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WESW‘

llHIIIIUIHHIIWI”WilliIIUIHHWIHIHIHIIHNUI

300910 1829

This is to certify that the

dissertation entitled

Organometallic, Coordination and Redox
Chemistry of Rhodium(II) Metalloradical
Species Supported by an Oxygen
Functionalized Triaryl Phosphine

presented by

Steven Christopher Haefner

has been accepted towards fulﬁllment
of the requirements for

Ph.D. Chemistry

degree in

 

 

TWQWL.

Major professor

Date August 18, 1992

 

MS U is an Afﬁrmative Action/Equal Opportunity Institution 0-12771

 

rWuemmv
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
c:\clrc\datedue.pm&p.1

 

 

ORGANOMETALLIC, COORDINATION AND REDOX
CHEMISTRY OF RHODIUM(II) METALLORADICAL
SPECIES SUPPORTED BY AN OXYGEN
FUNCTIONALIZED TRIARYL PHOSPHINE

By

Steven Christopher Haefner

A DISSERTATION

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

DOCTOR OF PHILOSOPHY

Department of Chemistry
1992

 

ABSTRACT

ORGANOMETALLIC, COORDINATION AND REDOX CHEMISTRY

OF RHODIUM(II) METALLORADICAL

SPECIES SUPPORTED BY AN OXYGEN

FUNCTIONALIZED TRIARYL PHOSPHINE

By

Steven Christopher Haefner

The study of mononuclear rhodium chemistry has focused primarily on

 

its monovalent and trivalent oxidation states with relatively little emphasis
on the chemistry of divalent species. The scarcity of mononuclear Rh(II)
complexes is due, in part, to the proclivity of these systems to either undergo
dimerization or disproportionation, consequently few paramagnetic Rh(II)
complexes have been the subject of comprehensive studies. Recent reports of
carbon monoxide and methane activation by Rh(II) metalloradicals have
sparked renewed interest in this under developed area of chemistry.

This work focuses on Rh(II) radical chemistry of the
multifunctionalized ether phosphine ligand tris(2,4,6-trimethoxyphenyl)-
phosphine (TMPP). The unique combination of steric bulk and chelating
abilities afforded by TMPP has allowed for the stabilization of a novel
mononuclear, six coordinate Rh(II) homoleptic phosphine complex, [Rh(n3-
TMPP)2][BF4]2. The steric bulk of the ligand precludes the formation of
dinuclear species, yet the presence of labile ether interactions permits the

complex to react with a variety of small molecules. Most notably, this

— i-_ A

 

metalloradical species readily reacts with n-acceptors such as carbon
monoxide and isocyanide ligands to form adducts of the type
[Rh(TMPP)2L2]2+. In the case of CO, the complex is highly unstable and
immediately undergoes a series of redox reactions involving the formation of
Rh(I) and Rh(III) intermediates that ultimately regenerate the original Rh(II)
complex. In contrast, the reaction of [Rh(n3-TMPP)2][BF4]2 with the weaker
n-acceptor ligands CNR (R = tBu, iPr) affords the stable four-coordinate Rh(II)
adduct [Rh(TMPP)2(CNR)2][BF4]2 .

In addition to the observed substitution chemistry with n-acceptors,
[Rh(n3-TMPP)2][BF4]2 reacts with nucleophiles, resulting in dealkylation of a
bound methoxy-group to give a new Rh(II) complex ligated by one phosphine
and one phosphino-phenoxide ligand. This complex, formulated as
[Rh(TMPP)(TMPP-O)][BF4], (TMPP-0 = [P{C6H2(0Me)3}2(CGH2(OMe)2O)]1'),
reacts with carbon monoxide to form unusual paramagnetic adducts that
have been detected by IR and EPR spectroscopies.

The series of d7 phosphine complexes isolated in these studies are
among the ﬁrst mononuclear Rh(II) complexes to be fully characterized by X-
ray crystallography, EPR and a variety of other spectroscopic techniques. An
account of the syntheses, characterization and reactivity of these rare
paramagnetic species is presented together with preliminary results

regarding the extension of this work to other radical systems.

 

 

To my wife Kelly, without whose love and patience, this work would not have

been possible.

"What is essential is invisible to the eyes. It is only with the heart with which
one can see rightly. "

Antoine de Saint Expuréy

 

 

ACKNOWLEDGEMENTS

The research presented invthis dissertation would not have been
possible without the assistance and support of numerous individuals whose
direct and indirect contributions are an integral part of this work. Foremost,
I would like to thank my research adviser, Professor Kim R. Dunbar. She

taught me through example, the importance of hard work and dedication in

 

achieving your goals. Her enthusiasm and love of chemistry encouraged me
to reach within myself and strive for excellence. Above all, she impressed
upon me to always be critical of oneself and work. I will always hold her
advice and insight in the highest regard, as I have the deepest respect and
admiration for Professor Dunbar as both a scientist and a friend. I extend my
gratitude to my contemporaries in the Dunbar Group for sharing the
successes and failures that go along with being a graduate student. Together,
they made being a member of the Dunbar Group a memorable and enjoyable
experience. In particular, I would like to thank Laura Pence for her help and
friendship, especially in the early years when I was only a young pup still wet
behind the ears. She impressed upon me the importance of promoting oneself
and chemistry. Special thanks go to Anne Quillevéré; the other half of "the
match made in heaven". Her support and encouragement went along way in
helping me to achieve my goals. Although she would never admit to it, she

truly shares my enthusiasm for chemistry. I will forever value and treasure

—

 

her friendship. I also thank my long time friends Gene, Jim and Rob for
providing that often needed outlet when things did not work quite they way
they should. Furthermore, I thank my parents for their love, support and for
never expecting anything less from me. In addition, I would like to
acknowledge Michigan State University, College of Natural Science, and Dow
Chemical for providing ﬁnancial support in the form of scholarships. Finally,
I thank the numerous collaborators whom I have worked with over the past

several years for their individual contributions to this work.

TABLE OF CONTENTS

 

 

 

 

_ Page
LIST OF TABLES xvi
LIST OF FIGURES xviii
LIST OF SYMBOLS AND ABBREVIATIONS xxii
LIST OF COMPOUNDS xxv
CHAPTER I. INTRODUCTION 1

 

A. Role of Tertiary Phosphines in Transition Metal Chemistry ........ 2
B. Development of Ether-Phosphines 7
C. Chemical Aspects of Tris(2,4,6-trimethoxyphenyl)phosphine ....... 11
D. Chemistry of Mononuclear Rh(II) Complexes ................................ 18
List of References 24
CHAPTER II. STRUCTURAL AND SPECTROSCOPIC
PROPERTIES OF TRIS(2,4,6-

TRIMETHOXYPHENYL)PHOSPHINE AND
TRIS(2,4,6-TRIMETHOXYPHENYL)PHOSPHINE

 

 

 

 

OXIDE 32
1_ Intrnrlnoﬁnn 33
2. Experimental 34
A. Synthesis 34

(1) Preparation of Tris(2,4,6-
trimethoxyphenyl)phosphine (TMPP) ...................... 34

(2) Preparation of Tris(2,4,6-
trimethoxyphenyl)phosphine Oxide (TMPP=O) ...... 35

 

 

 

(3) Preparation of Ni(CO)3TMPP ................................... 36
B. X-ray Crystallography 36
(1) TMPP 36
(i) Data Collection and Reduction 36
vii

 

 

 

 

 

 

 

 

 

(ii) Structure Solution and Reﬁnement. .............. 38
(2) TMPP=O - 2 H20 39
(i) Data Collection and Reduction 39
(ii) Structure Solution and Reﬁnement ............... 39
3. Results and Discussion 40
A. Synthesis and Characterization 40
B. Molecular Structures 44
(l) TMPP 44
(2) TMPP=O - 2 H20 49
List of References 54 i

 

CHAPTER III. ISOLATION, CHARACTERIZATION AND REDOX
CHEMISTRY OF THE MONONUCLEAR Rh( II)

 

 

 

 

 

COMPLEX [Rh(n3-TMPP)2][BF4]2 ................................. 56
1. Intrndrwﬁnn 57
2. Experimental 58
A. Synthesis 58
(1) Preparation of [Rh(n3-TMPP)2][BF4]2 (3) ................. 58
(2) Preparation of [Rh(n3-TMPP)2][BF4]3 (4) ................. 59
(i) Oxidation of [Rh(n3-TMPP)2][BF4]2 with
NOBF4. 59
(ii) Oxidation of [Rh(n3-TMPP)2][BF4]2 with
[CpZFe][BF4]. 60
(iii) Oxidation of [Rh(n3-TMPP)2][BF4]2 with
HBF4 . Et20 60

 

(3) Demethylation of [Rh(n3-TMPP)2][BF4]3:
Formation of [Rh(n3-TMPP)(CBH2(OMe)2-

 

 

 

 

 

OP{C6H2(OMe)3}2)][BF4]2 (5) 60
(i) Solution 60
(ii) Solid State 61
(iii) Reaction of [Rh(n3-TMPP)2][BF4]3 (4)
with TMPP 61
(iv) Reaction of [Rh(n3-TMPP)2][BF4]3 (4)
with [(Bu”)4N][I] 61
(4) Chemical Reduction of [Rh(n3-TMPP)2][BF4]2 ....... 62
(5) Preparation of [Rh(TMPP)2][PF6]2 ........................... 62
viii

B. X-ray Crystallography ........................................................... 63
(1) [Rh(n3-TMPP)2][BF4]2 (3) ......................................... 63
(i) Data Collection and Reduction ..................... 63
(ii) Structure Solution and Reﬁnement. ............ 65
(2) [Rh(n3-TMPP)2][PF6]2[BF4] ..................................... .65
(i) Data Collection and Reduction. .................... 65
(ii) Structure Solution and Reﬁnement. ............ 66
3. Results ............................................................................................... 67
A. Preparation and Spectroscopic Properties of [Rh(n3-

TMPP)2][BF4]2 (3) ................................................................ 68

B. Magnetic and EPR Spectroscopic Properties of [Rh(n3-
TMPP)2][BF4]2 (3) ................................................................ 7 1
C. Crystal Structure of [Rh(n3-TMPP)2][BF4]2 (3) ................. 75
D. Redox properties of [Rh(n3-TMPP)2][BF4]2 (3) ................... 80
(1) Oxidation of [Rh(n3-TMPP)2][BF4]2 (3) ................... 80

(i) Synthesis and Spectroscopic
characterization of [Rh(n3-TMPP)2]-

[BF4]3 ............................................................ 80
(ii) Crystal Structure of [Rh(n3-TMPP)2]-
[PF6]2[BF4] .................................................... 84
(iii) Decomposition of [Rh(n3-TMPP)2][BF4]3
(4) ................................................................... 84
(2) Reduction of [Rh(n3-TMPP)2][BF4]2 (3) ................... 92
4. Discussion .......................................................................................... 94
List of References ................................................................................... 96
CHAPTER IV REVERSIBLE CARBON MONOXIDE
CHEMISTRY OF [Rh(n3-TMPP)2][BF4]2 ...................... 100
1. Introduction ....................................................................................... 101
2. Experimental .................................................................................... 102
A. Synthesis ................................................................................ 102
(1) Reaction of [Rh(n3-TMPP)2][BF4]2 (3) with CO. ..... 103
(2) Reaction of [Rh(n3-TMPP)2][BF4]2 (3) with
1200/1300 (1:1). ....................................................... 103

(3) Preparation of [Rh(n2-TMPP)(TMPP)CO][BF4]

(7 ) ................................................................................ 104
(i) Reduction of [Rh(n3-TMPP)2][BF4]2 1n
the Presence of CO. ....................................... 104
(ii) Reaction of [Rh(cod)Cl]2 with TMPP in
the Presence of CO. ....................................... 104
(iii) Reaction of [Rh(CO)2Cl]2 with TMPP ......... 105
(4) Preparation of [Rh(TMPP)2(CO)2][BF4] (6) ............ 105
(5) Solid State Reactions of (3) - (7) with CO. ................ 106
(6) Redox Reaction of [Rh(nZ-TMPP)(TMPP)CO]-
[BF4] (7) with [Rh(n3-TMPP)2][BF4]3 (4). .............. .106
(7) Preparation of [Rh(TMPP)2][BF4] (3) from
[Rh(TMPP)2CO][BF4] (7) .......................................... 107
(8) Reaction of [Rh(TMPP)2CO][BF4] (7) with
small molecules .......................................................... 107
(i) N2, 02, C02, H2. ........................................... 107
(ii) CNR ................................................................ 107
(iii) pyridine. ......................................................... 108
B. X-ray Crystallography ........................................................... 108
(1) [Rh(TMPP)2(CO)2][BF4] oCHzClz (6). ..................... 109
(i) Data Collection and Reduction. .................... 109
(ii) Structure Solution and Reﬁnement. ............ 109
(2) [Rh(nZ-TMPP)(TMPP)CO][BF4Jo2C6H5 (7). ............ .109
(i) Data Collection and Reduction. .................... 109
(ii) Structure Solution and Reﬁnement. ............ 111
3. Results ............................................................................................... 111
A. Reactivity of [Rh(n3-TMPP)2][BF4]2 (3) with Carbon
Monoxide. .............................................................................. 111
B. NMR Spectroscopic Studies. ................................................. 112
C. Electrochemistry of [Rh(nz-TMPP)(TMPP)CO][BF4] (7)
and [Rh(TMPP)2(CO)2][BF4] (6). ...................................... 114
D. Crystal Structures of [Rh(TMPP)2(CO)n][BF4] (n=1,2) ...... 114
(1) [Rh(nZ-TMPP) (TMPP)CO][BF4] (7 ). ........................ 116
(2) [Rh(TMPP)2(CO)2][BF4] (6). .................................... 116
E. Solid State Reactions of (3) - (7 ) with Carbon
Monoxide. .............................................................................. 120
4. Discussion .......................................................................................... 123

List of References ................................................................................... 131

CHAPTER V REVERSIBLE CARBON MON OXIDE ADDITION
TO SOL-GEL DERIVED COMPOSITE FILMS
CONTAINING THE MOLECULE

[Rh(TMPP)2(CO)]1+ ......................................................... 133
1. Introduction ....................................................................................... 134
2. Experimental .................................................................................... 136
A. Preparation of Composite Films Containing
[Rh(TMPP)2(CO)][BF4] (7) ................................................. 136
(1) Zirconia Films ............................................................ 136
(2) Titania Films .............................................................. 136
B. Flow Rate Determination ...................................................... 136
C. Electrochemical Measurements ............................................ 138
3. Results ............................................................................................... 138
4. Discussion .......................................................................................... 144
List of References ................................................................................... 145
CHAPTER VI CHEMISTRY OF [RhH(n3-TMPP)2][BF4]2 WITH
ISOCYANIDE LIGANDS ................................................ 146
1. Introduction ....................................................................................... 147
2. Experimental .................................................................................... 147
A. Synthesis ................................................................................ 147
(1) Preparation of [Rh(TMPP)2(CNBut)2][BF4]2
(9) ................................................................................ 148
(2) Preparation of [Rh(TMPP)2(CNPri)2][BF4]2
(10) .............................................................................. 148
(3) Reaction of [Rh(n3—TMPP)2][BF4]2 (3) with
other isocyanides ....................................................... 149
(i) Methyl isocyanide .......................................... 149
(a) one equivalent. .............................................. 149
(b) ﬁve equivalents. ............................................. 149
(ii) Cyclohexyl isocyanide. .................................. 150
(iii) n-Butyl isocyanide. ........................................ 150
(4) Reactions of [Rh(TMPP)2(CNBut)2][BF4]2 (9) ......... 150
(i) Cobaltocene: Preparation of
[Rh(TMPP)2(CNBut)2][BF4] (ll) .................. 150

(ii) TMPP ............................................................. 151
(iii) tert-Butyl isocyanide. .................................... 151
(iv) Carbon monoxide. .......................................... 152
B. X-ray Crystallography ........................................................... 152
(1) [Rh(TMPP)2(CNBut)2][BPh4]2 (9) ............................ 152
(i) Data Collection and Reduction. .................... 152
(ii) Structure Solution and Reﬁnement. ............ 154
3‘. Results ............................................................................................... 154
A. Synthesis and Spectroscopic Characterization of
[Rh(TMPP)2(CNR)2][BF4]2 (R = But, Prl) ........................... 154
B. Magnetic and EPR Spectroscopic Properties of
[Rh(TMPP)2(CNR)2][BF4]2 (R = But, Pri) ........................... 157
C. Redox Chemistry of [Rh(TMPP)2(CNR)2][BF4]2 (R =
But, Pri) ................................................................................. 158
D. X-ray Crystal Structure of
[Rh(TMPP)2(CNBut)2][BPh4] .............................................. 163
4. Discussion .......................................................................................... 166
List of References ................................................................................... 169

CHAPTER VII CHEMISTRY OF Rh(II) AND Rh(III)
COMPLEXES LIGATED BY PHOSPHINO-

PHEN OXIDE LIGANDS ................................................. 172
1. Introduction ....................................................................................... 173
2. Experimental .................................................................................... 173
A. Preparation of eq-[RhH(TMPP)(TMPP-O)][BF4] (12) .......... 174

(1) Reactions of [Rh(n3-TMPP)2][BF4]2 (3) with
nucleophiles ............................................................... 17 4
(i) TMPP. ............................................................ 17 4
(ii) Tetra-n-butyl ammonium cyanide. ............... 17 4
(iii) Tricyclohexyl phosphine. .............................. 175

(2) Reduction ofax-[Rhmm3-TMPPXTI3-TMPP-
0)][BF4]2 (5) ............................................................... 175

B. Reaction of [Rh(n3-TMPP)2][BF4]2 (3) with excess
TMPP .................................................................................... 175

 

C. Oxidation of eq-[RhH(TMPP)(TMPP-0)][BF4] (12);
Synthesis of eq-[RhIH(n3-TMPP)(113-TMPP-0)][BF4J2

(l3) ......................................................................................... 176
D. Reaction of eq-[RhH(TMPP)(TMPP-0)][BF4] (12) with
CO .......................................................................................... 176
(1) Infrared study. ........................................................... 176
(2) NMR study. ................................................................ 177

E. Dealkylation ofax-[Rh111(n3-TMPP)(n3-TMPP-O)]
[BF4]2 (5): Formation of ax,eq- and ax,ax-[Rh(n3-

TMPP-0)2][BF4] (l4) and (15) ............................................. 17 7
(1) TMPP .......................................................................... 177
(i) Bulk reaction. ................................................ 17 7
(ii) NMR reaction. ............................................... 178
(2) Tetra-n-butyl ammonium iodide ................................ 17 8
F. Reactions of ax-[Rhm(n3—TMPP)(n3-TMPP-0)]
[BF412 (5) .............................................................................. 178
(1) HBF4oEt20 ................................................................ 178
(2) CO ............................................................................... 179
(3) H2 ................................................................................ 179
(4) MeOH ......................................................................... 179
3. Results ............................................................................................... 180
A. Synthesis and Characterization of eq-
[RhH(TMPP)(TMPP-0)][BF4] (12) .................................... 180
B. Redox behavior of eq-[RhH(TMPP)(TMPP-0)][BF4]
(l2) ......................................................................................... 182
C. Reactivity of ax-[Rhmm3-TMPP)(Ti3-TMPP-0)][BF4J2
(5) .......................................................................................... 185
D. Chemistry of eq-[RhH(TMPP)(TMPP-0)][BF4] (12)
with CO ................................................................................. 189
4. Discussion .......................................................................................... 192
List of References ................................................................................... 197

CHAPTER VIII CHEMISTRY OF TRIS(2,4,6-
TRIMETHOXYPHENYL)PHOSPHINE WITH
RHODIUM (I) AND IRIDIUM (I) OLEFIN
COMPLEXES ................................................................... 198

xiii

 

 

1. Introduction 199

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2. Experimental 199
A. Synthesis 199
(1) Preparation of [Rh(cod)(n2-TMPP)][BF4] (16) .......... 200
(2) Preparation of Rh(cod)(n2-TMPP-O) (l7) .................. 201
(3) Reaction of [Rh(C2H4)2Cl]2 with TMPP ................... 201
(4) Preparation of Ir(cod)(n2-TMPP-0) (18) ................... 202
(5) Preparation of [Ir(TMPP)2(CO)2][BF4] (l9) ............. 202
(6) Reaction of TMPP with [Ir(TMPP)2(CO)2][BF4]
(19) 203
(7) Oxidation of [Ir(TMPP)2(CO)2][BF4] (19) ................. 204
(i) Chemical oxidation with NOBF4 .................. 204
(ii) Chemical Oxidation with [(p-
BX'CGH4)3N][BF4]. 204
(iii) Bulk Electrolysis. 205
(8) Reaction of [Ir(TMPP)2(CO)2][BF4] (19) with
Iodine 205
B. X-ray Crystallography 207
(1) [Ir(TMPP)2(CO)2][BF4] (19) - CH2012 ..................... 207
(i) Data Collection and Reduction. .................... 207
(ii) Structure Solution and Reﬁnement. ............ 209
3. Results 209
A. Synthesis and Characterization of [M(cod)(n2-
TMPP)]1+ (M = Rh) and M(cod)(n2-TMPP-0) (M = Rh,
Ir) 209
B. Synthesis and Spectroscopic Characterization of
[Ir(TMPP)2(CO)2][BF4] (19) 213
C. Crystal Structure of [Ir(TMPP)2(CO)2][BF4] (19) ............... 215
D. Oxidation of [Ir(TMPP)2(CO)2][BF4] ................................... 215
4. Dionnum'nn 220
List of References 223
CHAPTER IX CONCLUSION 225
List of References 232
APPENDIX A PHYSICAL MEASUREMENTS 233

 

APPENDIX B CHEMISTRY OF TRIS(2,4,6-
TRIMETHOXYPHENYL)PHOSPHIN E WITH

 

1_ Intrnrlnr‘finn

 

2. Experimental

 

GROUP VI METALS 235

23.5

236

A. Synthesis 236

 

(1) Preparation of (n3-TMPP)MO(CO)3 (20) ................
(2) Reaction of TMPP with (n6-C7H3)W(CO)3 ............
(3) Reaction of TMPP with MoCl3(THF)3 ...................
(4) Reaction of TMPP with M02Cl4(MeCN)4 ..............

(i) 4 equivalents.

 

(ii) 6 equivalents.

 

 

B. X-ray Crystallography

(1) (n3-TMPP)M0(CO)3 (20) . CH2012 ........................
(i) Data Collection and Reduction. .................
(ii) Structure Solution and Reﬁnement. .........

3. Results

 

A. Synthesis and Characterization of (n3-TMPP)Mo(CO)3
(20)

 

B. Molecular Structure of (n3-TMPP)M0(CO)3 (20) ..............

C. Reaction of TMPP with (116-C7H8)W(CO)3 .......................

D. Reaction of TMPP with Solvated Molybdenum
Chloride Complexes

 

4_ Di cmrqeinn

List of References

 

APPENDIX C COMPILATION OF 31P NMR SPECTRAL DATA .....

236
237

237
237
238

238
238

238
240

240

240

244

247

247

248
25 1

...252

 

 

 

10.

11.

12.

LIST OF TABLES

Page
pKa values for methoxy substituted triphenylphosphines .................. 14
Summary of Crystallographic Data for TMPP (1) and TMPP=O -
2H20 (2) ................................................................................................. 37
Selected bond distances (A) and angles (deg) for TMPP (l). ............... 47
Dihedral angles (deg) between the planes of the aryl rings and
the plane described by Cipso for TMPP (l) and TMPP=O . 2H20
(2). ........................................................................................................... 48
Selected bond distances (A) and angles (deg) for TMPP=O .
2H20 (2). ................................................................................................ 52
Summary of crystallographic data for [RhH(n3-TMPP)2][BF4]2
(3) and [RhHI(n3-TMPP)2][BF4][PF6]2 (4a). ........................................ 64
EPR Spectroscopic and Magnetic Susceptibility Data for [Rh(n3-
TMPP)2][BF4]2 (3) ................................................................................. 74
Selected bond distances (A) and angles (deg) for [RhH(n3-
TMPP)2][BF4]2 (3). ................................................................................ 79
Selected bond distances (A) and angles (deg) for [Rhm(n3-
TMPP)2][PF6]2[BF4] . ........................................................................... 86
Summary of crystallographic data for [Rh(TMPP)2(CO)2][BF4] .
CH2012 (6) and [Rh(TMPP)2(CO)]lBF4] ° ZCGHG (7). .......................... 110
Selected bond distances (A) and angles (deg) for
[Rh(TMPP)2(CO)][BF4] . ZCGHG (7). .................................................... 119
Selected bond distances (A) and angles (deg) for
[Rh(TMPP)2(CO)2][BF4] . CH2C12 (6). ................................................ 122

13.

14.

15.

16.

17.

18.

19.

Summary of crystallographic data for [RhH(TMPP)2(CNBut)2]

[BPh4]2 (9a). .......................................................................................... 153
Selected bond distances (A) and angles (deg) for
[RhH(TMPP)2(CNBut)2][BPh4]2 (9a). ................................................... 165
Summary of crystallographic data for [IrI(TMPP)2(CO)2][BF4] .
CH2012 (19) ............................................................................................ 208
Selected bond distances (A) and angles (deg) for
[IrI(TMPP)2(CO)2][BF4] - CH2C12 (l9). ............................................... 217
Summary of crystallographic data for (n3-TMPP)Mo(CO)3 .

CH2C12 (20). ........................................................................................... 239
Selected bond distances (A) and angles (deg) for (n3-TMPP)-

M0(CO)3 (20). ......................................................................................... 246
Compilation of 31P NMR spectral data ................................................ 252

xvii

10.

11.

12.

13.

LIST OF FIGURES
Page

Schematic drawing of tris(2,4,6-trimethoxyphenyl)phosphine ........... 12
Tolman cone angles for various tertiary phosphines. .......................... 15
111, n2 and n3 bonding modes of TMPP. ................................................ 17
Plot of the cone angle versus the v(CO)A1 stretch for various
Ni(CO)3(PR3) complexes. ...................................................................... 43
ORTEP representations of the two crystallographically
independent molecules of TMPP. .......................................................... 46
ORTEP diagram of TMPP=O . 2H20. .................................................. 50
Packing diagram of TMPP=O . 2H20. ................................................. 51
EPR spectrum of [Rh(n3-TMPP)2][BF4]2 (3) in a CHgClz/Me-
THF glass at 77 K. ................................................................................. 72
ORTEP representation of the [Rh(n3-TMPP)2]2+ (3) molecular
cation with 40% probability ellipsoids. Phenyl ring atoms are
Shown as small spheres of arbitrary size for clarity. ........................... 76
ORTEP drawing emphasizing the coordination sphere of [Rh(n3-
TMPP)2]2+. ............................................................................................. 77
Cyclic voltammogram of [RhII(n3-TMPP)2][BF4]2 (3) in 0.1 M
TBABF4 in CH2C12. ............................................................................... 81
Cyclic voltammogram of [RhHI(n3-TMPP)2][BF4]3 (4) in 0.1 M
TBABF4 in Cchlz. ............................................................................... 83
ORTEP drawing of the [RhIH(n3-TMPP)2]3+ (4) molecular cation.
Phenyl ring atoms are shown as small spheres of arbitrary size
for clarity. ............................................................................................... 85

14.

15.

16.

17.

18.

19.

20.

21.
22.

23.

24.

25.

26.

27.

1H and 31P NMR spectra of ax-[Rhm(TMPP)(TMPP-0)][BF4]2 in

 

CD2012.

31P NMR spectrum of the product from the reduction of [Rhn(n3-
TMPP)2][BF4]2 (3) with cobaltocene

 

Cyclic voltammograms in 0.1 M TBABF4/CH2C12 for (a)
[RhIII(n3-TMPP)2][BF4]3 (4), (b) [Rh(TMPP)2(CO)2][BF4] (6), (c)
[Rh(TMPP)2(CO)][BF4] (7).

 

ORTEP representation of [Rh(TMPP)(n2-TMPP)(CO)]1+ (7 ). .............

PLUTO drawing of [Rh(TMPP)(n2-TMPP)(CO)]1+ (7)
emphasizing the coordination geometry about the Rh atom. ..............

ORTEP drawing of [Rh(TMPP)2(CO)2]1+ (6) with 30%
probability ellipsoids.

 

Proposed pathway for the reversible reaction between [Rh11(n3-
TMPP)2]2+ (3) and CO.

 

High yield synthetic routes to compounds 3-7. ....................................

Schematic diagram of the apparatus used to estimate CO
concentrations.

 

Infrared spectra of a zirconia composite ﬁlm of
[Rh(TMPP)2(CO)][BF4] (7): (a) in the absence of CO. (b) exposed

to CO.

Electronic absorption spectra of (a) [Rh(TMPP)2(CO)][BF4] and
(b) [Rh(TMPP)2(CO)2][BF4] in: (I) CH2C12. (II) a zirconia
composite ﬁlm. '

 

 

Cyclic voltammograms of a zirconia composite ﬁlm containing
[Rh(TMPP)2(CO)][BF4] (7) and LiCF3SO3 cast onto a platinum
disk electrode: (a) under a N2 atmosphere. (b) after purging the
cell for 30 seconds with CO. (c)-(e) after ﬂushing the CO
saturated cell with N2 for 2, 5 and 10 min.

 

EPR spectrum of [RhH(TMPP)2(CNBut)2][BF4]2 (9) in the solid-
state at 100 K.

 

EPR spectrum of [RhH(TMPP)2(CNBut)2][BF4]2 (9) in a
CHZClZ/Me-THF glass at 100 K.

 

88

93

115

117

118

121

124

125

137

139

141

143

159

160

 

 

28.

29.

