PLACE IN RETURN BOX to move this checkout from your record.
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DATE DUE DATE DUE DATE DUE

 

 

 

 

 

 

 

 

 

 

 

 

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Q_
Vii—1W1

 

 

 

 

 

 

 

 

 

   

 

 

 

 

 

 

 

 

 

 

 

 

MSU II An Affirmative AdlaVEqnl Opportunity lnditution

 

(I)

SYNTHETIC DESIGN OF THE ELECTRONIC
EXCITED STATES OF BINUCLEAR COMPLEXES

By

JoellvanDulebohn

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment of the nequirements
fiat the degree

DOCTOR OF PHILOSOPHY

DeparhnentofChemlstry
1990

ABSTRACT

SYNTHETIC DESIGN OF THE ELECTRONIC
EXCTTED STATES OF BINUCLEAR COMPLEXES

By

Joel IvanDulebohn

The usual excited state deactivation process for many binuclear
metal complexes is photodegradation of the metal-metal bond. The
metal-metal bond can be retained in the excited state by externally
anchoring the metal together with a bidentate ligand. Alternatively, the
metal-core may be preserved by internally anchoring the metals together
with a multiple metal-metal bond. The synthetic chemistry aimed at
designing excited states of externally and internally anchored binuclear
complexes is described herein.

A novel homologous series of Rh2(0,0), Rh2(0,II), and Rh2(II,II)
bis(difluorophosphino)methylamine complexes has been synthesized.
These binuclear rhodium compounds are all prepared from the reaction
of [RhCl(PF3)2]2 with CH3N(PF2)2. Under reducing conditions,
Rh2[CH3N(PF2)2]3(PF3)2, 1, is isolated. Conversely, reaction of
[RhCl(PF3)2]2 with CH3N(PF2)2 in the presence of the oxidant CIZICGHs
produces Rh2[CH3N(PF2)2]3Cl4, 3. In the absence of oxidants and

reductants
RhZECH3NiP
separations of
are consisten‘
absorption 5
chmcteristic
exhibit red, 1
State parents
The er.
Slim-lived (-
lifetime by ex
imes"L‘lgated.
Ealeonjmle
MLCT State-s
acceptOrs. L,
bond COTES 01
“mm [m
MOH'MO 591
“518mm “

Dron'deS a n

reductants, the unsymmetrical mixed-valence
Rh2[CH3N(PF2)2]3Clz(PF3), 2, complex is obtained. The Rh---Rh
separations of 2.841(2) A, 2.735(1) A, 2.707(1) A in 1, 2, and 3, respectively,
are consistent with the presence of a Rh-Rh single bond. Electronic
absorption spectra are dominated by intense bands, which are
characteristic of o -' do" transitions. Crystalline solids of 1, 2, and 3
exhibit red, long-lived emissions, and is consistent with a do“ excited
state parentage of primarily triplet character.

The emission of quadruply bonded dimers is characteristically
short-lived (1 ~ 100 us). An approach of increasing the excited state
lifetime by energetically lowering a MLCT state below the 1(88*) state is
investigated. Th n-acid ligands 4,4'-dimethyl-2,2'-bypridine (dmbpy) and
maleonitrile dithiolate mntz‘ can in principle give rise to lowest energy
MLCT states. We have found, however, that the ligands are too strong 7:-
acceptors. Introduction of these strong u-acid ligands to the quadruple
bond cores of M02(02CCH3)4 and MozCl4(CH3CN)4 yields the M02(V,V)
dimers [(C4H9)N]2[Mog(0XS)(u-S)2(mnt)2] and M0204Clz(dmbpy)2. The
Mo---Mo separation of 2.858(1) A, and 2.562(2) A, respectively, are
consistent with the presence of a Mo-Mo single bond. This chemistry

provides a new route to the preparation of M02(V,V) dimers.

Copyright by
JOEL NAN DULEBOHN
1990

To My Wife, and Son
Vicki and Matthew Dulebohn

iv

I M
counsel, a
Universit}
0:130an
‘0 thank '
Alexander

also would

ACWOWLEDGEMENTS

I would like to thank Dr. Daniel G. Nocera for his inspiration,
counsel, and friendship during my graduate study at Michigan State
University. I have learned from Dr. N ocera to look at obstacles as
opportunties, and to solve these obstalcles in different ways. I would like
to thank Dr. Carl H. Brubaker for being my second reader, and Dr.
Alexander Tulinsky and Dr. Eugene LeGoff for being on my committee. I
also would like to thank Dr. Donald L. Ward for helping me solve my X-
ray crystal structures.

I also want to thank the members of the group, who are one of the
finest research groups in the country, for inspiring me into new heights
in chemistry. A special thanks to Dr. Dunbar's group who has the best
distillation stills in the midwest.

I must express my deepest gratitude to my family. To my parents
who set no limitation on me, and instilled in me never to be afraid to ask
the question WHY? To my sister and brother who had to put up with 18
years of WHY? and who only occasionally told me God wanted it that way.
To my wife, Vicki who shares my dreams and helps me realize them, and
to my son, Matthew who shows me much of the mystery of life. I want to
also send a special thank-you to all my family and friends who supported
me through these years with their friendship and their prayers.

TABLE OF CONTENTS

Pam
LIST OF TABLES ......................................................................... IX
LIST OF FIGURES ....................................................................... XII
I INTRODUCTION ................................................................ 1
II. EXPERIMENTAL ................................................................ 30
A . Syntheses .................................................................. 31
1. General Procedures ........................................... 31
2. Syntheses of Dirhodium Complexes ..................... 31
a. Starting Materials ....................... 31
b. Rh2[CH3N(PF2)2]3(PF3)2, (1) ........................ 31
Rh2[CH3N(PF2)2]3012(PF3), (2) .................... 32

(1. Preparation of Rh2[CH3N(PF2)2]3Clz(PF3)
from Rh3(u-Cl)3[CH3N(PF2)2]3 .................... 33
e. Rh2[CH3N(PF2)2]2Cl4, (3) ............................ 33
3. Synthesis of Rh3(u-Cl)3[CH3N(PF2)2]3, (4) ............... 34
4. Syntheses of Diiridium Complexes ....................... 35
Starting Materials ..................................... 35

Attempt to Prepare

Ir2[CH3N(PF2)2l3C12(PF3). (5) ...................... 35
c. Attempt to Prepare Ir2[CH3N (PF2)2]3C14, (6) . 35
5. Synthesis of Dimolybdenum Complexes ................ $

vi

III.

Page

a. Starting Materials ...................................... 36
b. M02C14(dmbpy)2. (7) ................................... 36
c. [(C4H9)4N)]2-

{Moz(OXSXu-S)2[82C2(CN)2121. (8) ................. 36
d. M0201202(u-O)2(dmbpy)2. (9) ....................... 37
e. Attempt to Prepare M02(dppm)2(mnt)2. (10) . 37
B. Instrumentation and Methods ...................................... 38
1 Absorption Spectroscopy ..................................... 38
2 Infrared Spectroscopy ........................................ 38
3. Time—Resolved and Steady State Luminescence $
4 Mass Spectroscopy ............................................. :9
5 Nuclear Magnetic Resonance Spectroscopy ........... 39
C. Crystal Structure Determinations ................................. 4O
1. General Procedures ........................................... 40
2. Methods ............................................................ 40
a. Rh2[CH3N(PF2)2]3(PF3)2, (1) ....................... 40
b. Rh2[CH3N(PF2)2]3C12(PF3), (2) ..................... 41
Rh2[CH3N(PF2)2]3Cl4, (3) ............................. 41

d. Rh3(u-Cl)3[CH3N(PF2)2]3
in CZ/c Space Group, (4) ............................. a

e. Rh3(u-Cl)3[CH3N(PF2)2]3
in Pmna Space Group, (4') ......................... 43
f. [(C4H9)4N)]2{M02(O)(S)(u-S)2[5202(CN)2]2). (8). 43
g. M0201202(u-O)2(dmbpy)2. (9) ....................... 44
RHODIUM FLUOROPHOSPHINE CHEMISTRY .................... 46
A . Background ............................................................... 46

B. Dirhodium Complexes ................................................. 54
1. Results ............................................................. 54
Synthesis of Dirhodium Complexes ............. 54

b. Structural Interpretation ........................... 70

c. Electronic Absorption and

Emission Spectra ...................................... 91

2. Discussion ....................................................... 107

C. Trirhodium Complex ................................................. 120
1. Results and Discussion ..................................... 120

a. Synthesis of Rh3(u-Cl)3[CH3N(PF2)2]3 ......... 1120

b. Structural Interpretation .......................... 120

c. Electronic Absorption and

Emission Spectra ..................................... 129

IV. PREPARATION OF DIMOLYBDENUMWN) DIMERS .......... 133
A. Background .............................................................. 133

B. Results and Discussion .............................................. 134

1. Reaction of Mntz‘ with Quadruply Bonded Metal

Complexes ....................................................... 134
2. Reaction of Bipyridine with Quadruply Bonded
Metal Complexes .............................................. 156
APPENDIX Nuclear Magnetic Resonance Spectra ......................... 173
REFERENCES ............................................................................. 193

viii

1 Crystal Data:
I

RhglCH3N;P
RhglCH3N :

 

Atomic Posh

Parameters l

3 (tempos-1.

“Hummers

LIST OF TABLES

Paw
Crystal Data for halCH3N(PF2)2]3(PF3)2 (l),
halCH3N(PF2)2]3C12(PF3)' (2). and
Rh2[CH3N(PF2)2]3Cl4 (3) .................................................. 71
Atomic Positional and Isotropic Displacement (A2 )
Parameters for halCH3N(PF2)2]3(PF3)2 (l) ....................... 72
Atomic Positional and Isotropic Displacement (A2 )
Parameters for Rh2[CH3N(PF 2)2]3C12(PF3) (2) ..................... 73
Atomic Positional and Isotropic Displacement (A2 )
Parameters for Rh2[CI-I3N(PF2)2'I 3014 (3) ................................... 7 4
Selected Bond Distances (A) and Bond Angles (deg) for
Rh2[CH3N(PF2)2]3(PF3)2 (l) ................................................ 84
Selected Bond Distances (A) and Bond Angles (deg) for
Rh2[CH3N(PF2)2]3Clz(PF3) (2) ............................................. 85,86

ix

Selected Bo:
Rh2lCH3N

3 Emission Sp

‘9 Calculated d
1. 2, and 3

10 CD'Slal Dal;

 

H Atomic Post:
Paramems l

Slime GroL

‘l

AumllC P05
133rametel

Puma Sr;

13 Selected
for Rh3l

Se/ected l
léf‘flfiCH

l5
crystal D347

11

14

Page
Selected Bond Distances (A) and Bond Angles (deg) for

Rh2[CH3N(PF2)2]3Cl4 ......................................................... 87
Emission Spectral Data for Crystalline 1, 2, and 3 at 77 K ...... 99

Calculated decay rate constants and energy gaps for

Crystal Data for Rh3(u'Cl)3[CH3N(PF2)2]3 ............................ 121

Atomic Positional and Isotropic Displacement (A2 )
Parameters for Rh3[CH3N(PF2)2]3013 (4) in the CZ/c
Space Group ..................................................................... 122

Atomic Positional and Isotropic Displacement (A2 )
Parameters for Rh3(tl-Cl)3[CH3N(PF2)2]3 (4') in the
ana Space Group ........................................................... 123

Selected Bond Distances (A) and Bond Angles (deg)
for Rh3[CH3N(PF2)2]3(Cl)3 (4) in the CZ/c space group ............ 125

Selected Bond Distances (A) and Bond Angles (deg)
for Rh3[CH3N(PF2)2]3(C1)3 (4') in the ana space group ........ 1%

Crystal Data for [M02(O)(S)(p-S)2(mnt)2]2' and
M020 4Cl4(dmbpy)2 ............................................................ 144

X

1-
Li

£6

£5?

£3

Atomic

Paras)!

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Paras

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Sele<

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5&1
MO

17

21

Page
Atomic Positional and IsotrOpic Displacement (A2)

parameters for (M02(O)(S)(u-S)2(mnt)2)2' ............................. 145

Atomic Positional and Isotropic Displacement (A2 )

Parameters for the Tetrabutylammonium Ions .................... 146,147

Selected Bond Distances (A) and Bond Angles (deg) for
[M02(O)S(u-S)2(mnt)2]2‘ ..................................................... 150

Selected Bond Distances (A) and Bond Angles (deg) for
[N(C4H 9,)4]+ Ions .............................................................. 151,152

Relevant angles for complexes with M02(|.l-S)2XY
(X, Y = O, S) cores ............................................................. 154

Relevant angles for complexes with M02(|.l-S)2XY
(X, Y = O, S) cores ............................................................. 155

Atomic Positional and Isotropic Displacement (A2)
Parameters for M02(O)2Clz(u—O)2(dmbpy)2 .......................... 164

Selected Bond Distances (A) and bond Angles (deg) for
M0202012(u-O)2(dmbpy)2 ................................................... 167

xi

Schematic r
Processes. k
constants to

energy tran

The structu:

reaction Co
The Structu:
RQIatiVe em
R€1atjve ene:

M2andian

)1st comp,

 

l
[Rhflbrldgg

 

LIST OF FIGURES

Page

Schematic representation of excited state deactivation
processes. km, 11,, kwb, ken and ket are the rate
constants for nonradiative and radiative decay, substitution,
energy transfer and electron transfer, respectively ................... 3
The structure of triad that mimics the photosynthetic
reaction center ..................................................................... 6
The structure of [Ru(me(bpy)—3DQ2+)(me(bpy)—PTZ)2]4+ ............ 8
Relative energy diagram for Mn2(CO)10 ................................... 13
Relative energies of the d-derived molecular orbitals in Dab
M2 and in D4}: M2L8 complex. The ground state of the d4-d4
M2L8 complex is 1A1g(0215482) ................................................. 16
Production of hydrogen from the irradiation of

[Rh2(bridge)4]2"’ in acidic solution ........................................ 20

xii

(I)

12

Photo
it'll

are'll

Phot

acetl

H1115

in it

pho

Red

FA;

PA

We

FA

ind

Dre

14

Photoinduced bimolecular reaction of [Ir(COD)(u-pz)]2
with 1,2-dichloroethane and methylene chloride. Pyrazines

are indicated by bowed lines strapping the metal centers ......

Photoinduced catalytic conversion of isopropyl alcohol into

acetone and hydrogen by using Pt2(pop)4’ as a photocatalyst .

Illustration of the potential energy diagram for reagent
in its photoexcited state and its photoproduct with the

photoproduct having an photoexcited state ..........................

Redox chemistry of Coz[CH3N(PF2)2]3(CO)2 .........................

FAB mass speCtrum Of Rh2[CH3N(PF2)2]3(PF3)2, 1 ..............

FAB mass spectrum of Rh2[CH3N(PF2)2]3(PF3)Clz, 2
prepared by method i of Section II.A.2.c .............................

FAB mass spectrum of Rh2[CH3N(PF2)2]3Cl4, 3 ..................

Iostope peak-intensity pattern for ions containing the

indicated number of chlorine atoms (from ref. 123) ..............

FAB mass spectrum of Rh2[CI-I3N(PF2)2]3(PF3)C12, 2,
prepared by method ii of Section II.A.2.c ............................

xiii

22

Z

8

50

57

59

61

63

%

 

FAB mass 5

prepared by

ORTEP dra
RthCH3N$
ellipsoids. l
Selected bor

All ORTEP r

the Dunlber

at the 50% l

°f Clams. T

An ORTEP \
nulDberlng
Pmbability 1

7 lists Belem

A Skeletal Vi

0)) 2, and ((

StnlCtm-al fl‘
nearly along

 

17

Page
prepared by method ii of Section II.A.2.c ............................ 68

ORTEP drawing and numbering scheme of
Rh2[CH3N(PF2)2]3(PF3)2,1 with 30% probability thermal
ellipsoids. For clarity hydrogen atoms are not shown.

Selected bond distances and angles are listed in Table 5 ....... 76

An ORTEP view of Rh2[CH3N(PF2)2]3(PF3)Clz, 2, showing
the numbering scheme. Thermal parameters are shown
at the 50% level. Hydrogen atoms are omitted for the sake
of clarity. Table 6 lists selected bond distances and angles 78

An ORTEP view of Rh2[CH3N(PF2)2]3Cl4, 3, showing the
numbering scheme. Thermal ellipsoids are at the 50%
probability level; hydrogen atoms are not shown. Table

7 lists selected bond distances and angles .......................... so

A skeletal view of the inner coordination spheres of (a) 1,

(b) 2, and (c) 3 .................................................................. 82

Structural framework of (a) l. (b) 2, and (c) 3 as viewed
nearly along the Rh-Rh axis. Axial ligands are omitted ..... 90

xiv

13

£2?!

E)?

Elect
CH.,(

spect

Elec:

Elec
in C

Spec

Fit .

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F it

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Sta
ma

€163

Electronic absorption spectrum (—) of 1 dissolved in
CH2012 at room temperature, and corrected emission
spectrum (- - -) of crystalline 1 at 77 K ................................ 33

Electronic absorption spectrum (—) of 2 dissolved in
0112012 at room temperature, and corrected emission

spectrum (- - -) of crystalline 2 at 77 K ................................ 95

Electronic absorption spectrum (—) of 3 dissolved
in CH2012 at room temperature, and corrected emission

spectrum (- - -) of crystalline 3 at 77 K ................................ 97

Fit of the variation of the observed emission decay rate

constant to eq 16 of 1 in the 10 - 180 K temperature range ....... 101

Fit of the variation of the observed emission decay rate
constant to eq 16 of 2 in the 10 - 200 K temperature range ....... 103
Fit of the variation of the observed emission decay rate

constant to eq 16 of 3 in the 10 - 290 K temperature range ....... 105

Proposed energy diagram of the lowest energy excited
states of the Pt2(III,III)L2 phosphates. The state
manifold is derived from the spin-orbit coupling
perturbation of the 3E“ state arising from the one-

electron 3(dlr" -* do“) promotion ...................................... 111

Page

XV

51m

oft

are

Qu
Rh

Orb

8V1:
Rh
int,

am

Simple molecular orbital diagram for the interaction of

two C3,, Rh(O)P4 fragments. The dlt- and d8-symmetry

orbitals are filled and indicated by the shaded box. To

minimize level congestion, the o—o" splitting is shown

to be small and its manifold is isolated from that of

the (lit and d8 orbitals ...................................................... 115

Qualitative level diagram for the interaction between C3,,
Rh(O)P4 and 04‘, Rh(II)P301'2 fragments. The d1t- and
d8-symmetry orbitals are filled and indicated by the
shaded box. To minimize level congestion, the o-o*
splitting is shown to be small and the o-manifold is
isolated from that of the dn and d5 orbitals. Low
symmetry splitting within the (lit level of the 04‘,
Rh(II)P3C12 fragment is not considered; interactions

of the do and do" levels with the La and Lo" orbitals

are also not considered ..................................................... 117

Qualitative level diagram for the interaction of two C 4v
Rh(II)P3012 fragments. The du- and d5- symmetry

orbitals are filled and indicated by the shaded box.

The 0-0* splitting is shown to be small. Low

symmetry splittings within the (11: levels of the 04v

Rh(II)P3012 fragments are not considered;

interactions of the do and do" levels with the La

and Lo“ orbitals are also not considered .............................. 119

xvi

i=3

Ape

ellip

hyd:

lec
CH1.

