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This is to certify that the

dissertation entitled

lilililllllli

Solvated Cations with Metal-Metal Bonds:
Design Strategies and Reactivity of a
New Class of Coordination Compounds

presented by

Laura Ellen Pence

has been accepted towards fulfillment
of the requirements for

Ph . D . degree in Chemistry

 

 

L/ /.,, (A

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4
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M a jor professor

Date 7/77/13 .LJ 77772

MS U i: an Affirmative Action/Equal Opportunity Institution

 

 

 

042771

LIBRARY

Michigan State
University

 

PLACE IN RETURN BOX to remove this checkout from your record.
TO AVOID FINES return on or before date due.

 

DATE DUE DATE DUE DATE DUE

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

MSU Is An Affirmative Action/Equal Opportunity Institution
cm

NH. 1

SOLVATED CATION S WITH METAL-METAL BONDS:
DESIGN STRATEGIES AND REACTIVITY OF A
NEW CLASS OF COORDINATION COMPOUNDS

By

Laura Ellen Pence

A DISSERTATION

submitted to
Michigan State University
in partial fulfillment of the requirement
for the degree of

DOCTOR OF PHILOSOPHY

Department of Chemistry
1992

 

 

ABSTRACT

SOLVATED CATION S WITH METAL-METAL BONDS:
DESIGN STRATEGIES AND REACTIVITY OF A
NEW CLASS OF COORDINATION COMPOUNDS

By

Laura Ellen Pence

Historically, the use of starting materials possessing polar solvent
moieties in one or more coordination sites has enhanced reactivity of
transition metal species and allowed access to a variety of unusual molecules.
The numerous advantages of acetonitrile over water as the labile solvent
ligand include the reduced ability of the group to be transformed or act as a
bridging group as well as the utility of these materials in organic media
without decomposition. Homoleptic mononuclear acetonitrile species are .
known for roughly half of the transition elements, but prior to our research,
easily accessible dinuclear examples were limited to only the dimolybdenum
system. We proposed to further expand these examples by probing synthetic
procedures to access dinuclear species in other metal systems. In addition,
we sought to expand the chemistry of partially solvated systems such as
[M2 (OAc)2(MeCN)6]2+ (M = M0, Rh).

Exploration of the reactivity of the partially solvated dirhodium cation,
with a dimetal anion, [n-Bu4N]2[Re2C13], precipitated the unusual soft salt,
[Rh2(0Ac)2 (MeCN)6][RezClsl (l) by metathesis rather than ligand

 

Laura Ellen Pence
redistribution or cation-anion annhilation occurring. This unusual structure
suggests a wealth of reactivity for solvated dinuclear species.

Efforts to prepare fully solvated dinuclear species were very successful
in the dirhodium system as both the [BF4]‘ and [TFMS]' salts of
[Rh2(MeCN)1o]4+ were prepared as well as the [BF4]‘ salt of the longer chain
propionitrile species. The elementary reactivity of these species was
established along with solvent exchange behavior in both nitriles and water.
Spectroscopic studies revealed that the [Rh2(MeCN)1o][BF4]4 salt displays
reversible photochemistry with an extended lifetime before the original
species is reformed. The versatility of this metal system indicates excellent
potential for tailoring the starting material to various reaction conditions.

Elucidation of the general synthetic methodology to prepare solvated
transition metal species had mixed success. The dinuclear chromium species
is not produced in favor of the oxidized [Cr(MeCN)(;]31L species. Further
investigation will be required in iridium, ruthenium and the extremely
promising osmium chemistry to further generalized these preparative

methods.

 

    
  
   
 
  
   
  
 
 
   
 
  
    

M hi*“jp‘lli'o’in'y family, who always believed I could do anything I tried,
. We “Ml“ “ " ‘ and gave me the support and encouragement
7": 7 V M? WW to make this dream come true.
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ACKNOWLEDGEMENTS

I would like to express my gratitude to my parents and sisters who
have offered so much support and encouragement throughout my career.
They helped lay the strong foundations for the person I have become, and
have always believed that I could succeed at whatever I set my mind to.
Their understanding and whole-hearted pride in my accomplishments in
graduate school have helped me to persist in meeting new challenges and
pursuing my goals.

I am also extremely grateful to my research advisor, Kim Dunbar, who
taught me so much and made research so exciting. I appreciate her efforts to
mold me into a complete and well-rounded scientist and all our discussions
that may not have had any direct bearing on my latest reaction but have
served to shape my habits and attitudes and launch me into the next phase of
my career. Thanks also goes to my fellow members of the Dunbar group- .
especially Steve Haefner who made my first year bearable, Anne Quillevéré
who experienced the "joys" of writing up with me, and Julia Clements
Thomas who has helped me through many a rough spot.

My many mentors have also offered their encouragement and
understanding when I needed it- some via electronic mail like Dick Cornelius
and Don Dahlberg from LVC, and others in person like Gerry Babcock and
Jim Harrison at MSU. A special note of appreciation goes to John Stille who
found time for me during his own pursuit of tenure and efforts to run a
research group. A brief word must be said for all the staff people who helped
my work along. Many thanks go to Don Ward for his tireless efforts to teach

ii

me to run ORTEP, and to Tom Atkinson (TVA) for putting up with my efforts
to keep KDVS2O and ATHENA up and running. I also thank Teri Roache for
listening.

Graduate School could never be bearable without the friends to lighten
the hours- especially Happy Hours. First, I would like to acknowledge the
friends far away- Kathy Kleponis and Sonja Compton who answered late
night phone calls and have always let me be me, and Ross Hoffman and Dr.
Mary Beth Seasholtz who shared by electronic mail all the trials and
tribulations in the pursuit of a Ph.D. The friends here have partied with me
when I was up, and consoled me when I was down. They helped me grow in
many new directions and introduced me to skiing, hockey games, football
tailgates, intramural softball, and lunch dates. I'm thankful for Ed and
Maria Townsend, Beta Borer, Jeff Gilbert, Berly Sames, Mary Puzycki, Kurt
and Tracy Kneen, Nancy Barta, and all the rest of the crew.

One last very special person to me has been Martin Lodish. He lent
me his strength this past year through many rough spots, held me together,
and always believed in me. I thank him for all the sharing and all the

understanding that have helped me through the last of this long journey. -

iii

TABLE OF CONTENTS

 

 

 

 

 

 

 

 

Page
LIST OF TABLES ...................................................................... xv
LIST OF FIGURES .................................................................... xviii
LIST OF ABBREVIATIONS ............... xxiii
CHAPTER 1. INTRODUCTION .......................................... 1
Acetonitrile Versus Water in Coordination Complexes ........ 2
Acetonitrile Binding Modes . 8
Infrared Spectra] Properties of Nitrile Complexes ................ 9
Syntheses of Mononuclear Acetonitrile Compounds ............. 13
Syntheses of Dinuclear Solvated Cations ............................. 18
LIST OF REFERENCES, CHAPTER I 23
CHAPTER II. REACTIONS OF PARTIALLY SOLVATED
CATION S WITH A DIVALENT RHENIUM
ANION 22'
A. Introduction 28
B. Experimental, Synthesis 3)
(1) Synthesis of lRh2(OAc)2(MeCN)el[RezClgl (l)
(i) Method 1 3)
(ii) Method 2 31
(2) Reaction of [Rh2(OAc)2(MeCN)sl[RezClg]
in MeCN 31

 

(3) Reaction of [Rh2(OAc)2(MeCN)sllRegClgl

in CH2C12 ....................................................... 32
(4) Reaction of [Rh2(OAc)2(MeCN)ellRe2Clgl

in Acetone ..................................................... 3‘2
(5) Reaction of [Rh2(OAc)2(MeCN)6llRe2C18] in THF 33
(6) Thermal Reaction of [Rh2(OAc)2(MeCN)6][Re2C18]

in the Solid State 33

 

(7) Work-up of Layer Reaction solutions ..................... 34
(8) Reaction of [Rh2(OAc)2(MeCN)61[BF4]2 with

 

 

 

 

 

 

Chloride ion .................................................. 34

(9) Control reaction of lRe2Clgl2' with MeCN ............... 35

C. Experimental, Crystallography 35
[Rh2(OAc)2(MeCN)6llRe2Clg] 35

D. Results and Discussion ..... 38
Synthetic Methods ......... 38
Spectroscopy ........................................................... 38
Molecular Structure ................................................ 45
Reactions of [Rh2(OAc)2(MeCN)6][Re2018] .................. 58'

E. Summary and Future Directions 63
LIST OF REFERENCES, CHAPTER II 64

CHAPTER III. SYNTHESIS AND REACTIVITY OF SOLVATED

 

 

DIRHODIUM CATIONS ................................. 65

A. Introduction 66
B. Experimental, Synthesis 6'?
(1) Preparation of Rh2(OAc)4(MeCN )2 ......................... 67

V A

(2) Preparation of [Rh2(MeCN)1o][BF4]4 (2) .................. 67

(i) Method 1 ................................................... 67

(ii) Method 2 .................................................. 68
(3) Metathesis of [Rh2(MeCN)10]lBF4]4 with LiTFMS 6)
(4) Metathesis of [Rh2(MeCN)10][BF4]4 with Sodium

tosylate ......................................................... a)
(5) Metathesis of [Rh2(MeCN)1ollBF4]4 with

TBA(Tosyl) .................................................... 70
(6) Preparation of [Rh2(MeCN)10l[TFMS]4 (3) ............. 70
(7) Reaction of [Rh2(MeCN)1ollTFMS]4 with

Methanol ...................................................... 71
(8) Reaction of Rh2(OAc)4(MeOH)2 with HT FMS

and Acetonitrile ............................................. 72
(9) Reaction of Rh2(OAc)4(MeOH)2 with MeTFMS ........ 72
(10) Reaction of [Rh2(MeCN)101[BF4]4 with

Propionitrile 72
(11) Synthesis of [Rh2(EtCN)10]lBF4l4 (4) ...................... 73
(12) Reaction of Rh2(OAc)4(MeOH)2 with HTFMS and

 

Propionitrile 74
(13) Reaction of Rh2(OAc)4(MeCN)2 with MegSiTFMS and

 

Propionitrile 74
(14) Reaction of Rh2(OAc)4(MeCN)2 with HBF4 and

 

Butyronitrile 74
(15) Reaction of Rh2(OAc)4(MeCN)2 with Et30BF4 and

 

Butyronitrile 74
(16) Reaction of [Rh2(EtCN)1o][BF4]4 with

 

 

Butyronitrile 75

(17) Reaction of [Rh2(MeCN )1ollBF4l4 with

Benzonitrile ................................................... 75
(18) Reaction of [Rh2(EtCN)10][BF4]4 with

Benzonitrile ................................................... 76
(19) Reaction of Rh2(OAc)4(MeCN)2 with

Pentanedinitrile ............................................. 76
(20) Reaction of [Rh2(MeCN)1o][BF4]4 with

Pentanedinitrile ............................................. 76
(21) Reaction of [Rh2(MeCN)10]lTFMS]4 with

Pentanedinitrile ............................................. 76

(22) Reaction of [Rh2(MeCN)10llBF4]4 with

Propanedinitrile ............................................ 77
(23) Conversion of [Rh2(MeCN)10][BF4]4 to

Rh2(OAc)4(MeOH)2 ......................................... 77
(24) Reaction of [Rh2(MeCN)10][BF4]4 with 2 equivalents

of NaOAc ...................................................... 78

(25) Reaction of [Rh2(MeCN)10llTFMS]4 with

 

 

 

2 equivalents of NaOAc 78
(26) Reaction of [Rh2(MeCN)1o][BF4]4 with

NaOzCCFg 78
(27) Reaction of [Rh2(MeCN)1ollBF4]4 and

Na02CC3H7 79
(28) Reaction of [Rh2(EtCN)1o][BF4]4 with 2 equivalents

of [n-Bu4N][OAc] a)

 

(29) Axial Substitution Reactions of [Rh2(MeCN)10][BF4]4
with Donor Solvents a)

 

vii

(i) Reaction with MeOH. ................................. 80

 

(ii) Reaction with THF. 80
(iii) Reaction with Acetone. ............................. 81
(30) Preparation of Rh2(aq)nJr ( 5) ................................ 81
(31) Crystallization attempt of Rh2(aq)4+ ...................... 81
(32) Conversion of Rh2(aq)4+ to Rh2(OAc)4 ................... 82

(33) Aerobic Reaction of [Rh2(MeCN)1Ol[BF4]4 with Water

 

at Room Temperature 82
(34) Reaction of Rh2(aq)n+ with Acetonitrile ................ 83
(35) Reaction of [Rh2(MeCN)10]lBF4]4 with HClO4 ........ 83

(36) Preparation of a UV-visible sample of Rh2(aq)n+

in 3 M HClO4 83
(37) Control reaction of Rh2(OAc)4(MeOH)2 in Water 83
(38) Reaction of Rh2(OAc)4(MeOH)2 with Pyridine

and Et30BF4 84
(39) Reaction of [Rh2(MeCN)1n][BF4]4 with Pyridine ..... 84

 

 

(40) Reaction of [Rh2(MeCN)10][TFMS]4 with Pyridine .. 84

(41) Reaction of [Rh2(MeCN)10][BF4]4 with 2 equivalents

of Bipyridine in Acetonitrile ............................ 85
(42) Reaction of [Rh2(MeCN)10][BF4]4 with 2 equivalents

of Bipyridine in Acetone .................................. $
(43) Reaction of [Rh2(EtCN)10][BF4]4 with 2 equivalents

of Bipyridine %
(44) Reaction of [Rh2(MeCN)10][BF4]4 with 4 equivalents

 

of Bipyridine 87

 

viii

(45) Reaction of [Rh2(MeCN)10][BF4]4 with 2 equivalents

of dppm ......................................................... 87
(46) Reaction of [Rh2(MeCN)1o][TFMS]4 with 2 equivalents

of dppm ......................................................... 88
(47) Reaction of [Rh2(MeCN)1o][BF4]4 with 2 equivalents

of PMe3 ......................................................... 88
(48) Reaction of [Rh2(MeCN)10][BF4]4 with ppnCl ........ 88

(49) Crystallization of an oxygen derivative of
[Rh2(MeCN)10][BF4]4 89
(50) Long term reaction of [Rh2(MeCN)1o][BF4]4

 

 

 

 

 

 

 

 

 

 

 

 

 

 

with 02 and hv ............................................... $

C. Experimental, Crystallography ..................................... £1)
1. [Rh2(MeCN)1ol[BF4l4 (2) 90

2. [Rh2(MeCN)10][TFMS]4 (3) 90

3. [Rh2(EtCN)1ol[BF4l4 (4) 94

D. Results and Discussion ............... %
[Rh2(MeCN)1ollBF4l4 96
Synthesis ...................................................... EB-
Spectroscopy .. ............... 97

Molecular Structure 104
[Rh2(MeCN)1ollTFMS]4 ....... 111
Synthesis 111
Spectroscopy 113

Molecular Structure 114
[Rh2(EtCN)1ol[BF4l4 123
Synthesis 123
Spectroscopy 1%

 

ix

Molecular Structure ....................................... 129

 

Other Nitriles ......................................................... 129
Carboxylate Reactions ............................................. 142
Axial Substitution ................................................... 143
Solvent Exchange, Rh2(aq)4+ 146
Synthesis ...................................................... 146
Spectroscopy .................................................. 146
Interconversions of the Rhg4+ core ............................ 149
Pyridine and Bipyridine Reactions ............................ 152
Tertiary Phosphine Reactions ................................... 153
Chloride Reactions .................................................. 153
Oxygen Reactions .................................................... 154

E. Summary .................................................................... 154
LIST OF REFERENCES, CHAPTER III 156

 

CHAPTER IV. PHOTOCHEMISTRY OF [Rh2(MeCN)10][BF4]4 158

 

 

 

 

A. Introduction ............... 159
B. Experimental, Synthesis 159
(1) Thermal Decomposition of [Rh2(MeCN)10][BF4]4
in Solution 159
(2) Reduction of [Rh2(MeCN)1ol[BF4]4 with the
Tris(2,6-dimethoxyphenyl) Methyl Radical ........ 159
(3) Reduction of [Rh2(MeCN)1ol4+ with
Na-Acenaphthylenide (NaAce) ......................... 16)
(4) Reduction of [Rh2(MeCN)10][BF4]4 with
Cobaltocene 161
x

 

~ - _. - .—.--_——.———4-

(5) Reduction of [Rh2(MeCN)10]lBF4]4 with Cobaltocene
in the presence of Electrolyte ........................... 161
(6) Crystallization attempt of the Cobaltocene reduction
product ......................................................... 161
( 7) Reduction of [Rh2(MeCN )1ollBF414 by NaEt3BH ....... 162

(8) Bulk Electrochemical Synthesis of

 

"th4(MeCN)16]lBF416" 162
(9) Electrocrystallization of [Rh4(MeCN)16][BF4]6 ......... 163
(10) Reaction of [RhCl(COD)]2 with TlPF6 ................... 166
(11) Reaction of RhCl3 with TlPF6 .............................. 166
(12) Reaction of RhC13 with SbC15 ............................... 167
(13) Reaction of RhC13 with AgBF4 ............................. 167
(14) Reaction of RhC13 with AgTFMS ......................... 168
( 15) Synthesis of [RhC12(MeCN)4][BF4] ....................... 168
( 16) Synthesis of [RhC12(MeCN)4]lTFMS] .................... 163
( 17) Reaction of [RhCl2(MeCN)4l[BF4] with

 

[Na]+[mhpl' 169
(18) Reaction of [Rh2(MeCN)1ol[BF4]4 with co '

and light 170
(19) Aerobic reaction of [Rh2(MeCN)1o][BF4l4 with CO

 

in the Presence of light. .................................. 171
(20) Reaction of [Rh2(MeCN)_1ol[BF4]4 with CO

in the dark 171
(21) Thermal reaction of [Rh2(MeCN)1o][BF4]4 with CO

 

 

in the dark 172
(22) Reaction of [Rh2(MeCN)1o][BF4]4 with 10 equivalents
of i-PrNC 172

 

xi

 

 

(23) Reaction of [Rh2(MeCN)1o][BF4]4 with 10 equivalents

 

 

of n-BuNC ......... .......... 173

C. Experimental, Crystallography 174
[RhC12(NCCH3)4l[BF4] ............................................. 174

D. Results and Discussion ................................................. 176
Electronic Spectroscopy of [Rh2(MeCN )1ollBF4l4 .......... 179

Synthesis of Intermediates:

 

 

 

" [Rh4LILILI<MeCN)16]6+ " ............................... 182

"thI(MeCN)4]+" ............................................ 1%

"[Rhm(MeCN)6l3+" ......................................... 191

Synthesis of [RhClz(MeCN)4]“r 192

Molecular Structure of [RhC12(MeCN)4][BF4] ............. 196

Further Reactions with [RhC12(MeCN)4][BF4] ............ 196
Photochemical Reactions of [Rh2(MeCN)10][BF4]4

with CO ....... 203

Reactions with isocyanides 204

E. Summary .................................................................... 205

LIST OF REFERENCES, CHAPTER IV 206

 

CHAPTER V. STRATEGIES FOR THE SYNTHESIS OF OTHER
SOLVATED DINUCLEAR TRANSITION METAL

 

 

 

COMPLEXES 208

A. Introduction 209
B. Experimental 211
( 1) Reaction of Cr2(OAc)4 - H20 with Et30BF4 .............. 211

(2) Reaction of IrCl3 with TlPFs ................................. 211

xii

 

 

(3) Reaction of IrC13 with SbCl5 .................................. 212

(4) Reaction of IrCl3 with AgBF4 ............................... 212
(5) Reaction of IrCl3 with AgTFMS ............................ 213
(6) Reaction of IrC13 with HBF4 ................................. 214
(7) Reaction of RuC13 with TlPF6 ................................ 214
(8) Reaction of RuCl3 with AgBF4 .............................. 215
(9) Reaction of RuCl3 with HBF4 ................................ 215
(10) Reaction of RuC13 with HTFMS ........................... 216
(11) Reactions of anhydrous RuC13 ............................. 216
( 12) Reaction of Ru2(OAc)2(CO)4(MeCN)2

with Et30BF4 ................................................. 216
( 13) Reaction of Ru2(OAc)4Cl with HBF4 ..................... 217
(14) Reaction of Ru2(OAc)4Cl with HTFMS ................. 217

(15) Reaction of [Ru2(OAc)4(THF)2][BF4] with HBF4 ...... 217
(16,) Thermal reaction of [NH4]5[M02C19] with HBF4 ..... 218
(17) Reaction of [NH4]5[M02C19] with HBF4 at Room
Temperature ................................................. 218
(18) Reaction of K4[M02018] with HBF4 ........................ 218
(19) Reaction of K4[M02C18] with HTFMS in the presence

 

 

of Propionitrile 219
(20) Reaction of M02C14(Me28)4 with HBF4 .................. 219
(21) Reaction of K4[M02Clgl with NaBPh4 .................... 219
. (22) Reaction of OsCl3 and AgTFMS ........................... 22)
(23) Reaction of [n-Bu4N12IOszC13] and HBF4 ............... 220

(24) Reaction of [n-Bu4N]2[OszClgl with 8 equivalents
of AgBF4 220

xiii

 

 

 

(25) Reaction of [n-Bu4N]2[OszClg] with HTFMS in the

presence of Propionitrile .................................

(26) Reaction of 032(0Ac)4C12 with AgTF MS and

MegsiTFMS ...................................................

(27) Reaction of Os2(OAc)4C12 with HBF4 in the presence

of Propionitrile ............

(28) Reaction of 032(0Ac)4C12 with HBF4 in the presence

of Acetonitrile and Dichloromethane .................

C. Results and Discussion .................................................

Chromium .............................................................

Iridium .................................................................
Ruthenium ............................................................
Molybdenum ..........................................................

Osmium ................................................................

D. Summary ....................................................................

LIST OF REFERENCES, CHAPTER V

 

CHAPTER VI. CONCLUDING REMARKS AND FUTURE
DIRECTIONS

 

General .. .........................

 

Mixed-Metal Cation-Anion reactions

 

Synthesis and Reactivity of Dirhodium Solvates ...................
Photochemistry of [Rh2(MeCN)1o]4+

 

Other Transition Metal Systems

 

Conclusions
LIST OF REFERENCES, CHAPTER VI
APPENDICES

 

 

 

xiv

221

221

222

222
224
224

240
242
244

247
247
247

53535

 

 

10.

11.

LIST OF TABLES

Unit Cells of [Rh2(OAc)2(MeCN)6][Re2Clg] determined at

different temperatures.

 

Crystal Data for [Rh2(OAc)2(MeCN )ellRe2C18] .......................

Selected Bond Distances in A for
[Rh2(OAc)2(MeCN)6]lRe2Clg]

 

Selected Bond Angles in degrees for
th2(OAc)2 (MeCNlellRe2C181

 

Crystal Data for [Rh2(MeCN)10]lBF4l4

 

Crystal Data for [Rh2(MeCN)1ol[TFMS]4 .................................

Crystal Data for [Rh2(EtCN)1ollBF4l4

 

Selected Bond Distances in A for [Rh2(MeCN)10][BF4]4 ..........
Selected Bond Angles in degrees for [Rh2(MeCN)1o][BF4]4 .....
Selected Bond Distances in A for [Rh2(MeCN)10][TFMS]4 ......

Selected Bond Angles in degrees for [Rh2(MeCN)1ol[TFMS]4

page

36

39

91

93

95‘

105

106

117

118

 

 

12.

13.

14.

15.

16.

17.

18.

19.

Selected Bond Distances in A for [Rh2(EtCN)1ollBF4]4 ............. 130

Selected Bond Angles in degrees for [Rh2(EtCN)1o][BF4]4 ........ 131

Summary of Spectral and Reactivity Properties for halaq)4+ .. 147

 

Crystal Data for [RhC12(MeCN)4l[BF4] 175
Selected Bond Distances in A for [RhC12(MeCN)4l[BF4] ........... 197
Selected Bond Angles in degrees for [RhC12(MeCN)4][BF4] ...... 198
Selected Bond Distances in A for [Cr(MeCN)5][BF4]3 ................ 230
Selected Bond Angles in degrees for [Cr(MeCN)6][BF4]3 ........... 231

20. Atomic Positional Parameters (A2) and their estimated standard

deviations for [Rh2(OAc)2(MeCN)6l[Re2Clgl .............................. 256

21. Atomic Positional Parameters (A2) and their estimated standard

deviations for [Rh2(MeCN)1ollBF4l4 258

22. Atomic Positional Parameters (A2) and their estimated standard

deviations for [Rh2(MeCN)1ol[TFMS]4 260

 

23. Atomic Positional Parameters (A2) and their estimated standard

deviations for [Rh2(EtCN)10][BF4l4 262

 

24. Atomic Positional Parameters (A2) and their estimated standard

deviations for [RhCl2(MeCN)4l[BF4] 264

25. Atomic Positional Parameters (A2,) and their estimated standard

deviations for [Cr(MeCN)6][BF4]3 265

xvii

 

 

 

LIST OF FIGURES

 

 

Page
Known homoleptic aqueous mononuclear and dinuclear
transition metal cations 4
Known homoleptic acetonitrile mononuclear and dinuclear
transition metal cations 7

Model of electron flow during nitrile stretching vibration.

Upper white arrow represents flow of backbonding electrons from
metal to ligand. Lower white arrow represents flow of electrons
within ligand. Black arrow represents net electron flow.

(Ref 14) 12

Potential stacking configurations for the bridged and unbridged
dimetal units 41

 

Diagram of the dinuclear ionic components of

[Rh2(OAc)2 (MeCN)6llRe2Clgl 43

 

ORTEP unit cell packing diagram for
th2(OAc)2(MeCN)sl[RezCls] 52

 

Schematic diagram of the unit cell of
[Rh2(OAc)2 (MeCN)6][RezClsl showing the packing orientation

of the Re-Re units. The singly labeled locations are units

xviii .1.

 

 

10.

11.

12.

13.

parallel to the viewing angle. The Rh-Rh units are located in

the empty boxes 54

Representation of the three stacking modes of Re-Re units

in the crystal, [Rh2(OAc)2(MeCN)el[Re2C18] ............................... 56

Qualitative electronic spectra of the kinetic and

thermodynamic products of the reaction between
[Rh2(OAc)2(MeCN)sl[BF4]2 and ln—Bu4N]2[Re2Clgl . (1) Solution
from initial reaction in MeCN. (2) Kinetic salt in MeCN

for 24 h. (3) Thermodynamic products(s) in MeCN. .................. 60

1H NMR spectrum of [Rh 2(MeCN )10][BF4]4 recrystallized in the
absence of light 100

1H NMR spectrum of [Rh 2(MeCN) 10][BF4]4 recrystallized in the

 

presence of light 102

ORTEP diagram of the cationic component of
[Rh2(MeCN)1ol[BF4]4 viewed perpendicular to the
Rh-Rh bond 108

 

ORTEP diagram of the solvated cation of [Rh2(MeCN)1o][BF4]4
viewed parallel to the Rh-Rh bond depicting the staggered

nature of the molecule 1 10

 

 

14.

15.

16.

17.

18.

19.

20.

21.

22.

1H NMR of the orange form of [Rh2(MeCN)1ol[TFMS]4
in CD3N02 116

ORTEP packing diagram of [Rh2(MeCNl1ollBF4l4. The anions

have been omitted for clarity 120

ORTEP packing diagram of [Rh 2(MeCN) iollTFMS]4. The

anions have been omitted for clarity 122

Space filling model of the dimetal cation in

[Rh2(MeCN)1ol[BF4l4 125

Cyclic voltammetry of (a) [Rh 2(MeCN) 1ollBF4l4 and
(b) [Rh2(EtCN)10][BF4]4 128

ORTEP diagram of the dimetal cation in
[Rh2(EtCN)1ol[BF4l4 133

ORTEP packing diagram of [Rh2(EtCN)1ol[BF4]4. The

counterions have been omitted for clarity ................................... 135

Space filling diagram of the cationic component of

[Rh2(EtCN)1ol[BF4l4 137

Diagram of the postulated staggered vs. eclipsed conformational
differences with monodentate and bidentate nitriles ................. 140

 

 

- .vi‘--’ .

23.

24.

25.

26.

27.

28.

29.

30.

31.

Comparison of the infrared spectra of the axial solvent

 

 

substition with th2(MeCN)1o][BF4l4 145
Interconversions of the [hal‘l+ core 151
Diagram of a three compartment electrocrystallization cell ...... 165

Proposed mechanism for the reversible photochemistry of

[Rh2(MeCN)10l[BF4l4 178

Electronic spectra of an anaerobically irradiated sample of
[Rh2(MeCN)10][BF4l4 in MeCN solution recorded periodically

without additional exposure to light 181

Electronic spectra displaying the thermal access to the Rh-Rh
bond cleavage reaction in [Rh2(MeCN)10][BF4]4 in the absence
of light 184

 

Synthetic methods used to access the proposed mixed-valence

tetramer, "[Rh4IvH’H:I(MeCN)1614+" 187

Electronic spectrum of the proposed mixed-valence tetramer

synthesized by bulk electrolysis redissolved in MeCN ............... 189

Cyclic voltammetry of a halide abstraction product with RhCl3
containing significant Agt impurity, in Volts vs. Ag/AgCl ......... 194

 

 

ORTEP diagram of the cationic component of
[RhC12(MeCN)4l[BF4] 200

ORTEP unit cell packing diagram of [RhC12(MeCN)4][BF4],
viewed down the A axis 202

View of the discrete Cr2(OAc)4Lg molecule and the extended

interaction in the anhydrous form. (Ref 13) ................................. 226

Diagram of the crystallographically characterized
[Cr(MeCN)6]3+ 229

Cyclic voltammetry of a halide abstraction product with IrCl3

containing significant Agt impurity 236

Diagram of a condenser and water trap to dehydrate reaction

solutions 251

 

 

Ag/AgCl
nBu
Calcd
cm -1

CV

DMSO
dppm

E1/2
EPR
EtCN
FT-IR

HBF4

HTFMS

M-M

MeCN

mg

LIST OF ABBREVIATIONS

angstrom

silver-silver chloride reference electrode
n-butyl group

calculated

wavenumber

cyclic voltammetry

deuterated

dimethylsulfoxide
bis(diphenylphosphino)methane
electron

molar extinction coefficient
half-wave potential

electron paramagnetic resonance
propionitrile

fourier transform infrared
grams

tetrafluoroboric acid
trifluoromethanesulfonic acid
hertz

infrared

metal-metal

methyl group

acetonitrile

milligrams

methylhydroxypyridine

xxiii

 

 

 

- i £‘_‘.‘é-: ‘s

ox
ppn
pTS
Ph

red

TBAH
TBABF4
TFMS
THF
TMPP
tolsyl

minutes

milliliters

millimoles

Michigan State University

nanometer

nuclear magnetic resonance

observed

acetate

oxidation
bis(triphenylphosphoranylidene)ammonium
para-toluenesulfonate

phenyl group

reduction

shoulder

tetra-n-butylammonium hexafluorophosphate
tetra-n-butylammonium tetrafluoroborate
trifluoromethanesulfonate

tetrahydrofuran
tris(2,4,6-trimethoxyphenyl)phosphine, P[C6H2(0Me)3]3
para-toluenesulfonate

volts

xxiv

CHAPTER I

     
   
  
  
  
    
 

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Acetonitrile Versus Water in Coordination Compounds

The ability of polar solvents to participate in metal ligation plays an
important role in the synthesis of transition metal compounds. Common
donor solvents such as water, nitriles, acetone, THF, pyridine, alcohols,
and DMSO may be used to occupy otherwise vacant coordination sites on a
metal center lending stability to the solution species, but promoting
reactivity at the labile donor sites. This reduces the necessity for extreme
reaction conditions or additional reagents to displace strong ligands, and
allows reactions to be carried out under mild conditions and weaker donors
to be added. The use of solvated starting materials often allows access to
unusual compounds by avoiding undesirable thermal pathways. Examples
are common in the early transition metals, for which it is customary to take
advantage of partially solvated metal halide derivatives such as
MClg(THF)3, since these discrete mononuclear units react cleanly under
mild conditions whereas the anhydrous polymeric halides tend to lead to
disproportionation reactions.1

Of the examples of fully solvated transition metal cations, by far the
most widespread are the aqua complexes. Investigations of the syntheses
and subsequent chemistry of these complexes span from the beginnings of
inorganic coordination chemistry to the present. As indicated in Figure 1,
these species are known for nearly all of the transition metals,2 in fact,
hydrated species of Mn, Fe, Ni, Cu, Ag, and Hg are commercially
available.3

Unfortunately, the utility of aqua species is somewhat limited by the
difficulties of working in an aqueous medium. Many ligands and
coordination compounds decompose in the presence of water and are acid

sensitive.2 Additionally, water itself may undergo hydrolysis which alters

 

Figure 1. Known homoleptic aqueous mononuclear and dinuclear

transition metal cations

 

 

V
\\\

 

Au

 

 

 

 

 

Os

 

 

Tc

 

 

 

HfTaW

 

 

 

 

a
k

 

 

dinuclear and
mononuclear

Figurel

mononuclear

 

 

 

 

its binding mode, coordination ability, and most likely the structure of the
compound. Water is a strong donor in the aqueous medium and thus
access to compounds containing weaker donors is reduced.

The use of nitriles as ligands offers an excellent alternative to water.
Acetonitrile is a considerably weaker acid than water, and consequently a
stronger base to maintain excellent donor ability while being far less likely
to undergo decomposition under normal conditions.4 Furthermore, nitrile
complexes are soluble in a variety of organic systems which reduces solvent
self-exchange competition with an incoming ligand. Studies of the first row
metals have established the relative solvent labilities as NH3 > H20 >
CH3CN > CH30H5 and although less labile than water, the ease of
replacement of acetonitrile groups makes these complexes ideal synthetic
precursors.6v7

Methods of preparing these solvated compounds have varied widely
and have included efforts with most of the common transition metal
starting materials: metal halides, metal filings or powder, metal carbonyls,
and hydrated metal salts. Examples of these homoleptic acetonitrile
complexes are numerous, but not as widespread as the aqua analogs, as
shown in Figure 2.

Changing from an aqueous to a nitrile ligand environment has the
net effect of stabilizing lower oxidation states since the reducing nature of
nitriles prevents these groups from supporting more highly charged
metals.8 Redox changes also become less accessible in homoleptic
acetonitrile complexes as illustrated by the [Ru(NH3)5L]2+ system in which
replacement of the one L = H20 ligand by acetonitrile leads to a stabilization
of the +2 oxidation state by about 0.4 V.9 Although [Ru(H20)(;]3+ may be

 

 

Figure 2. Known homoleptic acetonitrile mononuclear and dinuclear

transition metal cations

i’ ‘

,
n4
,.
. ) c
Ill I.
. 0"

ii

14”
}.
4

Figure. 3

 

 

 

 

 

 

 

a \V \‘ a a

Sc §§ \Qs

Y Zr Nb Tc \ \\ \\
a Vg‘k

La Hf Ta W Os Ir \§ Hg

 

 

 

 

 

 

 

 

 

 

 

 

 

RV mOIlOnuclear

dinuclear

Figure 2

 

 

 

 

 

easily prepared by oxidation of [Ru(H20)6]2+, there is no corresponding

oxidation of the [Ru(NCCH3)6]2+ species observed out to +2.5 V.10

Acetonitrile Binding Modes

The potential binding modes of alkyl nitriles are through the lone
pair localized on the nitrogen atom (mode 1, below), or through 1r donation
from the C-N triple bond (mode 11). In all but a few isolated examples, the
bonding is of the former type.11 The o-bonded examples coordinate to the
metal in a nearly linear fashion which has been crystallographically
established (M-NEC angle close to 175 :t 5°) although significant deviations
down to values of 160° have been observed}8 The NEC-R linkage maintains

an angle much closer to linearity.
R—C=N:-—>M R—C=N:

1 M

II

The shift to lower energies of the v(ClN) stretch in the infrared
spectra has been used in the past to postulate side-on Tr-bonded nitrile
complexes,” but unequivocal identification may only be made through X-
ray crystallography, since these bands have been correlated with o-bonded
groups as well.8 Examples of definitively characterized n-bonded species
include Mon2(n2-NCCH3), [MoCl(n2-NCCH3)(dmpe)2l+, (n5-
CsMe5)(CO)Ir(n2-NCC(;H4CI), and a benzonitrile nickel clathrate
compound, [(112'C(;H5CN)(PPh3)Ni14,.133"d In one isolated case, that of
[W(bPY)(PMe3lzCl(n2-NCCH3)1+, structural and spectroscopic evidence

 

 

 

supports the coordination of the nitrile as a four-electron donor.11 The
more common preference of nitriles for a o—binding mode contrasts with the
coordination flexibility of CO and adds to the greater lability of the solvent

complexes.

Infrared Spectral Properties of N itrile Complexes

The infrared spectral properties of acetonitrile complexes have been
widely discussed.7,8’14,15 The CH3CN molecule itself possesses C3V
symmetry and has eight normal modes of vibration, all of which are active
in either the infrared or Raman spectra.16c Four modes are totally
symmetrical and belong to the species A1, while four are degenerate and
belong to the species E. The magnitude of the frequency shift upon
coordination provides valuable information about the bonding interaction
between the metal and the ligand. Unlike the negative shifts upon
coordination of groups containing the moieties, CEO, P=O, and 8:0 which
is due to a decrease in the force constant of the internal ligand bonding,
coordinated nitriles usually display positive frequency shifts, concomitant
with the increasing force constant of the stabilized C-N 0 bond.7 This trend
is general for nitriles coordinated to a Lewis acid via the nitrogen, although
negative shifts for this mode are known.

The most diagnostic nitrile stretches are between 2200 and 2300 cm'1
where few other groups are infrared active. Activity in this region is due to
the v(C-IN) stretch as well as a combination mode of the C-C stretch and a
CH3 deformation.16c Differentiation between these two modes is easily
made by comparing spectra of the CD3CN adduct to the CH3CN adduct.7
While the Yv(C-N) remains the same in both, the combination mode

containing the CD3 deformation experiences a shift due to the isotope effect.

 

 

 

l gum

10

Upon coordination to a metal, an additional IR active region for
nitrile ligands is found in the far infrared where v(M-N) stretches occur.
These strong modes, one for v(M-N) stretching and one for M-NCC
wagging, usually fall in the range 330 and 180 cm'1 in octahedral
complexes, and are higher (450-230 cm'l) in square planar species.16f
Incompletely substituted species may have values ranging from 100 cm'1
for trans-PdX2(NCPh)2 (X = Cl, Br) to a maximum of 525 cm'1 for
M(CO)3(NCCH3)3 (M = Cr, Mo, W).7 Specific assignment of modes to bands
in the far infrared is often complicated due to the symmetry interactions of
the ligand and metal-ligand modes, and the additional modes from the
ligands or counterions in this region.

It has been noted by Johnson and Taube14 that the observed infrared
intensity of the v(CEN) bands does not necessarily correlate with the
presence or number of nitrile ligands; in some cases the anticipated bands
are totally absent. There are major categories of metal-nitrile interactions
that give rise to differences in v(CEN)intensitites. In metal ions that lend
themselves well to back-bonding interactions, such as Ru(II) and Os(II),
this leads to net electron flow from the metal to the ligand generating a -
dipole with a positive charge on the metal and a negative charge on the
ligand which enhances the intensity. In cases where there is little or no 1r-
back-bonding, the net electron flow from the ligand to the metal through the
sigma bond predominates and the intensity is no longer enhanced by the
metal-ligand dipole. In intermediate cases, the ligand 0 and metal 1r back-
bonding electron flows are approximately equal, canceling any dipole
moment of the infrared mode and significantly reducing or eliminating the

intensity of the stretching mode. This situation is depicted in Figure 3.

 

11

Figure 3. Model of electron flow during nitrile stretching vibration. Upper
white arrow represents the flow of backbonding electrons from metal
to ligand. Lower white arrow represents the flow of electrons within

ligand. Black arrow represents net electron flow. (Ref 14)

_.A‘ [a

 

 

Ill III

(a) Weak back-bonding: Co(III), Rh(III), Ru(III)

:>
M—NE C—R
(1::

4.

(b) Intermediate back-bonding: Os(III), Ir(III)

z)
M—NE C—R

C:
‘

(c) Strong back-bonding: Ru(II), Os(II)

Figure3

 

 

 

 

 

13

Syntheses of Mononuclear Acetonitrile Compounds

The most comprehensive studies of acetonitrile compounds were
carried out by Groeneveld and coworkers in the mid 1960's.16 Since this
was prior to widespread use of X-ray crystallography, these complexes were
characterized primarily by elemental analysis, infrared spectroscopy, and
magnetic susceptibility measurements. The limitations of these methods
for allowing differentiation between coordinated and interstitial solvent
prompted researchers to propose several stoichiometries for a single
oxidation state of several of the transition metals. The correct formulations
were subsequently identified by other research groups, and reliable
stoichiometries were eventually established in questionable cases.

The principal synthetic strategy for these numerous studies involved
reactions of metal halides and with a halide abstraction reagent that
incorporates the liberated chloride into the counterion as exemplified by the

equation outlined below:

CH3CN
MClx + x SbCl5 ———> {M(NCCH3)nl[SbClslx
(x = 1-3, n = 2,4,or 6)
Equation 1

Antimony pentachloride proved to be an excellent reagent in the more
easily handled form, SbCl5 - CHgCN, and monovalent complexes of Li, Na,
K, Rb, Cs, Ag, Au and T1 as well as divalent species of Be, Mg, Ca, Sr, Ba,
Mn, Fe, Co, Ni, Cu, Zn, Pd and Cd were all prepared by reaction with the
appropriate metal halide in acetonitrile. 168 Trivalent species accessed by
this method include solvated compounds of Al, Ga, In, Cr, V, and La.16d

In the latter cases, potentiometric titrations supported the formulation of

 

 

14

each as fully solvated despite the increasing metal charge and number of
halides in the starting material which might be expected to retard or even
prevent the transfer of chloride ions. Reactions of TiClg, V012, and Cr012
with SbCl5 under the same conditions led to oxidation of the metal; no
complex could be isolated from the reaction with HgClz. Incomplete halide
transfer resulted from the reaction between Fe013 and Sb015.