30.

31.

32.

33.

34.

35.

36.

37.

38.

39.

40.

41.

EPR spectrum of [RhH(TMPP)2(CNPri)2][BF4]2 (10) in a
CH2C12/Me-THF glass at 100 K. ........................................................... 161

ORTEP representation of the molecular cation
[Rh(TMPP)2(CNBut)2]2+ in structure (9). Thermal ellipsoids are

Shown at the 40% probability level. Carbon and oxygen atoms of
the phosphine ligand are shown as small spheres of arbitrary
Size for clarity ......................................................................................... 164

EPR Spectrum of eq-[RhII(TMPP)(TMPP-0)][BF4] (12) at 100 K
in a 1:1 CH2C12/Me-THF glass .............................................................. 181

Cyclic voltammograms of ax-[RhHI(n3-TMPP)(n3-TMPP-
0)][BF412 (5) and eq-[RhH(TMPP)(TMPP-O)][BF4] (12) in 0.1 M
TBABF4/CH2012. ................................................................................... 183

Possible dealkylation products resulting from nucleophilic attack
on ax-[Rhm(n3-TMPP)(T]3-TMPP-0)][BF4]2 (5). ................................. 187

EPR spectrum of a frozen 1:1 CH2C12/Me-THF solution of eq-
[RhII(TMPP)(TMPP-O)][BF4] (12) exposed to CO. .............................. 191

Reaction scheme depicting the relationship between the various
isomers of dealkylated and non-dealkylated complexes 3, 4, 5, 12

and 13. .................................................................................................... 194
Schematic drawing of the electrolysis cell used in the

electrochemical oxidation of [IrI(TMPP)2(CO)2]1+ (l9). ...................... 206
ORTEP representation of the molecular cation

[IrI(TMPP)2(CO)2]1+ (19). ...................................................................... 216
Synthetic pathways to Rh-TMPP complexes ....................................... 227
n1 - n3 bonding modes for TMPP and [TMPP-01 ................................ 228
Variable temperature 1H NMR spectra of (n3-TMPP)M0(CO)3

(20) in d8-toluene. .................................................................................. 242
Variable temperature 1H NMR Spectra of (n3-TMPP)M0(CO)3

(20) in CD2C12. ....................................................................................... 243
ORTEP representation of (n3-TMPP)Mo(CO)3 (20) ............................. 245

42.

 

Proposed pathway for intramolecular exchange in (113-
TMPP)M0(CO)3 (20).

 

249

Ag/AgCl

br
But
ca.

cm-1
cod

esd

EtOH

tram

LIST OF SYMBOLS AND ABBREVIATIONS

Angstrtim

Silver-silver chloride reference electrode
aryl group

broad

tert-butyl

circa, about

centimeter

wavenumber

1,5-cyclooctadiene

cyclic voltammetry

cyclohexyl

degree centigrade

doublet (NMR), day, deuterated
parts per million (ppm)
bis(dicyclohexylphosphino)ethane
doublet of doublets

anodic peak potential

cathodic peak potential

electron paramagnetic resonance
electromagnetic unit

estimated standard deviation
ethyl

ethanol

molar extinction coefﬁcient

Fast Atom Bombardment

epr g-value, gram

Gauss

hour

xxii

 

TMPP-H
(M)Hz

NBA
NMR
ox
PCy3
PPh3
P(mes)3
Pp_m
Prl

psi

red

r.t.

s

sh
SQUID

 

tris(2,4,6-trimethoxyphenyl)phosphonium
(Mega)Hertz

infrared

Kelvin

wavelength

medium

moles per liter

methyl

acetonitrile

methanol

milligram

minute

milliliter

millimole

multiplet

bridging ligand

Bohr magneton

microliter

nanometer

frequency

3-nitrobenzyl alcohol
nuclear magnetic resonance
oxidation
tricyclohexylphosphine
triphenylphosphine
trimesitylphosphine

parts per million
isopropyl

pounds per square inch
reduction

room temperature

singlet (NMR), strong (IR)
shoulder
Superconducting Quantum Interference
Device

triplet

TBABF4
THF

TMPP
TMPP=O
TMPP-0
TMPP-CH3

TMPP-CH2CI

tetra-n-butylammonium tetraﬂuoroborate
tetrahydrofuran
tris(2,4,6-trimethoxyphenyl)phosphine
triS(2,4,6—trimethoxyphenyl)phosphine oxide
P[{C6H2(CH3O)3}2{C6H2(CH3O>2O}]'
tris(2,4,6-trimethoxyphenyl)methyl
phosphonium
tris(2,4,6-trimethoxyphenyl)chloromethyl
phosphonium

tetramethylsilane

ultraviolet

Volt

versus, very strong

weak

halide ligand

xxiv

 

LIST OF COMPOUNDS

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(l) TMPP

(2) TMPP=O

(’3) [Rh11(n3-TMPP)2][BF4]2

(4) [RhHI(n3-TMPP)2][BF4]3

(5) ax-[Rh111(n3-TMPP)(113-TMPP—O)][BF4]2
(6) [RhI(TMPP)2(CO)2][BF4]

(7) [RhI(TMPP)2(CO)l[BF4]

(8) [RhI(TMPP)2(CO)(CNBut)] [BF 4]

(9) [Rh11(TMPP)2(CNBut)2][BF4]2

(10) [RhH(TMPP)2(CNPri)2][BF412

(11> [RhI(TMPP)2(CNBut)2][BF4]

(12) eq-[RhH(TMPP)(TMPP-0)][BF4]

(13) eq-[RhHI(n3-TMPP)(113-TMPP-0)][BF4]2
(14) ax,eq-[Rhm(n3-TMPP-0)2][BF4]

(15) ax,ax-[Rhm(n3-TMPP-0)2][BF4]

(16) [Rh1(cod)(n2-TMPP)][BF4]

(17) Rh1(cod)(n2-TMPP-0)

(18) IrI(cod)(’n2-TMPP-O)

(l9) [IrI(TMPP)2(CO)2][BF4]

(20) (n3-TMPP)M00(CO)3

CHAPTER I

INTRODUCTION

2

A. Role of Tertiary Phosphines in Transition Metal Chemistry

The use of tertiary phosphines as ancillary ligands has played a
prominent role in the development of modern coordination and organo-
metallic chemistry. This statement is most evident from the observation that
phosphine complexes are known for virtually all transition metals.1 These
versatile ligands are capable of stabilizing a variety of metals in a range of
oxidation states. Furthermore, transition metal-phosphine complexes,

particularly those of the later transition metals, catalyze a number of

 

industrially important organic processes.2 These range from oleﬁn
hydrogenation, hydroformylation, hydrosilation and hydrocyanation to
polymerization and oligomerization of oleﬁns and acetylenes. In addition to
catalytic processes, transition metal phosphine complexes are also capable of
performing a number of stoichiometric organic transformations.3 An
important aspect in the use of transition metal-phosphine complexes in
catalysis has been the potential for controlling catalyst activity and
selectivity by modifying the substituent groups of the phosphine. By varying
the R groups of a phosphine, both the electronic and steric properties of the
ligand may be changed. This in turn will affect the reactivity properties of
the metal to which the phosphine is bound. It is important to realize,
however, that steric and electronic effects are not independent of each
another; often, substitution of one of the R substituents results not only in a
change in the steric properties of ligands, but also the electronic properties as
well. For instance as the size of a phosphine is increased, the C-P-C angle is
forced to expand which affects the percentage of S-character in the
phosphorus lone pair and hence the phosphine donor properties.
Furthermore, sterically hindering substituents also affect the ability of the

phosphine to interact strongly with the metal center and therefore inﬂuence

__——_—'—___4

 

3
the metal phosphorus bond strength. As a consequence of this
interrelationship, there is considerable interest in quantiﬁng and separating
the factors that inﬂuence transition metal-phosphorus bonding.

The electronic properties of a particular phosphine molecule are
dominated by the donor capacity of the phosphorus lone pair of electrons
Which is quite sensitive to the nature of the phosphine substituents, or R
groups. By altering the R groups, and therefore the donor ability of the
phosphine, the electron density at the metal center may be tuned in order to
increase or decrease the reactivity of the complex.

The electronic donating properties of phosphine ligands are typically
divided into C and 1: contributions.4 Generally, the 6 contribution dominates
and 1: contributions are only evident for phosphines with electronegative
substituents such as -F, -OR, -Cl and -OAr. The n-accepting behavior of
phosphines has been debated for many years. Conventional wisdom has
traditionally maintained that the n-acceptor capability is facilitated through
back donation from ﬁlled metal d-orbitals into empty low-lying phosphorus-
3d orbitals,5 but, recent theoretical calculations suggest that the frontier
orbitals responsible for n-interactions consist primarily of phosphorus 3p
character having local 6* symmetry with no involvement of phosphorus-3d
orbitals.6 Highly electronegative substituents, such as -F, Cl, and OR, result
in a highly polar P-X bond, which in turn, lowers the energy of the 6* orbitals,
making them more accessible for metal Tr-donation.

Unfortunately, it is difﬁcult to separate 6 and 1: contribution due to
their inherent synergistic relationship.7 Nonetheless, the importance of
understanding metal-phosphine bonding has prompted the search for
methods to quantify the individual contributions. Early methods of

measuring phosphine donor strength were performed by measuring the

 

4

energy of the A1 carbonyl stretching vibration for the monosubstituted Ni
carbonyl complex, Ni(CO)3PR3.8 The energy of this band provides an
indication of the total donor ability (0' + 1:) of the phosphine. The more
strongly donating a phosphine, the more electron density resides at the metal
center. Consequently, the degree of n-accepting of the carbonyl groups will
increase, resulting in a lowering of the carbonyl stretching frequencies.
Therefore, strongly donating phosphines will result in lower values of v(CO)
for Ni(CO)3L.

Another commonly used measure of phosphine donor strength is pKa.9
Values of pKa for the conjugate acid R3PH+ provide an indication of the o
donor strength of the phosphine. However, it is often argued that these
values are not a good measure of ligand donor strength because pKa is a
measure of the phosphine's afﬁnity for a hard acid and not a soft transition
metal. Furthermore, variations in phosphine size may result in differing
solvation energies that will effect the pKa. In spite of these considerations,
pKa values have been found to correlate well with other experimental
observations regarding phosphine basicity.1°aib

Others have sought more quantitative methods to measure ligand 6
and 1: effects; these have met with mixed success.11 In particular, Giering et
al. have divided phosphine ligands into three classes based upon the
correlation of oxidation potential for the phosphine complex 11 5-

MeCpMn(CO)2PR3 and phosphine pKa values:113

1 s I (cs-donor / n—donor)

PR3 (R = Et, Bu, Cy, etc.)

1 II (cs-donor only)

 

5
PR3 (R = Me), PPh3_n(R)n (n = 1, 2; R = Me, Et, Bu)
P(p-X-Ph)3 (X = H, Me, OMe)

Class III (cs-donor / n-acceptor)
P(OR)3 R = Et, Ph, Pri, Me
P(p-X-Ph)3 (X = F, Cl)

Similar correlations were found between pKa, on°, vco, cone angle and heats
of reactions for other systemslla'f Although Giering classiﬁed phosphine
ligands in terms of rt-acidity and n-basicity, it is important to stress that
these n-effects are relatively small compared to those observed for true 1:-
acids and bases such as CO or Cl'. Furthermore, the classiﬁcation system is
speciﬁc to the particular system under investigation, although in practice,
these class boundries differ only slightly.

In addition to varying the electronic properties of the phosphine,
substitution of the R groups often affects the size of the ligand. Altering the
size can have major ramiﬁcations on the subsequent chemistry of the
phosphine, as in the case of phosphines with unusually large steric
requirements, such as tricyclohexylphosphine, tri-tert-butylphosphine, tri-o-
tolylphosphine and trimesitylphosphine; these ligands are capable of
kinetically stabilizing metal complexes with low coordination numbers,
thereby engendering electronically unsaturated metal centers that are highly
reactive.12 For example, bulky phosphines have been used to isolate a series
of two-coordinate, 14-electron complexes of Pt0 and Pd0.13 Although these
systems are formally electron deﬁcient, their reactivity is governed by the

degree of steric overcrowding that is present.14

 

6

Another example of the use of bulky phosphines to stabilize
electronically unsaturated metal centers is provided by the recent work of
Kubas and co-workers who used sterically hindering phosphines to create
coordinatively unsaturated Mo0 and W0 complexes with the general formula
trans-M(CO)3(PR3)2 (R = cyclohexyl, iso-propyl).15 These 5-coordinate
complexes are stabilized by an "agostic" interaction of a C-H bond on one of
the phosphines with the metal center. This interaction is readily displaced in
favor of small substrates including N2 and H2. One consequence of steric
crowding is that these complexes tend to undergo intramolecular C-H bond
activation to form metallated phosphine complexes.16,17

Whitesides and co-workers have used steric effects to generate a highly
reactive "Pt0L2" fragment in situ by reductive elimination of neopentane from
cis-PtII(dCype)H(CH2C(CH3)3) (dCype = bis(dicyclohexylphosphino)ethane,
(Cy)2P(CH2)2P(Cy)2) (eq 1)18 14-electron complex activates a variety of aryl

Cy.P.Cy">1’ A cy‘ ,Cy
\ R
pt ————> Pt° + CH3 (1)

and aliphatic C-H bonds, but does not undergo intramolecular C-H activation,
due, presumably, to the bent conﬁguration of the dCype moiety, which
precludes interaction of the C-H bonds of the cyclohexyl groups and affords
the substrate better access to the metal center.18

The steric interactions of phosphines is also implicated in the activity
of many homogenous catalysts by facilitating ligand dissociation that opens
up coordination Sites for an incoming substrate. Mechanistic studies of

Wilkinson's catalyst, Rh(PPh3)Cl, have shown that initial dissociation of

 

 

 

7
phosphine is a key step in oleﬁn hydrogenation.19 The size of the ancillary
ligands also imparts selectivity to a number of reactions, a factor that is
particularly important in the development of homogeneous catalysts for
asymmetric synthesis.

Although it is certainly true that both steric and electronic properties
of phosphine play a role in the reactivity and catalytic behavior of transition
metal phosphine complexes, steric effects tend to dominate. In order to
address the size of a phosphine ligand, the concept of cone angle, (9, was
developed by Tolman.20 This method, which is based on rigid space ﬁlling
models, deﬁnes the cone angle of a phosphine as the apex angle of a solid
cone, centered 2.28 A from the phosphorus atom, that encompasses the
phosphine substituents atoms at their van der waal radii. Cone angles thus
described have been found to correlate well with a number of spectroscopic
and physical properties of transition metal phosphine complexes.21 A more
realistic description of ligand size was provided by the development of cone
angle proﬁles.22 This method, based on X-ray crystallographic data,
accounted for the clefts and intermeshing of the phosphine substituents and
gave a better indication of the true ligand requirements.

B. Development of Ether-Phosphines

An important aspect in any homogenous catalytic process is the
availability of accessible coordination sites in which an incoming substrate
molecule may bind.23 Typical catalysts employing tertiary phosphine
complexes rely on the steric bulk of the phosphine to promote ligand
dissociation in order to create a coordinately unsaturated metal center.
Another strategy is to incorporate weak donor atoms (e.g. solvent molecules)
into the metal coordination sphere that can be easily displaced in favor of the

incoming substrate. For example, recent work with zirconium

 

 

 

 

 

8
cyclopentadienyl systems has shown that replacement of a chloride ion with
THF in the coordination sphere creates a highly reactive cationic metal
center, capable of performing a variety of organic transformations.24 A
number of compounds that incorporate solvent into their coordination sphere
have been found to be quite effective hydrogenation, hydroformylation and
polymerization catalysts.25 Furthermore, although many catalyst precursors
use oleﬁns as supporting ligands, under catalytic conditions, the oleﬁns are

hydrogenated and replaced by solvent to form the active catalyst (eq 2).26

 

1
R3P\ /\—_| + solv R3P\ ,SOlV—l +1
M

p + 2 H2 —> ’M\ (2)
12,3]?I \/ R3P SOIV

An alternative approach to the design of a new catalyst with open
coordination sites is to structurally modify the ancillary phosphine ligands by
incorporating weak donor groups directly into the phosphine. A variety of
functional groups that are capable of acting as weak donors have been
combined with phosphines to form polydentate ligands;27 these provide
additional stabilization to the complex by chelating to the metal center, but
because of their weak donor nature these groups are easily displaced in favor
of the incoming substrate. After the transformations are complete and the
product dissociates, the tethered donor atom is able to quickly reassociate.28
As a result of this "arm on/arm off' behavior, these functionalized ligands
have been described as being hemi-labile.29 A further advantage of a
tethered donor group, particularly an oxygen donor atom, is the observed rate
enhancement for oxidative addition reactions, which are well recognized as

important steps in many catalytic processes.30 These reactions are facilitated

9
by the presence of the oxygen atom, which provides anchimeric assistance to
the oxidative addition of polar substrates.

The successful implementation of phosphine ligands bearing oxygen
donor substituents to catalysis has been demostrated in a number of
important reactions.27 For example, Shells Higher Oleﬁn Process (SHOP)
uses a nickel complex ligated by the phosphino-carboxylate ligand,
thPCH2C02H, to selectively polymerize ethylene in the presence of other
olefins.31 Further work in this area was carried out by Knowles and co-*
workers who developed a series of chiral phosphines (shown below) that
incorporate ortho substituted phenyl groups.32 Cationic rhodium complexes
supported by these chiral ligands successfully hydrogenate pro-chiral oleﬁns,
an important step in the synthesis of cr-amino acids, achieving enantiomeric

excesses > 95%. The asymmetric hydrogenation of

eMePQ
cm; «at; o no

MeO

pamp camp dipamp .

(N-acylamino)-cinnamic acid catalyzed by the diphosphine complex
[Rh(cod)(diPAMP)]1+ (diPAMP = (R,R)-1,2-bis[2-methoxyphenyl)phenyl-
phosphino]ethane), a key step in Monsanto's L-Dopa synthesis, represents
the ﬁrst commercialized catalytic asymmetric process.33 The development of
these catalysts ignited a revolution in the area of asymmetric hydrogenation
promoted by transition metal complexes. Although the role of the ortho-
methoxy group is primarily steric in nature, replacement of the methoxy

substituent with sterically equivalent groups resulted in lower catalytic

 

10
activity, suggesting that the methoxy groups provide an electronic inﬂuence
as well.34

Although a variety of donor groups, ranging from amines and
thioethers to enolates and diketonates, have been used as functional groups
on phosphine ligands, ether donors are the most prevalent.27a Being
uncharged, ethers are very weak donors, which renders them highly highly
labile, particularly for the soft late transition metals, with which they are
relatively incompatible.

The ﬁrst crystallographic evidence for the coordinating ability of a
pendent ether group was reported for the Rh(III) arsine complex
RhCl3[MeAs(o-CGH4(OMe)]2.35 Later in the mid 1970's, research on the
chemistry of ether-phosphines expanded due to the independent work of
Roundhill, Rauchfuss and Shaw.36 These groups were able to show that
ortho-substituted phenylphosphines were capable of binding a variety of late
transition metals through both the oxygen and phosphorus atoms. Rauchfuss
found that the coordinated methoxy groups were loosely bound and could
dissociate readily, thus generating vacant coordination sites. As a result,
many of these complexes exhibit stereochemically non-rigid behavior and
reversible substrate addition.

More recently, Lindner and co-workers have developed a
comprehensive series of ether-phosphines in which the ether group is
tethered by an ethylene group as opposed to a rigid phenyl ring.37 In addition
to being the ﬁrst to use a more ﬂexible linkage in a ether-phosphine ligand,
Lindner has also pioneered the use of chiral and cyclic ether donors such as
THF and dioxane. His work has focused on the use of these chelating
phosphines for the development of catalysts that assist in the carbonylation

of methanol to acetic acid. These phosphino-ether complexes, like others in

 

11

the same category, exhibit ﬂuxional behavior, reversible substrate binding,
and enhanced oxidative addition and reductive elimination chemistry
promoted by the on/off nature of the ether donors. Others have found similar
results with both mono- and di- phosphines containing pendent ether
substituents.33
C. Chemical Aspects of Tris(2,4,6-trimethoxyphenyl)phosphine

Although the use and development of ether-phosphines is extensive,
largely due to the recent efforts of Lindner et al., none of the aforementioned
systems have addressed the combined effects of high basicity and steric bulk
on complex stability and reactivity. We are interested in developing the
coordination chemistry of ether-phosphine ligands that combine steric bulk
and strong donor capability with chelating ability to stabilize complexes in
highly reactive and uncommon oxidation states. To this end, we have
undertaken the comprehensive study of the coordination chemistry of the
highly basic and sterically hindering ether-phosphine, tris(2,4,6-
trimethoxyphenyl)phosphine (Figure 1).

The molecule tris(2,4,6-trimethoxyphenyl)phosphine, which we refer to
as TMPP, was originally prepared by Soviet chemists in the late 1950's.39
The phosphine later reappeared in literature in the mid 1980's, when Wada
and co-workers described its extraordinarily high basicity (PKa = 11.2) and
steric properties (cone angle ~ 184°).40 Wada has exploited the phosphine's
unusual basic properties in a variety of organic transformations, including
mild ring opening reactions of terminal epoxides and facile dealkylations.“=1
More recently, the basicity and solubility properties of its phosphonium salts
have been applied in the extraction of metal ions.42 The unusual properties
of this phosphine are derived from the presence of the methoxy groups in the

ortho and para positions. The electron releasing nature of the methoxy

 

 

12

I
Me M

 

Figure 1. Schematic drawing of tris

(2,4,6-t1imethoxyphenyl)
phosphine (TMPP).

—

 

 

13
substituents increases the nucleophilicity of the phosphorous lone pair via the
mesomeric effect of the phenyl rings. The affect of sequential methoxy
substitution at the ortho and para ring positions on the phosphine's basicity
is Shown in Table 1.421) As the number of methoxy substituents is increased,
there is an increase in the basicity. Substitution of a -OMe group in the para
position has a greater effect on the basicity than substitution in the ortho
positions. In addition, the presence of two or more methoxy groups on one
ring results in a dramatic increase in nucleophilicity of the phosphine. For
example, the presence of one 2,6-dimethoxy substituted ring results in a
higher basicity (pKa = 5.39) than either of the tris 2-ortho- or para-
substituted phosphines (pKa = 4.47 and 4.75). In any event, there is an
overall increase in phosphine basicity as the number of substituted aryl

groups are increased. The electron donating effect of the methoxy groups is

dramatically illustrated by the comparison of the pKa of TMPP with that of
the unsubstituted phosphine, triphenylphosphine (pKa = 2.73).43

In addition to increasing the nucleophilicity, the presence of the
methoxy groups in the 2 and 6 positions also serves to augment the steric
bulk of the ligand. The cone angle of TMPP was reported by Wada and later
conﬁrmed by our group to be approximately 184°. Comparision with other
phosphines reveals that TMPP is one of the most sterically encumbering
phosphines known (Figure 2). The cone angle of TMPP is nearly 40° larger
than that of triphenylphosphine, commonly considered a sterically hindering
ligand (cone angle = 145°).20 As evidenced by the results of numerous
studies, sterically encumbering ligands lend kinetic stability to normally very
reactive transition metal centers, thus creating electron deﬁcient complexes.
Such complexes would be expected to exhibit unusual reactivity not normally

observed for complexes ligated by more conventional ligands.

 

 

14

Table l. PKa values for methoxy substituted triphenylphosphines (from

 

reference 42b).

 

 

2-MeO 4-MeO 2,6-MeO 2,4,6—Me0
(x-MeOPh)3P 4.47 4.75 9.33 11.2
(x-MeOPh)2PPh 4.01 4.06 7.28 8.22
(x-MeOPh)PPh2 3.33 3.67 5.39 5.77

 

 

Cone Angle

- P(mesityl)3

200° 4

180° — - PButa

160° - - PBu3s

 

140° -

120°-

 

Figure 2. Tolman cone angles for various tertiary phosphines.

 

 

16

Perhaps most importantly for the purpose of our research goals, the
presence of ether donors in the ortho positions provide the opportunity for
multidentate coordination modes for the phosphine. Unlike other ether-
phosphines that contain only one ether substituents, the presence of methoxy
groups in the ortho positions of all three aryl rings allow for TMPP to
participate in 111 through 113 bonding arrangements (Figure 3). Most other
ether-phosphines typically have only one ether donor and therefore may only
participate in n1 or 112 type bonding. The multiple chelating ability of TMPP
will be of paramount importance in this work for the stabilization of
coordinatively unsaturated, electronic deﬁcient, metal diphosphine
complexes. Moreover, the multidentate capabilities afford versatility, by
facilitating adjustments to the electronic requirements dictated by the metal
center. As the electronic requirements of the metal change, TMPP
compensates by altering its bonding mode. As a result, the phosphine may
accommodate a number of different metal oxidation states. The physical and
structural properties of TMPP and its corresponding oxide are fully described
in Chapter 2.

The combination of the chelate effect provided by the multidentate
capability of TMPP, together with the kinetic stability afforded by the steric
Size of the ligand, render TMPP an excellent ligand to stabilize highly
reactive metal centers in unusual oxidation states. Moreover, unlike other
transition metal complexes stabilized by bulky phosphines, the presence of
hemi-labile ether donors will provide these complexes with open coordination
sites in which further chemistry, particularly with small substrates, may
take place. Interestingly, with the exception of our work, only four reports of

transition metal TMPP complexes have appeared in the literature.“

 

 

Figure 3.

 

17

I Me
0
Me.
’0 0’ Me
\ ”Ar'
Ari

O,Me
M?
P 0' Me
Ni—PWAr'
Me’O OYMe
0‘Me

n3

111, n2 and n3 bonding modes of TMPP.

 

18

D. Chemistry of Mononuclear Rh(II) Complexes

Based on these considerations, our goal is to exploit the unique
properties of TMPP in order to stabilize, isolate and study the chemistry of
odd electron transition-metal systems. We are particularly interested in the
chemistry of mononuclear d7 rhodium complexes. A survey of the literature
reveals that the bulk of rhodium coordination and organometallic chemistry
is dominated by the catalytically important +1 and +3 oxidation states.45
These oxidation states form the basis for oxidative addition and reductive
elimination reactions that are key steps in many important catalytic
processes.46 In contrast to the ubiquity of mononuclear Rh(I) and Rh(III)
complexes, the chemistry of the paramagnetic, divalent oxidation state
remains relatively unexplored. Generally, these species exist only as highly
reactive, ﬂeeting intermediates or as impurities in the chemistry of d6 and d8
rhodium.47 The lack of stability of mononuclear Rh(II) complexes is a result of
the proclivity of these systems to (a) dimerize to form a metal-metal bonded
species and (b) disproportionate to the more stable Rh(I) and Rh(III)
oxidation states. These factors notwithstanding, a number of the Rh(II)
metallo-radicals are known.43

Under suitable conditions, sterically encumbering phosphines react
with Rh(III) trihalides in alcohol to yield paramagnetic Rh(II) compounds
formulated as trans-RhClz(PR3)2 (PR3: PCy3, P(o-tolyl)3, P(But)2Me).49 With
a few exceptions, these complexes are poorly characterized, prone to
decomposition and invariably contaminated with diamagnetic hydrido
species. As a result, there has been no comprehensive investigation of the
chemistry of these paramagnetic species undertaken and only a few studies

have appeared.50 In fact, only recently, the ﬁrst structural studies of such

19
species were ﬁnally reported.51,52 Paramagnetic mononuclear species of
rhodium that are coordinated by less sterically demanding ligands are also
known; these complexes , however, are ligated by "non-innocent" ligands in
which extensive delocalization of the unpaired electron is possible.56
Typically, such species are not considered to be authentic metallo-radicals.

In addition to the aforementioned phosphine complexes, a variety of
meta-stable mononuclear Rh(II) complexes have been generated by
electrochemical oxidation and reduction of Rh(III) and Rh(I) species.53 Few
stable mononuclear Rh(II) organometallic complexes have been reported;
many of these exist as short-lived intermediates and hence are poorly
characterized.54 A recent exception is the unprecedented Rh(II) dialkyl
Species Rh(2,4,6-Pri3CeH2)2(tht)2 (tht = tetrahydrothiophene) prepared by
Wilkinson and co-workers from Rthl3(tht)3.55 This complex is one of the
rare examples of a paramagnetic Rh(II) complex to be structurally
characterized by X-ray crystallography. .