Spa,

is

dis

Ell
[M

OI
Do
th.
sh

in

A perspective ORTEP view and labeling scheme of
Rh3(u-Cl)3[CH3N(PF2)2]3, 4, with the thermal
ellipsoids at the 50% probability level. For clarity

hydrogen atoms are not shown .......................................... 128

Electronic absorption spectrum (—) of 4 dissolved in

CH2012 at room temperature, and corrected emission

spectrum (- - -) of crystalline 4 at 77 K ...... , ........................... 131

Electronic absorption spectrum of [(C4H9)4]2Mo(mnt)3

dissolved in 0112012 .......................................................... 136

Mid IR spectrum of [(C4H9)4]2Mo(mnt)3 .............................. 138

Mid IR spectrum of [(04119)N]2[M02(O)(S)(u-S)2(mnt)2] .......... 141

Electronic absorption spectrum of [(C4H9)N]2

[M02(O)(S)(ll-S)2(mnt)2] dissolved in CH2012 .................... 143

ORTEP drawing and numbering scheme of

Mo2(O)(S)(u-S)2(mnt)2]2' with 50% probability

thermal ellipsoids. For clarity (C 4H9)4N"’ ions are not

shown. Selected bond distance and angles are listed

in Table 18 .................................................................. 149
Page

xvii

 

Mid IR 5pc

the reactio:

l
Far IR Spe:

the reactior

Electronic 2

dissolved i

 

ORTEP drg
M0202Cl2iL
elliPsoids.

shovm, Se?

llSlEd in '1
Mid IR Spe

MOQOQCl2l

3lp{1H} N‘
NMR Spec-

3lp{lH} N‘

prepared l

Mid IR spectrum of the green compound isolated from

the reaction of M02(Cl)4(dppm)2 with Na2(mnt) ..................

Far IR spectrum of the green compound isolated from

the reaction of M02(Cl)4(dppm)2 with Na2(mnt) .................

Electronic absorption spectrum of M02(dppm)2(mnt)2

dissolved in CH2C12 ........................................................

ORTEP drawing and numbering scheme of
M0202012(u-O )2(dmbpy)2 with 50% probability
ellipsoids. For clarity hydrogen atoms are not

shown. Selected bonddistances and angles are

listed in Table 23 ............................................................

Mid IR spectrum of M0202012(ll-O)2(dmbpy)2 ....................

Electronic absorption spectrum of

M0202012(u-0)2(dmbpy)2 dissolved in CH3CN ....................

31min) NMR spectrum of Rh2[CH3N(PF2)2]3(PF3)2, 1.

NMR spectrum was not reference to an external standard

31P(1H) NMR spectrum of Rh2[CH3N(PF2)2]3(PF3)Clz, 2

prepared by method i of Section II.A.2.c ...........................

xviii

..158

1H)

1w

.. 171

174

176

Page

F3

“Will NW
1917 NMR 5

19F NMR 5'

I

prepared b

19s ml 5

 

19W?) m.

prepal'Ed b

”Win N:

Displayed l:

19" {311’} N.
MIR Spec

alpllH} N

47

51

31P(1H) NMR spectrum of Rh2[CH3N(PF2)2]3Cl4, s .............. 178
19F NMR spectrum of Rh2[CH3N(PF2)2]3(PF3)2, 1 ................ 180

prepared by method i of Section II.A.2.c ............................. 182

191‘ NMR spectrum of Rh2[CH3N(PF2)2]3CI4, s .................... 134

19F(31P) NMR spectrum of Rh2[CH3N(PF2)2]3(PF3)Clz, 2
prepared by method i of Section II.A.2.c ............................. 186

19F(31P) NMR spectrum of Rh2[CH3N(PF2)2]3(PF3)Clz, 2
prepared by method i of Section II.A.2.c ............................. 188

NMR spectrum was not reference to an external standard 190

311mm NMR spectmm of Rh3(u-Cl)3[CH3N(PF2)2]3, 1 .......... 192

xix

CHAPTERI

Introduction

The at
first act of pl
many deacti
emission of E
photosubstit
1-3 are uni
and 3) occu
luminescem
molecule ha

electron tra

The absorption of a visible or ultraviolet photon by a molecule is the
first act of photochemistry. In the excited state, the molecule can undergo
many deactivation processes. As shown in Figure 1, these include (1)
emission of a lower energy photon (luminescence), (2) emission of heat, (3)
photosubstitution, (4) energy transfer, and (5) electron transfer. Cases
1-3 are unimolecular transformations, with radiationless decay (cases 2
and 3) occurring at the expense of luminescence. Typically, long-lived
luminescence is desirous because it is a signature that the excited
molecule has sufficient lifetime to participate in energy transfer and/or
electron transfer bimolecular transformations.

The utility of electronic excited states in many important processes
such as energy conversion chemistry, molecular electronics, and small
molecule activation chemistry are predicated on the oxidation-reduction
chemistry of the electronically excited molecule.1'10 In order for the
excited state to be useful in an electron transfer reaction it must be
reached with high efficiency following light absorption, last long enough
to undergo electron transfer reaction, and exhibit appreciable stability so
that catalytic cycles can be designed. There are many examples of one
electron transfer reactions over the past two decades. Yet recent
emphasis has turned to the multielectron chemistry of transition metal
excited states because the most useful system will necessarily rely on the
transfer of more than one electron. ”'15

One approach to a multielectron system is coupling one electron
transfer reactions. The best example of this approach is found in nature.
The photosynthetic eubacteria contains photochemical reaction centers

(RCs) containing one or more chlorophyll molecules.16 Each reaction

center consists of a primary electron donor P (bacteriochlorophyll), an
1

Figurel

Schematic representation of excited state deactivation processes. km, kr, Mthv

k ken and ket are the rate constants for nonradiative and radiative

sub’

decay, substitution, energy transfer and electron transfer, respectively.

ML!l

 

 

 

 

 

 

Radiationless Deactivation

 

 

 

MLx + Heat
l ML, + W
knr Luminescence
k7
ML, + hv 9 ML;
AID A
(A
ksub
ken
MLf” + A71)+ l MA,L,,, + yL
Electron Transfer . Photosubstitution
ML)( 4- A

Energy Transfer

Di

4

initial electron acceptor A (bacteriochlorophyll or bacteriopheophytin),
and one or more secondary acceptors X (Fe-S center, quinones).
Sometime a secondary electron donor D (Cyt c) is tightly bound to the RC.
The 'primary' chemical reaction sequence of photosynthesis occurs
in the RC and is summarized by eq 1.

DPAX + hv -> DP‘AX -> DPIA'X -> DP+D(' -> D+PAX" (1)

When the RC is photoexcited, the primary donor P is excited to P*, which
in turn transfers an electron to A. To prevent the back electron reaction to
P", an electron is transferred from A to secondary acceptor X and charge
separation is stabilized.

Photochemically induced separation has been accomplished in
synthetic molecules that contain a light absorber or chromophore, and
electron donors and acceptors held in appropriate spatial array.” Two
examples based on the photosynthetic RC are illustrated in Figures 2 and
3. The former is based on the x -' 71* chromophore of a porphyrin

18

derivative. The molecular triad consists of a tetraarylporphyrin (P)

covalently linked to both a carotenoid (C) and a quinone (Q). The
postulated electron transfer reactions are shown in Scheme 1. 19

C-P—Q _h"_.. C—P *-Q

31 4 11
"c—P-Q"<—-2— C—P‘"—Q"°

Scheme 1

FigureZ

The structure of triad that mimics the photosynthetic reaction center.

 

Figure2

 

Figure3

The structure of [Ru(me(bp)’)—3DQ2+)(me(bpy)—PTZ)2]4+.

 

Figure3

9

The excited P* undergoes electron transfer to yield C-PI-Q". and
subsequent electron transfer to the tetraarylporphyrin from the carotenoid
yields a long-lived charge separated state *‘C—P—Q". on the microsecond
time scale. The charge separated state recombines to form the starting
compound over 2 us. The yield of the charge separated species was low
due to the recombination reaction indicated by step 4.

Figure 3 shows charge-separation based on MLCT excitation of
[Ru(me(bpy)-3DQ2+)(me(bpy)-PTZ)2]4+.20 Two pathways are possible
depending on the initial quenching act of the MLCT, and are illustrated in
Scheme 2.

k:
[WVWW' 3F: [(DQ”b")Rum(b'PTZ)(bPTZ)l“°
A ‘3, 'i to, f
WWW [(Wbmqu-Prszprz")r*‘

.. \‘z *y

[(DQ’b-')Ru“(bPl‘Z"XbPTZ)l“°

——l:va—>[(DQ"V)&1"(bptz),l“ « 1:”
v.

 

 

 

 

 

Scheme 2

Irregardless of the initial pathway, the same charge-separated species
[(DQ'+b)Ru"(bPTz+*) (bPTZ)]4+* is ultimately produced. The charge
separated species storage time is not very long (165 ns) due to the
recombination reactions k5 and k'5.

The successful realization of overall multielectron chemistry
necessary relies on the integration of the charge separated states with
multielectron catalytic sites. This has not yet been achieved for

10

unimolecular charge transfer, but limited success has been recognized

21 The most prominent schemes

for some bimolecular inorganic systems.
have been predicated on Ru(bpy)32'*.22'23 The electronically excited
Ru(bpy)32+ ion, I"Ru(bpy)32“, can readily transfer one electron to
methylviologen. The reduced viologen reacts with protons in the presence

of colloidal platinum to produce hydrogen as described by the eqs 2-4.

Ru(bpy)32+ + hv —> Ru(bpy)32+* (2)
Ru(bpy)32+* + W“ —> Ru(bpy)33+ + MV+ (3)
MV*+H++Pt-—>MV2*+1/2H2 (4)

The photogenerated Ru(bpy)33+ ion is a strong oxidant. It has been shown
that Ru(bpy)33+ can react with water or hydroxide in the presence of a
Ru02 catalyst under suitable conditions to produce oxygen. Nevertheless,
the overall emciency of the Ru(bpy)32+ cycle is low, due to the fast back
reactions between Ru(bpy)33+ and MV+. One approach to avoid these back
reactions is to develop charge separated species of the type discussed in
the context of Figures 2 and 3. Other approaches have appeared in recent

24,25 26,27 26,28 and

years based on semiconductors, membranes, vesicles,

polymers.29'3o

Our group has been interested in addressing the issue of
multielectron photochemistry from a different conceptual basis. Charge
separation is circumvented if the multielectron chemistry of the excited
state occurs in single reaction sequence. This research has focussed on
polynuclear metal cores where the redox activity of individual metal

centers may be coupled in a discrete excited state molecule.

11

To date most effort has been directed towards coupling the redox
activity of metal centers in a bimetallic core. A major obstacle in this
chemistry is the photodegeneration of the bimetallic core along a
dissociative pathway. An example of a predominate photocleavage
pathway is that of Mn2(CO)10 (Figure 4), which has a single bond between
the metal atoms.31’34 Irradiation of the lowest-energy transition in this
dimer promotes an electron from the o bonding orbital to the antibonding
metal-metal o" orbital. This excitation reduces the metal-metal bond
order to zero yielding the °Mn(CO)5 radical fragment. Because the
dissociative pathway for o“ deactivation is very emcient, the electronically
excited lifetimes for single metal bonded dimers are short. Luminescence
from M-M complexes is extremely rare, and chemistry from the M-M
complexes is typically free-radical based.

The photodegradation of M-M complexes can be circumvented by
anchoring the two metal atoms together with the coordination of bidentate
ligands across the metal-metal bond. This is dramatically illustrated by a
comparison of Re2(CO)8L2 (L = co, 14061193) and Re2(CO)8(fl’) (PAP =
bis(dimethylphosphino)methane) (dmpm) and bis(diphenylphosphino)-
methane) (dppm)) photochemistry.35'39 The Re2(CO)8L2 dimers undergo
rapid and efficient metal-metal bond cleavage to produced 'Re(CO)4L
radicals. Coordinating a 1;} bidentate ligand across the metal-metal
bond circumvents photodegradation of the dimer, and a highly emissive
long-lived 60* excited state is observed at low temperature.40 This
emission is attributed to the bidentate ligand maintaining the metal
atoms in a rigid and proximate geometry in the 00* excited state. There
are other examples of d7- (17 species that also have 0* luminescence.

These include the Pt2(III,III) pyrophosphate Pt2(pop)4X24' (P0? =

12

Figure4

Relative energy diagram for Mn2(CO)10.

13

hv
Mn2(CO)1o —-. 2 (CO)5Mn-

2
xiyz 0—— ray

9

x2, yz

 

 

 

 

 

 

 

 

 

X)’

(CO)5Mn- Mn2(CO)1o -Mn(CO)5
Finn-=4

14

(H20P)2O, x = halides),41 and the Pt2(III,III) phosphate Pt2(HPO p412"-
[L = H20, 11: 2; L = Cl, Br, 11 = 4) complexes.42 The retention of the metal-
metal core in the excited state is a necessary condition of do“
luminescence in these complexes.

Another approach to prevent photofragmentation is to anchor the
metal atoms with multiple bonds. In this manner, even if one net bond is
annihilated, the remaining metal-metal interaction holds the binuclear
core intact. Numerous quadruply bonded metal-metal complexes have
been prepared. These complexes are comprised primarily of rhenium,
chromium, molybdenum, and tungsten metal atoms coordinated by a
variety of ligands.

Numerous theoretical“3'50 and experimental‘r’l'67 investigations
have led to the elucidation of L4MML4 electronic structure. A general
molecular orbital diagram of the quadruply bonded metal dimer is shown
in Figure 5.52 The L4MML4 quadruple bonded dimer is derived from an
M2 core (DM,l symmetry) in which the z axis of a right handed cartesian
coordinate system lies along the metal-metal bond. The linear
combination of dzz , (du, dyz), and (d,2_y2, dxy) atomic orbitals on each
metal forms the bonding and antibonding o, 7:, and 5 molecular orbitals,
respectively. In the Dab point group the 1: and 5 molecular orbitals are
each doubly degenerate. The symmetry of the M—M core is lowered from
Duh to D411 when the eight ligand of L4MML4 is placed in equal position of
the M-M core, and therefore, causes the splitting of the 5 orbital
degeneracy. By definition of the coordinate system in Figure 5, the
ligands lie along the x and y axis, and the four metal-ligand bonds are
generated by the overlap of the ligands orbitals with the metal dx2,y2
orbitals and the s, px, py orbitals (which are not shown in Figure 5). The

15

Pigln‘e5

Relative energies of the d-derived molecular orbitals in th M2 and in D4}1

M 2L8 complex.
1A1g(0'27:452).

The ground state of the d‘i-d4 M2L8 complex is

{not pry

I

 

(I

Energy

 

16

 

...............
O

 

 

 

 

y
{y
l/l/ .a
Ill-I
M22
/l xL/I ’Il‘
/ VLL

 

 

Figune5

azu(0*)
b2u(dx2 2
- )
b1g(dx2-;2)
ego-l“)
blu(5 *)
b2g(5)

Cu(Tl)

alg(o)

 

and thus cor
Re metal b0
Molt
linetallic I
there is n
properties
intensivel
cipectire
complexe
Pmmotior
the bondi
the excio
\l’
exCited
Mr
(it = it
“all ((
its (2.
one 0.
31120
Show
Prom:
l’ield
[Rh 1
Dhot
dish

Show

17

and thus compete with the expected rapid recombination to reform the Re-
Re metal bond.

Multielectron diradical chemistry has been also observed for
dimetallic centers strapped by a bidentate ligand across a core in which
there is no metal-metal bond. The spectroscopic and photochemical
properties of d8-d8 “'72‘78 and dm-d10 79'“ complexes have been studied
intensively. Simple molecular orbital arguments suggest that the
respective ground state and excited state configuration of the binuclear
complexes are represented as (dzz )2(d.‘2# )2 and (dzz )2(d22* )1(pz)1.
Promotion of an electron from the metal-metal antibonding framework to
the bonding orbital results in an increase of the metal-metal interaction in
the excited state.

With the structural integrity of the dimetal core preserved in d8-d8
excited states, a rich photooxidation chemistry of Pt2(pop)4"",81'85
[Rh2(bridge)4]2+ (bridge = 1,3-diisocyanopropane)86'87 and [M(COD)(u-L)]2
(M = Rh, Ir; COD = 1,5 cyclooctadiene; u-L = hp (2-hydroxypyridinate),
mhp (6-methyl-2-hydroxpyridinate), chp (6-chloro-2-hydroxypyridinate),
2hq (2-hydroxyquinolate), pz (pyrazolate)) has been observed.“"12335'88'89
One of the earlier examples is the reduction of H2 by the irradiation of
[Rh2(bridge)4]2+ in aqueous 12 M HCl solution.86'87 The reaction steps are
shown in Figure 6. In the reaction scheme [Rh2(bridge)4]2+ in 12 M HCl is
protonated to [(Cl')RhI---RhI(H+)]2+. Subsequent irradiation at l. > 530 nm
yields H2 and the two electron oxidative-addition product
[Rh2(bridge)4Clz]2+. Another example of oxidative-addition
photochemistry is the visible irradiation of [Ir(COD)(u-pz)]2 with 1,2-
dichloroethane (DCE) or methylene chloride.11c The reaction scheme is
Shown in Figure 7. Irradiation (A = 577 nm) of [MCODXu-pznz with DCE

mhwd
oi lhtCOl
electron o:
Otl
Ptzlpopl,
into acetl
The elect
isopropal
reaction
the prod
undergol
1hyclrid
“teen t
T
dimers
chelitist
0deati
(lemonE
110th
tnale t

The 0‘
result
MOgill
two ell

18

leads to the formation of ethylene and [Il(COD)(ll-pz)Cl]2. The irradiation
of [Ir(COD)(u-pz)]2 in methylene chloride leads to the formation of a two-
electron oxidative-addition product [MCODXn—pz)]2(ClCH2)(Cl).

Other examples of bimetallic free radical chemistry include
Pt2(pop)44". This dimer can photocatalytically convert isopropyl alcohol
into acetone and hydrogen by the reaction scheme shown in Figure 8.85
The electronically excited Pt2(pop)4‘“* 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)4H". The Pt2(II,III) mixed-valent Pt2(pop)4H4‘
undergoes disproportionation to give Pt2(pop)44" and Pt2(pop)4H4’. The
dihydride reacts with HCl to generate hydrogen and reacts rapidly with
oxygen to give Pt2(pop)44’.

The multielectron free radical chemistry of d7-d7, d8-d8, and dm-dm
dimers is not paralleled by the quadruple bond dimers. In this case, the
chemistry is dominated by electron transfer and discrete two electron
oxidation-addition transformations. In our laboratory we have
demonstrated the oxidation photochemistry of the quadruple bonded
Mo2(HPO4)44" dimer in an acid medium to produce hydrogen and the

triple bonded M02(HP04)42'.14‘

M02(HPO4)4" + 211* 3‘1» M02(I-IPO4)42’ + H2 (5)

The overall reaction is composed of sequential one electron transfer to
result in a formal oxidation change metal core from M02(II,II) to
M02(III,III). Alternatively nonaqueous soluble phosphates promote the
two electron reduction of halocarbons. 14b

l9

Figune6

Production of hydrogen from the irradiation of [Rh2(bridge)4]2+ in acidic

solution.

20

Rh2(b1'idge),,2+

2MHC1

[(Cl‘)Rh‘---Rh‘(H*)l

hv
546 nm

l(Cl")RhI “2---Rh‘ ”(14+)?”
12 M HCl
(Cl.—Rh"—.Rh"—Cll2+ + H2

FigureB

21

Figure7

Photoinduced bimolecular reaction of [Ir(COD)(u-pz)]2 with 1,2-

dichloroethane and methylene chloride. Pyrazines are indicated by the

bowed lines strapping the metal centers.

22

(:33!) h" = [cw/”Ml

RCl RC1

 

fl

. l ‘ I l
R + <|,Ir---Ir\|>

as... .,_/\.,va,

 

m7

 

leads to tilt
of llrtCOD
electron or
0th
Ptzlpopl;
into aceto
lhe electr
isopropanl
reaction 0
the Plodm
undergoes
dihl'tlride
Oil'gen to
To
dimsrs l
thermg
1111ng

T95”:
”has.

[’0 Glenn

23

leads to the formation of ethylene and [Ir(COD)(u-pz)Cl]2. The irradiation
of [Ir(COD)(u-pz)]2 in methylene chloride leads to the formation of a two-
electron oxidative-addition product [Ir(CODXll-pz)]2(ClCH2)(Cl).