The halide abstraction reagent, tetrachlorotin(IV) is also
conveniently handled as the acetonitrile solvate, Sn014 - 3 0H30N.
Complexes of [SnClelz‘ have been isolated for Li+, Na+, Be2+, Mg2+,0a2+,
Sr2+, Mn2+, Fe2+, 002+, Ni2+, Cu+, and Cu2+,16b although the existence of
identical Cu species in two different oxidation states seems unlikely.
Solvated species of K, Rb, Cs, and T1 were not accessible by this method
leading to the conclusion that Sn014 is a weaker halide abstraction reagent
than SbCl5. Extrapolation of this general extraction and incorporation
method was provided by the reagents B013, AlCl3, GaCl3, InCl3, T1013, and
FeCl3 with the divalent cations of Be, Mg, Ca, Sr, Ba, Mn, Fe, 00, Ni, Cu, '
Zn, Hg, and Pd.16‘3’hvi~1 These agents were ranked in order of chloride

acceptor ability to be:
T1013 > Sb015 > FeCl3 > Ga013 > In013 > A1013 > SnCl4 > B013

This general methodology has been explored by other researchers
since the early work of Groeneveld and coworkers. Several groups have
independently verified this general methodology for Sb015 with V and Cr,17
Zn,18 and Ni.19 An unusual halide incorporation reaction was reported by

Wilkinson and coworkers involving the reaction of V013 and ZnEtz. Halide

 

 

15

abstraction was accompanied by a reduction of the vanadium from III to II
to yield the product [V(MeCN)el[Zn014].2O

The strategy of sequestering the liberated halide in the counterion
has certainly been an extremely successful method of preparing salts of
acetonitrile cations, but difficulties arise from the relatively reactive nature
of the halide containing counterions. It has been noted in some cases, that
anions such as [SbClel' and [SnClelz' interfere with subsequent chemistry,
often by releasing halides back into the system. Thus total removal of the
abstracted Cl‘ ligand and substitution for a more innocuous anion would be
preferred. The use of silver or thallium salts as halide abstraction reagents
are viable alternatives, since the removed ligand is precipitated as T101 or
AgCl and may be separated from the product by filtration as indicated in
the equation below. Reports of this halide sequestering and removal

0H3CN
M01x + x AgBF4 —> [M(NCCH3)nl[BF6]x + X AgCl (s)
(X: 1-3, n=4 or 6)
Equation2

method are not as widespread as for halide abstraction and incorporation
into the anion, but it has been shown that the [Pt(N00H3)4][BF4]2 species is
easily prepared by the action of AgBF4 on Pt012(NCCH3)2,21 and
[00(N00H3)6][PF6]2 is cleanly isolated from the reaction of 00012 and
AgPF(-3.22

A strategy for preparing fully solvated acetonitrile cations that was
elucidated even before the metal halide investigations (vide supra), is that of
using metal filings or powder and an oxidant such as N OBF4 or N00104 in
CH3CN medium. These reactions yield solvated solids originally

 

16

formulated as M(BF4)2 - 4 CH30N for M = Cu, Mn, and Zn, and M(BF4)2 - 6
CH3CN for M = Ni, Fe, 00.23 This approach was later extended to other
metal systems by Sen and coworkers for the syntheses of

lPd(N CCH3)41[BF4]2,24 and [Eu(NCCH3)3(BF4)3]x25 according to the

following equation:

CH3CN
M(s) + x N0BF4 ——‘> [M(NCCH3)nl[BF4]x + X N0 (g)
(x = 1-3, n=4 or 6)
Equation3

Direct preparation of solvated species by electrochemical methods
was reported for [Au(NCCH3)2]+ which involves anodic dissolution of the
metal in acetonitrile.26 Addition of HBF4 to the CH3CN system was
required for the electrochemical oxidation of Ti and Cr metals to
[Ti(NCCH3)6][BF4]3 and [0r(N00H3)6l[BF4]2.27 The In3+ species was also
prepared in these experiments. This experiment represents the only report
of a 0r2+ system; other results indicate that the 0r3+ species is
preferred.16d’17

Metal oxides are occasionally used for the synthesis of mononuclear
acetonitrile cations. Dissolution of 0u20 in CH30N and 2 M H0104 at 100
0C deposits colorless crystals of [Cu(NCCH3)4][0104] upon cooling. 28 An
excess of Mn02 treated with aqueous H0104 and 30% H202 gives the

1 aqueous solvate which may be dehydrated with molecular sieves in a
Soxhlet extractor in the presence of acetonitrile to yield

[Mn(NCCH3)6][0104]2.29 The conversion of hydrated iron perchlorate to

 

 

 

17

[Fe(NCCH3)6]2+ may also be easily effected by this CH3CN / molecular sieve
dehydration method.5

Finally, synthesis of a few metal acetonitrile species is even more
straightforward. Recrystallization of [Ru(H20)6][TF MS] from acetonitrile
cleanly produces the CH3CN solvated species.10 The same simple
procedure is used to obtain crystals of [Ag(NCCH3)4][CIO4]3O and samples
of the Mn“, Fe2+, and 002+ perchlorate salts.31

In some instances, fully solvated species form serendipitously from
other reaction strategies. For instance, the interaction of trans-
[V(NCCH3)2(dmpe)2][BPh4]2 with HCPh(SO2CF3)2 produces
[V(NCCH3)6][BPh4]2 in low yield.13b It is apparent that the action of the
acid protonates the phosphine ligands which are good leaving groups, but
this was not a particularly direct or intuitive method of synthesis. An even
more unusual result occurred in the reaction between [{Ru(n6-06Me6)012}21
and [NEt4]2[B10H14] in CH3CN solution to give a high yield of
[Ru(NCCH3)(5][7-(r16-06Me6)-nido-7-RuB10H13]2.32 Both of these species have
been reported by other methods as well.10»20

Considering that the main body of the solvated transition metal
chemistry was carried out in the early 1960's, many of these species have
not had the benefit of modern methods of characterization. Subsequent
investigations have led to numerous examples characterized by X-ray
diffraction. These include Ni(II),19v33 Ru(II),32 V(II),20 Fe(II),34’35
0u(I),28 and Ag(I).30 Powder diffraction‘data indicate that the Mg( II) and
Ni(II) complexes are isostructural with Fe2+.34

Mononuclear carbonyls of the form, M(CO)6, are another common
starting material for solvated species. Unlike the aforementioned

reactions, this source does not appear to yield homoleptic solvent products.

 

 

 

18

For Cr, Mo, and W, a procedure involving refluxing the metal carbonyl in
acetonitrile removes three carbonyls in a facial arrangement to yield
partially solvated species of the form, fac-M(CO)3(NCCH3)3.36 The
analogous Re complex was prepared unexpectedly by a different synthetic
route. The reaction of [n-Bu4Nl2lRe4(00)15] with AgBF4 in 0H30N
unexpectedly deposits silver metal and yields [Re(CO)3(NCCH3)3l[BF4l in
another example of serendipitous formation of a solvated species.37

Chemical oxidation of the metal carbonyls of molybdenum and
tungsten with N0BF4 removes all the carbonyl ligands, but produces
nitrosyl species of the form, [M(N0)2(NCCH3)4][BF4]2 for M=Mo, W.24
Coordination of the N0 by-product prevents one from using this route to
access fully solvated species for these metals. Compounds of this general
type also exist for Rh. The reaction of N 0BF4 with [Rh(COD)(NCCH3)2l[X]
or [Rh(00D)2][X] (COD = 1,5-cyclooctadiene) in acetonitrile yields
[Rh(NCCH3)4(N0)]lX12.38

Syntheses of Dinuclear Solvated Cations

Although numerous examples of mononuclear acetonitrile and aqua
solvated species have been prepared, dinuclear complexes are rare. When
our research commenced, three solvated dinuclear species were known for
only two metals: the aqua cations of M02HvII and RhZIIJI, and the
acetonitrile species for M02113.

The dinuclear rhodium aquo complex was reported by Taube and

coworkers from the reduction reaction: 39

2 [Rh(H20)5Cl]2+ + 2 [Cl'(H20)(;]2+ —> [Rh2(H20)10]4+ + 2 [Cr(H2O)5Cl]2+
Equation 4

19

The resulting charged species were separated on a cation-exchange
column. The Cr containing fraction was eluted by 1 M H0104 while the
more highly charged Rh24+ complex was removed with 3 M H0104. Other
Rh(III) starting materials, [Rh(H20)6]3+ and [Rh(H20)5Br]2+ were later
found to yield the same products and a slight reduction of the acid
concentration was possible.39b Unfortunately, the constraints of the acidic
medium prevented a solid from being obtained without decomposition of the
compound.

Wilkinson and coworkers subsequently reported synthesis of the

same species by an alternative route:40

Rh2(OAc)4 + 4 HBF4 —> Rh24+ + 4 BF; + 4 HOAc
Equation5

The Taube group found some disparities between these reports and their
original work and proceeded to reproduce the reaction conditions reported
by Wilkinson. Their experiments suggested that protonation of the acetates
is incomplete in the above reaction leaving behind two or three bridging.
groups on the dirhodium, unit. Taube's conclusions were advanced on the
premise of the difference in the cation exchange behavior of the Wilkinson
species which eluted at much lower acid concentration and the lack of
decomposition of the solutions upon reduction of the volume.41 These
observations were in direct contrast to the properties of the "authentic"
Rh2(aq)4+ prepared from the reduction of Rh(III) complexes. Because of the
unresolved controversy regarding the acetate method, preparation from the

mononuclear Rh(III) species remains the only unquestionable method of

2)

synthesis for the Rh2(aq)4+; unfortunately the highly acidic medium
restricts exploration of subsequent chemistry.42

Several, largely unsuccessful, attempts to prepare M024+(aq) from the
same route as the Rh synthesis, namely the reduction of M0012+ with 0r2+
were reported by Taube and coworkers.43 A different strategy was then
employed, borrowing from the mononuclear examples to acidify the

chlorides of K4M02013 - 2 H20 with 0.25 M HTFMS. This appeared to be

successful, but isolation of the pure aqua material was not possible. The

K4Mo2013 + HTFMS MeC—Ny [M02(aq)][TFMS]4 + 4 KTFMS + 8 H01 (g)

Equation 6

red compound was strongly retained by cation—exchange resins to eluent
concentrations of l M HTFMS. A more highly charged anion was used in
the form of 3 M H2804 which leads to derivatization of the M0211:II unit to
give K4[M02(SO4)4].43 Barium triflate solutions may be used to convert this
sulfate species to the molybdenum(II) water compound, but no solid can be
isolated; furthermore the solutions are stable for only hours under N2
before decomposition ensues.“

Preparation of the dinuclear Mo( II) species of acetonitrile was first
reported by a two step synthesis from M02(02CH)4.4*5 The formate was
refluxed with HTFMS and trifluoromethanesulfonic anhydride to form the
very moisture sensitive compound, [M02(H20)4(TFMS)211TFMS]2. This solid
was then dissolved in acetonitrile with reduction of the volume and chilling
to produce blue crystals of "[Mo2(N00H3)3][TFMS]4". The species,
M02(TFMS)4 had earlier been prepared by action of HTFMS on M02(0Ac)4 in

the same laboratory,46 but the product was reported to be extremely difficult

21

to separate from an acetate impurity. Use of the formate takes advantage of
this ligand's decomposition in strong acid to carbon monoxide and water.
The acetonitrile species is insoluble in non-polar solvents and reacts with
coordinating solvents. It easily reverts back to M02(0Ac)4 upon reaction
with acetic acid. The species formulated as "[M02(NCCH3)8]4+" by Abbott is
very air sensitive and apparently loses solvent readily under vacuum.
Crystals were finally grown at a much later date by Cotton and
coworkers“?7 The compound was identified as [M02(N00H3)1ol4+ in the
form of the [BF4]' salt, indicating that solvent is present in the axial
positions contrary to the original formulation.

Finally, during the course of the research reported in this thesis,
Baranovskii and coworkers reported the synthesis of
[Rh2(NCCH3)8(H20)2][PF5]4 from either rhodium(II) acetate or
Na4[Rh2(SO4)4(H20)2] through the action of trifluoromethanesulfonic
acid.48 Recrystallization of the initial product in the presence of N aPFe
gave the [Rh2(N00H3)3(H20)2][PF5]4 salt. X-ray characterization and
further reactivity studies had not been carried out before our studies were
reported.49 A detailed investigation of other methods of synthesis and the
elementary reactivity of the cempound would prove quite advantageous to
future work.

The paucity of dinuclear solvated complexes and their potential
usefulness led us to explore the designed synthesis of other examples of
these cations. Primarily, we wished to establish general methods of
preparation to access these species, especially considering the problematic
syntheses described by others. Successful strategies beginning from the
mononuclear investigations may be applied when possible, but it is realized

that dinuclear species may not necessarily be prepared conveniently

22

through these routes. The possibility of using starting materials for
dinuclear species such as the carboxylates broadens the potential
techniques for producing different metal analogs. The broad goal of the
research described in this thesis was to investigate, in a general manner,
the possible methods for accessing various [M2(NCCH3)x]n+ species. In
addition, the full characterization and solution chemistry of these systems

was targeted.

959‘s“?

9°.“

10.

11.

12.

13.

14.
15.

16.

LISI‘ 0F REFERENCES, CHAPTER I

Manzer, L.E. Inorg. Synth. 1982, 21, 135.

(a) Burgess, J. in Comprehensive Coord. Chem. 1987, 2, 295. (b)
Brar, A.S.; Sandhu, H.S.; Sandu, S.S. Indian, J. Chem. Sect. A.
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Aldrich Catalog, Aldrich Chemical Company, 1992.

Walton, R.A. Q. Rev., Chem. Soc. 1965, 19, 126.

West, R.J.; Lincoln, S.F. Aust. J. Chem. 1971, 24, 1169.

Davies, J .A.; Hartley, F.R. Chem. Rev. 1981, 81, 79.

Storhoff, B.N.; Lewis, H.C.Jr. Coord. Chem. Rev. 1977, 23, 1.
Endres, H. in Comprehensive Coord. Chem. 1987, 2, 261.
Clarke, R.E.; Ford, P.0. Inorg. Chem. 1970, 9, 227.

Rapaport, I.; Helm, L.; Merbach, A.E.; Bernhard, P.; Ludi, A. Inorg.
Chem. 1988, 27, 873.

Barrera, J.; Sabat, M.; Harman, W.D. J. Am. Chem. Soc. 1991, 113,
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(a) Bland, W..;J Kemmitt, R.D..;W Moore, R..D J. Chem. Soc. Dalton
Trans. 1973,1292. (b)Farona,M.F.,Kraus, KF. Inorg. Chem. 1970,
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(a) Wright, T.C.; Wilkinson, 0.; Motevalli, M.; Hursthouse, M.B. J.
Chem. Soc. Dalton Trans. 1986, 2017. (b) Anderson, S.J.; Wells, F.J.;
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Organometallics, 1988, 7, 650. (d) Bassi, I..W ,Benedicenti, 0.;
Calcaterra, M. Intrito, R.; Rucci, G.; Santini, 0. J. Organomet.
Chem. 1978, 144, 225.

Johnson, A.; Taube, H. J. Indian Chem. Soc. 1989, 66, 503.

(a) Purcell, K.F.; Drago, R.S. J. Am. Chem. Soc. 1966, 88, 919. (b)
Purcell, K.F.; J. Am. Chem. Soc. 1967, 89, 247.

(a) Zurr, A.P.; Groeneveld, W.L., Part I, Recueil, 1967, 86, 1089. (b)
Reedijk, J.; Groeneveld, W.L., Part II, Recueil, 1967, 86, 1103. (c)

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Reedijk, J.; Zuur, A.P.; Groeneveld, W.L., Part III, Recueil, 1967, 86,
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Recueil, 1968, 87, 755. (e) Reedijk, J.; Groeneveld, W.L. Part V,
Recueil, 1968, 87, 513. (f) Reedijk, J.; Groeneveld, W.L., Part VII,
Recueil, 1968, 87 , 1079. (g) Reedijk, J. Part D(, Recueil, 1969, 88, 86.
(h) Reedijk, J .; Groeneveld, W.L. Part X, Recueil, 1968, 87, 1293. (i)
Reedijk, J.; Groeneveld, W.L. Part XI, Recueil, 1969, 88, 655. (j)
Reedijk, J .; Vervelde, J .B.; Groeneveld, W.L. Part XII, Recueil, 1969,
88, 42.

Gutman, V.; Hampel, 0.; Lux, W. Mh. Chem. Ed. 1965, 96, 533.

Billinger, P.N.; Claire, P.P.K.; Collins, H.; Willey, G.R. Inorg.
Chim. Acta 1988, 149, 63.

Bougon, R.; Charpin, P.; Christe, K.0.; Isabey, J .; Lance, M.;
Nierlich, M.; Vigner, J.; Wilson, W.W. Inorg. Chem. 1988, 27, 1389.

Chandrasekhar, P.; Bird, P.H. Inorg. Chim. Acta 1985, 97, L31.

De Renzi, A.; Panunzi, A.; Vitagliano, A. J. Chem. Soc. Chem.
Comm. 1976, 47.

Goldstein, A.S.; Drago, R.S. Inorg. Chem. 1991, 30, 4506.

(a) Hathaway, B.J.; Holah, D.G.; Underhill, A.E. J. Chem. Soc. 1962,
2444. (b) Hathaway, B.J.; Underhill, A.E. J. Chem. Soc. 1960, 3705.

Thomas, R.R.; Sen, A. Inorg. Synth. 1989, 26, 128.

Thomas, R. R.; Venkatasuryanarayana, 0.; Sen, A. J. Am. Chem.
Soc. 1986, 108, 4096.

Johnson, P.R.; Pratt, J.M.; Tilley, R.I. J. Chem. Soc. Chem. Comm.
1978, 606.

Habeeb, J.J.; Said, F.F.; Tuck, D.G. J. Chem. Soc. Dalton Trans.
1981, 118.

Csiiregh, 1.; Kierkegaard, P.; Norrestam. R. Acta Cryst. 1975, B31,
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Sisley, M.J.; Yano, Y.; Swaddle, T.W. Inorg. Chem. 1982, 21, 1141.
Nilsson, K.; Oskarsson, A. Acta Chem. Scand. A 1984, 38, 79.

Richert, S.A.; Tsang, P.K.S.; Sawyer, D.T. Inorg. Chem. 1989, 28,
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a

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25

Bown, M.; Fontaine, X.L.R.; Greenwood, N.N.; Kennedy, J .D.;
Thornton-Pett, M. J. Chem. Soc. Dalton Trans. 1987, 1169.

Leban, I.; Gantar, D.; Frlec, B.; Russell, D.R.; Holloway, J.H. Acta
Cryst. 1987, C43, 1888.

Stork-Blaisse, B.A.; Verschoor, G.C.; Romers, C. Acta Cryst. 1972,
B28, 2445.

Constant, G.; Daran, J-0.; Jeannin, Y. J. Organomet. Chem. 1972,
44, 353.

Tate, D.P.; Knipple, W.R.; Augl, J .M. Inorg. Chem. 1962, 1, 433.

Chan, L.Y.Y.; Isaacs, E.E.; Graham, W.A.G. Can. J. Chem. 1970,
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Connelly, N.G.; Draggett, P.T.; Green, M.; Kuc, T.A. J. Chem. Soc.
Dalton Trans. 1977, 70.

(a) Maspero, F.; Taube, H. J. Am. Chem. Soc. 1968, 90, 7361. (b)
Ziolkowski, J.J.; Taube, H. Bull de L'acad. Polonaise des Sciences.
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(a) Legzdins, P.; Rempel, G.L.; Wilkinson, G. Chem. Commun. 1969,
825. (b). Legzdins, R; Mitchell, R.W.; Rempel, G.L.; Ruddick, J.D.;
Wilkinson, G. J. Chem. Soc. A. 1970, 3322.

(a) Wilson, 0.R.; Taube, H. Inorg. Chem. 1975, 14, 2276. (b) Wilson,
C.R.; Taube, H. Inorg. Chem. 1975, 14, 405.

(a) Hills, E.F.; Moszner, M.; Sykes, A.G. Inorg. Chem 1986, 25, 339.
(b) Moszner, M.; Ziélkowski, J .J. J. Organomet. Chem. 1991, 411,
281. (c) Moszner, M.; Wilgocki, M. Ziolkowski, J.J.; J. Coord. Chem.
1989, 20, 219.

(a) Bowen, A.R.; Taube, H. J. Am. Chem. Soc. 1971, 93, 3287. (b)
Bowen, A.R.; Taube, H. Inorg. Chem. 1974, 13, 2245.

Richens, D.T.; Sykes, A.G. Inorg. Synth. 1985, 23, 130.
Mayer, J.M.; Abbott, E.H. Inorg. Chem. 1983, 22, 2774.

Abbott, E.H.; Schoenewolf, F.Jr.; Backstrom, T. J. Coord. Chem.
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Cotton, F.A.; Wiesinger, K.J. Inorg. Chem. 1991, 30, 871.

48.

49.

%

Baranovskii, I.B.; Golubnichaya, M.A.; Zhilyaev, A.N.; Shchelokov,
R.N. Soviet J. Coord. Chem. 1988, 13, 369.

Dikareva, L.M.; Andrianov, V.I.; Zhilyaev, A.N.; Baranovskii, I.B.
Russ. J. Inorg. Chem. 1989, 34, 240.

 

CHAPTER II

REACTIONS OF PARTIALLY SOLVATED CATIONS
WITH TRANSITION METAL AN IONS

27

28

A. Introduction

In the mid 1980's, two groups independently reported the partially
solvated acetonitrile species of the form, [M2(0Ac)2(MeCN)6]2+ for M = Rh,
Mo, 1 prepared under mild conditions by the action of Et30BF4 on the

tetracarboxylates, M2(0Ac)4. The structure, as shown below,

A 72+

possesses two bridging carboxylates in a cis disposition to each other with
coordinating solvent occupying the other four equatorial sites and the two
axial sites. The axial ligands exhibit longer M-N distances than the

equatorial groups, indicating these solvent molecules are less tightly held;

This is easily explained by the trans influence of the M-M bond.

The ligand distribution in these cations presents an unusual case
wherein one side of the molecule is stabilized by anionic bridging groups
while the other side, which possesses more labile neutral groups, is open for
reactivity. The equatorial solvent molecules bound to the molybdenum
species are actually substantially more labile than those of the rhodium
system, as evidenced by 1H NMR spectroscopy. The MeCN ligands on the
[M02 12+ unit rapidly exchange for deuterated solvent to give only a singlet in
the 1H NMR spectrum due to free uncoordinated solvent. 1c The [Rh2]2+

29

system displays two discrete MeCN resonances in the 2:1 ratio dictated by
the presence of four coordinated equatorial ligands and two free MeCN
ligands lost from the axial positions due to exchange with CD3CN.1c This
lability difference between dimetal units is not restricted to solvent ligands.
For M=Mo, rapid scrambling of carboxylates between molecules occurs in the
reaction of M2(02CR)4 and M2(02CR')4, while no such process is observed for
M=Rh.2

Chisholm and Cayton, in some recently published work, proposed this
difference in lability to be related to the difference in the M-M molecular
orbital configuration.2 The Mo-Mo tetra-cation has a configuration of 0371453
with low-lying, empty 6* and 11* orbitals localized on the metal centers which
allows ligand exchange to take place via an associative mechanism. By
contrast, the Rh-Rh single bond possesses an electron rich 03714636*37r*4
configuration which effectively shuts down the associative mechanism and
makes the equatorial sites relatively inert, similar to the octahedral t2g6 MLe
complexes whose reactivity is very slow.

The reactivity of [M02(0Ac)2(MeCN)6]2+ has not been extensively
investigated. The reaction of this species with the bidentate ligand, dmpe,
(dmpe = 1,2-dimethylphosiphinoethane) results in the displacement of four
solvent ligands with concomitant isomerization to the trans compound,

[M02 (0Ac)2 (dmpe)2 ][BF4]2.3 Tridentate donors such as the macrocyclic
thioether, 1,4,7-trithiacyclonone (T’I‘CN), displace three solvents in a facial
arrangement to chelate to a single metal center. Even with two such groups,
the metal-metal bond is preserved by the presence of the acetates.4

Reactivity is not limited to the solvated side of the Mo-Mo unit. The
acetate ligands may be selectively exchanged for a different carboxylate by

stirring the initial compound in the incoming carboxylic acid for 12 h. The

30

acetonitrile ligands remain on the molecule while ligand substitution occurs.5
It is interesting to note that the substituted species with menthoxyacetate
and propenoate prepared by this method are effective room temperature
catalysts for the polymerization of cyclopentadiene.5

Both the Mo and Rh systems offer a versatile combination of salvation
and cationic charge. Reactions of these dications with dinuclear transition
metal anions were investigated in the expectation that the resulting partially
solvated mixed-metal salts could be used as precursors for mixed-metal
clusters or extended solids. The isolation and characterization of several

products isolated from these reactions are described herein.
B. Experimental, Synthesis

(1) Synthesis of [Rh2(0Ac)2(MeCN)6][Re20lsl (1)

(i) Method 1

The starting materials, [Rh2(0Ac)2(Me0N)6][BF4]2 and [n-
Bu4N]2[Re2018] were prepared by literature methods“):6 In a typical
reaction, 116 mg of [Rh2(OAc)2(Me0N)5][BF4]2 (0.156 mmol) and 175 mg er
[n—Bu4N] 2[Re20131 (0.1531mmol) were stirred in 15 mL of MeCN for 90 min.
A lavender precipitate in a navy blue solution began forming within ca. five
min. After 30 min, the solid was allowed to settle out, the solvent was
decanted and the purple solid was washed with 4 x 10 mL of MeCN to remove
residual [n-Bu4N][BF4] and dried in vacuo; yield was 132 mg, (70%). IR (081,
Nujol), cm'lz 2329 (w), 2308 (w), 2304 (w), 2280 (w), 1555 (m), 1538 (m), 1057
(m), 1037 (m), 338 (s).

31

(ii) Method 2

A quantity of [Rh2(0Ac)2(MeCN)6][BF4]2 (116 mg, 0.156 mmol) was
dissolved in 5 mL of MeCN to give a burgundy colored solution which was
layered over a teal blue solution of [n-Bu4N]2[Re2013] (175 mg, 0.153 mmol)
dissolved in a mixture of MeCN and CH 2012. Ratios of this solvent mixture
varied from 18:0.2, to 10:1 to 3:2 mL of MeCN : CH2012. Air was accidentally
leaked into the first reaction through an open stopcock, and subsequent
experiments showed that this expedited crystal growth. Slow diffusion of the
two reactant solution layers afforded a large amount of black microcrystalline
needles that tended to grow from a single point in "bow tie" fashion from a
dark navy solution. Chilling the Schlenk tube increased the amount of
crystals formed, but generally they were not of X-ray quality. Crushing the
dark crystals produced a lavender powder. Single crystals of this compound
suitable for X-ray study were obtained from the first solvent ratio, but
repeated attempts to reproduce larger crystals led to only microcrystalline
product. After the solvent was removed by decanting, the crystals were
washed with several 10 mL portions of MeCN and dried in vacuo. Yield 60
mg, (32%). Reducing the volume of the decanted solution led to further
precipitation of the lavender product. IR data (081, Nujol), cm-l: 2327 (m),
2311 (w), 2301 (w), 2279 (m), 1557(8), 1538(8), 1029 (m), 335 (8). Anal. Calcd
for 016H24018N604Re2Rh2: 0, 15.67; H, 1.97; 01, 23.13. Found: C, 15.95; H,
2.05; 01, 24.59.

(2) Reaction of [Rh 2(0Ac)2(Me0N)61[Re2 013] in MeCN
A sample of 1 in the form of either the lavender solid or black

microcrystals was suspended in a small volume of MeCN (25 mL). The salt

32

was initially insoluble, but gradually dissolved to form an intensely blue
colored solution with no sign of residual solid. For 20 mg of solid, (0.016
mmol) this process was complete within 24 h. The solution was subjected to a
dynamic vacuum to produce a green residue which inevitably formed a glassy
coating on the walls of the vessel. The solid dissolved in MeCN and Me0H to
yield a blue solution and in acetone to yield a kelly green solution. The
product was slightly soluble in 0H2 012 in which it produces a pale green
solution and was insoluble in diethyl ether, benzene, hexanes, and toluene.
Crystallization attempts from MeCN/toluene appeared to yield two different
solid products. Other solvent combinations did not yield precipitates. IR
(CsI, Nujol), cm '1: 2330 (w), 2300 (w), 1554 (m), 1520 (m), 1030 (w), 720 (m),
350 (s), 320 (m).

(3) Reaction of [Rh 2(0Ac)2(MeCN)5][Re2 013] in 0112012

A small amount of the lavender solid 1 (20 mg, 0.016 mmol) was
suspended in ca. 4 mL of 0H2 012. After 2 h, the solid had become a slate blue
color without dissolving and the solution was a pale green color. After
standing for three weeks, the solution was yellow-brown, but the slate blue
solid remained. The 0H2 012 solution (Am = 670 nm) was decanted and
reduced to a yellow residue which did not dissolve in MeCN. After MeCN
was added to the blue solid, it reverted back to the original lavender color,

again without dissolving.

(4) Reaction of [Rh 2(0Ac)2(Me0N)s][Re2 013] in Acetone
A small volume of acetone (3-5 mL) was added to a sample of the
lavender solid, 1 (20 mg, 0.016 mmol). Within 2 h, the solvent had turned

turquoise green, but most of the lavender solid remained undissolved. After

33

standing for three weeks, the solution was emerald-green, but the suspended
solid was a forest green color. IR of the forest green solid, (CsI, N ujol), cm'lz
2330 (w), 2300 (w), 1680 (s), 1550 (s), 1238 (m), 1028 (w), 340 (vs). The
solution was decanted from the solid and reduced to a residue (IR on the
green residue from evaporation {0sI, Nujol}, cm '1: 2330 (W), 2300 (w), 1690
(w), 1600 (w), 1520 (w), 690 (m), 345 (m), 320 (m)) which turned to teal green
when redissolved in MeCN (Am: 610, 323 nm). The forest green solid

became a dark yellow-green upon exposure to MeCN.

(5) Reaction of [Rh 2(0Ac)2(Me0N)3][Re2 013] in THF

Addition of THF (ca. 25 mL) to a sample of the lavender product, 1, (20
mg, 0.016 mmol) produced a pale blue solution over a mixture of blue and
purple solids within 2 h. Prolonged exposure for ~3 weeks led to a greenish-
yellow solution and a blue solid. The solvent (Am = 600 nm) was decanted
and reduced to a residue of a quantity insufficient for analysis. The blue solid
(IR {0sI, Nujol}, cm‘1: 2325 (w), 2300 (w), 1565 (m), 1550 (In), 1030 (m), 880
(m), 338 (s)) was dried in vacuo and formed a pink/blue dichroic solution

when exposed to MeCN. (UV-visible in MeCN, Am at 680 and 545 nm.) '

(6) Thermal Reaction of [Rh2(0Ac)2(MeCN)6][Re2013] in the Solid
State

A quantity of 1 , ( 40 mg, 0.033 mmol) was placed in a Schlenk tube and
warmed gently in an oil bath. At 80°C, there was no apparent change. After
heating at 95° for an hour, the solid turned olive green. Further temperature
increase to 145° did not appear to effect any further reaction. The flask was
cooled to room temperature, and was subsequently evacuated under dynamic

vacuum to remove any liberated MeCN solvent. (IR on the olive green solid

34

{081, Nujol}, cm'1: 2335 (w), 2310 (W), 2284 (W), 1550 (s), 1030 (m), 340 (5)).
Addition of 3 mL of MeCN to the solid gave a green solution and a brown
solid. IR on the brown solid (CsI, Nujol), cm'1: 1540 (m), 1020 (W), 690 (W),
350 (m,br).

(7) Work-up of Layer Reaction solutions

After the product was isolated from the [Rh2(0Ac)2(MeCN )ellRe2Cl3l
crystallization reactions in Synthesis Method 2, the decanted solutions were
stored aerobically in vials. After several days, the MeCN/0H 2012 solutions
had turned dichroic wine/blue and had produced a small amount of white
precipitate. ( Amax = 605 nm, shoulder at 430 nm). The solid was removed by
filtration, and several combinations of CH2 012, acetone, diethyl ether and

MeCN were used in an attempt to isolate a pure compound without success.

(8) Reaction of [Rh 2(0Ac)2(Me0N )31[BF4]2 with Chloride ion

An amount of [Rh2(0Ac)2(Me0N)3][BF4]2 (0.040 g, 0.054 mmol) and
0.0505 g of ppnCl (0.088 mmol) was stirred in 5 mL of MeCN at room
temperature. Within two minutes, the solution had turned dark bluish-
purple. After 24 hours, a fluffy green solid settled out of a pinkish-purple
solution. (UV-visible of purple solution, Am = 550 nm). The solvent was
decanted, and the solid was washed with 5 mL of MeCN. (IR data for the
green solid {0sI, Nujol}, cm‘lz 2330 (W), 2300 (W), 1560 (s), 1020 (s), 370 (vw),
310 (vw).) The green solid was soluble in CH2012 to give a UV-visible Am of
610 nm.

35

(9) Control reaction of [Re2013]2' with MeCN

A sample of [n-Bu4N]2[Re2013] was dissolved in MeCN and stored at
room temperature for a week in order to note if any decomposition had
occurred. The electronic spectrum verified the presence only of the original

compound, (Am = 680 nm).

0. Experimental, Crystallography
[Rh2(0Ac)2(MeCN)s11Re20131

(i) Data Collection and Reduction.

Due to difficulties with refinement of the original data set, a second
crystal was also studied by X-ray methods and the results are given here. A
black single crystal of 1 grown by Synthesis Method 2 and having
approximate dimensions 0.7 x 0.2 x 0.2 mm was mounted with epoxy on a
glass fiber and was examined on a CAD-4 diffractometer at several
temperatures. A preliminary cell was established by centering on reflections
chosen from a rotational photograph and the final cell was indexed on 25
reflections with 21 < 20 < 28.30. Axial photographs, as well as photographs of
the [111] and [110] diagonals, verified the choice of orthorhombic symmetry
rather than tetragonal in spite of two axes exhibiting nearly identical
lengths. Five slightly different cells were obtained at various temperatures
and are shown in Table 1. Although the cell parameters varied as the
temperature was lowered, the symmetry did not reach tetragonal.

Intensity data were collected at -100i 2 °C over the range 4-45° using
the w- scan mode. Three periodically monitored standard reflections were

collected every hour and indicated the crystal decayed by an average of 6.7%.

36

Table 1. Unit Cells of [Rh 2(0Ac)2(Me0N )3][Re2 013] determined at

different temperatures

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Temp in °C 23 0 -8 -50 - 100
a 2723(1) 2795(1) 2752(4) 26.875(4) 27.42(1)
b 27.521(3) 27.504(7) 2700(4) 27.482(9) 26.736(4)
c 18.203(2) 18.193(5) 1822(5) 18.164(5) 18.137(5)
a 8999(1) 8992(2) 89.89( 4) 90.03( 2) 90.03( 2)
l3 8998(2) 8999(3) 8992(5) 90.03(2) 9004(3)
X 8986(2) 90.03(3) 9018(6) 8998(2) 9002(1)
volume 13643(6) 13535( 9) 13540(20) 13415(6) 13296(7)

 

 

37

9451 unique data were collected of which 5229 were observed with F022

30(F 012- Psi scans were applied to correct for absorption.

(ii) Structure Solution and Refinement.

All calculations were carried out on a VAXSTATION 4000 computer
using the TEXSAN software package. The positions of the rhenium and
rhodium atoms were established by application of the SHELXS-86 solution
program. 7 Most of the remaining non-hydrogen atoms were located using
DIRDIF;8 alternating least squares and Fourier maps were used to locate and
refine the rest of the atoms. A disorder of the Re4-Re5 metal unit became
apparent quite early in the refinement. The occupancy of both orientations
were experimentally determined and fixed for final refinement. Although
Psi-scans had been applied to correct for absorption, the large number of
heavy atoms in the structure led us to further correct for absorption by the
use of the program, DIFABS. Unfortunately, this did not entirely alleviate
the problem, as a large number of peaks in the final difference map were
above 1.0 e'/A3, 10 of which approached 2.0 e’/A3. The highest peaks in the
map were near the dirhenium units, but did not make chemical sense as .
disordered metal units. These peaks persisted even after anisotropic
refinement of all metal and chloride atoms. The N and 0 atoms in the
immediate coordination sphere of the two dirhodium units became non-
positive definite when refined anisotropically, so these and all the carbons
were left isotropic. There were no non-positive definite atoms in the final
refinement. The final full-matrix refinement involved 483 parameters and
5229 data for a data to parameter ratio of 10.8. The refinement converged
with residuals of R = 0.078 and Rw = 0.109 and quality-of-fit = 2.64. The

maximum shift/error in the last cycle was 0.05. The highest peak in the final

38

difference map was 2.88 e'/A3 with about ten peaks of 2.0 e'/A3or below. A

summary of the crystallographic data is found in Table 2.

D. Results and Discussion

Synthetic Methods

Our original intention in preparing mixed-metal salts was to pursue
the possibility of stacking dinuclear transition metal units in the solid state
in such a way that there would be conductivity along the M-M axes (See
Figure 4). The Rh unit is most likely to form extended metal-metal
interactions if the axial ligands could be removed.

The reaction to form 1 is a simple metathesis using very large
constituent ions, shown in Figure 5. The insolubility of the mixed-metal salt
not only in acetonitrile but all common solvents drives the reaction towards
the kinetic product which is easily isolated before it has time to convert to the
thermodynamic products. Since further reactivity upon prolonged exposure
to solvents makes it impossible to recrystallize the sample, crystals may be
grown only by slow diffusion of the reactants in solution. The slow rate of the
diffusing solution produces crystals rather than a powder.

It might be expected that ions of this size would possess a rather low
lattice energy, but this is apparently not the case. One consideration is that

the ions are very well matched in size which is favorable for salt formation.

Spectroscopy
Infrared data verify that the lavender bulk product and the black
microcrystalline product are the same compound. The IR spectrum is

essentially an overlap of the corresponding spectra for the two starting

39

Table 2. Crystal Data for [Rh 2(0Ac)2(MeCN )3][Re2 013]

Formula
Formula weight

Space group

a, A
b, A
c, A
(1, deg
13. deg
Y. deg
v, A3
Z

dcalc, g/cm3

p (Mo Ka), cm‘1

Data collection instrument
Radiation (monochromated in
incident beam)
Temperature, °C

Scan method

Trans. factors, max., min.
Ra

wa

Rh2C16N504RezC|8
1226.241

Pccn

27.42(1)
26.736(4)
18.137(5)
90

9O

90
13296(7)
16

2.450

90.05

CAD-4

Mo Ka().a = 0.71073A)

-100 i 2
cc

0.75 - 1.16
0.078

0.109

 

 

aR=£ l 11:01 " chll/ZlFol

bRW = [rwlr0 I- IFc |)2/Zw|F012]l/2; w = 1/oZ( lFol)

40

Figure 4. Potential stacking configurations for the bridged and unbridged

dimetal units

41

l l = Repeat Unit

No Bridges

 

 

cis-Bridges

 

 

Figure 4

 

42

Figure 5. Diagram of the dinuclear ionic components of

[Rh2(OAC)2(MeCNlellRe2C13l

43

‘72.

L L

l .\“\\L . ,8»

L—— Bil—RH;— L
0/ l o/ l
U 0
V

[Rh2(OAc)2(MeCN)6]2+

L

 

 

 

materials. The v(Re-Cl) mode falls at about 335 cm'1 , and the prominent
v(OCO) is centered at 1550 cm '1. The v(C EN) stretching region between 2250
and 2350 cm"1 is more complex than that of the [BF4 ]' precursor in this
spectrum, presumably due to solid-state splitting effects. The starting
material, [Rh2(0Ac)2(MeCN)3][BF4]2, exhibits three medium stretches of
equal intensity in this region between 2340 and 2277 cm '1. By comparison,
the infrared spectrum of 1 displays a pattern wherein the two outer stretches
are more intense than the inner stretches for a total of four bands in this
region.

The electronic spectrum of the bulk [Rh2(OAc)2(MeCN)3][Re2013]
reaction solution is, again, a combination of the individual spectra for the
starting materials, indicating a slight solubility of the salt in MeCN. The
main features are at 680 nm {55* for [Re2Cl3]Z} and 540 nm
{[Rh 2(0Ac)2 (MeCN 1612+}. Solid state resolution is not as great due to the
slaping baseline, but electronic spectra on finely divided N ujol mull samples '
verified the existence of these two features in the solid salt as well.

Due to the general insolubility of the compound, accurate
characterization was limited to the solid state. However, some information
was obtained from freshly prepared solutions of the mixed-metal salt.
Dissolution of the compound to produce a dilute sample in CD30N gave a 1H
NMR spectrum with primarily free CH 3CN at 5 = 1.95 ppm due to solvent
exchange with the deuterated solvent. Very small singlet resonances were
also observed at 5 = 3.23, 3.13, 2.12, 2.01, 1.99, and 1.97 ppm. None of these
match particularly well with the spectrum of the [Rh2(OAc)2 (MeCN)3][BF4]2
which has singlets at 5 = 2.54 ppm due to equatorial MeCN, 2.04 ppm for
bound acetate, and 1.95 ppm for the exchanged axial MeCN ligands. This

45

result suggests that decomposition has taken place and there is, most likely,

a mixture of products in the NMR sample.