By far the most promising results for Rh(II) complexes have been found
in the realm of Rh(II) porphyrin chemistry. Highly reactive, short-lived
Rh(II) Species have been implicated in a number of unusual and fundamental
organometallic reactions involving mononuclear and dinuclear rhodium
porphyrin complexes.57 Wayland and co-workers have reported strategies
that favor the existence of these paramagnetic intermediates that involve
introducing bulky substituents onto the porphyrins that discourage metal-
metal bond formation through steric interactions. By studying such key
systems, they were able to isolate Rh(II) radicals that reversibly couple
carbon monoxide to form dimetal-a-diketones (eq 3 - 5).58 Through the use of
even more sterically demanding porphyrins, dimerization of the carbonyl

radicals is disfavored and a 17 electron Rh(II) monocarbonyl species may be

 

20

 

 

 

 

 

(por)Rhm-CH3 h" r (por)RhH- (3)
benzene
0
||
(por)RhII - + CO ‘ ‘ (por)Rhn’ C ° (4)
O 0
II II
2 (pOI‘)RhII’ C 0 V — (130191111111, C\ C ’ Rhm(p or) (5)
ll
(por) = tetramesitylporphyrin 0

observed directly.“c In contrast to other 17 electron d7 carbonyl complexes,
the nonlinear RhCO’ unit behaves as an acyl radical, undergoing reactions at
the carbon and not at the metal center. Even more fascinating are the recent
reports that these Rh(II) porphyrin systems reversibly and selectively
activate methane to form Rh(III)-hydride and Rh(III)-alkyl Species (eq 6).59
Thermodynamic data point to the presence of a linear four-centered
transition state that precludes the activation of aromatic C-H bonds due to
steric inhibition by the porphyrin ligands. As a result, these Rh(II) porphyrin
systems are highly selective towards the activation of alkyl versus aryl C-H
bonds. Recent results by Wayland and co-workers indicate that these Rh(II)

metalloradicals also react with acrylates to produce C-C bonded oligomers.60

CH4 III (por)RhIII -C H3
2 (por)RhH° :2“— (P0r>Rh--,C\--H--Rh(por)= + (6)
If H (P0P)Rhm -H

(por) = tetramesitylporphyrin or tetraxylylporphyrin

In light of these recent results, the study of rhodium based odd electron

systems is becoming an important area of research. In general, these Species

 

21
must be afforded kinetic stabilization by sterically encumbering ligands that
preclude dimerization. In the case of Rh(II), further stabilization must be
provided by electronic factors that favor the +2 oxidation state relative to the
+1 and +3 oxidation states, hence avoiding undesirable disproportionation
reactions. The work in this thesis supports the use of TMPP as a good
candidate for providing the proper combination of kinetic and thermodynamic
stability for the isolation of mononuclear Rh(II) complexes. As a backdrop for
this work we note that Shaw and co—workers had earlier demonstrated that
mixed phosphorus and oxygen donor chelate ligands are capable of stabilizing
d7 metal centers.61 Reactions of P(But)3(o-MeOC6H4) with MCl3 oxHZO (M =
Rh, Ir) in reﬂuxing alcohol produced the paramagnetic mononuclear Rh(II)
and Ir(II) complexes, M(P(But)2(o-OC6H4))2 (eq 7). During the reaction the
metals are reduced from M(III) to M(II) and the phosphine undergoes
dealkylation to form a phosphino-phenoxide chelating ligand. The Ir complex
represents one of the few crystallographically characterized examples of a
stable Ir(II) mononuclear species. The remarkable stability of these species

illustrates the potential for other bulky mixed P,O chelates to stabilize odd

0\ 132
P i0H
M0130 xHZO + 2 R2P—Q ’—> 0 M’ I) (7)
A I \
P o
Moo 2
M = Rh, Ir R = Me, tBu

electron systems.
This thesis reports the results of investigations concerning the
isolation and reactivity of paramagnetic Rh(II) centers using the triaryl

phosphine ligand TMPP. Our entry into the area of mononuclear d7 rhodium

 

22

chemistry began with the synthesis of the bis-phosphine complex [cis-Rh(n3-
TMPP)2][BF4]2 from a reaction between TMPP and the solvated dinuclear
salt [Rh2(MeCN)10][BF4]4. The details of the synthesis and characterization
of this metalloradical is presented in chapter 3 together with its associated
redox chemistry. Chapter 4 examines in detail the reversible CO chemistry
that was observed for [cis-Rh(n3—TMPP)2][BF4]2. This rather complicated
chemistry involves the disproportionation of [RhH(n3-TMPP)2]2+ to give Rh(I)
carbonyl and Rh(III) species that upon loss of CO, recombine to form the
original Rh(II) species. The reversible CO behavior of the Rh(I) carbonyl
species is examined more closely in chapter 5, wherein we describe the
potential application of this process for the development of a CO sensing
composite material by incorporation the Rh(I) carbonyl species into a porous
sol-gel derived glass. As an extension of the reversible CO chemistry
discussed in chapter 3, reactions of [Rh(n3-TMPP)2][BF4]2 with isocyanide
ligands is presented in chapter 6. Unlike the complex electron transfer
chemistry that was observed with CO, the weaker n-acidity of isocyanides
allows for the isolation of paramagnetic Rh(II) diisocyanide complexes, rare
examples of stable organometallic Rh(II) complexes. The redox properties
and reactivity of these organometallic species are also discussed. Chapter 7
examines the effect of nucleophiles on the stability of Rh(II) and Rh(III) bis-
TMPP complexes. Nucleophilic attack on [Rh(n3-TMPP)2][BF4]2 results in
dealkylation of a coordinated methoxy substituent to yield a Rh(II) complex
stabilized by a phosphino-phenoxide interaction. The chemistry of this
species is discussed along with its relationship to analogous Rh(III)
compounds formed from dealkylation reactions of [Rh(n3-TMPP)2][BF4]3.
Chapter 8 discusses the formation of Rh(I) and Ir(I) oleﬁn species with TMPP

and the subsequent formation of the Ir(I) dicarbonyl cation

 

 

 

23
[Ir(TMPP)2(CO)2]1+, which, unlike its rhodium counterpart, is stable with

respect to CO loss. Attempts to oxidize this complex to yield an Ir(II) species

 

are also presented. Finally, the application of the n3-bonding mode to group
VI metal tricarbonyl fragments is presented in Appendix B. Speciﬁcally, the
isolation and characterization of the highly ﬂuxional molecular species (113-

TMPP)Mo(CO)3 is discussed.

 

 

 

 

 

10.

11.

24
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1990, 123, 1469. (g) Lindner, E.; Norz, H. Chem. Ber. 1990, 123, 459.
(h) Lindner, E.; Bader, A.; Braunling, H.; Jira, R. J. Mol. Catal. 1990,
57, 291. (i) Lindner, E.; Fawzi, R.; Mayer, H. A.; Eichele, K.; Pohmer,
K. J. Organomet. Chem. 1990, 386, 63. (j) McCann, G. M.; Carvill, A.;
Lindner, E.; Karle, B.; Mayer, H. A. J. Chem. Soc., Dalton Trans. 1990,
3107. (k) Lindner, E.; Karle, B. Chem. Ber. 1990, 123, 1464. (l)
Lindner, E.; Karle, B. Z. Naturforsch. 1990, 45b, 1108. (m) Lindner, E.;
Speidel, R. Z. Naturforsch. 1989, 44b, 437. (n) Lindner, E.; Norz, H. Z.
Naturforsch. 1989, 44b, 1493. (o) Lindner, E.; Reber, J. P.; Wegner, P.
Z. Naturforsch. 1988,43b, 1268. (p) Lindner, E.; Andres, B. Z.
Naturforsch. 1988, 43b, 369. (q) Lindner, E.; Schober, U.Inorg. Chem.
1988,27, 212. (r) Lindner, E.; Andres, B. Chem. Ber. 1988,12], 829.
(s) Lindner, E. ; Sickinger, A.; Wegner, P. J. Organomet. Chem. 1988,
349, 75. (t) Lindner, E.; Meyer, S. J. Organomet. Chem. 1988, 339, 193.
(u) Lindner, E.; Andres, B. Chem. Ber. 1987, 120, 761. (v) Lindner, E.;
Schober, U.; Fawzi, R.; Hiller, W.; Englert, U.; Wegner, P. Chem. Ber.
1987, 120, 1621. (w) Lindner, E.; Meyer, 8.; Wegner, P.; Karle, B.;
Sickinger, A.; Steger, B. J. Organomet. Chem. 1987, 335, 59. (x)
Lindner, E.; Schober, U.; Stangle, M. J. Organomet. Chem. 1987,33],
013. (y) Lindner, E.; Schrober, U.; Glaser, E.; Norz, H.; Wegner, P. Z.
Naturforsch. 1987, 42b, 1527. (z) Lindner, E.; Scheytt, C.; Wegner, P.
J. Organomet. Chem. 1986,308, 311. (aa) Lindner, E.; Scheytt, C. Z.
Naturforsch. 1986, 41b, 10. (bb) Lindner, E.; Mayer, H. A.; Wegner, P.
Chem. Ber. 1986,119, 2616. (cc) Lindner, E.; Sickinger, A.; Wegner, P.
J. Organomet. Chem. 1986,312, C37. (dd) Lindner, E.; Raudeler, H.;
Scheytt, C.; Mayer, H. A.; Hiller, W.; Fawzi, R.; Wegner, P. Z.
Naturforsch. 1984, 39b, 632.

(a) Mason, M. R.; Verkade, J. G. Organometallics 1992, 11 , 1514. (b)

, Mason, M. R.; Verkade, J. G.; J. Am. Chem. Soc. 1991,113, 6309. (0)

Mason, M. R.; Su, Y.; Jacobson, R. A.; Verkade, J. G. Organometallics
1991, 10, 2335. (d) Werner, H.; Stark, A.; Shulz, M. ; Wolf, J.
Organometallics 1992, 11 , 1126. (e) Werner, H.; Hampp, A.; Peters, K.;
Peters, E. M.; Walz, L.; von Schnering, H. G. Z. Naturforsch. 1990, 45b,
1548. (D Anderson, G. K.; Kumar, R. Inorg. Chem. 1984, 23, 4064. (g)
Anderson, G. K.; Corey, E. R.; Kumar, R. Inorg. Chem. 1987, 26, 97.

 

 

 

 

39.

40.

41.

42.

43.

44.

45.

46.

47.

48.

29

(h) Anderson, G. K.; Kumar, R. Inorg. Chim. Acta. 1988, 146, 89. (i)
Alcock, N. W.; Platt, A. W. G.; Pringle, P. G. J. Chem. Soc., Dalton
Trans. 1989, 2069. (j) Reddy, V. V. S.; Whitten, J. E.; Redmill, K. A.;
Varshney, A.; Gray, G. M. J. Organomet. Chem. 1989, 372, 207. (k)
Reddy, V. V. S.; Varshney, A.; Gray, G. M. J. Organomet. Chem. 1990,
391, 259.

(a) Protopopov, I. S.; Kraft, M. Y. Zhumal Obshchei Khimii 1963,33,
3050. (b) Protopopov, I. S.; Kraft, M. Y. Med. Prom. SSSR, 1959, 13, 5;
Chem. Abstr. 1960, 54, 10914c.

(a) Wada, M.; Higashizaki, S.; J. Chem. Soc., Chem. Commun. 1984,
482. (b) Wada, M.; Higashizaki, S.; Tsuboi, A. J. Chem. Res. 1985, (S),
38; (M), 0467.

(a) Wada, M.; Tsuboi, A. J. Chem. Soc., Perkin Trans. I 1987, 151. (b)
Wada, M.; Tsuboi, A.; Nishimura, K.; Erabi, T. Nippon Kagaku Kaishi
1987, 7, 1284.

(a) Yamashoji, Y.; Matsushita, T.; Wada, M.; Shono, T. Chem. Lett.
1988, 43. (b) Yamashoji, Y.; Matsushita, T.; Tanaka, M.; Shono, T.;
Wada, M. Polyhedron 1989, 8, 1053.

Streuli, C. A. Anal. Chem. 1960, 32, 985.

(a) Bowmaker, G. A.; Cotton, J. D.; Healy, P. C.; Kildea, J. D.; Silong,
S. B.; Skelton, B. W.; White, A. H. Inorg. Chem. 1989,28, 1462. (b) Li,
G. Q.; McAuliffe, C. A.; Mackie, A. G.; Mac Rory, P. P.; Ndifon, P. T. J.
Chem. Soc., Dalton Trans. 1992, 1297. (c) Kurosawa, H.; Tsuboi, A.;
Kawasaki, Y.; Wada, M. Bull. Chem. Soc. Jpn. 1987, 60, 3563. (d)
Baker, L.-J.; Bowmaker, G. A.; Camp, D.; Effendy,; Healy, P. C.;
Schmidbauer, H.; Steigelmann, 0.; White, A. H.Inorg. Chem. 1992, 31,
3656.

(a) Hughes, R. P. in Comprehensive Organometallic Chemistry;
Wilkinson, G.; Stone, F. G. A.; Abel, E. W., eds.; Pergamon: New York,
1982; vol. 5.

Collman, J. P.; Hegedus, L. 8.; Norton, J. R.; Finke, R. G. Principles
and Applications of Organotransition Metal Chemistry; University
Science Books: Mill Valley, Ca. 1987; chapter 5 and references therein.

Busetto, C.; D‘Alfonso, A.; Maspero, F.; Perego, G.; Zazzetta, A. J.
Chem. Soc., Dalton Trans. 1977, 1733.

For a review of Rh(II) complexes see Felthouse, T. R. Prog.Inorg.
Chem. 1982,29, 73 and references therein.

 

 

 

 

49.

50.

51.

52.

53.

54.

30

(a) Bennett, M. A.; Lonstaff, P. A. J. Am. Chem. Soc., 1969,91, 6266.
(b) Moers, F. G.; DeJong, J. A. M.; Beaumont, P. M. J. J. Inorg. Nucl.
Chem. 1973,35, 1915. (c) Masters, C.; Shaw, B. L. J. Chem. Soc. (A)
1971, 3679. (d) Empsall, H. D.; Heys, P. N.; Shaw, B. L. Transition
Met. Chem. 1978, 3, 165. (e) Empsall, H. D.; Hyde, E. M.; Pawson, D.;
Shaw, B. L.; J. Chem. Soc., Dalton Trans. 1977 , 1292.

(a) Vleck, A. Inorg. Chim. Acta 1980, 43, 35. (b) Holah, D. G.; Hughes,
A. N.; Hui, B. C. Can. J. Chem. 1975, 53, 3669. (c) Valentim', G.; Braca,
G.; Sbrana, G.; Colligiani, A. Inorg. Chim. Acta 1983, 69, 215. (d)
Valentini, G.; Braca, G.; Sbrana, G.; Colligiani, A. Inorg. Chim. Acta
1983, 69, 221.

(a) Harlow, R. L.; Thorn, D. L.; Baker, R. T.; Jones, N. L. Inorg. Chem.
1992, 31 , 993. (b) Rappert, T.; Wolf, J .; Schulz, M.; Werner, H. Chem.
Ber. 1992,125, 839.

The crystal structure of RhIIClz(PPh3)2 was reported recently, Ogle, C.

A.; Masterman, T. C.; Hubbard, J. L. J. Chem. Soc., Chem. Commun.
1990, 1733, but it was later shown to be that of the Rh(I) complex
Rh(PPh3)2(CO)Cl, Dunbar, K. R.; Haefner, S. C. Inorg. Chem. 1992, 31 ,

0000.

Examples of coordination and organometallic Rh(II) complexes
prepared electrochemically: (a) Cooper, S. R.; Rawle, S. C. ; Yagbasan,
R.; Watkin, D. J. J. Am. Chem. Soc. 1991,113, 1600. (b) Rawle, S. C.;
Yagbasan, R.; Prout, K.; Cooper, S. R. J. Am. Chem..Soc. 1987, 109,
6181. (c) Blake, A. J .; Gould, R. D. ; Holder, A. J .; Hyde, T. I.; Schroder,
M. J. Chem. Soc., Dalton Trans. 1988, 1861. ((1) Anderson, J. E.;
Gregory, T. P. Inorg. Chem. 1989, 28, 3905. (e) Scwarz, H. A.; Creutz,
C. Inorg. Chem. 1983,22, 207. (e) Mulazzani, Q. G.; Emmi, S.;
Hoffman, M. Z.; Venturi, M. J. Am. Chem. Soc. 1981, 103, 3362. (f)
Kalle, U.; Klaui, W. Z. Naturforsch. 199], 46b, 75. (g) Bianchini, C.;
Laschi, F.; Ottaviani, M. F.; Peruzzini, M.; Zanello, P.; Zanobini, F.
Organometallics 1989,8, 893. (h) Bianchini, C.; Meli, A.; Laschi, F.;
Vizza, F.; Zanello, P. Inorg. Chem. 1989,28, 227. (i) Pilloni, G.;
Schiavon, G.; Zotti, G.; Zecchin, S. J. Organomet. Chem. 1977, 134, 305.
(i) Fischer, E. 0.; Lindner, H. H. J. Organomet. Chem. 1964, 1, 307. (k)
Fischer, E. 0.; Wawersik, H. J. Organomet. Chem. 1966,5, 559. (1)
Keller, H. J .; Wawersik, H. J. Organomet. Chem. 1967,8, 185. (m)
Dessy, R. E.; King, R. B.; Waldrop, M. J. Am. Chem. Soc. 1966,88,
5112. (n) Dessy, R. E.; Kornmann, R.; Smith, C.; Hayter, R. J. Am.‘
Chem. Soc. 1968,90, 2001.

(a) Bianchini, C.; Laschi, F.; Ottaviani, F.; Peruzzini, M.; Zanello, P.
Organometallics 1988, 7, 1660. (b) Bianchini, C.; Laschi, F.; Meli, A.;
Peruzzini, M.; Zanello, P.; Frediani, P. Organometallics 1988, 7, 2575.

 

 

55.

56.

57.

58.

59.

60.

61.

31

Hay-Motherwell, R. S.; Koschmieder, S. U.; Wilkinson, G.; Hussain-
Bates, B.; Hursthouse, M. B. J. Chem. Soc., Dalton Trans. 1991, 2821.

(a) Billig, E.; Shupack, S.I.; Waters, J.H.; Williams, R.; Gray HE. J.
Am. Chem. Soc. 1964, 86, 926.(b) Pneumatikakis, G.; Psaroulis, P.
Inorg. Chim. Acta 1980, 46, 97. (c) Pandey, K. K.; Nehete, D. T.;
Sharma, R. B. Polyhedron 1990, 9, 2013. (d) Peng, S. M.; Peters, K.;
Peters, E. M.; Simon, A. Inorg. Chim. Acta 1985, 101, L35.

(a) Wayland, B. B.; Woods, B. A.; Cofﬁn, V. L. Organometallics 1986, 5,
1059. (b) Del Rossi, K. J .; Wayland, B. B. J. Am. Chem. Soc. 1985, 107,
7941. (c) Paonessa, R. S.; Thomas, N. C.; Halpern, J. J. Am. Chem.
Soc. 1985,107, 4333. (d) Kadish, K. M.; Yao, C.-L.; Anderson, J. E.;
Cocolios, P. Inorg. Chem. 1985,24, 4515. (e) Anderson, J. E.; Yao C.-L.;
Kadish, K. M. Organometallics 1987, 6, 706. (f) Anderson, J. E.; Yao,
C.-L.; Kadish, K. M. J. Am. Chem. Soc. 1987, 109, 1106. (g) Aoyama,
Y.; Yoshida, T.; Sakurai, K.—I.; Ogoshi, H. Organometallics 1986, 5, 168.

(a) Wayland, B. B.; Sherry, A. E.; Coffin, V. L. J. Chem. Soc., Chem.
Commun. 1989, 662. (b) Wayland, B. B.; Sherry, A. E. J. Am. Chem.
Soc. 1989, 111, 5010. (c) Wayland, B. B.; Sherry, A. E.; Poszmik, G.;
Bunn, A. G. J. Am. Chem. Soc. 1992,114, 1673.

(a) Wayland, B. B.; Ba, S.; Sherry, A. E. J. Am. Chem. Soc. 199], 113,
5305. (b) Sherry, A. E.; Wayland, B. B. J. Am. Chem. Soc. 1990,112,
1259.

(a) Bunn, A. G.; Wayland, B. B. J. Am. Chem. Soc. 1992, 114, 6917. (b)
Poszmik, G; Wayland, B. B. presented at the 203rd National meeting of
the American Chemical Society, San Francisco, CA, April 5-10, 1992;
paper INOR 67.

(a) Empsall, H. D.; Hyde, E. M.; Jones, C. E.; Shaw, B. L. J. Chem.
Soc., Dalton Trans. 1974, 1980. (b) Mason, R.; Thomas, K. M.;
Empasall, H. D.; Fletcher, S. R.; Heys, P. N.; Hyde, E. M. Jones, C. E;
Shaw, B. L. J. Chem. Soc., Chem. Comm. 1974, 612. (c) Empsall, H.
D.; Hyde, E. M.; Shaw, R. L. J. Chem. Soc., Dalton Trans. 1975, 1690.
(d) Empsall, H. D.; Heys, P. N .; McDonald, W. 8.; Norton, M. 0.; Shaw,
B. L. J. Chem. Soc., Dalton Trans. 1978, 1119.

 

 

 

 

 

CHAPTER II

STRUCTURAL AND SPECTROSCOPIC PROPERTIES OF TRIS(2,4,6-

TRIMETHOXYPHENYL)PHOSPHINE AND TRIS(2,4,6-
TRIMETHOXYPHENYL)PHOSPHINE OXIDE

32

 

 

33
1. Introduction

Before delving directly into the coordination chemistry of tris(2,4,6-
trimethoxyphenyl)phosphine (TMPP), a summary of its spectroscopic and
structural properties is in order. The initial synthetic report describing
TMPP and several related quaternary phosphonium salts did not report any
spectroscopic information about the phosphine; in fact only melting points
and elemental analyses were given.1 Later, Wada described a more
conventional route for the preparation of TMPP along with an account of its
1H NMR and infrared spectra] data.2 In addition to providing some spectral
data for TMPP and several of its quaternary phosphonium salts, Wada
estimated the pKa of the phosphine to be 11.2 by titration of the protic
phosphonium salt with a series of bases and monitoring the process by 1H
NMR spectroscopy. Later the value was redetermined by non-aqueous
titrimetry in nitromethane and found to be 1102.3 The high nucleophilicity
of TMPP is readily apparent from its observed chemical reactivity. Although
some of the physical and spectroscopic properties of TMPP appeared during
the course of our studies, a complete structural and spectroscopic study of
this ligand has not yet been published to our knowledge.4 In this chapter, we
describe the results of such studies completed in our laboratories.

In addition to the spectroscopic and structural properties of TMPP, we
are also interested in those of the corresponding phosphine oxide, in
particular the affect that phosphine oxidation has on the spectroscopic and
structural properties. Transition metal catalyzed oxidations of phosphines
are not uncommon, particularly in the chemistry of late transition metal-

phosphine complexes.5 An example of such a process was investigated in our

own laboratories. In this work, TMPP was reacted with FeCl3 in the presence

of oxygen to form a TMPP oxide-FeCl3 complex.6 In order to identify the

I “I.“

 

 

 

34
possible formation of any phosphine oxide by-products, it is necessary to
study their physical properties, thus the oxide derivative of TMPP was
prepared and fully characterized by spectroscopic and crystallographic
methods.

2. Experimental
A. Synthesis
( 1) Preparation of Tris(2,4,6-trimethoxyphenyl)phosphine (TMPP)

TMPP was prepared by using a modiﬁcation of the method previously
described by Wada.2b A 1 liter 3-necked ﬂask equipped with a condenser, a
gas inlet, and a 120 mL addition funnel, was charged with 1,3,5-
trimethoxybenzene (42.05 g, 0.25 mol) and diethyl ether (150 mL). The
resulting solution was cooled to 0°C with an ice bath and a 2.5 M solution of
butyllithium in hexanes (100 mL) was slowly added dropwise over a period of
1-2 h. The resulting suspension was stirred at 0°C for a total of 12 h. After
this time, triphenylphosphite (19.65 mL, 0.075 mol) was slowly added
dropwise to the suspension at 0°C. While stirring for 10-12 h, the reaction
was allowed to slowly warm to r.t. The solvent was decanted off and the
remaining white solid was ﬁltered in air and washed with a minimal amount
of cold EtOH (2 x 10 mL).

The product was recrystallized in air by dissolving the solid in a large
volume of hot EtOH (1 L), followed by ﬁltration of the hot solution and
concentration on a rotary evaporator. The resulting white crystalline solid
was ﬁltered, washed with cold EtOH (2 x 10 mL) and diethyl ether (4 x 10
mL), and dried in vacuo; yield, 26-28 g (65-70% based on P(OPh)3). IR (Nujol,
CsI) cm'1: 1596, 1581, 1398, 1321, 1293, 1223, 1202, 1182, 1154, 1120, 1086,
1036, 949, 919, 816, 805, 789, 675, 634, 474. Electronic absorption spectrum

 

 

 

 

35

(CHgCN) kmax, nm (e): 276 (21.6 x 103), 245 sh (~29.3 x 103), 214 (87.2 x 103).
1H NMR (CD30N) 5 ppm: 3.45 (s, 18H, o-OCH3), 3.75 (s, 9H, p-OCH3), 6.07
(d, 4JP-H = 2.51 Hz, 6H, m-H). 31P (0D30N) 5 ppm: -66.3 (s). 13C (CD30N) 5
ppm: 109.19 (d, 1JP-C = 22.78 Hz, 01), 163.01 (d, 02,6), 92.21 (3, 03,5),
164.12 (8, C4), 56.42 (s, o-OCH3), 55.80 (s, p-OCH3). Cyclic Voltammetry (0.1
M TBABF4/ CH3CN, vs Ag / Ag01): EM = + 0.49 v, Ep,c(chemical) = — 0.09 V.

(2) Preparation of Tris(2,4,6-trimethoxyphenyl)phosphine Oxide

(TMPP=O)

An excess of a 30% aqueous solution of H202 (1 mL) was added to a
solution of TMPP (1.00 g, 1.98 mmol) in 10 mL of acetone. The colorless
solution was gently reﬂuxed for 8 h, after which time the solution was cooled
and the volume was reduced to approximately 5 mL under reduced pressure.
Slow addition of diethyl ether (50 mL) effected the precipitation of a large
amount of white crystalline solid. The product was collected by suction
ﬁltration, washed with diethyl ether (4 x 5 mL) and dried in vacuo for 1 h;
yield, 0.897 g (87%). IR (Nujol, CsI), cm'1: 1596, 1579, 1406, 1333, 1293,

1230, 1206, 1184, 1159, 1120, 1096, 1028, 952, 921, 812, 789, 692, 665, 640,
594, 527, 520, 501, 482, 449. Electronic absorption spectrum (MeCN) km”,

nm (E): 285 sh (~10,100), 253 (36,600), 216 (114,600). 1H NMR (CD30N) 5
ppm: 3.46 (s, 18H, o-OCH3), 3.78 (s, 9H, p-OCH3), 6.08 (d, 4JP-H = 4.1 Hz,
6H, m-H). 31P (CD30N) 5 ppm: + 10.8 (s). 130 (CD30N) 5 ppm: 109.66 (d,
1Jp,C = 121.6 Hz, Cl), 164.40 ((1, 2Jp,C = 1.3, 02,6), 92.04 (d, 3JP-C = 6.9,
03,5), 164.00 (d, 4JP-C = 1.1, 04), 56.42 (s, o-OCH3), 55.98 (s, p-OCH3). Cyclic
voltammetry (0.1 M TBABF4/CH30N, vs Ag/AgCl): Ep,c(irrev.)1 = - 0.65 V,
Ep,a(chemical) = - 0.09 V, sz = -1.29 V, EMt = + 1.41 V. Mass spectrum

(El), m/z: 548 (TMPP=O), 517, 367, 335, 374, 197, 181, 167, 151, 136, 121.

   

36

(3) Preparation of Ni(CO)3TMPP

The compound Ni(CO)3TMPP was prepared in situ by a modiﬁcation of
the method described by Tolman for the preparation of other Ni(CO)3PR3
complexes.7 An amount of Ni(CO)4 (100 11L, 0.77 mmol) was syringed into a
ﬂask containing 15.4 mL of a 0.05 M solution of TMPP in benzene. The
reaction was shaken brieﬂy and allowed to stand for 15 min. The solution
was then diluted with fresh benzene. The infrared spectrum was obtained by
removing a small sample and transferring it to a 0.1 mm CaF2 solution IR
cell: v(CO) (cm-1); 2048 (A1) and 1963 (E).
B. X-ray Crystallography

The structure of tris(2,4,6-trimethoxyphenyl)phosphine (1) and
tris(2,4,6-trimethoxyphenyl)phosphine oxide (2) were determined by
application of general procedures which have been fully described elsewhere.8
Geometric and intensity data were collected on a Rigaku AFCGS
diffractometer with graphite-monochromated MoKa Ova = 0.71069 A)
radiation and were corrected for Lorentz and polarization effects. All
calculations were performed with VAX computers on a cluster network within
the Department of Chemistry at Michigan State University using the Texsan
software package of the Molecular Structure Corporation.9 Important
crystallographic parameters for the structures of 1 and 2 are summarized in
Table 2.
(l) TMPP
(i) Data Collection and Reduction. Large single crystals of TMPP (1)
were obtained by slow evaporation of a benzene solution of TMPP in air. A

suitable single crystal, with the approximate dimensions of 0.49 x 0.47 x

 

 

 

 

Table 2. Summary of Crystallographic Data for TMPP (1) and TMPP=O -

37

 

 

 

2H20 (2)
Cnmnmmd 1 L
Formula P09027H33 P0120.271'137
Formula weight 532.53 548.53
Space group Pn (#7) P-1 (#2)
a, A 8.092 (4) 12.768 (4)
b, A 24.871 (4) 13.506 (5)
c, A 13.455 (3) 9.927 (4)
01, deg 90 109.56 (3)
[3, deg 96.83 (3) 109.96 (3)
7, deg 90 98.99 (3)
v, A3 2689 (1) 1444 (2)
Z 4 2
dcalc, g/cm3 1.315 L262
0 (Mo Koc), cm-1 1.47 1.41
Temperature, °C 23 °C -80°0
Trans. factors, max, min. 1.00, 0.95 1.00, 0.95
R8 0.040 0.045
wa 0.038 0.051
Quality-of-ﬁt indicatorc 2.08 2.65

 

aR=2 I IFOI- chH/ZIFOI

bRw = [2w | F0 l— I Fc I)2/2w IF, I211/2; w = 1/62( IFo I)
CQuality of ﬁt = [XW( lFo I - [Fe |)2/(Nobs-Nparameters)]l/2

 

 

38

0.31 mm3, was cleaved from a larger crystal and mounted on the tip of a glass
ﬁber with epoxy cement. Cell constants were obtained from a least-squares
reﬁnement using 24 carefully centered reﬂections in the range 21 < 20 < 32°,
and were found to correspond to a monoclinic cell with the following cell
constants: a = 8.092 (4) A, b = 24.871 (4) A, c = 13.455 (3) A, a = y = 90°, (3 =
96.83 (3)°, V = 2689 (1) A3. Data were collected at 23i2°0, by using the (0—
scan method, in the range 4 .<_ 20 S 47°. Weak reﬂections, those with F <
10.06(F), were rescanned at a maximum of 2 rescans and the counts were

accumulated to ensure good counting statistics. A total of 4403 reﬂections

were collected. Equivalent reﬂections were averaged (Rmerge = 2.6%) to yield

a total of 4088 unique data and 3035 data with F02 2 36(F0)2. Periodic

measurement of three representative reﬂections at regular intervals showed
no loss of diffraction intensity had occurred during data collection. An
empirical absorption correction was applied on the basis of azimuthal scans of
3 reﬂections with x near 90° and resulted in transmission factors ranging
from 0.95 to 1.00.