Other examples of bimetallic free radical chemistry include
Pt2(pop)44’. This dimer can photocatalytically convert isopropyl alcohol
into acetone and hydrogen by the reaction scheme shown in Figure 8.85
The electronically excited Pt2(pop)4"* 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 Pt2(II,III)mixed-va1ent Pt2(pop)4H4’
undergoes disproportionation to give Pt2(pop)44' and Pt2(pop)4H4‘. The
dihydride reacts with HCl to generate hydrogen and reacts rapidly with
oxygen to give Pt2(pop)44'.

The multielectron free radical chemistry of d7-d7, d8-d3, and d1°-d1°
diners is not paralleled by the quadruple bond dimers. In this case, the
Chemistry is dominated by electron transfer and discrete two electron
oxidation-addition transformations. In our laboratory we have
demonstrated the oxidation photochemistry of the quadruple bonded
M°2(HPO4)44’ dimer in an acid medium to produce hydrogen and the

tn'Ple bonded Mo2(Hl>o,,),,2-.1481

Moza-IPO4)44’ + 211* 1'1» Mo,(HP04),2- + H2 (5)

The Overall reaction is composed of sequential one electron transfer to
result in a formal oxidation change metal core from M02(II.II) t0
M°2(II1,111). Alternatively nonaqueous soluble phosphates promote the

o ele(ttron reduction of halocarbons. 14"

24

Figure8

Photoinduced catalytic conversion of isopropyl alcohol into acetone and
hydrogen by using Pt2(pop)4‘ as a photocatalyst.

 

"1}
J
. -‘.

25

Pt2(l>or>)44- fl)”
CH3CECH3
ll Pt (POP) H4‘
CH3CCH3 2 4
cm
Pt2(l>oe)4li4- CH3?C”3
H
9:2(POP)44-*
P'2(POP)4H24'
P12(POP)44- hv
H2

111021.021j

When elec
electron rt
bonded W:
addition ;
anus

EWSQ
W2tlll,ll

As
Photorea.
Schemes
phliorea,
mllltielel

klllellc y

timed E

26

M02(02P(0R)2)4 + ClCH20H201 3‘5» M02(02P(0R)2)4Clz + 0211, (6)

When electron transfer from the metal core is avoided, concerted two
electron reactions are observed. The visible irradiation of the quadruply
bonded W2(dppm)ZCl4 dimer in the presence of CH3I yields the oxidative-
addition product W2(dppm)2Cl4(CH3XI), where CH3I adds to the W-W

bond in a discrete multielectron process.14c

w2(dppm)2014 + CH3 —’“1> w,(dppm)2Cl,(CH3)(l) (7)

In this case also the formal oxidation state changed from W2(II,II) to
W2(III,III).
As evidenced by the above discussion, guidelines for multielectron
photoreactivity have begun to emerge. However, few photochemical
schemes have been realized in which the initial multielectron
photoreagent is regenerated.”91 As is the case for the photoreactant, the
multielectron photoproduct typically resides in deep thermodynamic or
kinetic wells that hinders its conversion back to the desired photoreactive
state. It is not unreasonable to expect, therefore, that these kinetic or
thermodynamic barriers of the photoproduct, in principle, can be
surmounted by channeling the regenerative reaction via the excited state
of the photoproduct. In this manner, photocatalytic schemes can be
constructed. The success of this approach necessarily relies on the design
of a series of multielectron reagents, each of which possesses a long-lived
excited state. A scheme illustrating this strategy is shown in Figure 9.
The photoreactant R is excited to R*, which has enough energy to

27

Figure9

Illustration of the potential energy diagram for a reagent in its
photoexcited state and its photoproduct with the photoproduct having an
photoexcited state.

28

 

 

<——reaclion pathway—’-

Figure9

orercom
photoprc
photopn
accompl
barrier
'l
homolc
molds
emplo
cente
electl
State
and
and
lumi;
thesl

dEmc

29

overcome the thermodynamic and/or kinetic barrier to produce
photoproduct P. If the system is designed such that the multielectron
photoproduct possesses a long-lived excited state, regeneration of R can be
accomplished in principle by using P* to overcome the thermodynamic
barrier confronting the ground state.

To this end, we became interested in the possibility of synthesizing a
homologous series of excited state complexes, which are interrelated by
multielectron steps. Our strategy has depended primarily on the ability to
employ a ligating coordination sphere capable of stabilizing metal redox
centers in a variety of formal oxidation states while engendering
electronic structural properties that are compatible with a rich excited-
state redox chemistry. Chapter III reports the synthesis, and structural
and spectroscopic characterization of a homologous Rh2(0,0), Rh2(O,II),
and Rh2(II,II) bis(difluorophosphino)methylamine series. Long lived
luminescence characteristic of a do" excited state is observed for each of
these novel binuclear complexes, and their interconversion is
demonstrated. .

Chapter IV describe the attempts to synthetically control the excited
state in L4MML4 complexes. Our ultimate goal is to use excited state
design of catalytic multielectron photochemistry with the goal of lowering
a long-lived MLCT excited state below the 88* excited state. We
investigated the chemistry of M02(II,II) quadruple bond starting
complexes such as M02(OZCCH3)4 and (NH4)2M02019 with n-acid ligands
Nazmnt and dmpby. The reaction of mntz" with M02(OZCCH3)4 yields the
monomer [(C4H9)4]2Mo(mnt)3 and the M02(V,V) dimer Mo2(0)(S)(u-
S)2(mnt)2]2' . In the reaction of dmbpy with (NH4)2M02019 the complex

 

1:2(3 tldm
llosC 'dmh

3O

MozCl4(dmbpy)2 is behaved to be formed. The oxidation in air of
M02014(dmbpy)2 also yields a M02(V,V) dimer M0204C12(dmbpy)2.

CHAPTERII

Experimental

 

l Syn

can
methl

cobalt

A. Syntheses
1. Genm'alProcedures

The synthesis of all complexes, unless otherwise stated, were
performed under argon atmospheres by using standard Schlenk-line
techniques. All solvents were dried and deoxygenated before use.
Microanalyses were performed by Galbraith Laboratories, Inc., Knoxville,
TN.

2. Syntheses of Dirhodium Complexes

a. Starting Materials. The starting compounds [RhCl(PF3)2]2,
CH3N(PF2)2, Rh2(OzCCH3)4, and 0121C6H5 were prepared by literature
methods.91'95 Phosphorus trifluoride (Ozark-Mahoning Company) and

cobaltocene (Strem Chemical Company) were used as received.

b. halcnamrrawslrrgm 1. A 15 mL dichloromethane solution
containing 0.701 g of [RhCl(PF3)2]2 (1.12 mmol) was charged with 0.58 mL
of CH3N(PF2)2 (0.83 g, 5.0 mmol) via syringe. The orange solution turned
red with liberation of PF3. Upon cessation of PF3 liberation, 0.422 g of
Co(C5H5)2 (2.23 mmol) was added from a side-arm. The mixture was
stirred for 10 min under PF3 and then filtered to remove cobaltocenium
that had formed. Addition of 10 mL of hexane led to further precipitation
of cobaltocenium and excess cobaltocene. Filtration under argon yielded a
red solution that was vacuum distilled to dryness to produce 0.691 g of an

orange-brown precipitate. The solid was introduced onto a Florisil®
31

fi/__

column
The frat
precipit
on
P, 28.0'
(litiract
with lit

32

column and eluted with a hexane:dichloromethane (85%:15%) solution.
The fraction was distilled to dryness under vacuo to give an orange-yellow
precipitate. Yield 0.312 g (0.35 mmol, 32%). Anal. calcd (found) for
03119F18N3P8Rh2: C, 4.08 (5.67); H 1.03 (1.40); F, 38.74 (33.02); N, 4.76 (6.17);
P, 28.07 (25.49); Rh 23.32 (22.08). Orange crystals of 1, suitable for x-ray
difi'raction studies, were obtained from dichloromethane solution layered

with hexane.

c. Rh2[CH3N(PF2)2]3C12(PF3), 2. The Rh2(II,0) complex
Rh2[CH3N(PF2)2]3C12(PF3) was prepared by three methods:

Method i. To a solution of 0.300 g of [RhCl(PF3)2]2 (0.477 mmol) in 15
mL of benzene, 0.24 g of CH3N(PF2)2 (1.4 mmol) was added. The yellow
solution turned red and PF3 gas was liberated. After stirring for 20 min,
the solution was filtered, and the filtrate volume was reduced to
approximately 10 mL. Addition of 15 mL of hexane led to the precipitation
of an orange solid, which was collected by filtration under an argon flow,
washed with benzene, and dried in vacuo. Yield 0.102 g (0.118 mmol,
25%). Anal. calcd (found) for C3H9012F15N3P7Rh2: C, 4.16 (4.73); H, 1.05
(1.07); F, 30.14 (32.92); N, 4.68 (4.85). Red-orange crystals of 2 were
obtained by layering a CH2012 solution of the complex with hexane under
argon. Method ii. [RhCl(PF3)2]2 (0.201 g, 0.32 mmol) was dissolved in 5
mL of benzene. The solution was saturated with PF3 and 1.8 mL of
CH3N(PF2)2 (0.27 g, 1.6 mmol) was added by syringe. The mixture was
bubbled with PF3 for 30 min during which a precipitate formed. The
orange-red precipitate was filtered, washed with benzene, and dried
under vacuo. Yield 0.184 g. (0.213 mmol, 67%).

 

this
0.04

red-

lick

33

Method iii. To an orange solution containing 0.251 g of
[RhCl(PF3)2]2 (0.40 mmol) in 5 mL of benzene was added 0.28 mL (0.41 g,
2.4 mmol) of CH3N(PF2)2. The solution turned red and PF3 was liberated.
After a few minutes of stirring, a precipitate began to form. The mixture
was heated to reflux for 16 h under argon. The mixture was cooled to
room temperature, and the red-orange precipitate was filtered under
argon, washed with benzene, and dried under vacuum. Yield 0.290 g

(0.335 mmol, 84%).

(1. Preparation of Rh2[CH3N(PF2)zlsClz(PF3) from Rh3(u-
Cl)3[CH3N(PF2)2]3. Rh3(u-Cl)3[CH3N(PF2)2]3 (see Section II.A.3) was
dissolved in 5 mL of a benzene solution saturated with PF3. Addition of
0.040 mL of CH3N(PF2)2 (0.34 mmol) with stirling prompted a precipitate
to form after a few minutes. The suspension was refluxed for 4 h. The
red-orange precipitate was filtered, collected, and dried under vacuum to
yield 0.038 g of halCH3N(PF2)2]3Clz(PF3) (0.044 mmol, 89%).

e. Rh2[CH3N(PF2)2]3014, 3. Q'ystalline [RhCl(PF3)2l2 (0.394 g, 0.627
mmol) was dissolved in 15 mL of benzene. The solution turned dark and
PF3 was liberated immediately upon the addition of 0.44 mL (3.83 mmol) of
CH3N(PF2)2. The solution color lightened within seconds. After a few
minutes of stirring, 0121(06H5) (0.700 g, 2.55 mmol) was added from a
side-arm, causing the solution to turn yellow. The suspension was
refluxed for 4 h and a yellow precipitate was filtered off under argon,
washed with benzene, and dried under vacuum. Yield 0.315 g, (0.371
mmol, 59%). Anal. calcd (found) for C3H9014F12N3P6Rh2: c, 4.25 (4.73);
H, 1.07 (1.11); CI 16.71 (16.80); F, 26.87 (27.10); N, 4.95 (4.80); Rh, 24.25

34

(23.67). X-ray quality yellow crystals of 3 were prepared by diffusing ether
into a THF solution of the complex while under argon.

Alternatively 3 can be prepared by adding 0.010 mL (0.11 g, 0.68
mmol) of CH3N(PF2)2, and 195 11L of Me3SiCl to a suspension of 0.151 g
(0.344 mmol) Rh2(OchH3)4 in 20 mL of CHZClz. The suspension was
refluxed for 12 h under argon, cooled, charged with an additional 130 11L
(1.02 mmol) of Me3SiCl and refluxed for an additional 20 h. The
suspension was cooled again and an additional 0.010 mL (0.68 mmol) of
CH3N(PF2)2 and 165 11L (1.30 mmol) of Me3SiCl was added. The
suspension was brought to reflux for 24 h. Alter cooling the suspension,
the solution was filtered to remove unreacted Rh2(OchH3)4 leaving a
dark red filtrate. The volume of the solution was reduced to 10 mL by
vacuum distillation and the resulting filtrate was placed in the
refrigerator for 12 h. An orange-yellow precipitate was collected by
filtration and dried under vacuo. Yield 0.0247 g (0.0291 mmol, 8.5%).

3. Synthesis othsol-Cl)3[CH3NCPF2)2]3, 4

A 5.0 mL solution of benzene was charged with 0.225 g of
[RhCl(PF3)2]2 (0.36 mmol), and 0.050 mL of CH3N(PF2)2 (0.43 mmol). The
red solution was refluxed for 5 h under argon. The solution was cooled to
room temperature and the solution volume was decreased by 2.0 mL by
vacuum distillation. To the filtrate, 5.0 mL of hexane was added, and a
red precipitate formed. The precipitate was filtered under argon, and
dried under vacuum. Yield 0.0274 g (0.030 mol, 8%). X-ray crystals of 4
were obtained by diffusing hexane into a CH2012 solution of the complex

ondnlal

noopsc

1.8ynt

menu)

asrec

bobbi
ofbo
mL I
and
Arc
fihl
yel
Die

02

35

under argon. This method yielded crystals with CZ/c and ana space
groups on separate preparations.

4. Syntheses ofDiiridium Complexes

a. Starting Materials. [IrCl(CsH14)2]2 was prepared by literature
methods.96 IrCl3-H20 and 08H“ were purchased from Aldrich and used

as received.

b. Attempt to Prepare Ir2[CH3N(PF2)2]3Clz(PF3), 5. PF3 was
bubbled into a suspension of 0.401 g [IrCl(C8H14)2]2 (0.447 mmol) in 10 mL
of benzene until all the solid had dissolved. To this yellow solution, 0.26
mL (2.2 mmol) of CH3N(PF2)2 was added. The solution turned orange,
and then returned to yellow. PF3 was passed into the solution for 30 min.
A small amount of a grey-yellow precipitate was removed from solution by
filtration and the filtrate was reduced to 4 mL by vacuum distillation. A
yellow precipitate formed with the addition of 8 mL of hexane. The
precipitate was collected by filtration and dried under vacuum. Yield

0.229 g of this product.

c. Attempt to Prepare Ir2[CI13N(PF2)2]3Cl4, 6. 0.200 g of
[IrCl(CsH14)2]2 (0.223 mmol) was suspended in 10 mL of benzene. PF3 was
bubbled into the solution and after all of the solid had dissolved, 0.23 mL of
CH3N(PF2)2 (1.86 mmol) was added. The yellow solution turned to light
orange for a few seconds, and then back to yellow. The solution was
saturated with PF3 and 0.303 g of CIZI(CGH5) (1.10 mmol) was delivered

from a side-arm. The solution was heated to reflux under argon for 2 h.

The

argc
prec
und
5. S
disc

Me:
by l

glee

dice

36

The yellow solution was cooled to room temperature and filtered under
argon. Hexane was added to the yellow filtrate and a light yellow
precipitate formed. The solution was filtered, and the solid was dried
under vacuo. Yield 0.0764 g of this product.

5. SynthesesofDimolybdenumComplexes

a. Starting Materials. The starting compounds (NH4)5M02019-H20,
disodium 1,2-dicyanoethylene-2,2-dithiolate (Naznmt), M02(OZCCH3)4, and
M02C14(dppm)2 [dppm = bis(diphenylphosphino)methane] were prepared
by literature methods.97'100 (C4H9)4NBr (Aldrich) and 4,4'-dimethyl-2,2'-
dipyridyl (dmpby) (Ald1ich) were used as received.

b. M02C14(dmbpy)2, 7. This preparation was modified from the
preparation of M02014(bpy)2l4 . 2.00 g (10.9 mmol) of dmbpy was dissolved
in 30 mL of methanol. With the subsequent addition of 0.500 g of
(NH4)5M02019°H20 (0.826 mmol), the solution was stirred for 90 min and a
solid formed. The product was filtered, washed with one 10 mL portion of
water, and methanol, and three 10 mL portions of ether. The solid was
dried in vacuum. Yield 0.406 g (0.578 mmol, 70%).

c. [(C4H9)4N)]2[M02(O)(S)(u-S)2[82C2(CN)2]21, 8. A suspension of
M02(OchH3)4 (0.604 g, 1.41 mmol) in 50 mL of dry methanol was charged
with 1.06 g of Nazmnt (5.67 mmol). The yellow suspension turned dark
green. After 5 h of stirring, the mixture was filtered and the volume of
the green filtrated was reduced to 35 mL by rotary evaporation. To the
green solution, 1.81 g (5.62 mmol) of (C4H9)4NBr was added. The flask

 

fiher

susD91

37

was cooled for 12 h in a dry ice-acetone bath and a green precipitate
formed. The precipitate was filtered in air and washed with methanol.
Upon dissolution of this precipitate in boiling methanol, the green
precipitate dissolved and a red precipitate remained. The red precipitate
was filtered and recrystallized from hot methanol and acetone. Yield
0.278 g (0.264 mmol, 19%). Anal. calcd (found) for C40H72M02N 60 187: C,
44.91 (45.38); H, 6.78 (6.86); N, 7.86 (7.60); S, 20.98, (20.35); Mo, 17.98 (18.19).
X-ray quality crystals of 8 were grown from a solution of CHzclz layered
with hexane. Green crystals, grown from the green filtrate, were
identified by IR and absorption spectrum to be [(C4H9)4N]2Mo(mnt)3.
Yield 0.500 g (0.494 mmol, 35%). Anal. calcd (found) for C40H72M0NGS4:
C, 52.77 (55.38); H, 7.25 (7.89); N, 11.19 (10.61); S, 19.21 (16.20); Mo, 9.85
(10.56).

d. M0201202(u-02)(dmbpy)2, 9. A suspension of 0.384 g (0.547
mmol) M02Cl4(dmbpy)2 in 50 mL of oodichlorobenzene was heated to reflux
for 5 h under air. The suspension was cooled to room temperature, and
filtered. Hexane was added to the red filtrate and a red precipitate
formed. The precipitate was filtered, washed with ether, and dried under
vacuum. The precipitate was recrystallized from CH2012 and hexane.
Yield 0.080 g (0.115 mmol, 21%). Anal. calcd (found) for
CuH24014M02N402: C, 41.46 (36.06); H, 3.48 (3.30); N, 8.06 (6.43); CI 10.20
(23.46); Mo, 27.60 (23.54). X—ray quality crystals of 9 were grown from slow
evaporation of CH30N solution.

e. Attempt to Prepare M02(dppm)2(mnt)2, 10. To a green
suspension of M02014(dppm)2 (0.210 g, 0.190 mmol) in 30 mL of acetone,

50

5P
of

38'

Nazmnt (0.0694 g, 0.380 mmol) was added. The suspension turned dark
green, and was refluxed for 3 h under argon. The solution was cooled to
room temperature and stirred for an additional 2 h. The suspension was
filtered, the filtrate volume was reduced to 10 mL by vacuum distillation,
and 20 mL of hexane was added. The solution was cooled to 10 °C for 12 h
and a dark green precipitate (10) formed. The precipitate was filtered,
washed with hexane, and dried under vacuum. Yield 0.0761 g (0.0613
mmol, 32%) based on M02(dppm)2(mnt)2. IR (KBr pellet) 2207 (s, CaN);
1482 (m), 1152 (8). (0:0 stretch); 797 (m). 737 (s) (out-of-plane c-h bend); 688
(s out-of-plane ring C=C bend).

B. Instrumentation and Methods

1. Ahsmption Spectroscopy

Electronic absorption spectra were recorded on dichloromethane
solutions (HPLC grade) of the compounds with a Cary 17
spectrophotometer. Extinction coefficients, which were determined by use
of 1-cm quartz cuvette, were calculated from the Beer-Lambert equation.

2. lnfraredSpectroscopy

Infrared spectra were obtained as KBr pellets on a Perkin Elmer
599 and a Nicolet 740 FT IR spectmphotometer.

