 -t 66* absorption (xexc - 530 nm) are
shown in Figure 45. An initial decrease in the intensity of the 62 -’

6 6 "' absorption band is accompanied by a concomitant increase in

the near-infrared region. It is noteworthy that the near-infrared

absorption arising from the photoreaction of M02(02P(0C6H5)2)4 in
C1CH2CH2C1 is not identical to the absorption of
M02(02P(0C6H5)2)4+BF4'. The band maximum of the photolyzed
product is red-shifted (1470 nm for M02(02P(0C6H5)2)4+BF4' and
1501 nm for photoproduct) and more intense. Because Cl' is the
most possible anion generated in dichloroethane solution, the
oxidized photoproduct is very likely to have chloride in the axial
coordination position. This assumption has been verified by Fast

Atom Bombardment Mass Spectroscopy (FABMS). The highest
molecular weight cluster found in photolyzed CICHZCHzCl solutions of

M02(02P(0C6H5)2)4 displays a parent ion peak at 1225 amu (Figure
46) which is consistent with the molecular weight of
M02(02P(0C6H5)2)4Cl (mol wt = 1223.33). We attribute the red-

shifted and increased absorption intensity of the 6 -* 6* transition of

the photolyzed product to coordination of chloride in the axial

coordination site. This hypothesis is confirmed with the independent
synthesis of the M02(02P(0C6H5)2)4Cl. The absorption spectrum of

CH2C12 solutions of M02(02P(0C6H5)2)4 recorded after addition of
0.5 equivalents of the oxidant C6H51C12 exhibits a strong absorption

band at 1494 nm (5 ~ 362 M‘1 cm'l) as shown in Figure 47. The

intensity and energy of this band closely match that of the final

near-infrared photolysis spectrum ()‘max = 1501 nm, 8 ~ 391 M'1

cm'l). The photochemically and chemically produced spectra also

197

Figure 45. Electronic absorption spectral changes during irradiation
()‘exc > 530 nm) of M02(02P(0C6H5)2)4 in ClCH2CH2Cl solution at 25

0C. The visible absorbance scale is 2 times greater than the near-

infrared absorbance scale.

 

 

198

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199

Figure 46. FABMS spectrum of a photolyzed ClCHzCHZCl solution of
M02(02P(OC6H5)2)4.

 

200

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201

Figure 47. Electronic absorption spectrum of the product of the

reaction between M02(02P(0C6H5)2)4 and 0.5 equivalent of
C6H51C12 in CH2C12 solution at room temperature.

 

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203

are in close agreement in the visible except that the 450-nm band is
much more pronounced in the latter. Additionally, the 605-nm band
present in the spectrum of photolyzed solutions is not as pronounced
for solutions containing chemically oxidized product. Interestingly
the M02(02P-(0C6H5)2)4BF4 species does exhibit a band at 605 nm
and more intense absorption to the ultraviolet. These results suggest
that photolyzed solutions consist of a mixture of
M02(02P(0C6H5)2)4C1 and M02(02P(0C6H5)2)4+ owing to the
establishment of an equilibrium between coordinated chloride and

free mixed-valence species,

M02(02P(0C6H5)2)4C1 3
M02(02P(0C6H5)2)4+ + Cl‘ (34)

Since the absorption of M02(02P(0C6H5)2)4Cl is much stronger than
M02(02P(0C6H5)2)4BF4 in the near infrared region, the final
spectrum of photolyzed solutions in the near infrared region will be
dominated by the appearance of M02(02P(0C6H5)2)4Cl with minor
blue-shifts of the band maximum. Additionally, the visible

absorption features of photolyzed solutions match those obtained
from the sum of the absorption of M02(02P(0C6H5)2)4Cl and

M02(02P(0C6H5)2)4+BF4'.
The FABMS data in conjunction with chemical oxidation results

suggest that the initial photoreaction step is

M02(02P(0C6H5)2)4 + ClCH2CH2Cl .__Lv_. -CH2CH2C1 +
[M02(02P(0C6H5)2)4+ a M02(02P(0C6H5)2)4Cl] (35)

 

204

The photogenerated chloroethane radical is a very reactive species,
and thermal reaction with unreacted quadruple bond species is
thermodynamically favorable, to yield the chloro-substituted product

and ethylene,

'CH2CH2C1 + M02(02P(OC6H5)2)4 4
M02(02P(0C6H5)2)4C1 + CH2CH2 (36)

As discussed above, the equilibrium described by reaction (34) is
quickly established. Consistent with this proposed mechanism is our
measurement of ethylene above reacted solutions. Gas
chromatographic analysis reveals ethylene as the only detectable
organic product, and manometric measurements determine 0.51 i
0.03 equivalents is produced. The slightly higher measurement of
ethylene (according to reaction (35) and (36) only 0.50 equivalents
of ethylene should be produced) is indicative of the secondary

reaction,

'CH2CH2C1 + M02(02P(OC6H5)2)4C1 -*
M02(02P(0C6H5)2)4C12 + CH2CH2 (37)

which should become important as the concentration of
M02(02P(0C6H5)2)4 is depleted. 0ur observation of the loss of the
initial isosbestic point at long photolysis times in Figure 45 is
consistent with reaction (37) becoming a significant pathway as the

quadruple bond photoreacts.