(ii) Structure Solution and Refinement. The positions of all non-
hydrogen atoms were obtained by direct-methods10 and reﬁned by successive
full-matrix least-squares cycles. All non-hydrogen were reﬁned with
anisotropic thermal parameters. Hydrogen atoms were treated as ﬁxed
contributors at idealized positions and were not reﬁned. Final least squares
reﬁnement of 665 parameters resulted in residuals of R = 0.040 and Rw =
0.038 and a goodness-of-ﬁt = 2.08. Reﬁnement of both enantiomorphs
established the correct one at the 97.5% conﬁdence level by application of the
Hamilton Signiﬁcance test.11 A ﬁnal difference Fourier map revealed no

peaks above 0.42 e'/A3.

 

 

 

 

39

(2) TMPP=O - 2 H20

(i) Data Collection and Reduction. A large crop of colorless crystals were
grown in air by slow evaporation of a benzene solution of TMPP=O. A single
crystal of approximate dimensions 0.36 x 0.13 x 0.10 mm3 was taken up with
silicon grease on the end of a glass ﬁber and placed in a cold N2 stream.
Least squares reﬁnement of 21 orientation reﬂections in the range 20 < 20 <
38° resulted in cell constants consistent with a triclinic cell. Intensity
measurements were performed at -80 i 2°C using the (1)-20 scan technique.
Routine measurement of three check reﬂections at regular intervals
throughout data collection revealed that the crystal had experienced only a
slight decay (1.6%) in diffraction intensity. Nonetheless, a linear decay
correction was applied to compensate for the observed loss. In addition, an
empirical absorption correction was applied based on azimuthal scans of 3
reﬂections with Eulerian angle 1 near 90°. A total of 4476 data were collected
in the range of 7.36 S 20 S 47 °. After averaging equivalent reﬂections (Rmerge
= 2.1%), there remained 4254 unique data of which 3045 were in the category
F02 2 36(F0)2.

(ii) Structure Solution and Refinement. The positions of all non-
hydrogen atoms were obtained by application of the direct methods programs
MITHRIL and DIRDIF.12 Hydrogen atoms of the phosphine oxide were
placed at calculated positions, while the positions of the hydrogen atoms of
the interstitial H20 molecules were located from difference Fourier maps. All
hydrogen atoms were treated as ﬁxed contributors to the structure factor
calculation and were not reﬁned. Final least-squares reﬁnement of 361

parameters gave residuals of R = 0.045 and RW = 0.051 and a goodness-of-ﬁt

of 2.65. The highest peak remaining in the Fourier difference map was 0.22
e'/A3.

 

 

40

3. Results and Discussion
A. Synthesis and Characterization

The tertiary phosphine compound tris(2,4,6-trimethoxyphenyl)-
phosphine (TMPP) (1)can be prepared in good yield (60-7 0%) by lithiation of
1,3,5-trimethoxybenzene followed by coupling with triphenylphosphite at 0°C
(eq 8). The phosphine was isolated as a white solid after recrystallization
from hot ethanol. Alternatively, TMPP can be prepared in an unusual one-

pot synthesis following a Soviet literature procedure.131 This method entails

 

OMe OMe OMe
"BuLi P(oph)3
W W P @- OMe (8)
MeO OMe 0°C MeO OMe 12h OMe 3
12h

reﬂuxing 1,3,5-trimethoxybenzene with zinc chloride in neat P013 for 8 hours
(eq 9). Yields are comparable to those obtained from the lithiated arene.

Both procedures are currently used in our laboratories.

OMe OMe
O P013 06H“ Zn012°©30M
211012
MeO OMe A, 8h OMe
(9)
2) H20 P Q OM
3) Na2SO4
OMe 3

Despite the known sensitivity of many phosphine lingands towards
aerial oxidation, TMPP is air-stable both in solution and in the solid state.

The phosphine is highly soluble in acetone and acetonitrile and moderately

 

 

 

41
soluble in THF, MeOH and EtOH. The molecule exhibits limited solubility in
both benzene and toluene, is relatively insoluble in diethyl ether, and is
completely insoluble in water and hydrocarbon solvents. Although soluble in
methylene chloride, TMPP readily decomposes (t1/2 < 15 min) to form the
chloromethyl phosphonium salt [(06H2(0Me)3)3P-CH201+][01'] (eq 10).2

MeO MeO +
CH2C12 _
MeO P ——-> MeO P-CH2C1 Cl (10)
MeO 3 MeO 3

The infrared spectrum of TMPP reveals a number of vibrations
between 400 and 1600 cm'1 that render the presence of TMPP in a sample
readily apparent. The 1H NMR spectrum of TMPP in MeCN exhibits three
resonances: 3.45 (s,18H, o-OMe); 3.75(s, 9H, p-OMe); 6.07(d, 4Jp_H = 2.51 Hz,
6H, m-H) corresponding to the meta-ring protons, and the ortho- and para-
methoxy protons, respectively. The appearance of only 3 resonances indicates
that fast rotation about the P'Cipso bond is equilibrating all-exchangeable
meta-ring and methoxy protons. This rotation is quite facile, in so much as
the 1H NMR spectrum remains unchanged down to -100°C in d8-toluene.

The 31P{1H} NMR spectrum in CD3CN exhibits a single resonance at
-66.3 ppm. The extreme upﬁeld chemical shift is attributed to an unusually
large 7 effect caused by the presence of the ortho-methoxy substituents on the
aryl rings of TMPP.12 The 7 effect is believed to arise from the additional
shielding provided by the proximity of an ortho substituent to the phosphorus
lone pair. The effect is lost upon quaternarization of the phosphine and the
31P chemical shifts of the corresponding phosphonium salts are similar to

non-ortho substituted phosphonium salts.13

 

 

 

 

42

In addition to the spectroscopic properties of TMPP, we are also
interested in its nucleophilicity. In order to ascertain the donating
capabilities of TMPP to metal centers, we measured the A1 mode of the
carbonyl stretching vibration of the nickel complex Ni(CO)3TMPP. The
complex was prepared in situ by the method described by Tolman to measure
the donor properties of a host of other phosphines.7 The IR spectrum of
Ni(CO)3TMPP measured in benzene registers 2 00 bands, 2047.9 cm'1 and
1962.7 cm'1 which are assigned to the A1 and E stretching modes
respectively. 14 A comparison of the A1 stretching mode of TMPP with those

 

of other phosphines is shown in Figure 4.15 The stretching frequency is the
lowest reported for a phosphine complex of Ni(CO)3 and is indicative of the

highly donating nature of TMPP. This result agrees well with the reported
pKa value for TMPP (pKa = 11.02)3 and further supports that TMPP is one of
the most strongly donating tertiary phosphines.

The oxide derivative of TMPP was prepared by reﬂuxing an acetone
solution of TMPP with aqueous H202 and was isolated as a white crystalline
solid in high yield (87%) (eq 11). The TMPP oxide molecule is air-stable and
has similar solubility properties as those of the parent phosphine.Attempts to
prepare TMPP=O by reaction of TMPP and Me3NO under reﬂuxing
conditions met with failure, yielding only unreacted phosphine. Other work
has shown that TMPP oxide can be prepared by catalytic oxidation of TMPP
with 02 in the presence of FeCl3.16

TMPP + 30% H202(aQ) $7 TMPP=O (11)

1 2

The infrared spectrum of TMPP=O displays a number of vibrations
between 400 and 1600 cm'1 that are slightly shifted from those found for the

 

 

 

43

 

 

.TMPP B , P
180° - ' U 3 . (Measi)3P
: 0
< 160 -
2
O . Ph3P
0 140° — .MePhZP
Et3P .
. (PhO)3P
o _ ° MeZPhP
120 MeaP .
l l 01(MeO)3P
2050 2060 2070 2080 (cm")
vA1(CO) for LNi(c0)3
Figure 4. Plot of the cone angle versus the v(CO)A1 stretch for various

Ni(CO)3(PR3) complexes.

 

 

44

parent phosphine. The v(P-O) band is obscured by the other vibrations of
TMPP in this region and is not discernible. The purity of the solid product
was conﬁrmed by 1H and 31P NMR spectroscopy. The 1H NMR spectrum of
TMPP=O in 0D3CN exhibits three resonances at 5 ppm, 3.46 (s, 18H, o-Me),
3.78 (s, 9H, p-OMe) and 6.08 (d, 4JP-H = 4.1 Hz, 6H, m-H). As was observed
for TMPP, the appearance of only three resonances suggests that the aryl
groups are free to rotate about the P'Cipso bond. In spite of the change in
phosphorus oxidation state, these resonances are only slightly shifted from
the free phosphine. In sharp contrast, the 31P chemical shift difference
between TMPP and TMPP=O is over 75 ppm, as the 31P displays a single
resonance at +10.8 ppm. This dramatic chemical shift difference is a result of
deshielding of the phosphorus nucleus upon oxidation from +3 to +5 and
illustrates the sensitivity of 31P NMR spectroscopy to changes in phosphorus
oxidation state.
B. Molecular Structures
(1) TMPP

Single crystals of TMPP were grown by slow evaporation from a
benzene solution. The compound was found to crystallize in the acentric
space group P2n (#7), resulting in the presence of two independent TMPP
molecules per asymmetric unit. Reﬁnement of the structure in the centric
space group P21/n (#13) proved unsatisfactory as evidenced by abnormally
high residuals. Therefore, the ﬁnal reﬁnement was performed in the acentric
group. Further evidence for the acentric space group was provided by the
computer program MISSYM,17 which showed that the two independent
molecules are not symmetry related. ORTEP drawings for both independent
molecules are depicted in Figure 5. Pertinent bond distances and angles are

Summarized in Table 3.

 

 

 

45

As expected, the aryl rings of the phosphine adopt a pseudo-propeller
arrangement about the phosphorus atom. However, the angles that the aryl
rings adopt with respect to the plane described by the ipso-carbon atoms vary
considerably from ideality, ranging in value from 36.61° to 74.12° (Table 4).
The average P'Cipso bond length, 1.842 A, is slightly longer than that
reported for triphenylphosphine (P-Cave = 1.828 A),13 but comparable to that
reported for the structure of tris(2,6-dimethoxyphenyl)phosphine (P-Cave =
1.844 A)19 and trimesitylphosphine (P'Cave = 1.837 A).20 As expected, the
average C-P-C bond angle of TMPP (C-P-Cawe = 105.2°) is larger than that of
triphenylphosphine (C-P-Cave = 103.0°), yet smaller than trimesitylphosphine
(C-P-Cawe = 109.7°). Although the presence of the ortho-methoxy groups
contributes to the deformation of the C-P-C bond angle, the steric interactions
between these groups are not nearly as great as those observed for the
analogous methyl substituted phosphines tris(2,6-dimethylphenyl)phosphine
and trimesitylphosphine. As a result, the cone angle of TMPP is not as large
as those observed for the methyl substituted analogs.

Just as was observed in the structure of the 2,6-dimethoxy substituted
phosphine, the ortho-methoxy groups of TMPP are bent slightly towards the
phosphorus lone pair. For example, the O-C-C angle, O(3)—C(6)-C(5) is
122.3(8) while the O-C-C angle O(3)-C(6)-C(1) is 116.0(8), indicating a
bending towards the P-C bond. This effect is opposite to that observed for
trimesitylphosphine, in which the methyl groups bend away from the P-C
bond. The methoxy groups of phosphine are approximately in the plane of
the phenyl ring with the methoxy group torsion angles relative to the phenyl
rings varying from 032° to 21.42°.

 

 

 

 

 

NEE mo 318538 use :25
bﬁoEnmuMozaamio 23 on... no macs—Sawmemouvmmemam .m 959.,—

46

 

 

 

 

 

47

 

Table 3. Selected bond distances (A) and angles (deg) for TMPP (1).

 

Atom 17 Atom 2 Distance Atom 1 Atom 2 Distance

 

 

 

P(1) C(l) 1.823(8) P(2) C(28) 1.835(7)
P(1) C(10) 1.847(8) P(2) C(37) 1.850(7)
P(1) C(19) 1.842(7) P(2) C(46) 1.853(7)
0(1) C(2) 1.377(9) 0(10) C(29) 1.342(8)
0(1) C(7) 143(1) 0( 10) C(34) 1.38(1)
C(2) C(4) 140(1) 0(11) C(31) 1.368(8)
0(2) C(8) 135(1) 0(11) C(35) 142(1)
0(3) C(6) 135(1) 0(12) C(33) 1.372(9)
0(3) C(9) 134(1) 0(12) C(36) 1.442(9)
Atom 1 Atom 2 Atom 3 Bond angle

C(1) P(1) C(10) 106.4(3)

0(1) P(1) C(19) 101.6(3)

C10) P(1) C(19) 108.1(3)

C(28) P(2) C(37) 102.5(3)

C(28) P(2) C(46) 106.5(3)

C(37) P(2) C(46) 106.2(3)

 

 

 

48

Table 4. Dihedral angles (deg) between the planes of the aryl rings and
the plane described by Cipso for TMPP (1) and TMPP=O - 2H20,

 

 

(2).
TMPP (1) TMPP=O - 2 H20 (2)
0(1) - 0(6) 74.12 89.66
C(10) - C(15) 56.41 28.81
C(19) - C(24) 40.05 41.71
C(28) - C(33) 72.09 -------
C(37) - C(42) 53.55 .......

C(46) - C(51) 36.61 .......

 

 

 

 

 

49

(2) TMPP=O - 2 H20

An ORTEP drawing of TMPP oxide is presented in Figure 6. A listing
of selected bond distances and angles is given in Tables 5. In contrast to the
propeller-like geometry of phenyl rings of TMPP, only two of the phenyl rings
of TMPP=O are tilted at 28.81° and 41.71° with respect to the plane described
by the ipso-carbon atoms. The third phenyl group is perpendicular to this
plane and hence parallel to the P=O bond. (Table 4). A similar arrangement
of the phenyl rings was observed in the structure of tris(2,6 dimethoxyphenyl)
phosphine. The average P'Cipso distance (1.817A) is slightly contracted from
that of the parent phosphine and the average C-P-C angle has opened up
from 105.20 in TMPP to 107.8°. However, the ﬂattening of the C-P-C angles
is not as extensive as that observed for the metal bound phosphine oxide in
the structure of FeCl3(O=TMPP) (C-P-Cavez 111.2[5]°).6 Not surprisingly the
P-O bond distance is much shorter in the free phosphine oxide than in the
coordinated ligand (Rom: 1.497(2) A vs P-oc,,,d= 1.550(7) A), but
nevertheless, it is slightly longer than that observed for other documented
structures of phosphine oxide ligands. One explanation for this is the
presence of two hydrogen bonded water molecules in the structure of
TMPP=O (vide infra). Finally, the observed torsion angles involving the
methoxy groups and the phenyl rings vary from 286° to 9.66°, placing the
methoxy substituents in an essentially co-planar arrangement with the
phenyl rings.

Interestingly, although the TMPP=O compound was crystallized from
dry benzene, the structure shows that there are two H20 molecules per
molecule of TMPP=O. Obviously, these were carried along in the crude

product prepared in 30% H202 solution. The location of the H20 molecules

 

 

 

 

50

 

.ONEN . Dung mo Ear—MSU mmﬁmo .@ 953%

km:

NS

 

 

 

 

 

 

 

 

 

Figure 7. Packing diagram of TMPP=O - 2H20.

 

 

 

52

Table 5. Selected bond distances (A) and angles (deg) for TMPP=O .
2H20 (2).

 

Atom 1 Atom 2 Distance Atom 1 Atom 2 Distance

 

P(1) O(10) 1.497(2) 0(1) 0(2) 1.367(4)
P(1) 0(1) 1.813(4) 0(1) 0(7) 1.421(4)
P(1) C(10) 1.810(3) 0(2) 0(4) 1.375(4)
P(1) C(19) 1.828(4) 0(2) 0(8) 1.433(5)
0(1) 0(2) 1.401(5) 0(3) 0(6) 1.369(4)
0(1) 0(6) 1.397(5) 0(3) 0(9) 1.398(5)
C(10) C(11) 1.394(5) 0(4) C(11) 1.365(4)
C(10) C(15) 1.401(5) 0(4) C(16) 1.421(4)
C(19) C(20) 1.403(5) 0(5) C(13) 1.373(4)
C(19) C(24) 1.410(4) 0(5) C(17) 1.432(5)

 

Atom 1 Atom 2 Atom 3 Angle

 

O(10) P(1) C(1) 114.2(2)
O(10) P(1) C(10) 107.9(1)
O(10) P(1) C(19) 111.5(2)
0(1) P(1) C(10) 110.0(2)
0(1) P(1) C(19) 101.5(2)

C(10) P(1) C(19) 11.8(2)

 

 

53
suggests that they are both hydrogen bonded to the terminal oxygen atom of
the phosphine oxide. Their positions relative to the phosphine is clearly seen
in the packing diagram (Figure 7). The hydrogen atoms of the water
molecules were located in the Fourier difference map but were treated only as
ﬁxed contributors to the structure factor calculation and their positions were
not reﬁned. Hydrogen atoms, H(34) and H(36), lie at a distance of 1.82 A and

1.86 A from O(10) and are clearly interacting with the oxygen lone pairs. The

 

resulting P-O---H bond angles are approximately 124.5° and 141.7°.
Apparently, the occurrence of a phosphine oxide involved in two hydrogen
bonds is rare, as only three other cases have been reported.21 In each of the
reported structures, the P-O bond is slightly longer than that of the non
hydrogen bonded phosphine oxide.

 

 

 

10.

11.
12.

13.

54
List of References

(a) Protopopov, I. S.; Kraft, M. Y. Zhurnal Obshchei Khimii 1963, 33,
3050. (b) Protopopov, I. S.; Kraft, M. Y. Med. Prom. SSSR, 1959, 13, 5;
Chem. Abstr. 1960, 54, 10914c.

(a) Wada, M.; Higashizaki, S.; J. Chem. Soc., Chem. Commun. 1984,
482. (b) Wada, M.; Higashizaki, S.; Tsuboi, A. J. Chem. Res. 1985, (S),
38; (M), 0467.

Yamashoji, Y.; Matsushita, T.; Wada, M.; Shono, T. Chem. Lett. 1988,
43.

A preliminary structural report of TMPP has appeared: Wada, M. J.
Syn Org. Chem. Jpn. 1986, 957.

For examples of phosphine oxidation promoted by platinum group
metals see: (a) Sen, A.; Halpern, J. J. Am. Chem. Soc. 1977, 99, 8337.
(b) Van Vungt, B.; Koole, N.; Drenth, W.; Kuijpers, F. P. Rec. Trav.
Chim., Pays-Bus 1973, 92, 1321. (c) Takao, K.; Fujiwara, Y.; Imanaka,
T.; Teranishi, S. Bull. Chem. Soc. Jpn. 1970,43, 1153. (d) Wilke, G.;
Schott, H.; Heinbach, P. Angew. Chem. Int. Ed. 1967, 6, 92.

(a) Dunbar, K. R.; Haefner, S. 0.;1990, 9, 1695. (b) Dunbar, K. R.;
Quillevéré, A. Polyhedron in press.

Tolman, C. A. J. Am. Chem. Soc. 1970, 92, 2953.

(a) Bino, A.; Cotton, F. A.; Fanwick, P. E. Inorg. Chem. 1979, 18, 3558.
(b) Cotton, F. A.; Frenz, B. A.; Deganello, G.; Shaver, A. J. Organomet.
Chem. 1973,50, 227.

TEXSAN - TEXRAY Structure Analysis Package, Molecular Structure
Corporation, 1985.

(a) MITHRIL: Integrated Direct Methods Computer Program,
Gilmore, C. J. J. Appl. Cryst. 1984, 17, 42-46. (h) DIRDIF: Direct
Methods for Difference Structure, An Automatic Procedure for Phase
Extension; Reﬁnement of Difference Structure Factors. Beurskens, R.
T. Technical Report 1984.

Hamilton, W. C. Acta Cryst. 1965, 18, 502.

(a) Grim, S. O.; Yankowsky, A. W. Phosphorus and Sulfur 1977,3, 191.
(b) Quin, L. D.; Breen, J. J. Org. Mag. Resonance 1973,5, 17 .

Grim, S. O.; Yankowsky, A. W. J. Org. Chem. 1977,42, 1236.

 

 

 

14.

15.
16.
17.
18.
19.
20.
21.

55
Band assignment is based on the work of (a) Stromeier, W.; Muller, F.
J. Chem. Ber. 1967, 100, 2812. (b) Bigorgne, M. Bull. Soc. Chim. Fr.
1960, 1986.
Tolman, C. A. Chem. Rev. 1977, 77, 313.
Dunbar, K R.; Quillevéré, A. submitted for publication in Polyhedron.
Le Page, Y. J. Appl. Cryst. 1988, 21 , 983.
Daly, J. J. J. Chem. Soc. 1964, 3799.
Livant, P.; Sun, Y. J .; Webb, T. R. Acta Cryst. 1991, C47, 1003.
Blount, J. F.; Maryanoff, C. A.; Mislow, K. Tetrahed. Lett . 1975, 913.
(a) Llamas-Saiz, A. L.; Foces-Foces, C.; Elguero, J.; Molina, P.;
Alajarin, M.; Vidal, A. J. Chem. Soc., Chem. Commun. 199], 1694. (b)
Jones, R.; Warrens, C. P.; Williams, D. J.; Woollins, J. D. J. Chem.
Soc., Dalton Trans. 1987, 907. (c) Antipin, M. Yu.; Akhmedov, A. I.;

Struchkov, Yu. T.; Matrosov, E. I.; Kabachnik, M. I. Zh. Strukt. Khim.
1983,24, 888.

 

 

 

 

CHAPTER III

ISOLATION, CHARACTERIZATION AND REDOX CHEMISTRY OF
THE MONONUCLEAR Rh(II) COMPLEX [Rh(n3-TMPP)2][BF4]2

56

 

 

57
1. Introduction

This chapter addresses the synthesis, solution properties, redox
chemistry, and solid-state structure of a remarkable rhodium(II) phosphine
complex. These ﬁndings are of considerable interest in the general context of
small molecule binding to paramagnetic metal centers, which has been well
investigated in the case of 00(II), but for which parallel studies are
essentially non-existent for Rh(II).1 The reason for this is undoubtedly the
paucity of stable mononuclear complexes Rh(II) that have been established
as authentic metal-based radicals.2 The uncertainty as to the location of the
unpaired electron is especially high in the reported Rh(II) compounds with
non-innocent sulfur- and nitrogen-based ligands.3 Only recently, through the
work of Wayland and co-workers,4 has the tremendous potential of Rh(II)
metallo-radical systems become apparent.

Our entry into mononuclear Rh(II) chemistry came about by a less
traditional method than previous approaches, but it is one that holds great
promise for preparing many previously unknown rhodium compounds.
Typically mononuclear Rh(II) species are prepared by either reduction of
hydrated Rh(III) halides in alcohol5 or by oxidation of Rh(I) complexes“,7
Often such routes yield impure products that are inevitably contaminated
with diamagnetic species. In contrast, there have been few reports of
paramagnetic Rh(II) species being prepared from dinuclear Rh(II)
complexes.8 This is, in part, due to the lack of suitable dinuclear starting
materials. Currently, a major focus of our research group is the development
of solvated metal systems, particularly dinuclear species, as synthetic
precursors for radical mononuclear complexes.9 To this end we are exploring
the chemistry of the solvated Rh211,11 complex [Rh2(MeCN)10][BF4]4.10 The

use of solvated metal systems in conjunction with TMPP have produced a

 

 

 

58

number of homoleptic phosphine complexes. For example, acetonitrile

 

complexes of 002+ and Ni2+ react with TMPP to give neutral bis-phosphino-
phenoxide compounds.11 More significantly, the reaction of
[Rh2(MeCN)10][BF4]4 with TMPP has resulted in the ﬁrst structurally
characterized six-coordinate Rh(II) complex without a metal-metal bond.
Herein, the synthesis, spectroscopy, structural characterization, and redox

chemistry of the remarkably stable Rh(II) complex [Rh(n3-TMPP)2][BF4]2 (3)

is presented.

2. Experimental
A. Synthesis

All reactions were carried out under an argon atmosphere by the use of
standard Schlenk-line techniques unless otherwise stated. The solvated
dirhodium complex, [Rh2(Me0N)1o][BF4]4, was prepared as described in the
literature.12 Tris(2,4,6-trimethoxyphenyl)phosphine (TMPP) (1) was
prepared by the reaction of triphenylphosphite with thevlithium salt of
trimethoxybenzene as described in chapter II. [szFe][BF4] was prepared by
oxidation of szFe with hydroﬂuoroboric acid in the presence of p-
benzoquinone.13 The reagents NOBF4, NOPFe, and szCo were purchased
from Strem Chemicals and used without further puriﬁcation.

(1) Preparation of [Rh(n3-TMPP)2][BF4]2 (3)

An amount of [Rh2(MeCN)10][BF4]4 (0.650 g, 0.675 mmol) was
dissolved in 10 mL of MeCN and cooled to -15°C with a dry ice/ethylene glycol
bath. The reaction was covered with aluminum foil to avoid photochemical
generation of Rh(I) and Rh(III) species.14 A solution of TMPP (1.437 g, 2.698
mmol) in 20 mL of MeCN was added dropwise over a 15 minute period. The
resulting purple solution was stirred for 1 h at -15°C and then evaporated to

 

 

59

a residue. The crude purple solid was washed several times with copious
amounts of THF. It was then redissolved in 50 mL of CH2012, ﬁltered, and
treated with THF (approximately 10-15 mL). The volume of the solution was
reduced with the use of a rotary evaporator to approximately 10 mL to yield a
purple crystalline solid. The solid was ﬁltered, washed with 4 x 10 mL of
THF, followed by 4 x 10 mL of diethyl ether, and dried in vacuo.; yield 1.647
g (91%). Anal. Calcd for C55H66F8P2018B2Rh: C, 48.34; H, 4.96. Found: C,
48.56; H, 4.86. IR (Nujol, CsI) cm'l: 1599 vs, 1585 vs, 1412 s, 1380 m, 1230
vs, 1207 vs, 1190 m, 1161 vs, 1135 s, 1110 s, 1049 vs/br, 950 m, 920 w, 908 m,
820 m, 785 w, 720 w, 700 w, 680 w, 673 w, 637 w, 520 w, 490 m, 450 w, 435 w,
415 w. Electronic absorption spectrum (0H2012) )tmax, nm (e): 537 (2050), 420
sh, 329 (13,400), 298 (17,500), 233 (62,400). 1H NMR (CDzClz) 5 ppm: Broad.
31P NMR (CD2012) 5 ppm: not observed.