39

3. 'I‘lme-ResolvedandSteadyStateInminescence

Time-resolved and steady-state luminescence spectra were
recorded on previously described Nd:YAG pulsed laser system (lexc = 355
nm, fwhm = 8 ns) and high-resolution emission spectrometer “em = 365
nm) respectively,1°2v103 constructed at Michigan State University. All
luminescence spectra were corrected for the instrument response
function. Variable temperature luminescence and lifetime
measurements were recorded on a sample cooled with an Air Product

closed-cycle cryogenic system by methods described elsewhere.102

4. MasSpectroscopy

The mass spectra were taken at the NIH/MSU Mass Spectrometry
Facility. The mass spectra were measured on a double focusing (EB
geometry) JEOL HX-110 mass spectrometer. The acceleration potential
used for this work was 10 kV. All data were acquired, stored, and
processed on a JEOL DA5000 data system. Fast Atom Bombardment
(FAB) ionization was preformed with a 6 kV neutral beam of xenon atoms

impinging on a o-nitrobenzyl alcohol (NBA) matrix.

5. Nuclear Magnetic Resonance Spectroscopy

NMR spectra were performed at the Max T. Roger NMR Facility at
Michigan State University. The 31P{1H} NMR spectra were recorded at
202.334 MHz and 121.421 MHz with a Varian VXR-SOOS and a Varian
VXR-3OOS, respectively. The 19F, and 19F{31Pl NMR spectra were recorded
at 470.268 MHz on the VXR-5OOS spectrometer. 31PIIH] chemical shifts

 

were 1
H3PC
l9}: l)
chem
recol
Lah-
lah

CE

ST

Zl

a. i
(dint

paT'cll

refine

Was lc

40

were reported in parts per million (5-scale), and measured relative to 85%
H3PO4. Positive chemical shifts are downfield from the standard. The
19F NMR were referenced to an external standard of CF013 with positive
chemical shifts downfield from the standard. All NMR spectra were
recorded at 25 °C. Deuteriochloroform (99.8%, Cambridge Isotope
Laboratories) and deuterioacetonitrile (99%, Cambridge Isotope

Laboratories) solvents were used as received.

C. Crystal Structm'e Determinations

1. GeneralProcedures

Crystal structure determinations were performed by Dr. Donald L.
Ward of the X-ray Crystal Structure Facility at Michigan State University.
The diffraction data were collected on a Nicolet P3F difl‘ractometer using
graphite-monochromated MoKa (A = 0.71073 A) radiation. All
calculations were performed on a VAX 11/750 computer using

SDP/VAX.104

2.Methods

a. RhZICH3N(PF2)2]3(PF3)2. 1. An orange irregularly shaped crystal of 1
(dimensions 0.14 x 0.32 x 0.44 mm) was mounted on a glass fiber. The cell
parameters and orientation matrix were obtained from least-squares
refinement of 25 reflections in the range 15 < 26 < 24°. The space group
was found to be PT.

and:
ever

con

too
531

CO

e. RI

dime:

41

A total of 4552 reflections was collected, of which 4172 were unique
[(1 not systematically absent. Three standard reflections were measured
ery 93 reflections and indicated a decrease of intensity of 15.2%; a decay
rrection was applied.

The structure was solved by Patterson heavy-atom methods. The
ordination sphere was completed by a series of difference Fourier
ntheses. The hydrogen atoms were placed in idealized positions and
nstrained to ride on the carbon atoms. Anisotropic refinements were
ed to give R = 0.067 and Rw = 0.078. The largest shift-to—error was 0.110,
1d the largest peak in the final difference map was 1.01(13) e/A3.

Rh2ICH3N(PF2)2]3C12(PF3), 2. An irregular shaped red-orange crystal
2 (dimensions 0.36 x 0.45 x 0.56 mm ) was mounted in a glass capillary
a random orientation. The cell constants and orientation matrix were
tained from least-square refinement of 15 reflections in the range 20 <
l< 25°. From systematic absences and from refinement, the space group
18 determined to be P212121.

A total of 9542 reflections were collected, of which 4889 were unique
id not systematically absent. Corrections were applied and the
ructure was solved as described for 1 above. The nonhydrogen atoms
are refined with anisotropic parameters, and hydrogen atoms were
need in idealized positions and constrained to ride on the carbon atoms.
1e largest shift-to-error was 0.150, and the largest peak in the final
fi‘erence map was 0.57 (7) e/A3.

RhZICH3N(PF2)2]3014, 3. A yellow transparent needle of 3 (crystal

mansions 0.04 x 0.10 x 0.42 mm) was mounted with Paratone-N oil and

 

cool
orie
refl.
fro:

at

0.]

go:
fro
Fn

det

am
Stu
Wer
We:

{ecu

42

cooled with a nitrogen gas stream to -70°C. The cell constants and an
orientation matrix were obtained from least-square refinement of 23
reflections in the range 15 < 20 < 20°. From the systematic absences and
from refinement, the space group was determined to be C2/c.

A total of 2957 reflections were collected. Corrections were applied
and the structure was solved as described above for l. The nonhydrogen
atoms were refined with anisotropic thermal parameters, and hydrogen
atoms were placed in idealized positions and constrained to ride on the
carbon atoms. Atoms N(1) and C(1) lie along a crystallographic two-fold
axis that passes midway between the two rhodium atoms; only half of the
atoms are crystallographically unique. The largest shift-to-error was
0.140, and the largest peak in the final difference map was 0.68(8) e/A3.

d. Rhsm-Cl)3[CH3N(PF2)2]3 in C2/c Space Group, 4. A dark red-orange
prism crystal of 4 (dimensions 0.40 x 0.52 x 0.82 mm), was mounted in a
glass capillary with its long axis roughly parallel to the phi axis of the
goniometer. The cell constants and orientation matrix were obtained
from least-square refinement of 19 reflections in the range 15 < 20 < 20°.
From systematic absences and from refinement, the space group was
determined to be C2/c.

A total of 8658 reflections were collected, of which 8058 were unique
and not systematically absent. Corrections were applied and the
structure was solved as described for 1 above. The nonhydrogen atoms
were refined with anisotropic thermal parameters. The hydrogens atoms
were calculated in two disordered sets of positions, assigned occupation

factors of 0.5, and were included in the refinement but restrained to ride

on the
0.030, s
e. Rhs
irregul:
mountt
on'enta
reflectic
refinem
A
and not
every 95
decay q

T
SHE“.
“mail

sEmlhe

f [(04,
We“,

W103] c

43

on the atom to which they are bonded. The largest shift-to-error was
0.030, and the largest peak in the final difi‘erence map was 0.91(9) e/A3.

e. Rhsm-Cl)3[CH3N(PF2)2]3, in Pmna space group 4'. A deep red
irregular shaped crystal of 4 (dimensions 0.20 x 0.36 x 0.56 mm) was
mounted on a fiber in a random orientation. The cell constants and
orientation matrix were obtained from least-square refinement of 25
reflections in the range 20 < 20 < 25°. From systematic absences and fiom
refinement, the space group was determined to be Pmna.

A total of 2952 reflections were collected, of which 2679 were unique
and not systematically absent. Three standard reflections were measured
every 93 reflection and indicated an increase of intensity of 2.6%; a linear
decay correction was applied.

The structure was solved using the direct methods program
SHELXSS6. A total of 17 atoms were located from an E-map; the
remaining atoms were located in succeeding difference Fourier
syntheses. The hydrogen atoms were included in the refinement to ride
on the atom to which they are bonded. Atoms Rh(2), C1(3), M3), and C(3)
lie on a crystallographic mirror plane; only half of the atoms are
crystallographically unique. P(2), F(2a), and F(2b) are disordered and
were refined in two sets of 0.5 occupancy positions [P(2a), F(2aa), F(2ab);
and P(2b), F(2bb), F(2ba)]. The largest shifi-to-error was 0.260, and the
largest peak in the final difference map was 0.53(9) e/A3.

f. [(C4H9)4N)]2 Moz(0)(S)ot-S)2[SZC2(CN)2]2}. 8. A red crystal platelet of 8
(dimensions 0.11 x 0.30 x 0.50 mm) was mounted on a glass fiber in a

random orientation. The cell parameters and orientation matrix were

obtaine
28 < 20

and n
stmctu
nonhyc
The by
ride on

0.200, a

3. Mo:
(dimeng
orienta‘
least-s:
system

09mm

and I
Gl‘er:

aniSl

A to;
Were
aims

are 5011.

44

obtained from least-square refinement of 21 reflections in the range 15 <
20 < 20°. The space group was determined to be P21/c.

A total of 7767 reflections were collected, of which 7168 were unique
and not systematically absent. Corrections were applied and the
structure was solved as described for 4 (see Section II. C. 2e) above. The
nonhydrogen atoms were refined with anisotropic thermal parameters.
The hydrogens atoms were included in the refinement but restrained to
ride on the atom to which they are bonded. The largest shift-to-error was
0.200, and the largest peak in the‘final difference map was 0.83(7) e/A3.

g. MOZClgozm-02debpylz, 9. A clear red diamond shape crystal of 9
(dimensions 0.10 x 0.15 x 0.20 mm) was mounted on a fiber in a random
orientation. The cell constants and orientation matrix were obtained from
least-square refinement of 12 reflections in the range 15 < 20 < 20°. From
systematic absences and from refinement, the space group was
determined to be P3121.

A total of 9353 reflections were collected, of which 2109 were unique
and not systematically absent. Three standard reflections were measured
every 93 reflection and indicated an increase of intensity of 16.5%; an
anisotropic decay correction was applied.

The structure was solved using the Patterson heavy-atom methods.
A total of 17 atoms were located from an E-map; the remaining atoms
were located in succeeding difference fourier syntheses. The hydrogen
atoms were included in the refinement to ride on the atom to which they
are bonded. Complex 9 possesses a crystallographic two-fold axis that
passes between the Mo(1), Mo(1)’ atoms; only half of the atoms are

cyst:

45'
crystallographically unique. The largest shift-to-error was 0.010, and the
largest peak in the final difiemnce map was 1.13( 12) e/A3.

CHAPTERIII

Rhodium Fluorophosphine Chemistry

 

M

5m
inte
the
11161
eng
rich
to 1
ace:
fluo
cent

the

Oxid
fluor

A. Background

Our interest in developing multielectron schemes is centered on
synthesizing a homologous series of excited state complexes, which are
interrelated by multielectron steps. The strategy depends primarily on
the ability to employ a ligating coordination sphere capable of stabilizing
metal redox centers in a variety of formal oxidation states while
engendering electronic structural properties that are compatible with a
rich excited-state chemistry. For these reasons, we turned our attention
to the chemistry of fluorophosphine metal complexes. The good 1:-
accepting and moderate 0-donating abilitylos'110 of bidentate
fluorophosphine ligands is manifested in their ability to stabilize metal
centers in low and high on’dation states, respectively.7 As illustrated by
the schematic represented by 10 for PF3, phosphorus lone pair electrons
can be donated to the metal 0 orbital.

 

O
."”’”
”/1

F
F

10

This 0-donor ability of the phosphorus will stabilize metals in high
oxidation states. Conversely, the u-accepting properties of
fluorophosphines will stabilize metals in low oxidation states. The 1:-
accepting properties of PF3 can be understood within two bonding

frameworks. The low-lying 3d orbitals of phosphorus can accept electron

45

47

density from the metal d1: orbitals by the conventional n-back bonding
picture illustrated by 11.

 

More recently Marynick has suggested orbitals with 0* symmetry with
respect to the P-F bond axis can accept electron density from the metal.106
This accepting orbital is formed from the P-F 0* bonding interaction

shown in 12.

 

There are several reasons for the enhanced n-accepting ability of PF3 over
other phosphines like PH3 and PR3 (R = alkyl). Firstly, phosphorus-
fluorine bonds are highly polar bonds, and these characteristically have

48

low-lying 0* accepting orbitals. Secondly, because the 0 P-F bond is highly
polar toward F, the 0* orbital must necessarily be highly polar toward P,
therefore, increasing the 0*-metal d1: overlap.

On the basis of these arguments, we became intrigued with the
bidentate bis(difluorophosphino)alkyl/arylamines, RN(PF2)2, (R = methyl,
phenyl) owing to the studies of King and coworkers,112 which establish the
precedence for this ligand to give rise to a diverse redox chemistry of
bimetallic cores. The unique redox pr0perties of metal coordinated by this
ligand is best exemplified by the cobalt complex C02[CH3N(PF2)2]3(CO)2,
(see Figure 10) which upon treatment with Br2, gives the 002(II,II)
derivative Coz[CH3N(PF2)2]3(CO)2Br4,113 and upon its electrochemical
reduction yields the Coz(-I,-I) anion, C02[CH3N(PF2)2]3(CO)22’.114 In this
system, the bis(difluorophosphino)methylamine ligand has stabilized the
metal core to an overall six electron change in formal oxidation state!
Moreover, the cobalt complex illustrates the ability of the RN(PF2)2 ligand
to coordinate across singly bonded metal-metal (M-M) cores while
accommodating very different coordination geometries about the metal

center.115'116 For

instance, the binuclear compound
Fe2[CH3N(PF2)]2(CO)6 13, has a square-pyramidal geometry rather than

the usual trigonal-bipyramidal for five-coordinated iron (0) atoms. 115

49

Figure 10

Redox chemistry of Coz[CH3N(PF2)2]3(CO)2.

 

3C

0 O O
l C\ ‘0
oc—m—oJ—co
E 0
0° 0 o
(I3Ha
N
/ \
sz PF2

 

 

 

P P\ P
_ VJ
26 28r2

A A

P P P

I - I -1 69- Brut... 'u

oc—Co Co—CO 4 , 0°
/r=- /"= 3'7
p RP F P E\P I:

Figure 10

 

Be

51'

CH3

h
F \T,
C O— ep.00 CO

e—CO

\tco 00'
F2 F2

on3
13

Subsequent loss of CO from 13 forms Fe2[CH3N(PF2)]2(CO)5 14, which
possesses a geometry about each iron atom that can be considered as
either a distorted octahedron or distorted trigonal bipyramid depending if

the metal-metal bond is considered as a coordination position.116

Because multielectron transformations usually prompt a significant

change in structure of the coordination sphere of a metal, the ability of the

0n

 

52

uorophosphine ligand to permit different geometries could be important
I the design of multielectron schemes.

Studies were initiated with group VIII metals because of Kruck
rld coworkers reported117 that UV photolysis of HIr(PF3)4 yields Ir2(PF3)3
ad H2. The Ir2(PF3)8 photoproduct was subsequently observed to react
ith water to regenerate HIr(PF3)4. The fate of oxygen is unknown. This
eaction is important because ( 1) the photochemical reaction is at least a
vo electron process; and (2) the photochemical reaction represents half of
le water splitting cycle with implied oxygen activation.

The photochemical reaction of HIr(PF3)4 may proceed by three
lausible mechanisms: radical, acid-base, or oxidative addition/reduction

.imination. A radical mechanism is shown by eqs 8-11,

HIr(PF3)4 + hv ——> H + 'Ir(PF3)4 (8)
-Ir<PF3>4 + lama), —> h2<PF3)8 (9)
'H + 'H —> H2 (10)

radiation produces the -H and -Ir(PF3)4 radicals as primary

“8 Simple recombination of products will not be

hotoproducts.
lnstructive. However, combination radical crossover yields the observed
.‘0d110t8. Alternatively, excitation could significantly change the pKa of
Ir(PF3)4 producing H+ and Ir(PF3)4". Nucleophilic attack of Ir(PF3)4"

lHIr(PF3)4 will yield H2 and the observed dimer.

 

Hurry, + hv ———> HIr(PF3)3)4* (11)
mam-*3); ——> H+ + Ir(PF3)4' (12)
HIr(PF3)4 + Ir(PF3)4’ —> Ir2(PF3)3 + H- (13)
H+ + H“ H2 (14)

 

Wit

53

Finally and perhaps most reasonably is the intermolecular reductive

elimination show by equations 15-17.119

HITCPF3)4 '1' 11V ——"’ MF3)3 + PF 3 (15)
In\
maple?)4 + maple?)3 ——> (PF3)3I|i' ---l’r(PF3)4 (16)
R H
(PF3)3I|i-’- - 313113113)4 + PF3 ——> H2 + 1120193)8 (17)
H

Excitation causes PF3 to photodissociate from the metal center to produce
a vacant coordinated site.12° Reaction of the coordinatively unsaturated
HIr(PF3)3 with HIr(PF3)4 yields the binuclear hydride intermediate which
can undergo subsequent reductive elimination to produce H2 and
Ir2(PF3)8.

Irregardless of the particular mechanism, it is necessary to bring
together monomeric iridium centers to ultimately yield products.
Certainly, the low photoreaction yield of the reaction is consistent with a
rate determining step for hydrogen production involving a bimolecular
reaction between monomeric iridium species. These observations suggest
that binuclear iridium (and rhodium) complexes with bidentate
CH3N(PF2)2 ligands may retain the interesting multielectron chemistry,
while exhibiting an increased quantum yield for photoreactivity. Because
the observed photochemistry of the M2(PF3)8 (M: Rh, Ir) establishes
reactive excited state chemistry for Group VIII fluorophosphines
complexes, we decided to explore the chemistry of CH3N(PF2)2 beginning

with bimetallic rhodium complexes.

‘1.

9X}

55’!

54

B. Dirhodium Complexes
1. Results

a. Synthesis of Dirhodium Complexes. The [RhCl(PF3)2]2 dimer
reacts smoothly with bis(difluorophosphino)methylamine to yield
Rh2[CH3N(PF2)2]3CIZ(PF3), 2, and under reducing and oxidizing
conditions to yield Rh2[CH3N(PF2)2]3(PF3)2, 1 and Rh2[CH3N(PF2)2]3Cl4, 3,
respectively. The formation of 2 corresponds to an intramolecular
disproportionation of the Rh2(I,I) starting material. However, the
intimate mechanism, appears to be more complicated owing to our
isolation and characterization from reaction mixtures of the recently
reported trimer, Rh3(ll-Cl)3[CH3N(PF2)2]3.121 We have observed that
reaction of this trimetallic complex with excess ligand leads to 2 in good
yields.

The unsymmetrical binuclear complex can be viewed as a synthon
for l and 3. The former is derived from the reduction of the Rh(0) center of
2 with subsequent trapping by PF3, whereas 3 can be envisaged to form
from the oxidation of the Rh(0) center by chlorine. Indeed, although 1 and
3 are prepared from the aforementioned [RhCl(PF3)2]2 starting compound
in high yields, we have been able to directly convert among 1, 2, and 3 with
the appropriate choice of oxidizing and reducing conditions. For
instance, oxidation of 1 by ClzICsHs readily affords 3. Conversely, sodium
borohydride reduction of 3 produces 1, but admittedly in low yields. As
expected, both 1 and 3 can directly be obtained from 2. The ability to

synthetically interconvert among these ha complexes underscores the

 

fl(
0%

D8

Whg

One

55

fluorophosphine ligand's capacity to readily accommodate metal cores in a
variety of formal oxidation states.

The complexes 1, 2, and 3 where characterized by elemental analysis,
mass spectroscopy, and x-ray crystallography. The elemental analysis was
not reliable; even samples from which crystals were chosen for x-ray
structural determination yielded poor analysis.

A more reliable method for routine analysis was mass spectroscopy.
The mass spectrum of l, 2, and 3 were recorded by employing Fast Atom
Bombardment (FAB) methods. The FAB mass spectra of l, 2, and 3 are
shown in Figures 11 - 13. The molecular ion observed for 1 occurs at a m/z
of 882.95 and is labeled P in Figure 11. Loss of one and two PF3 is indicated
by a m/z of 794.95 and 706.96, respectively. Unknown fragments at m/z of
755, 688, and 609 are labeled with an U. The mass spectraof 2 and 3 differ
from 1 in that fragmenation peaks display isotope peak-intensity patterns
owing to the presence of chlorines. Figure 14 shows the peak intensity
pattern for different numbers of chlorine atoms. This pattern was

calculated from the binomial expansion122

(a + b)“ = an + nb(“ - 1h + n(n - 1)a(“ ' 2>h2/2! + n(n - 1)(n - 2)a(“ ' 3’h3/3! +..(1s)

where a represents the relative intensity of the first isotope (assumed to be
one), b represents the relative intensity of the next highest mass isotope,
and n is the number of isotope atoms present to determine the magnitude of
the abundance contribution for a given isotope. Comparison of Figure 14 to

the mass spectrum of 2 and 3 shows that the correct chlorine isotope

56

Figure 11

FAB mass spectrum of Rh2[CH3N(PF2)2]3(PF3)2, l.