Molecular Structure

Selected bond distances and angles for [Rh 2( OAc)2(MeCN)3][Re 2013]
are given in Table 3 and 4. Atomic positional parameters are included in the
Appendix. An ORTEP packing diagram of the unit cell is shown in Figure 7
with a simplified representation in Figure 8. The asymmetric unit contains
two unique dirhodium cations and one dirhenium anion on general positions,
and two dirhenium units, one along a C2 and the other bisected by a C2 axis.
Both of the dirhodium bis-acetate cation units gave similar bond distances
and bond angles as the original structure with [BF4]' as the counterion.1c
The average Rh-Rh bond distance is 2.504(4)A compared to 2.534(1)A, the Rh-
0 is 2.01(3)A compared to 2.015(4)A, Rh—equatorial N of 1.97(3)A compared to
1.983(4)A, and Rh-axial N of 2.23(3)A compared to 2.232(4)A. One [Re201312'
anion lies along a crystallographic C2 axis rendering the two Re atoms half-
occupied, each bound to two unique chlorides. Another [Re2 013]2 is bisected
perpendicular to the Re-Re bond, therefore only one Re atom and its attached
four chlorides are unique.’ As far as we are aware, the present structure is
the only one in which a [Re2X3]2r unit is present in the same asymmetric unit
with three different environments.9'11

A closer look at the packing diagram reveals the importance of cation
shape on the local environments of the three unique [Re 2013 ]2' units. One
type of [Re2 12, unit can be seen to stack in an end-to-end fashion (Figure 8b)
in a four-point star-shaped channel in which the chlorides point between the
MeCN ligands of the [Rh2]2+ cations and toward the Rh atoms. The second

independent [Re2 013]?” unit forms a stack with perpendicularly aligned metal

atom
REl
REl
REl
R81
R81
RE2
RE2
RE2
RE2
R82
R83
RE3
RE3
R33

RE4
RE4a

RE4
R84
RE4
R84

RES

46

Table 3. Selected Bond Distances in A for
[Rh2(OA012 (MeCN)61[Re2 C13]

atom
REI'
CL1
CL2
CL3
CL4
RE3
CLS
CLS'
CL6
CL6'
CL?
CL7'
CL8
CL8'

RES
RESa

CL11
CL12
CL14

CL15
0L9

CLIO

distance

2.225(4)

2.31(1)
2.28(1)
2.33(1)
2.32(1)

2.216(3)

2.31(1)
2.31(1)
2.32(1)
2.32(1)
2.30(1)
2.30(1)
2.32(1)
2.32(1)

2.215(4)

2.20(1)
2.31(1)
2.32(1)
2.29(1)

2.30(1)
2.35(1)

2.31(1)

atom
RES

RES
RHl

R81
R81
RHl
RHl
RBI
RH2
RHZ
RHZ
R82

RHZ

atom
CL13

CL16
R32

02
04
N4
N5
N6
01
03
N1
N2

N3

distance
2.32(1)
2.33(1)
2.509(4)
1.99(2)
2.01(3)
1.9e(3)
1.99(3)
2.21(3)
2.03(3)
1.99(2)
1.98(3)
1.97(4)
2.23(3)

47

Table 3. continued

atom atom distance atom atom distance
01 C1 1.40(4) RH4 N7 1.90(3)
02 c1 1.29(4) 334 N8 1.99(3)
03 c3 1.24(4) 334 N9 2.22(3)
04 C3 1.37(5) 05 C17 1.31(5)
N1 C5 1.11(S) 06 C17 1.21(4)
N2 C7 1.14(5) 07 C19 1.26(4)
N3 C9 1.12(4) 08 C19 1.21(4)
N4 c11 1.12(4) N7 C21 1.10(5)
N5 C13 1.11(4) N8 C23 1.15(5)
N6 C15 1.10(4) N9 C25 1.15(4)
c1 c2 1.42(6) N10 C27 1.22(5)
C3 C4 1.34(5) N11 C29 1.14(4)
C5 C6 1.51(5) N12 C31 1.07(5)
C7 C8 1.50(7) C17 C18 1.50(5)
C9 C10 1.55(5) C19 C20 1.52(7)
C11 C12 1.57(6) C21 C22 1.58(6)
C13 C14 1.46(5) C23 C24 1.44(6)
C15 C16 1.53(6) C25 C26 1.49(5)
RH3 R84 2.498(4) C27 C28 1.42(5)
R83 06 2.03(2) C29 C30 1.53(5)
R83 07 2.01(2) C31 C32 1.38(5)

Raz N10 2.00(3)
RH3 N11 1.96(3)
333 N12 2.26(4)
334 05 2.07(3)
Ra4 O8 1.97(3)

atom
881
881
881
881
CL1
CL1
CL1
CL2
CL2
CL3
R83
883
R83
883
CLS
CLS
CLS
CLS
CLS
CL6
882
R82
882
R82
CL7
CL7

Angles are in degrees.
Significant figure are 9

Table 4. Selected Bond Angles in degrees for
[Rh 2(0Ac)2 (MeCN )61[Re2 013]

atom
R81
R81
R81
881
881
881
881
881
881
881
882
882
882
882
882
882
882
882
882
882
883
883
883
883
883
883

atom
CL1
CL2
CL3
CL4
CL2
CL3
CL4
CL3
CL4
CL4
CLS
CLS
CL6
CL6
CLS
CL6
CL6
CL6
CL6
CL6
CL7
CL7
CL8
CL8
CL7
CL8

angle
104.9(3)
103.2(3)
104.6(3)
104.2(3)
86.3(4)
150.4(4)
86.4(4)
87.1(4)
152.7(5)
86.4(5)
103.9(2)
103.9(2)
104.0(2)
104.1(2)
152.2(5)
86.3(4)
87.0(4)
87.0(4)
86.3(4)
151.9(4)
102.9(2)
103.0(2)
103.8(2)
103.9(2)
154.0(5)
87.0(4)

atom

CL7
CL7
CL7
CL8
885
885
885
885
CL11
CL11
CL11
CL12
CL12
CL14
885A
0L9
0L9
0L9
CLIO
CLIO
CL14
884
884
884
884

atom

883
883
883
883
884
884
884
884
884
884
884
884
884
884
884A
884A
884A
884A
884A
884A
884A
885
885
885
885

atom
CL8
CL8
CL8
CL8
CL11
CL12
CL14
CL15
CL12
CL14
CL15
CL14
CL15
CL15
CL15
CLIO
CL14
CL15
CL14
CL15
CL15
0L9
CLIO
CL13
CL16

angle
86.8(4)
86.8(4)
137.0(4)
152.3(5)
101.8(4)
102.7(4)
103.0(3)
104.1(3)
85.5(5)
88.7(5)
154.1(5)
154.4(5)
87.5(4)
137.0(4)
107.0(2)
87.2(5)
87.4(5)
152.4(6)
153.8(6)
88.0(5)
84.9(5)
101.5(3)
103.0(3)
105.5(4)
104.3(4)

Estimated standard deviations in the least

iven in parentheses.

atom
CL9
CL9
CL9
CLIO
CL10
CL13
884A
884A
884A
884A
CL11
CL11
CL11
CL12
CL12
CL13
882
882
882
882
882
02
02
02

Angles are in degrees.

atom

885
885
885

885
8858
885A
8858
885A
8858
8858
885A
8858
8858
RESA
881
881
881
881
881
881
881
881

atom
CL10
CL13
CL16
CL13
CL16
CL16
CL11
CL12
CL13
CL16
CL12
CL13
CL16
CL13
CL16
CL16
02
04
N4
NS
N6
04
N4
NS

Table 4. continued

angle
87.1(4)
87.4(4)
154.2(5)
151.5(5)
87.5(4)
85.3(4)
96.1(6)
96.5(6)
106.0(6)
109.0(6)
78.0(5)
89.5(6)
153.3(7)
155.3(7)
89.6(6)
92.5(7)
86.1(6)
83.4(9)
95.0(8)
97.1(7)
174.0(9)
88(1)
179(1)
90(1)

49

atom
02
O4
O4
04
N4
N4
NS
881
881
R81
881
R81
01
01
01
01
03
O3
03
N1
81
N2

atom
881
881
881
881
881
881
881
882
882
882
882
882
882
882
882
882
882
882
882
882
882
882

atom
N6
N4
N5
N6
N5
N6
N6
01
03
N1
N2
N3
03
N1
N2
N3
N1
N2
N3
N2
N3
N3

angle
89(1)
91(1)
178(1)
94(1)
91(1)
90(1)
86(1)
87.0(6)
84.9(6)
95.2(9)
98.3(9)
174.0(8)
88.0(9)
178(1)
87(1)
89(1)
93(1)
174(1)
90(1)
92(1)
89(1)
86(1)

. . _ . gstimated standard deviations in the least
Significant figure are given in parentheses.

atom
884

884
884
884
884
06
O6
O6

Angles are in degrees.
significant figure are given in parentheses.

atom
883

883
883
883
883
883
883
883

atom
06

07

N10
N11
N12
07

N10
N11

Table 4. continued

angle
83.6(6)
84.6(6)
98.0(9)
96.4(7)
173.5(9)
85.0(9)
178(1)
93(1)

50

atom
06
O7
O7
07
N10
N10
N11
883
883
883
883
883
05
OS
05
OS
08
08
08
N7
N7
N8

atom
883
883
883
883
883
883
883
884
884
884
884
884
884
884
884
884
884
884
884
884
884
884

atom
N12
N10
N11
N12
N11
N12
N12
05
08
N7
N8
N9
08
N7
N8
N9
N7
N8
N9
N8
N9
N9

angle
92(1)
95(1)
177(1)
91(1)
87(1)
87(1)
88(1)
86.2(7)
83.3(8)
99.3(9)
95.5(9)
172.8(8)
89(1)
174(1)
92(1)
87(1)
88(1)
178(1)
95(1)
90(1)
87(1)
87(1)

Estimated standard deviations in the least

51

Figure 6. ORTEP unit cell packing diagram for
[Rh2(OAc)2 (MeCN)6][Re2C18 ]

52

 

 

   
  
 

 
  
  

 

C OCT. ,ij/C)
I .— 3.’ .-; — =3

Figure 6

53

Figure 7. Schematic diagram of the unit cell of [Rh2(OAc)2(MeCN)6][Re2Clg]
showing the packing orientation of the Re-Re units. The singly labeled
locations are units parallel to the viewing angle. The Rh-Rh units are

located in the empty boxes.

54

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Re Re Re—Re Re _ e
Re—Re Re —Re e—l
Re Re
x 5%?
Re Re 8
Re—Re Re—Re Re—Re
Re—Re Re—Re Re—Re
£859 Re .Re
Re’ Re
Re —Re Re—Re Re — 9
—Re Re—Re Re— e

 

 

 

 

 

 

 

 

 

 

Figure 7

55

Figure 8. Representation of the three stacking modes of Re-Re units in the

crystal, [Rh2(OAc)2(MeCN)6][Re2Clgl

\“‘\\

\ ‘0“

 

 

56

 

3:: [A ..z...

  

  

 

 

 

\\ I \\ I \ I
I I \
\\\s .1." \\\~ .11] \\\~ .1!!!
\ \ \
\\ I; \s I/ x l/
\s. .11] \\\ .II/ \\\.. III
a O . o

 

 

 

Figure 8

57

units (Figure 8c) in a second smaller star-shaped channel. Finally, the third
unique rhenium atom (8a) displays a parallel stacking of M-M units in a
larger, ill-defined channel bordered by acetate groups of the surrounding
dirhodium cations. This unit is the only type of octachlorodirhenate in the
structure to exhibit disorder of the metal unit, a feature found for the
[RezClglz' groups in all of the salts in which it has been crystallized. The
minor orientation has an occupancy of 19% which is within the range
reported for other examples.9

A comparison of the Re-Re separations in the three different types of
[R62Clgl2' ions are of interest in this study. The three values of 2.225(4) for
Rel—Rel, 2.216(3) for Re2-Re3, and 2.215(4) for Re4-Re5 are at the lowest
end of the range of values for Re-Re bond which vary from 2.46(8)8 for [2,4,6-
(CH3)3C5H2NH]2R82C18 to 2.21(1)A for [(DMF)2H] 211320812 Ofthe two
units in the current structure that are shorter than any of the known values,
Re2-Re3 possesses no disorder, and Re4-R85 possesses the aforementioned
disorder of 81% / 19%( vice supra).

Study of structures containing the [M 21.8]?r core with various ligands
has recently been the focus of attention by Cotton and coworkers. It is their
goal to establish the influence of counterion and ligand on the crystal packing
forces of the molecule. In no cases previous to our work has a dication of this
large size been used. It has been established from many independent X—ray
results that the M-M unit in M2118 systems tends to pack in a disordered
manner; the magnitude of which depends on the counterion and the
supporting ligands. At one extreme is [n-Bu4N]2[Re213] where the salt is
quite loosely packed and the metal-metal unit disorders equally over all three
possible orientations.9 The other extreme is represented by [n—Bu4N]2[Re2F3]

for which no disorder was observed. 10 These two disparate results

58

underscore the influence of the ligands; the large, highly polarizable iodides
provide no discrimination for the M-M orientation while the small, hard
fluorides distinctly prefer only one site for the dimetal unit. More commonly,
crystal structures of [M2X8]2' complexes where X = Br, or Cl tend to have one
major orientation with one minor orientation being occupied between 5 and
40 %.11 This maximum value was obtained for the crystal structure of [n-

BU4N12IRezBrsl .11°

Reactions of [Rh2(OAc)2(MeCN)3][Re2 C13]

Monitoring the electronic spectrum of the reaction solution of
[Rh2(OAc)2(MeCN)5][BF4]2 and [n-BU4N]2[Re2Clg] showed that over time,
the characteristic features of these two compounds disappeared and a broad
new feature grew in at 610 nm, shown in Figure 9. This feature similarly
appeared when the lavender solid of l was dissolved over time in MeCN. The
infrared spectra of the kinetic and redissolved products also differed, further
supporting the formation of a new thermodynamic product upon dissolution
of the kinetic product 1. The original complicated pattern in the v( C 'N)
region of the [Rh2(OAc)2(MeCN)6][Rezclg] solid state infrared spectrum '
collapsed into two bands, and the v(Re-Cl) stretch in the far-IR region
broadened and changed from one stretch at 338 cm '1 to one at 350 with a
shoulder at 320 cm '1. A 1H NMR spectrum of the residue displays, once
again, principally free MeCN at 5 = 1.95 ppm, but also exhibits singlet
resonances at 3.26, 3.13, and 2.54 ppm. It is assumed that the 2.54 feature is
due to equatorial MeCN. There are a number of other features reminiscent of
the earlier NMR spectrum (0 = 2.48, 2.37, 2.14, 2.01, 1.99, and 1.98 ppm), but

these are of much lower intensity than the others. We concluded from this

59

Figure 9. Qualitative Electronic Spectra of the Kinetic and Thermodynamic
Products of the Reaction between [Rh2(OAc)2(MeCN)6][BF4]2 and
[n-BU4N]2[882C18]. (1) Solution from initial reaction in MeCN. (2)
Kinetic salt in MeCN for 24 h. (3) Thermodynamic product(s) in
MeCN.

Absorbance

b)

 

Figure 9

2
1
_4 .5’ 1 L
400 500 600 700
Wavelength (in nm)

800

61

result that the chemistry is complicated, and is quite likely producing a
mixture of several species.

Numerous efforts to elucidate the identity of the new compound met
with limited success. If ligand redistribution were occurring, it would seem
that either [Re2C18]2 was not stable in MeCN, or the [Rh2 12+ unit was
abstracting chloride from the [Re2]?r anion. To test the first hypothesis, a
sample of [n-Bu4N]2[Re2C18] was dissolved in MeCN and stirred for a week
after which time the electronic spectrum was measured which showed no sign
of decomposition (Am = 680 nm). The second hypothesis was tested by
reacting two equivalents of a soluble Cl' source, namely ppnCl, to the [Rh2]2+
unit. This chemistry did not proceed cleanly, nevertheless we noted that the
product's properties did not match those of the thermodynamic product. An
IR spectrum of the lime-green compound displayed two weak stretches in the
v(C IEN) region, but did not show substantial v(Rh-Cl) features. The electronic
spectrum of this solid in MeCN exhibited one absorption at 550 nm, which is
not the same as that seen for the thermodynamic product. Subsequent
experiments in our labs revealed that the reaction of [Rh 2(OAc)2 (MeCN)6]2+
with ppnCl yields three different products including Rh2(OAc)2 MeCN)4Clz,
Rh2(OAc)4 (MeCN)2, and [’ppn][RhCl4(MeCN)2] which are purple, green, and
orange respectively. 13 None of these compounds resembles the blue
thermodynamic product of the mixed-metal reaction.

The difference between the experiments involving redissolution of the
purified product in fresh solvent and working up the layer reaction solutions
lies in the presence of [n-Bu4N][BF4] in the solution. If the kinetic salt is
isolated before further reaction occurs, only the dimetal units, their ligands,
and the solvent may take part in further reaction. The layer reaction

solutions contain the soluble by-product, [n-Bu4N][BF4] which may also

62

participate in further chemistry. The presence of [ n-Bu4N][BF4] may alter
the subsequent reaction pathway due to the enhanced electrolyte
concentration or it may participate in the transformation of the

[Rh 2(0Ac)2 (MeCN)6 llRe 2013] salt.

The most likely site of reactivity in the mixed-metal ion salt 1 is at the
labile axial position of the dirhodium cation. Such a reaction would be
accompanied by color changes with different donor types, which indeed was
observed with the various dissolution attempts of 1. We decided to further
explore the reactivity of 1 without solvent which would encourage
condensation reactions of the two metal units. The lavender solid was heated
to 145 0C in the solid state. An olive compound was produced, and washing
with MeCN gave a green solution and an insoluble brown solid. Successful
removal of coordinated solvent ligands was confirmed by infrared
spectroscopy, as the olive solid displays very weak v(C IN) stretches and the
brown compound does not give any evidence for MeCN in the 2200-2300 cm '1'
region. The loss of the MeCN ligands in the solid-state reaction required
forcing conditions. Such a product is not forming in the solution
decomposition reactions of l, as the resulting solids from these experiments
reveal several medium intensity features in the acetonitrile region. Thus it is
reasonable to assume that solvent plays an important role in the chemistry
upon redissolution of l and that this reaction pathway is inaccessible without
this medium. In light of these experiments, the reactivity of the kinetic
product 1 with other solvents is more easily explained. The solvent, CH2C12,
with little coordinating ability, does not substantially alter the identity of the
original salt as verified by the solid reverting to lavender upon exposure to
MeCN. Only a very small amount of the solid is dissolved. For THF, acetone,

and MeCN, the coordinating ability is more significant so more of the salt

63

dissolves (all in the case of MeCN) and the lavender solid can not be
regenerated by addition of MeCN. All three solids display IR activity in the
V(C sN) region around 2300 cm '1 indicating that some, but not total solvent
exchange has occurred. It is apparent that the ability of the solvent to act as

a ligand contributes to the further reactivity of the salt.

E. Summary and Future Directions

The unusual soft salt [Rh 2(OAc)2 (MeCN)6][Re 2013] was isolated by
taking advantage of the insolubility of this compound in common solvents.
Although this product was fully characterized, the product or mixture of
products formed by its decomposition in MeCN were not identified. Straight-
forward ligand redistribution does not seem to be responsible for the new
products. Of the solvents studied other than MeCN, redissolution of l in
acetone gives the best solubility; perhaps the products of this reaction may be
isolated more easily and then compared to the inconclusive MeCN data.

Preparation of the Mo analog with [Re 2013 19' would be an extremely
valuable comparison. Several researchers in our group have verified that
there is no initial precipitation of product in this reaction as is seen in the.
aforementioned work. Most of these experiments have been accidentally
exposed to air after about an hour, so repeating these reactions under
rigorously anaerobic conditions should lend more insight into the general

nature of this system.

9‘99”.”

10.

11.

12.

13.

LIST OF REFERENCES, CHAPTER II

(a) Cotton, F.A.; Reid, A.H.Jr.; Schwotzer, W. Inorg. Chem. 1985, 24,
3965. (b) Clegg, W.; Plimbett, G.; Garner, C.D. Polyhedron, 1986, 5, 31.
(c) Plimbett, G.; Garner, C.D.; Clegg, W. J. Chem. Soc. Dalton Trans.
1986, 1257.

Casas, J.M.; Cayton, R.H.; Chisholm, M.H. Inorg. Chem. 1991 , 30, 360.
Farrugia, L.J.; McVitie, A.; Peacock, R.D. Inorg. Chem. 1988, 27 , 1257.
Kiippers, H-J.; Wiegardt, K. Polyhedron, 1989, 8, 1770.

(a) McCann, M.; Guinan, P. Polyhedron, 1991, 10, 2283. (b) Guinan, P.;
McCann, M.; Ryan, H. Polyhedron, 1992, 11, 205.

Barder, T.J.; Walton, R.A. Inorg. Synth. 1985, 23, 116.

Sheldrick, G.M.; SHELX86, Program for the solution of crystal
structures, 1986. Univ. of Gottingen, Germany.

Beurskens, P.T. DIRDIF: Direct Methods for Difference Structures- An
Automatic Procedure for Phase Extension and Refinement of
Difference Structure Factors. Technical Report 1984/1
Crystallography Laboratory, Toernooiveld, 6525, Ed Nijmegen,
Netherlands.

Rempel, G.A.; Legzdins, P.; Smith, H. Wilkinson, G. Inorg. Synth.
1972, 13, 87.

Henkel, G.; Peters, G.; Preetz, W.; Skowronek, J. Z. Naturforsch. 1990,
45b, 469.

(a) Cotton, F.A.; Harris, C.B. Inorg. Chem. 1965, 4, 330. (b)Koz'min,
P.A.; Kotel'nikova, A.S.; Surazhskaya, M.D.; Larina, T.B.; Baglrov,
S.A.; Misailova, T.V. Koord. Khim. 1978, 4, 1557. (c) Huang, H.W.;
Martin, D.S.; Inorg. Chem. 1985, 24, 96. ((1) Cotton, F.A.; Frenz, B.A.;
Stults, B.R.; Webb, T.R. J. Am. Chem. Soc. 1976, 98, 2768.

Cotton, F.A.; Walton, R.A. "Multiple Bonds Between Metal Atoms,"
Wiley: New York, 1982.

Dunbar, K.R.; Thomas, J .L.C. unpublished results.

CHAPTER III

SYNTHESIS AND REACTIVITY OF

SOLVATED DIRHODIUM CAT ION S

66

A. Introduction

Metal complexes ligated solely by neutral solvent groups have the
advantage of lacking strong donors that direct the location and number of
incoming groups through electronic trans effects.1 The relatively small size
of common weak donor solvents that act as ligands such as water, nitriles,
alcohols, and THF, also reduces the cis-steric effects near the reaction site.
Since they lack anionic or bulky ligands such as halides, phosphines, and
carboxylates, fully solvated complexes may be used as synthons in the
systematic design of new molecules based on number of equivalents and the
relative trans effects of the new ligands rather than on the stoichiometry and
disposition of substituents on the starting material.

The importance of [Rh2(MeCN)1014+ as an excellent unhindered
starting material was first verified in its reaction with the highly bulky and
basic functionalized phosphine, TMPP (TMPP = tris(2,4,6-
trimethoxyphenyl)phosphine).2 The Rh2 unit undergoes non-redox homolytic
bond cleavage to yield a rare example of a stable mononuclear Rh(II) d7
radical, [Rh(TMPP)2][BF4]2. In comparison, reaction of the same phosphine
with the tetra-bridged rhodium acetate, a more common starting materialfor
this metal, leads to the removal of only one bridging group while the Rh2 unit
remains intact.3 Clearly, the absence of bridging ligands in the
[Rh 2(MeCN) 10] 4+ complex promises to enable access to many new compounds.

A comprehensive investigation of synthetic routes into the [Rh2 L10]4+
core, in addition to study of the elementary reactivity trends, is vital to the
full realization of the potential of this system as a synthon, and will also

assist our further efforts to design and prepare new compounds.

67

B. Experimental, Synthesis
(1) Preparation of Rh2 (OAc)4 (MeCN )2

RhCl3 was obtained from either Sigma or through a precious metal
loan from Johnson-Matthey. In the course of experimentation, it was found
that the commonly used Rh 2(OAc)4 (MeOH)2 was often contaminated with
excess sodium acetate not removed during the MeOH recrystallization step in
the synthesis.4 Thus later reactions with this starting material utilized a
different axial adduct, Rh2(OAc)4 (MeCN ) 2, which was formed by an extra
recrystallization of the tetraacetate from MeCN. The free acetate impurity is
far less soluble in MeCN than in MeOH, therefore more effective separation
is achieved. The crude dirhodium product can not be directly converted to

the MeCN adduct as the HOAc axial adduct is insoluble in MeCN.

(2) Preparation of [Rh2(MeCN)1()][BF4l4 (2)

(i) Method 1

Rhodium tetraacetate was prepared by literature methods.4 In a
typical reaction, Rh2(OAc)4(MeOH)2 (200 mg, 0.395 mmol) was refluxed in a
mixture of 5 mL of a 1 M solution of Et3 OBF4 in CH2 C12 (excess) and 10 mL
of MeCN under N 2 at atmospheric pressure. Large orange crystals began to
form in the red-orange solution after several days. After seven days of reflux,
the crop of crystals was removed by filtration under an inert atmosphere,
washed with two 5 mL amounts of CH2C12 followed by diethyl ether, and
dried in vacuo. The volume of the filtrate was reduced to a low volume (~ 1
mL) and chilled to -20°C. Additional product was precipitated from the
concentrated filtrate by addition of CH2012 (about 4 mL). The remaining pale
orange solution was decanted from the mixture of crystalline and solid

product. The orange [Rh2(MeCN)1o][BF4]4 salt was recrystallized from pure

68

acetonitrile by alternating cycles of reducing the volume and chilling; yield,
260 mg (70%). The solid complex is hygroscopic as evidenced by its facile
reaction with moist air to form the pink axial bis-water adduct. The salt is
soluble in MeCN, H2 O, DMSO, and CH3N02 , but is insoluble in THF,
alcohols, acetone, and CH2C12. Anal. Calcd for C20H 33B 4F15N 10Rh2; C,
24.93; H, 3.13; F, 31.55. Found: C, 25.44; H, 3.58; F, 31.37. IR (CsI, Nujol),
cm°1: 2342 (m), 2317 (m), 2300 (w), 1062 (vs, br), 1024 (vs, br). 1H NMR
(CD3CN, anaerobic): 6 = 1.95 ppm (singlet). UV-visible (MeCN, anaerobic):
1mm, nm (e in _M_'1cm'1) = 468 (570), 277 (24,400).

(ii) Method 2
In a typical reaction, a mixture of Rh2(OAc)4(MeOH)2 (200 mg, 0.395
mmol), 10 mL of MeCN, and 1 mL of HBF4/diethyl ether complex in diethyl

ether (excess) were refluxed for ten days. After ca. seven days, an additional

quantity of HBF4- Eth complex (0.7 mL) was added to ensure complete
reaction. The initial solution was a dark purple and gradually changed to a
reddish orange color. (If the solution remains red, more HBF4 - Eth should
be added.) The reaction solution was allowed to cool, after which hexanes.
and diethyl ether were layered on top to precipitate the product. Hexanes
and MeCN are immiscible so placing a layer of hexanes between the MeCN
and diethyl ether insures a good interface. The diethyl ether diffuses into the
hexanes, and the resulting mixture is miscible with MeCN so complete
diffusion occurs with the production of large rod-shaped crystals. The
original reaction solutions often contain brown or black impurities, so the
solid must be recrystallized by the acetonitrile/hexanes/diethyl ether method.
Yields averaged between 60 and 70% (228 to 266 mg). The product was

characterized as in Method 1.

69

(3) Metathesis of [Rh 2(MeCN)1o][BF4]4 with LiTFMS

A quantity of 2 (164 mg, 0.170 mmol) and a ten-fold excess of lithium
trifluoromethanesulfonate, (1.125 g, 7.20 mmol, 42.4 equiv.) were dissolved in
20 mL of MeCN. The solution was warmed gently to dissolve the last of the
rhodium starting material. Alternating volume reduction and chilling yielded
a crop of bright orange microcrystals which were washed with diethyl ether
after the supernatant solution was removed by decantation. IR data
indicated that only a small amount of [BF4]‘ remained, but the product yield
was greater than theoretical, implying that the product was contaminated by
excess LiTFMS (yield = 225 mg, theoretical yield = 206 mg). Subsequent
recrystallizations did not appear to effectively purify the product which was
visibly admixed with a white contaminant. IR (KBr, Nujol), cm '1: 2345 (s),
2319 (m), 2293 (m), 1306 (vs), 1252 (vs), 1235 (vs), 1178 (vs), 1055 (vs), 643
(vs), 517 (s). 1H NMR (dG-acetone, 5 min. after sample preparation) 5 = 2.91

ppm (s).

(4) Metathesis of [Rh 2(MeCN )10][BF4]4 with Sodium tosylate
Compound 2, (282 mg, 0.293 mmol) and sodium p-toluenesulfonate
(1.117g, 5.75 mmol, 19.6 equiv.) were warmed gently in 20 mL of MeCN for
24 h. At this point there remained a large quantity of undissolved white
solid, but the solution color had darkened with dissolution of the orange Rh
salt. The solution was filtered by cannula, and the volume of the solution was
reduced. The white solid was washed with 5 mL of MeCN which was
combined with the solutions. The volume of the combined solutions was
reduced again causing more white solid to precipitate. This was removed by

filtration, and the volume of the resulting solution was reduced by one-half

70

and chilled to -20°C which led to the precipitation of an orange solid. Diethyl
ether (3 mL) was added to encourage complete precipitation. IR data
indicated no metathesis had occurred as evidenced by the presence of [BF4]‘
and the absence of [TFMS]'. IR (KBr, Nujol), cm'1: 2344 (s), 2316 (m), 2281
(m), 1069 (vs, br).

(5) Metathesis of [Rh 2(MeCN)1ol[BF4]4 with TBA(Tosyl)

A sample of2 (211 mg, 0.219 mmol) and 362 mg of
tetrabutylammonium p-toluenesulfonate (0.875 mmol, 4.0 equiv.) was
dissolved in 20 mL of freshly distilled MeCN. Upon stirring in room light, the
solution turned a red-brown color with a dark precipitate at the bottom. The
reaction was then covered with foil and left to stir overnight. The next
morning, the orange solution had yielded a large quantity of brown
precipitate which was removed and discarded. Precipitation of the product
from the filtrate was induced by reduction of the volume and addition of
ether. The resulting orange solid was isolated by decanting off the
supernatant and washing with several amounts (5 mL each) of diethyl ether.
The product was dried in vacuo. The IR was the same as for the LiTFMS .
metathesis demonstrating that the [BF4]‘ salt prevailed.

(6) Preparation of [Rh2(MeCN)1o][TFMS]4 (3)

An amount of Rh2(OAc)4(MeCN)2 (200 mg, 0.382 mmol) was refluxed
together with 0.5 mL of Me 3SiTFMS (excess) and 10 mL of MeCN for two
weeks. After ~ 1 week, an additional amount of Me3 SiTFMS (0.5 mL) was
added to aid in complete reaction. The red-orange solution was layered with
hexanes and diethyl ether which produced large amounts of the crystalline

product. The solution was decanted and the crystals were washed with

71

copious amounts of CH2C12 and diethyl ether (2 x 5 mL each). The solid was
dried by passing N2 gas over the solid. Application of a dynamic vacuum to
the solid for long periods of time led to a loss of crystallinity and conversion of
the orange product to a purple solid. Orange product: IR (CsI, N ujol), cmilz
2345 (s), 2316 (s), 2286 (m), 1263 (vs), 1227 (vs), 1157 (s), 1030 (s), 756 (m),
640 (vs), 572 (m), 518 (vs). 1H NMR (CD3N02): 5 = 2.79 (s, 6H, equatorial
CH3CN), 2.02 (s, 1H, free CHgCN). Integration is imprecise due to the
breadth of the second resonance. Purple product: IR (CsI, Nujol), cm'l: 2341
(m), 2316 (w), 1309 (m, split), 1267(8), 1228 (s), 1205 (s), 1157 (m), 1032 (s),
1018 (m), 1008(8), 640 (s), 572 (w), 518 (m). 1H NMR (CD3N02): 5 = 2.79 (s,
equatorial CH 3CN).

(7) Reaction of [Rh2(MeCN)1ol[TFMS]4 with Methanol

Dissolution of 3 (30 mg, 0.025 mmol) in MeOH yielded a cherry red
solution which turned green within hours with concomitant precipitation of .
black Rh metal. The solvent was removed by vacuum to produce a residue
which dissolves in MeCN to form an orange solution and a black insoluble
precipitate. The residue was completely soluble in THF to give a red-brown
solution, while acetone, like MeCN, does not dissolve the black component.
No spectral data were collected from this sample due to the very small
residues remaining.

In separate experiments, an anaerobic UV-visible sample of 3 was
aged in MeOH in room light for two hours. The initial spectrum (lmax = 514
nm, shoulder at 381 nm) totally disappeared after two hours giving a
featureless spectrum with no absorption maxima distinguishable from the
tail into the ultraviolet region. An anaerobic solution IR in MeOH after 10
min. displayed three weak features (CaF2 cells) cm'1: 2359 (w), 2336 (m), and

72

2313 (w). These features faded into the baseline after the solution sat

overnight in ambient light.

(8) Reaction of Rh2(OAc)4(MeOH)2 with HTFMS and Acetonitrile
Rh2(OAc)4(MeOH)2, (111 mg, 0.219 mmol) was reacted with 0.5 of mL
HTFMS (excess) in 5 of mL MeCN under reflux conditions for 18 h. The
orange solution was cooled then filtered to remove a small amount of brown
solid. The solution volume was reduced by one-half and 4 mL of CH2C12 were
added to induce precipitation. The solution separated into two phases; the
CH 2C12 was removed by vacuum until the two phases mixed. Chilling the
solution to ~40 0C did not precipitate any solid, but instead produced an oil.

Subsequent efforts to extract a solid from this solution were not successful.

(9) Reaction of Rh2 (OAc)4(MeOH)2 with MeTFMS

A sample of Rh2(OAc)4(MeOH)2 (72 mg, 0.142 mmol) was reacted with
5 mL of freshly distilled MeCN and 0.3 mL of MeTFMS (excess). After six
days of refluxing, an additional 1.5 mL of MeTFMS was added. The reaction
was heated for ten days at which time the solution was opaque and oily red-
orange. Efforts to isolate a product by addition of diethyl ether, CH 2C12, or
toluene to the oil and chilling to -40 0C did not prove helpful as all solvents

tended to form immiscible layers with the reaction solution.

(10) Reaction of [Rh2(MeCN)1()][BF4]4 with Propionitrile

A quantity of 2 (54.2 mg, 0.056 mmol) was stirred and warmed gently
in 10 mL of EtCN for 12 h at which time the solution volume was reduced
slightly and 15 mL of diethyl ether were added. Chilling to -40 0C

precipitated a tan-orange solid. The solution was decanted, and the crystals

73

were dried in vacuo. IR (KBr, Nujol), cm'l: 2345 (w), 2314 (w), 2288 (w), 1030
(m). 1H NMR (CD3CN,), ppm: 5 = 2.35, (q, 2 H, CH3CH2CN), 1.19 (t, 3 H,
C_I_IQCH2CN), 1.95 (s, unintegrated, free CilgCN ) indicating incomplete ligand

exchange.

(1 1) Synthesis of [Rh 2(EtCN ) lollBF4l4 (4)

Rh2(OAc)4(MeCN)2 (205 mg, 0.391 mmol), 10 mL of EtCN, and excess
(1 mL) HBF4/diethyl ether complex were refluxed together for two days. The
solution changed from purple to a bright red-orange. The solution was
allowed to cool, whereupon hexanes and diethyl ether were carefully layered
on top and allowed to diffuse slowly. A mixture of large crystals and orange
oil resulted. The solution and the oil were decanted, and the crystals were
washed with three 5 mL portions of 1:1 ether : propionitrile to remove the
sticky residue. The washings were added to the decanted solution and the
entire amount was reduced to a low volume, after which EtCN added to
dissolve the oil, and finally hexanes and diethyl ether (1 mL and 10 mL
respectively) were added to precipitate more product. This procedure was
repeated until there was a negligible amount of oil. Combined yield of all .
crystals, 290 mg, 67%. Anal. Calcd for C30H50B4F15N10Rh2: H, 4.57; C,
32.60; N 12.69. Found: H, 4.56; C, 31.08; N, 12.07. IR (CsI, Nujol), cm‘l:
2324 (s), 2287 (m), 1315 (w), 1285 (w), 1055 (vs, hr), 783 (m), 561 (w), 521 (m).
1H NMR (CD3 CN, anaerobic): 5 = 3.03 ppm (q, 8H, equatorial-CH ggflgCN),
2.35 (q, 2H, free CH 3QI_IgCN), 1.36 (t, 12H, equatorial-flgCHzCN), 1.19 (t,
3H, free figCHgCN).

74

(12) Reaction of Rh2(OAc)4(MeOH)2 with HTFMS and Propionitrile
An amount of Rh2(OAc)4(MeOH)2 was combined with excess HTFMS

(2 mL) and 5 mL of EtCN and refluxed for 24 h to yield an orange solution.

Unfortunately, all attempts to isolate a solid by addition of either CH 2C12 or

diethyl ether/hexanes led only to oils or no precipitation at all.

(13) Reaction of Rh 2(OAc)4(MeCN )2 with MegSiTFMS and
Propionitrile

A sample of Rh2(OAc)4(MeCN)2 (100 mg, 0.191 mmol) was refluxed
with an excess of MegSiTFMS (1 mL) in 5 mL of propionitrile for two weeks.
After 7 days, an additional 0.5 mL of Me3 SiTFMS was added to the solution.
The resulting red-brown solution was cooled, layered with hexanes and
diethyl ether (1 mL and 10 mL respectively), and chilled to ~20 0C. A brown
oil formed which did not resemble the color of the [BF4]' salt; no solid product

was obtained.

(14) Reaction of Rh 2(OAc)4 (MeCN)2 with HBF4 and Butyronitrile

An amount of Rh2(OAc)4(MeCN)2 ( 102.5 mg, 0.196 mmol) was reacted
with an excess of HBF4 (l.mL) in 6 mL of deoxygenated PrCN under refluxing
conditions. After 2 h, the solution was orange, nevertheless the reaction was
heated overnight (~ 12 h) to ensure complete reaction. Addition of hexanes
and diethyl ether ( 1 mL and ~ 10 mL) and subsequent chilling of the solution

did not precipitate any solid.

(15) Reaction of Rh2(OAc)4 (MeCN)2 with Et3 OBF4 and Butyronitrile
A quantity of Rh2(OAc)4(MeCN)2 (102 mg, 0.196 mmol) was combined
with 8 mL of PrCN and 4 mL of EthBF4 solution in CH2C12 (l M) and

75

refluxed for one day. The solution was allowed to cool at r. t., after which
hexanes (1 mL) and diethyl ether (~ 10 mL) were added to initiate

precipitation. An orange oil resulted, but no solid was isolated.

(16) Reaction of [Rh 2(EtCN) 10][BF4]4 with Butyronitrile

An amount of 4 (106 mg, 0.096 mmol) was stirred in 5 mL of PrCN at
r. t. overnight. The compound readily dissolved to give an orange solution.
Hexanes (1 mL) and diethyl ether (~ 10 mL) were layered on the solution

after 1 day, but these efforts did not yield a tractable product.

(17) Reaction of [Rh2(MeCN)1ol[BF4]4 with Benzonitrile

A sample of 2 (50 mg, 0.052 mmol) was stirred in 5 mL of PhCN for 12
h. The solution (UV-visible shoulder at 450 nm) was filtered in air to yield
47 .1 mg of a gold-yellow product which was dried by washing with diethyl
ether. The 1H NMR spectrum revealed an absence of benzonitrile (CD3 CN):
5 = +1.95 ppm (free CH 3CN). IR (CsI, Nujol), cm'1: 2343 (s), 2314 (m), 2270
(w), 2243 (w), 1672 (m), 1603 (m), 1060 (vs, br). Redissolving the solid in
benzonitrile and heating for 4 h led to the formation of an orange-brown
solution. Addition of diethyl ether and chilling to 0 0C precipitated a pale tan
solid which was washed with diethyl ether and dried in vacuo. The 1H NMR
spectrum of the product displayed no free MeCN (CD3CN), ppm: 5 = 7.74 (m),
7.72 (d), 7.67 (t), 7.65 (t), 7.56 (In), 7.54 (m), 7.51 (m), and the IR spectrum
showed all v(C lIN) bands shifted from those of the starting material (CsI,
Nujol), cm'l: 2241 (m), 2226 (w), 1070 (s, br). Efforts to recrystallize the solid

were not successful.

76

(18) Reaction of [Rh2(EtCN)1o][BF4]4 with Benzonitrile

A quantity of 4 (120 mg, 0.109 mmol) was dissolved in 15 mL of warm
deoxygenated PhCN to produce a dark orange-red solution. Addition of
hexanes (1 mL) and diethyl ether (10 mL) did not lead to precipitation.

(19) Reaction of Rh 2(OAc)4 (MeCN )2 with Pentanedinitrile

An amount of Rh 2(OAc)4 (MeCN ) 2 (200 mg, 0.381 mmol), NC(CH2 )3CN
(1 mL) and 8 mL of Et3OBF4 were refluxed together for 9 days with 1 mL of
additional nitrile added at two different intervals. The resulting solution was
an intense orange color. Addition of CH2 C12 yielded a very oily solid that was
not well characterized. Further attempts to obtain a solid from the solution

led only to oils.

(20) Reaction of [Rh 2(MeCN )1ollBF 414 with Pentanedinitrile

A sample of 2 (100 mg, 0.104 mmol) was stirred in 2 mL of warm
NC(CH2)3CN which resulted in the production of a brown solution within
several hours. Upon work-up consisting of addition of diethyl ether, the
solution produced a brown solid whose 1H NMR spectrum displayed the
broad features indicative of a paramagnetic compound. Efforts to measure
the infrared spectrum led to solids that did not mull well with N ujol, and KBr
pellet samples that were not concentrated enough to register. The presence
of [BF4]' was observed in the Nujol mull spectra, but the insignificant nitrile

stretches were inconclusive as to the presence of coordinated dinitrile.

(21) Reaction of [Rh 2(MeCN)1ol[TFMS]4 with Pentanedinitrile
A small amount of 3 was dissolved in 3 mL of fresh NC(CH2)3CN that

was not deoxygenated. Stirring at room temperature initially produced a red

77

solution then an orange-brown solution. Hexanes and diethyl ether layer

attempts failed to mix with the reaction solution; toluene and CH2 C12 did not

precipitate any solid.