(2) Preparation of [Rh(n3-TMPP)2][BF4]3 (4)

(i) Oxidation of [Rh(n3-TMPP)2][BF4]2 with NOBF4. A quantity of
[Rh(n3-TMPP)2][BF4]2 (0.200 g, 0.14 mmol) and NOBF4( 0.016 g, 0.14 mmol)
was dissolved in 5 mL of MeCN which resulted in the immediate formation of
a deep red solution. The reaction was stirred at -40 °C for 45 min with
periodic pumping to remove the evolved N0 gas, after which time 20 mL of
diethyl ether was added to precipitate the product. The red solid was
collected by ﬁltration under argon, washed with 3 x 5 mL of diethyl ether and
dried in vacuo; yield, 0.085 g (80%). Due to thermal instability, solid and
solution forms of 4 must be stored anaerobically at -20°C. Anal. Calcd for
C54H66F12P2018B3Rh: C, 45.40; H, 4.66. Found: C, 44.39; H, 5.14.
Electronic absorption spectrum (CH2012) Amax, nm (6): 363 (22,000), 260 sh,
245 (65,000). 1H NMR (CD2012) 5 ppm: -OCH3, 2.93 (s, 6H), 3.57 (s, 6H, ),
3.59 (s, 6H), 3.64 (s, 6H), 3.90 (s, 6H), 3.92 (s, 6H), 3.97 (s, 6H), 4.19 (s, 6H),

 

 

 

 

60

4.69 (s, 6H); m-H, 5.73 (dd, 2H), 6.05 (dd, 2H), 6.18 (dd, 2H), 6.32 (dd, 2H),
6.50 (dd, 2H), 6.68 (dd, 2H). 31P NMR (CD2012) 5 ppm: 37.4 (d, lJRh_P= 107
Hz).
(ii) Oxidation of [Rh(n3-TMPP)2][BF4]2 With [Cp2Fe][BF4]. A solution of
[Rh(n3-TMPP)2][BF4]2 (0.100 g, 0.07 mmol) and [Cp2Fe][BF4] (0.014 g, 0.07
mmol) in 5 mL of 0H2012 was stirred at 40°C for 30 min. The red solution
was treated with 20 mL of diethyl ether to precipitate a solid which was
collected by ﬁltration, washed with 2 x 10 mL of diethyl ether to remove
unreacted ferrocene and ﬁnally dried in vacuo; yield, 0.091 g (85%).
(iii) Oxidation of [Rh(n3-TMPP)2] [BF4J2 with HBF4 . Et20. An amount
of [Rh(n3—TMPP)2][BF4]2 (3) (0.015 g, 0.011 mmol) was dissolved in ~ 0.7 mL
of CD3CN. The purple solution was then transferred to a NMR tube and ~
0.1 mL of HBF4 o EtZO. Within 12 h, the solution colored had changed from
purple to red. A 31P NMR spectrum of the solution at this time revealed the
presence of [RhHI(n3-TMPP)2][BF4]3 (4).
(3) Demethylation of [Rh(n3-TMPP)2][BF4]3: Formation of [Rh(n3-

TNIPP)(C6H2(0Me)20P{06H2(0Me)3}2)] [BF412 (5)
(i) Solution A quantity of [Rh(n3-TMPP)2][BF413 (4) (0.150 g, 0.11 mmol)
was dissolved in 10 mL of acetone to give a red solution which was stirred
overnight at r.t. The pale orange solution was ﬁltered through Celite and
treated with 20 mL of diethyl ether to produce an orange solid which was
collected by suction ﬁltration, washed with 2 x 5 mL of diethyl ether, and
dried under a reduced pressure; yield, 0.085 g (61%). Anal. Calcd for
053H63F8P201882Rh: C, 47.98; H, 4.79. Found: C, 47.28; H, 5.21. Electronic
absorption spectrum (CH2012) Amax, nm (e): 329 (25,000) 260 sh, 242 sh. 1H
NMR (CD2012) 5 ppm: -OCH3, 2.94 (s, 3H), 3.06 (s, 3H), 3.32 (s, 3H), 3.41 (s,
3H), 3.46 (s, 3H), 3.52 (s, 3H), 3.53 (s, 3H), 3.55 (s, 3H), 3.68 (s, 3H), 3.84 (s,

 

 

   

 

61
3H), 3.85 (s, 6H), 3.90 (s, 3H), 3.92 (s, 3H), 4.16 (s, 3H), 4.33 (s, 3H), 4.51 (s,
3H); m-H, 5.60 (mult, 3H), 5.84 (dd, 1H), 5.89 (dd, 1H), 5.98 (dd, 1H), 6.11
(dd, 1H), 6.19 (dd, 1H), 6.23 (dd, 1H), 6.36 (dd, 1H), 6.53 (dd, 1H), 6.96 (dd,
1H). 31P NMR (CD2012) 5 ppm: + 31.5 (dd, 1JRh_p= 140 Hz, 2JPA_PB= 13.7
Hz), + 37.9 (dd, 1JRh_P= 139 Hz, 2JPA_PB= 13.7 Hz).
(ii) Solid State. An amount of [Rh(n3-TMPP)2][BF4]3 (4) (0.015 g, 0.010
mmol) was placed in a Schlenk tube and heated to 50°C for 36 hrs in an oil
bath, during which time the color of the solid changed from deep red to pale
orange. After cooling to room temperature, the 1H NMR spectrum of the solid
was recorded and compared to an authentic sample of 5 prepared as in (1).
Conversion of 4 to 5 was quantitative based on this result.
(iii) Reaction of [Rh(n3-TMPP)2][BF4]3 (4) with TMPP. A quantity of
[Rh(n3-TMPP)2][BF4]3 (4) (0.015 g, 0.010 mmol) and TMPP (0.006 g, 0.011
mmol) was dissolved in ~ 0.7 mL of CD3CN. The solution color immediately
went from red to orange. The solution was pippetted into a NMR tube and
the conversion of 4 to 5 was conﬁrmed by 1H NMR spectroscopy.
(iv) Reaction of [Rh(n3-TMPP)2][BF4]3 (4) with [(Bun)4N] [I]. A
mixture of [Rh(n3-TMPP)2][BF4]3 (4) (0.015 g, 0.010 mmol) and tetra-butyl
ammonium iodide (0.004 g, 0.011 mmol) were dissolved in ~ 0.07 mL of d6-
acetone. The solution color changed from red to orange within 5 min. The
solution was transferred to a NMR tube in order to monitor the reaction
progress by 1H and 31P NMR spectroscopy. Conversion of 4 to cis-[RhHI(n3-
TMPP)(n3-TMPP-O)][BF4]2 (5) was found to be quantitative by 31P NMR
spectroscopy. The presence of 0H3I (5 = 2.16 ppm), presumably formed as a

by-product, was detected by 1H NMR.

 

 

62

(4) Chemical Reduction of [Rh(n3-TNIPP)2][BF4]2

In a typical reaction, a mixture of [Rh(n3-TMPP)2][BF4]2 (0.100 g,
0.07 5 mmol) and szCo (0.014 g, 0.075 mmol) was dissolved in acetonitrile (5
mL). The resulting purple solution was stirred for 5 h at r.t. During this
period the solution color transformed from purple to red-brown. The solvent
was removed under vacuum and the resulting residue was taken up in THF
(5-10 mL) leaving behind undissolved [Cp2Co][BF4]. The red-brown solution
was ﬁltered through Celite and subsequently pumped to dryness. Finally, '
the brown residue was washed with diethyl ether (2 x 10 mL) and dried
under reduced pressure. 1H NMR (d6-acetone) 5 ppm: -OCH3: 3.46 (s, 3H),
3.47 (s, 6H), 3.52 (s, 18H), 3.67 (s, 3H), 3.80 (s, 12H), 3.86 (s, 6H), 3.90 (s, 3H);
m-H: 4.83 (mult, 1H), 5.18 (mult, 1H), 5.87 (dd, 1H), 6.08 (t, 4Jp_H = 4JHa-Hb =
2.1 Hz, 1H), 6.17 (d, 4JP_H= 2.7 Hz, 4H), 6.26 (d, 4Jp_H= 3.6 Hz, 2H), 6.40 (d,
4Jp_H= 3.0 Hz, 2H). 31P NMR (CD30N) 5 ppm: -8.9 (dd, 1JRh_p= 131.8 Hz,
2JpA_PB= 392.4 Hz), -134 (dd, 1JRh_p= 120.9 Hz, 2JPA_pB= 392.4 Hz).
Samples were contaminated with a small amount of TMPP-CH3+, 1H NMR
(d6-acetone) 5 ppm: 2.47 (d, 2Jp,H= 15 Hz, 3H, P-CH3), (3.66 (s, 18H, o-OME),
3.84 (s, 9H, p-OMe), 6.34 (d, 4JP-H= 4.8, 6H, m-H); and [Cp200]+: 1H NMR
(d6-acetone) 5 ppm: 5.92 (s, 10H, Cp-H). Similar results were also obtained
when acetone was used as the reaction solvent.
(5) Preparation of [Rh(TMPP)2][PF6]2

In an Erlenmeyer ﬂask, a quantity of [Rh(TMPP)2][BF4]2 (0.100 g,
0.975 mmol) and NaPF6 (0.125 g, 0.750 mmol) was dissolved in 5 mL of
acetone. The solution was stirred for 15 min, ﬁltered and an additional
amount of NaPF6 was added (0.125 g, 0.750 mmol). Again, the solution was

stirred for 15 minutes and ﬁltered. The solvent was evaporated and the

purple solid was redissolved in 20 mL of CH2012. After ﬁltration of the

 

 

 

 

63

solution to remove the undissolved sodium salts, the solution was treated
with 10 mL of THF and concentrated on a rotary evaporator to approximately
5 mL. The resulting purple crystalline solid was collected by suction
ﬁltration, washed with cold THF (2 x 3 mL), diethyl ether (2 x 5 mL) and
ﬁnally dried under reduced pressure; yield, 0.085 g (78%).
B. X-ray Crystallography

The structures of the complexes [Rh(n3-TMPP)2][BF4]2 (3) and [Rh(n3-
TMPP)2][BF4]3 (4) were determined by application of general procedures
which have been fully described elsewhere.15 Geometric and intensity data
were collected on a Nicolet P3/F diffractometer with graphite monochromated
MoKa (15 = 0.71073 A) radiation and were corrected for Lorentz and
polarization effects. All calculations were performed on a VAXSTATION
2000 computer. Data reduction and the initial reﬁnement were performed
using the programs from the Enraf-Nonius Structure Determination Package
(SDP).16 The modeling of the [BF4]' ion and ﬁnal reﬁnement for 4 were
carried out with the use of the Texsan crystallographic software package of
Molecular Structure Corporation.17
(1) [Rh(n3-TMPP)2][BF4]2 (3).
(1) Data Collection and Reduction. Single crystals of 3 were grown by a
careful layering of toluene on a solution of the compound in 0H2012. A red-
purple parallelepiped with approximate dimensions 0.67 x 0.35 x 0.22 mm3
was mounted on the tip of a glass ﬁber with epoxy cement. Geometric and
intensity data were collected at 22 i 2°C. Indexing and reﬁnement of 16
reﬂections in the range 4 S 20 S 12° selected from a rotational photograph

gave unit cell parameters for an orthorhombic crystal system. The cell was

 

 

64

 

 

 

Table 6. Summary of crystallographic data for [Rh11(n3-TMPP)2][BF4]2
(3) and [Rhmm3-TMPP)2][BF4][PF6]2 (4a).
3 4a

Formula RhP2F8018054BzH66 RhP4F16018Cs4BiH66

Formula weight 1341.5 1544.68

Space group Pbcn Pcca

a, A 15.938(5) 21.205(7)

b, A 17.916(7) 11.694(6)

c, A 21.015(8) 2910(2)

0, deg 90 90

B, deg 90 90

7, deg 90 90

V, A3 6001(6) 7216(6)

Z 4 4

dcalc, g/cm3 1.427 1.422

M, cm'1 4.20 4.17

Temperature, °C 22 i 2 -90 i 3

Trans. factors, max, min. 1.00, 0.87 1.00, 0.38

Ra 0.073 0.118

wa 0.089 0.139
ﬂuality-of-ﬁtc 2.89 4.57

 

aR=2 l IFOI- chH/ZIFOI
bRw = [2w( | F0 I— In, l)2/Zw F0 |2]1/2; w = 1/0'2( lFo I)
cQuality-of-ﬁt = [EW( ”:0 I ' [ Fc I)2/(Nobs‘NparameteerU2

 

 

65

further reﬁned by a least squares ﬁt of 20 reﬂections in the range of 13 S 20 S
25°. The Laue class was determined to be mm by axial photography. A
total of 3952 unique data were collected in the range 4 S 29 S 45° by using the
0-20 scan technique. Three standard reﬂections, measured at regular
intervals every 97 reﬂections, decayed by 2%; a decay correction was applied
to the data by using the program CHORT in SDP. After data reduction, a
total of 2462 reﬂections remained with F02 > 30(F02).

(ii) Structure Solution and Reﬁnement. The position of the Rh atom was
located by the direct methods program in SHELXS-86.18 The remaining non-
hydrogen atoms were located through successive cycles of least-squares
reﬁnements and difference Fourier maps. After isotropic convergence had
been achieved, an empirical absorption correction based upon the program
DIFABS was applied to the data.19 In the end, reﬁnement of 369 parameters
gave residuals of R = 0.080 and Rw = 0.098 and a quality-of-ﬁt index of 2.89.
The largest shift/esd was 0.58 and the highest peak in the ﬁnal difference
Fourier map was 1.91 e-/A3.

(2) [Rh(n 3-TMPP )2] [PF612IBF4]

(1) Data Collection and Reduction. Dark red crystals of [Rh(n3-
TMPP)2][PF6]2[BF4] were prepared by oxidation of [Rh(n3-TMPP)2][BF4]2 (3)
with N OPF6 and were grown from a mixture of CHgCN/toluene/Etgo at -5°C
in the presence of excess PF6: A crystal with the approximate dimensions
0.52 x 0.39 x 0.31mm3 was selected, taken up on the tip of a glass ﬁber with
viscous oil and immediately placed in a cold stream of N2 at -90°C. Indexing
and reﬁnement of 13 reﬂections selected from a rotational photograph gave
cell parameters consistent with an orthorhombic space group. After further
reﬁnement of 20 reﬂections in the range 13 < 20 < 25°, the Laue symmetry

was determined to be m by axial photography. Data were collected using

 

 

66

the co-scan technique. Three check reﬂections were monitored at regular
intervals throughout data collection and showed an average decay of 6%. The
data were corrected for the observed decay by application of the program
CHORT in SDP. After averaging equivalent reﬂections, 4722 unique data
remained of which 2390 were observed with F02 2 30(F0)2.
(ii) Structure Solution and Reﬁnement. The position of the Rh atom
was located by the direct methods program in SHELXS-86. The remaining
non-hydrogen atoms were found after a series of alternating least-squares
reﬁnements and difference Fourier maps. The positional and thermal
parameters of the hydrogen atoms were calculated and treated as ﬁxed
contributors to the structure factor calculation. An absorption correction was
applied using the program DIFABS,19 which resulted in relative minimum
and maximum transmission factors of 0.38 and 1.00. All non-hydrogen atoms
of the [Rh(TMPP)2]3+ cation were reﬁned anisotropically with the exception of
C(10), C(13), C(20), C(21), C(24), and C(25) which were reﬁned isotropically.
Thermal parameters for the [PF6]' and [EMT anions were also reﬁned
isotropically. Treatment of the [BF4]' ion is described in the following section.
The ﬁnal reﬁnement converged with residuals R and Rw of 0.118 and 0.139,
respectively, and a quality-of-ﬁt of 4.60. After convergence, the largest
shift/esd was 0.58 and the highest peak in the difference Fourier map was
2.23 e-/A3 which is associated with the boron atom of the [BF4]- counterion.
The ﬁrst counterion located in the difference map occupied a general
position in the cell and was determined to be a [PF6]' ion based on the initial
height of the peak and the presence of six other peaks in the difference map
distributed in an octahedral arrangement about the central atom. The
central atom of the second counterion was found to reside on a

crystallographic two-fold rotation axis and was accompanied by two other

 

 

67
atoms situated with a bond angle that approximated that of a tetrahedron.
Reﬁnement of the group as a [BF4]' ion led to a satisfactory thermal
parameter for boron, but unfortunately, the ﬂuorine atoms could not be
successfully reﬁned. Therefore the entire [BF4]' anion was treated as a rigid
group at a population of 0.5 using the rigid group parameters located in the
Texsan software package. The group was ﬁxed on the two-fold axis and
allowed to rotate relative to this axis. This resulted in the formation of two
rigid tetrahedral [BF4]' units, at 0.5 occupancy each, that shared a central
boron atom. Isotropic reﬁnement of the [BF4]' moiety led to the formation of
a pseudo-cage of eight ﬂuorine atoms at a population of 0.5 each about the
boron atom. Other attempts to reﬁne this group as part of a PF6' ion failed,
leading to an unsatisfactorily high isotropic thermal parameter (> 35) for the

central atom.

3. Results

The highly unusual mononuclear rhodium(II) complex, [Rh(n3-
TMPP)2][BF4]2 (3) was isolated in high yield from the reaction of TMPP and
[Rh2(MeCN)1o][BF4]4, and although the complex possesses extraordinary air
and thermal stability for a radical species, it nonetheless exhibits a rich and
varied chemistry. Chemical or electrochemical oxidation of 3 produces the d6
Rh(III) complex [Rh(n 3-TMPP)2][BF4]3 (4) with a ligand arrangement
identical to that found in the parent complex. In striking contrast to the
remarkable stability of the divalent complex [Rh(n3-TMPP)2][BF4]2 (3), the
Rh(III) species 4 is air-sensitive and thermally unstable, eventually forming
the demethylated complex Rh(III) complex, [Rh111(n3-TMPP)(n3-TMPP-
0)][BF4]2 (5), where TMPP-O = [(006H2(OMe)2P{C6H2(OMe)3)2]1'.

 

 

68

The structures of compounds 3 and 4 have been determined by single
crystal X-ray diffraction methods. Crystal parameters and basic information
pertaining to data collection and structure reﬁnement are summarized in
Table 6. ORTEP representations of [Rh(n3-TMPP)2][BF4]2 (3) and [Rh(n3-
TMPP)2][PF6]2[BF4] are depicted in Figures 9 and 13. Selected bond
distances and angles for each structure are listed in Tables 8 and 9. Full
tables of positional and anisotropic thermal parameters for compounds 3 and
4 are located in the appendices.

A. Preparation and Spectroscopic Properties of [Rh(n3-TMPP)2]
[BF4l2 (3)

Slow addition of TMPP to [Rh2(Me0N)1o][BF4]4, both dissolved in
CH3CN, produces a dark red-purple solution of 3, with the reaction being
complete within one hour (eq 12). The reaction is performed in the dark in
order to avoid formation of Rh(I) and Rh(III) species that result from room
light photolysis of [Rh2(MeCN)1o]4+ in acetonitrile.20 Free TMPP reacts with
[Rh(n3-TMPP)2][BF4]2 to dealkylate one of the coordinated methoxy

M N
[Rh2(MeCN)10][BF4]4 + 4TMPP —%p 2 [RhII(n3-TMPP)2][BF4]2 (12)
1 3

groups. This results in the formation a new Rh(II) complex that is ligated by
one neutral TMPP ligand and a phenoxy-phosphine derivative of TMPP.
Therefore, in order to maximize yields of 3, the presence of excess phosphine
must be avoided during the reaction. As a result, dilute solutions of TMPP
are added dropwise to highly concentrated of [Rh2(MeCN)10][BF4]4 to

maintain an excess of [Rh2(MeCN)10][BF4]4. Moreover, the reaction is kept

at 0° C to further retard nucleophilic attack by free phosphine. Unlike

 

 

 

69
[RhH(n3-TMPP)2][BF4]2 (3), the demethylated Rh(II) complex is air-sensitive
and readily decomposes to an intractable mixture of diamagnetic species.
Details of the deliberate synthesis of this species by reaction of 3 with various
nucleophiles is presented in chapter VII.

Initially, [Rh(n3-TMPP)2][BF4]2 was synthesized by addition of a
methanolic solution of TMPP to a suspension of [Rh2(MeCN)10][BF4]4 in
MeOH. This method was attractive because the relatively low solubility of 3
in MeOH leads to the precipitation of the product. However, the yields were '
typically lower than those found for reactions carried out in MeCN; this is
presumably due to the low solubility of the Rh24+ salt in MeOH which serves
to keep the phosphine ligand in excess and increases the likelihood of the
demethylation side-reaction. Furthermore, samples of 3 obtained from
methanolic solutions were invariably contaminated by a gray solid with
unusual properties. When redissolved in acetonitrile, the solid produces
orange solutions that analyzed as [Rh2(MeCN)10]4+ by 1H NMR. This gray
solid is believed to be a mixed-valence linear tetramer formed by one electron
reduction of the dirhodium species.21 Evidence for the formation of this
species is found in the photochemistry of [Rh2(MeCN)10][BF4]4 and a similar
species has been prepared directly by electrochemical reduction of
[Rh2(MeCN)10][BF4]4 in high ionic strength media by another member of this
research group.20’21

As a ﬁnal synthetic approach to preparing Rh(II) complexes of TMPP,
we reacted TMPP with RhCl3'xH20 in reﬂuxing ethanol, a method that has
been widely used to prepare many of the reported Rh(II) species with bulky
phosphines.5 In the present case, however, an intractable mixture of

diamagnetic products was detected by 1H NMR spectroscopy. We rationalize

 

 

 

70

that this is largely due to the demonstrated instability of [Rh(n 3-
TMPP)2][BF4]2 in the presence of free halides.

The compound [Rh(n3-TMPP)2][BF4]2 is soluble in methylene chloride
and acetonitrile, partially soluble in methanol, chloroform and acetone, and
completely insoluble in water, tetrahydrofuran, diethyl ether and
hydrocarbon solvents. The complete lack of solubility of 3 in THF is
important, since the reaction by-products formed in the synthesis of 3 exhibit
limited solubility in THF and can therefore be separated from 3 by repeated
washings with THF. Recrystallization of [Rh(n3-TMPP)2][BF4]2 also relies on
its insolubility in THF; recrystallization is accomplished by concentrating 3:1
CH2012 / THF solutions of 3 on a rotary evaporator until the solutions are
sufﬁciently rich in THF to precipitate the paramagnetic complex as a purple
crystalline solid. In contrast to the [BF4]' salt, the [PF6]' salt of [RhH(n3-
TMPP)2]2+ is slightly solubile in THF.

Remarkably, [Rh(n3-TMPP)2][BF4]2 is air-stable, but CH2012 solutions
decompose slowly over periods of several weeks, depositing a ﬁnely divided
insoluble gray solid presumed to be rhodium metal. Solid samples, on the
other hand, are stable for indeﬁnite periods in air. These observations are
somewhat surprising considering the documented, sensitive nature of Rh(II)
monomers. Typically these molecules can only be synthesized by
electrochemical methods and subsequently decompose within short periods of
time even under anaerobic conditions.22

The infrared spectrum of [Rh(n 3-TMPP)2][BF4]2 displays bands
assignable to coordinated TMPP ligands and a broad feature at 1050 cm-1
which is indicative of the [BF.,]' counterion. The electronic spectrum recorded
in CH2012 exhibits a d-d transition at Am” = 537 run (2050 M-1 cm'l) which is

responsible for the intense purple color of the compound. Several higher

 

 

71

energy charge-transfer transitions are located at Amax = 329 nm (1.34 x 104 M-
1cm-l), 298 nm (1.53 x 104 M-1cm'1) 285 nm (1.75 x 104 M'1cm'1), 233 nm (6.24
x 104 M-1cm-1), 420 nm (sh).

Not unexpectedly for a d7 metal complex, the 1H and 31P NMR signals
are broad and reveal no information about the structure. In general, Rh(II)
phosphine complexes have been characterized by their EPR activity and their
broad, largely unidentiﬁable NMR features. We note that this data and ours
are in sharp contrast to several puzzling instances in which paramagnetic
Rh(II) compounds have evidently exhibited sharp and unshifted NMR signals
as well as EPR signals.3°:23
B. Magnetic and EPR Spectroscopic Properties of [Rh(n 3-

TMPP)2] [BF412 (3)

The paramagnetism of 3 was investigated by EPR spectroscopy and
magnetic susceptibility, the results of which are presented in Table 7. EPR
measurements of 3 were carried out in a variety of solvents at 298 K and at
77 K. At room temperature, solutions of 3 in CH2012, acetonitrile/toluene
(1:1) and CH2012/Me-THF (1:1) produce identical spectra with a single
isotropic homogeneous line at g z 2.20. The EPR spectrum of [Rh(n3-
TMPP)2][BF4]2 in a 0H2012/MeTHF glass at 77 K exhibits a rhombic signal
as shown in Figure 8. The g—values of the CH2012MeTHF system are gxx =
2.26, gyy = 2.30 and gzz = 1.99. The proximity of gzz to that of the free-
electron value is consistent with a dz2 ground state for the unpaired electron
and conﬁrms the radical nature of 3. The average of the three anisotropic g-
values equals that of the isotropic line at room temperature, therefore the

change in line shape is due to the rapid tumbling of the complex in solution,

 

 

 

.m E as 33» BE.
525636 d 3 5 «3833329355: .5 8583 Em

-:
u
I..-

mmzmo «N l_

72

 

 

 

 

 

 

 

 

73

as opposed to being frozen in random orientations with respect to the applied
ﬁeld at 77 K. In the anisotropic spectrum (77 K), gzz is split into a doublet
(Au: 2.0 x 10'3 cm'l) due to the hyperﬁne interaction with the I=1/2 nucleus
of 103Rh. Since the linewidths along gxx and gyy are broader than the
hyperﬁne tensor components along these axes, hyperﬁne coupling is not
observed. Similar behavior has been reported for Rh(II) species trapped in
zeolites?4 In the zeolite study, a doublet splitting of gzz of ~32 G was
reported and a dynamic J ahn-Teller effect was observed at high temperatures
(400°C). Even so, examples of 103Rh hyperﬁne coupling along all three
principal directions are not unknown; Wilkinson and co-workers observed
hyperﬁne coupling along all three axes (Ax = 4.7 x 10'3 cm'l, Ay = A2 = 5.6 x
10'3 cm'l) in the organometallic species Rh(2,4,6-Pri306H2)2(tht)2.25

The magnetic moment of [Rh(n3-TMPP)2][BF412 (3) was measured both
in solution and the solid state. The results of these studies are listed in Table
7. Solid state susceptibility measurements were performed over a
temperature range of 5-286 K at a variety of ﬁeld strengths. Compound 3
exhibited Curie-Weiss behavior over the entire range of temperatures. A plot
of Xm vs 1/T yielded a temperature independent paramagnetism (T.I.P.)
contribution of 424 x 10'6 cgsu, which was applied, along with a correction for
diamagnetism, to the average magnetic moment giving a value of 1.80(3) HB-
Solution studies were carried out using the Evans NMR method26 and
resulted in a Heff of 2.10 03. Both measurements are consistent with a S=1/2
ground state, as is expected for a low spin d7 ion. Similar values have been

reported for several other mononuclear Rh(II) complexes?

 

 

 

74

Table 7. EPR Spectroscopic and Magnetic Susceptibility Data for [Rh(n3-

 

 

TMPP)2][BF412 (3)
[Rh(n3-TMPP)2][BF4]2 (3)
EPR (CHzClg/Me-THF glass)a
gxx 2.26
gyy 2.30
gzz 1.99
Azz, G (cm-1) 22 (2.0 x 10-3)
magnetic moment, peg, 1113b
solution0 2. 10
solidd 1.80

3 Measured at 77 K. b A diamagnetic correction of -724 x 10'6 was applied
based on -20 x 10'6 for Rh2+, -39 x 10'6 for BF4' and -313 x 10'6 for TMPP. 0
Measured in CH2012 at 294 K using Evans methodls. d Average magnetic
moment measured over a temperature range of 5-286 K at several ﬁeld
strengths.

 

 

 

75

0. Crystal Structure of [Rh(n3-TMPP)2][BF4]2 (3)

The molecular cation shown in Figure 9, consists of two phosphine
ligands bonded to the Rh atom in a face capping tridentate mode through the
oxygen atoms of two pendant methoxy-groups and the phosphorus atom.
This bonding arrangement of TMPP is identical to that observed in the
molybdenum tricarbonyl complex (n3—TMPP)Mo(CO)3?7 The geometry about
the Rh center is a distorted octahedron, with the Rh atom residing on a two-
fold axis that bisects the P-Rh-P' angle (Figure 10). Perhaps the most
striking feature of the structure is the fact that the phosphines lie cis to each
other rather than trans, as expected on the basis of steric arguments. The
consequences of steric repulsion resulting from the cis geometry are evident
in the P(1)-Rh-P(1)' bond angle of 105.2(1)°. Indeed Rh(II) complexes with
the formula RhX2L2 where L is a bulky tertiary phosphine have been shown
to exist in a transconformation.6 While it is conceivable that the cis complex
is a kinetic product, we have been unsuccessful in isomerizing the compound
to a transgeometry at higher temperatures. It is interesting in the context of
this discussion to note that Rauchfuss et al. also observed a cis arrangement
of PPh2(o-Me006H4) groups for Ru012(PR3)2, in which the ether-phosphine
also participated in a chelating interaction with the metal?3 Furthermore,
Shaw noted similar cis isomers in the reaction of PPh2(06H4OH) with Pd and
Pt chlorides.29 Apparently, electronic factors play a prominent role in
stabilizing the cis conformation relative to the sterically favored trans
geometry for these mixed ether-phosphine donors.

In the case of 3, the chelation results in the formation of a ﬁve-
membered metallacyclic ring, the geometric requirements of which distort the
molecule as evidenced by the strained P(1)—Rh(1)-O(6) bond angles of 77 .7(2)°
and 80.5(1)° (Table 8). This distortion is of the same magnitude as that

 

 

 

 

 

       
   
     

76
C(17) 0(5)
0..— C(16)
C12
C(9)0 ( )
0(4)
C(26)
C(22) Q)
C(27) C(23) 0“”
(Iowan) 3 50(21)
., t’——-OC(25)
C(19) C(4)) 0(7)
W

Figure 9. ORTEP representation of the [Rh(n3-TMPP)2]2+ (3)
molecular cation with 40% probability ellipsoids. Phenyl
ring atoms are shown as small spheres of arbitrary size for
clarity.

 

 

 

77

 

Figure 10. ORTEP drawing emphasizing the coordination sphere of
[Rh(n3-TMPP)2]2+.