Relative Abudance

S7

 

 

 

 

 

 

1 0°) |P| -2PF3
* 706.
8 0: IPI = Rh2L3(PF3)2 96 IPI _ PF3
} I" CH3N(PF2)2 794.95
. M = unknown
601
J
J
40“
‘ P
1 M 'U' u aslz I95
20.509 6 8 ll .
1 I 755
‘ 4‘. ‘4' J: . V 1“]; TLL L- ‘Tr , 1 cl a s Lrh‘ U s -
600 700 800 900

Figure 11

58

Figure 12

FAB mass spectrum of Rh2[CH3N(PF2)2]3(PF3)Clz, 2 prepared by method i
of Section II.A.2c.

 

Relative Abudance

59

 

 

 

 

 

100
I Pl-(PF +01)
P =Rh L (PF Cl ' 3
1 I l 2 3 3) 741.58
3° , L = CH3N(F’F2)2
j M sunknown
601
40* .
1|PI-(L+PF3) 'P' ”3
J 609 51 776.45 M
2 0] |P| - (Pl:3 + 20!) 84°
‘ L 706
.,, - -Lxl-f-L +~3L Lfilhrecutfieef- e:
600 700 800
m]:

Figure 12

Figure 13

FAB mass spectrum of Rh2[CH3N(PF2)2]3Cl4, 3.

 

 

61

 

6 0i |P| . haLacu
L I CH3N(PF2)2
8 50': M 3 unknown

'° “1 |P| - (L + 201)

 

 

|P| . 3CI

 

 

 

g 301 i
2 i |P| - (1+ LA)
3 20. ll
g 3
1 03 IPI-4C'
: Lfilltfi 1 A? an. vx-fififiu—
600 700

Figure 13

ml:

|P| - Cl

  

800

 

900

62

Figure 14

Isotope peak-intensity pattern for ions containing the indicated number of

chlorine atoms (from ref. 123).

 

Cl

 

Cl2

 

 

 

Cl3

 

 

 

cu4

 

 

 

 

Figure 14

Cl.5

 

 

 

Cls

 

 

 

 

X 0400

' 64

peak intensity pattern is observed. Molecular ions were not observed for 2
and 3. The mass spectrum of 2 shows fragments due to the loss of PF3 at
m/z of 776.45 and the loss of chlorines from the fragments at m/z of 741.50
and 706.48, respectively. Also the fragment due to the loss of CH3N(PF2)2
and a PF3 ligand is observed at m/z of 609.51. For 3 the FAB mass
spectrum clearly shows the successive loss of chlorines (m/z of 811.46,
776.54, 741.65, and 707.66). Fragments due to the loss of CH3N(PF2)2 and
chlorine, and CH3N(PF2)2 and two chlorines (m/z of 644.57, and 605.59 ,
respectively) are also apparent.

In 2 and 3 unknown peaks at a m/z of 874 and 840 are observed. The
peak at m/z of 874 show no chlorine isotope pattern, and the peak at m/z of
840 indicates one chlorine atom is present. However no other fragments
arising from this ion were detected. It is likely that these unknown peaks
arise from matrix contributions.

The mass spectral analysis provides a convenient analytical tool for
assessing the optimal conditions for the preparation of compounds 1-3. To
prepare 2 in good yield, a mole ratio of [RhCl(PF3)2]2 to CH3N(PF2)2 of at
least 1:5 was needed (Section II.A.2.c). If the ratio is approximately 1:2, a
mixture of 2 and Rh3(u-Cl)3[u-CH3N(PF2)2]3 (see Section II.A.3) was
obtained. Several attempts were made to purify 2 by column
chromatography, but decomposition on Florisil® or silica gel was
observed. Pure 2 could only be obtained by several recrystallizations from
CHzClzlhexane mixtures. The FAB mass spectra of 2 prepared by
methods ii and iii are shown in Figures 15 and 16. The mass spectra of
product 2 prepared by these alternative methods are similar to those
obtained for compound prepared by method i

65

Figure 15

FAB mass spectrum of Rh2[CH3N(PF2)2]3(PF3)012, 2, prepared by method
ii of Section II.A.2.c.

Relative Abundance

 

 

 

 

 

 

1 00‘ v
1 |P|=Rh2L3(PF3)Ci 741-53
803 L=CH3N(PF2)2 |P|'(PF3+C')
M = unknown
‘ U
60'] 84|0 I56
1 IPI ' PFa .
401 776.53
: IPI '(L+ PFs)
. 609.59
2 0? |P| - (PF3 + 201)
‘ 706.60
1 w 1 -htLr‘J: -‘L. ‘: , Lr wL‘rfi , 117—v .4.“ y—lr t“: fffirhv—w—J
600 700 800 900
m/z

Figure 15

67

Figure 16

FAB mass spectrum of Rh2[CH3N(PF2)2]3(PF3)Clz, 2, prepared by method
iii of Section II.A.2.c.

100

b
0

Relative Abudanoe

20

68

 

 

 

 

1 |P| . Rh2L3(PF3)CI
L - CHaNlPinz IPI - ”‘3 WI
1 IPI ‘ (PF3 + Cl)
(IUI - unknown 741.50 776.45 840.50
1 H - (l- + PFa)
11509.55
1 IPI - (PF3 + 201)
‘1 706
00 700 800
m/z
Figure 16

 

900

69

A series of reducing reactions from higher valent Rh2 complexes
were employed in an attempt to increase the yield of 1. In early attempts,
2 was used as the starting material with sodium borohydride (N aBH4) as
the reducing agent in the presence of PF3. The product was 1, as
determined by absorption spectroscopy, but only oils could be isolated. The
same result was obtained by reducing 3 with NaBH4 in ethanol. In
benzene, no reaction took place with NaBH4 or butyllithium [CH3(CH2)3Li]
as the reducing agents. With sodium amalgam (N aHg) as the reducing
agent in THF at 0°C, a sticky purple precipitate formed when the solvent
was removed under vacuum. The precipitate was not identified. The
reducing agent tetrabutylammonium iodide (Bu4NI) was alsoused under
various conditions. The solvent for all reactions was CH2012 and
solutions of [RhCl(PF3)2]2, CH3N(PF2)2, and Bu4NI at a mole ratio of 1:6:2
were stirred under PF3, refluxed under argon, or heated to 43 °C for 12 h
in the presence of PF3. An unknown red precipitate or a red oil was
typically obtained from these various reactions. The result was the same
when the reaction was performed in benzene contained in a high .
pressure vessel heated to 65 °C for 12 h under PF3. Alternatively sodium
acenaphthylenide (NaAc) reduced solutions of [RhCl(PF3)2]2 in the
presence of PF3 and CH3N(PF2)2 in THF. A brown precipitate obtained
from this reaction. The electronic absorption spectrum from this reaction
indicated the brown precipitate to be a mixture of 2, NaAc, and
presumably 1 indicated by a shoulder at 310 nm in the electronic
absorption spectrum. The mixture could not be seperated by column
chromatography.

The most convenient way to prepare 3 was to oxidize 2 with a slight
excess of the oxidizing agent ClzlcsHs. The complex can also be prepared

70

by starting with Rh2(02CCH3)4. Me3SiCl is used to remove the carboxyl
group, and as a source of chloride ligands. The absorption spectrum of
the product from this method was the same as 3, but the yield was much
lower. An unsuccessful attempt to prepare 3 was undertaken when 012
gas was bubbled into a solution of 1.

b. Structural Interpretation. The crystal parameters and details of
intensity collection for ‘1, 2, and 3 are listed in Table 1. Positional
parameter for 1, 2, and 3 are listed in Table 2 - 4, respectively. The
molecular structures adopted by 1, 2, and 3 are unique for dirhodium
chemistry; they are represented by the ORTEP diagrams reproduced in
Figures 17, 18, and 19. The presence of three bidentate fluorophosphine
ligands in these structures is unusual and in contrast to the trans
coordination of two bidendate ligands of the ubiquitous A-frame
structures for dirhodium complexes.124'125 Complexes 1, 2, and 3 are
structurally distinguished among themselves by their rhodium
coordination environments of which there are two basic types. As
illustrated by the inner coordination spheres depicted in Figure 20,
trigonal bypyrimidal and octahedral coordination gives rise to the
symmetrical structures of 1 and 3, respectively, and to the unsymmetlical
structure of 2. In the case of 1, the coordination geometry about each
Rh(0) is nearly an ideal trigonal bipyramid whose equatorial plane is
comprised of three phosphorus atoms from each of the bridging
fluorophosphine ligands, and whose apices are capped by the phosphorus
of a terminal trifluorophosphine and by the neighboring Rh(0) of the
binuclear core. The trigonal bipyramidal coordination of the Rh(O)
centers is contrasted by the pseudooctahedral ligation sphere of Rh(II).
In 3, the three phosphorus atoms of the individual fluor0phosphine

71

 

 

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72

Table 2. Atomic Positional and Isotropic Displacement (A2 ) Parameters for
halcnsnmrz)213(l>rs)2 (1) “

 

 

Atom 1 y z BIA2
Rh(l) 0.1433(1) 0.7525(1) 0.72377(8) 4.19(3)
Rh(2) 0.2718(1) 0.9001(1) 0.79385(8) 4.12(3)
P(l) 0.2870(5) 0.7771(5) 0.5849(3) 6.4(1)
P(2) 0.4753(4) 0.7929(5) 0.6908(3) 5.5(1)
P(3) -0.0786(4) 0.9434(5) 0.7599(3) 5.1(1)
P(4) 0.0807(5) 1.1139(4) 0.7367(3) 5.0(1)
P(5) 0.2357(4) 0.5608(4) 0.8454(3) 5.1(1)
P(6) 0.2353(4) 0.7682(4) 0.9400(3) 4.9(1)
P(7) 0.0472(5) 0.6428(5) 0.6695(4) 6.9(1)
P(8) 0.3708(5) 1.0138(5) 0.8463(4) 6.6(1)
F(l) 0.334(1) 0.665(1) 0.5174(8) 10.9(4)
F(2) 0.223(1) 0.923(1) 0.5013(8) 11.1(5)
F(3) 0.581(1) 0.875(1) 0.6520(9) 9.1(3)
F(4) 0.605(1) 0.628(1) 0.7260(9) 9.1(4)
F(5) -0.214( 1) 0.978(1) 0.7059(8) 8.3(3)
F(6) -0. 170(1) 0.937(1) 0.8620(7) 7.9(3)
F(7) 0.030(1) 1.2552(9) 0.7847(8) 8.2(3)
F(8) 0.098(1) 1.199(1) 0.6290(7) 7.7(3)
F(9) 0.150(1) 0.463(1) 0.8860(8) 8.3(3)
F( 10) 0.392(1) 0.425(1) 0.8320(8) 8.4(4)
F(ll) 0.351(1) 0.726(1) 1.0128(7) 8.5(3)
F( 12) 0.092(1) 0.838(1) 1.0133(7) 8.5(4)
F( 13) 0.147(1) 0.478(1) 0.6463(9) 11.1(4)
F( 14) -0.0807(9) 0.608(1) 0.7324(9) 8.9(3)
F( 15) -0.028( 1) 0.716(1) 0.5756(8) 13.2(4)
F(16) 0.411(1) 1.136(1) 0.7778(9) 9.8(4)
F(17) 0.524(1) 0.921(1) 0.884(1) 11.4(4)
F( 18) 0.278(1) 1.107(1) 0.9268(8) 9.6(3)
N(1) 0.453(1) 0.777(1) 0.584(1) 6.3(4)
N(2) -0.088(1) 1.113(1) 0.7419(8) 5.3(3)
N(3) 0.240(1) 0.600(1) 0.9514(9) 5.4(3)
C(1) 0.580(2) 0.742(2) 0.504(1) 10.2(7)
0(2) -0.233(2) 1.259(2) 0.735(1) 7.6(5)
C(3) 0.257(2) 0.489(2) 1.045(1) 8.6(7)

 

a Anisotropically refined atoms are given in the form of the isotropic equivalent

displacement parameter defined as 413(02811 + 52822 + c2333 + ab(cos 79812 + ac(cos

73

Table 3. Atomic Positional and IsotrOpic Displacement (A2 ) Parameters for
Rh2[CH3N(PI-‘2)2]3012(PF3) (z) ‘

 

 

 

Atom 1 y z B/A2
Rh(l) 0.27066(5) 0.0068564) 0.07903(3) 3.020(7)
Rh(2) 0.10328(5) 0.07458(4) 0.18646(2) 2.934(7)
(31(1) 0.4119(2) 0.0632(2) 0.0133(1) 575(4)
01(2) 0.4552(2) 0.1142(2) 0.1206(1) 4.36(4)
P(l) 0.1040(2) 0.0907(2) 0.0379(1) 435(4)
P(2) 0.0719(2) 0.0123(2) 0.14032(9) 332(3)
P(3) 0.3668(2) 0.1162(2) 0.1473(1) 4.61(4)
P(4) 0.2632(2) 0.0029(2) 0.25600(9) 4.17(4)
P(5) 0.2086(2) 0.1511(2) 0.0202(1) 398(4)
P(6) 0.1393(2) 0.2383(1) 0.1468(1) 341(3)
P(7) 0.0261(3) 0.1256(2) 0.2709(1) 512(5)
F(I) 0.0737(6) 0.0770(5) 0.0388(2) 8.0(1)
F(2) 0.1319(6) 0.2101(4) 0.0360(4) 7.9(1)
P(3) 0.2015(5) 0.0526(4) 0.1190(3) 6.4(1)
F(4) 0.1563(5) 0.0903(4) 0.1855(3) 6.4(1)
P(5) 0.3211(7) 0.2324(4) 0.1391(3) 7.4(2)
F(6) 0.5224(5) 0.1346(5) 0.1350(4) 8.3(2)
P(7) 0.2082(6) 0.0451(4) 0.3241(2) 6.4(1)
F(8) 0.3748(6) 0.0732(5) 0.2911(3) 7.0(1)
F(9) 0.0881(7) 0.1439(5) 0.0310(3) 7.7(1)
F(lO) 0.3184(7) 0.1906(4) 0.0307(3) 7.0(1)
F(ll) 0.0138(5) 0.3143(4) 0.1539(3) 5.5(1)
F(12) 0.2456(5) 0.3147(3) 0.1805(3) 5.4(1)
F(13) 0.1496(8) 0.1924(7) 0.2555(4) 14.7(2)
F(14) 0.0945(7) 0.0494(5) 0.3182(3) 9.7(2)
F( 15) 0.0331(9) 0.1919(6) 0.3257(3) 12.3(2)
N(1) 0.0524(6) 0.0831(5) 0.0701(3) 3.9(1)
N (2) 0.3614(7) 0.0963(6) 0.2291(4) 5.2(2)
N(3) 0.1749(6) 0.2571(5) 0.0646(3) 3.8(1)
C(1) 0.1715(8) 0.1436(7) 0.0391(5) 5.8(2)
C(2) 0.449(1) 0.159(1) 0.2783(6) 9.7(3)
C(3) 0.186(1) 0.3651(6) 0.0349(5) 5.6(2)

 

I

' Anisotropically refined atoms are given in the form of the isotropic equivalent

displacement parameter defined as 4/3[0an + 02822 + 02833 + ab(cos $812 + ac(cos
(”313 + 5“” “Baal

Table 4. Atomic Positional and Isotropic Displacement (A2 ) Parameters for

74

 

 

wcngwrzblscu (3) ‘

Atom x y z BIA2
Rh(l) 0.03788(4) 0.25217(7) 0.17586(4) 1.824(9)
C1(1) 0.0943(1) 0.250Z3) 0.0352( 1) 318(4)
01(2) 0.1755(1) 0.1618(2) 0.2759(2) 325(5)
P(l) 0.0044(2) 0.0537(2) 0.1500(2) 2.61(4)
P(3) 0.0846(2) 0.3444(2) 0.0873(2) 2.71(4)
P(4) 0.1147(2) 0.4203(2) 0.2637(2) 301(5)
F(l) 0.0586(4) 0.0229(5) 0.1078(3) 3.4( 1)
F(2) 0.0952(4) 0.0165(5) 0.0756(4) 3.7(1)
P(5) 0.0708(4) 0.4334(5) 0.0092(4) 3.9(1)
F(6) 0.1588(3) 0.2650(6) 0.0166(4) 4.2(1)
P(7) 0.0747(4) 0.5480(5) 0.1973(4) 4.5( 1)
F(8) 0.2071(4) 0.4332(6) 0.2189(4) 4.8(1)
N(l) 0.000 0.0257(9) 0.250 3.0(2)
N(2) 0.1389(4) 0.4340(7) 0.1439(5) 2.7(1)
C(1) 0.000 0.163(1) 0.250 6.3(5)
C(2) 0.2091(6) 0.5243(9) 0.0866(8) 4.0(2)

 

a Anisotropically refined atoms are given in the form of the isotropic equivalent

displacement parameter defined as 4/3[02311 + b2822 + c2833 + ab(cos ”812 + ac(cos

75

Figure 17

ORTEP drawing and numbering scheme of Rh2[CH3N(PF2)2]3(PF3)2, l
with 30% probability thermal ellipsoids. For clarity hydrogen atoms are

not shown. Selected bond distances and angles are listed in Table 5.

76

 

 

Figure 17

77

Figure 18

An ORTEP view of Rh2[CH3N(PF2)2]3(PF3)C12, 2, showing the numbering
scheme. Thermal parameters are shown at the 50% level. Hydrogen
atoms are omitted for the sake of clarity. Table 6 lists selected bond

distances and angles.

'78

 

 

Figure 18

79

Figure 19

An ORTEP view of Rh2[CH3N(PF2)2]3Cl4, 3, showing the numbering
scheme. Thermal ellipsoids are at the 50% probability level; hydrogen
atoms are not shown. Table 7 lists selected bond distances and angles.

80

a '
3 our

CH
F7
J IVE
P3 ’ P4 Cl2
e 2435 Q
F5 ’ -
“ F6 6) [~12 ‘9 F8
:3
C2

Figure 19

81

Figm20

A skeletal view of the inner coordination spheres of (a) 1. (b) 2, and (c) 3.

82

 

(a) p1

P8

 

 

(b)

 

 

(C)

CI2 P3'

C11

 

 

Cl1'

 

Figure20

 

83

ligands adopt a meridional arrangement with a chlorine atom occupying
the fourth site of the equatorial coordination plane. The
pseudooctahedron about the Rh(II) is completed with the axial
coordination of a chlorine atom and a Rh(II). The mutually trans
arrangement of chlorines on adjacent Rh(II) centers has previously been
postulated to be present in trans-Rh2C12(CO)2(dmpm)2 (dmpm =
bis(dimethylphosphino)methane) and indeed observed for its chlorine
oxidation product, trans-Rh2C12(CO)2(dmpm)2.126 With 1 and 3 as
structural benchmarks, we see that the unsymmetrical congener, 2, is
simply a structural composite of the Rh2(0,0) and Rh2(II,II) complexes in
which the trigonal bipyramidal coordination about Rh(O) and octahedral
coordination about Rh(II) is preserved.