(22) Reaction of [Rh2(MeCN)1o][BF4]4 with Propanedinitrile

A sample of 2 (50 mg, 0.052 mmol) was added to a small amount of
NCCH3CN. (This material is a highly toxic solid with a melting point of 32-
34 0C, therefore it melts rapidly at warmer temperatures.) An internal finger
condenser was used during the heating of this reaction which induced
solidification of the malononitrile, and unfortunately, the addition of diethyl
ether to lower the melting point resulted in the precipitation of a product.
Subsequent washing with diethyl ether (5 x 5 mL) produced a dry red-brown
solid. IR (CsI, Nujol), cm'1: 3321 (3, br), 3259 (3, br), 2219 (8, br), 1661 (s),
1621 (s), 1597 (s), 1566 (s), 1062 (8, br). 1H NMR (CD3CN, in air, not well-
phased) ppm: 5 = 3.88(s), 3.76 (s) in ca. a 1 : 20 ratio.

(23) Conversion of [Rh2(MeCN)1ol[BF4]4 to Rh2 (OAc)4 (MeOH)2

An amount of 2 (50.0 mg, 0.052 mmol) and an excess of NaOAc (80.8
mg, 0.985 mmol) were refluxed in glacial acetic acid for 1 h. The solution
color quickly reverted from orange to green, and after cooling a blue solid
formed which was removed by filtration and recrystallized from MeOH. The
identity of the solid as Rh2(OAc)4 (MeOH )2 was confirmed by a comparison of

its UV-visible spectrum in MeOH to that of an authentic sample of
Rh 2(OAc)4(MeOH)2 (Am, 580 and 435 nm). Recrystallized yield: 9.9 mg,

37.6%.

78

(24) Reaction of [Rh 2(MeCN)1()][BF4]4 with 2 equivalents of NaOAc

A quantity of 2 (51 mg, 0.053 mmol) and anhydrous N aOAc (8.5 mg,
0.104 mmol, 1.96 equiv.) was stirred in 5 mL of MeCN and gently heated.
The reaction solution remained orange with some undissolved N aOAc, and
did not convert to red or purple, the characteristic colors of either pure
[Rh2(OAc)2(MeCN)6]2+ or a mixture of that and the starting material.

In early reactions, the expected reddish-purple solutions resulted and
the identity of cis- [Rh2(OAc)2(MeCN)6]2+ was verified spectroscopically.
{Amax = 525 nm, IR (KBr, Nujol, air), cm-1: 3591 (m, br), 3538 (m, br), 2355
(m), 2306 (m), 2278 (m), 1644 (w), 1552 (s), 1075 (vs, br), 1H NMR (CD3CN)
ppm: 5 = 2.53 (s, 4H, equatorial CH3CN), 2.17 (s, 1H), 2.03 (s, 2H, O2CCH3),
1.95 (s, free CH3CN).} Unfortunately, these reactions were not reproducible.
Several instances of impure starting materials were found during that time
period, so the apparent success of these reactions may be attributed to
contamination of [Rh 2(MeCN )1o][BF4]4 with amounts of
[Rh2(OAc)2(MeCN)5]2+.

(25) Reaction of [Rh 2(MeCN )1()][TFMS]4 with 2 equivalents of NaOAc
A sample of3 (51.1,mg, 0.042 mmol) and NaOAc - 3H2O (11.7 mg, .086

mmol, 2.05 equiv.) was dissolved in 5 mL of acetone. Stirring at r. t. initially

produced a red solution, but within 30 min., the solution became brown.

Prolonged reaction yielded an oily green solid and a red solution.

(26) Reaction of [Rh2(MeCN)1o][BF4]4 with NaOzCCF3
A quantity of 2 (50 mg, 0.052 mmol) and 11.7 mg of Na02 CCF3 (0.086
mmol, 1.65 equiv.) was stirred in 4 mL of MeCN for several days. The

resulting orange solution was layered with 10 mL of diethyl ether to produce

79

a brown precipitate and a small quantity of green solid. The electronic
spectrum of the filtrate did not agree with that of [Rh2(OAc)2(MeCN)(;]2+
(Am = 512 nm). The solution was decanted, the solid was washed with
diethyl ether, and dried in vacuo. IR spectral data for the brown solid did not
resemble that of the target complex, viz., [Rh 2(O2 CR)2 (MeCN )6]2+ in
particular, there was an absence of a carboxylate stretch in the spectrum.
(KBr, Nujol, air), cm'lz 2340 (w), 2315 (w), 1070 (m), 1020 (m). The brown
solid was redissolved in 5 mL of MeCN to yield an orange solution that

eventually produced orange microcrystals.

(27) Reaction of [Rh2(MeCN)1ol[BF4]4 and NaOzCC3H7

A sample of 2 (63 mg, 0.065 mmol) was dissolved in 5 mL of MeCN. To
this was added 15 mg of sodium butyrate (0.014 mmol, 2.2 equiv.) suspended
in 8 mL of MeCN after which the mixture was heated gently with stirring.
The color of the solution was red-orange indicating incomplete reaction,
therefore an additional 21 mg of Na02 CC3H7 (0.191 mmol, 2.94 equiv.) was
added. The solution then became purple, after which diethyl ether (15 mL)
was added to precipitate a white solid from the purple solution. The solution
was filtered and reduced to a purple residue by application of dynamic
vacuum. This solid turned green after prolonged exposure to vacuum. 1H
NMR verified the product to be Rh 2(02 CC3H7)4. (CD 30D) ppm: 5 = 4.87 (s),
2.03 (t, 2 H Rh2O2CQ12CH2CH3), 1.94 (s, 0.5 H, RhNCQHg), 1.43 (m, 2 H,
Rh2OzCCH2C_HgCH3), 0.73 (t, 3 H, Rh2O2CCH2CH2Q113) (This product
consists of mostly the anhydrous species which is why the axial CH3CN
integrates to less than two.) IR (CsI, N ujol, air), cm'1: 1661 (m), 1576 (vs),
1414 (s), 1316 (m), 1097 (m), 1076 (m, br), 801 (w), 667 (w), 456 (w). No

nitrile stretches are observed.

80

(28) Reaction of [Rh2(EtCN)1o][BF4]4 with 2 equivalents of
[n-Bu4 N llOAcl

A quantity of 4 (40 mg, 0.036 mmol) was stirred with 2 equiv. of
[n-BU4N][OAc] (21.8 mg, 0.072 mmol) in 5 mL of EtCN. The acetate salt
appeared to insoluble in this medium, therefore the solution was reduced to a
residue under vacuum, whereupon 5 mL of acetone was added. A brown
precipitate and a red-brown solution ensued. The solid was isolated by
filtration of the solution in air. IR data did not indicate significant activity in
the nitrile region implying that this was not either the cis or trans isomer of
the target complex, [Rh2(OAc)2(EtCN)6l2+. IR (CsI, Nujol, air), cm‘l: 1576
(w), 1306 (m), 1033 (vs, br), 771 (w), 534 (s), 522 (s).

(29) Axial Substitution Reactions of [Rh2(MeCN)1o][BF4]4 with Donor

Solvents.

(i) Reaction with MeOH. A quantity of 2 (20 mg, 0.021 mmol) was
stirred in 5 mL of MeOH for 8 h. While the solid did not dissolve appreciably,
it nevertheless gradually changed from orange color to red as the axial MeCN
groups were extracted and replaced by MeOH. The solution was removed by
decantation, and the solid was dried in vacuo. IR (N ujol, CsI), cm '1: 3409 (s,

br), 2341 (s), 2317 (s), 1066 (3, br), 1027 (8, br), 522 (m), 454 (w).

(ii) Reaction with THF. A 20 mg sample of 2 (0.021 mmol) was
stirred in 5 mL of THF for 8 h. The solid was insoluble in THF, but the
powder eventually turned a more pale orange than the starting material.
The solvent was decanted and solid was dried under vacuum. IR (Nujol,

KBr), cm’1: 3404 (w, br), 3305 (w), 2345 (s), 2316 (s), 1061 (8, br), 1034 (s).

81

(iii) Reaction with Acetone. An amount of 2 (20 mg, 0.021 mmol)
was stirred in 5 mL of acetone for 8 h. The solid was slightly soluble in
acetone, and gradually formed a pale green solution and an olive green solid.

The solution was decanted and the solid was dried under reduced pressure.

IR (Nujol, KBr), cm'l: 2332 (w), 2311(w), 1635 (w), 1057 (8, br), 1032 (3, br).

(30) Preparation of Rh 2(aq)11+ (5)

A sample of 2 (100 mg, 0.104 mmol) readily dissolved in 10 mL of
deoxygenated milli-q water, purified by the Millipore system from Waters
Chromatography, to give a red solution. Under refluxing conditions with
periodic pumping, the reaction color changed from red to purple to blue and
finally to green after 8 h. The solution was heated as it was pumped to
dryness to ensure complete reaction through removal of free acetonitrile from
the system. The product obtained in this manner was a dry green solid.
(Purity of the starting material from residual acetate sources is crucial as
evidenced by numerous reactions that produced Rh 2( OAc)4(HzO)2 which was
verified by crystallography or by inability of the dissolved material to bind to
a cation exchange column.) IR (N ujol, KBr), cm'1: 3445 (s,br), 3325 (s,br),-
3213 (s, br), 1661 (s), 1597 (s), 1088 (8, br), 722 (s).

(31) Crystallization attempt of Rh2(aq)4+

The p-toluenesulfonic acid that was used in this experiment was
obtained from Fluka- The Aldrich samples of this reagent contain significant
amounts of HCl. After preparing the green aqua complex, 5, by the above
method, the solution was transferred to a Dowex cation-exchange column
containing ca. 2 cm3 of resin that had been washed with ca. 30 mL of pure

H2 0. The Rh-containing solution was then applied to the column; a green

82

band remained bound to the resin and was washed with several 5 ml.
portions of water. The column and the new receiving flask were
deoxygenated by a N2 purge for several hours as was the 4 M solution of p-
toluenesulfonic acid. The green compound was eluted easily by anaerobic
transfer of the acid. The resulting solution was evaporated under a nitrogen
purge. While large amounts of the acid crystallized, no green crystals of the
Rh2(aq)4+ species were observed. The solution did not totally evaporate, and

the product was still dissolved in the acidic solution.

(32) Conversion of Rh2(aq)4+ to Rh2(OAc)4

Quantities of 5 (20 mg) and excess N aOAc (37 mg, 0.449 mmol) were
dissolved in 10 mL of glacial acetic acid and heated for 12 h. The resulting
blue precipitate was identified as Rh2(OAc)4 by the UV—visible spectrum ( vide

supra ).

(33) Aerobic Reaction of [Rh2(MeCN)10][BF4]4 with Water at Room
Temperature

A quantity of [Rh2(MeCN)10llBF4l4 (24.5 mg, 0.025 mmol) was
dissolved in 4 mL of H20 [in air. After 20 min., the solution was orange with
a hint of red (Am = 503 nm). Within 21 h, the solution had changed to
orange-brown, (UV-visible shoulder at 523 nm). The solution was allowed to
stand for two weeks, after which the color was a very pale yellow. The
solvent was removed under vacuum to yield a green residue. This solid was
dissolved in water to give a yellow solution with a Am = 548 nm. 1H NMR
(CD 3CN ), gave a large number of resonances superimposed on a broad
feature between 2.0 and 2.8 ppm. The presence of free CH3CN was apparent
at 5 = 1.95 ppm.

83

(34) Reaction of Rh2(aq)1n+ with Acetonitrile

Dissolution of 5 in MeCN with heating produced a red solution that did
not revert to an orange color that is characteristic of [Rh2(MeCN)1()]4+ (Amax
= 506 nm). IR spectral data reveal only very tiny v(C EN) and intense v( OH)
stretches- (KBr, Nujol), cm '1: 3377 (vs, br), 2333 (vw), 2302 (vw), 2273 (vw),
1622 (m, br), 1053 (vs, br).

(35) Reaction of [Rh2(MeCN)1o][BF4]4 with HClO4

A 20 mg sample of 2 (0.021 mmol) was refluxed in 5 mL of 3 M HClO4
for ca. 12 h. The pale green solution was transferred anaerobically to a
quartz UV-visible cell and the spectrum was recorded. Once the visible
spectrum had been measured, the solution was exposed to air. After seven

days, the solution had displayed no signs of decomposition.

(36) Preparation of a UV-visible sample of Rh2(aq)11+ in 3 M HClO4
A residue left from a preparation of 5 in H2O was redissolved in fresh
H20 and loaded onto a cation exchange column. The column was purged with

nitrogen for several hours, and the compound was eluted anaerobically with 3

M HClO4 .

(37) Control reaction of Rh2(OAc)4(MeOH)2 in Water
Rh2(OAc)4(MeOH)2 (50 mg, 0.099 mmol) was refluxed in 5 mL of H20
for three days. While a minor amount of rhodium metal was produced, the

dirhodium tetraacetate molecule was essentially unchanged as evidenced by

the its visible spectrum (Amax = 586 and 441 nm).

84

(38) Reaction of Rh2(OAc)4(MeOH)2 with Et3OBF4 and Pyridine

An amount of Rh2(OAc)4(MeOH)2 (200 mg, 0.395 mmol) was refluxed
with 8 mL Et3OBF4 in 5 mL of pyridine for 4 days. A pink solid precipitated
from the orange solution. Decanting followed by reduction of the volume of
the orange solution and addition of CH 2Cl2 yielded a large amount of yellow
precipitate. Crystals of the yellow precipitate were grown and examined by
X-ray crystallography, but these proved to be twinned. IR (CsI, N ujol, air),
cm'1: 3287 (s), 3202 (s), 3147 (s), 3118 (s), 1638 (s), 1609 (s), 1540 (s), 1216
(m), 1067 (vs, br), 763 (s), 750(3), 697 (s), 679 (s), 510 (m). 1H NMR (CD2C12),
ppm: 5 = 8.48 (d), 8.06 (t), 7.44 (t), 5.32 (d), 3.39 (s), 1.14 (t). The pink solid
was insoluble in all common solvents including pyridine; yield, 336 mg. IR
(KBr, Nujol), cm'1: 3448 (vs), 3400 (vs), 3095 (s), 1635 (vs), 1582 (m), 1498
(vs), 1302 (s), 1188 (vs), 1075 (vs, br), 785 (s), 697 (s), 575 (s), 477 (m).

(39) Reaction of [Rh 2(MeCN)1o][BF4]4 with Pyridine

A sample of 2 (86 mg, 0.089 mmol) was stirred at r. t. in 5 mL of
pyridine that had been freeze-pump-thawed over 48 molecular sieves to
remove residual oxygen and water. The reaction was covered with aluminum
foil and allowed to react for three days. After this time, the stirring was
halted, and the solution, which had appeared to be orange, was merely a

suspension of the starting material in the solvent.

(40) Reaction of [Rh2(MeCN)lo][TFMS]4 with Pyridine
A sample of 3 (25 mg, 0.021 mmol) was stirred in 3 mL of pyridine
that had been freeze-pump-thawed to remove residual oxygen. The solution

immediately turned bright yellow. When stirring was ceased, a small amount

85

of solid settled out, therefore 4 mL of additional pyridine was added. The
solid did not dissolve even with warming. Subsequently, 5 mL of CH2C12 was
layered under the pyridine solution in an attempt to crystallize the product.
A large amount of solid was produced which when redissolved in MeCN,
became a deep red-brown color. By layering hexanes and diethyl ether on
this solution, olive green feathery crystals and a yellow solution were

obtained. No spectral characterization was carried out.

(41) Reaction of [Rh 2(MeCN )1()][BF4]4 with 2 equivalents of
Bipyridine in Acetonitrile

Quantities of 2 (100 mg, 0.104 mmol) and bpy (33 mg, 0.212 mmol, 2.04
equiv.) were dissolved separately in 5 mL MeCN each and combined by
cannula transfer of the second solution into the first. The combined solution
immediately turned dark green with a small amount of flocculent precipitate.
Addition of diethyl ether produced a brown solid, and chilling to -40 0C
precipitated the entire contents of the solution. The solvent was decanted to
give a brown solid admixed with white microcrystals. This residue was dried
under vacuum and then redissolved in 5 mL of MeCN to give a red solution
and a brown suspension. The vessel was wrapped in aluminum foil, filtered
by cannula without exposure to light, and dissolved in MeCN. The solution
was layered over CH2 C12 which produced a powdery olive green solid and an
orange solution. The solid was collected by cannula filtration and dried in
vacuo. IR(CsI, Nujol), cm'1: 2362 (w), 2334 (m), 1610 (w), 1577 (w), 1320 (w),
1248 (w), 1058 (vs, br), 768 (m), 523 (m). 1H NMR (CD3CN). 13me 5 = 8.69 (d,
2H), 8.44 (m, 2.5H), 7.70 (m, 4H), 5.43 (s, .6H), 2.82 (s, .5H), 1.95 (s, ~15H).
The orange solution was reduced to a mixture of orange and white solids

under vacuum. IR(CsI, Nujol, very dilute), cm'1: 1059 (8, br). 1H NMR

86

(CD3CN), P1311125 = 8.68 (d, 2H), 8.43 (t, 3H), 7.68 (m, 6H), 7.19 (m, 2H), 5.44
(s, 0.5H), 3.26 (s, 2H), 2.65 (s, 2H), 1.95 (s, ~ 50H).

(42) Reaction of [Rh2(MeCN)1()][BF4]4 with 2 equivalents of
Bipyridine in acetone

Samples of 2 (82 mg, 0.085 mmol) and bpy (21 mg, 0.134 mmol, 1.56
equiv.) were dissolved separately in about 5 mL of acetone each and then the
bpy solution was transferred under N2 into the Rh-containing solution. A
finely divided gray suspension immediately resulted. After stirring
overnight, there was a large amount of gray precipitate and a very pale
orange solution. The solution was decanted and discarded. After the gray
solid had been washed with MeCN, it dissolved to give an olive solution with

a gray suspension. The solution was reduced to a residue under vacuum, and

a 1H NMR in CD3CN displayed only free MeCN at 5 = 1.95 ppm.

(43) Reaction of [Rh 2(EtCN ) 10][BF4]4 with 2 equivalents of Bipyridine
Quantities of 4 (74 mg, 0.067 mmol) and bpy (21 mg, 0.135 mmol, 2.01
equiv.) were dissolved separately in ca. 3 mL of CH 3N 02 each. The bpy
solution was transferred anaerobically by cannula into the rhodium solution
which immediately changed from orange to red-brown. The reaction was
stirred for two hours, after which the solution was layered with hexanes and
diethyl ether (1 mL and 10 mL respectively). After 5 days, this had produced
an orange solution and an oily green-black product similar in color to the
product from [Rh2(MeCN)1o][BF4]4 with 4 equiv. of bpy. The solution was
decanted from the solid, which was washed with diethyl ether (3 x 5 mL) and
dried in vacuo to isolate a dry forest green powder. 1H NMR (CD3N02 ),ppm:
5 = 8.81 (d, 2H, bpy), 8.51 (t, 2H, bpy), 7.92 (d, 2H, bpy), 7.78 (t, 2H, bpy), 4.35

87

(s, 1H, free CH 3NO2 ). Additional multiplets appear in the baseline between
1.0 and 3.5 ppm which are attributed to free EtCN.

(44) Reaction of [Rh 2(MeCN)1ol[BF4]4 with 4 equivalents of
Bipyridine

Amounts of 2 (100 mg, 0.104 mmol) and bpy, (65 mg, 0.416 mmol, 4.0
equiv.) were combined. To this mixture was added 5 mL of CH 3NO2. Rapid
color changes ensued: red to green to brown to purple to green to finally teal,
all within ~ 2 min. 24 h after no further color changes had been observed, the
solution was layered with diethyl ether (10 mL). The solution was decanted
from the gray-black solid. After washing with diethyl ether (2 x 5 mL), the
solid was still very sticky. Subsequent washing with CH 2012 and THF did
not improve the nature of the oily residue. The solid was redissolved in
CH 3NO2, to which diethyl ether was added, and the flask was chilled to 0 oC.
The solution was then decanted, and the solid was dried in vacuo. IR(CsI,
Nujol), cm'lz 2332 (w), 2307 (w), 1608 (w), 1554 (w), 1502 (w), 1321 (w), 1284
(w), 1250 (w), 1059 (vs, br), 767 (m), 520 (m). 1H NMR (d6-acetone) ppm: 5 =
9.05 (d), 8.60 (t), 8.25 (d), 7.88 (t), 4.42 (s).

(45) Reaction of [Rh2(MeCN)1()][BF4]4 with 2 equivalents of dppm
Samples of 2 (100 mg, 0.104 mmol) and dppm (80 mg, 0.208 mmol, 2.0
equiv.) were stirred at r. t. in ca. 7 mL freshly distilled MeCN. The solution
immediately turned an intense ruby red color. After ~ 2 h, the solution
darkened in color and a brown solid was seen to be suspended in the solution.
A dark brown solid was precipitated from the reaction solution by the
addition of 10 mL of diethyl ether. A 1H NMR spectrum in d6-acetone and

CD 3CN displayed no proton resonances in the phenyl region.

88

(46) Reaction of [Rh2(MeCN)lo][TFMS]4 with 2 equivalents of dppm
A small amount of 3 (20 mg, 0.017 mmol) was stirred at r. t. in 5 mL of
acetone together with 13 mg of dppm (0.034 mmol, 2.0 equiv.) The solution
immediately turned a bright orangered. After 90 min., hexanes and diethyl
ether were layered on the solution. Three days later, a few small yellow
crystals had formed. Attempts to produce any more compound by layering
with toluene met with failure.
In a separate reaction, samples of [Rh2(MeCN)10][TFMS]4
(53 mg, 0.044 mmol) and dppm (33 mg, 0.086 mmol, 1.95 equiv.) were
dissolved in 6 mL of acetone and 3 mL of acetone respectively before being
combined under anaerobic conditions. The color of the combined solution

remained that of the original Rh2 starting material.

(47) Reaction of [Rh2(MeCN)10][BF4]4 with 2 equivalents of PMe3

A sample of 2 (50 mg, 0.052 mmol) and 0.1 mL of a 1 M solution of
PMe3 (0.100 mmol, 1.93 equiv.) in toluene was stirred at r. t. in 5 mL of
MeCN for 1 day. The solution immediately turned ruby-red, went through a
dark brown phase, and after 1 more day was once again the pale orange of
the starting material with a white precipitate at the bottom. This
reappearance of the original solution color indicated that the original

compound had reformed.

(48) Reaction of [Rh2(MeCN)1o][BF4]4 with ppnCl
Samples of 2 (100 mg, 0.104 mmol) and 238 mg of ppnCl (0.415 mmol,
3.99 equiv.) were stirred at r. t. for ~ 12 h. Immediately after addition of the

solvent, the solution turned gray-white with a dingy white precipitate. The

89

next morning, the solution was decanted and allowed to evaporate, producing
a large amount of white crystals. The precipitate from the reaction was
washed with 20 mL of MeCN. The compound was insoluble in THF and
CH2Cl2 but was quite soluble in water. This reaction was not reproducible.

In all subsequent chloride reactions, the solutions remained pale orange and

formed no precipitate.

(49) Crystallization of an oxygen derivative of [Rh2(MeCN)1o][BF4]4
An amount of [Rh2(MeCN)10][BF4]4 was dissolved anaerobically in
MeCN and then covered with foil for an hour. 02 was bubbled for 15 min.
after which CH2 C12 was layered under the solution. Orange block-shaped
crystals resulted and were examined on an X-ray diffractometer. These

proved to exhibit the same unit cell as an authentic sample of

[Rh2(MeCN)10llBF4]4 .

(50) Long term reaction of [Rh2(MeCN)1o][BF4]4 with O2 and hv

A sample of 2 (22 mg, 0.023 mmol) was dissolved in 30 mL of MeCN
and purged with 02 during irradiation with a broad band UV-visible source
for 3 days after which the/solution was yellow. The volume of the solution
was reduced and layered with hexanes (2 mL) and ether (20 mL). A yellow oil
was produced; unfortunately subsequent attempts to isolate a solid led only
to further oiling of the solution.

90

C. Experimental, Crystallography

l. [Rh 2(MeCN )1ollBF4l4 (2)

Data Collection. Pertinent crystal data are summarized in Table 5.
Large orange block-shaped crystals were obtained from the reaction solution;
one of these with dimensions 0.75 x 0.78 x 0.80 mm was mounted with epoxy
cement on the end of a glass fiber. The crystal was examined on a N icolet
P3/F diffractometer with graphite monochromated MoKa (7(01 = 0.710738)
radiation. A rotational photograph indicated that the crystal diffracted well.
The unit cell was determined by 25 reflections in the range 20 S 20 S 30°. The
crystal indexed in the monoclinic crystal system which was verified by axial
photography.

Structure Solution and Refinement. The intensity of three periodically
monitored check reflections decayed by ~40% thus the program CHORT was
applied to compensate for this loss of intensity. The position of the unique Rh
atom was determined by application of SHELXS-86,5 and the rest of the
atoms were located by an alternating series of least squares refinement and
difference Fourier maps. Anisotropic refinement of all non-hydrogen atoms
gave residuals of R = 0.0520 and RW = 0.0807 with the quality-of-fit index
equal to 2.33. In the last cycle, 235 parameters were refined with 2802
unique data and Fobs>3o of 2395. The shift/esd of the final cycle was 0.13
and the highest remaining peak in the final difference Fourier map was

1.7 e' /A3.

2. [Rh2 (MeCN)1ol[TFMS]4 (3)
Data Collection. X-ray quality crystals were grown by layering a
solution of 3 that had been standing in the dark for at least 90 min. with

91

Table 5. Crystal Data for [Rh 2(MeCN )1o][BF 414 (2)

Formula
Formula weight

Space group

a, A

b, A

c, A

01, deg

6. deg

Y. deg

v, A3

Z

dcalc, g/cm3

)1 (Mo Ka), cm‘1
Data collection instrument

Radiation (monochromated in
incident beam)

Temperature, °C
Scan method

Trans. factors, max., min.

R3

aR=£ l lFol ' chll/ElFol

bRw = [ZwlFo l- IFC l)2/£w|F0l2]l/2; w

Rh2C20H30N1034F16
963.51

C2/c

18.123(2)
11.920(1)
18.243(3)
90
9958(1)
90
3886(1)

4
1.647

9.383

Nicolet P3/F

Mo Ka(la = 0.710738); graphite
monochromated

98.8, 87.9 %

0.0520

1/oZ( lFo I)

92

hexanes and diethyl ether. The flask was covered with foil for the duration of
the diffusion experiment. A crystal of approximate dimensions 0.54 x 0.39 x
0.39 mm was mounted with Dow Corning grease on a glass fiber and
examined on a N icolet P3/F diffractometer at -90 :t 2 0C. A rotational
photograph indicated that the crystal diffracted well. An initial cell
determined by centering on 15 film spots, and indicated that the crystal was
monoclinic primitive which was verified by axial photography. An accurate
data collection cell was obtained by centering on 25 reflections in the range
20 S 20 S 30°. 020 scans were used to collect 7354 final data (6977 unique)
between 20 values of 4-450. Check reflections were collected every 150
reflections and indicated a positive decay of 5%. Several independent data
sets that did not refine satisfactorily also displayed this increase in intensity
during data collection. Pertinent crystal data are summarized in Table 6.

Structure Solution and Refinement. The structure was solved by the
direct-methods program, SHELXS-86 which led to location of the two Rh
atoms. The rest of the structure was determined by a series of alternating
DIRDIF and Fourier maps until the majority of the atoms had been found.
Least-squares analysis and Fourier maps were used to complete the
structure. After isotropic convergence was achieved, the program, DIFABS
was applied to correct for absorption. The four CF3 groups did not refine well
as individual atoms, so the three atoms were refined together as a group. All
atoms other than these groups and the interstitial MeCN solvent molecule
were refined aniostropically to give final residuals of R = 0.075 and Rw =
0.091 with the quality-of-fit indicator equal to 2.35. A final difference map
gave the highest remaining peak as 1.5 e'/A3, located in the lattice. All other
peaks were less than 1.0 e'/A3.

93

Table 6. Crystal data for [Rh2(MeCN)1ollTFMS]4

Formula Rh2C24H30N1oS4O12F12
Formula weight 1212.578
Space group P2 1 / n

a, A 1 2.1 95(8)

b, A 22.797(3)

c, A 1 8.686(3)

a, deg 90

(3, deg 9757(2)

y. deg 90

v, A3 5150(3)

Z 4

dcalc, g/cm3 1.616

)1 (Mo Ka), cm’1 8-87

Data collection instrument Nicolet P3/F
Radiation (monochromated in M0 Ka(la = 0.71073A); graphite
incident beam) monochromated
Temperature, °C -90 1 2°

Scan method I 0 - 26

Trans. factors, max., min. 0.89 - 1.11
Ra 0.075

wa 0.091

aR=£ l lFol ' chll/ElFol

bRw = [ZwlFo l- ch |)2/ZwlFol2]l/2; w = 1/02( lFol)

94

3. [Rh2(EtCN)1ol[BF4l4 (4)

Data Collection. Large single crystals of 4 were grown by placing a
layer of hexanes on the reaction solution and layering diethyl ether on top of
this. Slow diffusion produced hexagonal rod-shaped crystals. A crystal of
dimensions 0.31 x 0.36 x 0.89 mm was mounted on a glass fiber and
examined on a Rigaku AFC6S diffractometer equipped with graphite
monochromated Mo Ka (la = 0.71073 8) radiation. A random search routine
located initial reflections to determine a cell. The pre-cell was determined by
25 reflections with 29 .<. 20 S 30, and the symmetry was confirmed as
monoclinic C-centered by an automatic Laue check. Data were collected to a
maximum 20 value of 500 at -90 0C. Three intensity standards collected every
150 reflections displayed no significant decay. Pertinent crystal data are
summarized in Table 7.

Structure Solution and Refinement. The structure was solved by
application of direct methods. The unique Rh atom and immediate
coordination sphere were located from the initial solution whereas all other
non-hydrogen atoms were located and refined from a series of alternating
least squares and difference Fourier maps. Two reflections were found to be
extremely poor fits to the model, therefore these were eliminated from the
calculations. The program, DIFABS, was applied to correct for absorption
problems. Two of the ethyl groups were disordered, therefore two
orientations were each refined at half occupancy. All atoms were refined
anisotropically. Final refinement gave residuals of R = 0.047 and RW = 0.072
with the quality-of-fit indicator = 2.27. The last cycle refined with 316
parameters and 3353 data with Fobs > 30; the maximum shift/esd was 0.05,
and a final difference Fourier showed the highest peak to be 1.04 e'/A3 in the

map.

95

Table 7. Crystal Data for [Rh 2(EtCN )1ollBF4l4

Formula Rh2C3oH50N1084F16
Formula weight 1103.82

Space group C2/c

a, A 1 9.920(4)

b, A 1 2.646(2)

c, A 1 8.646(2)

(1, deg 90

[3,deg 9255(2)

y, deg 90

V, A3 3886(1 )

Z 4

dcalc, g/cm3 1-574

)1 (Mo Ka), em-1 7.93

Data collection instrument Rigaku AFC6-S
Radiation (monochromated in M0 K0100: = 0.710738); graphite
incident beam) monochromated
Temperature, °C , -90 :t 2°

Scan method 0: - 26

Ra 0.047

wa 0.072

aR=£ l lFol ' chll/ZlFol

bRw = [1“.wlrO |— IFC ))2/1:w lFo|2]l/2; w = 1/02( lFoi)

96

D. Results and Discussion

[Rh 2(MeCN)10l[BF4]4
Synthesis

Preparation of fully solvated dirhodium cation species has been
established by several routes. The first successful method employed
triethyloxonium tetrafluoroborate as an alkylating reagent to remove the
carboxylates from rhodium tetraacetate. This reaction is done in excess of

the oxonium reagent

 

A
MeCN
Rh2(OAc)4(MeOH)2 + [Et3OllBF4] 10 d > [Rh2(MeCN)10][BF4]4
ays
alkylation 1M in CH2C12 60-70% Yield
Equation 7

to drive the reaction to completion mainly because of a competing side
reaction of Et3 OBF4 with MeCN which yields the iminium salt,
[MeCNEtHBF4l‘. This preparative method involves a rather long reaction
time (up to two weeks) and maximizing the yield is tedious due to a
propensity of the reaction solutions to produce oils. Shorter reaction times,
addition of CH2 C12, and lower temperatures yield a mixture of the desired
product with the [Rh2(OAc)2(MeCN)5]2+ complex. Incomplete reaction occurs
most often when excessive CH 2Cl2 is present because the intermediate
precipitates and does not redissolve to undergo further reaction. The product
mixtures often appear red by casual examination, but careful scrutiny reveals
the presence of admixed purple and orange crystals. The mixtures are easily

identified by 1H NMR spectroscopy where the resonances at 5 = 2.31 and 5 =

97

2.03 ppm appear for the intermediate; large amounts of this impurity may be
observed in the IR spectrum as four bands in the v(C EN) region between 2340
and 2249 cm'1 due to the spectral overlap of the two compounds. No sign of
either a tris-acetate or a mono-acetate intermediate was detected, suggesting
that the bridging ligands are removed in pairs and cis to each other.

A second successful strategy for the synthesis of [Rh2(MeCN)1o][BF4]4
was found from the reaction of tetrafluoroboric acid with Rh 2( OAc)4 (MeOH)2.
The acid protonates the carboxylates, liberating acetic acid. Solvent replaces

the vacant coordination sites as in the equation below:

A

M CN
Rh2(OAc)4(MeOH)2 + HBF4 ' E120 '—e—_> [Rh2(MeCN)10][BF4l4

protonation 7 days 60-70% Yield

Equation 8

This reaction proceeds slightly faster than the triethyloxonium method, and
high yields of the crystalline product may be achieved by layering hexanes
and diethyl ether on top of the reaction solution. The product must be
recrystallized once again prior to use, nevertheless this procedure has proved
to be a very convenient method for accessing the solvated compound in

relatively high yields.

Spectroscopy

The 1H NMR spectrum of 2 displayed rather surprising results. If the
salt is recrystallized in the dark, one observes only one resonance at 5 = 1.95
ppm for free MeCN indicating that all the solvent ligands have exchanged for
CD3CN. If the solid is recrystallized in room light or if the sample is

98

prepared in air, a second resonance at 5 = 2.65 ppm appears in a ratio of
about 4:1 with the 1.95 resonance. (See Figures 10 and 11) The former
resonance was initially mistaken as a resonance for the equatorial ligands in
[Rh2(MeCN)10]4+, but it appears more likely that it is due to an impurity that
co~crystallizes with [Rh2(MeCN)10][BF4]4. In studies of the pure crystals
grown in the dark, we were not able to record a spectrum containing bound
solvent within the time constants of sample preparation and setting up the
experiment. A sample prepared anaerobically and immediately purged with
02 displays an intermediate amount of the 5 = 2.65 ppm resonance. The
extremely rapid ligand exchange from all positions in the pure crystals
contradicts the postulation of Chisholm and Cayton who suggested that
Rh2(II,II) species should be relatively substitutionally inert in the equatorial
positions.6

The infrared spectrum of [Rh2(MeCN)10][BF4]4 displays a large broad
feature centered around 1065 cm'1 due to the [BF4]' stretches. The most
sensitive region for identification is the v(C EN) region (2345, 2315, and 2287
cm'1 ); these are in order of decreasing intensity. These three features are
predicted by group theory treatment of an idealized D41, molecule. The lowest
intensity
stretch is due solely to the axial ligands since this band is absent in solution
spectra, while the other two stretches remain unchanged (2342, 2317 cm'1 ).
A solution spectrum in CD3 CN helped identify the remaining two stretches as
v(C EN) and the CH3 deformation/C-C stretching combination mode. The
infrared data in solution show a greatly reduced intensity of the highest
energy band (2361, m), an intense intermediate energy band (2334, vs), a

weaker lower energy band (2307 w), and a new feature at 21 12 due to the

99

Figure 10. 1H NMR spectrum of [Rh2(MeCN)1ol[BF4l4 recrystallized

in the absence of light

100

End a; cd ~.~ YN 9N ad

bub-bbbbhhrpprbbptPrhppbppppbpr-phthLbbbpphrprpbpl—bpbprstnbn.
uh)

 

 

 

 

 

1! 1 J/lllll )1 < lil<lll

Figure 10

 

101

Figure 11. 1H NMR spectrum of [Rh2(MeCN)1o][BF4]4 recrystallized

in the presence of light

102

End a.—

hbh—pb

ll

9N ~.N

RN

ppLL.lp_bLLlr_l» PlbphLleL—LLIPLF_L|L_—IL

hub

ed wd

 

5:) )<)

mmé

 

ll‘

p?..LlPLb-Fb)tppphp

 

 

mad.

 

)4

Figure 11

103

exchanged CH3CN. The latter is unobservable in other solution spectra since
the experiments are carried out in CH 3CN . The exact position of the bands
has shifted slightly, presumably due to the remote isotope effect on the force
constants, however the pattern of three bands remains the same suggesting
that the stretches still parallel those of the protic solvent spectrum. This
particular solid sample was recrystallized in room light and thus is likely to
undergo solvent exchange at a much slower rate as verified by 1H NMR
experiments, so the incomplete disappearance of some stretches is
understandable. Still, this is enough evidence to assign the three features of
the protic samples in this region: the highest energy band around 2350 cm'1
is the combination mode due to CH3 deformation and CC stretching, the
center band around 2315 cm '1 is the v(CEN) of the coordinated equatorial
ligands, and the last is due to the axial ligands.

Surprisingly, complete solvent exchange does not appear to take place
in CH3 N02. Solution infrared spectra show that the stretches of the
equatorial ligands do not change (2342 vs, 2314 s) while a single additional
band appears at 2254 cm ’1 which is apparently the solvent that is exchanged
from the axial positions. A 1H NMR spectrum of [Rh2(MeCN)10][BF4]4 in,
CD 3NO2 displays two resonances, one singlet at 5 = 2.74 ppm due to
equatorial MeCN, and a very broad feature at 5 = 2.42 ppm attributed to the
exchanged axial groups. This spectrum is invariant over the course of 7 days.
The breadth of the resonance at 2.42 ppm indicates rapid exchange is
occurring at the axial sites. Apparently nitromethane is sufficiently polar to
dissolve the tetracation, but competes poorly for all but axial coordination
sites.

Electronic spectra of anaerobic solutions of [Rh2 (MeCN) 1014+ prepared

in the dark display two absorptions, one at Amax = 468 nm and a second

104

feature at 270 nm. The influence of light on these spectra will be discussed in
Chapter IV. A cyclic voltammogram of [Rh2(MeCN)10][BF4]4 in 0.1 M
TBABF4 in MeCN displays a single irreversible reduction at EN: = -0.05V vs
Ag/AgCl (-0.538V vs. F c/Fcr). The absence of an accessible oxidation, which
is usually observed for Rh2(II,II) species,1 is not surprising due to the high

charge localized on the metal centers.

Molecular Structure

A summary of selected bond distances and angles are presented in
Tables 8 and 9. Positional parameters are included in the Appendix. The
ORTEP diagram in Figure 12 clearly shows the dirhodium unit with each
metal in an octahedral environment consisting of five ligands with the sixth
position being occupied by the other metal. The center of the Rh-Rh bond
resides on a crystallographic C2 axis. The equatorial planes of CH 30N
ligands are twisted with respect to each other with a torsion angle xav of
44.8°. The axial CH3CN groups deviate somewhat from linearity which
reduces the molecular symmetry from the idealized D4d to C2. This
essentially perfectly staggered arrangement (depicted in Figure 13) reflects
the low steric demand of acetonitrile as a ligand. The few previously reported
unbridged dinuclear rhodium compounds that have been structurally
characterized exhibit torsional twist angles less than 45° due presumably to
the bulky size of the ligands as in Rh2(dmg)4(PPh3 )2, x = 42.6°,7 (ding =
dimethylglyoximate anion) and Rh2(p-CH 3C5H4NC)3122+, x = 26°.3

While this is not the first example of an unbridged Rh-Rh bond, it
represents a rare example of this unit in the absence of any significant
repulsion between ligands. The longest distance for a Rh( II)-Rh( II) bond to
date is 2.936 A found in the Rh2(dmg)4(PPh3 )27 molecule in which the

105

Table 8. Selected Bond Distances in A for [Rh2(MeCN)1o][BF4]4

AtomZ Distance

Atoml

11
(((((( 123451357911112222
hhhhhh ((((((((((((((((((
RRRRRRNNNNNCCCCCBBBBBBBB

1
1
1
1

))))))))))))))))))

Numbers in parentheses are estimated standard deviations

in the least significant digits.

106

Table 9. Selected Bond Angles in degrees for [Rh2(MeCN)1()][BF4]4

8tom3 Angle

Atomz

Atoml

))))))))))))))))))))))))) )))) )) \II )

1111111222222224444555667)7769)67\I7)8
((((((((((((((((((((((((( 1((((l((1(1(

2551107979412642472119924<7636(6q4(9(3

OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO

070089818974.9086466675778053102506372
9899787988798987777677777210100129297
1 1 1 111111111111111111 1

(((((((((((((((((((((((((((((((((((((

)))))))))))))))

l
1
l
1
1
l
1
1
1
l
l
l
l
1
l

))))))))))))))))))))))

hhhhhhhhhhhhhhh ((((((((((((((((((((((
RRRRRRRRRRRRRRRNNNNNCCCCCBBBBBBBBBBBB

IIII
))))) )))))

11111 llllllllll 11111 )))))))))))))))))
((((( 1111222334((l\((12345111223555667
hhhhh (((((((((( hhhhh (((((((((((((((((
RRRRRNNNNNNNNNNRRRRRNNNNNFFFFFFFFFFFF

Numbers in parentheses are estimated standard deviations

in the least significant digits.

107

Figure 12. ORTEP diagram of the cationic component of
[Rh 2(MeCN )10][BF4]4 viewed perpendicular to the Rh-Rh bond

108

CH)

'\
on?) '
) "3821 N (l )

 

l/ 8h(l)'

:7 [I F,

, {-1) v.4 a\ ..1' a.

p: "I ,4 ‘11

[3 ‘/§ ,.

V 6.)) W)

\ | M3) 8‘ ”7) 0(3)
3),", w .

0(5) (3.71 ')7/I' C‘“
5 /

gsK
C18) :3
'L".