 

 

 

 

78

observed for other complexes containing similar chelate rings, such as
As013(PR3)230 Ru012(PR3)228 and (n3-TMPP)Mo(CO)327

The distances within the immediate coordination sphere of Rh are
worth commenting upon as they show some unusual features. The Rh(I)-P
bond distance of 2.216(2)A is signiﬁcantly shorter than the corresponding
distances found in most Rh(I) and Rh(III) phosphine complexes, which are
generally in the range of 2.28 A to 2.37 A31 As for the Rh-P bond distances

 

in other mononuclear Rh(II) compounds, the only other example which we
are aware, trans-Rh012(PPri3)2, exhibits Rh-P bonds that are slightly longer,
2.366(1) AG» 32 This is not unexpected, since in Rh012(PPri3)2 the phosphines
exert a signiﬁcant trans effect on each other. In contrast, complex 3 contains
ether donors in positions opposite to the phosphorus donors; these would be
expected to exhibit little if any trans effect.33 Indeed the Rh-O bond
distances in 3 are quite long as compared to those of metal alkoxides or other
anionic oxygen donor ligands and are indicative of the weak nature of the Rh-
ether bond.34 Rauchfuss arrived at a similar conclusion upon observing that
Ru012(PR3)2, with phosphorus atoms trans to methoxy-groups, exhibits Ru-P
bond distances that are much shorter than in other Ru(II) phosphine
complexes?8

Of further importance in the solid state structure of [RhH(n3-
TMPP)2]2+ is the presence of two chemically distinct metal-ether interactions.
The oxygen atoms that are trans to phosphorus are bonded at a distance of
2.201(6) A, whereas the mutually trans oxygen atoms exhibit Rh-O distances
of 2.398(1) A. The ain'al elongation is undoubtedly the result of the unpaired
electron residing in the dz2 orbital and constitutes the ﬁrst structural
evidence for such an effect in d7 Rh(II) chemistry. Removal of the odd

electron via chemical oxidation results in shortening of the Rh-OMe

 

 

‘l

79

 

 

 

 

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80
distances, so that both sets of methoxy bonds to the metal, axial and
equatorial, are essentially equivalent in length (vide infra). Interestingly,

although the axial ether groups are located at long distances, they afford

 

protection from attack by donor solvents such as MeCN or oxidizing solvents
such as CH2Clz. Similar protective properties of ortho methoxy substituents
were also noted by Rauchfuss?8
D. Redox properties of [Rh(n3-TMPP)2][BF412 (3)

A cyclic voltammogram of [Rh(n3-TMPP)2][BF4]2 (3) (shown in Figure
11) exhibits two accessible redox couples, one reversible oxidation at Em = +
0.46 V and a quasi-reversible reduction at E1/2 = ~0.65 V (vs Ag/AgCl).
Accordingly, we expected that 3 would undergo chemical redox reactions to
form Rh(I) and Rh( III) compounds with minimal structural rearrangement.
(l) Oxidation of [Rh(n3-TMPP)2][BF412 (3)
(1) Synthesis and spectroscopic characterization of [Rh(n3-
TMPP)2][BF4]3

Oxidation of 3 with either [NO][BF4] or [0p2Fe][BF4] in MeCN or
0H2012 solutions results in the formation of a deep red solution of [Rh(n3-
TMPP)2][BF4]3 (4). The product was isolated as an oily solid by addition of
Et20 or evaporation of the solvent. Complex 4 is both air sensitive and
thermally unstable. The latter point is illustrated by the fact that solutions
of 4 readily decompose at ambient temperatures (vide infra). To circumvent
this problem, the synthesis of 4 was carried out at -40°C. The cyclic
voltammogram of 4 is identical to that of 3 except that the two redox couples
correspond to one-electron reductions to form Rh(II) and Rh(I) species
(Figure 12). Shriieder and Cooper reported analogous electrochemical
behavior for the homoleptic Rh(III) thioether complexes [Rh(9S3)2]3+,
[Rh(lOS3)2]3+ and [Rh(1283)2]3+, for which both the Rh(II) and Rh(I)

 

 

 

81

Elam) = + 0.46 v

     
    

RhIII / RhII

SpA

Rh“ / Rh!

E1/2(red) = ' 0.65 V

_1 1

+1.5 +1.0 +0.5 do -o.'5 -|:O -u.'5
VOLTS vs Ag/AgCl

Figure 11. Cyclic voltammogram of [RhH(n3-TMPP)2][BF412 (3) in 0.1 M
TBABF4 in CH2C12.

 

 

 

 

 

 

 

 

 

 

82

oxidation states are accessible.22""c Interestingly, acetonitrile solutions of 3
may also be oxidized to 4 by the addition of an excess of HBF4 - Et20. The
reaction, which occurrs slowly over a period of 12 hours, was monitored by
both 1H and 31P NMR spectroscopy. Presumably, H+ is reduced to H2 during
the course of the reaction, although the formation of H2 was not readily
apparent from the 1H NMR spectrum.

Infrared spectral measurements of 4 show a typical pattern for

coordinated TMPP in addition to a strong broad band at 1050 cm'1 which

 

corresponds to the [BF4]' counterion. The 1H NMR spectrum of 4 in CD2012
exhibits nine resonances between 5 = 2.93 and 5 = 4.69 corresponding to the
12 methoxy substituents for two magnetically equivalent TMPP ligands.
Further downﬁeld there are a set of six poorly resolved doublets of doublets of
equal intensity found in the region of the meta-protons of the phenyl rings.
The ABX pattern of the resonance corresponds to coupling of one ring proton
to the other ring proton as well as to the phosphorus. The 31P NMR spectrum
in CD2012 shows a doublet centered at 5 = + 37.4 ppm (JRh-p = 107 Hz),
further indicating that the two TMPP ligands are equivalent. Therefore, it
appears that the ABX pattern of the meta-protons arises from coupling of
proximal and distal protons on the individual chelating rings, which then
further couple to a phosphorus nucleus to produce the observed ABX splitting
pattern. Although only two rings participate in the chelation to the metal, it
is apparent that the meta-protons on the third non-interacting ring are
magnetically inequivalent as these protons also exhibit an ABX splitting
pattern. These results suggest that the non-interacting ring is sterically
locked into an asymmetric environment and is unable to rotate freely about

the P-C bond, thereby rendering the ring protons inequivalent.

 

 

 

83

EH2 (red); = +0.46 V EH2 (red)2 = -0.65 V

 

I
SpA
1

 

l 1

+15 +10 +05 do -0.' -If0 -I.'5

VOLTS VS Ag/AgCl

Figure 12. Cyclic voltammogram of [Rhm(n3-TMPP)2][BF4]3 (4) in 0.1
M TBABF 4 in CH2C12.

 

 

 

84

(ii) Crystal Structure of [Rh(n3-TMPP)2][PF6]2[BF4]

Difﬁculties encountered while modeling a disorder of the [BF4]- anions
have precluded a satisfactory reﬁnement of the structure in these regions.
Attempts to model the highly disordered counterions are described in the
experimental section. These problems notwithstanding, the Rh(III)
molecular cation itself is well behaved, and therefore, a brief description of

this species is presented here. The preliminary X-ray results show that the

 

Rh atom lies on a two-fold axis that bisects the cis phosphine ligands as in
the structure of the parent complex [Rh(n3-TMPP)2]2+ (3) (Figure 13). As a
result of this symmetry, the two phosphines are crystallographically
equivalent just as was observed in solution by 1H and 31P NMR studies. Both
TMPP ligands bind to the metal in the familiar capping tridentate fashion
observed in the structure of 3 and in the molecule (n3-TMPP)M0(CO)3?7 The
most striking feature of the structure 4 compared to 3 is the dramatic
contraction of the axial Rh-OMe bonds while the equatorial distances remain
essentially unchanged (Table 9). The Rh(1)-O(1) bond distance has shortened
from 2.398(5) A in [Rh(n3-TMPP)2][BF4]2 to 218(2) A in the structure of 4,
and is now is roughly equivalent to the equatorial Rh-O(6) distance (222(1)
A). Thus, removal of the odd electron from the dz2 orbital of 3 alleviates the
observed axial distortion. The above result is important because it illustrates
the TMPP ligand's ability to adjust to and accommodate electronic changes at
the metal center.
(iii) Decomposition of [Rh(n3-TMPP)2][BF4]3 (4)

As mentioned earlier, solutions of [Rh(TMPP)2][BF4]3 are thermally
unstable with respect to loss of a methyl group to form [Rh(n3-TMPP)(n3:
TMPP-0)][BF4]2 (5) ( n 3-TMPP-O = P{06H2(OMe)3}2{06H2(OMe)2O}) as

shown in equation 13. At room temperature, solutions of 4 in CD3CN show

 

 

 

85

 

0/

Figure 13. ORTEP drawing of the [Rh111(n3-TMPP)2]3+ (4) molecular
cation. Phenyl ring atoms are shown as small spheres of
arbitrary size for clarity.

 

 

 

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87
evidence of demethylation within minutes by NMR spectroscopy, but upon
cooling to -20 °C the process is slowed considerably Dealkylation eventually
occurs regardless of solvent choice, but the reaction is much faster in acetone
and acetonitrile than in methylene chloride. The process is quite facile, as
evidenced by the fact that even solid samples of 4 slowly convert to 5 over a
period of weeks at room temperature. If one heats solid samples of 4 to 50 °C

under vacuum, nearly quantitative conversion to 5 occurs within 36 hours.

 

-CH;
[Rhm(n3-TMPP)2][BF413 a? [Rhm(n3—TMPP)(n3-TMPP-0)][BF412 <13)
2 2
4 r. t. 5

Compound 5 was prepared in bulk by stirring acetone solutions of
Rh(n3-TMPP)2[BF4]3 at room temperature for 12 hours; during this time the
solution color changed from deep red to orange. The product was isolated as
a pale orange solid by addition of diethyl ether, and while its infrared
spectrum is nearly indistinguishable from that of [Rh(n3-TMPP)2][BF4]3 (4),
the 1H NMR spectrum reveals the magnetic inequivalence of the two
phosphine ligands (Figure 14). The spectrum, shown in Figure 14, displays
nine distinct resonances between 6 = 5.84 and 6.96 with an ABX coupling
pattern and a set of three overlapping signals centered about 5 = 5.60 ppm.
Upﬁeld resonances between 8 = 4.51 and 8 = 2.94 ppm correspond to the ortho
and para methoxy substituents on the phenyl rings and number 17 by
integration, as compared to 18 in the parent complex [Rh(n3-TMPP)2]3+,
thereby conﬁrming the loss of a methyl group from one of the methoxy
substituents. The appearance of the 1H NMR spectrum is highly solvent
dependent.

 

 

 

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89

The conversion of [Rh(n3-TMPP)2][BF4]3 (4) to [Rh(TMPP)(TMPP-
0)][BF4]2 (5) has been monitored by 1H NMR in d6-acetone, d3-acetonitrile
and d2-methylene chloride; each of these studies revealed the onset of new
resonances for 5, but none that could be assigned to the protons of a
methylated by-product were observed. This observation coupled with the
solid state transformation of 4 to 5 is consistent with the formation of a

volatile species that we believe to be CH3F resulting from attack by [BF4]- on

 

the complex (eq 14).35 Although uncommon, such transformations are not

unprecedented; In fact, several examples of activation of [BF4]' by transition

OMe OMe
IF _I -1
f" Me F- B‘u" F
MeO ,0 F MeO ,0 CH8 F T
,‘..P-Rhm —> ,‘,.P-Rhm + (14)
Ar / Ar / BF3T
Ar' Ar‘

metal complexes have been reported.36 Generally these transformations
involve highly charged electrophilic metal centers. Such complexes are so
susceptible to activation that researchers are resorting to highly deactivated
ﬂuoro-substituted anions such as [(C6F5)4B]' and [(3,5-CF3-CGH3)4B]‘ in
order to stabilize these highly electrophilic metal centers.37 It is apparent
that in order to further expand the chemistry of [Rh(n3-TMPP)2][BF4]3,
similar strategies must be adopted in our laboratories.

The 31P NMR spectral data provide information regarding the
coordination geometry about the metal center.3 8 31P NMR spectral
measurements of 5 (Figure 14) reveal two resonances with an ABX splitting

pattern at 8 = + 37.9 ppm for PA (1JRh-p=138.9 Hz, 2Jp.p=13.73 Hz) and 5 = +

 

 

90
31.5 ppm for P3 (1JRh_p=140.4 Hz, 2Jp_p=13.73 Hz). The small magnitude of
the P-P coupling is indicative of a cis disposition of phosphines.39
Furthermore, the similarity in the two Rh-P coupling constants suggests that
the phenoxide interaction is cis to both phosphorus atoms, in an axial (ax)
position relative to the plane containing the phosphorus atoms. Presence of a
Rh-phenoxide bond in a trans disposition to one of the phosphorus nuclei
would be expected to result in a larger difference in the Rh-P coupling
constants. The structure of 5, shown below, is rationalized by considering
that a methoxy interaction with a metal center weakens the O-CH3 bond thus
rendering the carbon susceptible to nucleophilic attack. It follows that the
stronger the metal-oxygen interaction, the more electrophilic the methyl

group becomes. Methoxy groups exert a weaker trans inﬂuence than the

Me Me 72+
in ff)
OMe
M
PI_:,,, 11111in Fo’ _ea —{ yOMe
(j): OMe

ax-[Rhm(n3 -TMPP)(T1 3-TMP1>-0)]2+

phosphorus atoms, resulting in stronger M-O interactions for the mutually
trans methoxy groups than for those trans to the phosphines. It is worth
noting that dealkylation of ether groups in methoxy-phosphines has been
previously observed, including for a dirhodium carboxylate complex prepared
in our laboratories, but in contrast to the present study, these were found to
occur under more forcing conditions and in the presence of much stronger

nucleophiles.4°’41 The instability of [Rh(n3-TMPP)2][BF4]3 (4) may, in part, be

 

 

 

91

explained by the relatively hard nature of the Rh3+ cation compared to that of
other platinum group metals in the +1 and +2 oxidation states. Furthermore,
from their work with ether-phosphines, Shaw and co-workers observed that
the ease of dealkylation increased with increased steric bulk of the ancillary
substituents on the phosphine.41b It is reasonable to assume that the
considerable bulk of the methoxy substituted phenyl rings of TMPP
contributes to facile demethylation of [Rh(n3-TMPP)2][BF4]3 (4).

 

We found that dealkylation of a coordinated methoxy-group
signiﬁcantly alters the electrochemistry of the complex. Electrochemical
measurements performed on CH2C12 solutions of 5 show that the molecule
undergoes an irreversible oxidation at Em = +1.55 V vs Ag/AgCl that most
likely corresponds to a ligand-based process based on the fact that a number
of other TMPP species undergo similar processes near this potential. More
importantly, 5 exhibits an irreversible reduction at E10,c = -0.80 V vs. Ag/AgCl
with a coupled chemical oxidation wave at -0.02 V vs Ag/AgCl. Chemical
reduction with cobaltocene yields a deep red solution that exhibits a broad
1H NMR signal characteristic of a paramagnetic compound. The latter species
resembles the product obtained by the direct reaction of the Rh(II) complex
[RhH(n3-TMPP)2][BF412 (3) with additional TMPP. In fact, comparison of the
EPR spectroscopic properties of this newly formed Rh(II) complex with that of
the product obtained from nucleophilic attack on [RhII(n3-TMPP)2][BF4]2 (3)
reveal that the two compounds are identical. We propose that the two
products obtained by both routes are the Rh(II) analog of 5, i.e. the Rh(II)
phosphino-phenoxide complex [Rh(TMPP)(TMPP-0)][BF4]. Details of the
chemical and structural relationship between 5 and its Rh(II) analog are

presented in chapter VII.

 

92

(2) Reduction of [Rh(n3-TMPP)2][BF4]2 (3)

In light of the synthesis of [Rh(n3-TMPP)2][BF4]2 and [Rh(n3-
TMPP)2][BF4]3, isolation of a analogous Rh(I) complex ligated solely by
TMPP has proven elusive. Chemical reduction of [Rh(n3-TMPP)2][BF4]2 (3)
by cobaltocene in either acetone or acetonitrile produces a red-brown solution
within several hours at room temperature. No reaction occurs at

temperatures less than -15°C. The product was isolated from THF as a

 

brown diamagnetic solid. Samples were invariable contaminated with minor
amounts of salts containing [TMPP-CH3]+ and [Cngo]+. The complex series
of multiplets and doublets observed for the meta proton resonances in the 1H
NMR spectrum of the solid, clearly indicates that the two phosphines are
magnetically inequivalent. Moreover, integration of the spectrum reveals
that only 17 methoxy resonances are observed, clearly supporting that a
dealkylation reaction occurs following reduction of the metal. This is not
surprising, in light of the demonstrated presence of [TMPP-CH3]+ in isolated
samples. While the complexity of the 1H NMR spectrum precludes further
analysis of the coordination geometry, important information may be gleaned
from the 31P NMR spectrum. The 31P NMR spectrum of the product in
CD3CN exhibits two resonances with an ABX spin system at 8 = -8.9 ppm (dd,
1JRh_p= 131.8 Hz, 2JpA_PB= 392.4Hz) and -13.4 ppm (dd, 1JRh_p= 120.9 Hz,
ZJPA-pB= 392.4 Hz) (Figure 15). The magnitude of the PA-PB coupling
constant is in agreement with the presence of two magnetically inequivalent
TMPP ligands that have a trans relationship to each other. Based on the the
NMR spectral data presented, it appears that the reduction of 3 ultimately
results in the formation of trans- RhI(n2—TMPP)(n2-TMPP-O) (eq. 15).
Apparently, the Rh(I) cation, [Rh(TMPP)2]1+, formed upon initial reduction of

 

 

 

 

 

 

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94

3, is unstable with respect to phosphine dissociation. Upon dissociation, the

 

[Rhn(n3-TMPP)2]2+ ﬂ. "[RhI(TMPP)2]1+" (15a)
Me Me
O Afar
TMPP 0Q?"
"[RhI(TMPP)2]1+" Me, 0‘1th 0 (15b)
[TMPP-CH3]+ Arm‘fﬁ
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Me Me

free TMPP then acts as a nucleophile resulting in the demethylation of a
coordinated phosphine. Further evidence for phosphine dissociation was
provided by the appearance of [TMPP-CHzcl]+ impurities in samples
prepared in CH2012.

4. Discussion

As far as we are aware, [Rh(n3-TMPP)2]2+ represents the only case of a
structurally characterized six—coordinate mononuclear Rh(II) complex. The
utility of [Rh2(MeCN)10][BF4]4 as a synthon for unusual mononuclear and
polynuclear rhodium species is illustrated by the observation that chemistry
of TMPP with other dinuclear starting materials, such as
Rh2(OAc)4(MeOH)2, yield only partially substituted dinuclear TMPP
complexes“, 42 The stability of the (formally) 19 e' species is attributed to
the presence of four interacting ether groups which serve to stabilize the
complex both electronically and sterically. The presence of these pendent

groups serve two purposes: (1) to kinetically stabilize the metal center from

 

 

 

 

 

 

95
dimerization and (2) to provide thermodynamic stability for the +2 oxidation
state relative to either Rh(I) or Rh(III) oxidation states. Without the "built-
in" coordinative saturation afforded by the tethered "solvent" molecules, one
would expect the molecule to undergo the usual disproportionation or
dimerization reactions which are commonly observed in divalent Rh
chemistry. However, unlike many complexes kinetically stabilized by
sterically encumbering ligand sets, the lability of the ether groups opens up

coordination sites so that ﬁirther reactivity may occur. Moreover, the ready

 

availability of the dangling ether groups allows for reversible substitution
reactions with a variety of substrates, a situation which is generally not
possible with complexes that undergo dissociative loss of a ligand. We have
exploited these aspects of the ligand in order to further develop the
potentially rich area of mononuclear Rh(II) chemistry, results of which are

presented in subsequent chapters.

 

 

 

10.

96
List of References

(a) Kemmit, R. D. W.; Russel, D. R.; in Comprehensive Organometallic
Chemistry; Wilkinson, G.; Stone, F. G. A.; Abel, E. W., eds.; Pergamon:
New York, 1982; vol. 5. (b) Smith, T. D.; Pilbrow, J. R. Coord. Chem.
Rev. 198], 39, 295. (c) Stoppioni, P; Dapporto, D.; Sacconi, L. Inorg.
Chem. 1978, 17, 718. (d) Klein, H.-F.; Karsch, H. H. Chem. Ber. 1976,
109, 1453. (e) Mu, X. H.; Kadish, K. M. Inorg. Chem. 1989, 28, 3743.

Felthouse, T. R. Prog. Inorg. Chem. 1982, 29, 73 and references
therein.

(a) Billig, E.; Shupack, S.I.; Waters, J.H.; Williams, R.; Gray HE. J.
Am. Chem. Soc. 1964, 86, 926.(b) Pneumatikakis, G.; Psaroulis, P.
Inorg. Chim. Acta 1980, 46, 97. (c) Pandey, K. K.; Nehete, D. T.;
Sharma, R. B. Polyhedron 1990, 9, 2013. (d) Peng, S. M.; Peters, K.;
Peters, E. M.; Simon, A. Inorg. Chim. Acta 1985, 101, L35.

(a) Wayland, B. B.; Sherry, A. E.; Cofﬁn, V. L. J. Chem. Soc., Chem.
Commun. 1989, 662. (b) Wayland, B. B.; Sherry, A. E. J. Am. Chem.
Soc. 1989, 111, 5010. (c) Wayland, B. B.; Sherry, A. E.; Poszmik, G.;
Bunn, A. G. J. Am. Chem. Soc. 1992, 114, 1673. (d) Wayland, B. B.;
Ba, 8.; Sherry, A. E. J. Am. Chem. Soc. 199], 113, 5305. (e) Sherry, A.
E.; Wayland, B. B. J. Am. Chem. Soc. 1990, 112, 1259. (f) Bunn, A. G.;
Wayland, B. B. J. Am. Chem. Soc. 1992, 114, 6917.

(a) Bennett, M. A.; Lonstaﬂ‘, P. A. J. Am. Chem. Soc., 1969,91, 6266.
(b) Moers, F. G.; DeJong, J. A. M.; Beaumont, P. M. J. J. Inorg. Nucl.
Chem. 1973,35, 1915. (c) Masters, C.; Shaw, B. L. J. Chem. Soc. (A)
1971, 3679. (d) Empsall, H. D.; Heys, P. N.; Shaw, B. L. Transition
Met. Chem. 1978, 3, 165. (e) Empsall, H. D.; Hyde, E. M.; Pawson, D.;
Shaw, B. L.; J. Chem. Soc., Dalton Trans. 1977, 1292.

(a) Harlow, R. L.; Thorn, D. L.; Baker, R. T.; Jones, N. L. Inorg. Chem.
1992, 31, 993. (b) Rappert, T.; Wolf, J .; Schulz, M.; Werner, H. Chem.
Ber. 1992,125, 839.

Van Gaal, H. L. M.; Verlaak, J. M. J .; Posno, T. Inorg. Chim. Acta.
1977, 23, 43.

(a) Zhilyaev, A. N.; Rotov, A. V.; Kuz'menko, I. V.; Baronovskii, I. B.
Doklady Akad. Nauk 1988,302, 275. (b) Pneumatikakis, G.; Psaroulis,
P. Inorg. Chim. Acta. 1980, 46, 97 .

Dunbar, K. R. Comments Inorg. Chem. 1992, 13, 0000.

(a) Dunbar, K. R. J. Am. Chem. Soc. 1988, 110, 8247. (b) Dunbar, K.
R.; Pence, L. E. manuscript in preparation.

 

 

 

 

 

 

 

11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

21.
22.

97

(a) Dunbar, K. R.; Quillevéré, A. Inorg. Chem. submitted for
publication. (b) Quillevéré, A. Ph. D. Dissertation, Michigan State
University, 1992.

Dunbar, K. R.; Pence, L. E. Inorg. Synth. 1992,29, in press.

Gray, H. B.; Hendrickson, D. N.; Sohn, Y. S. Inorg. Chem. 197 1, 10,
1559.

Morris, D. E.; Dunbar, K. R.; Arlington, C. A.; Doom, S. K.; Pence, L.
E.; Woodruff, W. H. submitted to J. Am. Chem. Soc.

(a) Bino, A.; Cotton, F. A.; Fanvvick, P. E. Inorg. Chem. 1979, 18, 3558.
(b) Cotton, F. A.; Frenz, B. A.; Deganello, G.; Shaver, A. J. Organomet. '
Chem. 1973, 50, 227.

Structure Determination Package: Enraf-Nonius, Delft, The
Netherlands (1979).

TEXSAN-TEXRAY: Structure analysis package, Molecular Structure
Corporation (1985).

SHELXS-86: Sheldrick, G. M. in Crystallographic Computing 3;
Sheldrick, G. M.; Goddard, R. Eds; Oxford University Press, 1984; p.
175.

Walker, N.; Stuart, D.Acta. Cryst. 1983,A39, 158.

Morris, D. E.; Dunbar, K. R.; Arlington, C. A.; Doorn, S. K.; Pence, L.
E.; Woodruff, W. H. submitted to J. Am. Chem. Soc.

For other tetranuclear Rh46+ species see (a) Miskowski, V. M.; Gray, H.
B. Inorg. Chem. 1987,26, 1108. (b) Mann, K. R.; DiPierro, M. J .; Gill,
T. P. J. Am. Chem. Soc. 1980, 102, 3965. (c) 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, 4384-5. (d) Mann, K. R.; Gray, H. B.
Adv. Chem. Ser. 1979, 173, 225. (e) Mann, K. R.; Lewis, N. S.;
Miskowski, V. M.; Erwin, D. K.; Hammond, G. 8.; Gray, H. B. J. Am.
Chem. Soc. 1977, 99, 5525.

Pence, L. E. Ph. D. Thesis, Michigan State University, 1992.

Examples of coordination and organometallic Rh(II) complexes
prepared electrochemically: (a) Cooper, S. R.; Rawle, S. C.; Yagbasan,
R.; Watkin, D. J. J. Am. Chem. Soc. 1991,113, 1600. (b) Rawle, S. C.;
Yagbasan, R.; Prout, K.; Cooper, S. R. J. Am. Chem. Soc. 1987, 109,
6181. (c) Blake, A. J .; Gould, R. D.; Holder, A. J .; Hyde, T. I.; Schroder,
M. J. Chem. Soc., Dalton Trans. 1988, 1861. (d) Anderson, J. E.;
Gregory, T. P. Inorg. Chem. 1989, 28, 3905. (e) Scwarz, H. A.; Creutz,

 

 

 

 

 

23.

24.

25.

26.

27.

28.
29.

30.

31.

32.

33.

98

C. Inorg. Chem. 1983,22, 207. (e) Mulazzani, Q. G.; Emmi, S.;
Hoffman, M. Z.; Venturi, M. J. Am. Chem. Soc. 198], 103, 3362. (f)
Kblle, U.; Klaui, W. Z. Naturforsch. 199], 46b, 75. (g) Bianchini, C.;
Laschi, F.; Ottaviani, M. F.; Peruzzini, M.; Zanello, P.; Zanobini, F.
Organometallics 1989,8, 893. (h) Bianchini, C.; Meli, A.; Laschi, F.;
Vizza, F.; Zanello, P. Inorg. Chem. 1989,28, 227. (i) Pilloni, G.;
Schiavon, G.; Zotti, G.; Zecchin, S. J. Organomet. Chem. 1977 , 134, 305.
(i) Fischer, E. 0.; Lindner, H. H. J. Organomet. Chem. 1964, 1, 307. (k)
Fischer, E. 0.; Wawersik, H. J. Organomet. Chem. 1966,5, 559. (1)
Keller," H. J.; Wawersik, H. J. Organomet. Chem. 1967,8, 185. (m)
Dessy, R. E.; King, R. B.; Waldrop, M. J. Am. Chem. Soc. 1966,88,
5112. (n) Dessy, R. E.; Kornmann, R.; Smith, C.; Hayter, R. J. Am.
Chem. Soc. 1968,90, 2001.

Ogle, C. A.; Masterman, T. C.; Hubbard, J. L. J. Chem. Soc., Chem.
Commun. 1990, 1733.

Goldfarb, D.; Kevan, L. J. Phys. Chem. 1986,90, 264; 2137; 5787.

Hay-Motherwell, R. S.; Koschmieder, S. U.; Wilkinson, G.; Hussain-
Bates, B.; Hursthouse, M. B. J. Chem. Soc., Dalton Trans. 1991, 2821.

(a) Evans, D. F. J. Chem. Soc. 1959, 2003. (b) Deutsch, J. L.; Poling, S.
M. J. Chem. Ed. 1969, 46, 167.

Dunbar, K. R.; Haefner, S. C.; Burzynski, D. J. Organometallics 1990,
9,1347.

Jeffrey, J. C.; Rauchfuss, T. B. Inorg. Chem. 1979,18, 2658.

Empsall, H. D.; Shaw, B. L.; Turtle, B. L. J. Chem. Soc., Dalton Trans.
1976, 1500.

Granziani, R.; Bombiere, G.; Volponi, L.; Panattoni, 0.; Clark, R. J. H.
J. Chem. Soc. (A) 1969, 1236.

Corbridge, D. E. C. The Structural Chemistry of Phosphorus; Elsevier:
Amsterdam, 1974, chapter 11.

The structure Rh(PPh3)2Cl(s-qui) (s-qui = semi-o-benzoquinone
diimine) of has appeared. However, no magnetic evidence was
presented to support the assignment of the rhodium oxidation state:
Peng, S. M.; Peters, K.; Peters, E. M.; Simon, A. Inorg. Chim. Acta
1985, 101, L35.

Cetinkaya, B.; Lapport, M. F.; McLaughlin, G. M.; Turner, K. J. Chem.
Soc., Dalton Trans. 1974, 1591.

 

 

 

 

 

34.

35.

36.

37.

38.
39.

40.

41

42.

99

For example see: (a) Chebi, D. E.; Fanwick, P. E.; Rothwell, I. P.
Polyhedron 1990, 9, 969. (b) Mehrotra, R. C.; Agarwal, S.K.; Singh, Y.
P. Coord. Chem. Rev. 1985, 68, 101. (c) Boyar, E. B.; Robinson, S.D.
Coord. Chem. Rev. 1983, 50, 109. (d) Graham, D.E.; Lamprecht, G. J .;
Potgieter, I. M.; Roudt, A.; Leipoldt, J. G. Trans. Met. Chem. 199], 193.
(e) Bryndza, H. E.; Tam, W. Chem. Rev. 1988,88, 1163.