Inspection of the selected bond angles and distances of l, 2, and 3
listed in Tables 5 - 7 reveals several interesting trends. It is noteworthy
that the Rh—Cl bonds trans to the rhodium-rhodium axis in 2 («Rh-Cl”)
= 2.431(2) A) and 3 («Rh—Cl”) = 2.416(2) A) are significantly longer than
the Rh-Cl bonds cis («Rh-019,) = 2.385(2) A in 2 and d(Rh—Cleq) = .
2.392(2) A in 3) to it. Long axial Rh-Cl bonds have previously been
attributed to the trans influence of the metal-metal bond.127'128 However,
in the case of 2 and 3, it is not that the Rh—Clu bonds are unusually long
but rather that the Rh—Cleq bonds are short. These observations are
congruent with simple bonding considerations. The good n-accepting
ability of the fluorophosphine ligands should selectively enhance 1t-
' donation from trans chlorine atoms. Accordingly, the n-backbonding
between the rhodium and chlorine will be strengthened thereby resulting
in a shortening of the Rh-Cleq bond. This simple bonding model is
further supported by comparison of Rh—P distances. The resulting

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88

synergism established between the n-donating chlorine and the 7:-
accepting fluorophosphine should not only strengthen the Rh-Cleq bond
but should also be manifested in increased n-backbonding interactions
between rhodium and the phosphorus trans to chlorine. This appears to
be the case. As observed in Tables 6 and 7, the Rh-P distances for
phosphorus trans to chlorine (d(Rh-Pt,m) = 2.183(2) A in 2, d(Rh-Pt,CI) =
2.184(2) A in 3) are at least 0.07 A shorter than those for phosphorus trans
to another phosphorus (d(Rh-Pt,p)av = 2.255(2) A in 2, d(Rh'Pt,P)av =
2.252(25) A in s). The latter distances are more typical of normal Rh(II)—P
bonds.126'129'13°

As a concluding issue, we consider the Rh---Rh bond distances.
Separations of 2.841(2) A, 2.785(1) A, 2.707(1) A in 1, 2, and s, respectively,
are indicative of a normal Rh-Rh single bond. The decrease along the
series Rh2(0,0) > Rh2(0,II) > Rh2(II,II) is expected in view of the larger
atomic radii of Rh(O) as compared to Rh(II). Although the Rh—Rh
distances are considerably longer than those observed for rhodium
carboxylates,131'133 they are typical of Rh-Rh cores ligated by bidentate
phosphines containing one bridgehead atom.126'134’136' The presence of a
single bond in 1, 2, and 8 induces significant rotation of the bridging
fluorophosphine ligands away from an eclipsed conformation. The view
along the nearly linear L-Rh-Rh-L' (L, L’ = Cl or PF3) bond axis of l, 2,
and 3 (Figure 21) clearly reveals a twisted conformation of the bidentate
ligands. An average torsional angle 1“ = 30.6° for 1 is comparable to the
33.3° twist angle reported for the bis(difluorophosphino)methylamine
ligands in Coz[CH3N(PF2)2]3(CO)2,145 and is virtually identical to 3'8
average torsional angle of 30.5°. This remarkable flexibility of the

bis(difluorophosphino)methylamine ligand allows the coordination

89

Figure 21

Structural framework of (a) 1, (b) 2, and (c) 3 as viewed nearly along the

Rh-Rh axis. Axial ligands are omitted.

 

 

 

 

 

(C) '

 

 

Figure 21

 

91

asymmetry about the dirhodium core in 2 to be accommodated with
facility and, indeed, undoubtedly plays a crucial role in the stabilization of
the unusual mixed-valence Rh2(0,II) core.

c. Electronic Absorption and Emission Spectra. The electronic
absorption spectra of the dirhodium fluorophosphine complexes 1, 2, and
3 are shown in Figures 22 - 24. The spectra exhibit pronounced
absorptions in the ultraviolet spectral region with less intense absorption
bands in the visible. The electronic absorption spectrum of 2 by methods ii
and iii (Section A. 2. c) are similiar to method i except that the former do
not exhibt the low energy band at 570 cm’l. Red luminescence is
observed from solids and low temperature glasses of 1, 2, and 3 upon
excitation with frequencies coincident with absorption manifold. In
Figures 22 - 24 we show the emission spectra recorded on crystalline
solids of l, 2, and 3 at 77 K. Each displays an intense band in the red
spectral region, and 1 and 3 feature an additional emission to higher
energy. The intensity of the emissions monotonically decrease with
increasing temperature. Although luminescence from 3, which is the
brightest lumophore of the series, can be detected to room temperature,
the luminescence from 1 and 2 reach the limits of our instrumentation by
~ 220 K. This temperature dependence in emission intensity is
accompanied by an extreme temperature sensitivity of the emission half-
width, increasing by approximately 1000 - 1500 cm"1 from 77 K to the
highest temperatures at which emission can be detected. Emission
spectra are vibrationally featureless, and remain so to temperatures as
low as 10 K. The emission maximum of 2 by methods i, ii and iii vary

from 780, 750, and 730 nm, respectively. The reason for the variations are

92

Figure22

Electronic absorption spectrum (—) of 1 dissolved in CH2C12 at room
temperature, and corrected emission spectrum (- - -) of crystalline l at 77

K.

93

Emission Intensity

 

 

 

 

 

 

 

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“Gleam
“flu-o

t-mo t-N 1701/3

W22

750

450 550

350

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94

W23

Electronic absorption spectrum (——) of 2 dissolved in 01-12012 at room
temperature, and corrected emission spectrum (- - -) of crystalline 2 at 77

K.

)om

£77

95

Emission Intensity

 

 

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ld
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/
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750

 

 

450 550

350

 

 

250

 

teem
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96

Figure24

Electronic absorption spectrum (——) of 3 dissolved in CH2C12 at room
temperature, and corrected emission spectrum (- - —) of crystalline 3 at 77

K.

97

Emission Intensity

 

 

 

C

 

 

 

1 1 2
‘0. O. “2 O.
N N H II!

t-UIO t-N 701/3

0.5 -

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950

350 450 650 650 750
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98

not understood. Luminescence was not detected from fluid solutions of
any of the dirhodium complexes.

Insight into the nature of luminescence comes from time-resolved
spectroscopic measurements. The red emissions from 1, 2, and 3 display
similar behavior in that the lifetime decays are monoexponential and
long. Microsecond lifetimes for 1, 2, and 3 (Table 8) are a signature of
phosphorescence. Conversely, the decay of the higher energy emissions of
1 and 3 within the 8-ns temporal profile of the 355-nm excitation pulse
from a Nd:YAG laser suggests to us that the red phosphorescence is
accompanied by companion fluorescence. Interestingly, fluorescence
from 2 could not be detected even at our instrument's highest sensitivity.
In regard to the phosphorescence, the observed lifetime exhibits a
pronounced attenuation of emission from 1, 2, and 3 with increasing
temperature. Figures 25 - 27 display the temperature dependences of the
excited state decay rate constant of l, 2, and 3, respectively. In each case
there is a low temperature regime in which the decay rate constant
exhibits little variance then is followed by a sharp monotonic increase of
the rate with increasing temperature. The observed rates for 1 - 3 are fit
well (see Figures 25 - 27) by an expression for the decay constant based on
a two-state Boltzmann distribution,138

k1 + k2 GM‘AE/kBT)
0‘” ' 1 + exp(-AE/kBT)

 

(16)

where k1 and R2 are the decay constants for the two states in thermal
equilibrium separated by an energy gap AE. The calculated rate constant

for and the energy gaps are summarized in Table 9.

'99

Table 8. Emission Spectral Data for Crystalline 1, 2, and 3 at 77 K

 

 

Complex 11m”, nm 1', ns Am“, nm 7', us
1 540 < 8 840 53
2 ‘ --- --- 780 79
3 580 < 8 81) 287

 

' N o fluorescence is detected from solids of this compound.

100

Figure25

Fit of the variation of the observed emission decay rate constant to eq 16 of
l in the 10 - 180 K temperature range.

k obs/(*10-4)

101

 

 

 

 

 

160
TEMPERATURE (K)

Figure25

200

102

Figune26

Fit of the variation of the observed emission decay rate constant to eq 16 of

2 in the 10 - 200 K temperature range.

k obs/(*10-3)

103

 

20

10'

 

 

O
0.0.

 

 

 

TEMPERATURE (K)

100

Fis'tn'e26

200

104

Figure 27

Fit of the variation of the observed emission decay rate constant to eq 16 of
3 in the 10 - 290 K temperature range.

k obs/(*10-3)

40

105

 

 

 

 

100 200

TEMPERATURE (K)

Efinue27

 

300

106

Table 9. Calculated decay rate constants and energy gaps for 1, 2, and 3

 

 

Complex k1 / 8'1 k2 / s'1 AE / cm'1
1 1.1 x 104 1.1 x 106 903
2 3.5 x 103 6.6 x 106 12.58

s 3.0 x 103 2.3 x 105 1070

 

107

2. Discussion

Although cursory inspection of l, 2, and 3 suggests disparate
molecular and electronic structures, closer examination shows these
complexes to be quite similar. Trigonal bipyramidal and octahedral
coordination geometries, which are exclusively determined by the formal
oxidation state of the rhodium metal centers, are preserved almost
identically among the three complexes. Moreover, each of the binuclear
complexes possesses a rhodium-rhodium single bond; and it is this
bimetallic core and its associated axial ligands that appear to determine
the electronic structure of this homologous series of complexes.

Absorption bands characteristic of the allowed transitions for M-M
complexes dominate the electronic spectra of each of the dirhodium
complexes. The absorption spectrum of 1, typical of most single bonded
metal-metal dimers,138'139 is dominated by an intense isolated band in the
ultraviolet that is flanked by a broader less intense band to lower energy.
The intensity, bandwidth, and position of the 305-nm band are signatures
of a o 4 da“ transition. The significant mixing of the metal-metal do -+
do“ transition with axial ligand-to-metal charge transfer (LMCT)
transition, Lo -9 do“, which has previously been observed for a variety of
metal complexes containing L-M-M-L (M = Rh, Ir, Pt; L = halide, SCN ,
OH) cores,140 is not likely significant in 1 owing to the much higher lone
pair ionization potential of trifluorophosphine as compared to halides and
pseudohalides. Consequently, the o -9 do“ transition of 1 should be of
relatively pure ‘metal character. The lower energy, less intense
absorption is analogous to the dx‘ -> do“ transitions of a variety of M-M
complexes including Pt2(III,III) sulfates139 and pyrophosphites,14°a ‘

108

Rh2(II,II) acetates and isocyanides,14°b and dimanganese and dirhenium
carbonyls.14°‘144

The absorption profiles of 2 and 3 are more complicated. Because
the complexes are structurally unique, reliable assignment of the spectral
features in Figures 24 and 25 is difficult without undertaking detailed
spectroscopic measurements. Nevertheless, the spectra do display some
useful qualitative information. Transitions arising from the ligand based
o-orbitals of the axial chloride ligands, in addition to the primarily metal
center a -' do“ band, should be present. Inspection of Figures 24 and 25
indicates this to be the case; uv-absorptions possessing band shapes and
energies consistent with 6 transitions are observed. The appearance of
bands between 270 - 350 nm in the spectra of 2 and 3 is compatible to the
location of ligand-based and configurationally mixed metal-ligand
transitions of Pt2(III,III)Cl2 and Rh2(II,II)012 complexes.14o Moreover,
the shift of a -* do“ transitions to lower energies upon metal-ligand
mixing has been observed to increase the absorptivity of the dn" -’ do“
transition via intensity stealing mechanisms.140 The larger intensity of
the absorption bands lying in the spectral region in which du“ -> do“
transitions typically occur (400 - 450 nm) for complexes 2 and 3 as
compared to l is certainly consistent with the operation of similar a ->
du“ intensity stealing mechanisms.

Our observation that electronic absorption is governed by the o
M-M framework of 1, 2, and 3 is complemented by the luminescence
properties of these complexes. The pronounced narrowing of the
emission bands at lower temperatures is characteristic for luminescence
of do“ parentage.41’1"’1 Moreover, the invariance of the emission lifetime

at low temperatures followed by a monotonic decrease with increasing

109

temperature is a trend previously observed by our group for the do"
emission from Pt2(III,III)L2 (L = Cl, Br, and H20) tetraphosphates.42 An
excited stated model proposed for Pt2(III,III)L2 (L = Cl, Br, and H20)
tetraphosphates, and consistent with the observed temperature
dependance of the ha complexes, is shown in Figure 28. Photophysical
analysis of these Ptz complexes have revealed that the emissive excited
state results from promotion of an electron from the dn“ level to the do"
level. In this case the lowest energy excited state is 3(d1t*d6*)(3Eu), which
is split to yield a lowest energy (Blu,B2u) spin-orbit component. The
temperature dependence arises from the thermal population of a higher
energy deactivating state corresponding to the Eu(3Eu) spin-orbit
component. Similar behavior for the observed lifetime of the Pt2(III,III)L2
(L = Cl, Br, and I) pyrophosphites has been observed as well, and indeed,
nonradiative deactivation by Boltzmann population within the 3(dn“ -'
do“) spin-orbit manifold appears to be an emerging trend of do'*
luminescence for many M-M complexes. In the case of the dirhodium
series, measured energy gaps of 1,200 cm"1 shown in Table 9 conform
well with those experimentally determined for the (Blu,B2u) - Eu gap of
Pt2(III,III)L2 complexes and are in agreement with the 1000 cm'1
splittings predicted from first-order spin-orbit coupling calculations.42
Finally, our observation that luminescence is not detected from solutions
of 1, 2, and 3 at temperatures equivalent to those at which the crystalline
solids emit is consistent with recent photophysical studies demonstrating
the importance of medium rigidity as a crucial controlling factor of do“
luminescence."‘l'1“l

Insight into the structural and spectroscopic properties of

dirhodium complexes is provided by analysis of their electronic structure.

110

Figure28

Proposed energy diagram of the lowest energy excited states of the
Pt2(III,III)L2 phosphates. The state manifold is derived from the spin-
orbit coupling perturbation of the 3Eu state arising from the one-electron

3(d1t’“ -> do“) promotion.

 

111

 

 

Eu

1Eu (dfl*dc*)—

——(A lu’A‘lu)

 

Eu

3E1: (dfl*d0*)—_ —_<_
——1—-— (31,133“)

 

Figure28

112

We begin by considering the level structure of the C3,, Rh(0)P4 and C4,,
Rh(II)P3012 fragments. Molecular orbital treatments145'146 suggest that
eight electrons of the d9 Rh(0)P4 fragment reside in orbitals of 7t(dn, dyz)
and 5(dn, dx2_ ,3) symmetry with the remaining electron occupying the
o(d,2) orbital. In the case of the d7 Rh(II)P3C12 fragment, the d,2_ ,3 level
is displaced to very high energies owing to the destabilizing 0*
interactions of the metal with the ligands in the equatorial plane.
Consequently, the odd electron of the d7 Rh(IDP3012 fragment, as was the
case for Rh(0)P4, resides in the 0(d22) orbital with the remaining six
electrons residing in the lower energy 1|:(dxy, d32_ yg) and 8(dxy) orbitals;
low symmetry splittings within the 1|: orbitals have been ignored.
Construction of the level diagrams for 1, 2, and 3 is achieved by the orbital
mixing of the appropriate rhodium fragments. These results are shown
in Figures 29 - 31. The orbitals that will interact most strongly are those
that extend perpendicularly from the equatorial plane of the fragments,
namely the spatially directed 0(d22) orbitals. To a lesser extent the filled
lt-symmetry orbitals will interact and negligible overlap will occur
between the d-orbitals, which are aligned parallel to each other. In each
case, formation of a Rh—Rh single bond results from the pairing of the dzz
electrons of the individual fragments in the a orbital. To this end, the d-
electron configurations of 1, 2, and 3 are best represented as (d8)d1-d1(d8),
(d3)d1-d1(d5), and (d6)d1-d1(d6), respectively. Within this framework, we
see that l and 3 are isoelectmnic with the M—M prototypes Coz(CO)3147 and
Mn2(CO)m,148 respectively, and the existence of 2 formally completes the
Rh-Rh single bonded series.

On the basis of this molecular orbital model, our observation of

lowest energy excited states derived from the Rh-Rh core is not

113

surprising. The lowest energy allowed transitions are predicted to be do
-' do" and dx“ -* do“, and for 2 and 3 0(Cl) 4 do“. Although the do
manifold is purposely isolated from the levels of x- and 8-symmetries to
minimize congestion in Figures 29 - 31, absorption spectra of the ha
complexes clearly suggest a c-splitting of considerable magnitude such
that the lowest energy allowed transition in l, 2, and 3 is dlr“ -’ do". In
accordance with these considerations, our observation of emission from
3(dx" -* da“) states follows directly from the level diagrams illustrated in
Figures 29 - 31. Thus the similarity of the electronic absorption and
emission properties of the seemingly disparate ha complexes is easily
understood by the electronic structure models described in Figures 29 - 31.

The homologous series presented herein provides a basis for the
synthetic design of luminescent multielectron congeners. With the two-
electron mixed-valence Rh2(0,II) complex as a benchmark, the
preparation of its multielectron Rh2(0,0) and Rh2(II,II) counterparts can
be achieved such that a long-lived emissive da“ excited state can be
preserved. As is explicitly shown in Figures 29 and 30, conversion of the
octahedral Rh(II) center of 2 to the trigonal bipyramidal Rh(O) center of 1
stabilizes a formally highly energetic level that can accommodate the
addition of two electrons to the metal core. In this manner, the overall
electronic structure necessary for do" luminescence is preserved.
Furthermore, two-electron oxidation of the Rh(0) center of 2 does not
significantly perturb the o-framework because the formation of an
octahedral coordination geometry destabilizes the formally occupied d6
orbital. This strategy should be completely general for several M—M
systems contingent upon the successful preparation of the appropriate

mixed-valence intermediate. We believe that the torsional flexibility and

114

Figure29

Simple molecular orbital diagram for the interaction of two 03‘, Rh(O)P4
fragments. The dn- and d8-symmetry orbitals are filled and indicated by
the shaded box. To minimize level congestion, the 0-0* splitting is shown

to be small and its manifold is isolated from that of the dlt and d8 orbitals.

115

 

xz-y?‘ <——___> XZ-y2

 

xz,yz -1_L—i$= —1_L—’;{:xz,yz
xv —th—— % +xy

Figumzs

116

Figure 30

Qualitative level diagram for the interaction between C3v Rh(0)P4 and C4,,
Rh(II)P3C12 fragments. The dlt- and d5-symmetry orbitals are filled and
indicated by the shaded box. To minimize level congestion, the 0-0*
splitting is shown to be small and the o- manifold is isolated from that of
the dlt and d8 orbitals. Low symmetry splitting within the dlt level of the
C 4V Rh(II)P3012 fragment is not considered; interactions of the do and
do“ levels with the L0 and Lo“ orbitals are also not considered.

117

X2_y2

 

 

118

Figure 31

Qualitative level diagram for the interaction of two 04v Rh(II)P3012
fragments. The dn- and d8-symmetry orbitals are filled and indicated by
the shaded box. The o-o* splitting is shown to be small. Low symmetry
splittings within the (11: levels of the C4,, Rh(II)P3C12 fragments are not
considered; also the interactions of the do and do* levels with the Lo and

Lo* orbitals are not considered.