Figure 12

C(2)

C(10)

C(9)
N (5)

(III/3'
~ I

109

Figure 13. ORTEP diagram of the solvated cation of
[Rh 2(MeCN)1()][BF4]4 viewed parallel to the Rh-Rh bond

depicting the staggered nature of the molecule

110

Z:
8

N(1)

Rh( 1)

N(3)

O

Rh(] )'

§<

C

2
A
A
V

Figure 13

111

equatorial ligands are close to achieving the maximum torsion angle, but the
added presence of bulky axial PPh3 groups prevent the ligands from further
relieving steric repulsion by bending away from each other; as a result, the
metal-metal distance lengthens. The Rh-Rh separation in

[Rh 2(MeCN ) 1ollBF4l4 of 2.624(1)A is much shorter than the other unbridged
examples, but is still longer than the average length of 2.35-2.45 A in tetra-
bridged systems.9 It must be noted that the bridged systems generally employ
anionic n-donors as ligands in contrast to the neutral o-donors in this

situation, so comparisons must necessarily be limited.

[Rh 2(MeCN)10][TFMSl4
Synthesis

Tetrafluoroborate salts display only limited solubility in common
solvents. The [Rh2(MeCN)10][BF4]4 complex is completely insoluble in THF,
CH 2012, acetone, and alcohols, and dissolves only in MeCN, water, CH 3NO2,
and other polar solvents such as DMSO. This limited range of solubilities
serves to restrict subsequent reaction conditions, so preparation of a more
soluble form of the salt was required. The most straightforward method of

achieving this would be to exchange the anion by simple metathesis. Efforts

to substitute p-toluenesulfonate for the [BF4]' led to incomplete exchange as
judged by the FT-IR spectrum which exhibited a substantial [BF41' stretch
but no features due to the incoming anion. Metathesis did occur with

LiTFMS as verified by the IR spectrum which displayed stretches typical for
uncoordinated TFMS" v[SOg(E)] and v[SO3(A1)l at 1270 and 1043

respectively, and v[CF3(Al)] and v[CF3(E)] at 1237 and 1167 cm'1
respectively. 10 The band near 1280 cm’1 for ionic CF3SO3' generally shifts to

higher energy around 1380 cm‘1 for monodentate coordination of the anion.

112

The characteristically broad [BF4]’ stretch at ca. 107 0 cm:1 was absent.

Unfortunately, the new Rh salt and the original LiTFMS exhibit virtually
identical solubilities, so even with repeated recrystallizations, the orange
solid remained contaminated with the white LiTFMS salt.

Since metathesis displayed such poor results, we proposed to
synthesize a different salt starting from the original Rh2(OAc)4 (MeOH) 2.
Several reagents were used including MeTFMS and HTFMS. Both of these
reactions yielded red-orange solutions, but these oiled extensively and did not
produce solids in any cases. This is a persistent difficulty with acidic media
that generally results from water in the hygroscopic reagents. A slightly
different method was successful in producing [Rh2(MeCN)10][TFMS]4 from
the action of Me3 SiTFMS on the tetraacetate. This is a silylating reagent as

shown in the equation below:

A
, MeCN
Rh2(OAc)4(MeOH)2 + M€3SI(TFMS) _——d——'> [Rh2(MeCNllollTFMSl4
7 a s
silyl esterification y 60-70% Yield
Equation 9

This type of reagent has proven to be very effective at removing carboxylate
ligands from dinuclear units, but typically the reagent is of the general form
Meg SiX for X=Cl, Br, or 1.11 The halide replaces the liberated anionic
carboxylate to maintain the original charge on the compound. In the present
case, an anion that is generally non-coordinating is used, which allows
solvent to replace the transformed carboxylate. Isolation of the product was

achieved in the same manner as the HBF4 synthesis by layering hexanes and

diethyl ether on the reaction solution. One difference between the two salts

113

was observed when the [TFMS]' salt was subjected to a dynamic vacuum for
several hours. The orange microcrystalline compound turned into a purple
powder which redissolves in MeCN to give an orange solution. Preparing

crystals of this derivative was not possible.

Spectroscopy

Substitution of trifluoromethanesulfonate for tetrafluoroborate
increases the solubility of the solvated cation. [Rh2(MeCN)10][TFMS]4 is
soluble in acetone and alcohols, and although it appears to undergo
transformations in these solvents judging by the observed color changes, the
compound is sufficiently stable in these media to carry out reactions. A 1H
NMR spectrum in d6-acetone supports the conclusion of long term instability.
If the sample is measured immediately after preparation, there is a single
resonance at 5 = 2.91 ppm. Within 24 h, the red solution has turned olive
green and gives multiple resonances at 2.95, 2.91, 2.85, and 2.76 ppm as the
most prominent features. The conversion is not particularly clean, but is
reproducible. Prolonged exposure to MeOH at high concentrations tends to
result in precipitation of a black solid (Rh metal) from an olive-green solution

The 1H NMR of the [TFMS]‘ salt mimics that of the [BF4]' salt,
especially with respect to its sensitivity to light. A sample recrystallized in
light exhibits the same two resonances at 5 = 2.68 and 1.95 ppm as found for
the [BF4]' compound in CD3CN. Deuterated acetonitrile is not very
diagnostic for the ligand environment due to the solvent self-exchange, so
several measurements were recorded in other solvents.

The difference between the orange and purple compounds with the
[TFMS]' anion is easily seen in the 1H NMR spectra recorded in CD3N02.

The purple species displays only one resonance at 5 = 2.79 ppm with no

114

second feature in the baseline due to axial MeCN, while the spectrum of the
orange compound under the same conditions shows the axial feature at 5 =
2.02 ppm. The latter spectrum is shown in Figure 14. This suggests that
exposure to vacuum removes the axial ligands from the [Rh 2( MeCN ) 10] 4+ core
since it is the exchange of the axial positions that produces the very broad
resonance. A solution IR spectrum adds to this evidence because there are
only two stretches present in the v(C EN) region of the purple compound at
2340 and 2311 cm'1 while in the orange compound these two stretches are
present in addition to free MeCN at 2253 cm'l. The solid state infrared
spectra are the most convincing, as the orange compound displays the three
V(C EN) stretches typical of the decakisacetonitrile cations at 2345, 2316, and
2286 cm'1 while the spectrum of the purple compound is missing the lowest

energy stretch which is assigned to the axial ligands.

Molecular Structure

Selected bond distances and angles are presented in Tables 10 and 11
respectively. Positional parameters are included in the Appendix. The
difference in counterion is enough to reduce the symmetry of the crystalline
salt from a C-centered to a primitive monoclinic lattice. Considering the
lower symmetry of [TFMS]' compared to the tetrahedral [BF4]', this is not
particularly surprising. The ORTEP packing diagrams of the two salts,
shown in Figures 15 and 16 show the much more loosely packed lattice of the
[TFMSI' salt which also incorporates an interstitial solvent molecule unlike
the more closely packed [BF4]' structure. The [Rh2]4+ molecule itself, is on a
general position so the entire unit is unique in the asymmetric unit in

contrast to the earlier structure in which the center of the Rh-Rh bond lies on

115

Figure 14. 1H NMR of the orange form of [Rh2(MeCN)1()][TFMS]4
in CD3 N02

116

P

h

h

9m
1
Ida 9m

T _ _

(I‘ll

b

 

Figure 14

117

Table 10. Selected Bond Distances in A for [Rh2(MeCN)1o][TFMS]4

atom atom distance atom atom distance
881 RHZ 2-616(2) C11 C12 1.51(3)
881 N6 2-01(1) C13 C14 1.54(2)
881 N7 2-00(1) C15 C16 l.48(2)
881 N8 1-97(1) C17 C18 1.51(2)
RHI n9 2-01(1) C19 C20 1.53(3)
881 N10 2.15(1)

882 N1 1.97(1)

882 N2 1.96(1)

382 N3 1.97(1)

882 N4 2.03(1)

882 N5 2.14(1)

N1 C1 1.12(2)

N2 C3 1.13(2)

N3 C5 1.14(2)

N4 C7 1.06(2)

N5 C9 1.12(3)

N6 C11 1.10(2)

N7 C13 1.06(2)

N8 C15 1.15(2)

N9 C17 1.08(2)

N10 C19 1.15(2)

C1 C2 1.55(2)

C3 C4 1.51(3)

C5 C6 1.52(3)

C7 C8 l.48(3)

C9 C10 1.56(3)

118

Table 11. Selected Bond Angles in Degrees for [Rh 2(MeCN)1o][TFMS]4

atom atom atom angle atom atom atom angle
882 881 N6 91.3(4) N2 882 NS 91.5(6)
882 881 N7 90.4(3) N3 882 N4 90.2(5)
882 881 N8 88.7(3) N3 882 N5 90.2(5)
882 881 N9 90.2(4) N4 882 NS 86.8(5)
882 881 N10 179.0(4) 882 81 C1 178(1)
N6 881 N7 88.3(5) 882 82 C3 177(1)
N6 881 N8 178.6(5) 882 N3 C5 179(1)
N6 881 N9 92.7(5) 882 N4 C7 172(1)
N6 881 N10 89.6(5) 882 N5 C9 160(1)
N7 881 N8 90.3(5) 881 N6 C11 176(1)
N7 881 N9 178.8(5) 881 N7 C13 176(1)
N7 881 N10 90.2(5) 881 N8 C15 176(1)
N8 881 N9 88.7(5) 881 N9 C17 173(1)
N8 881 N10 90.5(5) 881 N10 C19 174(2)
N9 881 N10 89.2(5) N1 C1 C2 176(2)
881 882 N1 89.8(3) N2 C3 C4 178(2)
881 882 N2 91.2(4) N3 C5 C6 176(2)
881 882 N3 89.7(4) N4 C7 C8 179(2)
881 882 N4 90.5(4) N5 C9 C10 174(2)
881 882 N5 177.3(4) N6 C11 C12 180(2)
N1 882 NZ ' 88.2(5) N7 C13 C14 177(2)
N1 882 N3 179.5(5) N8 C15 C16 178(1)
81 882 N4 90.1(5) N9 C17 C18 177(2)
N1 882 N5 90.3(5) N10 C19 C20 178(2)
N2 882 N3 91.5(5)

82 882 N4 177.6(5)

119

Figure 15. ORTEP packing diagram of [Rh 2(MeCN )1ollBF4l4. The

anions have been omitted for clarity

120

 

 

 

 

Figure 15

121

Figure 16. ORTEP packing diagram of [Rh2(MeCN)1o][TFMS]4. The

anions have been omitted for clarity

122

 

 

 

 

 

 

 

 

 

Figure 16

123

a C2 axis. With the exception of the lower crystallographic symmetry, the
two [Rh2 (MeCN) 10] 4+ cations are virtually identical. The Rh-Rh bond of
2.617(2) is comparable to the value of 2.624(1) in the original salt. The
torsion angle is slightly less than ideal at 42.8(5), but otherwise, crystal
packing forces do not appear to substantially influence either the

conformation or the structure of the dirhodium cation.

[Rh 2(EtCN)10l[BF414
Synthesis

Another strategy for increasing the solubility of the solvated dirhodium
species involves the preparation of a compound with a longer alkyl chain on
the nitrile. The space-filling model of [Rh2(MeCN)10][BF4]4 shown in Figure
17 shows that there is no significant steric repulsion between the staggered
ligands, therefore it is possible to substitute a longer chain without
disruption of the metal unit. Attempts to synthesize [Rh2(EtCN)1ol[BF4]4 by
solvent exchange of propionitrile with the MeCN ligand did not proceed
cleanly, judging by the weak v(CEN) stretches in the infrared, and by the
presence of free CH30N in the 1H NMR of the product. Since this route was
not very productive, the best approach proved to be synthesis of the desired
compound from Rh2(OAc)4(MeCN)2. The HBF4 method which is also the

preferred synthesis for the MeCN species was employed as in the equation

below:
A
EtCN
Rh2(OAc)4(MeCN)2 +HBF4-Et20 —Td——> [Rh2(EtCN)1ollBF41.
ays

Equation 10

124

Figure 17. Space filling model of the dimetal cation in

[Rh 2(MeCN710] [BF4 l4

125

    

\ u
) r

\\ \\
V § \
\ Q.
\\\\\\\\ A \ \\\ \
\\\\“
W
\\\x. \\ I
\

48.

 

126

The reaction in propionitrile proceeds much faster than with acetonitrile, but
work-up involves repeated iterations of crystallization due to a tendency of
the solution to oil. Synthesis of the triflate salt of the propionitrile
substituted cation was not successful as reactions of HTFMS or Me3SiTFMS
with Rh2(OAc)4 (MeCN)2 did not yield tractable products. The triflic acid
reaction gave a red-orange colored solution, but the reaction with
trimethylsilyltriflate produced a brown solution. The triflic acid is even more
hygroscopic than tetrafluoroboric acid; this fact along with the increasing
solubility of triflate salts probably complicates all reactions using this

reagent.

Spectroscopy

The 1H NMR of the [Rh2(EtCNl10llBF4l4 compound is more
straightforward than the corresponding spectrum of the MeCN compound.
Immediately after dissolution in CD3CN, the expected 4:1 ratio of equatorial
ligands to exchanged axial solvent ligands was observed; full exchange of all
sites was complete within 24 hours. The FT-IR spectrum displays only two
stretches in the CEN region, at 2324, and 2287 cm'1 since the additional .
carbon removes the combination mode. The broad v(B-F) stretch occurs at
1055 cm'1 .

The electrochemistry of the EtCN and MeCN species are compared in
Figure 18. [Rh2(EtCN)1o][BF4]4 displays an irreversible reduction at EN =
-0.20V vs Ag/AgCl (-.625 V vs Fc), at a slightly more negative potential than
[Rh2(MeCN)1ol[BF4]4, and an additional second irreversible reduction at
lower potential (Eps = -0.95V vs Ag/AgCl, -l.375 vs Fc) that is not observed
out to -2.0 V for the MeCN salt. As in the MeCN complex, the lack of an

accessible oxidation is not surprising for this highly charged system.

127

Figure 18. Cyclic voltammetry of (a) [Rh2(MeCN)10][BF4]4 and
(b) [Rh2(EtCN)1ol[BF4l4

128

 

(A)

 

 

 

 

 

(B)
- a n J . n .
‘ ‘ ‘ v 1
+ 1.0 +0.5 0.0 -0.5 -1.0
Volts vs Ag/AgCl

Figure 18

129

Molecular Structure

Tables 12 and 13 present selected bond distances and angles.
Positional parameters are included in Appendix 2. An ORTEP diagram in
Figure 19 depicts the dirhodium cation with only one orientation of the
disordered atoms presented for clarity. In spite of the longer chain EtCN
ligand, the homologous acetonitrile and propionitrile salts pack in the
identical space group with the same symmetry as is effectively demonstrated
in the packing diagrams depicted in Figures 15 (vide supra) and 20. The
center of the Rth bond lies on a two-fold axis rendering one-half of the
molecule and two of the [BF4]‘ ions unique. The two planes of equatorial
ligands are almost perfectly staggered with respect to each other with the
average N -Rh-Rh-N torsion angle equal to 44.9(2)°. Two equatorial ethyl
groups are disordered in two orientations- C5 and C6, and C11 and C12. It is
somewhat surprising that only two of the five groups displayed any
significant disorder, even though multiple orientations become more likely
with the increasing length of the alkyl group. The space filling diagram of
the [Rh2(EtCN) 1014+ molecule, depicted in Figure 21, shows that the longer
chain nitrile does not increase the steric demands around the dimetal unit.
The Rh-Rh bond of 2.6040(9) A falls in the same range as the other
crystallographically characterized nitrile salts which suggests that the Rh-Rh
bond is quite robust and is not highly influenced by the identity of the ligand

or crystal packing forces due to different counterions.

Other Nitriles
In an effort to test the stability limits of the Rh-Rh bond with more
sterically demanding ligands, the preparation of homoleptic butyronitrile and

benzonitrile complexes was attempted both by solvent exchange with the

130

Table 12. Selected Bond Distances in A for [Rh2(EtCN)1ol[BF4l4

atom
881
8h1
8h1
Rhl
Rhl
Rhl
F1
F2
F3
F4
F5
F6
F7
F8
N1
N2
N3
N4
N5
C1
C2

atom
8h1'
N1
N2
N3
N4
N5
BI
Bl
Bl
BI
82
82
82
82
C1
C4
C7
C10
C13
C2
C3

distance

2.6040(9)

1.991(4)
1.977(4)
1.982(4)
1.987(4)
2.180(6)
1.408(8)
1.374(9)
1.374(9)
1.367(8)
1.39(1)

1.44(1)

1.33(1)

1.31(1)

1.128(7)
1.122(8)
1.139(7)
1.101(8)
1.148(9)
1.460(8)
1.54(1)

atom

C4
C4
C5
C58
C7
C8
C10
C11
C118
C13
C14
C12

atom
C5
C58
C6
C68
C8
C9
C11
C12
C128
C14
C15
C128

distance

1
1
1
l
1
l
1

.55(2)
.50(2)
.49(2)
.53(2)
.456(8)
.528(9)
.61(2)

.50(3)
.51(2)
.54(1)
.43(1)
.79(3)

 

131

Table 13. Selected Bond Angles in degrees for [Rh 2(EtCN) lollBF4]4

atom atom atom angle atom atom atom angle
Rhl 8h1' N1 91.2(1) C4 C5 C6 107(2)
Rhl 8h1' N2 91.0(1) C4 C58 C6A 103(1)
Rhl 8h1' N3 88.6(1) N3 C7 cs 178.1(6)
Rhl 8h1' N4 91.6(2) C7 C8 C9 111.8(5)
Rhl 8hl' N5 176.9(2) N4 C10 C11 165(1)
N1 8h1 N2 91.3(2) N4 C10 C118 161(1)
N1 Rhl N3 179-0(2) C10 C11 C12 101(2)
N1 Rhl N4 89-1(2) C10 C118 C128 96(2)

N1 Rhl N5 87.1(2) N5 C13 C14 177.7(9)
Nz 8h1 N3 89.8(2) C13 C14 C15 108.4(8)
N2 8h1 N4 177.4(2)

N2 Rhl N5 86.4(2)

N3 8hl N4 89.9(2)

N3 8h1 N5 93.1(2)

N4 8hl N5 91.1(2)

Rhl N1 C1 171.9(4)

Rhl N2 C4 170.6(6)

Rhl N3 C7 178.0(4)

8h1 N4 C10 175.2(7)

8h1 N5 C13 157.4(6)

N1 c1 C2 178.5(6)

Cl c2 C3 112.5(6)

N2 c4 C5 166(1)

N2 C4 C58 165(1)

C5 C4 C58 28.8(7)

132

Figure 19. ORTEP diagram of the dimetal cation in
[Rh2(EtCN)1ol[BF 414

 

133

   

C(12) C(11)

f7, cum/.7"
C(15) Ex ‘ N“) a?“
C(14) C(13) Nt5) ”(J

mm

Figure 19

134

Figure 20. ORTEP packing diagram of [Rh2(EtCN)1ol[BF4]4 . The

counterions have been omitted for clarity

135

 

 

 

 

 

 

 

 

 

 

 

 

Figure 20

136

Figure 21. Space filling diagram of the cationic component of

[Rh 2(EtCN)1ol[BF4l4

137

‘- \\\\\\\\‘\

\
\9- ' \\ “E ~\\\§\
\\.'

 

Figure 21

138

MeCN and EtCN species, and by synthesis from Rh 2( OAc)4(MeCN)2, but
solids were not isolated. This is largely due to the increased solubility of
these species as well as the typical problems with the acidic medium,
although the use of Et3 OBF4 was not any more successful than HBF4.
Synthesis of a butyronitrile complex does appear to proceed from the action of
HBF4 on Rh 2( OAc)4 (MeCN)2, judging from the orange color of the reaction
solution, but no solid was obtained. The chemistry of Rh2(OAc)4(MeCN ) 2 and
HBF4 with benzonitrile produces a brown intractable solution. Solvent
exchange with the acetonitrile salt is not clean, but dissolving the
propionitrile salt in benzonitrile produces an orange solution similar to those
of the other nitrile salts. The extreme solubility of the product has thus far
prevented isolation of the compound, but it appears that [Rh2(EtCN)1ol[BF4l4
may be a useful route to compounds that are inaccessible from the MeCN
analog.

Substitution reactions were also carried out with bidentate nitriles.
We proposed that if monodentate nitriles adopt a perfectly staggered
disposition across the M-M bond, perhaps the unit could be forced into an
eclipsed mode by tethering together two of the ligands. Pentanedinitrile, or
glutaronitrile, is exactly that- two acetonitrile ligands are tied together by a
methylene bridge. Figure 22 depicts the ligands and the proposed products of
these with the [Rh 2]4+ unit. It is important to pursue the X-ray structures of
these compounds, as determining the Rh-Rh bond length crystallographically
would afford the first opportunity to compare the metal-metal distances of
bridged and unbridged metal systems with similar ligand sets. The neutral
o-donors such as MeCN cannot be validly compared with anionic n-donors

like carboxylates.

139

Figure 22. Diagram of the postulated staggered vs. eclipsed
conformational differences with monodentate and bidentate

nitriles

140

 

 

N
N \sN N
N=—C —CH3 N—Rh Rh———N
..,,”
Acetonitrile
N
N N

 

 

 

 

C C
N N
H H N N
N N
Pentanedinitrile

Figure 22

 

141

On initial examination, it appears that bridging a dimetal unit by
pentanedinitrile is very unlikely due to the necessity of forming a nine-
membered ring. This is actually not quite the case since the C-CEN unit is
relatively rigid so flexibility at each atom will not be observed.
Propanedinitrile (malononitrile) was also investigated, but there is less
flexibility in this ligand due to the presence of only one sp3 hybridized carbon;
furthermore, the bite size may be too large for this dimetal unit.

Our main interest in these complexes, is that bidentate nitriles should
help to stabilize the Rh-Rh bond in substitution reactions that led to metal-
metal bond cleavage. The bridging nitrile ligands should not alter the
electronic structure of the molecule substantially, but may change the
environment sufficiently to allow more reactive species to be stabilized.

Bifunctionalized nitriles have been previously used as ligands that
either bridge or coordinate monofunctionally.12 In the present case of
homoleptic nitrile systems, there is good potential for polymeric networks to
be established through two units sharing an axial ligand. To encourage the
isolation of discrete molecules, acetonitrile was added upon work-up in an
attempt to substitute the axial positions and thereby break up any existing
networks. 2

Unfortunately in our synthetic work, substitution for bidentate nitriles
has encountered many of the same problems as with the longer mono-
functionalized nitriles. Solvent exchange is incomplete with the starting
material, [Rh2(MeCN)1o][BF4]4, and isolation from acidic media was not
possible. Additional problems were encountered when the reactions were
carried out in neat dinitrile, as this liquid displays limited miscibility with
other organic solvents which further restricts options for subsequent work-

up.

142

Carboxylate Reactions

Our efforts to determine the stability of the metal-metal bond in
[Rh 2(MeCN) 10] 4+ were not limited to substitution of MeCN for other nitrile
ligands. Elementary reactivity studies were performed with a variety of
different donors to elucidate the ability of this starting material to be used in
designed synthesis. The most obvious starting point was to attempt to
convert the complex back to Rh2(OAc)4(MeOH)2. This reaction is quite
straight forward; indeed reaction of [Rh2(MeCN)1ol4+ with acetic acid and
sodium acetate leads to a green solid that after recrystallization from MeOH,
was verified as dirhodium tetraacetate.

During the course of our studies, Chisholm and coworkers reported the
findings on the lability of dirhodium and dimolybdenum carboxylate
complexes, which they supported with a theoretical argument. 13 They found
that the equatorial MeCN ligands in [Rh2(OAc)2(MeCN)6]2+ were not
substitutionally labile unlike the Mo analog.13:14 It was of interest to their

studies as well as ours to ascertain the lability of the equatorial MeCN groups

in [Rh2(MeCN)1()] 4+, especially with respect to replacement with carboxylate
ligands. While carboxylate removal in going from Rh2(OAc)4(MeCN)2 to .
[Rh2(MeCN) 10] 4+ appears to occur with cis geometry to give the intermediate
cis-[Rh 2(OAc)2(MeCN)6]2+, this may be a kinetic rather than thermodynamic
product. Trans carboxylates are not out of the question in reactions of the
cis-carboxylate compounds as seen in the cis to trans isomerization in the
reaction of [M02(OAC)2(MeCN)6]2+ and dmpe.15

Many attempts were made to prepare a bis-carboxylate product from
the [Rh2 (MeCN ) 10] 4+ in order to observe its disposition of carboxylate
ligands. Reactions with sodium acetate and its hydrate are hampered by

their low solubility, and more soluble carboxylates such as sodium butyrate

 

143

and sodium trifluoroacetate led to brown solutions. In one reaction with
sodium acetate, a purple solution resulted that gave electronic spectra similar
to that of cis-[Rh2(OAc)2(MeCN)6]2+, but considering both the
irreproducibility of this result as well as the diagnosed impurity of the
starting material we strongly suspect that the product was merely an
impurity in the solvated compound.

One unusual result emerged from the reaction of [Rh2(MeCN)1o][BF4]4
with Na[O2C4H7]. Two equivalents were added originally, and due to a
persistent lack of solubility in MeCN, slightly more than two more
equivalents were added to favor the forward reaction. The solution turned
purple and was then pumped to a solid. The solid turned green after
prolonged vacuum. A 1H NMR spectrum revealed that this produce was
Rh2(O2C4H7)4, signifying that the reaction had gone beyond the desired bis-

carboxylate stage.

Axial Substitution

In spite of its relative insolubility in most common solvents,
[Rh2(MeCN)1()]4+ undergoes axial exchange reactions in various media. The
most marked example is axial solvent exchange in the solid state. Upon
addition of a small volume of MeOH, THF, or acetone to a sample of the
[BF4]' salt, the color was seen to change without dissolution of the compound.
In MeOH, the solid turns red, in THF- pale orange, and in acetone- olive
green. Figure 23 shows the v(C-N) region in the infrared spectra of the new
solids. With the exception of the axial MeCN spectrum which is presented for
comparison, each spectrum shows loss of the axial MeCN stretch. Features of

the incoming axial donor may also be observed in each spectrum.

144

Figure 23. Comparison of the infrared spectra of the axial solvent

substitution with [Rh2(MeCN)1o][BF4l4

mmoz EB 2032
3-8.3 ..mnEscch

88 cc: 8: .58 SE .58 88 as: as:

L
J

 

-
ld

M5

 

 

 

 

5-. mm

saw

206:

8: 8:

db

 

 

_m+mmw

Figure 23

146

Solvent Exchange, Rh2(aq)4+
Synthesis

Solvent exchange is of course more facile when the salt is soluble in the
new medium. It occurred to us that [Rh 2(MeCN) 10] 4+ was an excellent entry
point into Taube's aquarhodium species in the absence of acid.
[Rh 2(MeCN ) 10][BF4]4 is completely soluble in water and turns red upon
initial dissolution. After refluxing, the solution converts to a purple color;
subsequent periodic pumping to remove the liberated MeCN results in
further color changes to blue, and finally green. These color changes are
typical of the substitution of nitrogen donors for oxygen donors. The reaction
to produce the MeCN solvate proceeds in exactly the opposite order; the
green compound, Rh2(OAc)4(MeOH)2, turns purple when dissolved in MeCN
as the axial positions exchange oxygen-donor MeOH for MeCN, and goes
through red to orange as the ligand environment becomes rich in nitrogen
donors. One advantage of our method to access an aqua compound that
differs from Taube's is that we use water as opposed to acid, therefore our
solvent may be removed in vacuo to yield a green solid. We attempted to
crystallize the aqua compound by eluting it with strong acid from a Dowex
cation exchange column. The compound was eluted anaerobically with 4 M p-
toluenesulfonic acid, and the resulting solution was purged with N2 to
encourage evaporation. No crystals have been obtained to date, but the

method appears to be very promising.

Spectroscopy
Since crystallization of aqua species is difficult, more practical methods
of characterization and comparison were employed. Table 14 compiles the

values obtained for the electronic spectrum of our product along with those

 

147

Table 14. Summary of Spectral and Reactivity Properties for

 

 

 

 

 

 

 

 

Rh 2(aq)4+
Synthesized from Synthesized from
[Rh 2(MeCN)1ol4+ [Rh(H2O)5Cll2+ and
Cr2+
lmax in H20 587 nm a
max in 3M. 80104 600 nm b 648 or 630 nm
580 nm C
converts back to
Rh2(OAc)4 in yes yes
CH 3COOH
(a) a value was not reported in H2O as the sample can not be isolated from
3 M HC104

(b) sample was prepared in H20
(0) sample was prepared in 3 M HClO4

148

reported by Taube.16 Since Taube's group was unable to separate the
compound from the acid, there is no report of a spectrum of that species in
water; Instead their data were reported in 3 M HClO4. In order to make a
valid comparison of our work to theirs, we also collected data in 3 M HClO4.
A Am value of 600 nm was observed for the complex prepared in water and
redissolved in acid before the spectrum was taken. This was quite
reproducible, and is most likely of a pure compound because the sample was
anaerobically eluted off a cation exchange column prior to spectroscopic
characterization. The second value was obtained from the compound isolated
from the synthesis of the aqua species from [Rh2(MeCN)1o][BF4]4 in acid. We
attribute the slight variations in the values among all methods of preparation
to the different pH values for each preparation. Varying degrees of
hydrolysis of the H20 ligands would alter the structure and therefore affect
the electronic spectrum of the compound. Taube verified that the Rh 24+ core
was intact in his Rh2(aq)4+ compound by its ease of conversion back to

Rh 2(OAc)4. The same procedure was repeated with our green solid with
identical results.

The infrared spectrum of the sample of "Rh2(aq)4+" prepared in this
study displays both stretches and bends due to the presence of water; these
are 3449, 3356, 1657, 1601, and a large feature due to [BF4]' at 1070 cm'l.
There was no evidence for MeCN apparent in the spectrum. A cyclic
voltammogram in 0.1 M KBF4 in water displayed a single quasi-reversible
reduction at E172 = 0.29V. The 1H NMR spectrum in CD3CN has three
significant resonances, one at 5 = 1.87 ppm ( 8,), a doublet at 5 = 1.78 ppm,
and a third feature at 5 = 1.69 ppm. The spectrum in D20 is slightly
different with four singlet resonances at 5 = 1.82, 1.79, 1.74 and 1.68 ppm in
approximately a 5:2:4:4 ratio. The 1H NMR spectrum of

149

[Rh2(MeCN)1ol[BF4]4 dissolved in D2O displays several singlet resonances
close together at 5 = 2.47, 2.45, 2.43, and 2.42 ppm as well as a doublet at
1.93 and a large singlet at 1.87 ppm. These features are attributed to a
combination of free solvent and coordinated solvent in several different

environments.

Interconversions of the Rh 24+ core

Figure 24 summarizes the variety of interconversions we have found
possible for the [Rh2]4+ compounds in this study. The tetraacetate may be
used to synthesize the decakisacetonitrile compound from which the water
compound may be prepared, and from there, conversion back to the acetate is
quite facile. The reverse reactions are not as accessible. The
decakisacetonitrile species can be converted back to dirhodium tetraacetate,
but whether or not the water compound may be accessed from there is
questionable. As mentioned in the Introduction of this dissertation, _
Wilkinson and coworkers had reported that it is possible, 17 but Taube's group
challenged these findings.18 Certainly our success with acidic syntheses in
our research prove that it is possible to remove all the carboxylates from -
Rh2(OAc)4 by this method. The final exchange of acetonitrile for water does
not proceed to completion. This is not surprising for several reasons. Firstly,
the pH of the reaction solution is quite low, suggesting that the rhodium
compound has undergone considerable hydrolysis. If this is the case, OH '
ligands as well as H2 0 ligands are present in the coordination sphere, and it
is highly unlikely that neutral acetonitrile groups will replace anionic
hydroxides. Secondly, the forward reaction is driven by periodic pumping to
remove the liberated MeCN ligands. This is possible because acetonitrile has

a lower boiling point than water. In the reverse direction, this situation

150

Figure 24. Interconversions of the [Rh 2]“ core

151

[Rh2(MeCN),O)4+

 

4 N aOAc/HOAC +
Rh2(OAc)4 r Rh2(aq)n
?

 

Figure 24

I52

prevents the free water from being removed from the system thus setting up

an equilibrium between the two fully solvated species.

Pyridine and Bipyridine Reactions

Efforts to prepare a [Rh2(py)1o]4+ such as simple solvent exchange
have not met with substantial success. [Rh2(MeCN)10][BF4]4 is not soluble in
pyridine even with gentle warming. Other attempts to prepare the complex
from Rh2(OAc)4(MeOH)2 led to a pink insoluble polymer, and yellow crystals
whose color suggests a Rh(III) mononuclear species. 1 Two X-ray structures
were carried out on these crystals, but due to severe twinning problems, the
structures were never solved.

In the case of the 2,2'-bipyridine chemistry, the best solvent proved to
be CH 3NO2 which neither degrades the starting material as acetone does, nor
competes strongly for the coordination sites as does MeCN. Reaction in this
medium of 4 equivalents of bpy with [Rh 2(MeCN )10][BF4]4 produced a green
compound with an unusual 1H NMR spectrum. The resonances in the bpy
region are shifted with respect to the free ligand as expected for coordinated
bpy, but the sets of bpy protons fall at different chemical shifts with respect
to one another than in most dinuclear bpy complexes.19 Perfection of the
synthesis and isolation of this product is extremely desirable so that X-ray
quality crystals may be grown of the pure compound.

Reaction with 2 equivalents of bpy also yields a product with a similar
pattern in the 1H NMR spectrum, but appearing at different chemical shifts
than the 4 bpy reaction, although we are aware that the difference in
deuterated solvent makes comparison a bit less valid. Further study of this

system is in progress.

153

Tertiary Phosphine Reactions

Reactions between tertiary phosphine ligands and the
[Rh2(MeCN) 1014+ complex pairs two relatively incompatible types of systems.
The highly charged dirhodium unit is a hard acid whereas the phosphines are
softer bases. The inability of these two to form a stable complex is evidenced
by the reaction solution initially turning red, but quickly reverting back to
the orange color of the pure starting material upon work-up. This result was
the same for monodentate (PMe3) or bidentate (dppm) phosphine ligands.
Orange solids isolated from these reactions displayed 1H NMR spectra typical
of the original fully solvated tetracation.

The one phosphine that readily produces a new complex is the highly
basic and bulky functionalized phosphine, TMPP, mentioned in the
introduction of this Chapter. This ligand apparently binds so strongly in the
axial position that the M-M bond is disrupted, and the bulk of the pendant
groups prevents reformation of the dinuclear unit.2 In the absence of bulk
and pendant groups, phosphines evidently do not compete effectively for
coordination sites occupied by the solvent, so there is no net reaction. This

was also confirmed independently by Baranovskii and coworkers in their ,

study of PPh3 with [Rh2 (MeCN)3(H2O)2 14+.20

Chloride Reactions

It was anticipated that the cation would readily bind the anionic
halides easily to balance the charge, but the reactions carried out with
chloride donors have produced inconclusive results. Further work is

necessary to elucidate this chemistry.

154

Oxygen Reactions

It is apparent that oxygen reacts with the [Rh2(MeCN)10]4+ only in the
presence of light. In the absence of photochemical conditions, the reaction of
MeCN solutions of the complex with O2 produce only the pure compound as
verified by a unit cell of a crystal grown in the dark from one of these reaction
solutions. A further discussion of the photochemistry of this compound is

found in Chapter IV of this dissertation.

E. Summary

Several different forms of homoleptic nitrile dirhodium species were
prepared that possess subtle differences in their reactivity.
[Rh2(MeCN)10][BF4]4 is the least soluble of the three, and exhibits extensive
photochemistry, detailed in Chapter IV of this dissertation.
[Rh2(MeCN)10][TFMS]4 crystallizes in a loosely packed lattice that facilitates
removal of the axial groups in the solid state. The salt, [Rh2(EtCN)10][BF4]4,
is the most stable of the group and the most readily prepared. Preliminary
results of the chemistry of [Rh2(EtCN )101 4+ indicates that it does not undergo
the same photochemistry as the acetonitrile complex. Solvent exchange
occurs readily with [Rh2(EtCN)10]4+, although the Rh2(aq)4+ species is
conveniently prepared from the [Rh2(MeCN)10][BF4]4

The fundamental reactivity of these species was broadly explored.
Reactions with bidentate anions with the exception of acetate has not been
investigated since these ligands will not test the subtle changes induced by
substitution of neutral acetonitrile donors for other neutral donors.
Phosphine ligands are generally incompatible with this hard acid, and the
results with chlorides are not conclusive. The reactions of two and four

equivalents of bipyridine are quite interesting, and underscore the

155

compatibility of the [Rh2]4+ core for neutral nitrogen donor ligands. These
substitution reactions occur much more cleanly in a non-nitrile medium.
Further study of these reactions would be particularly interesting.

A very useful tool in much of this chemistry would be the application of
103Rh NMR. As the products become less symmetrical in their electron
distribution due to mixed-donor ligand sets, the possibility for observing a
103 Rh signal increases. This method could be extremely helpful in indicating
oxidation state and coordination environment in systems for which the 1H
NMR spectrum shows only exchanged solvent. Future directions of this

research will make use of this metal NMR tool.

10.
11.

12.

13.
14.

15.

156

LIST OF REFERENCES, CHAPTER 111

Cotton, F.A.; Wilkinson, G. "Advanced Inorganic Chemistry," Wiley:
New York, 1988.

(a) Dunbar, K.R.; Haefner, S.C.; Pence, L.E. J. Am. Chem. Soc. 1989,
111, 5504. (b) Haefner, S.C.; Dunbar, K.R.; Bender, C. J. Am. Chem.
Soc. 1991, 113, 9540.

Chen, S—J.; Dunbar, K.R. Inorg. Chem. 1991, 30, 2018.

Rempel, G.A.; Legzdins, P.; Smith, H. Wilkinson, G. Inorg. Synth.
1972, 13,87.

Sheldrick, G.M.; SHELX86, Program for the solution of crystal
structures, 1986. Univ. of Gottingen, Germany.

Casas, J .M.; Cayton, R.H.; Chisholm, M.H. Inorg. Chem. 1991,30, 360.

(a) Caulton, K.G.; Cotton, F.A. J. Am. Chem. Soc. 1969, 92, 6517. (b)
Caulton, K.G.; Cotton, F.A., 1971 , 93, 1914.

Olmstead, M.M.; Balch, A.L. J. Organomet. Chem. 1978, 148, C15.

Cotton, F.A.; Walton, R.A. "Multiple Bonds Between Metal Atoms,"
Wiley: New York, 1982.

Lawrance, G.A.; Chem. Rev. 1986, 86 , 17.

See for example, (a) Cotton, F.A.; and Dunbar, K.R. J. Am. Chem. Soc.
1987, 109, 3142. (b) Cotton, F.A.; Dunbar, K.R.; Verbruggen, M.G. J.
Am. Chem. Soc. 1987, 109, 5498. (c) Cotton, F.A.; Dunbar, KR.;
Matusz, M. Inorg. Chem. 1986, 25, 3641. (d) Campbell, F.L.; Cotton',
F.A.; Powell, G.L. Inorg. Chem. 1984, 23, 4222. (6) Cotton, F.A.;
Falvello, L.R.; Harwood, W.S.; Powell, G.L.; Walton, R.A. Inorg. Chem.
1986, 25, 3949. (f) Cotton, F.A.; Dunbar, K.R.; Poli, R. Inorg. Chem.
1986, 25, 3700.

(a) Endres, H. in Comprehensive Coord. Chem. 1987, 2, 261. (b)
Johnson, A.; Taube, H. J. Indian Chem. Soc. 1989, 66, 503.

Casas, J .M.; Cayton, R.H.; Chisholm, M.H. Inorg. Chem. 1991 , 30, 360.

Plimbett, G.; Garner, C.D.; Clegg, W. J. Chem. Soc. Dalton Trans.
1986, 1257 .

Farrugia, L.J.; McVitie, A.; Peacock, R.D. Inorg. Chem. 1988, 27 , 1257.

16.

17.

18.

19.

20.

157

(a) Maspero, F.; Taube, H. J. Am. Chem. Soc. 1968, 90, 7361. (b)
Ziolkowski, J .J .; Taube, H. Bull de L'acad. Polonaise des Sciences.
1973, 21, 113.

(a) Legzdins, P.; Rempel, G.L.; Wilkinson, G. Chem. Commun. 1969,
825. (b). Legzdins, R; Mitchell, R.W.; Rempel, G.L.; Ruddick, J.D.;
Wilkinson, G. J. Chem. Soc. A. 1970, 3322.

(a) Wilson, C.R.; Taube, H. Inorg. Chem. 1975, 14, 2276. (b) Wilson,
C.R.; Taube, H. Inorg. Chem. 1975, 14, 405.

(a) Matonic, J.H.; Chen, S-J.; Perlepes, S.P.; Dunbar, K.R.; Christou, G.
J. Am. Chem. Soc. 1991, 113, 8169. (b) Perlepes, S.P.; Huffman, J.C.;
Matonic, J.H.; Dunbar, K.R.; Christou, G. J. Am. Chem. Soc. 1991 , 113,
2770.

Zhilyaev, A.N.; Rotov, A.V.; Kun'menko, I.V.; Baranovskii, I.B. Dokl.
Akad. Nauk SSSR, 1988, 302, 614.

CHAPTER IV

PHOTOCHEMISTRY OF [Rh 2(MeCN) 10llBF4l4

158

159

A. Introduction

In an effort to further elucidate the properties of the solvated dinuclear
rhodium compound, [Rh2(MeCN)1o][BF4]4 , we collaborated with scientists at
Los Alamos National Laboratories to examine the Raman stretching
frequency of the Rh-Rh single bond. During the Raman experiment, the
spectrum bleached, indicating that the compound had decomposed. Later, a
reexamination of the sample revealed that the original electronic spectrum
had been reestablished. Subsequent experiments proved that this compound
undergoes reversible photochemistry on the kilosecond time scale. We
proposed to further study the synthetic aspects of this chemistry to further

elucidate the photochemical process.