Igau, A.; Gladysz, J. A. Polyhedron 1991,10, 1903. (b) Ferandez, J. M.;
Gladysz, J. A. Organometallics 1989,8, 207.

(a) Gorrel, I. B.; Parkin, G. Inorg. Chem. 1990,29, 2452. (b) Thomas, R.
R.; Chebolu, V.; Sen. A. J. Am. Chem. Soc. 1986, 108, 4096. (c)
Crabtree, R. H.; Hlatky, G. G.; Holt, E. M. J. Am. Chem. Soc. 1983,
105, 7302. (d) Hitchcock, P. B.; Lappert, M. F.; Taylor, R. G. J. Chem.
Soc., Chem. Commun. 1984, 1082. (e) Reedijk, J. Comments Inorg.
Chem. 1982, 6, 379 and references therein.

For instance see (a) Brookhart, M.; Rix, F. C.; DeSimone, J. M.;
Barborak, J. C. J. Am. Chem. Soc. 1992, 114, 5894. (b) Yang, X.; Stem,
C. L.; Marks, T. J. Organometallics 1991, 10, 840. (c) Yang, X.; Stem,
C. L.; Marks, T. J. J. Am. Chem. Soc. 1991,113, 3623. (d) Horton, A.
D.; Orpen, A. G. Organometallics 1991, 10, 3910. (e) Brookhart, M. ;
Volpe, A. F. Jr.; DeSimmone, J. M.; Lamanna, W. M. Polym. Prepr.
1991, 32(1), 461.

Meek, D. W.; Mazanec, T.J.Acc. Chem. Res. 198], 14, 226.

(a) Pregosin, P. S. Phosphorus-31 NMR Spectroscopy in Stereochemical
Analysis; Verkade, J. G.; Quin, L. D. ed., VCH, 1987. (b) Pregosin, P.
S.; Venanzi, L.M. Chem. Br. 1978, 14, 276.

(a) Chen, S. J .; Dunbar, K. R. Inorg. Chem. 199], 30, 2018. (b) Chen, S.
J .; Dunbar, K. R. Inorg. Chem. 1990, 29, 588.

(a) Kurosawa, H.; Tsuboi, A.; Kawasaki, Y.; Wada, M. Bull. Chem. Soc.
Jpn. 1987, 60, 3563. (b) Empsall, H. D.; Heys, P. N.; McDonald, W. S.;
Norton, M. 0.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1978, 1119. (c)
Empsall, H. D.; Heys, P. N.; Shaw, B. L. J. Chem. Soc., Dalton Trans.
1978, 257. (d) Empsall, H. D.; Hyde, E. M.; Shaw, B. L. J. Chem. Soc.,
Dalton Trans. 1975, 1690. (e) Jones, C. E.; Shaw, B. L.; Turtle, B. L.J.
Chem. Soc., Dalton Trans. 1974, 992. (f) Empsall, H. D.; Hyde, E. M.;
Jones, C. E.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1974, 1980.

Dunbar, K. R.; Saharan, V.; Matonic, J. M. manuscript in preparation.

 

 

 

CHAPTER IV

REVERSIBLE CARBON MONOXIDE CHEMISTRY OF
[Rh(n3-TMPP)2] [BF4] 2

100

 

 

 

101

1. Introduction
The synthesis and isolation of [Rh(n3—TMPP)2][BF4]2 provided us with

 

a unique opportunity to explore the chemistry of authentic Rh(II) d7 metallo-
radicals. Typically, radical species are highly reactive and only observed as
transients resulting from homolysis of M-M and M-L bonds.1 Such species
have been implicated in a number of important stoichiometric and catalytic
transformations.2 In contrast to the well-documented chemistry of early
transition metal radicals, radical species of the platinum group are less
understood. Recent work with Rh(II) porphyrin complexes by Wayland and
co-workers, which revealed the ability of such species to activate C-H bonds
and couple CO grOups, underscores the fact that platinum group radical
complexes are as important an area of investigation as their early transition

metal counterparts.3

 

Early studies of Rh(II) phosphine complexes revealed the proclivity of
these systems to undergo rapid disproportionation reactions.4 For example,
reaction of RhClZ(PCy3)2 with CO initially yields a paramagnetic species
believed to be the Rh(II) carbonyl complex, RhC12(PCy3)2(CO), which, in
solution, rapidly disproportionates to the more stable Rh(I) and Rh(III)
counterparts.4e The initial Rh(II)CO adduct is so unstable with respect to
disproportionation that it can only be detected in solid state reactions. One
method of preventing such a reaction is by introducing a more rigid ligand
environment that disfavors ligand redistribution. This concept is elegantly
illustrated by the chemistry of Rh(II) porphyrins. The porphyrin macrocyclic
framework prevents ligand redistribution from occurring and therefore
kinetically stabilizes the Rh(II) oxidation state with respect to
disproportionation. As a consequence, reactivity is dominated by ligand

association reactions and not electron transfer processes. When bulky

 

 

 

 

102

substituents were added to the porphyrin macrocycle, dimerization of the d7
metal fragments was disfavored resulting in the isolation of metal centered
radicals.3

Similar results may be obtained for Rh(II) phosphine complexes by
incorporating chelating groups into the phosphine that deter ligand
dissociation. Some work in the past has supported this concept, for example
that of Shaw et al. who used this approach to isolate moderately stable Rh(II)
and Ir(II) species with the general formula MH(OC6H4P(But)2)2.5:6 In fact,
they found that the Ir(II) complexes formed stable adducts with Oz and CO.7
The corresponding small molecule chemistry of the Rh(II) species was not
reported.

With these fascinating but limited results as a backdrop, we set out to
explore the reactivity of [Rh(n3-TMPP)2][BF4]2 by exploiting the weak nature
of its Rh-ether bonds to study its potentially rich small molecule substitution
chemistry, particularly with CO, in a quest for forming paramagnetic adducts
that may exhibit novel reactivity. This chapter investigates the redox and

substitution chemistry of [Rh(n3-TMPP)2l[BF4]2 with CO.

2. Experimental
A. Synthesis

All reactions were carried out under an argon atmosphere by the use of
standard Schlenk-line techniques unless otherwise stated. Reactions at
pressures greater than 1 atm were performed in a 450 mL stainless steel Parr
mini reactor (model 4560) equipped with a magnetic drive stirrer and an
automatic temperature control. The starting materials tris(2,4,6-
trimethoxyphenyl)phosphine (1) (TMPP) and [Rh(n3-TMPP)2][BF4]2 (3) were

prepared as described in Chapters II and III, respectively. The starting

 

 

 

103

material [Rh(cod)Cl]2 was purchased from Strem Chemicals. [Rh(CO)2Cl]2
was synthesized from [Rh(cod)Cl]2 by displacement of cod with CO in THF.
The salt [szFe][BF4] was prepared by oxidation of Cnge with
hydroﬂuoroboric acid in the presence of p-benzoquinone.8 The reagents
CNBut, NOBF4, and Cngo were purchased from Strem Chemicals and used
without further puriﬁcation. Carbon monoxide was obtained from Matheson
Gas Products and was used as received. Labeled 13C0 (99%) was purchased
from Aldrich.

(1) Reaction of [Rh(n3-TMPP)2][BF4I2 (3) With CO. In a typical reaction,
[Rh(n3-TMPP)2][BF4l2 (3) (0.100 g, 0.07 mmol) in 10 mL of CH2C12 was
purged with CO gas for 20 min. While maintaining a CO atmosphere, 20 mL
of CO-purged diethyl ether was added to the solution which effected the
separation of an oily red solid from a yellow solution. The solution was
decanted from the solid, the volume of the solution was reduced to 5 mL and
diethyl ether (20 mL) was slowly added to precipitate a yellow
microcrystalline solid. The yellow product was ﬁltered in air and washed
with 2 x 10 mL of diethyl ether and dried in vacuo; yield, 0.038 g (40%). The
red oil was dissolved in 5 mL of CH2C12 and layered with 20 mL of diethyl
ether. A red-orange precipitate formed which was ﬁltered off, washed with 3
x 5 mL diethyl ether, and dried in vacuo; yield 0.043 g (41%).

(2) Reaction of [Rh(n3-TMPP)2][BF4]2 (3) with 1200/1300 (1:1). A
quantity of [Rh(n3-TMPP)2][BF4]2 (3) ( 0.050 g, 0.035 mmol) was dissolved in
10 mL of CH2C12 and the solution was degassed by several freeze/pump/thaw
cycles on a high vacuum line. An equimolar volume of 12CO and 13CO was
delivered to the reaction ﬂask with the use of a Toepler pump. The pressure
above the solution was calculated to be in the range of 1-2 atm at room

temperature. The reaction vessel was allowed to slowly warm to room

 

 

 

 

104
temperature, during which time the solution color changed from purple to
murky red. After 30 min, a small amount of solution was syringed out and its
infrared spectrum was measured; v(CO) (cm'l): 2011, 1985, 1968 in an
approximate 1:211 intensity ratio.
(3) Preparation of [Rh(nZ-TMPP)(TMPP)CO][BF4] (7)
(i) Reduction of [Rh(n3-TMPP)2][BF4]2 in the Presence of CO. A
solution of [Rh(n3-TMPP)2][BF4]2 (0.100 g, 0.07 mmol) and a 3-fold excess of
szCo (0.042 g, 0.22 mmol) in 5 mL of CH2012 was stirred under a moderate ‘
purge of carbon monoxide. After 20 min, 30 mL of diethyl ether was added
and the solution was cooled to -5 °C for 24 h. The yellow product was
collected by suction ﬁltration, washed with diethyl ether and recrystallized
from CH2C12 (10 mL) by reduction of the volume and slow addition of diethyl
ether (0.5 mL). A crop of yellow-orange crystals was collected by ﬁltration in
air, washed with diethyl ether, and dried in vacuo; yield: 0.057 g (60%). Anal.
Calcd for C55H66F4P2019BRh: C, 51.50; H, 5.19. Found: C, 50.57; H, 5.02.
IR (Nujol, CsI): v(CO) 1958 cm'l; (CH2C12)I v(CO) 1970 cm'l. 1H NMR
(CD2012) 5 ppm: 3.48 (s, 36H, o-OCH3), 3.80 (s, 18H, p-OCH3), 6.04 (t, 4JP_H =
2.1 Hz, 12H, m-H). 31P NMR (CDZCIZ) 5 ppm: -11.1 (d, lJRh_P = 128 Hz).
Electronic absorption spectrum (CHZCIZ) Xmax, nm (e): 415 sh, 345 (7770), 305
sh, 285 sh, 254 (64,400). Cyclic voltammogram (0.1 M TBABF4 / CH2012, vs
Ag/AgCl): Elm”) = + 0.50 V.
(ii) Reaction of [Rh(cod)Cl]2 with TMPP in the Presence of CO. A
mixture of [Rh(cod)Cl]2 (0.500 g, 1.01 mmol) and AgBF4 (0.395 g, 2.03 mmol)
was dissolved in 5 mL THF for 15 min to give a yellow solution. The yellow
solution was then ﬁltered through a Celite plug into a 3-neck ﬂask equipped
with a gas inlet and an addition funnel containing a solution of TMPP (2.160
g, 4.056 mmol in 30 mL of THF). The entire apparatus was then cooled to 0°C

 

 

 

 

105

with an ice bath. The TMPP solution was added dropwise over a period of 10
min during which time the solution was gently purged with CO. After
addition of the phosphine, the CO purge was discontinued and the reaction
was stirred at 0°C under a CO atmosphere for 1 h. The volume was reduced
to 15—20 mL and the reaction solution was stirred for another hour at 0°C to
give a yellow precipitate. The yellow solid was ﬁltered in air and washed
with diethyl ether (4 x 10 mL). The product was recrystallized by
redissolving the solid in 50 mL of CH2C12 followed by ﬁltration and addition
of 20 mL of THF. The volume of the solution was reduced to 10-15 mL and
the solution was refrigerated at -5°C. A large crop of yellow crystalline solid
was collected by ﬁltration in air, washed with 4 x 10 mL of diethyl ether and
dried under reduced pressure; yield: 2.250 g (86%).
(iii) Reaction of [Rh(CO)2Cl]2 with TMPP A solution ﬂask was charged
with [Rh(CO)2Cllz (0.100 g, 0.257 mmol), TMPP (0.548 g, 1.03 mmol) and
NaBF4 (0.056 g, 0.514 mmol) and 10 mL of MeCN. The cloudy yellow
solution was stirred under reduced pressure for 30 min, after which time, the
solvent was removed under vacuum. The yellow product was redissolved in
10 mL of CH2012 and ﬁltered through a Celite plug. THF (10 mL) was added
to the solution, and the volume of the solution was reduced to approximately
3-5 mL, yielding a yellow crystalline solid. EtZO (10 mL) was added to
precipitate additional product. The solid was ﬁltered in air, washed with 3 x
5 mL of EtzO and dried in vacuo; yield: 0.530 g (80%).
(4) Preparation of [Rh(TMPP)2(CO)2][BF4] (6)

A quantity of [Rh(n2-TMPP)(TMPP)CO][BF4] (7) (0.100 g, 0.08 mmol)
in 5 mL of CHzclg was treated with CO gas for 5 min, after which time 30 mL
of CO-saturated diethyl ether was slowly added to induce precipitation of the

product. The yellow crystalline solid was collected by suction ﬁltration in air,

 

 

 

106

washed with 2 x 5 mL of diethyl ether and dried. Prolonged drying under
vacuum must be especially avoided to prevent potential loss of CO.
[Rh(TMPP)2(CO)2][BF4] was isolated as its CHgClz solvate; yield: 0.10 g
(90%). Anal. Calc'd for RhC12P2057OzoBF4H68: C, 49.05; H, 4.91. Found: C,
48.22; H, 4.96. IR (Nujol, CsI): v(CO) 2006 cm'l; (CHZCIZ): v(CO) 2011 cm'l.
1H NMR (CD2C12) 5 ppm: 3.41 (s, 36H, o-OCH3), 3.79 (s, 18H, p-OCH3), 6.02
(t, 4JP-H = 2.1 Hz, 12H, m-H). 31P NMR (CD2C12) 6 ppm: -23.8 (d, lJRh-P =
116 Hz). Electronic absorption spectrum (CHZCIZ) kmax, nm (e): 438 (3400),
sh, 350 (6400), 288 sh, 256 (61,500). Cyclic voltammogram (0.1 M TBABF4 /
CH2C12, vs Ag/AgCl): Ep’a = + 0.80 v.

(5) Solid State Reactions of (3) - (7) with CO.

In a typical experiment, a small amount of ﬁnely divided starting
compound (5-10 mg) was loaded in a polyethylene vial and placed in a Parr
reactor. After several ﬁllings and subsequent purgings with CO, the reactor
was pressurized to approximately 50 psi. Reaction times were varied from 15
min to 5 days. After the vessel was depressurized, a small drop of Nujol oil
was added and the sample was quickly transferred to C51 plates for infrared
spectral measurements. Alternatively, the reactions were performed by
purging a ﬁnely divided sample suspended in Nujol with CO at atmospheric
pressure for an extended period of time.

(6) Redox Reaction of [Rh(n2-TMPP)(TMPP)CO][BF4] (7 ) with [Rh(n3-
TMPP)2l [BF4ls (4).

A equimolar mixture of [Rh(TMPP)zCO][BF4] 7 (0.012 g, 0.009 mmol)
and [Rh(n3-TMPP)2][BF4]3 4 (0.013 g, 0.009 mmol) was dissolved in 10 mL of
CH2C12 and the reaction was stirred for 90 min under an argon purge to
liberate CO. During this time, the solution color changed from red to red-

purple. After stirring overnight under an Ar atmosphere, the solvent was

   

 

 

107

evaporated and a crop of purple crystals was collected, washed with a
mixture of CH2C12/Et20 (1:1 v/v) and dried in air. The product was identiﬁed
as [Rh(n3-TMPP)2][BF4]2 by a comparison of its electrochemical and infrared
spectral properties to those of an authentic sample; yield: 0.011 g (90% based
on [Rh(nZ-TMPP)(TMPP)CO][BF4l).

(7 ) Preparation of [Rh(TMPP)2] [BF4] (3) from [Rh(TMPP)2CO][BF4]
(7)

A solution of [Rh(TMPP)2(CO)][BF4] (0.500 g, 0.390 mmol) and
[Cp2Fe][BF4] (0.107 g, 0.390 mmol) in 50 mL of CH2C12 was purged with N2
overnight at -15°C. CH2C12 was added periodically to maintain the reaction
volume at approximately 50 mL. During this time, the solution color slowly
changed from red to purple. After the reaction was complete, the solvent was
evaporated and the crude purple solid was recrystallized as previously

described in Chapter III; yield: 0.460 g (88%).

(8) Reaction of [Rh(TMPP)2CO][BF4] (7 ) with small molecules

(i) N2, 02, C02, H2 . In a typical experiment, a solution of
[Rh(TMPP)2(CO)][BF4l (7) (0.025 g, 0.020 mmol) in 5-10 mL of CH2C12 was
gently purged with the appropriate gas at r.t. for 15-30 min. An aliquot was
removed by either syringe or cannula and transferred to a solution IR cell.
For each of the above gases, only one carbonyl stretch (v(CO) = 1970 cm'l)
was observed indicating the presence of unreacted [Rh(n3-TMPP)2][BF4]2 (7).
(ii) CNR: preparation of [Rh(TMPP)2(CO)(CNBut)][BF4] (8). To a 5 mL
solution of [Rh(TMPP)2(CO)][BF4] (0.250 g, 0.20 mmol) in CH2C12, was added
22 ML of CNBut. The yellow solution was stirred for 15 min. at r.t. A yellow
crystalline solid was precipitated by slow addition of 40 mL of diethyl ether.
The solid was ﬁltered in air, washed with 3 x 5 mL EtZO and dried in vacuo;
yield: 0.251 mg (94%). IR (CH2C12): v(CO) 1998 cm‘l, v(CN) 2188 cm'l. 1H

 

 

 

 

 

108

NMR (CD2C12) 5 ppm: 0.96 (5, -But), 3.36 broad (s, -OCH3), 3.78 broad (s,
-OCH3), 5.99 broad (s, m-H). 31P NMR (CD2C12) 8 ppm: -23.8 (d, lJRh-P = 116
Hz). Cyclic voltammogram (0.1 M TBABF4 / CH2C12, vs Ag/AgCl): E1/2(ox) = +
0.47 V, Ep,c = -o.01 V.

(iii) pyridine. An amount of [Rh(TMPP)2(CO)][BF4] (50 mg, 0.04 mmol) was
dissolved in 5 mL of CHZCIZ. One equivalent of pyridine (3.2 uL, 0.04 mmol)
was syringed into the solution. The yellow solution was stirred for 15 min at

r.t., after which time an infrared spectrum of the solution revealed a single

CO band at 1970 cm'1 corresponding to unreacted [Rh(TMPP)2(CO)][BF4] (7).

B. X-ray Crystallography

The structures of [Rh(TMPP)2(CO)2][BF4 ] (6) and
[Rh(TMPP)2(CO)][BF4] (7) were determined by application of general
procedures that have been fully described elsewhere.19 Geometric and
intensity data for 6 was collected on a Nicolet P3/F diffractometer with
graphite monochromated MoKa (la = 0.71073 A) radiation and were
corrected for Lorentz and polarization effects. Calculations for 6 were
performed on a VAXSTATION 2000 computer by using programs from the
Enraf-Nonius Structure Determination Package (SDP) programs.9

Crystallographic data for the compound [Rh(nz-
TMPP)(TMPP)CO][BF4] (7) were collected by Molecular Structure
Corporation on a Rigaku AFC5R diffractometer with monochromated CuKa
radiation and a 12KW rotating anode generator. Calculations were
performed by using the Texsan crystallographic software package of
Molecular Structure Corporation.10 Important crystallographic parameters
pertaining to data collection and structure reﬁnement for compounds 6 and 7

are summarized in Table 10.

 

 

 

 

 

109
(1) [Rh(TMPP)2(CO)2] [BF4]-CH2C12 (6).

(i) Data Collection and Reduction. Single crystals of 6 that were suitable
for X-ray diffraction studies were grown by slow diffusion of Et20 into a
CH2C12 solution of 6 under a carbon monoxide atmosphere to prevent loss of
CO and formation of 7. A yellow platelet of approximate dimensions 0.89 x
1.0 x 0.37 mm3 was taken up in viscous oil at the end of a glass ﬁber and
placed in a cold stream of N2(g) at -96i3°C. The cell parameters were reﬁned
from a ﬁt of 15 reﬂections in the range 20 .<_ 20 S 30° and indicated a triclinic
crystal system; axial photography conﬁrmed the low symmetry of the lattice.
A total of 8801 unique data were collected in the range 4 S 20 3 45° by using
the co-scan method. No signiﬁcant decay in the data was noted, as evidenced
by three check reﬂections that were monitored periodically throughout data
collection.

(ii) Structure Solution and Reﬁnement. The position of the Rh atom was
determined from a Patterson Fourier synthesis. The remaining non-
hydrogen atoms were located and reﬁned by a series of alternating least-
squares cycles and difference Fourier maps. Of the 8801 unique data, 5934
with F02 > 36(F0)2 were used in the reﬁnement of 799 parameters to give
residuals ofR = 0.059 and RW = 0.084.

(2) [Rh(nz-TMPP)(TMPP)CO] [BF4]°2C¢;H6 (7).

(i) Data Collection and Reduction. Yellow platelets of 7 were grown by
slow evaporation of a mixture of CH2C12 and CsHs. A small crystal of
approximate dimensions 0.25 x 0.10 x 0.10 mm3 was selected and mounted on
the end of a glass ﬁber with epoxy cement. Least-squares reﬁnement of 25
well-centered reﬂections in the range 58.5 S 20 S 77.20 gave cell parameters

that belong to a triclinic cell. Based on an analysis of intensities,

 

 

 

 

 

 

110

Table 10. Summary of crystallographic data for [Rh(TMPP)2(CO)2][BF4] -
CH2C12 (6) and [Rh(TMPP)2(CO)][BF4] . 2CeH5 (7).

 

 

6 7
Formula RhC12P2F4020C57BH68 RhP2F4019067BH73
Formula weight 1395.73 1432.02
Space group P—1 P-1
a, A 13.318(4) 14.898(5)
b, A 13.404(2) 18.060(8)
c, A 18.164(4) 14.343(4)
(1, deg 95.908(3) 9656(4)
[3, deg 97.037(3) 113.84(2)
% deg 90.711(3) 104.80(4)
V, A3 3200(2) 3308(2)
Z 2 2
dcalc, g/cm3 1.448 1.438
M, cm'1 4.74 32.60
Temperature, 0C -96 i 3 23 i 1
Trans. factors, max., 1.00, 0.82 1.00, 0.95
min.
Ra 0.059 0.067
wa 0.084 0.069
quality-of-ﬁtc 2.59 2.32

 

aR=X l lFol' chlIElFol

bRw = [2w( | F0 I- We |)2/2w lFo l211/2; w = 1/02( lFo I)
cquality-of—ﬁt = [2W( ”:0 I ' l I:c |)2/(Nobs‘Nparameter)lV2

 

 

 

 

 

 

111

the space group was determined to be P-l. Data were collected at 23i1°C by
using the co-20 scan technique to a maximum 20 value of 120.3°. Weak
reﬂections, those with F02 < 106(F02), were rescanned at a maximum of two
rescans and the counts were accumulated to assure good counting statistics.
A total of 10304 reﬂections were collected, of which 9855 were unique.
Intensity measurements of three standard reﬂections every 150 data points
indicated that the crystal had not decayed. An empirical absorption
correction based upon azimuthal scans of several reﬂections was applied to
the data; transmission factors ranged from 1.00 to 0.95. Data were also
corrected for Lorentz and polarization effects.

(ii) Structure Solution and Refinement. The structure was solved by
MITHRIL and DIRDIF structure solution programs11 and reﬁned by full-
matrix least squares reﬁnement. With the exception of the carbon atoms of
the benzene molecule in the lattice, all non-hydrogen atoms were reﬁned with
anisotropic thermal parameters. A total of 6393 observations with
F02>36(F02) were used to ﬁt 757 parameters to give R = 0.067 and RW = 0.069.
The quality-of-ﬁt index is 2.59 and the peak of highest electron density in the
ﬁnal difference map is 1.09 e'/A3.

3. Results

A. Reactivity of [Rh(n3-TMPP)2] [BF4]2 (3) with Carbon Monoxide.
Solutions of 3 in CH2C12 react smoothly with CO at ambient

temperatures and pressures. Within 15—20 minutes, the initial deep purple

color of 3 converts to a red-orange hue with concomitant growth of a band at

v(CO) = 2011 cm'l. Reactions of 3 with 1:1 mixtures of 12C0 and 1300 gave a

three band intensity pattern in the v(CO) region, leading to the assignment of

the 2011 cm:1 band to a dicarbonyl species. Subsequent purging with Ar or

 

 

 

 

 

112

N2 produces a new species that exhibits a lower energy CO stretch at 1970
cm'l. IR monitoring studies carried out during the period of inert gas purging
revealed a direct correlation between the disappearance of the species at 2011
cm'1 and the appearance of 1970 cm'l, without the observation of other CO-
containing intermediates. With long purging times, both bands gradually
decrease in intensity and ﬁnally disappear; at the same time the solution
color converts from red-orange to purple. The purple product, obtained as a
residue, was identiﬁed as the parent Rh(II) complex by cyclic voltammetry,
electronic spectroscopy and epr spectroscopy. Intrigued by the indication that
we were observing reversible binding of CO to the radical d7 metal complex,
we undertook a variety of experiments to elucidate the reaction pathway.

B. NMR Spectroscopic Studies.

NMR spectroscopy of CO-saturated CD2C12 solutions of 3 revealed that
the paramagnetic starting material reacts rapidly to form two diamagnetic
species at the same rate. Upon work-up with Et20, bulk reactions in CH2012
produce a red solid and a yellow ﬁltrate. The red solid was identiﬁed as
[Rh(n3-TMPP)2][BF4]3 4 by 1H and 31P NMR spectroscopies (vide supra).
Concentration of the yellow solution produced a crop of air-stable yellow
microcrystals displaying an infrared band at v(CO) = 1970 cm—1. The 1H
NMR spectrum of 7 in CD2012 shows 3 resonances; a virtual triplet at 5 = 6.04
(4Jp_H = 2.1 Hz) and singlets at 5 = 3.80 and 5 = 3.48 that correspond to the
meta protons of the phenyl rings, the p-methoxy protons and the o-methoxy
protons respectively. A companion 31P NMR study conﬁrmed that the
phosphorus nuclei are equivalent, with a doublet appearing at 5 = -11.1 ppm
(J Rh-p = 128 Hz). In light of the solid state structure (vide infra), the 1H NMR
can be interpreted to mean that a low energy ﬂuxional process occurs in

solution to exchange all ortho-methoxy groups. The exchange mechanism is

 

 

 

 

 

113

envisioned to occur through association of a free methoxy group, leading to
formation of a 5-coordinate trigonal bipyramidal structure, which is followed
by labilization of a methoxy interaction as shown in the schematic diagram
below.12 Upon methoxy group dissociation, the phenyl ring is free to rotate
about the P-C bond thereby bringing a second o-methoxy group into a
proximal position to the metal. Rotation about the Rh-P bond eventually
allows all o-methoxy groups on the phosphine ligands to interact with the
metal center, thus rendering the PR3 ligands equivalent on the NMR time
scale. Variable temperature 1H and 31P NMR experiments reveal that this
process continues below -90°C, which attests to the unusually high lability of
the ether interactions in Rh-TMPP complexes. Similar behavior has been
reported for other ether-phosphine complexes of both early and late transition
metals, but unlike the present case, low temperature limiting spectra were

observed for these ﬂuxional complexes.13

0 O , I
‘0’21‘35 ‘Q’Ap’\ O’\ Q0 ,
l ‘\\O‘ I \\O~ q; Pi O
R __i_ OC-Rh‘ —¥ I
v ‘— v — ‘— Rh
OC 1'3 o— 3 0 I ‘0’

Q ko 4v

The ease of Rh-O bond dissociation in these complexes is further
evidenced by the reaction of [Rh(n 2—TMPP)(TMPP)CO]1+ with a second
equivalent of CO. Upon exposure to an atmosphere of CO, a pale yellow
solution of [Rh(nZ-TMPP)(TMPP)CO]1+ converts to an intense yellow color,
signifying the formation of the trans dicarbonyl species [Rh(TMPP)2(CO)2]1+.

This reaction is entirely reversible with loss of carbonyl ligand occurring after

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

m

114

a brief period of purging with Ar or N2. The reversible addition of CO was
followed by 1H, 31P NMR, cyclic voltammetry and electronic spectroscopy.
31P{1H} NMR measurements reveal that the 31P resonance undergoes a shift
from 5 = -11.1 (JRh-P = 128 Hz) ppm for 7 to 5 = -23.8 (d, JRh_p = 116 Hz) for 6
upon addition of the second carbonyl ligand. The 1H NMR spectrum of
[Rh(TMPP)2(CO)2][BF4] (7) in CD2C12 shows magnetically equivalent TMPP
ligands with a triplet appearing at 5 = 6.02 (Jp_H = 2.1 Hz) due to the meta
protons and two singlets at 5 = 3.41 and 3.79 which integrate in the correct
ratio for ortho and para methoxy groups. The symmetrical nature of the
resonances indicates that rotation about the Rh-P bond is not sterically
hindered by the presence of the two carbonyl ligands.