 

119

 

xz,yz:u:‘d,: Z #xzyz
x2- 2,>< # —1_L4¢:x2- 2.x
y y //// V Y

figure 31

120
the electronic properties of the bis(difluorophosphinoyalkylamine ligand

is crucial to our success in isolating the unusual ha mixed-valence

dimer.
C. Trirhodium Complex
1. Results and Discussion

a. Synthesis of Rh3(u-Cl)3[CH3N(PF2)2]3. We observed in the
preparation of 2 , the formation of small amounts of Rh3(ll-
Cl)3[CH3N(PF2)2]3, 4, when the reaction was not preformed in a large
excess of the ligand CH3N(PF2)2121. Pure Rh3(u-Cl)3[CH3N(PF2)2]3 is
obtained with a [RhC1(PF3)2]2:CH3N(PF2)2 ratio of 1:1, but the yield from
this reaction is low and unreacted [RhCl(PF3)2]2 is recovered. In 4, the
formal oxidation state of the rhodium atoms is Rh(I) and thus reaction
from the [RhCl(PF3)2]2 does not involve a formal change in oxidation state
of the Rh3 core. During our studies the formation of 4 was reported by
reaction of CH3N(PF2)2 with [RhCl(CO)2]2 with a mole ratio of 1:2.121

b. Structural Interpretation. The Rh3(ll--Cl)3[CH3N(PF2)2]3
crystallizes in two different forms, one being in the space group CZ/c, and
the other in the space group ana. Mague's crystal structure of Rh3(ll-
C1)3[CH3N(PF2)2]3 was for the monoclinic space group CZIc.121 In our
solution of 4 in the ana space group, P(2)and F(2) atoms were modeled
in two different sets of position with each disordered atom assigned an
occupation of 0.5. The crystal parameters and detail of intensity for Rh3(ll-
Cl)3[CH3N(PF2)2]3 in the two different space groups are listed in Table 10.
The positional parameters for 4 are listed in Table 11 and 12 (CZ/c and

Table 10. Crystal Data for Rh3(u-Cl)3[CH3N(PF2)2]3

121

 

 

4 4'
“nma“ 31131532P 6N3C3H9 Rh3F12P 6N 303119
formula weight 916.02 916.02
crystal dimensions, mm 0.40 x 0.52 x 0.82 0.20 x 0.36 x 0.56
crystal system monoclinic orthorhombic
space group C 2/ c P nma
unit ce parameters

a, 17 339(2) 10.055( 1)

b, A 11.028(1) 17.167(2)

c, 23.264(4) 13.010(3)

a. deg 90 90

B. deg 9300(1) 90

7, deg 90 90

V, A3 4442.3(10) 2245.7(6)

Z 8 4
p,,1,,, g/cm3 2.77 2.71
ll(Mo Kn), cm‘1 15.3 30.3
radiation 0., A) Mo Ks(0.71073) Mo Ka(0.71073)
temp. °C 24(1) 24(1)
scan method to 0 - 20
scan rate, deg min' 4 2
no. of unique data, total
with F,2> 3o(F,2) 8085,6369 2679,2188
no. of parameters refined 272 170
trans. factors, min, max 0.275, 0.722 0.934, 0.996
R “ 0.041 0.026
17,,” 0.046 0.027
GOF c 2.79 1.86

 

“R = 2| IF,I-IF,I I/SIF,I. ”12w: [2w(IF,I-IF,I)2/zwIF,I2]1’2; w =
1/02( IF, I ). ° Goodness of fit = [2w( IF, I - IF, I )2/(N,,,, - N,,,,,,,,,,,)]1’2.

122

Table 11. Atomic Positional and Isotropic Displacement (A2 ) Parameters
for Rh3[CH3N(PF2)2]3C13 (4) in the CZ/c Space Group ‘

 

 

Atom x y z BIA2
Rh(l) 0.19476(2) 0.29074(4) 0.31801(2) 2.449(6)
Rh(2) 0.36732(2) 0.34791(4) 0.34005(2) 2.464(6)
Rh(3) 0.25356(2) 0.40886(4) 0.43425(2) 2.585(7)
C1(1) 0.30475(9) 0.1597(1) 0.31423(7) 324(3)
C1(2) 0.36920(9) 0.2870(2) 0.43896(6) 344(3)
01(3) 0.17685(9) 0.2304(1) 0.41562(6) 324(3)
P(l) 0.20988(8) 0.3145(1) 0.22814(6) 236(2)
F(2) 0.36852(8) 0.3925(2) 0.25044(6) 245(2)
P(3) 0.43548(8) 0.4990(2) 0.36965(6) 2.92(3)
P(4) 0.32523(9) 0.5628(2) 0.45477(6) 270(3)
P(5) 0.14685(9) 0.5051(2) 0.43418(7) 2.91(3)
P(6) 0.09801(8) 0.4099(2) 0.32091(6) 2.53(2)
F(la) 0.1498(2) 0.3832(4) 0.1886(2) 3.63(8)
F(lb) 0.2052(3) 0.1942(4) 0.1927(2) 420(8)
F(Za) 0.3937(3) 0.5201(4) 0.2310(2) 5.04(9)
F(2b) 0.4282(2) 0.3219(5) 0.2163(2) 5.1(1)
F(3a) 0.4611(3) 0.5996(4) 0.3282(2) 4.73(9)
F(3b) 0.5185(2) 0.4649(5) 0.3928(2) 5.4(1)
F(4a) 0.3523(3) 0.5737(5) 0.5191(2) 5.2( 1)
F(4b) 0.2923(3) 0.6946(4) 0.4496(2) 5.2(1)
F(Sa) 0.0953(3) 0.4755(6) 0.4852(2) 5.7(1)
F(5b) 0.1447(3) 0.6448(4) 0.4434(2) 5.7(1)
F(6a) 0.0167(2) 0.3486(4) 0.3129(2) 430(9)
F(6b) -0.0637(3) -0. 1279(2) 0.8524(2) 4.80(6)
N( 1) 0.3561(4) 0.0793(2) 1.1421(3) 422(9)
N(2) 0.4085(3) 0.5826(6) 0.4244(2) 3.4(1)
N(3) 0.0624(5) -0.250 0.9543(4) 3.5(1)
C(1) 0.3704(7) 0.0138(3) 1.2193(4) 6.4(2)
C(2) 0.4611(5) 0.6820(8) 0.4465(4) 5.8(2)
0(3) 0.2109(6) .0350 0.9683(6) 4.8(2)

 

‘ Anisotropically refined atoms are given in the form of the isotropic

equivalent displacement parameter defined as #3[a2811 + 52322 + c2333 +

ab(cosy)312 + ac(cos ”13 + bc(cos (1)823].

123

Table 12. Atomic Positional and Isotropic Displacement (A2)
Parameters for Rh3[CH3N(PF2)2]3CI3 (4') in the ana Space Group“

 

 

Atom 1: y z BIA2
Rh(l) 0.22593(3) 0.15806(2) ’ 0.93327(2) 3.001(5)
Rh(2) 0.44321(5) 0.250 1.4612(4) 3.288(9)
C1(1) 0.4634(1) 0.15253(8) 0.91400(9) 405(2)
01(3) 0.2093(2) 0.250 0.7958(1) 360(3)
P(l) 0.2500(1) 0.06797(7) 1.0450(1) 361(2)
P(Za) 0.4156(3) 0.1658(2) 1.1665(2) 359(6)
P(2b) 0.4546(2) 0.1567(1) 1.1535(1) 356(4)
P(6) 0.0141(1) 0.164550) 0.94420(8) 300(2)
F( 1a) 0.1285(3) 0.0336(2) 1.1032(3) 591(7)
F(lb) 0.2985(4) 0.0113(2) 1.0013(3) 597(8)
F(2aa) 0.342(1) 0.1887(5) 1.2632(6) 6.2(2)
F(2ab) 0.5488(9) 0.1370(6) 1.2203(7) 6.6(2)
F(2ba) 0.4399(7) 0.1724(3) 1.2705(3) 7.2(2)
F(2bb) 0.5930(5) 0.1182(4) 1.1613(5) 7.0(1)
F(6a) 0.0608(3) 0.1189(2) 1.0294(2) 4.78(6)
F(6b) 0.0637(3) 0.1279(2) 0.8524(2) 480(6)
N( 1) 0.3561(4) 0.0793(2) 1.1421(3) 422(9)
N(3) 0.0624(5) 0.250 0.9543(4) 3.5(1)
C(1) 0.3704(7) 0.0138(3) 1.2193(4) 64(2)
0(3) 0.2109(6) 0.250 0.9683(6) 4.8(2)

 

“ Anisotropically refined atoms are given in the form of the isotropic

equivalent displacement parameter defined as 413012811 4» 52322 + c2333 +

ab(cos 30812 + ac(cos ”13 + bc(cos 00323].

124

ana), respectively. Whereas the crystal structure with the space group
CZ/c possesses no symmetry element, the ana space group possesses a
crystallographic mirror plane passing through atom Rh(2), 01(3), N(3),
and 0(3).

The bond distances and bond angles for 4 are listed in Tables 13 and
14 (CZ/c and ana), respectively. Because there is only a slight difference
between the bond distances and angles for the two crystal structure the
crystal structure of 4 with the space group CZ/c will be only described in
detail.

The ORTEP of 4 is shown in Figure 32. The complex is constructed
from a triangle of rhodium atoms, which are linked together by a bridging
bidentate CH3N(PF2)2 ligand, and a chlorine atom. The average Rh---Rh
bond separation is 3.099 A, and is consistent with Rh3 trimers [Rh3(u3-
8),(p,sxp,-CI)2(pEt3),lpr,149 and [Rb3(lIg-E)2(CO)6]' (E = s or 85150 (3.202
and 3.050 A) which do not have a single metal-metal bond. The average
bridging Rh-Cl bond distance is 2.402 A, which is similar to the bridging
Rh-Cl distance of 2.38 A in the MG) dimer of [RhCl(CsH 1912.151 The
coordination about each rhodium atom is approximately square planar.
The average P—Rh—Clcis, P—Rh-P
values 87.21°, 89.88°, 95.59°, and 175.03° respectively) are consistent with

cis’ and P—Rh—Pmm, angles (average
the square planar geometry of regular Rh(I) monomers. The Rh atom
lies effectively in the RhP2C12 plane with the average displacement out of
the square planar unit RhP2012 of only 0.07 A.

The closest structural analogy to 4 is Rh3(u-H)3[P(0Pri)3]6, where
the geometry around each rhodium atom is also square planar.152 This
contrasts 4, in which all the chlorine atoms are on the same side of the

Rh3 plane. Thus the dihedral angles between the square-planar RhClsz

125

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127

Figure 32

A perspective ORTEP view and labeling scheme of Rh3(u-
Cl)3[CH3N(PF2)2]3, 4, with the thermal ellipsoids at the 50% probability

level. For clarity hydrogen atoms are not shown.

128

Figure32

129

and Rh3 triangular plane are fairly consistent (120.38(2)°, 120.83(3)°, and
121.04(3)°, respectively) and show little variance.

c. Electronic Absorption and Emission Spectra. The electronic
absorption and emission of 4 is shown in Figure 33. The absorption
spectrum of 4 displays a pronounced absorption in the ultraviolet spectra
region, 1 = 266 nm, e = 1.31 x 104 W1 cm-1 and x = 285 nm(sh), s =
1.10:103 M‘1 cm’l, and a broad absorption band in the visible region (1 =
415 nm, e = 1.24 x 104 M"1 cm’l). Since there are no rhodium metal-metal
bonds in 4, none of the absorption bands can be ascribed to o -* 0*
transitions centered in the Rh3 core. Indeed, the absorption spectrum of 4
is similar to the Rh(I) d8 monomers.153'155 The low energy visible

absorption band of Rh(CO)Cl(PPh3)2154 and TBA[Rh(P(OPPh3)2)mnt] (TBA

)155 were assigned

= tetra-n-butylammonium, mnt = maleonitriledithiolate
as metal-to-ligand charge transfer transitions (d -’ 1t and d-+ 3*),
respectively. For the case of square planar dimers Rh(I) dam (dam =
bis(diphenylphosphino)methane) and dppm (bis(diphenylphosphino)-
methane) the assignment is not as clear. Several groupsl56'157 initially
assigned the low energy absorption as MLCT transition primarily because
of their high intensities. Latter the assignment was changed to metal-
centered [0*(4dz2) 4 0(5Pz)] transition,158'159 because the low-energy
absorption maximum is not greatly affected by replacement of the dppm
with dam, by change of environment from solid to solution, or by a change
in temperature. In the case of 4 the visible absorption is probably due to a

metal-to-ligand transition because there is little or no overlap of the
metals 4(122 and 5pz orbitals.
The emission of 4 is shown in Figure 26. Red luminescence is

observed from solids at low temperatures, upon excition with frequencies

130

Figure33

Electronic absorption spectrum (—) of 4 dissolved in CH2C12 at room
temperature, and uncorrected emission spectrum (- - -) of crystalline 4 at

77K.

131

EMISSION INTENSITY

 

 

A
"m

 

 

 

 

1.5 -

1
°. '9
p o

g-WO t-W '0 U3

Mom

 

132

coincident with absorption manifold. The emission is quite broad and is
centered at 850 nm. The broading is probably due to the difl'erent isomers

of Rh3(p-Cl)3[CH3N(PF2)213-

CHAPTERIV

Preparation of
Dimolybdenum(VN) Dimers

A. Background

The study of quadruply bonded dimers is an active area of research
in inorganic chemistry. The spectroscopic properties of quadruply bonded
dimers have been intensely investigamd.33’64 A general result to emerge
from these studies is that the lowest electronic transition is the 82 -* 1(825*)
transition, which typically falls in the visible region (3. = 500-700 nm). The
complexes [M2X’8]“’ (M = Re, Mo; X = Cl, Br) do not exhibit mirror image
absorption and emission spectra. Two possible explanations for this are
(1) that the emitting state is not 1A2,,(155m) but rather a 3E8 or 3A28 state or
(2) the absorption transition in [MzXBP' produce 1A2“ (88*) MMCT states
with eclipsed geometry and the emission originates from 1(85*) MMCT
states of staggered (D4 or D4,!) geometry.

In support of the second hypothesis are the spectra of M02X4(PR3)4
complexes (X = Cl, Br, I; R = alkyl).61 The emission bands in these
molecules are mirror images of their 82 -* 1(55".) absorption profiles. The
bulky phosphine ligands in these quadruply bonded M02 dimers are
believed to create a large steric barrier to rotation about the metal-metal
bond. Hence, the 1(55*) excited state is likely to retain the eclipsed ground
state geometry and produce well-behaved luminescence. Athough
knowledge of the luminescence properties is emerging, a reliable
synthetic route to quadruple bond complexes possessing long-lived
luminescence has not been developed.

Spectroscopic studies during the recent years have demonstrated
metal complexes bonded by strong n-acids having low-lying metal-to-
ligand charge transfer states which are long-lived and highly
emissive.155'16°'162 The u-acid ligand 2,2'-bipyridine (bpy) and

133

134

maleonitrile dithiolate mntz' can induce long-lived, low lying metal-to-
ligand charge transfer excited states owing to their good electron
accepting properties. The metal-to-ligand charge transfer excited state of
Ru(bpy)32+ is at 590 nm with a quantum yield of 0.042 and a lifetime of 580
us at 25 °C.160 The metal-to-ligand charge transfer excited state for
Pt(PPh3)2mnt is at 650 nm with a lifetime of 24 us at 77 K155 Thus we
were interested in ascertaining whether the emissive properties of the
quadruply bonded dimers could be enhanced by incorporation of lowest
energy MLCT states in the excited state manifold of quadruply bonded
cores ligated by (bpy) and mntz’.

B. Results and Discussion

1. Reacfionoantz'withQuadruplyBondedMetal Complexes

A common precursor in the preparation of molybdenum quadruple
bonded dimers is M02(OZCCH3)4. Substituted complexes are generally
synthesized by using Me3SiCl to remove the acetate ligands.162 However
the reaction of Nazmnt with M02(0200H3)4 in the presence of Me3SiCl
yields decomposition products. In the absence of Me3SiCl, green and
orange precipitates form. The electronic and infrared spectra of the green
precipitate is shown in Figures 34 and 35, respectively. The absorption
spectrum is identical to that of [(C4H9)4]2Mo(mnt)3.163 The orange
precipitate, which was characterized by infrared, U.V.-visible
spectroscopy, elemental analysis, and X-ray crystallography, is unique.
Infrared, U.V.-visible spectroscopy, and elemental analysis all support a
complex with a M02(O)(S)(u-S)2 core.

135

Figum34

Electronic absorption spectrum of [(C4H9)4]2Mo(mnt)3 dissolved in CHZClz.

 

/

 

136

4!

X
N

_‘:':>

1
750

250

 

 

SONVSHOSSV

Figure34

Mum

137

Figure35

Mid IR spectrum of [(C4H9)4]2Mo(mnt)3.

 

138

can

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2: 8. 2...: 82 8: 82 .8. 8.. 82 83 88
q q q d 4 1 J 4
1 8
1 3
guild 8
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BONVLLMSNVUL '56

Figure35

139

The IR spectrum (Figure 36) shows a band at 955 cm“1 and 530 cm'
1, which are typical bands for terminal Mo=0 and Mo=S stretching
vibrations.164'165 A weak band at 470 cm’1 and a band at 340 cm'1 are
assigned to the stretching vibration of di-u-sulfido bridge. The typical
stretching vibrations due to the di-u-sulfido bridge are medium intensity
bands at ~ 450 cm'l, and ~ 350 cm‘l.164

The electronic spectrum of the proposed M02(0)(S)(u-S)2 species is
shown in Figure 37. The spectrum exhibits strong absorption in the
ultraviolet region with less intense and unresolved absorption bands in
the visible region. The electronic spectrum is similar to other compounds
containing a M02(O)(S)(u-S)2 core.166'167

The structure of the complex was unequivocally determined by
solving the X-ray crystal structure. The crystal parameters and details of
intensity collections are listed Table 15; the positional parameters for the
anion and cations are listed in Table 16 and 17, respectively. The ORTEP
of the structure is shown in Figure 38. Consistent with spectroscopic
data, the compound was determined to be [(C4H9)N]2[M02(O)(S)(u-
S)2(mnt)2]. The structure of the [Mo2(0)(S)(u-S)2(mnt)2]2’ dianion is very
similar to that of the dianion [M02(O)2(u-S)2(i-mnt)2]2' reported by Gelder
and Enemark.168 The geometry about each of the molybdenum atoms in
[M02(O)(S)(u-S)2(mnt)2]2' is best described as a distorted square pyramid
with terminal oxygen and sulfur atoms in the apical positions; the two
bridging sulfur atoms and the two sulfur atoms from 1,2-dicyanoethylene-
2,2-dithiolate (mnt) ligand form the basal plane of each square pyramid.
The bond distances and bond angles of [M02(O)(S)(u-S)2(mnt)2]2' and the
cations are listed in Tables 18 and 19, respectively. A Mo(V)---Mo(V)
separation of 2.858(1) A, is typical of other Mo(V)---Mo(V) dimers and

140

Figune36

Mid IR spectrum of [(C4H9)N]2[M02(O)(S)(p-S)2(mnt)2].

141

 

 

 

1 I I

250

 

 

8 3 3

BONVLIIRSNVUJ. 96

Figure36

ICU")

WAVENUMBEH

142

Figure 37

Electronic absorption spectrum of [(C4H9)N]2[M02(O)(SXu-S)2(mnt)2]
dissolved in CHzClz.

143

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144

Table 15. Crystal Data for [M02(0)(SXu-S)2(mnt)2]2’ and
M0204C14(dmbpy)2

 

 

8 9
formula M02N6087C40H72 M02C1204N4C24H24
formula weight 1069.35 695.27
crystal dimensions, mm 0.11 x 0.30 x 0.50 0.10 x 0.15 x 0.20
crystal system monoclinic trigonal
space group P2 1/ c P 3121
unit cell parameters

a, A 19.547(4) 16.135(4)

b, A 15210(4) 16.135(4)

c, A 18.754(6) 10.709(3)

0:, deg 9) 90

,6, deg 101.69(2) 90

7, deg 90 120

V, A3 5460(2) 2414.4(13)

Z 4 3
pcalcd’ g/cm3 1.24 1.43
mm Ka), cm'1 6.3 9.6
radiation (A, A) Mo 114071073) M0 Ka(0.71073)
temp, °C 27(1) 25(1)
scan method 9 - 20 0 - 26
scan rate, deg min'1 2 4
no. of unique data, total
with F,2> 30(F,2) 7168,4834 2109,1243
no. of parameters refined 505 167
transmission factors, min, max 0.844, 0.935 0.877, 0.916
R“ 0.046 0.042
R,” 0.049 0.051
GOFc 3.16 1.68

M
“R=Z| IF,l-IF,| l/ZIF,|. bRw=[Zw(lF,l-IF,|)2/ZwlF,l 1 2;w=
1/02( IF, I). c Goodness of fit = [Zw( IF, I - IF, I )2/(N,,,, - N,,,,,,,,,,,)]1/2.