205

The M02(02P(0C6H5)2)4 dimer also reacts with other
halogenated reactants. The spectral changes of M02(02P(0C6H5)2)4

in CH2C12 during visible excitation (A exc > 530 nm) are shown in

Figure 48. An absorption profile exhibiting a moderately intense
band at 542 nm and a strong band at 420 nm combined with
absorption in near infrared spectral region at 1494 nm. Although
the final spectrum in the near infrared is very similar to the one- I
electron oxidized species with Cl', the visible spectrum clearly differs

from that obtained for ClCHZCHzCl photochemistry. These

observations suggest the following photoreaction scheme,

M02(02P(0C6H5)2)4 + CH2C12 JV...
M02(02P(OC6H5)2)4C1 + °CH2C1 (38)

'CH2C1 + 'CH2C1 -’ C1CH2CH2C1 (39)

The generated 1,2-dichloroethane can then photoreact with
quadruply bonded species as described by reactions (35) and (36).

However that the visible absorption spectrum is not consistent with
exclusive production of M02 ( 0 2 P ( 0 C 6 H 5 )2 )4 C1 or

M02(02P(0C6H5)2)4+ clearly identifies a more complicated

photochemical scheme. The chloromethane radical is extremely
reactive and it is reasonable to assume that the following reactions

are competitive with dimerization,
'CH2C1 + M02(02P(OC6H5)2)4 -’ M02(02P(OC6H5)2)4CH2C1 (40)

'CH2C1 + M02(02P(OC6H5)2)4C1 4 CH2C1M02(02P(0C6H5)2)4C1 (41)

 

206

Figure 48. Electronic absorption spectral changes during irradiation
()‘exc > 530 nm) of M02(02P(0C6H5)2)4 in CH2C12 solution at 25 °C.

 

207

 

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208

The M02(III,III) organometallic products would definitely exhibit

intense absorptions in the visible spectral region. However, the
inorganic products of CH2C12 photolysis have not yet been isolated

and thus it is impossible to unequivocally identify the photoreaction

pathway from the data available at this time.

CHAPTER VI

FINAL REMARKS

The studies described herein demonstrate that M-A-M
complexes are good multielectron photoreagents. The redox activity
of the individual metal centers of the binuclear metal core can be
coupled to drive overall multielectron photoprocesses. This principle
is demonstrated by the photochemistry of M02(HP04)4n' (n = 2, 3, 4)
complexes in acidic solution in which the sequential one-electron
chemistry of the metal centers provides the two-electron reduction
of protons to hydrogen. An important discovery from these studies
is that the 1(66"‘) excited state is efficiently quenched by proton
transfer. Therefore, in acidic solution, the 1(66*) excited state is not
preserved for bimolecular reaction. Our approach to circumvent this
quenching process by employing synthetic strategies to design M-4-M
complexes with similar structures but readily soluble in nonaqueous
solutions was a successful one. The newly synthesized
M02(02P(0C6H5)2)4 complex luminesces from the 1(66*) excited
state in nonaqueous solutions at room temperature with an
appreciably long lifetime of 68 ns. This long-lived singlet excited

state dominates the photoreactivity of this molecule and in contrast

209

 

210
to M02(HP04)44', M02(02P(0C6H5)2)4 exhibits low energy two-
electron photochemistry. This system is significant because it
represents the first example of a multielectron process originating
from the 1(66*) excited state.

Although the M02(II,II) diphenyl phosphate photochemistry
constitutes a two-electron process driven by a single electron, the
reaction is not a concerted multielectron process of the 1(6 6 1")
excited state. Molecular models of M02(02P(0C6H5)2)4 reveal that
the metal-metal core, important for multielectron reactivity is
shielded by the diphenyl phosphate ligands. These results suggest
that M02(02P(0C6H5)2)4 photochemistry is confined to attack by
substrate at the axial coordination sites of the metal-metal core.
Quantum yield data support this analysis. The photoreaction
quantum yield of M02(02P(0C6H5)2)4 with 1,2-dichloroethane
decreases dramatically with increasing ligating ability of solvent.
These data are displayed in Table 14. The quantum yield decreases
from 0.040 to 0.001 by changing the solvent from 1,2-dichloroethane
to acetonitrile. Computer generated molecular models of the
M02(02P(0C6H5)2)4°2THF crystal structure shows that the two THF
molecules occupying the axial sites insulate the metal-metal core
from substrate. Consequently solvent coordination is reflected in the
reduction of the photoreaction quantum yield. Therefore, substrates
directly attack only one metal instead of the binuclear metal-metal
core. Hence the advantages of using M-4-M complexes are attenuated
by axial attack and one-electron photoredox chemistry is observed.

This analysis suggests that M-A-M multielectron photo-

chemistry will only be observed for substrates which can approach

211

Table 14. Quantum Yield Data for Photoreaction of
M02(02P(0C6H5)2)4 and ClCHZCHZCl in Various Nonaqueous

Solutions

 

 

Solvent [C1CH2CH2Cl]/M (Pa
ClCHzCHzCl 13(neat) 0.040
C6H6 9 0.031
THF 9 0.014
CH3CN 9 0.001

 

a Measured by using ferrioxalate actinometer. xexc = 546 nm.

212

the metal core in an equatorial position. Thus the steric congestion of
the equatorial planes of the MA-M dimers must be reduced. The
best structures would have two bridging ligands to prevent the free
rotation of the 1(66 a") excited state and four small monodentate
ligands to minimize steric crowding. The bridging ligands could be in
either trans or cis positions.

In summary, quadruply bonded MAM dimers are promising
multielectron photoreagents. The photochemistry described herein
represents the first examples of multielectron 1(66*) excited state
chemistry. The singlet character of the 1(66*) excited state appears
to be important in promoting multielectron reactivity but energy
wasting quenching reactions, such as proton quenching, must be
precluded. Additionally, attack by substrate in the equatorial plane
of the metal-metal core appears to be a crucial feature for concerted
multielectron reactivity. If these conditions are met, photochemical

studies of M-n-M dimers should continue to open new avenues of

excited state chemistry.

 

10.

11.

12.

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