B. Experimental, Synthesis
(1) Thermal Decomposition of [Rh 2(MeCN )1()][BF4]4 in Solution

A quantity of 2 (33.7 mg, 0.035 mmol) was dissolved in 3 mL of a 1:1
solution of MeCN : tetraglyme. The solution was wrapped in foil and
refluxed. The Rh starting material was not completely soluble, therefore the
solution was decanted, and 10 mL of MeCN was added to the solution. The
solution was refluxed overnight after which the solution was immediately
decanted into a quartz UV-visible cell for further examination. Spectra were

recorded at frequent intervals as the solution cooled.

(2) Reduction of [Rh 2(MeCN )10][BF4]4 with the Tris(2,6-
dimethoxyphenyl) Methyl Radical

Two solutions were prepared for this experiment. In the first, 80 mg
(0.083 mmol) of 2 and 450 mg (1.37 mmol) of TBABF4 were stirred in 10 mL
of MeCN until everything had dissolved. The second solution contained 77

160

mg of tris(2,6-dimethoxyphenyl) methyl radical (0.083 mmol, 1.0 equiv.) in 20
mL of MeCN. The solution containing the radical was added to the metal
solution, with the appearance of the dark purple of the organic cation
masking any new compounds. Approximately 2 g of TBABF4 was added to
the solution to encourage precipitation of a gray solid which was collected by
filtration in air. A Nujol mull IR spectrum did not reveal any characteristic
features other than the [BF4]' anion at 1070 cm'l. The solid was collected on
a frit and washed with CH 2C12 until the washings were clear (5 x 10 mL). A
small amount of the solid dissolved in 2 mL of MeCN to give an orange
solution. At time intervals of approximately 5-10 min., the solid was again
washed with CH2C12, and each time the washings began as a purple color

and were continued until the solutions were colorless.

(3) Reduction of [Rh 2(MeCN )1o]4+ with Na-Acenaphthylenide (NaAce)
Quantities of 2 (104 mg, 0.108 mmol) and 632.6 mg [n-Bu4N][BF4]
( 1.921 mmol) were dissolved in MeCN after which the solution was wrapped
in foil and chilled in a MeCN/CO2 bath to -42°C. To this solution was slowly
added 10.8 mL of recrystallized NaAce (0.010 M in THF) and THF (10 mL)
which were also chilled to' -42 °C. The solution was then allowed to settle
revealing a yellow solution and a dark brown solid which was filtered to
collect the solid. The yellow solution, with the characteristic color of
acenaphthylene and containing excess [n—Bu4NllBF4] was discarded. The
brown solid was washed with diethyl ether (3 x 10 mL) and dried in vacuo.
Yield 91 mg, (94% yield based on a theoretical yield of 86 mg of
[Rh4(MeCN)16][BF4]6 and an additional 11 mg of NaBF4 ). 1H NMR
(CD3CN), ppm: 5 = +1.95 (free CHgCN). IR(KBr, Nujol), CITl'li 2337 (s), 2316

161

(In), 2281 (m), 1070 (3, br). UV-visible (MeCN, anaerobic, nm (e in M'1 cm'1))
425 (1080), 305 (20,200).

(4) Reduction of [Rh2(MeCN)1o][BF4]4 with Cobaltocene

A quantity of 2 (49.9 mg, 0.052 mmol) was added to solid cobaltocene
(9.9 mg, 0.052 mmol, 1.0 equiv.) in a Schlenk tube. Without addition of
solvent, the mixture of solids turned black within ten minutes. Acetone was

added which gradually dissolved the solids, but no product was ever isolated.

(5) Reduction of [Rh2(MeCN)1o][BF4l4 with Cobaltocene in the
presence of Electrolyte

An amount of 2 (95 mg, 0.99 mmol) was dissolved in 5 mL of MeCN
together with 200 mg of N aBF4 ( 1.822 mmol). To this solution was added a
solution of Con2 (19 mg, 0.100 mmol, 1.0 equiv.) and 463 mg of [n-
Bu4NllBF4] ( 1.406 mmol) dissolved in 5 mL of acetone. The two solutions
instantly produced a dark green solid suspended in a red-brown solution.
The solution was filtered and the solid was dried under vacuum; yield, 263
mg. UV-visible spectrum of the solid redissolved anaerobically in MeCN, '

71mm, = 414 nm, shoulder at ~470 nm.

(6) Crystallization Attempt of the Cobaltocene Reduction Product
Two solutions were prepared, one containing 102.5 mg of 2 (0.106
mmol) and 210 mg of NaBF4 in 5 mL of MeCN, and a second solution of 20
mg of Con2 (0.106 mmol, 1.0 equiv.) and 464 mg of TBABF4 in 5 mL of 1:1
toluene : acetone. The flasks containing these solutions were wrapped in foil.
The first solution was layered on the second solution by cannula transfer. A

black-green powder immediately appeared at the solution interface, but the

 

162

layers did not mix further at this time. Over ten days, needle-like
microcrystals began growing from the interface into the upper layer.
Diffusion was complete two days later, yielding a large amount of fluffy green
precipitate and a yellow solution. When the solution was filtered, the mass of
solid at the bottom proved to contain a large amount of red-orange crystals of
the starting material and only microcrystals of the green product. An EPR
carried out on a solid isolated in air displayed a very broad signal suggesting
that the compound was paramagnetic, although this may have been caused
by exposure to air which occurred during sample preparation. A 1H NMR
displayed principally free CH 3CN (5 = +1.95 ppm, s) and a small amount of
butylammonium contamination, (3.05, (m), 1.58 (p), 1.33 (sextet), 0.96 (t)).

(7 ) Reduction of [Rh 2(MeCN)1()][BF4]4 with NaEt3BH

A sample of 2 (56 mg, 0.058 mmol) and 663 mg of TBABF4 (2.021
mmol) were stirred together and warmed gently in 10 mL of MeCN until they
dissolved. To this was added 0.058 mL of NaEt3BH (0.058 mmol, 1.0 equiv.)
A flocculent brown precipitate and a red solution immediately resulted. The
solution was filtered by cannula and the solid was collected, washed with '
diethyl ether (2 x 5 mL), and dried in vacuo. A solid state EPR sample was
prepared anaerobically and displayed no signal. The brown solid was not
particularly soluble in MeCN, so a 1H NMR measurement was not possible.

IR(CsI, Nujol), cm‘l: 2361(m), 2339 (m), 1051 (s), 1020 (s).

(8) Bulk Electrochemical Synthesis of "[Rh4(MeCN)16][BF4]6"
A sample of 2 (60 mg, 0.062 mmol) was dissolved in a .2 M solution of
TBABF4 in MeCN ( ~ 10 mL) and placed in a four compartment

electrochemical cell with coarse frits separating each compartment. After a

 

163

clean blank was established, the potential was set to ~0.50V and the sample
was electrolyzed for 45 minutes. The solution was filtered in air and pumped
to dryness. Solids from this reaction displayed a simple EPR signal, at g =
2.158

(9) Electrocrystallization of [Rh4(MeCN)16][BF4]6

A volume of MeCN (20 mL) was used to dissolve a sample of
[Rh2(MeCN)1ol[BF4l4 (103 mg, 5 X 103M) and 1.323 g TBABF4 (0.2 M
solution). This solution was placed in both sides of an electrochemical cell
designed for electrocrystallization (see Figure 25). The experiment was
carried out at a constant current of 10 )1A with two Pt wire electrodes. No
reaction was observed to occur initially, therefore the solution was removed
from the cell and 5.292 g of TBABF4 was added to bring the total
concentration up to 1 M. The solution volume was reduced to ca. 8 mL, and
then returned to the cell. Additional MeCN solvent was added until the
solution level was just above the level of the Pt wires (~ 2 mL). A small
quantity of [Rh2(MeCN)1ol4+ precipitated due to the extreme salt
concentrations, so the cell was warmed in an attempt to resolve this problem.
Crystals grew at the electrode in the form of tiny needles in several days.
Electrolysis continued for four more days until the current setting was
increased above 30 uA at which point all the solid dissolved to form one
chamber of dark green solution that did not diffuse through the frits. In a
separate attempt, the electrode with the attached crystals was removed to
view the crystals under the microscope. The very sensitive and hygroscopic
crystals quickly turned into an oily residue after exposure to air and the heat

of the microscope lamp.

164

Figure 25. Diagram of a three compartment electrocrystallization

cell

165

Pt wire /
Electrodes
/ \A )

septum
port

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

\

ground glass
frits

Figure 25

166

(10) Reaction of [RhCl(COD)]2 with TlPFg

A sample of [RhCl(COD)]2 (200 mg, 0.406 mmol) was reacted with
TlPFe (300 mg, 0.859 mmol, 2.12 equiv.)) in 5 mL of MeCN under mild
heating conditions. The orange solid immediately dissolved to form a golden
yellow solution which produced a large amount of pale yellow precipitate.
The solution was decanted and reduced to a residue after efforts to cause
precipitation by addition of CH2 C12 failed. Yield of the white TlCl was 170
mg, (0.709 mmol, 1.75 equiv.) 1H NMR of the residue from the yellow
solution supported the conclusion that both chlorides were removed and both
COD ligands were bound. (CD3CN), ppm: 5 = 4.47 (m, 2 H), 2.42 (m, 2H),
1.95 (s, free CH3CN). Further refluxing in MeCN/toluene solution for 1 week

did not alter the composition of the solid.

(1 1) Reaction of RhC13 with TlPFs

A quantity of RhC13 - 3H2O (100 mg, 0.380 mmol) and TlPFs (500 mg,
1.43 mmol, 3.76 mmol) was refluxed in 8 mL of MeCN for 24 h. The RhClg
was not in a soluble form, so it merely turned lighter as the particle size was
reduced by stirring. The solution was pumped to dryness, after washing the
residue with MeCN, a pale yellow solution ensued even though the solid did
not dissolve significantly.

When the same reaction was carried out by first dissolving the RhC13
in MeOH, reducing it to a residue under vacuum, and then adding the MeCN
and the TlPFs, a yellow solution and a white solid resulted. The yellow
solution was decanted from the AgCl and layered with 15 mL of diethyl ether
to obtain a bright yellow precipitate which was washed with diethyl ether
and dried in vacuo; yield, 233 mg, (2.56 equiv.) 1H NMR (CD3 CN, dilute),
ppm: 5 = 1.95 (free CH3CN). UV-visible (Amax = 382 nm, shoulder at 303 nm)

167

IR(CsI, Nujol), cm'1: 2347 (vs), 2317 (m), 1292 (m), 1128 (m), 1039 (m), 835
(vs), 559 (S), 470 (S), 364 (m), 270 (W).

(12) Reaction of RhClg with SbCl5

A sample of RhC13- 3H2O (206 mg, 0.078 mmol) was refluxed in 8 mL
of MeCN overnight. To this solution was added 812 mg SbC15- 1.14 MeCN
(0.235 mmol, 3.01 equiv.) dissolved in 5 mL of MeCN. The mixture was
refluxed for 4 days, after which the tan-orange solution was cooled to yield a
small crop of crystals and a small amount of brown residue was removed by
cannula. The resulting solution was reduced to a low volume to precipitate a
bright yellow solid. The solid dissolves in CH2Cl2, and turns brown in
contact with diethyl ether. The yellow product was redissolved in MeCN, and
toluene was layered under this solution to produce X-ray quality crystals. 1H
NMR (CD3CN), ppm: 5 = 2.62 (s, 12 H, coordinated CH3CN), 1.95 (s, 1 H, free
CH 3CN)

(13) Reaction of RhC13 with AgBF4

RhC13- 3H20 (101.4 mg, 0.385 mmol) and AgBF4 (247 mg, 1.27 mmol,
3.30 equiv.) were stirred at r. t. in 8 mL of MeCN for three weeks. The
initially insoluble reactants eventually dissolved to yield a bright yellow
solution over a dingy white solid. The white solid was carefully removed by
cannula. Addition of 15 mL of diethyl ether and chilling the solution to -40
°C led to precipitation of more white solid and orange crystals. UV-visible
(MeCN): Amax = 372 nm, shoulder at 304 nm. IR(CsI, Nujol), cm '1: 2349 (m),
2314 (m), 2284 (In), 1716 (w), 1020 (8, br), 375 (w).

168

(14) Reaction of RhClg with AgTFMS

A sample of RhC13 hydrate (200 mg, 0.760 mmol) was stirred in 5 mL
of MeOH until it dissolved, at which point the solution was pumped to a
residue. The residue was redissolved in 8 mL of MeCN and transferred
under N2 into a vessel containing 585 mg of AgTFMS (2.277 mmol, 3.0
equiv.) This solution was refluxed for five hours, after which the yellow
solution was decanted from a white solid (AgCl) and chilled at -20 °C to
induce precipitation. The product was not extremely soluble in MeCN as
evidenced by the layer of yellow crystals that were admixed with the AgCl
after the solution had been decanted. MeCN was added to the solids and the
solution was warmed gently to dissolve the product. Upon cooling, a fine
yellow powder precipitated. Both the original reaction solution and the
MeCN washings of the white product produced yellow solids that were
invariably contaminated by AgCl which we found could not be completely
separated from the desired product. When the combined products were
isolated in the solid state, decomposition occurred within several days upon
exposure to light, regardless of aerobic or anaerobic storage, evidenced by a
dark gray color of the original yellow solid. This implies that the light
sensitive Ag+ was present in significant quantities. UV-visible (Am = 412
nm) 1H NMR (CD3 CN): 5 = 2.62 (8, low intensity, coordinated CH3CN), 1.95
ppm (s, free CH3CN). IR(CsI, Nujol), cm'1: 2351 (s), 2323 (m), 1280 (s), 1224
(s), 1154 (s), 1034 (s), 752 (w), 639 (s), 569 (w), 520 (w), 463 (w), 362 (s).

(15) Synthesis of [RhCl2(MeCN)4][BF4l
An amount of RhCl3 ~3H2O (203 mg, 0.771 mmol) was dissolved at r. t.
in 5 mL of freshly distilled MeOH. After the solid had dissolved (~ 5 min.)

the solution was reduced to a residue under vacuum. The sticky solid was

169

redissolved in 5 mL of MeCN, and the resulting solution was transferred
under N2 into 0.7 mL of HBF4/diethyl ether solution and refluxed without
stirring for 1 day. After this, some product had already precipitated. When
the solution cooled, it was layered with 1 mL of hexanes and 10 mL of diethyl
ether to induce additional precipitation. The solid was recrystallized from
large amounts of MeCN; combined yield, 235 mg (72%). Anal. Calcd for
C3H12BC12F4N4Rh: H, 2.84; C, 22.62; N, 13.2. Found: H, 2.77; C, 22.93; N,
12.82. 1H NMR (CD3CN) ppm: 5 = 2.62 (s, coordinated CH 3CN), 1.95 (s, free
CH3CN). IR(CsI, Nujol), cm'1: 2355 (vs), 2322 (m), 1101 (vs), 1064 (vs), 1026
(vs), 960 (m), 517 (w), 469 (m), 451 (w), 360 (s), 270 (w). UV-visible (MeCN,
nm (e in M‘1 cm’1), 414 (130), 235 (32,000), shoulder at 300 11m.

(16) Synthesis of [RhC12(MeCN)4l[TFMSl

An amount of RhC13-3H2O (208 mg, 0.790 mmol) was stirred in 5 mL of
MeOH for 5 min. until the solid had dissolved to form a red solution. This
was reduced to a residue under vacuum and redissolved in 5 mL of MeCN,
after which HTFMS (0.5 mL) was added anaerobically, and the reaction was
refluxed for 4 h. Cooling the solution resulted in the deposition of a yellow
solid on the bottom of the reaction vessel. The volume was reduced slightly
and then 5 mL of 1:1 CH 2012 : diethyl ether was added to precipitate large
amounts of the yellow solid. The product was washed with toluene and
pumped to dryness; yield, 360 mg (94%). 1H NMR (CD3CN), ppm: 5 = 2.62 (s,
coordinated CH gCN), 1.95 (s, free CH3CN).

(17) Reaction of [Rh012 (MeCN)4][BF4] with [Na]+[mhp]'
Two solutions were prepared, one containing 50 mg of unrecrystallized

[RhC12 (MeCN )4]IBF4] (0.190 mmol) in 5 mL of MeCN, and one containing 13

170

mg of NaOMe (0.241 mmol) and 51 mg of Hhmp (6-methylhydroxypyridine,
0.467 mmol) in 5 mL of MeOH. The rhodium starting material was evidently
not pure as we observed some white solid that was insoluble in MeCN. The
solution was filtered before use, but the stoichiometry of the reaction was
altered by this impurity. The solution of [Na][mhp] was transferred
anaerobically to the filtered Rh solution which first turned a gold-yellow color
and then changed to a pale olive green in 24 h. Reduction of the volume
caused the precipitation of a pale gray solid and layering of diethyl ether (~ 8
mL) produced more solid. The solution was decanted and layered with
toluene to produce additional dirty white solid. No colored products were

isolated.

(18) Reaction of [Rh2(MeCN)1o][BF4]4 with CO in the Presence of
Light

A quantity of 2 ( 50 mg, 0.052 mmol) was dissolved in 15 mL of MeCN.
Carbon monoxide gas was bubbled through the solution while it was
irradiated with a broad band UV—visible lamp for 2 h. The solution turned
pale yellow within ca. 30 minutes;. An infrared spectrum taken after 10 min.
showed both the v(C EN) stretches of the starting material, and CO bands.
IR(CaF2, MeCN), cm'l: 2343 (m), 2316 (w), 2121 (w), 2062 (w). The final
solution after 2 h indicated the absence of the v(C IiEN) bands characteristic of
the starting material and the presence of two very intense CO bands.
IR(CaF2, MeCN), cm'1: 2121 (vs), 2062 (vs). A vacuum was applied to remove
the solvent, and as the volume was reduced, a dark blue solid began
depositing on the sides of the vessel from a pale yellow solution. This easily
redissolved in the pale solution upon agitation. The solution was reduced to a

blue residue without a trace of yellow even though the solution did not

171

change color or darkened until dryness. The blue solid redissolves in MeCN
to reform the yellow solution and vacuum application allows for cycling
through the transformation repeatedly. After several cycles, the IR spectrum
begins to change, indicating that the original compound is converting to a
new more stable species. IR(CaF2, MeCN),cm'1: 2121 (m), 2062 (m), 2035
(vs). The reaction is extremely sluggish under ambient light conditions, but
does eventually proceed to give a pale yellow solution at low concentration
(e.g. 0.01 M is too concentrated). For reactions performed in room light,
conversion to the pale yellow color occurred in ~ 2 h rather than in 30 min.
The blue solid {IR(KBr, Nujol, very weak), cm‘1: 2330 (vw), 2302 (vw), 2048
(W), 1062 (s), 1034 (s)} is insoluble in THF and MeOH, but is soluble in
acetone to give a solution IR with one band in the CO region (CaF2, MeCN),
cm'1: 2032 (m). 1H NMR of the blue solid (anaerobic, CD3CN) showed only
free CH3CN at 5 = 1.95 ppm within 10 min. of sample preparation and data

collection.

(19) Aerobic reaction of [Rh 2(MeCN)1()][BF4]4 with CO and light.
A solution of 2 that had been exposed to 02 was placed in an uncapped
test tube and purged with CO in room light for 5 h. Infrared data indicated

that the reaction was occurring in the same manner as the anaerobic

photolysis reactions. IR(CaF2, MeCN), cm'1: 2121 (s), 2063 (s). The solution

was allowed to stand for 9 days during which time the spectral properties

changed. IR(CaF2, MeCN), cm'1: 2342 (s), 2121 (vs), 2063 (vs).

(20) Reaction of [Rh 2(MeCNl10][BF4]4 with CO in the dark.
A quantity of 2 (20 mg, 0.021 mmol) was dissolved in 10 mL of MeCN
and covered with foil for 2 h, after which CO was bubbled through the

172

solution in the continued absence of light for 4 h. IR data indicated that only

a small amount of any CO products had formed. (CaF2, MeCN ), cm'l: 2121
(vw), 2063 (vw).

(21) Thermal Reaction of [Rh2(MeCN)101[BF 414 with CO in the dark.
An amount of 2 (20 mg, 0.021 mmol) was added to 10 mL of MeCN and
the solution was covered with foil to keep out light for 2 h. The solution was
then refluxed for 2 h with CO purging in the absence of light. A yellow
solution was produced by addition of MeCN to the resulting residue formed
by solvent evaporation. Infrared data indicated that the previously observed
CO product was formed in moderate amounts. (CaF2, MeCN), cm'1: 2121 (m),

2063 (m).

(22) Reaction of [Rh 2(MeCN)10][BF4]4 with 10 equivalents of i-PrNC

A quantity of 2 (52 mg, 0.054 mmol) was dissolved in 5 mL of MeCN to
which was added 49 uL of isopropylisocyanide (0.540 mmol, 10.0 equiv.)
whereupon the solution immediately turned bright yellow. A small amount of
yellow solid precipitated within ca. 5 min. The solution was decanted andthe
solid was dried in vacuo. ,IR of the yellow solid, (CsI, N ujol), cm '1: 2344(m),
2217 (m), 1062 (s, br). After standing for ca. 1 day, the solvent from the
original solution was removed under vacuum to give a residue that appeared
to be a mixture of orange and yellow solids; addition of MeCN to this residue
produced an orange solution. IR(CaF2, MeCN), cm'l: 2343(m), 2317 (m), 2276
(vw), 2220 (w), 2175 (w). Hexanes (1 mL) and diethyl ether (~ 15 mL) were
layered on the solution to produce a red microcrystalline solid tinged with a

blue oil and a pale yellow solution. The solution was decanted and the solid

was washed with THF and CH 2Cl2, and finally MeOH to remove the blue

173

component. Finally the residue was washed with diethyl ether (2 x 5 mL)
and dried under vacuum. Addition of MeCN to the microcrystals gave a dark
red solution. IR(CaF2, MeCN), cm'1: 2221 (w), 2193 (m), 2155 (m), 1954 (m),
1713 (m), 1276 (m), 1154 (vs, hr). The red solution was layered over toluene
to give a yellow solution and a small quantity of microcrystalline orange solid.
The solution was decanted and subjected to a dynamic vacuum. IR of the
orange solid, (CsI, Nujol), cm '1: 2359 (m), 2342 (m), 2315 (m), 2286 (w), 2269
(m), 2258 (m), 1060 (s, br). As the solution volume was reduced, rings of a
very dark solid reminiscent of the CO reactions filmed out on the sides of the
flask until only a green-black solid remained. IR(CsI, Nujol), cm‘1: 2210 (w, .
br), 2144 (w), 1062 (s), 1030 (s). Redissolving this dark material in
acetonitrile exhibited the IR properties (CaF2, MeCN), cm'1: 2192 (m), 2157
(s), 1273 (m), 1103 (w, br).

(23) Reaction of [Rh2(MeCN)1o][BF4]4 with 10 equivalents of n-BuNC

A sample of 2 (27 mg, 0.028 mmol) was dissolved in 3 mL of MeCN. To
this solution was added 29 uL of n-butylisocyanide. The mixture was
agitated to produce a yellow solution which was then layered with hexanes
and diethyl ether (1 mL and ~ 8 mL respectively) to induce precipitation of a
yellow solid. The solution was decanted and the solid was washed with
diethyl ether (2 x 5 mL) and dried under a N2 purge. IR(CsI, Nujol), cm '1:
2337 (m), 2320 (shoulder), 2307 (shoulder), 2226 (m), 1061 (3, br). The
decanted solution was reduced to a pink residue under vacuum. Addition of
MeCN to this pink solid yielded a yellow solution which could once again be
pumped to a bright pink residue. Layering with CH2 Cl2 followed by diethyl
ether (10 mL of each) did not produce a crystalline product.

174

C. Experimental, Crystallography
[RhC12(NCCH3)4l[BF4]

Data Collection. Pertinent crystal data are summarized in Table 15.
Large single crystals were grown by slow diffusion of toluene into a solution
of the compound in acetonitrile. A hexagonal yellow crystal of dimensions
0.75 x 0.41 x 0.31 mm was sealed in a quartz capillary and examined on a
Rigaku AFC6S diffractometer equipped with graphite monochromated Mo Ka
(la = 0.71073A) radiation. A random peak search indicated that the crystal
diffracted well. The cell was indexed on 10 preliminary reflections which
indicated a body-centered orthorhombic crystal system. An automatic Laue
check verified this choice. An accurate high-angle cell used for data reduction
was determined by 24 reflections in the range 38<20<40°. Three intensity
standards were gathered every 150 reflections and displayed no decay during
the course of data collection.

Structure Solution and Refinement. The data were collected in a non-
standard setting, and rotated into a standard setting using the matrix
supplied by the PROCESS output in TEXSAN. The automatic TEXSAN
software chose the acentric space group Iba2, but refinement in this space.
group revealed a large amount of correlation. Therefore the structure was re-
solved in the centric space group Ibam. The position of the Rh atom was
determined by application of SHELXS-86, 1 and the rest of the atoms were
located by initial alternating DIRDIF2 refinement and Fourier maps until
most of the structure was established, after which the least squares
refinement procedure was used. All non-hydrogen atoms were refined
anisotropically. Final refinement gave residuals of R = 0.032, and Rw = 0.055
with the quality-of-fit indicator of 2.05 In the last cycle, 50 parameters were
refined with 822 unique data and Fobs > 3oFobs of 659. The shift/esd of the

175

Table 15. Crystal Data for [RhC12(MeCN)4][BF4]

Formula RhCl2N4C8H128F4
Formula weight 424.83

Space group lbam

a, A 6.239(1 )

b, A 1 2.147(2)

c, A 20.61 1 (5)

(1, deg 90

)3. deg 90

Y. deg 90

v, A3 1 562( 1 )

Z 4

dcalc, g/cm3 1.806

)1 (Mo Ka), cm'l 14.55

Data collection instrument Rigaku AFC6$
Radiation (monochromated in M0 K0100: = 0.71073A); graphite
incident beam) monochromated
Temperature, °C 1 23 i 2°

Scan method to - 20

Trans. Factors, max., min. 0.85, 1.00

R3 0.032

wa 0.055

 

aR = 2 I IFOT- lair/3:75;) --------------------------------------

bRW = [ZwlFo l- ch l)2/£wlFol2]l/2; w = 1/oz( lFol)

176

final cycle was 0.02, and the highest remaining peak in the final difference

Fourier map was 0.54 e'/A3.

D. Results and Discussion

The proposed mechanism of the reversible photochemistry is presented
in Figure 26. The solvent ligands have been left out for clarity, but each
species is assumed to be fully solvated. The first step is homolytic bond
cleavage in the presence of light to give two Rh(II) - radicals which have a
lifetime of about 50 s according to in situ transient EPR experiments.3 The
subsequent chemistry involves a series of redox reactions rather than radical
reactions. The Rh( II )- species is expected to become solvated by a MeCN
ligand in the vacant coordination site thus inhibiting recombination. The
reducing Rh(II) - species is capable of reducing an intact dinuclear (II,II)
species to form Rh(III) and a reduced mixed-valence dinuclear Rh(I,II)-. This
reduction by a photogenerated intermediate has been noted previously for
Ru 3(CO)12 and CpMo( CO)3C1 (Cp = cyclopentadienyl).4 Based on a
combination of observation and procedure, we postulate that two of these
mixed-valence species associate to form a mixed valence tetramer,
Rh4(I,II,II,I) This is driven by the metal-metal bond formation between the
d7 centers of an unstable Rh(I)Rh(II) species. Precedence for this linear
species is found in previous studies of dirhodium(I,II) isocyanides.5 In this
work, the d8 and d7 centers were bridged so that the Rh(I) pieces could not
dissociate after the metal-metal bond had‘ formed. In the present system,
unless the mixed-valence tetramer is isolated by precipitation, it quickly
dissociates to form two Rh( I) fragments and the original Rh2 II,II) species.
Other reactions to reform the starting material include conproportionation of

the Rh(I) and Rh(III), and radical recombination of Rh(II)-.

177

Figure 26. Proposed mechanism for the reversible photochemistry

of [Rh2 (MeCN )1ollBF 414

 

178

Initiation:

RhII—Rhll —-——> ZRhII-

Oxidation-Reduction:
Rhl]. + RhII_RhII ——-> RhIII + RhL—RhII
2Rh1—RhII ———>Rh1—RhH—RhH—RhI
RhI—RhII_RhII_—Rhl ——> RhII_RhII +2RhI

Termination:
RhI + RhIII ———> RhII—RhII
2 RhII - ———> RhH—RhII

Figure 26

179

Much of this work was elucidated using sophisticated spectroscopic
techniques and fast kinetic measurements at Los Alamos and the University
of Texas at Austin. Our goal was to approach the same chemistry from the
synthetic point of view in an attempt to reproduce these results under
thermal conditions and to prepare the proposed intermediates to lend support

to the postulated mechanism.

Electronic Spectroscopy of [Rh2(MeCN)1()][BF4 14

It became apparent very early in this work that strict anaerobic
conditions were required to avoid side reactions with 02. Electronic spectra
of samples prepared in air display not only the two authentic features (nm (c,
M‘1 cm'1)) at 468 (570) and 277 (22,000) for [Rh2(MeCN) 1014+, but also an
additional feature at Amax = 365 nm (1200). Samples prepared anaerobically
in the dark display only the first two features. The conversion from the
original spectrum to the aerobic spectrum was monitored by exposing a pure
anaerobic sample to air and light and then recording the spectrum.

We also performed the irradiation experiment anaerobically. Figure 27
shows the visible region of a sample prepared in this manner. The solution
was stored in the dark sample chamber and spectra were recorded every 5
min. The extra feature at Am = 414 nm was seen to disappear over time as
the photochemical reaction pathway is shut down and the intermediates
convert back to the starting material. These results are the same as those
found at Los Alamos. Under their irradiation conditions, the 414 band was
significantly more intense to the point that it overwhelmed the less intense
468 feature. Our results demonstrate the extreme photosensitivity of the
compound, considering we used only ambient room light as a source of

photons. The 414 band has been assigned to the Rh(I) species due to its

180

Figure 27. Electronic spectra of an anaerobically irradiated sample

of [Rh2(MeCN)1()][BF4]4 in MeCN solution recorded periodically

without additional exposure to light

181

Wavelength (in nm)

Figure 27

182

extremely high 5 values and the location of the transition, both of which are
typical of Rh(I) compounds.6

Although the photochemical reactivity had been established, we were
curious as to whether the homolytic Rh-Rh bond cleavage and subsequent
chemistry could be accessed thermally in the absence of light. Figure 28
depicts the spectra from a solution that had been refluxed in MeCN for
several hours in the absence of light. Spectra were recorded on the hot
solution and then monitored periodically as the solution cooled in the dark
sample chamber. The transition at 414 nm was apparent early in the study
and subsequently disappeared as the intermediates reacted to reform the

original material.

Synthesis of Intermediates:

" [Rh 41.11.11.1(MeCNncht "

Elucidation and further support for the proposed photochemical
mechanism would be most effectively achieved by using synthetic methods to
access proposed intermediates. Individual preparation of the three most
stable intermediates, Rh(I), Rh(III), and the proposed mixed-valence
tetramer, would add convincing evidence for the proposed mechanism.

As mentioned in Chapter III of this dissertation, [Rh2(MeCN)1o][BF4]4
possesses a single accessible irreversible reduction in the cyclic
voltammogram. We attempted to exploit this process for the synthesis of a
mixed-valence dinuclear species by bulk electrolysis. During this experiment,
we were surprised to find that instead of a color change of the solution, a
large amount of gray-green solid formed as a coating on the electrode and
eventually settled to the bottom leaving behind a nearly colorless solution. It

became apparent that the only difference between this experiment and

183

Figure 28. Electronic spectra displaying the thermal access to the

Rh-Rh bond cleavage reaction in [Rh2(MeCN)1()][BF4]4 in the

absence of light

184

 

 

wavelength in nm

Figure 28

185

normal synthetic conditions was the large amount of supporting electrolyte
that was present in the reaction. A variety of chemical reducing agents were
used with the Rh 211,11 species while maintaining ~ 0.1 M salt concentrations
Figure 29 summarizes the various methods used to produce dark colored
precipitate that we postulate is the mixed-valence tetramer intermediate due
to its similarity in color and luster to Gray and Mann's linear isocyanide
species.5 That the starting material is supported solely by solvent
complicates the isolation of the desired product, since there is a high
probability of incorporating a non-innocent reducing agent into the
coordination sphere. This was apparently the case with the tris(2,6-
dimethoxyphenyl) methyl radical as well as cobaltocene. Dave Morris at Los
Alamos reported similar incorporation problems with using ferrocene as an
internal standard in his electrochemistry experiments.

Two separate bulk electrolysis experiments were carried out to obtain
enough of product to study by various spectroscopic means. Anaerobic
isolation was virtually impossible with the electrochemical set-up in our
laboratories, therefore the resulting products were filtered in air. Anaerobic
dissolution of a sample of the brown solid in MeCN gave an orange solution
whose electronic spectrum, shown in Figure 30, contained the same two
components as the photochemical spectra, one absorption at Am = 410 nm
due to Rh(I), and a shoulder at about 465 nm due to the haII,II cation,
indicating that the brown solid contains a compound that dissociates into
these compounds upon redissolution. Unfortunately, aerobic work-up clouded
the issue of whether the authentic sample is paramagnetic or diamagnetic.
Two separate samples of the aerobically isolated bulk electrolysis product
gave EPR signals suggesting that they were paramagnetic. This was finally

attributed to oxygen interaction when a rigorously aerobic sample prepared

186

Figure 29. Synthetic methods used in preparation attempts of the

proposed mixed-valence tetramer, "[Rh4I.II,Il,I(MeCN)16]4+"

[Rh2(MeCN)10][BF4]4 was combined with one of the following reducing agents

at an elevated electrolyte concentration with the indicated results:

 

incorporation of the
> reducing agent into
the product

 

 

3 J
+ N aAce ——> gray/green solid
+ 1 e' ——' gray/green solid

+ N aEt 3BH --——> brown solid

NaAce = Sodium acenaphthylenide

Figure 29

 

188

Figure 30. Electronic Spectrum of the proposed mixed-valence

tetramer synthesized by bulk electrolysis redissolved in MeCN.

 

189

RhH-Rhll

300 400 500 600 nm

Figure 30

190

from the chemical reduction of [Rh2(MeCNllollBF414 with NaEt3BH
displayed no EPR signal. This particular product is actually a slightly
different form than the electrochemically generated compounds in that it is
insoluble in MeCN.

Crystallization of a sample of reduced [Rh 2(MeCN >10] 4+ was attempted
by two different methods. Layering a cobaltocene solution over a solution of
[Rh 2(MeCN ) 10][BF4]4 produced crystals of the starting material and dark
microcrystals of the product. The second technique employed
electrocrystallization using the specialized cell represented earlier in Figure
25. This method was moderately more successful than the first, but gave only
tiny needles that were not of x-ray quality. The latter procedure appears to

be the most promising and will be pursued in future studies.

"[Rh I(MeCN)4 1""

One of the most commonly employed Rh(I) starting materials is
[RhCl(COD)]2. We proposed to abstract the halides from the complex and
attempt to remove the cyclooctadiene ligands in the strongly coordinating
MeCN solvent. A 1H NMR of the product proved that the reaction did not
proceed as hoped; although the halides were removed, the compound was the
partially solvated cation, [Rh(MeCN)2(COD)]+. Future efforts should include
substitution of the diene for two poorly ligating alkenes such as l-hexene.
These groups are removed more easily, or may be hydrogenated off to afford a
better chance of accessing the Rh(I) solvate, [Rh(MeCN)4]+. This Rh(I)
species ligated solely by solvent is likely to be extremely reactive and may be
difficult to isolate, therefore extreme care and low temperatures must be used

in future experiments.

191

"[Rh III(MeCN )6 13*"

Of the three target intermediate compounds, the Rh(III) species,
[Rh(MeCNlefl3+ should be the most stable, due to the inert behavior of (16
species. Also, we rationalize that it would be easily prepared considering that
[Rh(H20)(3]3+ is already known. 7 The obvious starting material for the
preparation of a non-aqueous solvated Rh(III) species was the RhCl3 hydrate.
A variety of halide abstraction reagents were reacted with RhCl3 - 3H20,
including those that incorporate the halide into the counterion as well as
those that precipitate the halide or remove it in a gaseous form. The
reactions generally proceeded more easily if the RhC13 was first "digested" in
MeOH to break up the polymeric network thereby creating a more reactive
form of the complex.

The reaction with SbCl5 produced bright yellow solids MeCN solutions
of RhC13. A 1H NMR spectrum of the product displayed two resonances, one
at a = 2.62 ppm due to coordinated MeCN, and one at a = 1.95 ppm due to the
ligands that had exchanged with the deuterated solvent. Two different X-ray
data sets were collected on well-diffracting crystals, but we were unable to
solve these data sets. The solution by various methods was always the same.
Two large peaks which were likely the metals were bridged by a two atom
unit in a linear fashion. The metals refined as either Sb or Rh, so these
structures were abandoned. Antimony reagents are known to bind nitriles in
the case of SbF5,8 thus we decided to move to a slightly less potentially
interfering agent for removing the halides.

Reactions with Ag+ salts produced bright yellow solutions from which
yellow solids were isolated. It soon became apparent that these solids were
contaminated with substantial amounts of Ag+ as evidenced by the color

changes upon prolonged exposure to light, the reaction with the CsI windows

192

during infrared studies, and the deposition of Ag on the Pt electrodes during
cyclic voltammetry experiments. A representative example of the latter is
depicted in Figure 31. The silver salts, whether AgBF4 or AgTFMS proved to
have nearly identical solubility with the Rh-containing compound, so
repeated recrystallizations were not successful at purifying the product. 1 H
NMR spectral measurements, which are unaffected by the silver impurities,
showed coordinated MeCN at 6 = 2.62 ppm and free MeCN at 6 = 1.95 ppm in
various ratios depending on the time lapse between sample preparation and
measurement. The infrared spectra taken on N ujol mulls on KBr windows
showed, in addition to the [BF4]' or [TFMS] ’ counterion, two stretches in the
v(C =EN) region, one very strong at 2351 cm'1 and one medium intensity
feature at 2323 cm'l. Our inability to isolate a clean product from these

reactions led to the use of acids as halide abstraction reagents.

Synthesis of [RhClg(MeCN)4l+

The reaction of HBF4 or HTFMS with RhCl3 - 3H20 results in dark
yellow solutions that produce a large amount of bright yellow solid upon
layering with hexanes and diethyl ether. A successful X-ray crystal structure
revealed that only one chloride had been removed from the starting material
to give the compound, [RhClz(MeCN)4][BF4]. This cation was reported
previously, but had not been characterized crystallographically.9 Extended
reaction times up to two weeks did not drive the reaction to a different
product. Apparently the substitutionally inert nature of this Rh( III)
compound inhibits the removal of the other two halides.

The [RhClg (MeCN)4]+ species is most likely the only product formed in
the silver reactions and the antimony reactions as suggested by comparison of

the spectra of the residues of these reactions with an authentic sample of

193

Figure 31. Cyclic voltammetry of a halide abstraction product with

RhCl3 containing significant Ag‘l' impurity, in Volts vs. Ag/AgCl

cm-

+1.0

 

 

194

Volts

Figure 31

195

[RhC12(MeCN)4][BF4]. 1H NMR spectral measurements on a pure sample
isolated from acidic preparation of this compound displayed the two
resonances at 5 = 2.62 and 6 = 1.95 ppm observed for the other products, and
the v(CEN) region of the infrared spectrum displayed the combination of the
very intense stretch at 2355 and the medium intensity mode at 2322 cm'l.
This lower intensity mode was identified as the v(C EN) mode in the earlier
report.9 Since this synthesis does not produce silver residues, CsI windows
may be used to record the full IR spectrum; the v(Rh-Cl) stretch is visible in
the far—infrared as a strong feature at 360 cm ‘1. The quantitative electronic
spectrum displays a transition at Amax = 414 nm with a low a value typical of
Rh(III) complexes in the absence of ligands such as bpy10 as well as a higher
energy transition at 235 nm ( e = 32,000). The 414 nm band compares
favorably with 410 nm band reported earlier.9 Cyclic voltammetry
experiments in 0.1 M TBABF4 in MeCN displays two irreversible reductions
at EN = -.23V and Ep,c = -.54V referenced to Ag/AgCl. The irreversibility of
these processes is logical since a Rh(II) species would be most stable as a
dinuclear compound, and the structural rearrangement required would result
in a completely irreversible redox process.

Several likely methods have been discussed for future efforts to
prepare the Rh(III) solvate. One possibility is the dehydration of the
[Rh( H 20)6l3+ species. The difficulty with this is the isolation of this material
in a pure form with no chlorides, and achieving total dehydration of the
relatively inert metal center. A second possibility suggested by a collaborator
at Los Alamos, is the use of RhBr3 since the larger, more polarizable halides
should be more easily removed. It should be noted that the [RhBr2(MeCN)4]+
species is prepared by the reaction of RhBr3 with AgClO4, so forcing acidic

conditions may be required to removed the additional bromides.9

196

Molecular Structure of [RhC12(MeCN)4][BF4]

Selected bond distances and angles are presented in Tables 16 and 17
respectively. Atomic positional parameters are contained in the Appendix.
The ORTEP diagram in Figure 32 shows the molecular structure of
[RhC12(MeCN)4l[BF4]. Both the Rh and B atoms reside on crystallographic
special positions- Rh is on a 2/m site, and B is on a 222 site, rendering the
atoms one quarter occupied. The high symmetry of the octahedral rhodium
and tetrahedral boron greatly reduces the number of unique atoms in each
asymmetric unit. One F atom generates the other three in the anion, and the
same is true for the acetonitrile ligand in the cation. The single unique C1
atom lies on a 2-fold axis at half occupancy. Within the octahedral
coordination sphere of the cation, the chlorides occupy trans positions while
the four acetonitrile ligands lie in the square equatorial plane as was
proposed from the Raman spectroscopy in the original report.9 The [BF4]'
counterion is extremely well-determined with none of the common disorder
associated with it. The high symmetry of this crystal can be easily seen by
inspecting the ORTEP unit cell packing diagram, viewed down the a axis

depicted in Figure 33.