145

Table 16. Atomic Positional and Isotropic Displacement (A2) Parameters for
Mo2(OXSXuS)2(mnt)2]2' ’

 

 

Atom x y z B/A2
Mo( 1) 0.25023(3) 0.12794(5) 0.04924(3) 4.49( 1)
140(2) 0.395446) 0.12021“) 0.04492(3) 4.28( 1)
8(1) 0.1784(1) 0.2575(2) 0.0499(1) 6.29(6)
8(2) 0.193280) 0.0852(2) 0.1482(1) 5.91(5)
8(3) 0.3173(1) 0.2341(1) 0.0063(1) 456(4)
8(4) 0.3370(1) 0.0463(1) 0.1228(1) 5.19(5)
8(5) 0.4742(1) 0.2407(1) 0.0338(1) 5.18(5)
8(6) 0.4919(1) 0.0782(1) 0.1423(1) 535(5)
0(1) 0.2095(3) 0.0630(4) 0.0249(3) 6.4( 1)
8(7) 0.3998(2) 0.0371(3) 0.0338(2) 258(8)
N (1) 0.0143(4) 0.3357(6) 0.0985(6) 11.8(3)
N(2) 0.0378(4) 0.1201(7) 0.2253(5) 10.1(2)
N(3) 0.6599(4) 0.3047(5) 0.0702(5) 9.3(2)
N(4) 0.6833(4) 0.0976(5) 0.2063(5) 8.3(2)
C( 1) 0.0604(5) 0.2885(7) 0.1009(6) 8. 1(3)
C(2) 0.0757(4) 0.1371(7) 0.1883(5) 7.2(2)
C(3) 0.1172(4) 0.2290(6) 0.1019(5) 5.9(2)
C(4) 0.1242(4) 0.1564(5) 0.1438(4) 5.6(2)
C(5) 0.5561(4) 0.2088(5) 0.0823(4) 50(2)
0(6) 0.5627(4) 0.1389(5) 0.1284(4) 5.1(2)
C(7) 0.6145(4) 0.2620(5) 0.0751(5) 6.2(2)
C(8) 0.6303(4) 0.1165(6) 0.1712(5) 6.1(2)

 

' Anisotropically refined atoms are given in the form of the isotropic equivalent
displacement parameter defined as 4/3lazB 11 + 62322 + c2B33 + ab(cos ”312 + ac(cos

”13 + Moos @823].

Table 17. Atomic Positional and Isotropic Displacement (A2 ) Parameters for the

146

Tetrabutzlammionium Ions"

 

Atom x y z BIA2
N(5) 0.3848(3) 0.3007(4) 0.2723(3) 5.2(2)
N(6) 0.1311(3) 0.2880(5) 0.2379(4) 6.1(2)
C(1) 0.0604(5) 0.2885(7) 0.1009(6) 8.1(3)
C(2) 0.0757(4) 0.1371(7) 0.1883(5) 7.2(2)
C(3) 0.1172(4) 0.2290(6) 0.1019(5) 5.9(2)
C(4) 0.1242(4) 0.1564(5) 0.1438(4) 5.6(2)
C(5) 0.5561(4) 0.2088(5) 0.0823(4) 5.0(2)
0(6) 0.5627(4) 0.1389(5) 0.1284(4) 5.1(2)
C(7) 0.6145(4) 0.2620(5) 0.0751(5) 6.2(2)
C(8) 0.6303(4) 0.1165(6) 0.1712(5) 6.1(2)
C(9) 0.3260(4) 0.3346(6) 0.3070(4) 6.1(2)
C(10) 0.2538(5) 0.3074(8) 0.2719(5) 9.6(3)
C(11) 0.1966(6) 0.3434(9) 0.3065(6) 12.4(4)
C(12) 0.171(1) 0.417(1) 0.2832(9) 27.1(9)
C(13) 0.4530(4) 0.3341(6) 0.3165(4) 6.2(2)
C(14) 0.5182(5) 0.3060(7) 0.2899(5) 8.2(3)
C(15) 0.5824(5) 0.3296(8) 0.3409(7) 10.9(4)
C(16) 0.6481(6) 0.3099(8) 0.3131(8) 13.2(5)
C(17) 0.3752(5) 0.3329(5) 0.1938(4) 6.0(2)
C(18) 0.3760(5) 0.4322(6) 0.1838(4) 7.3(3)
C(19) 0.3644(7) 0.4575(6) 0.1063(5) 9.9(3)
C(20) 0.3656(7) 0.0946(6) 11.2(4)

0.5549(7)

147

 

Table 17 (cont).

Atom 2 y z BIA2
C(21) 0.3841(5) 0.2014(5) 0.2695(4) 5.8(2)
C(22) 0.3975(6) 0.1551(6) 0.3432(5) 7 .9(3)
C(23) 0.4059(7) 0.0579(6) 0.3363(5) 9.8(3)
C(24) 0.4236(7) 0.0135(8) 0.4060(6) 12.8(5)
C(25) 0.1155(4) 0.2367(6) 0.1731(5) 6.8(2)
C(26) 0.1782(5) 0.2051(7) 0.1185(5) 8.9(3)
C(27) 0.1549(7) 0.1631(8) 0.0550(6) 12.5(4)
C(28) 0.2099(8) 0.149(1) 0.0067(7) 17.0(6)
C(29) 0.1664(4) 0.2299(7) 0.2851(5) 7.7(3)
C(30) 0.1248(6) 0.1536(7) 0.3211(6) 10.4(4)
C(31) 0.1634(7) 0.1057(9) 0.3705(7) 14.5(5)
C(32) 0.126(1) 0.042(1) 0.4169(9) 25(1)
C(33) 0.1795(4) 0.3651(6) 0.2130(5) 7 .0(2)
C(34) 0.1532(5) 0.4316(6) 0.1661(5) 8.2(3)
C(35) 0.2091(6) 0.5009(7) 0.1431(6) 10.3(4)
C(36) 0.1897(8) 0.5699(9) 0.0977(7) 16.3(6)
C(37) 0.0613(4) 0.3193(5) 0.2794(5) 7.2(3)
C(38) 0.0642(5) 0.3708(8) 0.3475(6) 10.8(4)
C(39) 0.0173(8) 0.382(1) 0.3905(8) 28.2(7)
C(40) 0.0162(9) 0.424(1) 0.435(1) 33(1)

 

‘ Anisotropically refined atoms are given in the form of the isotropic equivalent

displacement parameter defined as “[028“ + 52322 + c2833 + ab(cos M12 + ac(cos 81813

+ M008 (1)823].

148

Figure38

ORTEP drawing and numbering scheme of [(C4H9)N]2[M02(O)(S)(u-
S)2(mnt)2] with 50% probability thermal ellipsoids. For clarity (C4H9)4N+
ions are not shown. Selected bond distance and angles are listed in Table

18.

149

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Figure38

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153

indicative of a single Mo—Mo bond. An average Mo—Sb—Mo (Sb = bridging
sulfur) bond angle of 7599", an average bond distance of 2.322 A, and an
average Sb—Mo—Sb angle of 100.47° are in the range of those observed for
other compounds with M02(S)2 bridges.165'170 Dihedral angles between
the two Mo(Sb)2 planes, the two Mo(Sb)2 triangles, and the two basal
planes of the dimer are 148.43(11)°, 25.51(11)°, and 154.60(10)°, respectively.
Each molybdenum atom is 0.707 A above the basal plane towards the axial
atoms. Similar values were observed for previously reported examples of
complexes with [M02XY(u-S)2]2+- (X,Y = O and/or S) cores, as illustrated in
Table 20 and 21.165’166'170 Despite these similarities, the apical Mo=X (X =
0,8) distances of the [M02(O)(S)(u-S)2(mnt)2]2' dimer are different from
typical [M02XY (1.1-Sb]2+ cores. The observed Mal-01 = 1.759(5) A, M02-S2 =
1.958(4) A) are much shorter then average Mo-O and Moss bond distance
of 1.687 A and 2.111 A, respectively. The good x-accepting properties of
the mntz' ligand will accept electron density from the metal thereby
enhancing the Mo-O and M-S n-donating interaction. This synergistic
effect between the u-accepting and terminal n-donor ligands may well
explain the short terminal bond distances.

The origins of the bridging sulfur and terminal sulfur and oxygen
atoms are not resolved. An unidentified intermediate could be oxidized
with molecular oxygen to form the terminal oxygen bond. However, when
the reaction was performed under strict air free condition, a small
amount of [M02(O)(S)(u-S)2(mnt)2]2' still formed indicating that the
oxygen for the molybdenum oxo bond may be delivered upon oxidation of
the acetate ligand of the starting complex. Similarly the bridging and
terminal sulfur must come from the oxidation of the Nazmnt ligand. The
solution of Mo(mnt)32and [M02(O)(S)(u-S)2(mnt)2]2 do not interconvert

 

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156

and, therefore it is unlikely that they are produced along a concerted
reaction pathway.

To avoid oxygen addition to the quadruple bonded metal core
M02(02CCH3)4 was replaced by M02C14(dppm)2 as a precursor complex.
The reaction of M02014(dppm)2 with Nazmnt yields a dark green solid
which has not been identified. The mid IR spectrum of this unknown
solid is shown in Figure 39. A band at 2200 cm'1 is assigned to the CEN
stretching vibration. The far IR spectrum of the compound is shown in
Figure 40. No Mo-Cl stretching vibration or bands due to Mo=0, Mo-O,
Mo=S, or Mo-S stretching vibrations are observed.

The absorption spectrum of the product is shown in Figure 41, and
is similar to M02C14(dppm)2.107 The band at 634 nm (e = 2490 M‘lcm’l) in
M02Cl4(dppm)2, which is attributed to the 82 -’ 88" transition, is red shifted
to 650 nm (e = 1307 M‘lcm’l). Higher energy bands are not as well defined
as in M02C14(dppm)2. Nevertheless, these data suggest that the
quadruple bond core is preserved and mnt appears to be attach to the core.

2. Reaction of Bipyridine with QuadruperondMetal Complexes

Reaction of bypyridine with M02014(CH3CN)4 leads to the generation
of insoluble products. To this end, we turned our attention to dmbpy with
the goal of inducing greater solubility. The preparation followed
procedures similiar to those for the preparation of M02014(bpy)2.
Although the coumpound is believed to be M02014(dmbpy)2, it was not fully
characterized. The compound was very susceptible to oxidation and
refluxing in o-dichlorobenzene led to the formation of a red solution from
which crystalline red solid is isolated. A single X-ray quality crystal was

157

Figure39

Mid IR spectrum of the green compound isolated from the reaction of
M02Cl4(dppm)2 with Naz(mnt).

158

 

 

4A

 

 

400

 

 

woz<._..::mz<m._. 8

WAVENUMBEH (CM‘l)

Figure39

159

Figm‘e 40

Far IR spectrum of the green compound isolated from the reaction of
M02C14(dppm)2 with Na2(mnt).

% TRANSMITTANCE

160

 

50

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35*

30"

 

 

 

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Figure40

350
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250

161

figure 41

Electronic absorption spectrum of M02(dppm)2(mnt)2 dissolved in CH2C12.

162

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163

obtained, and was characterized as M0202C12(u-0)2(dmbpy)2. The
crystallographic data is listed in Table 15, and the positional parameters
are listed in Table 22. The ORTEP diagram of M0202C12(u-0)2(dmbpy)2 is
shown in Figure 42. The M0204 core of the dinuclear complex consists of
two terminal oxo ligands in a cis-arrangement and two bridging oxygen
forming a non-planar ring. The coordination around each Mo atom is
completed by a Cl atom and a chelating 4,4'-dimethy1-2,2'-bipyridy1 ligand.
The bond lengths and bond angles for M0202C12(u-O)2(dmbpy)2, listed in
Table 23, compare favorably to the complex M0202012(u-O)2(bpy)2 which
has been prepared by several methodsrn'173 The Mo-Mo bond distance of
2.562(2) A in M0202C12(u-0)2(dmbpy)2 indicates a strong Mo-Mo single
bond.173 The Mo-Ob-Mo (Ob = bridging oxygen) angle (83.2°) and Ob-Mo-Ob
angles (91.7°) are similar to M02C14(bpy)2, as are the MoOt (01; = terminal
oxygen) and MoCl bond distances of 1.682(7) A, and 2.451(4) A,
respectively.173 The dihedral angle between the two Mo(Ob)2 planes of the
dimer is 144.89°(30) between that of the two Mo(Ob)2 triangles is 35.11°(30).
With this x-ray structure, the infrared and absorption spectra are
easily interpreted. The infrared spectrum of M0202012(u-0)2(dmbpy)2
(Figure 43) shows a band at 965 cm which is typical of Mo=O stretching
vibration. The bands at 760 cm'1 and 440 cm'1 are due to the
antisymmetric and symmetric stretching vibration of the Mo-(Ob) bond,
respectively.171'172 The electronic spectrum for M0202C12(u-O)2(dmbpy)2 is
shown in Figure 44 and is similar to compounds containing a M0204
core071,172
There has been a great interest in coordination compounds of

molybdenum, because of their possible relationship to redox-active

molybdo- enzymes for which there is evidence that two molybdenum

 

 

 

Table 2

 

for M02

 

N(10)
C(2)
C(3)
C(4)
C(5)
C(Saf
C(63)
C(7)
0(8)
0(9)
C(10;
C(41;
C(71)

a

equiv;

ab(COS

164

Table 22. Atomic Positional and Isotropic Displacement (A2) Parameters
for M02(0 )2012(u-0)2(dmbpy )2‘

 

 

 

Atom x y z BIA2
Mo( 1) 0.56661(7) 0.61177(7) -0.1041(1) 337(2)
C1( 1) 0.5769(3) 0.5762(3) 0.3242(4) 7.1(1)
0(1) 0.6588(5) 0.7238(6) 0.1224(9) 5.4(3)
0(2) 0.6070(5) 0.5212(5) 0.0647(8) 3.6(2)
N( 1) 0.4168(7) 0.4853(7) -0.140(1) 4.5(3)
N(10) 0.4645(6) 0.6653(6) 0.1456(9) 3.3(2)
C(2) 0.400(1) 0.3989(9) 0.150(2) 68(5)
0(3) 0.313(1) 0.328(1) -0.173(2) 8.1(6)
C(4) 0.2328(8) 0.3370(8) -0.179(1) 4.4(3)
C(5) 0.2508(9) 0.4279(8) -0.168(1) 4.3(4)
C(5a) 0.3419(8) 0.501(1) -0.151(1) 3.7(3)
C(6) 0.3040(8) 0.6384(8) -0.153(1) 4.0(3)
C(7) 0.3353(8) 0.7331(7) -0.153(1) 3.2(3)
C(8) 0.4319(7) 0.7944(8) 0.154(1) 3.5(3)
C(9) 0.494(1) 0.7586(9) -0.148(1) 4.3(4)
C( 103) 0.3681(8) 0.6018(9) -0.145(10 3.5(3)
C(41) 0.134(1) 0.254(1) -0.189(2) 8.3(6)
C(71) 0.2651(7) 0.7667(9) -0.160(1) 5.1(4)

“ Anisotropically refined atoms are given in the form of the isotropic
equivalent displacement parameter defined as 4/3[azB 11 + b2822 + c2333 4-

ab(cos M12 + ac(cos [£313 + bc(cos 00323].

165

Figqu

ORTEP drawing and numbering scheme of M0202C12(u-O)2(dmbpy)2 with

50% probability ellipsoids. For clarity hydrogen atoms are not shown.
Selected bond distances and angles are listed in Table 23.

166

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Figure43

Mid IR spectrum of M0202012(u-O)2(dmbpy)2.

 

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170

Figure44

Electronic absorption spectrum of M0202012(u-O)2(dmbpy)2 dissolved in
CH3CN.

171

 

 

2X

J

 

 

moz<mm0mm<

600

400

300

Mum

Figure“

172

atoms are at the active site.174 The Mo(V) dimers (MoYL)2X2 (Y = 0, X2 =
02, S2, OS and Y = X = S) are of interest as models for such enzymes.
There are few general synthetic designs to prepare these M02(V,V)
dimers. Initial preparations employed Mo(V) monomers MoC15 and
(NH 4)2Mo(S)4 as starting reagents under inert atmosphere to form sulfur
bridging complexes.175 Oxygen bridge complexes also have relied on the
monomer M0015 upon addition of large excess of chelating compounds
such as sodium N,N-dialkyldithiocarbamates (Na(dtc)) to yield
M0203(dtc)4.176 A problem with using molybdenum monomers as
starting reagents is the preoponderance of different side reactions
competing with dimer formation. In this respect the synthetic design
using molybdenum quadruple bond dimers as starting reagents is useful
because the dimetallic core acts as a template.

The reaction of quadruple bond M02(II,II) cores to M02(V,V) is
unique. Although the M02(II,II) core can be oxidized while maintaining
the bimetallic core, the formal oxidation state of the core is
M02(III,III).177'178 The quadruple bonded molybdenum, tungsten, and
rhenium dimers have been shown to oxidatively add acids (HCl, HBr)
and/or halogens. In most cases, extensive ligand rearrangements occurs
with concomitant reduction of bond order. In the reaction of alkyl and
aryl disulfides edge sharing bioctahedra are obtained.179'180 Our
discovery of the conversion of M02(II,II) to M02(V,V) (MoYL)2X2 (Y = 0,
X2 = 02, 82, OS and Y = X = S) cores is without precedent and provides a
convenient route to the preparation of these interesting biological model

compounds.

APPENDIX

Nuclear Magnetic Resonance Spectra

173

 

Figure45

31P{1H} NMR spectrum of Rh2[CH3N(PF2)2]3(PF3)2, 1. NMR spectrum

was not reference to an external standard.

174

u?

 

 

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Mao

mmo

an!

“M45

175

Figure46

31P{1H} NMR spectrum of Rh2[CH3N(PF2)2]3(PF3)C12, 2 prepared by
method i of Section II.A.2c.

176

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177

Figure 47

31P{1H} NMR spectrum of Rh2[CH3N(PF2)2]3Cl4, 3.

 

178

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179

Figure48

19F NMR spectrum of Rh2[CH3N(PF2)2]3(PF3)2, 1

180

 

 

 

 

 

 

 

 

 

 

 

 

01.-

 

 

 

 

181

 

Figure 49

19F NMR spectrum of Rh2[CH3N(PF2)2]3(PF3)Clz, 2 prepared by method i
of Section II.A.2.c.

   

182

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..

 

 

 

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183

Figure50

19F NMR spectrum of Rh2[CH3N(PF2)2l3CI4, 3.

 

 

 

 

 

 

 

 

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14111111134443: 111.444 ijiqjifili 344—4111113443
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185

 

Figure 51

19F{31P} NMR spectrum of Rh2[CH3N(PF2)2]3(PF3)Clz, 2 prepared by
' method i of Section II.A.2.c.

   

186

 

 

 

 

 

   

  

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Figurefil

187

Figure 52

method i of Section II.A.2.c.

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188

 

 

 

 

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50 N0 0 two I ..0 1&0 100 .. ~00 .. .M0 t “50 t 30 00-

189

Figure53

19F{31P} NMR spectrum of Rh2[CH3N(PF2)2]3C14, 3. NMR spectrum was

not reference to an external standard

 

190

 

 

 

 

 

 

 

 

191

 

Figure54

31P{1H} NMR spectrum of Rh3(p.-Cl)3[CH3N(PF2)2]3, l

   

192

 

 

 

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10.

11.

12.

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Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.;
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Porter, G. B. In Concepts of Inorganic Photochemistry; Adamson,
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(a) Chang, I.-J.; Nocera, D. G. J. Am. Chem. Soc. 1987, 109, 4901-
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Caspar, J. V. J. Am. Chem. Soc. 1985, 107, 6718-6719.

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Moore, T. A.; Gust, D.; Mathis, P.; Mialocq, J. C.; Chachaty, C.;
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Danielson, E.; Elliott, C. M.; Merkert, J. W.; Meyer, T. J. J. Am.
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