Further Reactions with [RhClz(MeCN)4][BF4]

Although the partially solvated mononuclear species was not the
desired product, this Rh(I) species is a versatile combination of stabilizing Cl'
ligands and more labile solvent. We propose that two of these units could be
tethered by a bridging ligand such as mhp (mhp = methylhydroxypyridine) or
dppm (dppm = l,2-bisdiphenylphosphinomethane), and the two metals could
then each be reduced by one electron to form a d7-d7 system with a M-M

bond. This type of reaction should prove quite interesting in the future and

Table 16. Selected Bond Distances in A for [RhC12(MeCN)4][BF4]

atom
Rhl
Rhl
Rhl
Rhl
Rhl
Rhl

atom
C11
C11'
N1
N1'

N1!’

N1!!!

distance
2.331(1)
2.331(1)
1.986(3)
1.986(3)
1.986(3)
1.986(3)

197

atom
N1
C1
Bl
Bl
Bl

Bl

atom
C1
C2
F2
F2'
F2”

F2",

distance
1.125(5)
1.463(6)
1.339(4)
1.339(4)
1.339(4)
1.339(4)

198

Table 17. Selected Bond Angles in degrees for [RhClz(MeCN)4l[BF4l

atom atom atom angle
C11 Rhl Cll' 180.00
C11 Rhl N1 90.56(8)
C11 Rhl Nl' 90.56(8)
C11 Rhl N1" 89.44(8)
C11 Rhl N1"' 89.44(8)
Cll' Rhl N1 89.44(8)
Cll' Rhl Nl' 89.44(8)
Cll' Rhl N1" 90.56(8)
Cll' Rhl N1"' 90.S6(8)
N1 Rhl Nl' 88.3(2)
N1 Rhl N1" 91.7(2)
N1 Rhl. N1"' 180.00
Nl' Rhl N1" 180.00
Nl' Rhl N1"' 91.7(2)
Nl" Rhl Nl"’ 88.3(2)
Rhl N1 C1 176.2(3)
N1, Cl C2 177.9(4)
F2 Bl F2' 114.0(4)
F2 Bl F2" 106.8(4)
F2 Bl F2"' 107.7(4)
F2' Bl F2" 107.7(4)
F2' 81 F2"' 106.8(4)

r2" 31 92"' 114.0(4)

199

Figure 32. ORTEP diagram of the cationic component of
[RhC12(MeCN)4][BF4]

200

 

@190)

Cl(l)

@ C(l)

Figure 32

201

Figure 33. ORTEP Unit Cell Packing Diagram of
[RhC12(MeCN)4][BF4l. viewed down the A axis

202

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 33

203

may present a different way of accessing dinuclear species for the late

transition metals.

Photochemical Reactions of [Rh2(MeCN)lo][BF4]4 with CO

Since the preparation of the proposed photochemical intermediates had
not proceeded unequivocally in any case, we set out to trap these
intermediates in a derivatized form. A photolyzed solution of
[Rh2(MeCN) 10][BF4]4 reacts with CO (g) at r. t. to give a pale yellow solution.
A solution IR of this species indicates that the product that has been formed
is the previously reported but very poorly characterized cis-
[Rh(CO)2(MeCN)2l+.11 Reducing this solution to a residue under vacuum
deposits a blue solid as the square planar Rh(I) species associate.11 The
transformation from the yellow solution species to the blue solid may be
effected several times, but with each cycle of pumping, a new stretch in the
IR at 2035 cm ‘1 grows in intensity. The identity of this new species has not
yet been elucidated. Such extremely high energy stretches for v(CO) (2121
and 2062 cm'1 for cis-[Rh(CO)2(MeCN)2]+) are uncommon, but not unknown.
Extremes of 2204 cm '1 have been reported for Ag(CO)B(OTeF5)4 in which the
CO is said to be acting entirely as a Lewis base with no n-acid character. 12

The importance of this reaction is that it requires access to the
photochemical pathway in order to occur as evidenced by the lack of
appreciable reactivity observed for the solutions kept in the dark. Ambient
light results in much slower reactions than the strongly photolyzing
conditions with a broad band UV-visible source. The same results may be
achieved thermally by purging a refluxing MeCN solution of
[Rh2(MeCN)10][BF4]4 in the dark, but photolysis remains the best method of

preparing the yellow solution. The presence of the Rh( I) species in the

204

photolyzed solutions was verified by trapping out the cation with CO, and the
Rh(III) species is necessarily present for charge balance. Successful
separation of the two species and characterization by X-ray analysis is

important as these products strongly support the photochemical mechanism.

Reactions with isocyanides

Reactions with isocyanides which are isoelectronic with CO were
carried out in hopes of discovering similar chemistry. Indeed 10 equivalents
of either isopropylisocyanide or n-butylisocyanide react with solutions of
[Rh2(MeCN)1ol[BF4l4 to instantaneously produce pale yellow solutions as in
the CO reactions. The isocyanides modify the nature of the products
sufficiently such that MeCN ligands were observed in the solution IR spectra
in contrast to the CO spectra in which these stretches were conspicuously
absent, even though crystallography has established their presence. The
nitrile and isocyanide stretches fall close together, so specific formulations
are not easily derived from these spectra other than concluding that the
compounds contain both ligand types. Evaporation of the isocyanide reaction
solutions does not produce the dark colors of the CO solutions, instead, red (i-
PrN C ) or pink (n-BuN C) microcrystalline solids were obtained. The i-PrNC
solutions finally produced dark solids similar to the solid CO products, but it
is apparent that the chemistry of these ligands is not identical to the CO
reactions. Firstly, light is not required to initiate the reaction, and secondly,
the [Rh(NCCH3)2(CNR)2]+ compounds are not known. The most common
isocyanide Rh(I) species are homoleptic compounds of the form, [Rh(CNR)4]+
which are easily prepared by addition of excess isocyanide to
[Rh(CO)2C12 ]2.13 Partial adducts may be prepared by stoichiometric

substitution of the carbonyl starting material, or by measured addition of a

205

new ligand to the homoleptic isocyanide material,13a,14 but the only
thermodynamically stable bis-isocyanide complexes are the dinuclear bridged
systems of Balch where the ligands are forced to adopt a trans disposition. 15
Square planar Rh(I) isocyanide compounds do interact in the solid state along
a metal-metal vector which may explain the final dark solid produced in the
current reactions.5a Unfortunately, the spectral data of any of the
aforementioned compounds does not resemble that of the present chemistry,
so identification of the products has not yet been established. Additional

studies to elucidate these compounds are in order.

E. Summary

The photochemistry of [Rh2(MeCN) 10][BF 414 was seen to be accessible
under ambient light and heat conditions in our laboratories. Although the
synthesis of the proposed intermediates was not entirely successful thus far,
the results achieved will be invaluable in guiding future efforts in these
areas. The Rh(III) species is expected to be the most likely intermediate to be
isolated and fully characterized; crystallization of the mixed-valence tetramer
is likely to be much more difficult. In another approach to verifying the
presence of solvated Rh(I) and Rh(III) cations, we used CO trapping
experiments to selectively react with the Rh(I) intermediate which stabilizes
it but does not interfere with the Rh(III) complex. Reactions with isocyanides
have not been as promising in these derivative reactions. Both these and the

subsequent chemistry of the CO solutions will be pursued.

10.

11.

12.

206

LIST OF REFERENCES, CHAPTER IV

Sheldrick, G.M.; SHELX86, Program for the solution of crystal
structures, 1986. Univ. of Gottingen, Germany.

Beurskens, P.T. DIRDIF: Direct Methods for Difference Structures- An
Automatic Procedure for Phase Extension and Refinement of
Difference Structure Factors. Technical Report 1984/1 .
Crystallography Laboratory, Toernooiveld, 6525, Ed Nijmegen,
Netherlands.

Morris, D.E.; Dunbar, K.R.; Arrington, C.A.Jr.; Doorn, S.K.; Pence.
L.E.; Woodruff, W.H. submitted to J. Am. Chem. Soc. 1992.

(a) Stiegman, A.E.; Goldman, A.S.; Leslie, D.B.; Tyler, D.R. J. Chem.
Soc. Chem. Commun. 1984, 632. Stiegman, A.E.; Tyler, D.R. J. Am.
Chem. Soc. 1985, 107, 967.

(a) Mann, K.R.; DiPierro, M.J.; Gill, T.P. J. Am. Chem. Soc. 1980, 102,
3965. (b) Mann, K.R.; Lewis, N.S.; Miskowski, V.M.; Erwin, D.K.;
Hammond, G.S. Gray, H.B. J. Am. Chem. Soc. 1977, 99, 5525. (c)
Sigal, I.S.; Mann, K.R.; Gray, H.B. J. Am. Chem. Soc. 1980, 102, 7252.
(d) Miskowski, V.M.; Sigal, I.S.; Mann, K.R.; Gray, H.B.; Milder, S.J.;
Hammond, G.S.; Ryason, P.R. J. Am. Chem. Soc. 1979, 101, 4383.

(a) Mann, K.R.; Gordon II, J.G.; Gray, H.B. J. Am. Chem. Soc. 1975, 97,
3553. (b) Isci, H.; Mason, W.R.; Inorg. Chem. 175, 14, 913. (c) Mann,
K.R.; Lewis, N.S.; Williams, R.M.; Gray, H.B.; Gordon II, J.G. Inorg.
Chem. 1978, 1 7, 828.

Burgess, J. in Comprehensive Coord. Chem. 1987, 2, 295.

Tornieporth-Oetting, I.C.; Klapotke, T.M.; Cameron, T.S.; Valkonen, J .;
Rademacher, P.; Kowski, K. J. Chem. Soc. Dalton Trans. 1992, 537.

Gillard, R.D.; Heaton, B.T.; Shaw, H. Inorg. Chim. Acta 1973, 7, 102.

Cotton, F.A.; Wilkinson, G. "Advanced Inorganic Chemistry," Wiley:
New York, 1988.

(a) Ugo, R.; Bonati, F.; Fiore, M. Inorg. Chim. Acta. 1968, 2, 463. (b)
Epstein, R.A.; Geoffroy, G.L.; Keeney, M.E.; Mason, W.R. Inorg. Chem.
1979, 18, 478.

Hurlburt, P.K.; Anderson, O.P.; Strauss, S.H. J. Am. Chem. Soc. 1991,
113, 6277.

13.

14.

15.

207

(a) Dart, J.W.; Lloyd, M.K.; McCleverty, Mason, R. J. Chem. Soc.
Chem. Commun. 1971, 1197. (b) Dart, J.W.; Lloyd, M.K.; Mason, R.;
McCleverty, J .A. J. Chem. Soc. Dalton Trans. 1973, 2039. (c) Branson,
P.R.; Green, M. J. Chem. Soc. Dalton Trans. 1972, 1303.

Kaneshima, T.K.; Kawakami, K.; Tanaka, T. Inorg. Chem. 1974, 13,
2198.

Balch, A.L. J. Am. Chem. Soc. 1976,98, 8049.

CHAPTER V

STRATEGIES FOR THE SYNTHESIS OF OTHER SOLVATED

DINUCLEAR TRAN SITION-METAL COMPLEXES

208

209

A. Introduction

The success that was encountered in the preparation of the dirhodium
acetonitrile system spurred us to investigate the synthetic applicability of our
general methodology to other transition metals to establish practical methods
of preparing a wide number of dinuclear solvated starting materials.

Extrapolation to the dirhenium system was accomplished by other
group members who established three different routes to access the
decakisacetonitrile dirhenium system. The initial approach took advantage
of the first successful method of synthesizing [Rh2(MeCN)10][BF4]4 from
Rh2(OAc)4(MeOH)2. The dirhenium tetraacetate complex is extremely
insoluble, so the tetrabutyrate was used. Re2(02 CR)4 C12 compounds possess
axial chlorides which must be removed with AgBF4 prior to reaction of the
carboxylate with the triethyloxonium reagent. A reduction occurs during this
reaction to yield the triply bonded solvated species, [Re2(MeCN)1()]4+ in

moderate yields:1

MeCN
Rez(02CR)4C12 + 2AgBF4 -————> [Re2(02CR)4(MeCNl2llBF4]2+ 2 AgCl(s)
Equation 11

MeCN
[Re2(020R)4(MeCN)2l[BF4]2 + Et3OBF4 ——> [Re2(MeCN)1ol[BF4l4
Equation 12

The tetrapositive charge is common among the three known dinuclear
acetonitrile cations, Mo, Rh, and Re, in spite of the differing M-M bond
multiplicities. It may be that the dimetal units are unable to stabilize

charges of higher magnitude than 4+.

210

Protonation reactions also may be used to access the dinuclear
rhenium solvate. The first method involved halide abstraction from the

common starting material, [RezClglz , with HBF4 as is shown below.2

MeCN
[R82Clgl2' + HBF4 ——> [Re2(MeCN)10][BF4l4 + 8 HO] (g)
Equation 13

This chemistry also involves in situ reduction from the Re(III) starting
material; some advantages of this reaction are that it is a higher yield one-pot
synthesis and that. it requires one less step as it begins with [RezClglz' and
not with Re2(02 CR)4C12 which itself must be synthesized from [Re2Clg]2' .
Without question, however, the best method for preparing [Re2(MeCN) 1014+ is
directly, in the correct oxidation state, by protonation of all the ligands on

mixed halide-phosphine compounds of the form, Re2C14(PR3)4.2

MeCN
RegCl4(PR3)4 + HBF4 ——> [Re2(MeCN)1o][BF4]4 + 4 HCl (g) + 4 HPR3
Equation 14

No single synthetic'method is likely to be appropriate for preparing
solvated dinuclear cations for every transition element, and as demonstrated
in both the Rh and Re chemistry, and there is often more than one successful
route to the final target complex. Investigation of other metal systems
including divalent Cr, Ir, Ru, Mo, and Os has seen the application of a variety
of synthetic methods as the identity of convenient starting materials changes.

The results of these studies are presented herein.

211

B. Experimental, Synthesis
( 1) Reaction of Cr2(OAc)4 - H20 with Et3OBF4

Rigorously anaerobic and moisture-free conditions were maintained for
this extremely air-sensitive reaction. An amount of anhydrous Cr2(OAc)4
(100 mg, 0.294 mmol) obtained from Aldrich was stirred at room temperature
with 5 mL of Et3 OBF4 (1 M in CH2 C12) and 10 mL of freshly distilled MeCN
for a week. The rose colored starting material was insoluble in MeCN,
nevertheless, it reacted and turned blue upon addition of the triethyloxonium
solution, and became dark purple over the course of several days. The
solution was then heated for 3 days after which it was cooled which produced
peach colored crystals at the neck of the flask and a red-purple solution.
Layering diethyl ether (~ 15 mL) on the decanted solution produced more
peach crystals admixed with purple needles. Separation of these two species
to get a single pure product was achieved by addition of MeCN which
selectively dissolved the purple product but left behind the orange crystals.
The solution was decanted and the orange solid pumped to dryness. No pure
sample of the purple species was isolated. IR of the peach crystals, (KBr,
Nujol), cm'1: 2330 (vs), 2301 (vs), 1023 (vs, br), 959 (vs), 470 (m), 445 (vs). 1H
N1V[R(CD3CN,): 6 = 1.95 ppm (singlet, free CH3CN). Brief exposure to air
turns the orange solutions and solids green, presumably due to oxidation

reactions.

(2) Reaction of IrClg with TlPFs

Iridium trichloride trihydrate was obtained from Aldrich. A quantity
of IrCl3-3H20 (110 mg, 0.312 mmol) was reacted with TlPFe (380 mg, 1.09
mmol, 3.5 equiv.) at r.t. in 10 mL of MeCN. Ambient conditions did not

promote reaction, therefore the reaction was heated to reflux for 3 days. The

212

resulting yellow solution was separated from a large amount of white solid by
filtration. The solution was then layered with 10 mL of diethyl ether which

caused an oily solid to form.

(3) Reaction of IrCl3 with SbCl5

The acetonitrile complexed form of antimony pentachloride was
prepared according to the literature.3 An amount of IrCl3 - 3H20 (200 mg,
0.567 mmol) was refluxed in 5 mL of MeOH overnight and pumped to a
residue. To this was added 5 mL of MeCN and 559 mg of SbCl5 - 1.14 MeCN
(1.616 mmol, 2.85 equiv.) and the combined reagents were stirred at reflux
temperatures for 4 days. This produced a brown solution with a small
amount of brown precipitate. The solution was filtered anaerobically.
Addition of 10 mL of CH2 C12 did not form a precipitate, and 10 mL of diethyl
ether oiled the product. After these nonvolatile solvents were removed by
evaporation, the volume of MeCN was increased by 3 mL and CH2 C12 (~ 12
mL) was layered underneath to precipitate a brown solid and a deep orange
solution. Decanting the solution and reducing the volume produced more
brown solid. This reaction did not appear to be particularly promising for the

formation of Ir(III) species which are typically pale yellow compounds.4

(4) Reaction of IrClg with AgBF4

In a typical reaction, a quantity of IrClg - 3H20 (200 mg, 0.567 mmol)
was refluxed in MeOH for 12 h and reduced to a residue under vacuum. The
solid was dissolved in 5 mL of MeCN and decanted into 331 mg of AgBF4
(1.70 mmol, 3.0 equiv.) This mixture was refluxed for 24 h, which produced a
yellow solution and a tan solid. Several isolation procedures were attempted,

each involving the precipitation of additional AgCl by either reduction of the

213

volume or by addition of a precipitating solvent. Final isolation of the Ir-
containing product required pumping the solution to a residue. IR (CsI,
Nujol), cm'l: 2305 (m), 2280 (m), 1290 (m), 1070 (s, br), 520 (m), 385 (w). 1H
NMR (CD3CN), 5 = 2.84 (s, coordinated C_HgCN), 1.95 ppm (s, free CH3CN),
approximately 1:10 ratio. The presence of Ag-containing impurities was
obviated in the cyclic voltammetry by a catalytic reaction with the Pt

electrode at Ep,c = + 0.26V.

(5) Reaction of IrCl3 with AgTFMS

A sample of IrCl3 - 3H20 (309 mg, 0.876 mmol) was refluxed in 5 mL of
MeOH for two hours to effect dissolution. The resulting olive green solution
was reduced to a residue under vacuum and redissolved in 5 mL of MeCN.
This solution was then transferred into a vessel containing AgTFMS (.676
mg, 2.63 mmol) and refluxed for 1 week. (It is important that the Ag reagent
is not predissolved and added to the IrCl3 because silver salts will decompose
in light.) After heating was ceased, the solution was a yellow color and a pale
tan solid was present. The solution was decanted from the AgCl solids, and
the volume was reduced to about ~ one-half. Toluene was added (8 mL) and
the solution was chilled to ~200C. After precipitation, the bright yellow solid
was seen to be admixed with white AgTFMS. Separation was not achieved in
spite of repeated recrystallization. IR (KBr, Nujol), cm°1: 2330 (s), 2309 (m),
1414 (s), 1365 (m), 1273 (vs), 1224 (s), 1146 (vs), 1034 (vs), 639 (vs), 560 (In),
510 (m). UV-visible (Am 316 nm, shoulder at 280 nm). The far-infrared was
not recorded as the silver-containing impurities react with the CsI cells. 1H
NMR (CD3CN, anaerobic): 6 = 2.82 ppm (s, coordinated QHgCN), 1.95 ppm (s,
free CH3CN), approximately 10:1 ratio.

214

(6) Reaction of IrC13 with HBF4

To an amount of IrC13 - 3H20 (213 mg, 0.604 mmol) was added 10 mL
of MeOH which was refluxed until the solid had completely dissolved. This
solution was reduced to a residue under vacuum to which 10 mL of MeCN
and 0.8 mL of HBF4 were added. This solution was refluxed for 4 days after
which the reaction was cooled, reduced to ca. 2 mL, and filtered to remove a
minor amount of gray precipitate. To the filtrate was added 5 mL of CH2C12,
and the solution was chilled to -40 0C. Further attempts to precipitate a solid
with toluene, ether, CH 2C12, or combinations thereof led only to oils or sticky
products. IR (CsI, Nujol), cm'1: 2350 (W), 2312 (w), 1070 (m, br), 1040 (m),
330 (m).

(7 ) Reaction of RuClg with TlPFg

Ruthenium trichloride hydrate was obtained through a platinum metal
loan from Johnson-Matthey. The amount of hydration was calculated to be
2.17 waters per ruthenium based on the metal analysis listed on the bottle.
The same starting material was used in subsequent reactions unless
otherwise indicated.

A quantity of RuCl3 hydrate (100 mg, 0.406 mmol) was refluxed with
TlPFe (523 mg, 1.50 mmol, 3.69 equiv.) in 10 mL of MeCN for 5 days. The
original dark brown solution and suspension was converted to an intense
blue-purple color before finally becoming an intense blue color. (Am = 578
nm, shoulder at 318 nm) TlCl was observed as a precipitate at the bottom of
the flask, and additional amounts came out of the solution as the volume was
reduced. Diethyl ether (8 mL) was layered on the MeCN solution in an
attempt to isolate a blue solid, but exposure to air led to decomposition as

evinced by the solution changing to dark purple. This purple solution

215

produced both purple and white solids which were not possible to separate.
Separation attempts were carried out in air, and the purple solution

eventually decomposed to a peach solution.

(8) Reaction of RuC13 with AgBF4

Quantities of RuC13 hydrate (110 mg, 0.446 mmol) and AgBF 4 (259 mg,
1.33 mmol, 2.98 equiv.) were stirred in 8 mL of MeCN. The initial solution
was dark brown in color, but eventually turned olive green within 5 h. The
final dark blue solution was filtered and chilled after the addition of 8 mL of
diethyl ether. A large amount of dark blue solid precipitated after several
additions of diethyl ether to give a dichroic purple/red solution. After the
mother liquor was decanted from the solid, the product was washed with
diethyl ether (3 x 5 mL) and dried in vacuo. A 1H NMR spectrum (CD3CN,
anaerobic): 6 = +2.49 ppm (s), displayed no sign of free CH3CN, and the
sample appeared to be diamagnetic. IR (CsI, N ujol), cm '1: 2334 (w), 2305 (w),
1282 (In), 1050 (vs, br), 520 (m), 260 (vw). UV-visible (MeCN) Amax at 579
nm, shoulders at 319 and 267 nm. The solid was free of Ag“ impurities as
judged by cyclic voltammetry; dilute solutions displayed no interaction with

the Pt electrode.

(9) Reaction of RuCl3 with HBF4

A sample of RuClg hydrate (113 mg, 0.458 mmol) 0.5 mL of
HBF4/diethyl ether complex, 5 mL of MeCN, and 20 mL of CH 2C12 were
stirred together and refluxed for 7 days. The resulting solution which was
red with a brown precipitate was filtered and extracted with hexanes to
remove grease; finally the volume of the solution was reduced. CH2 C12 (3

mL) was added to encourage precipitation, but this turned out to be

216

immiscible with the solution. MeCN was then added until the layers mixed.

No solid was obtained from these efforts.

(10) Reaction of RuC13 with HTFMS

An amount of RuCl3-hydrate (254 mg, 1.03 mmol) was stirred and
refluxed in 8 mL of MeCN with 1 mL of HTFMS for 4 days. A dark red
solution and a brown solid resulted. Chilling the solution to 0 0C yielded an
orange-brown solution with a finely divided black suspension. The solution
was filtered, the volume reduced to about one-half, and a 1:1 mixture of
CH2C12 : diethyl ether (~ 10 mL) was added with chilling to -40 0C to
encourage precipitation. After this treatment, the solution turned orange and
a beige precipitate formed. No further work was done since the prior work-up

seemed to have changed the identity of the original product.

(11) Reactions of anhydrous RuCl3
RuC13 anhydrous (160 mg, 0.771 mmol) was refluxed in 5 mL of MeOH

for two days. The solid was totally insoluble in this medium.

(12) Reaction of Ru 2(OAc)2(CO)4(MeCN)2 with Et30BF4

The ruthenium starting material was prepared according to literature
methods.5 A sample of Ruz(OAc)2(CO)4(MeCN)2 (108 mg, 0.210 mmol) was
refluxed with 5 mL of a 1 M solution of EthBF4 in CH2012 and 10 mL of
MeCN for 4 days. The original bright yellow solution turned a bright orange
color. The volume of the solution was reduced and diethyl ether (~ 8 mL) was
added, but this did not produce a solid. Further efforts to isolate a solid by
addition of toluene or CH2C12 yielded intractable oils.

217

(13) Reaction of Ru 2(OAc)4Cl with HBF4

Ruthenium tetraacetate monochloride was prepared according to the
literature method.6 A quantity of Ru2( OAc)4 Cl (7 2 mg, 0.152 mmol) was
refluxed with 1 mL of HBF4/diethyl ether complex in diethyl ether and 5 mL
of MeCN for 2 weeks. The resulting red solution was filtered to remove a
small amount of brown precipitate at the bottom, the solution volume was
reduced slightly and finally layered with hexanes and diethyl ether (1 mL
and 10 mL respectively). This work-up produced a small amount of colorless
crystals. Further attempts to isolate a colored product from the solution led

only to oils. (UV-visible of the red solution, Amen. = 367 nm).

(14) Reaction of Ru 2(OAc)4Cl with HTFMS

Ru2(OAc)4Cl (102 mg, 0.215 mmol) was refluxed with 5 mL of MeCN
and 0.5 mL of HTFMS for 24 h. The resulting red-orange solution was
reduced in volume by one-half and 3 mL of CH 2012 was added to induce
precipitation. Additional CH2C12 (2 mL) was added with no effect, but 3 mL of
diethyl ether precipitated a tan-orange solid. Additional amounts of diethyl
ether and subsequent chilling to -40 0C yielded a brown solid, an orange
solution, and small gold crystals likely to be the known Ru( II) cation,
[Ru(MeCN)5]2+, judging by the pale color. No further efforts were made to

isolate a pure product from this complicated mixture.

(15) Reaction of [Ru 2(OAc)4(THF)2l[BF4] with HBF4

The literature procedure was followed to prepare the axially solvated
diruthenium tetraacetate.7 [Ru 2(OAc)4(THF)2][BF4], (102 mg, 0.154 mmol),
0.5 mL of HBF4, and 5 mL of MeCN were refluxed together for 2 days to yield
a translucent red solution. Addition of 5 mL of CH 2C12 followed by 5 mL of

218

diethyl ether and chilling of the solution to -40 OC precipitated orange-brown
and white solids from an orange solution. As in Reaction ( 14 ), no solid could

be isolated.

(16) Thermal Reaction of [N H4]5[M02019] with HBF4

A quantity of [N H4]5[M02 C19] (205 mg, 0.034 mmol), prepared
according to the literature procedure,8 was refluxed with 0.5 mL of HBF4 and
5 mL of MeCN. The reaction turned green within ~ 10 min. and after 16 h of
reflux, the solution color was yellow-gold. Presumably the extended heating

period led to decomposition.

(17 ) Reaction of [N H 4]5[M02019] with HBF4 at Room Temperature

An amount of [NH4 ]5[M02C19] (144 mg, 0.240 mmol) was stirred at r.t.
with 0.5 mL of HBF4/diethyl ether complex and 5 mL of MeCN. After 24 h,
the solution had turned green-blue, which did not resemble the deep blue of
the target complex, [M02(MeCN) 1014+. Extended stirring for several weeks
still did not produce any further color change. Work-up with CH2 012 and
diethyl ether produced an extremely oily forest green solid that could not be

dried adequately for characterization purposes.

(18) Reaction of K4lM02Clg] with HBF4

A sample of K4[M02 C18] (100 mg, 0.158 mmol), synthesized by
literature methods,9 was reacted with 0.5 mL of HBF4 and 5 mL of MeCN
and stirred at r.t. for 2 days to yield a green solution over a large amount of
light blue solid. The green solution was decanted from the solid which was
washed with diethyl ether and pumped to dryness. A 1H NMR spectrum of
the solid in CD3 CN displayed too many resonances to be the pure solvated

219

complex. Addition of diethyl ether ( ~ 8 mL) to the green solution yielded a
dark blue-green solid that did not resemble the color of [M02(MeCN ) 1014+.

(19) Reaction of K4[M02Clg] with HTFMS in the presence of
Propionitrile

An amount of K4[M02 C18] (100 mg, 0.158 mmol) was combined with 5
mL of EtCN and 1.5 mL of HTFMS. The mixture was refluxed for 1.5 h to
yield a green solution. Addition of 20 mL of CH2 012 led to a green solution

and a pale colored suspension.

(20) Reaction of M02Cl4(Me2 8).; with HBF4
Tetrachlorotetrakis(dimethylsulfide)dimolybdenum( II) was prepared
according to the literature method. 10 An amount of M02C14(Me28)4 (.105 mg,
0.180 mmol) was refluxed with 10 mL of MeCN and 0.8 mL of HBF4/ diethyl
ether complex for 11 days. Prior to addition of HBF4, the initial solution was
blue, but within 24 h of reacting with HBF4 under reflux conditions, the
solution began to turn green. Prolonged reaction did not cause formation of a
blue solution. Hexanes and diethyl ether (1 mL and 10 mL respectively) were
added to precipitate a sticky solid that contained several different shades of
green. Attempts to grind a sample for an infrared spectrum led to a sticky

solid that was immiscible with N ujol.

(21) Reaction of K4[M02Cl 3] with NaBPh4
A sample of K4[M02013] (99 mg, 0.157 mmol) and 430 mg of NaBPh4
(1.26 mmol, 8.02 equiv.) was stirred at r.t. for 3 days. An intense blue

solution and a large amount of white precipitate ensued. The NaCl was

220

removed by filtration (184 mg, 20.0 equiv.), but the solution was accidentally

exposed to air and it subsequently decomposed.

(22) Reaction of 05013 with AgTFMS

A quantity of commercially available OsC13 (200 mg, 0.674 mmol) was
stirred at r.t. with AgTFMS (572 mg, 2.23 mmol, 3.31 equiv.) in 10 mL of
MeCN. Within 10 min., the solution turned brown, and within 24 h, the
solution was a very dark purple color with no precipitate present. Addition of
toluene and chilling to -40 0C precipitated a large amount of blue-gray solid.
The solution was filtered and reduced under vacuum to a sticky residue.

Recrystallization from MeCN/CH2C12 did not yield a dry solid.

(23) Reaction of [n-Bu4N] 2[OS2C13] with HBF4

Octachlorodiosmate was prepared according to the method reported by
Walton et al. 11 A sample of the green [n—BU4N]2[OS2C18] (91 mg, 0.079
mmol) was treated at r.t. with 0.5 mL of HBF4 and 5 mL of MeCN for 4 days
with no change in color. Chilling the solution to -40 0C produced a tan
precipitate and a green-gold solution. Addition of CH 2C12 and diethyl ether
(58 mL each) produced only immiscible oils.

(24) Reaction of [n-Bu 4N]2[052C13] with 8 equivalents of AgBF4

The solids, [n—Bu4N] 2[OszC13] (85 mg, 0.074 mmol) and AgBF4, (115
mg, 0.591 mmol, 7.99 equiv.) were stirred in 5 mL of MeCN for 6 days. A
brown precipitate formed in an opaque solution. The solution was filtered
through Celite several times to remove the precipitate, and then layered with
hexanes (1 mL) and diethyl ether (8 mL). A finely divided brown solid

precipitated, leaving behind a gold solution which was decanted and saved.

221

An IR spectrum of the first brown solid displayed a small amount of MeCN
and a small feature in the region for an v(Os-Cl). The solution was extracted
three times with hexanes (5 mL portions) to remove oil that contaminated the
solution, and then layered with toluene to again yield a brown precipitate. A
1H NMR spectrum of the brown solid displayed only a small amount of free

MeCN and a substantial amount of [ n-Bu4N]+.

(25) Reaction of [n-Bu4N]2[Os 2C13] with HTFMS in the presence of
Propionitrile

A sample of [n-Bu4N]2[Os:gClgl (132 mg, 0.115 mmol) was refluxed
with 5 mL of EtCN and 1.5 mL of HTFMS for 90 min. The resulting solution
was golden brown. Addition of 20 mL of CH 2C12 caused only a small amount
of brown oil to form at the bottom; addition of 10 mL of diethyl ether had no

effect.

(26) Reaction of 082(OAc)4Cl2 with AgTFMS and Me 3SiTFMS
Diosmium tetraacetate dichloride was prepared according to the
method of Wilkinson et al. 12 A sample of OS2(OAc)4C12 (52.6 mg, 0.077
mmol) was stirred with 39 mg of AgTFMS (0.152 mmol, 1.97 equiv.) in 5 mL
of MeCN. This produced an opaque blue-black solution with a slight red
dichroism. A fine precipitate was present, but no significant amount of AgCl
was observed. Filtration to remove the finely divided precipitate did not work
well because of the large porosity of the frit. Me3 SiTFMS (1 mL) was added,
and the solution was refluxed for a week. The resulting solution was an
opaque purple-black that appeared red in color if light was passed through
the solution from behind. The volume was reduced to ~ 2 mL and toluene (3

mL) was layered underneath. Black microcrystals formed upon chilling the

222

solution to -40 0C. The solution was decanted and the crystals were pumped
to dryness. IR (KBr, Nujol), cm'lz 2295 (m), 1710 (m), 1273 (vs), 1234 (vs),
1196 (vs), 1168 (s), 1034 (vs), 639 (vs).

(27) Reaction of Osz(OAc)4C12 with HBF4 in the presence of
Propionitrile

A sample of Osz(OAc)4 C12 (102 mg, 0.148 mmol) was refluxed with 0.7
mL of HBF4 and 5 mL of EtCN for 24 h with stirring to effect complete
dissolution of the starting material. The resulting solution which was
green/red dichroic was layered with hexanes and diethyl ether (1 mL and ~ 8
mL respectively) with no precipitation. The solvent was then removed under
vacuum and 10 mL of diethyl ether was added to the solution without any
noticeable change. When this solvent was removed under vacuum, 0.5 mL of
triethylorthoformate was added to aid in dehydration of the solution.
Subsequent layering with hexanes (1 mL) and diethyl ether ( ~ 10 mL) on this
solution produced two layers; the upper layer was brown and the bottom
layer was green. A small amount of oily solid precipitated, but the 1H NMR
spectrum showed it to be a highly impure compound. Subsequent work-up

did not yield a clean product.

(28) Reaction of Osz(OAc)4C12 with HBF4 in the presence of
Acetonitrile and Dichloromethane

In an effort to isolate a solid from these reactions, dichloromethane
was added to the reaction solution in hopes of precipitating the product as it
was formed. A quantity of 082(OAc)4C12 (137 mg, 0.199 mmol) was refluxed
and stirred together with 16 mL of either 1:1 or 3:1 CH2012 : MeCN and 1 mL

of HBF4 for 6 days. The initial brown insoluble starting material formed a

223

dark purple solution after ca. 1 day. The volume of the reaction solution was
reduced under vacuum, and hexanes and diethyl ether (1 mL and 20 mL
respectively) were layered on the MeCN/CH2C12 solvent mixture to isolate
the product. A large amount of dark solid precipitated, leaving behind a
relatively dilute red-purple solution which was decanted off. Continued work-
up was carried out in air. Acetone (10 mL) was added to partially dissolve
the solid, producing a dark purple solution which was decanted and filtered.
This procedure was repeated to obtain two similar purple solutions which
were combined. Hexanes were added to the solution to precipitate product A;
yield, 25 mg. The remaining original solid was dissolved in copious amounts
of acetone to yield a blue solution. The volume was reduced, and a suspended
solid was easily collected by filtration; yield of product B, 65 mg.

Product A: IR (CsI, Nujol), cm'1: 2294 (m), 1641 (m), 1531 (m), 1064
(vs), 1022 (vs), 781 (m), 688 (m), 520 (m). 1H NMR (CD3CN): 6 = 8.58 ppm (s,
3.74 H), 6.07 (s, 7.65 H), 2.71 (s, 0.96 H), 2.19 (s, unintegrated), 2.08 (s,
unintegrated), 1.95 ppm (s, unintegrated). UV-visible (MeCN) Amax = 572
nm. Product B: IR (CsI, Nujol), cm'lz 2330 (w), 2298 (m), 1653 (w), 1539 (s),
1064 (vs), 1028 (vs), 680 (m), 522 (w). 1H NMR (CD3CN): 6 = 12.15 (8, br),
2.48 H), 9.7 (8, br, 6.65 H), 2.73 (s, sharp, unintegrated), 2.71 (s, sharp,
unintegrated), 2.19 (s, sharp, unintegrated), 2.08 (s, sharp, unintegrated).
UV-visible (MeCN) Am = 583 nm. The unintegrated resonances were
extremely minor compared to the other features, so were not included in the

integration.

224

C. Results and Discussion
Chromium

Metal-metal bonds are scarce among the first row transition elements
due to the relatively "hard" nature of these metals and the contracted orbital
size. Cr(II) is the only element among this group to form dinuclear
coordination compounds containing metal—metal bonds. These examples are
primarily with bridging ligands such as carboxylate and amidinate ligands to
hydroxypyridine anions. 13 Among these examples are found the extremely
short quadruple bonds known as chromium "super shorts." Much discussion
about the factors influencing these short distances has been put forth, 13 and
we rationalize that the preparation of unbridged systems if possible, would
assist in identifying some of the factors that influence Cr-Cr bonds.

Although this particular dimetal unit seems extremely unlikely to be
stabilized by solvent with a high charge which would further serve to contract
the orbitals, we decided that this was the only possibility for a dinuclear
solvated system among the first row elements, so we targeted
"[Cr2(MeCN)10]4+" to be synthesized by our established methods. Our
earliest successful efforts in dirhodium chemistry focused on the
decarboxylation reactions'with Et3 OBF4; thus we used this reagent on
Cr2(OAc)4 , in hopes of retaining the M-M band while several bridging groups
were removed. Anhydrous chromium acetate does not possess axial solvent
ligands. Instead, the oxygen atoms on one acetate ligate the equatorial site of
one Cr2 unit as well as the axial position of another Cr2 unit thereby setting
up an extended network as presented in Figure 34. Disrupting this network
is not particularly difficult- it involves merely dissolution in a coordinating

solvent to produce discrete dinuclear units. This reactive form exists in

225

Figure 34. View of the discrete Cr2(OAc)4L2 molecule and the

extended interaction in the anhydrous form. (Ref 13)

226

 

 

L

 

 

MeCN, therefore decarboxylation with the reagent EthBF 4 proceeds easily
to form a purple solution from which peach crystals may be grown.

X-ray crystallography on the pale orange crystals carried out by
Siemens indicated that the compound was actually the previously reported
but uncrystallized mononuclear solvated species, [Cr(MeCN )6][BF4]3.14
Preliminary data clearly showed the molecule as presented in Figure 35.
Selected bond distances and angles are found in Tables 18 and 19. Positional
parameters are located in the Appendix. The chromium atom is ligated by six
MeCN ligands in a perfectly octahedral disposition about the metal. Three
[BF 4]' counterions in the lattice indicate that an oxidation from Cr( II) to
Cr(III) has occurred during the course of the reaction. This is not particularly
surprising, given the extreme sensitivity of the starting material and the
established preference for a solvated complex of this oxidation state. 14
Excessive air exposure of this sensitive compound invariably produces green
solids and solutions. Evidently, the dinuclear [Cr214+ species is far too
unstable and the reaction proceeds to form the thermodynamically more
stable Cr(III) solvate.

Although there have been previous reports of the homoleptic MeCN
Cr3+ complex,14 characterization in each case was limited to electronic
spectroscopy and elemental analysis. We were able to detail the solid state
properties of [Cr(MeCN)6][BF4]3. The 1H NMR spectrum indicated the
presence of free acetonitrile ligands with the usual resonance at 6 = 1.95 ppm.
This is anticipated both because the ion is d3 and should possess no inherent
inhibition to solvent self-exchange, and because a coordinated nitrile signal
would be relaxed due to the compound's paramagnetism, even if exchange
was incomplete. The FT-IR spectrum of [Cr(MeCN)6][BF4]3 displayed two
intense stretches in the CEN region at 2330 and 2301 cm'1 due

228

Figure 35. Diagram of the crystallographically characterized
[Cr(MeCN)sl3+.

229

Figure 35

230

Table 18. Selected Bond Distances in A for [Cr(MeCN)6][BF4]3

atom 1

Crl
Crl
Crl
Crl
Crl
Crl
N1
N2
N3
N4
N5
N6
ClA
C2A
C3A
C4A
C5A
C6A

amm2

N1
N2
N3
N4
N5
N6
C1A
C2A
C3A
C4A
C5A
C6A
C1B
C2B
C3B
C4B
C5B
C6B

distance

1.97(2)
1.97(1)
2.00(1)
2.01(1)

1.995(6)
1.990(7)

108(2)
1.12(2)
1.14(2)
1.15(2)
1.11(1)
1.13(1)
1.53(3)
1.53(3)
1.41(2)
1.41(2)
1.48(1)
1.45(1)

atom 1

B1
B1
B1
B1
B2
B2
B2
B2
B3
B3
B3
B3

awm2

F1A
FlB
FlC
F1D
F 2A
F2B
F2C
F 2D
F3A
F3B
F3C
FBD

distance

139(2)
1.33(2)
1.35(2)
1.27(2)
1.35(1)
1.35(2)
1.36( 1)
1.38(2)
1.36(2)
1.34(2)
1.35(2)
1.36(2)

231

Table 19. Selected Bond Angles in degrees for [Cr(MeCN)sl[BF4]3

atoml atom2 atom3 angle
N1 Crl N2 179.7(5)
N1 Crl N3 90.2(5)
N1 Crl N4 90.3(6)
N1 Crl N5 90.0(6)
N1 Crl N6 87.3(6)
N2 Crl N3 90.1(5)
N2 Crl N4 89.4( 5)
N2 Crl N5 89.8(6)
N2 Crl N6 92.9(5)
N3 Crl N4 177.9(5)
N3 Crl N5 89.3(5)
N3 Crl N6 89.7( 5)
N4 Crl N5 88.6(5)
N4 Crl N6 92.4( 5)
N5 Crl N6 177.1(6)
Crl N1 C1A 169(2)
Crl N2 C2A 169(1)
Crl N3 C3A 171(1)
Crl N4 C4A 173(1)
Crl N5 C5A 176(2)
Crl (N6 C6A 171(1)
N1 ClA ClB 177(2)
N2 C2A C2B 175(2)
N3 C3A C3B 175(2)
N4 C4A C4B 176(2)
N5 C5A CSB 179( 1)
N6 C6A C6B 180(2)

232

to the v(CEN) stretch and the combination CH3 deformation/C-C stretch. The
expected broad [BF4]’ stretch appeared at 1030 cm '1. Several additional
intense features were observed at 959, 470, and 445 cm '1 , of which the first
can be identified as the CC stretch by comparison to the extensive
Groeneveld literature.15 The other two features are higher than the reported
values for both the C-CEN bend and M-N stretch in divalent octahedral
solvates, so these remain unassigned in the absence of a database of infrared
spectra of trivalent acetonitrile compounds with which these values may be

compared.

Iridium

In contrast to rhodium chemistry, only one dinuclear iridium (II,II)
complex has been reported.16 Diiridium tetracarboxlyates are not known;
neither are the octahalide supported metal-metal bonds for the late transition
metals with the exception of osmium, 13 thus available starting materials for
accessing the Ir 2( 11,11) core are few. Since the acetonitrile solvated Ir(III)
complex is also unknown, we proposed to approach through an alternative
method. We planned to first prepare [Ir(MeCN)6]3+, and then reduce the
cation to Ir( II) which is d7' and should have a proclivity to form a metal-metal
bond. We argue that the charge on the metals may even improve the stability
of the diiridium solvate since this may help to stabilize the lower oxidation
state Ir(II) over Ir(III). An encouraging factor for the hypothesized success in
the use of acetonitrile to stabilize this M-M bond is that the only known
Ir2(II,II) species is ligated by N-donors rather than the ubiquitous O-donor
ligands. 16

Efforts to prepare the Ir3+ solvated complex followed much the same

course as the halide abstraction reactions with RhCl3. The IrC13 polymeric

233

network is more robust than that of Rh, so refluxing MeOH was required to
prepare a reactive form of the Ir(III) starting material. Acetonitrile was not
sufficient to render the material soluble, therefore methanol was required in
order to observe any reaction. Reactions of IrCl3 with SbC15 did not seem to
proceed cleanly, judging by the large amounts of brown solid that
continuously precipitated upon work-up.

Halide abstraction using silver reagents was much more productive,
although these reactions proved to suffered from the same impurity problems
as the rhodium analogs. The contaminated solids decomposed in light due to
the photosensitivity of the Ag+ impurities. The presence of Ag+ impurities
was also verified by cyclic voltammetry where a catalytic reaction with the
electrode occurred at +0.26 V, and by the deterioration of CsI windows during
IR studies as the Ag+ reacted with the salt plates.

The presence of these impurities should not affect qualitative
spectroscopic results substantially since the silver salts will be silent in the
NMR and IR. The 1H NMR spectra displayed two resonances, one
presumably due to coordinated MeCN at 6 = 2.82 ppm, and one of various
intensity due to exchanged solvent at 6 = 1.95 ppm. The relative intensities
of these two features varies with the time elapsed between sample
preparation and data collection. The observation of the coordinated ligand is
due to the relatively inert nature of the d6 Ir(III) metal center; solvent self-
exchange is not facile at room temperature. 17 The coordinated solvent
resonance is near that of [RhC12(MeCN)4]+ and the slight shift may merely
reflect the change in metal center. The IR spectrum in the v(C =N) region is
also quite similar to that of [RhC12(MeCN)4]+, suggesting that the compound
that is admixed with the silver impurities may be [IrC12(MeCN)4]+ which is

unknown. Separation of the iridium and silver containing salts was not

234

possible due to the extremely similar solubilities of the components. Even
multiple fractional recrystallizations were not successful at producing a pure
compound.

Efforts to bypass these purity problems involved the use of HBF 4 to
remove the halides from IrCl3. Unfortunately, dry products could not be
isolated from these reactions which displayed a distinct tendency to oil. The
cyclic voltammetry on a sticky residue displayed complicated behavior of
several irreversible reductions at Ep,c = - 0.88 V, - 1.05 V, and - 1.55 V. No
oxidations were observed out to the solvent limit of + 2.0V. A typical
voltammogram in 0.1 M TBABF4 /MeCN with a glassy carbon working

electrode and a Ag/AgCl reference electrode is shown in Figure 36.

Ruthenium

Preparing a dinuclear ruthenium compound presented several new
challenges. The first is the choice of oxidation state. The majority of Ru-Ru .
bonds are mixed-valence II-III complexes; fewer examples of II-II and III-III
species exist.13 The demonstrated stability of [Ru(MeCN)5]2+ prevents any
consideration of forming a dinuclear II,II species, therefore efforts centered
on the +3 and mixed valence starting materials. Our initial approach
involved halide abstraction from the RuCl3 starting material since no
dinuclear octahalide of this metal is known. Several possible results of these
reactions were considered; the previously unknown mononuclear 3+ solvated
species could result, or adventitious reduction of half of the ruthenium and
dimerization to form a mixed valence complex was proposed.

The first strategy was to remove the halides via reaction of RuCl3 with
a thallium or silver reagent to precipitate the chloride. These reactions

produced bright blue solutions. The solids isolated from the AgBF4 reactions

235

Figure 36. Cyclic voltammetry of a halide abstraction product with

IrCl3 containing significant Ag+ impurity

236

 

0.0 -1.0 -21)

Volts vs. Ag/AgCl

Figure 36

237

were thoroughly characterized spectroscopically. The FT-IR spectrum
displays two weak features in the v(CEN) region at 2334 and 2305 cm'l. The
low intensity of these stretches is not unexpected for a weakly back-bonding
Ru(III) compound as discussed in the Introduction of this dissertation. A
large [BF4 ]' stretch is present with no substantial v(Ru-Cl) stretches in the
far-infrared region. The intense blue color of the solutions is reflected by the
absorption in the visible region at Amax = 579 nm which is accompanied by
two distinct features in the ultraviolet at maxima of 319 and 267 nm. The 1H
NMR spectrum displays only one resonance at 6 = 2.49 ppm which is
attributable to coordinated CH3CN.

All of the aforementioned data support the formulation of a fully
solvated species, except the reactions in acidic media to protonate the
chlorides from RuCl3 which do not give solutions that in any way resemble
these colors. It seems reasonable to assume therefore, that silver has become
incorporated into the product since it will not be detected by any of the
aforementioned methods. Protonation reactions of RuC13 with HBF4 produce
dark red solutions, but isolation of red products without further
transformation was not possible. It is important to note that all the above
reactions must be carried out using hydrated starting materials. The
anhydrous form proves to be totally unreactive even under prolonged
refluxing conditions.

After these halide abstraction reactions had not successfully produced
an isolated solvated ruthenium system, additional information about the
purity and composition of RuClg starting materials came to light. 18 The
solids that are commercially available are purified either for the Ru(III)
oxidation state in which case large amounts of HCl contaminate the samples,

or if the HCl is removed, the solid contains a mixture of Ru oxidation states.

238

Neither situation lends itself well to designed syntheses from this material.
Thus most Ru reactions are carried out on derivatives whose preparation
does not rest on the purity of the RuCl3.

The first reaction attempted for this metal was decarboxylation of
Ru 2(02 CC2 H5)4C12 with Et3 OBF4 in MeCN, carried out by K.R. Dunbar. 19
These conditions did not proceed to remove the carboxylates as evidenced by
a crystal structure of the only product isolated from the reaction which
turned out to be the axial aquo adduct of the original species. Although the
solvated bis-carboxylate complex, Rh2(OAc)2(MeCN)64+ does not exist, a
derivative of this system possessing four equatorial CO groups is known, and
it was hoped that this might change the solubility and open up the unit for
reactivity. A reaction was carried out with Ru2(OAc)2(CO)4(MeCN)2 and
Et3 OBF4, and while a color change did occur from yellow to orange, the
subsequent product proved to be intractable.

Acid reactions with the mixed-valence tetraacetate of ruthenium, with
or without the axial halide, appear to proceed more rapidly and completely
than the corresponding reactions with Et3OBF4, which does not easily attack
the relatively insoluble starting material. Red solutions result from the
reaction with either HBF4 or HTFMS, leaving behind only a small amount of
unreacted starting material. As in the acidic reactions with RuCl3, no red
product can be isolated from these reactions; the only recognizable component
is the pale cation [Ru(MeCN)(;]2+ which is apparently a thermodynamic sink
for these reactions and is likely to be formed from most mixed valence
starting materials. The appearance of this product was seen regardless of

whether or not the axial halides were removed prior to acidification.

239

Molybdenum

The dimolybdenum acetonitrile solvated species, [M02(MeCN)10]4+ ,
was the first example of a readily prepared dinuclear MeCN species and is
generally accessed directly or indirectly from the tetracarboxlyate dimetal
complex.20 As mentioned earlier, [Re2(MeCN)10]4+ may be prepared either
from the tetrabutyrate or from the octahalide. We wished to further
generalize our methodology to include the preparation of the Mag“ species by
halide abstraction from [M02Clgl4' as well.

Unlike the [Re2 C1812 case, these reactions did not produce the bright
blue solutions of the well-known fully solvated dimolybdenum complex. One
problem was quite obviously the acidic conditions in which most of these
reactions were performed. If the HBF4 or HTFMS are not perfectly dry, the
reaction solutions lead to intractable oils. The green colors resulting from
most of the reactions suggest that protonation ceases at the partially solvated
compound, M02 Cl4(MeCN )4. Starting material purity may be affecting the
formation of this intermediate compound since the preparation of both
K4 [M02 C18] and [N H4]5[M02 C19] are prone to contamination by KCl and
NH4C1 respectively. The presence of additional Cl ' would certainly retard the
progress of the reaction. The one difficulty with this hypothesis is that the
same reaction conditions were employed with M02 C14 (M82 S)4 but these also
gave green solutions, although there were only four halides to be removed.

One reaction that was promising in the [M02]4+ chemistry was the
reaction of K4[M02 C13] with NaBPh4. The room temperature reaction
produced a blue solution that unfortunately decomposed to a green solution
upon accidental exposure to air, but this sensitivity is typical of the target
complex, [M02(MeCN) 10] 4+. More careful treatment of this reaction or

possible use of silver reagents may assist the preparation of the solvated

240

cation from this halide route. At this point, a great deal of effort has not been
expended on this reaction since the dinuclear solvate of molybdenum is

already known.

Osmium

In contrast to ruthenium, there are no homoleptic acetonitrile species,
either mononuclear or dinuclear, known for osmium. As a third row metal,
higher oxidation states are more stable so the common carboxylate is
OS2(III,III) instead of the mixed valence Ru2(II,III) system.13 This would
necessitate the initial formation of a solvated Os compound with an [0S2 16+
core, but as seen for rhenium, another third row element, a reduction was
observed from Re2(III,III) to Re2( 11,11), therefore, it would not be surprising if
[Oszl4+ was the more stable dinuclear solvated molecule.

The halide abstraction reactions with OsCl3 produced similar results
as the RuC13 chemistry. The inky blue solutions gave sticky solids upon
which no characterization was obtained. This reaction was not followed up
due to a lack of a convenient source of OsCl3. We had received a large
quantity of OsO4 from Johnson -Matthey and decided that the dinuclear
starting materials [n-Bu4N] 2[Os 201.3] and Osz(OAc)4 C12 , prepared from
0304, would be even better starting materials for the fully solvated systems.

Halide abstraction reactions with the green [Os 2Clgl?r generated
brown solutions. The three reagents employed, AgTFMS, HBF4, and HTFMS
all produced slightly different results. The lack of white precipitate in the
silver reaction combined with the very weak nitrile stretches in the infrared
and persistence of substantial [n-Bu4N]+ in the 1H NMR spectrum suggested
that this product was not the desired fully-solvated species. The acidic

reactions did not produce solids and displayed a marked propensity towards

241

oiling. The combination of propionitrile and triflic acid in particular is less
likely to produce a solid owing to the enhanced solubility of both the triflate
anion and the longer chain nitrile.

Considering the difficulties encountered in the attempted synthesis of
[M02(MeCN)10]4+ from the dinuclear halide, more attention was focused on
Osz(OAc)4 C12 than on [Os2C1812m Early reactivity surveys indicated that the
insolubility of the starting material precluded reaction with Et30BF4 .19
Indeed, most reactions employing osmium tetracarboxylates as starting
materials are generally carried out with a longer chain R-groups to enhance
the solubility.12 Thus it is quite surprising that the brown insoluble
Osz(OAc)4 C12 reacts with HBF4 in MeCN to produce a green solution.
Unfortunately, preliminary efforts to isolate a solid from this tantalizing
solution proved fruitless, but this reaction is very promising and should be
pursued. The combination of a silver reagent to remove the axial chlorides
followed by Me3SiTFMS to remove the carboxylates was adopted from the
demonstrated success of a two-step method in the rhenium chemistry;
nevertheless, a lack of nitrile stretches in the infrared suggested that this
was not a viable method.

In the first reaction that we used to produce [Rh2(MeCN)1o][BF4]4, the
triethyloxonium reagent was introduced as a CH 2C12 solution, the presence of
which served to precipitate the product from the MeCN reaction solution.
This strategy was also tried in the osmium chemistry. Addition of CH2C12 to
the MeCN reaction solutions does not precipitate a green product, but instead
causes the reaction to turn color, in this case purple. Two compounds were
isolated from this reaction, the principal products are a dark blue species and
a bright purple compound. Neither compound displays v(Os-Cl) stretches

down to 200 cm'1 in the infrared, but both contain carboxylate stretches

242

around 1550 cm'1 with the feature in the blue sample more prominent than
that of the purple. The purple spectrum displays only one v(C-=-N) stretch at
2294 cm'1 compared to the blue compound which displays both a stretch at
2298 and an additional feature at 2330 cm'l. The 1H NMR of the blue
compound displays several very broad resonances shifted downfield to 6 =
12.2 and 9.8 ppm suggesting the presence of a paramagnetic component
which contact shifts these signals. The intense blue color also implicates a
mixed valence species. Thus, it is anticipated that the blue species is
partially reduced to Os25+ and is ligated by a mixture of acetate and
acetonitrile. The 1H NMR spectrum of the purple compound displays several
singlets at 6 = 8.58, 6.07, 2.71, 2.19 and 2.08 ppm. The sharp nature of the
resonances indicates a diamagnetic compound which presumably is ligated by
OAC' and MeCN. A methodical approach to separation of these two
compounds is necessary before more emphasis is placed on spectral
identification. Crystallization of the two species would greatly assist in
understanding the influence of the CH2C12 and assessing the next step in

successfully accessing a fully solvated complex.

D. Summary

Several mononuclear compounds resulted from attempts to prepare
dinuclear solvated species. The chromium system proved the efficacy of
triethyloxonium reagents in producing solvated compounds, even if the
[Cr(MeCN)6]3+ species was not what we had initially attempted to prepare.
Efforts to access [Ir(MeCN)6]3+ as a precursor to an [Ir2(MeCN) 10] 4+
compound were met with lability problems in the IrC13 system. Future
investigations would benefit from slightly different strategies. One

possibility is the use of IrBr3 instead of the chlorides since the larger, more

243

polarizable halides should be easier to remove. Another possibility would be
to prepare one of the examples of an assembled Ir2(II,II) core and try to
protonate the ligands in the presence of acetonitrile. The dehydration of
[Ir(H2O)6]3+ has been discussed as an option, but the difficulty of completely
removing all the water coupled with the extreme substitutional inertness of
this complex makes this method more attractive. 17

Synthesis of any solvated Ru complex other than [Ru( MeCN )6 12+ seems
unlikely at this point considering both the thermodynamic stability of that
compound and the impurities contained in the only convenient starting
material, RuC13. Continuation of this work should receive a low priority,
although elucidation of the inky blue solutions resulting from the halide
abstraction reactions with Ag+ reagents are quite intriguing. Likewise, the
[M02 l4+ chemistry is not as important to generalize as that of the other
metals since the fully solvated compound is already known.

Of the metal systems discussed, [OS2 14+ has shown the most promise
thus far. The dark blue and purple compounds resulting from the reactions
involving the‘addition of CH2 C12 may not be fully solvated complexes, but
they definitely present more soluble forms of the acetate starting material
from which it may be easier to finally prepare the sought after
[032(MeCN) 10] 11+. The ease of isolating dry solids of these compounds is also
extremely promising. Alternately, rigorously anhydrous conditions may
allow isolation of a solid from the oily acid solutions. Addition of the acid

anhydride should also assist in this process.

9‘99.”

10.

11.

12.

13.

14.

15.

16.
17.

244

LIST OF REFERENCES, CHAPTER V

Dunbar, K.R.; Quillevere, A. unpublished results.
Bernstein, S.N.; Dunbar, K.R. submitted to Angew. Chem. 1992.
Zurr, A.P.; Groeneveld, W.L., Part I, Recueil, 1967,86, 1089.

Cotton, F.A.; Wilkinson, G. "Advanced Inorganic Chemistry," Wiley:
New York, 1988.

Crooks, G.R.; Johnson, B.F.G.; Lewis, J .; Williams, I.G.; Gamlen, G. J.
Chem. Soc. A, 1969, 2761.

Mitchell, R.W.; Spencer, A.; Wilkinson, G. J. Chem. Soc. Dalton Trans.
1973, 846.

Urbanos, F.A.; Barral, M.C.; Jiménez-Aparicio, R. Polyhedron, 1988, 7,
2597.

Brencic, J .V.; Cotton, F .A. Inorg. Chem. 1970, 9, 346.
Brencic, J .V.; Cotton, F.A. Inorg. Chem. 1970, 9, 351.

San Filippo, J .Jr.; Sniadoch, H.J.; Grayson, R.L. Inorg. Chem. 1974, 13,
2121.

Johnson, T.W.; Tetrick, S.M.; F anwick, P.E.; Walton, R.A. Inorg. Chem.
1991 , 30, 4146.

Behling, T.; Wilkinson, G.; Stephenson, T.A.; Tocher, D.A.;
Walkinshaw, MD. J. Chem. Soc. Dalton Trans. 1983, 2109.

Cotton, F.A.; Walton, R.A. "Multiple Bonds Between Metal Atoms,"
Wiley: New York, 1982.

(a) Zuur, A.P.; Van Houte, J .J.; Groeneveld, W.L., Part IV,
Recueil, 1968, 87, 755. (b) Gutman, V.; Hampel, G.; Lux, W. Mh.
Chem. Ed. 1965, 96 , 533.

Reedijk, J .; Zuur, A.P.; Groeneveld, W.L., Part III, Recueil, 1967 , 86,
1127.

Cotton, F.A.; Poli, R. Polyhedron, 1987, 6, 1625.

Castillo-Blum S.E.; Sykes, A.G.; Gamsjager, H. Polyhedron, 1987, 6,
101.

18.

19.
20.

245

(a) Keppler, B.K.; Rupp, W.; Juhl, U.M.; Endres, H.; Niebl, R.; Balzer,
W. Inorg. Chem. 1987, 26, 4366. (b) Schroder, M.; Stephenson, T.A. in
"Comprehensive Coordination Chemistry," 1987, 4, 27 7.

Dunbar, K.R. unpublished results.

(a) Mayer, J .M.; Abbott, E.H. Inorg. Chem. 1983, 22, 2774. (b) Cotton,
F.A.; Wiesinger, K.J. Inorg. Chem. 1991 , 30, 871.

CHAPTER Vl

CONCLUDING REMARKS AND FUTURE DIRECTIONS

246

247

General

The chemistry of partially and fully solvated transition metal cations
was largely unexplored prior to the results in this dissertation. The versatile
combination of a positively charged dinuclear core and labile ligands may be
exploited in designing new molecules that are inaccessible through more
common starting materials; future applications such as incorporation of
dimetal units into porphyrins and materials would be attractive future
directions. The small number of dinuclear solvated complexes prompted our
investigation into establishing general methods of preparing other examples

and to study the fundamental reactivity of these species.

Mixed-Metal Cation-Anion reactions

The partially solvated dirhodium cation [Rh2(OAc)2(MeCN)6l2+ reacts
with the anionic [Re2C1812' to yield the unusual soft salt,
[Rh 2(OAc)2 (MeCN )6][Re 2Clg] . The partially solvated molybdenum dication
does not produce the same kinetic product as the rhodium analog, so the
elucidation of a thermodynamic product in this second system will probably
lend more insight into the identification of redissolution products of
th2(OAc)2(MeCN)el[R82,Clsl in MeCN. The reactions of transition metal
cations and anions have generally not been extensively investigated and are
rich with potential for ligand redistribution, clusterification through cation-
anion annihilation, preparation of unusual mixed-metal salts, or possible use

as molecular precursors for materials.

Synthesis and Reactivity of Dirhodium Solvates
Several methods for preparing solvated dinuclear transition metal

cations have been elucidated. The use of triethyloxonium tetrafluoroborate

248

as a carboxylate alkylation agent was the first method that led to clean
preparation of the solvated species [Rh2(MeCN)1o][BF4]4. Acidification of the
carboxylates with tetrafluoroboric acid was also found to be an excellent route
to prepare this nitrile cation in high yield. Preparation of a more soluble salt
was also established through the silylation reaction of dirhodium tetraacetate
with trimethylsilyltriflate. Solvent exchange on the acetonitrile compound is
quite facile for both the axial and equatorial positions as observed by 1H
NMR. This characteristic allows the substitution of water for MeCN to
prepare the elusive Rh2(aq)4+ in a non-acidic medium. The synthesis of
homoleptic dirhodium cations with different solvents and counterions allows

for the use of these synthons in specifically reactions.

Photochemistry of [Rh2(MeCN)10]4+

The photochemistry of the dirhodium decakisacetonitrile complex is
fascinating due both the reversibility and to the half-life of the process. That
this process occurs under normal synthetic conditions has been established by
examining the electronic spectra of samples exposed to ambient light or
refluxing conditions in the absence of light. Although independent synthesis
of the intermediates has not been possible to this point, verification of the
Rh( I) and Rh(III) intermediates was established by trapping the Rh(I)
intermediate by purging a photolyzed solution of [Rh2(MeCN)1o][BF4]4 with
CO gas to obtain cis-[Rh(CO)2(MeCN)2l+.

Other Transition Metal Systems

Extrapolation of the synthetic methods to access [Rh 2(MeCN ) 10] 4+ to
other transition metals was met with mixed success. Attempts to work with

chromium tetraacetate lead to an oxidation to form the stable mononuclear

249

[Cr(MeCNm]3+ species. Ruthenium is problematic because RuCl3 is an
impure starting material and a thermodynamic sink for the reactions is the
mononuclear species, [Ru(MeCN)5]2+. Preparation of a diiridium complex
will necessarily be more complicated than dirhodium due to a paucity of
appropriate starting materials and the inertness of IrCl3. Halide abstraction
from IrBr3 and subsequent reduction, or protonation of the ligands on the
Ir2(form)4(II,II) (form = formamidinate) complex should definitely be
investigated. Attempts to access the diosmium system have discovered a
surprisingly rich chemistry from the reactions of OS2(OAc)4 C12; this is
currently the most promising metal for future work.

General accessibility to solvated systems was established in both
dirhodium and dirhenium chemistry through the use of trialkyloxonium
reagents and acids. The osmium system, however, demonstrated that
Et3 OBF4 is not as forcing a reagent as either tetrafluoroboric acid or
trifluoromethanesulfonic acid since Osz(OAc)4 C12 does not react in the
presence of triethyloxonium. The difficulties with the acidic media are the
complications encountered in isolating the products. Several suggestions are
proposed to alleviate these difficulties. The first is the addition of both the
acid and the acid anhydride to the reaction which should help to scavenge the
water. The second requires distillation of all reagents involved prior to
combination. A third possibility involves the use of a specialized piece of
glassware designed specifically for this purpose, depicted in Figure 37. This
combination condenser/ water trap allows the reaction to be refluxed as the
solvent is distilled through molecular sieves which should dehydrate the
solution. The efficacy of this process has already been demonstrated for
organic reactions,1 and extrapolation to inorganic reactions should also prove

very effective. Some difficulty may be encountered since MeCN refluxes at a

250

Figure 37. Diagram of a condenser and water trap to dehydrate

reaction solutions

251

Gas inlet

 

 

 

 

 

Molecular sieves

i

To
Reaction
Flask

Figure 37

252

lower temperature than H2O, but the substitution of higher boiling nitriles

should alleviate this problem.

Conclusions

Preparation of a fully solvated dirhodium cation has been established
for two different nitriles with two different anions, and the fundamental
reactivity of these complexes with the aim of designed synthesis has been
established. Decakisacetonitrile dirhodium itself exhibits unusual reactivity
in the form of reversible photochemistry. Synthetic exploration of other
related transition metal compounds has provided useful insight into the
future selection of the most likely metals to form these dimetal solvates and
which particular starting materials will most likely produce the best results.
This research has effectively laid the ground work for establishing general

routes into homoleptic acetonitrile dinuclear transition metal species.

253

LIST OF REFERENCES, CHAPTER V1

1. Stille, J .R.; Barta, N.S. manuscript in preparation.

 

APPENDICES

254

255

Synthetic Methods

All experiments were carried out using standard vacuum and Schlenk
line techniques unless otherwise noted. Starting materials were obtained
from commercial sources where no other source is indicated. All nitriles
except for acetonitrile were used as received. MeCN, MeOH, diethyl ether,
hexanes, toluene, THF, CH2 C12, and acetone were distilled under N2 from an
appropriate drying agent prior to use. Water was purified by the Millipore
system from Waters Chromatography and was deoxygenated by a purge of

inert gas.

Physical Measurements

Infrared spectra were measured on either a Nicolet 740 FT-IR or a
Perkin-Elmer 599 Spectrophotometer. 1H NMR experiments were performed
on a Varian Gemini 300 MHz instrument. Electronic Spectroscopy was
carried out on a Hitachi U-2000, a Cary 17, or a Cary 2300 spectrophotometer
in the indicated solvents in quartz cells. Electrochemical measurements were
carried out using an EG&G Princeton Applied Research Model 352 scanning
potentiostat in conjunction with a BAS Model RXY recorder and were
uncorrected for junction potentials. Platinum working electrodes were used
with potentials referenced to a Ag/AgCl reference electrode an internal
Ferrocene standard was not used. EPR experiments were carried out on a
Varian E-4 spectrometer at room temperature. Elemental Analyses were

performed by either Galbraith Laboratories, or Desert Analytics.

256

Table 20. Atomic Positional Parameters (A2) and their estimated

standard deviations for [Rh 2(OAc)2(MeCN)sl[Re2 Clg]

3(89)

atom

8 ))))))))))) 9 ))))))))))))))))))))))))) ) ‘I )
(66661155551(4555776657115 ‘ 7776577 8986 7 8 a

7 (((((((((((7 ((((((((((((()()((((((()(((()’))(’()(

697a376‘26718207648‘83078517120357241092021219.1011
O

O O O. 0.... .00... O 0.0.0....(0‘. O O I O O .(O 000((((0(0(.

23334112222212422544335111203222102132221535231.4232

)))))))))))))))))))))))))

1665711555511565576666722 )))))))))))))))))))))))))
((((((((((((((((((((((((( 1112222112222222332222222
8770121836572694726949732( ((((((((((((( II‘ ((((((( (((
9614719120811140265218075218290152286901931455‘135
31463417798837326497932101144150291‘3273468523“06
767a743"22222231312321100100001112000000001211232
..000000.0.000000000000000000000.00.00.00.00. 0000
) 3)

7))\!\.! )\!))98 ))))))))))))))

(5455 4‘44((344465455511)88 )))))))))))))))))))))))
7(((( (((‘74 ((((( (((!\((!\1((1111111122221222212121
56527 833366436342384357‘85“ (((((( ((l\ ((((((( (((l‘
80.937 1204034326372068995881461212796654269083034
10405 3353326169405701629151600231269227977596182
22122“3223°°0000000010222323122223332310111111213
.00..//000000000000.00....0000000.000.00.000000000
) )’

7)))\! )\!))88 )))))))))))) \ul

(5554 4‘45((354455554511))8 ))))))))))))))))))))))
7(((!\ ((((19(!\((!\ ((((( ((11(1111111122221222212121
35088 389611945282726110((8( (((((((( ((II‘ (((((((( ((
30277 291914583141472085037236536945451056040488‘
21704 36348521067389S471535‘s?4214583075165694243
7777644213245556544544545554445545455444466553355‘
..... //000.00.00.000. 0 .000 0000.. 0.0000000000.0
00000110000000000000oooooooooooooooo00000000000000
))))))))))))) aa)0123456)) ))))))
1123423567845459111111112 ))))))))))))))))))) 012345
((((((((( ((((((((((((((((123‘12345612345678911.1111
elllleelllleeeelllllllluu(((( ((((((( I‘ ((((( I‘ (((((( l.‘
RCCCCRRCCCCRRRRCCCCCCCC OOOONNNNNNCCCCCCCCCCCCCCC

257

Table 20. continued

3(39)

atom

)) ))))” ) )) ’))))))) )

11 446666 5 87 99089978 7

)(()(((((()())(()))(((((((()(

16718941.092721941119052892015

(O .(O C. O O .(.((O .(((.. O O O O O .(O

311200222120431.015332322221231

)21 ))))))))))))))))))))))))))
2((111.122221.29.223222222222222
(4 7 (((((((( (((((((( ((((((!\!\((
2327887824078617233405340.1642
42144105290211.755793441523374
31000100002120000000012231123
0000.00.00..0.0.0 0000.00.00.

(o 9 (8 6 (((((((l\!\l\(l\l\((((l\(((((
1.1.225542244543078021873682705
482643589512441-38278668239442
34544555455444455664455553344.
000 00.00.0000 0 0.000.00.00.
00000000000000000000000000000

)11888 )))))))))))))))))))))))
2 ((((( 11111112112222221221121
(4 2 6 1 1 (((((((!\I\((I\ (((((((((((
60698010221701.536372340402791
13638809397790772027214920376
42221122322212.111333433233311
0.0.0.000... 0.00... 0.00....
00000000000000000000000000000
))) )))))))))))))))))))
634 lllllll 0 90123456789012

12222222222333

(( (((((((((((((((((((((((

1278
11112367878911.1511
NNCC

258

Table 21. Atomic Positional Parameters (A2) and their estimated

standard deviations for [Rh 2(MeCN)10][BF4l4

Atom x y z 3(A2)

Rh(l) 0.55984(3) 0.07960(4) 0.30154(2) 2.62(1)
N(1) 0.4161(3) -0.0754(4) 0.2269(3) 3.0(1)
N(2) 0.4967(3) 0.0160(4) 0.3700(3) 3 1(1)
N(3) 0.5356(3) 0.2340(4) 0.3324(3) 3.3(1)
N(4) 0.6189(3) 0.1460(4) 0.2300(3) 3.3(1)
N(5) 0.6621(4) 0.0783(5) 0.3847(3) 4 2(1)
C(l) 0.9019(4) 0.3349(5) 0.2390(4) 3.2(1)
C(2) 0.3853(6) -0.2801(6) 0.2586(5) 6.0(2)
C(3) 0.4547(4) -0.0177(6) 0.4062(3) 3.4(1)
C(4) 0.3989(5) -0.0589(7) 0.4462(4) 5.0(2)
C(S) 0.5256(4) 0.3217(6) 0.3500(4) 3.6(1)
C(6) 0.5093(5) 0.4379(6) 0.3720(5) 5.4(2)
C(7) 0.6532(4) 0.1895(6) 0.1913(4) 3.4(1)

Table 21. continued

Atom

C(8)
C(9)

C(10)

8(1)
F(1)
F(2)
F(3)
F(4)
8(2)
F(5)
F(6)
F(7)

F(8)

X

.6963(5)
.7221(5)
.7979(7)
.2932(6)
.2137(5)
.2361(6)
.6380(4)
.2499(6)
.5593(9)
.5874(4)
.5536(4)
.5339(6)
.4752(9)

0

0.

-0

-0.

0
0
0
0.
0
0

259

Y

.2419(8)
.0784(7)
.082(l)

4351(8)

.0125(6)
.1624(7)
.4265(5)

473(1)

.276(1)

3714(5)

.l953(5)
.2730(7)
.3125(8)

o o o 0

00000

2

.1382(4)
.4156(5)
.4542(8)
.1907(8)
.3592(4)
.3251(6)
.2662(4)
.2405(7)
.4579(5)
.4897(3)
.5022(3)
.3879(4)
.4465(8)

12.
20.

260

Table 22. Atomic Positional Parameters (A2) and their estimated

standard deviations for [Rh 2(MeCN )1ollTFMS]4

B(eq)

atom

 

55 ))))))))))))))))))) ) ))\I! \l! ) )))
((5666826667899 8 8 389778 3 378 s 4 768
75 ((((((((((((((((((((((((((((((((((((((((((((((((
5564124276.134691711112346?44111611126910211722134000
O O O O O O O O O O O O O O .(.(O(((.. O O O .(((.(((O O .(.(((O(((.
22233343223434363635513342335485038557907338672170
1. 1111. 111.1. 1.
66 ))))))))))) ) \.! \I ))))) \! )))))) \.!
((67677667789)8)9)8)))88)888)))3))837874)))4 llllll
39 ((((((((((( 1(1(1(111((1(((111(11 (((((( 111(111111
5975118722492(1(4(7(((80‘174(((9l\(768749((l\9 ((((((
05556518543007520347818921692525701179806599633056
10456588031315820430042999300820445252838677801570
232332322121213444224431100220032335455‘3446676990
O O O O O O C O O ..... O O O O O ..... O O O O O O O O O O O O O O O O O I O
00000000000000000000000000000000000000000000000001
66 )))))))))))))) )) ))))))))))))))))) \I!) )
((666676656779l88l88)l78867787)389726684ll73ll8lll
83 (((((((((((( 1((1((11 (((((((( 1 ((((((((( 11((11(111
16430709648700(22l\83((56793865(202046223((18I\l\7(l\l\
95664846791562862708665967475720731090102272369264
18818546667688728705843399393307622789768211252583
99999008899899889911018879090886676333311211101001
00000 00000000000000.00000.00000000000 0 000000 .0
00000110000000000011110000101000000111100000000000
)\! ) ) \I )
1 1 ))))))))))))))))))))))))))))) 4 )))4 )))7 )))6 ))))))
((1111111111111211112311111.2112(111(111(222l‘222222
7 0 ((((((((((((((((((((((((((((( 8 (((6 (((o (((1 ((((((
19988412280574.1020656251611857094210343105079403253
23501732561721515540604210488276358253732040284729
0122002010103423010023023111211121132429099‘533344
O O ........... O O 000000000000 O ......... O O O O O C O O O O O O
)) \I )))))))))) ))\I )\-I)
12 ))))))))) o ))))))))) 0234567890 ))))))))) 012))))156
((123456789112345678911111111122456112341113789122

((((((((((((((((((((((((((((((( :(( (((((((((((((((

@MNNNNNNNNNNCCCCCCCCCCCCCCCCCC CSOOOSOOOSOOOSOOONCC

Table 22. continued

atom

”OQNHHHNOtflfiNldNO-iNN

-~ONNWO~WNUH~NOO
AAAAAAAAAAAAAAAAA
vvv “NHOfivvv vaqu

261

- (8)
1.2924(3)
(6)

(6)

(6)

(5)

(7)

- (5)
0.6094(5)
(6)

(8)

(6)

(7)

(5)

(7)

(8)

(8)

0.1555

0.2977
0.2194

0.7501(5)
0.7261(9)
0.759(1)

0.8121(9)

B(eq)

HHHHHHHHHHH
UNUDhfihNNNNQQOQb

O O O O O 0 C O O C O C O O I
O‘OOOOOOOOOO‘OOOOO
AAAAAAAAAAAAAAAAA

13.6

uuwwuwuwwuwwmwwwa
vvvvvvvvvvvvvvvvv

Table 23. Atomic Positional Parameters (A2) and their estimated

262

standard deviations for [Rh 2(EtCN) 10][BF4]4

atom

Rh(l)

F(l)
F(2)
F(3)
F(4)
F(5)
F(6)
F(7)
F(8)
N(l)
N(2)
N(3)
N(4)
N(S)
C(1)
C(2)
C(3)
C(4)
C(5)

C(SA)

C(6)

C(GA)

C(7)
C(8)
C(9)

O.

-00
-O.

05269(2)
.0619(2)
.0268(2)
.1294(3)
.1093(2)
.3518(2)
.2698(3)
.3008(3)
.2437(3)
.1090(2)
.0758(2)
.0040(2)
.0326(3)
.1437(3)
.1448(3)
.1915(3)
.2243(4)
.0946(3)
.111(1)

.130(1)

.1158(9)
.079(1)

.0364(3)

0778(3)
1308(4)

Y

0.04196(3)
0.1866(3)
0.2750(4)
0.2011(4)
0.3423(3)
0.1394(4)
0.2496(5)
0.1202(7)
0.0897(7)
0.1032(3)

—0.1034(3)

-0.0164(3)
0.1875(4)
0.0366(4)
0.1441(4)
0.1991(6)
0.1245(6)

-0.1858(5)

-0.297(1)

-0.291(2)

-0.370(1)

-0.367(1)

-o.0472(4)

-0.0830(5)

-0.1627(6)

0.29423(2)
0.0230(2)
—0.0789(3)
-0.0717(2)
-0.0041(3)
0.7383(3)
0.6949(4)
0.6253(4)
0.7234(4)
0.2185(2)
0.2648(2)
0.3700(2)
0.3275(3)
0.3640(3)
0.1821(3)
0.1365(3)
0.0826(4)
0.2561(4)
0.227(1)
0.263(1)
0.289(1)
0.228(1)
0.4146(3)
0.4727(3)
0.4459(4)

9(BQ)

2.83(2)
6.5(2)
7.3(2)
7.8(3)
7.0(2)
7.3(2)
11.0(4)
13.4(5)
13.4(5)

L.)

.2(2)

LJ

.2(2)
3.0(2)
.O(2)

.1)

4.9(3)
3.7(2)
5.0(3)
5.8(4)
5.2(3)
4.5(8)
6(1)

6.0(8)
9(1)

3.3(2)
4.0(2)
6.3(4)

263

Table 23. continued

atom x y 2 B(eQ)

C(10) 0.0258(4) 0.2683(5) 0.3482(4) 6.4(4)
C(11) 0.000(1) 0.387(1) 0.363(1) 5.0(8)
C(llA) 0.035(1) 0.371(1) 0.397(1) 5.4(9)
C(13) 0.1981(4) 0.0155(8) 0.3814(4) 6.3(4)
C(14) 0.2710(5) -0.0156(8) 0.4020(6) 7.9(5)
C(15) 0.3016(6) 0.0688(9) 0.4427(6) 9.3(6)
C(12) 0.0448(9) 0.417(1) 0.428(1) 6.3(8)
C(12A) -0.020(1) 0.435(2) 0.359(1) 9(1)

8(1) 0.0815(4) 0.2525(6) -0.0337(4) 4.7(3)

8(2) 0.2929(5) 0.1422(9) 0.6950(6) 7.1(5)

264

Table 24. Atomic Positional Parameters (A2) and their estimated

standard deviations for [RhCl2 (MeCN)4][BF4]

atom x y z B(eq)

Rh(l) 0 1/2 0 2.42(3)
C1(1) -0.2211(2) 0.3453(1) 0 3.73(6)
N(1) 0.1860(4) 0.4319(2) 0.0671(1) 3.0(1)
C(1) 0.2984(5) 0.3912(3) 0.1024(2) 3.1(1)
C(2) 0.4378(8) 0.3369(4) 0.1496(2) 4.8(2)
8(1) 0 1/2 1/4 3.8(5)

 

F(2) -0.1169(6) 0.4343(4) 0.2117(2) 10.4(3)

265

Table 25. Atomic Positional Parameters (A 2) and their estimated

standard deviations for [Cr(MeCN)6][BF4]3

Atom x y z

Crl 0.3619 0.0333 0.1 780
N1 0.2682 -0.0092 0.0820
CIA 0.2123 -0.0167 0.0301
ClB 0.1306 -0.0316 -0.0398
HlA 0.1484 -0.0846 -0.0863
HlB 0.1175 0.0384 -0.0687
HlC 0.0739 -0.0568 -0.00 79
N2 0.4551 0.0762 0.2744
C2A 0.4993 0.1 157 0.3307
C2B 0.5622 0.1762 0.4012
H2A 0.5657 0.1326 0.4567
H2B 0.5340 0.2468 0.4153
H2C 0.6259 0.1869 0.3767
N3 0.4554 0.0802 0.0795
C3A 0.5050 0.1212 0.0266
C3B 0.5627 0.1651 -0.0449
H3A 0.6146 0.2058 -0.0171
H3B 0.5240 0.2143 -0.0817
H3C 0.5886 0.1078 -0.0837
N4 0.2669 -0.0079 0.2777
C4A 0.2058 -0.0204 0.3298
C4B 0.1324 -0.0353 0.3963
H4A 0.1274 -0.1097 0.4192
H4B 0.0731 -0.0149 0.3665
H4C 0.1447 0.0140 0.4471
N5 0.3076 0.1862 0.1793
C5A 0.2797 0.2722 0.1 757
C5B 0.2447 0.3881 0.1 704
H5A 0.2061 0.4092 0.2227
H5B 0.2071 0.3942 0.1 147
H5O 0.2998 0.4359 0.1662
N6 0.4131 -0.1203 0.1705
C6A 0.4354 -0.2096 0.1 760
C6B 0.4639 -0.3250 0.1827
H6A 0.4815 -0.3540 0.1232
H6B 0.4105 -0.3665 0.2069

H60

0.5178

-0.3309

0.2241

 

266

Table 25. continued

Atom x y 2

B1 0.1998 -0.3268 0.0009
FlA 0.1784 -0.4393 0.0026
FIB 0.1240 -0.2787 0.0410
FlC 0.2060 -0.2906 -0.0875
F1D 0.2805 -0.3064 0.0390
B2 0.5177 0.3787 0.1787
F2A 0.5782 0.4660 0.1784
F2B 0.4605 0.3776 0.2548
F2C 0.5699 0.2837 0.1736
F2D 0.4599 0.3838 0.1010
B3 0.3031 0.1647 0.8525
F3A 0.3388 0.0614 0.8681
F3B 0.2199 0.1527 0.8061
F3C 0.3663 0.2288 0.8053

F3D

0.2860

0.2064

0.9384

 

"1111111111111111

 

- - - - v