C. Electrochemistry of [Rh(nZ-TMPP)(TMPP)CO][BF4] (7 ) and
[Rh(TMPP)2(CO)2][BF4] (6).

The cyclic voltammogram of [Rh(nZ-TMPP)(TMPP)(CO)]+ shows a
quasi-reversible oxidation at Elam) = +0.50 V (Figure 16c). Not surprisingly,
this oxidation process is less accessible than the Rh(II)/Rh(I) couple of the
parent complex, [Rh(n3-TMPP)2][BF4]2 (3).14 The addition of a second CO
ligand to give the dicarbonyl [Rh(TMPP)2(CO)2]+ results in a large positive
shift in the Rh(I)/Rh(II) couple to +0.80 V (Figure 16b) indicative of further
destabilization of the Rh(II) oxidation state. The ramiﬁcations of this shift on
the overall reaction pathway will be detailed in the discussion section.

D. Crystal Structures of [Rh(TMPP)2(CO)nl[BF4l (n=1,2)

Crystallographic parameters and information regarding data collection
and reﬁnement for the structures of [Rh(TMPP)2(CO)2][BF4] (6) and
[Rh(TMPP)2(CO)][BF4] (7) are summarized in Table 10. Tables 11 and 12

contain a listing of pertinent bond distances and angles for 6 and 7.

 

 

 

 

at

(O)

 

H0 +05 0.0 ~05 -I:o
VOLTS VS Ag/AgCl

Figure 16. Cyclic voltammograms in 0.1 M TBABF4/CH2C12 for (a)

[RhIII(n3-TMPP)2][BF413 (4), (b) [Rh(TMPP)2(CO)2][BF4l (6).
(c) [Rh(TMPP)2(CO)][BF4] (7).

   

 

 

 

116

(l) [Rh(n2-TMPP) (TMPP)CO][BF4l (7).

The solid state structure of 7, shown in Figure 17 , consists of a [Rh(nz-
TMPP)(TMPP)CO]+ cation and a [BF4]- anion in the asymmetric unit. The
geometry about the Rh center is that of a highly distorted square planar
arrangement of ligands with trans phosphine ligands and a CO ligand that is
approximately trans to an oxygen atom from an interacting o-methoxy group
(O(1)-Rh(1)-C(1)=150.2(4)°). To effect this bonding, a highly strained ﬁve-
membered chelate ring is formed: M-P-C-C-O (P(1)-Rh(1)-O(1)=78.1(2)°). The
presence of this rather acute angle results in a gross distortion of the
structure from an ideal square planar arrangement. Related molecules with
this ligand and other ether-phosphines show similar structural features.15

The distance for Rh(1)—O(1) in the present case of 2.319(7) A is
intermediate between the values for the axial and equatorial Rh-O distances
found in [Rh(n3-TMPP)2][BF4]2 (3). The observed high lability of the ether
group in solution reﬂects the weakness of the bond. Of further interest in the
crystal structure of 7 is the presence of a second methoxy group from the
monodentate phosphine ligand at a distance of 2.611(7) A, which is outside
the sum of the covalent bonding radii for Rh and 0. As the PLUTO drawing
in Figure 18 emphasizes, this oxygen atom is poised to occupy an equatorial
site of a trigonal bipyramidal structure. We rationalize that, in solution, the
methoxy group readily coordinates to the metal to form such a ﬁve coordinate
intermediate in order to exchange all six ortho-methoxy groups in a ﬂuxional
process (vide supra).

(2) [Rh(TMPP)2(C0)2l[BF4l (6)-
Compound 6 crystallizes as a symmetrical molecule ligated by two

phosphine ligands and two carbonyl groups in a square planar geometry. As

 

 

 

 

 

 

 

117

C(17)
«ﬁg/C C(18)

  
  
       

 

pom)
0(5) C(15)
0(2) C10
duh) C(21) C(22)
ﬂ\ 0(8)
0(4) )1.—
jP—C(19)C(24)ﬁ(23‘)j\j C(26)
C(8) 0(3) 1) 0(9) C(27)
“5%
C(19)
ch C(18) £33066) C(45)
C(32)C(33) / C(12)
Gamma“) 4(7 P(2) C(15) C(42) C(41)
C(11) ’ M C(44)
cl M
C(30) C(29) W
, C(14)
C(10) C(50) 0““) C(39)
C(34) C(49) ~ 0(16)
C(53)

Figure 17. ORTEP representation of [Rh(TMPP)(TI2-TMPP)(CO)]1+ (7 ).

 

 

 

 

 

118

   

Figure 18. PLUTO drawing of [Rh(TMPP)(nZ-TMPPXCOH 1+ (7)
emphasizing the coordination geometry about the Rh atom.

 

 

 

“§

 

 

 

 

 

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120

was observed in the structure of 7, the two TMPP ligands are trans to each
other, but in this case each TMPP is bound through only the phosphorus
atom (Figure 19). Unlike for the mono-carbonyl compound, the closest
approach of ortho-methoxy groups is well outside the coordination sphere of
rhodium, and there are no major structural distortions as evidenced by the
nearly ideal angles P(1)-Rh(1)-P(2) = 178.46(6) A and C(55)-Rh(1)-C(56) =
179.0(3) A. The second CO ties up the fourth coordination site thus
eliminating the need for additional donation from an ether oxygen atom.
Both the Rh-P and Rh-C bond distances fall well within the range of values
expected for Rh(I) complexes and are in themselves quite unremarkable.

E. Solid State Reactions of (3) - (7 ) with Carbon Monoxide.

Finely divided samples of [Rh(TMPP)2(CO)][BF4] susPended in
mineral oil react with CO after purging for several minutes as evidenced by
the appearance of a second CO band corresponding to
[Rh(TMPP)2(CO)2][BF4]. The process, however, is slow and not quantitative
as a result of poor CO diffusion into the sample. The transformation can also
be effected by pressurizing powder samples with CO at 50 psi. The percent
conversion of 7 to 6 was improved by exposing the sample for longer periods
of time. Presumably, this is due to increased amounts of CO diffused into the
solid. Just as was observed in solution, replacement of the CO atmosphere by
an inert gas results in loss of coordinated CO and reformation of 7.

Solid samples of [Rh(n3-TMPP)2][BF4]2 (3) also react with CO in much
the same manner as was observed for solutions of 3. Samples of 3
pressurized to 50 psi for extended time periods partially convert to [Rh(n2-
TMPP)(TMPP)(CO)][BF4] and [Rh(TMPP)2(CO)2][BF4] as evidenced by the

appearance of weak CO bands at 1958 and 2006 cm‘1 in the IR spectrum.

 

 

 

 

 

 

121

 
  
  
  

C(27) C(9)
‘ 1 C(23) C(8)
as
. C(5
’. 0(3) 4' C(22) ' 0‘2)
C(17) 0(9) (3' C(26)
0(5) C(24) »
o.
C(13) C(14) C(4) C(8)
r C(15)C(18)C(19) 0(3)
\ ,- .‘
C(12) '2 N1. "‘05) '
C y — C(11)C(10“')P)(1 C(20)
(16) 0(4) '. ‘ 0(1)
0(7) I 0(7)
? *"
Rh(1) 19) /
ORG/‘1», C(25)
,‘ C(36) 1“”? C(55)
C(45)! O(12) C(55)
r)
0(15) C(33) C(16) C(52)
C(32) .C(28) P9 ) 4
C(41) " k ’ 0‘18’9’7‘14 "‘ C(49)
“ ‘h t 'i‘ " 5
C(35) C(29)" C(51) 4- 1: SI
. 54
C(11) ,-‘ ) C(50) O(17)"C,5)
C(40) \‘ ’) C(30) C(10) ‘
014 ‘ - C(38) 3.
( '\ n C(13) C(34)
C(39)
C(43) ;,
g C(44)

Figure 19. ORTEP drawing of [Rh(TMPP)2(CO)2]1+ (6) with 30%
probability ellipsoids.

 

 

 

 

 

 

122

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123
Interestingly, upon exposure of 3 to CO for longer than 16 hours, a very weak
high energy CO stretch was observed at 2088 cm‘l. This species is directly
related to reaction of CO with [Rh(n3-TMPP)2][BF4]2 , as neither samples of
[Rh(TMPP)2(CO)][BF4] nor [Rh(n 3-TMPP)2][BF4]3 produced this band under

identical conditions.

4. Discussion

The reversible CO chemistry of [Rh(n3-TMPP)2][BF4]2 at ambient
temperatures and pressure proceeds through a series of reactions that involve
the formation of Rh(III) and Rh(I) complexes. The interrelationships of the
ﬁve compounds involved in the cyclic reaction are depicted in Figure 20.
These conclusions were arrived at by establishing independent synthetic
routes to intermediates 4 - 7 and subsequently investigating their
spontaneous redox behavior and reactivity with CO. Figure 21 outlines
rational pathways to the key compounds 4 - 7 as well as their
interconversions and reactions with CO.

Curiously, although [Rh(n3-TMPP)2]2+ undergoes ether-group
dissociation and subsequent cis to trans isomerization in favor of n-acceptors
such as CO and CNR (R = Me, Pr-i, Bu—t),44 it is unreactive towards donors
such as 02. This is evidently a consequence of the electronic environment of
the metal rather than a steric effect, since identical behavior has been noted
for ether-phosphine complexes in which the ligand is not particularly
bulky.15b

There are several key features of the chemistry in Figure 20 that must
be emphasized in any discussion of this work. The initial attack of CO on the
Rh(II) monomer 3 is slow, therefore unreacted starting material is in large

excess during the early stages of the reaction. While we never actually

in

 

 

 

 

124

_ OI _
A 2+ 2+

(50 :3 2 f 9,!
(loch?) -L->O E: ”I'll
8

[Rhnm3 -TMPP)2]2+
(3)

 

 

 

 

—3[Rhn(n -'I‘MPP)2]2+

(3a)

[Rhﬂ(“3‘TMP ”212+ ‘ -Co [RhH(113-TMPP)2]2+

(3) (3)
[Rhm (113-Twang]3+ [Rhmm3_TMPP)2]3+

<4) (4)
E1/2(red) = +0.46 V E1/2(ox) = +0.46 V

\ 1+
‘0 1 1+ 0 0' —|

I

f 8 O
[9,“!th O) 0' E €914.1th J 0..

O V
I o | ' P\/ 7 ' I 5‘)
I IO +CO 8Q
[RhI(TMPP)2(CO)]1+ [RhI(TMPP)2(CO)2]1+
(7) (6)
v(CO) = 1970 cm‘1 v(CO) = 2011 cm'1
E1/2 (ox) = +0-50 V 131/2(0):) = +0.80 v

Figure 20. Proposed pathway for the reversible reaction between
[RhH(113-TMPP)2]2+ (3) and CO.

 

 

 

Figure 21. High yield synthetic routes to compounds 3-7.a

[hau'H(NCMe)10]4+ [RhI(cod)Cl]2
(i) (vii)
[RhH<n3-TMPP>21‘"* "[RhI(cod)(thf)2]1+'
(3)

   

[RhIH(n3-TMPP)2]3+ [RhI(nZ-TMPP)(TMPP)(CO)]1+
(4) (7)
(iii) (v) (vi)
[Rhm(n3-TMPP)(n3-TMPP-0)]2+ [RhI(TMPP)2(CO)2]1+
(5) (6)

2*(i) TMPP (4 equiv), MeCN, 0°C. (ii) NOBF4, MeCN, -40°C or [Cnge][BF4],
CH2C12, -40°C. (iii) acetone, r. 15., 24h. (iv) Cp2Co (excess), CO, CH2C12. (v)
CO, r. t. (vi) Ar, r. t. (Vii) AgBF4 (2 equiv), THF. (viii) CO, TMPP (2 equiv),
0°C. (ix) [Cp2Fe][BF4] (1 equiv), N2, CH2C12, -15°C.

 

 

 

 

126

observed the adducts Rh(II)(CO)x (x: 1,2) (although we attempted to do so by
in situ infrared or epr studies), their existence is predicated on the basis of
the available redox properties of the other intermediates along the reaction
pathway. The fact that (33) is too short-lived to be observed by ordinary
spectroscopic methods is explained by the very fast electron transfer reaction
that takes place between 3 and the strong oxidant (3a) to produce a 50:50
mixture of [Rh(n3—TMPP)2][BF4]3 (4) and [Rh(TMPP)2(CO)2][BF4] (6).
Indeed, experiments carried out in NMR tubes under a 12CO atmosphere
showed that two diamagnetic products were being produced at the same rate
and that they were stable with respect to further reaction - provided the CO
atmosphere was maintained. A parallel study using a 50:50 mixture of
12CO/13CO provided solution evidence for the assignment of 6 as a dicarbonyl
species. A subsequent X-ray structural study veriﬁed this formulation.
Apparently electron transfer is faster than initial CO addition to [Rh(n3—
TMPP)2]2+, thus, there is never a detectable build-up of a Rh(II)CO+ species
and only the diamagnetic products are observed. Under conditions of
pumping or purging with an inert gas, [Rh(TMPP)2(CO)2][BF4] (6) readily
loses a CO ligand to form [Rh(TMPP)2(CO)2][BF4] (7). However, 7 is
unstable with respect to oxidation by the Rh(III) complex, [Rh(n 3-
TMPP)2][BF4]3 (4), and a second redox reaction occurs producing [Rh(n3-
TMPP)2][BF4]2 (3) and presumably, a Rh(II) monocarbonyl species that
readily gives up CO to regenerate 3. The proposed spontaneous redox
reaction between 4 and 7 was conﬁrmed by a deliberate 1:1 reaction of the
pure compounds as shown in equation 16. Note that no reaction occurs
between [Rh(n 3—TMPP)2][BF4]3 and [Rh(TMPP)2(CO)2][BF4 ]. The
intermediate Rh(II) carbonyl species 3a is postulated to be a dicarbonyl and

not a monocarbonyl complex based on the recognized redox properties of 3 - 7;

 

 

 

 

127

the monocarbonyl species 7 is known to spontaneously react with [Rh(n3-

TMPP)2][BF4]3 (4), therefore the reverse electron transfer process must be

 

unfavorable.
I 1+
[RhIII (ns'TMPP)2]% 1 1 (3) (16)
TM +
(4) [Rh ( PP)2(CO)2L‘ No Reaction

 

(6)

The reversible CO chemistry of [Rh(n3-TMPP)2][BF4]2 that has been
observed is primarily driven by electronic considerations. That is to say, the
relative stabilities of the +1, +2, and +3 oxidation states are controlled by the
modulation of electron density at the metal center. The redox reactions of the
cycle are driven by the it acceptor strength of CO. The strong n-accepting
nature of CO greatly favors the lower oxidation states for which 1:-
backbonding is maximized, therefore the effect of CO coordination to 3 is to
destabilize the Rh(II) oxidation state relative to Rh(I). Electrochemically,
this effect is manifested in a positive shift of the Rh(I)/Rh(II) redox couple,
hence the resulting Rh(II) carbonyl complex is more susceptible to reduction.
In the case of 3a, the redox couple has moved to well past the Rh(II)/Rh(III)
couple for 3. As a result, electron transfer takes place between 3 and 3a to
produce [Rh(TMPP)2(CO)2]+ (6) and [Rh(n3-TMPP)2]3+ (4). At that point the
reaction is essentially complete, provided a CO atmosphere is maintained.
However, upon removal of the CO atmosphere, a coordinated CO ligand is
displaced by a TMPP ether group to give [Rh(nZ-TMPP)(TMPP)(CO)]1+. The
loss of CO makes the Rh(II) oxidation state more accessible, as evidenced by
the less positive oxidation potential for 7 (+ 0.50 V) relative to that of 6 (+
0.80) (Figure 16). The change in redox properties upon CO dissociation

 

 

 

128

triggers an electron-transfer reaction between [Rh(712-TMPP)(TMPP)(CO)]1+
and [Rh(n3-TMPP)2]3+ that ultimately results in the regeneration of [Rh(n3-
TMPP)2][BF4]2 (3). From this, we conclude that while kinetic stabilization is
probably an important factor in dictating the course of the reactions with 1:-
acceptors ligands, the modulation of the electron density at the metal center
(which controls the accessibility of the Rh(I)/Rh(II) and Rh(II)/Rh(III) couple)
is also a primary determinant.

The preceding analysis provides some insight into ways in which stable
paramagnetic adducts of [Rh(n3-TMPP)2][BF4]2 might be prepared. In the
current example, the initial disproportionation reaction between 3 and 33 is
facilitated by the n-acidity of the incoming ligand. It follows then, that by
reducing the n-accepting strength of the incoming ligand, such that the
Rh(II)/Rh(I) redox couple occurs at less positive potentials than the
Rh(II)/Rh(III) couple of (3), other stable Rh(II) complexes may be isolated.
With this in mind, we extended our studies of [Rh(n3-TMPP)2][BF4]2 to
include reactions with isocyanides. The weak n-acidity of isocyanide ligands
is well documented,16 and as such, they are excellent candidates for
preparing stable adducts of [Rh(n3-TMPP)2][BF4]2. The results of these
studies are presented in Chapter VI.

At this point, we now turn to a discussion of the major inﬂuence on the
observed chemistry of [Rh(n3-TMPP)2]2+, viz. the ﬂexibility of the ligand.
Unlike most other sterically hindered ligands, TMPP allows the metal
complex to undergo labilization of weakly held groups and isomerization to
more stable geometries. The structures of the trans—Rh(I) complexes 6 and 7
nicely demonstrate this point. It is precisely the lack of structural rigidity
that permits complexes of ether-phosphines such as TMPP to undergo

reversible small molecule addition reactions. Moreover, the ﬂexibility of the

 

 

 

129

ligand aids in the stabilization of different electronic and structural
conformations. As a consequence, complexes of TMPP are expected to exhibit
rich redox behavior; a fact that is nicely illustrated by the chemistry of
[Rh(n3-TMPP)2][BF4]2.

In order to make [RhH(n3-TMPP)2][BF4]2 (3) a viable system for
promoting C-C bond formation and other small molecule transformations,
disproportionation pathways must be shut down. A strategy for achieving
this goal is to tailor the ligand so that it is less accommodating to changes in
metal oxidation state and structural conformation but not too rigid to prevent
chemistry from occurring at the metal center. A possible solution to this
dilemma would be to anchor the TMPP ligand at two points rather than just
at the M-P bond, a situation that would prevent free rotation of the
phosphine. One such reaction to effect this result is nucleophilic attack on a
coordinated methoxy group to produce a stronger metal-phenoxide
interaction. An anionic oxygen is not as susceptible to dissociation as an
ether substituent, and as a result, the accessibility of the metal center to an
incoming substrate is limited. Thus, it is unlikely that substitution
chemistry with CO would follow the same pathway as that found in this
study. Furthermore, demethylation of a methoxy group to form a phenoxide
donor substantially increases the electron density at the metal center.
Consequently, this should better stabilize a Rh(II) carbonyl radical with
respect to reduction and CO dissociation.

Finally, while the overall pathway described in Figure 20 does not
represent a simple procedure for reversible CO binding, the reaction between
[Rh(TMPP)2(CO)2][BF4] (6) and [Rh(TMPP)2(CO)][BF4] (7 ) is such a process.
The high lability of the o-methoxy interaction for [Rh(TMPP)2(CO)][BF4] (7),

as evidenced by its solution ﬂuxionality and the facile addition of CO to form

 

 

 

 

130

6, motivated us to further explore the lability of these weak donor groups.
Previously, we observed that solid samples of [Rh(TMPP)2(CO)2][BF4] slowly
convert to 7 when exposed to a vacuum for extended periods. This behavior
prompted us to examine the reverse process, namely the addition of CO to
solid state samples of 7 to form 6. Indeed, powder or Nujol mull samples of
[Rh(n3-TMPP)2][BF4]2 (7 ) were observed to reversibly uptake CO at
atmospheric pressure by IR spectroscopy. The only impediment to uptake
appeared to be the ability of CO to diffuse into the medium. This behavior is-
intriguing in the context of CO sensors.1 7 Studies of [Rh(nz-
TMPP)(TMPP)CO][BF4] (7) indicate that the complex is unreactive to all
other atmospheric gases besides CO. In light of these properties, we set out
to investigate the incorporation of [Rh(TMPP)2(CO)][BF4] into porous
polymer matrices with the goal of developing a CO sensing material. The

results of this endeavor are presented in Chapter V.

 

 

 

10.

 

131

List of References

(a) Trogler, W. C. ed. ”Organometallic Radical Processes"; Elsevier:
Amsterdam, 1990. (b) Baird, M. C. Chem. Rev. 1988,88, 1217.

(a) Lappert, M. F.; Lendor, P. W. Adv. Organomet. Chem. 1976, 14, 345.
(b) Kochi, J. K. "Organometallic Mechanism and Catalysis"; Academic:
New York, 1978. (c) Connelly, N. G.; Gieger, W. E. Adv. Organomet.
Chem. 1984, 23, 1. (d) Connelly, N. G.; Gieger, W. E. Adv. Organomet.
Chem. 1985, 24, 87. (e) Connelly, N. G. Chem. Soc. Rev. 1989,18, 153.

(a) Wayland, B. B.; Sherry, A. E.; Coffin, V. L. J. Chem. Soc., Chem.
Commun. 1989, 662. (b) Wayland, B. B.; Sherry, A. E. J. Am. Chem.
Soc. 1989, 111, 5010. (c) Wayland, B. B.; Sherry, A. E.; Poszmik, G.;
Bunn, A. G. J. Am. Chem. Soc. 1992, 114, 1673. (d) Wayland, B. B.;
Ba, 8.; Sherry, A. E. J. Am. Chem. Soc. 199], 113, 5305. (e) Sherry, A.
E.; Wayland, B. B. J. Am. Chem. Soc. 1990, 112, 1259. (f) Poszmik, G;
Wayland, B. B. presented at the 203rd National Meeting of the
American Chemical Society, San Francisco, CA, April 5-10, 1992; paper
INOR 67.

(a) Bennett, M. A.; Lonstaff, P. A. J. Am. Chem. Soc., 1969,91, 6266.
(b) Vleck, A. Inorg. Chim. Acta 1980, 43, 35. (c) Holah, D. G.; Hughes,
A. N.; Hui, B. C. Can. J. Chem. 1975,53, 3669. (d) Valentini, G.;
Braca, G.; Sbrana, G.; Colligiani, A. Inorg. Chim. Acta 1983, 69, 215.
(e) Valentini, G.; Braca, G.; Sbrana, G.; Colligiani, A. Inorg. Chim. Acta
1983, 69, 221.

Empsall, H. D.; Hyde, E. M.; Jones, C. E.; Shaw, B. L. J. Chem. Soc.,
Dalton Trans. 1974, 1980.

(a) Mason, R.; Thomas, K. M.; Empsall, H. D.; Fletcher, S. R.; Heys, P.
N.; Hyde, E. M. Jones, C. E; Shaw, B. L. J. Chem. Soc., Chem. Comm.
1974, 612.

(a) Empsall, H. D.; Hyde, E. M.; Shaw, R. L. J. Chem. Soc., Dalton
Trans. 1975, 1690. (b) Empsall, H. D.; Heys, P. N.; McDonald, W. S.;
Norton, M. C.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1978, 1119.

Gray, H. B.; Hendrickson, D. N.; Sohn, Y. S. Inorg. Chem. 197], 10,
1559.

SDP: Structure Determination Package, Enraf-Nonius, Delft, The
Netherlands, 1979.

TEXSAN - TEXRAY Structure Analysis Package, Molecular Structure
Corporation, 1985.

 

 

 

 

11.

12.

13.

14.

15.

16.
17.

 

132

(a) MITHRIL: Integrated Direct Methods Computer Program,
Gilmore, C. J. J. Appl. Cryst. 1984, 17, 42. (b) DIRDIF: Direct
Methods for Difference Structure, An Automatic Procedure for Phase
Extension; Reﬁnement of Difference Structure Factors. Beurskens, R.
T. Technical Report 1984.

For evidence supporting similar exchange processes see (a) El-Amouri,
H.; Baltsoun, A. A.; Osborn, J. A. Polyhedron 1988, 7, 2035. (b)
English, A. D.; Meakin, P.; Jesson, J. P. J. Am. Chem. Soc. 1976, 98,
7590 (c) Volger, H. C.; Vrieze, K.; Praat, A. P. J. Organomet. Chem.
1968, 14, 429.

Bader, A.; Lindner, E. Coord. Chem. Rev. 199], 108, 27. and references
therein.

Treichel, P. M.; Mueh, H. J.; Bursten, B. E. Isr. J. Chem. 1976/77, 15,
253.

For instance see (a) Dunbar, K. R.; Haefner, S. C.; Burzynski, D. J.
Organometallics 1990, 9, 1347. (b) Jeffrey, J. C.; Rauchfuss, T. B.
Inorg. Chem. 1979, 18, 2658. (c) Granziani, R.; Bombiere, G.; Volponi,
L.; Panattoni, C.; Clark, R. J. H. J. Chem. Soc. (A) 1969, 1236. ((1) De
V. Steyn, M. M.; English, R. B.; Ashworth, T. V.; Singleton, E. J. Chem.
Res. (S) 198], 267.

Treichel, P. M. Adv Organomet. Chem. 1973, 11 , 21.
Biosensors and Chemical Sensors: Optimizing Performance Through

Polymeric Materials, ACS Symposium Series 487; Edelman, P. G.;
Wang, J. Eds., American Chemical Society: Washington, D. C., 1992.

 

 

CHAPTER V

REVERSIBLE CARBON MONOXIDE ADDITION TO SOL-GEL

DERIVED COMPOSITE FILMS CONTAINING THE MOLECULE
[Rh(TMPP)2(CO)]“

 

 

 

 

134

1. Introduction

The reversible solution chemistry of the novel mononuclear Rh(II)
complex [Rh(n3-TMPP)2][BF4]2 (TMPP = tris(2,4,6-trimethoxyphenyl)-
phosphine) with carbon monoxide was presented in Chapter 4. The reaction
was found to proceed by a redox pathway that involves the formation of the
Rh(I) carbonyl intermediates [Rh(TMPP)2(CO)][BF4] (7 ) and
[Rh(TMPP)2(CO)2][BF4] (6); these complexes were fully characterized by X-
ray crystallography, cyclic voltammetry, as well as by infrared and NMR
spectroscopies. In solution, complex 7 rapidly and reversibly binds carbon
monoxide under ambient conditions to form the dicarbonyl species 6 (eq 1).
In the absence of a CO atmosphere, 6 loses CO to reform 7. The facile nature
of the carbon monoxide addition to [Rh(TMPP)2(CO)][BF4] (7) is exempliﬁed
by the observation that ﬁnely divided powder and Nujol mull samples of 7 are
also capable of reversibly uptaking CO. Not surprisingly, these reaction

Me Me- Me

1 o’ “I 1*
Me- O ‘ArArTI 131-Ar
15‘ + C: 0 Me— 0 P‘

OC—lith— co (17)

Me’ O‘Iid’l— CO ‘7—
Arv\\\ E‘AI‘I

Ar' \" E‘ ' - CO
Ar' Ar'
OMe
[Rh(TMPP)2(CO)]1+ (7) Ar‘ = GOMe [Rh(TMPP)2(CO)2]1+ <6)
OMe

rates are sluggish due to poor diffusion of CO into the solid; this situation
prompted us to investigate the incorporation of 7 into a porous material that
would trap the molecular cationic species yet facilitate diffusion of CO into

the matrix. Low temperature sol-gel techniques can be used to create porous

 

 

 

 

 

 

135

oxide glasses (pore size < 100 A) that encapsulate large complex molecules yet
allow diffusion of small substrates into the matrix.1 Such oxide galsses
fromed by the hydrolysis and polycondensation of metal alkoxides are
optically transparent and can be readily probed by a number of spectroscopic
techniques. Previous work has demonstrated the general usefulness of SOI-
gel derived glasses for the immobilization of guest molecules such as
inorganic clusters, porphyrins, and lanthanide cryptate complexes.”
Recently, the process has been adapted for the incorporation of biochemically
active molecules that may lead to the development of novel biosensing
materials.4

The use of sol-gel techniques provides for the immobilization of a
sensing molecule, in this case [Rh(TMPP)2(CO)][BF4] (7), in an environment
that can be probed both by spectroscopic and electrochemical techniques,
while the porous nature of the ﬁlms permits facile diffusion of an exogenous
substrate, such as CO, that can then bind to the sensing molecule. Herein we
report the synthesis and CO binding properties of zirconia and titania glasses
impregnated with a rhodium ether-phosphine compound, the results of which
are promising for future adaptations of such molecular species as chemical
sensors. The work described in this chapter was performed in collaboration
with Professor Kris A. Berglund and Dr. Joel I. Dulebohn in the Departments
of Chemical and Agricultural Engineering, Michigan State University.

 

 

 

136

2. Experimental
A. Preparation of Composite Films Containing

[Rh(TMPP)2(C0)] [BF4] (7)
(l) Zirconia Films

An amount of lauric acid (0.161 g, 0.80 mmol) was added to 1 mL of
70% Zr(OPrn)4 in propanol (2.23 mmol) in a glass vial. The mixture was
placed in a sonicator (Bransonic 220) until the lauric acid had completely
dissolved. To this solution, was added an amount of valeric acid (1.7 mL, 15.6
mmol) and 0.2 mL of water. A solution of [Rh(TMPP)2(CO)][BF4] (7 ) (5.9 mg,
4.6 x 10‘3 mmol) in 50 uL of CH2C12 was added to 1 mL of the zirconia ﬁlm
solution. The resulting yellow solution was ﬁltered through a 0.22 mm
Millex-GS millipore ﬁlter.
(2) Titania Films

A quantity of of lauric acid (0.25 g, 1.25 mmol) was dissolved in neat
Ti(OPri)4 (1 mL, 3.36 mmol). The solution was sonicated to ensure that the
lauric acid was completely dissolved. A mixture of valeric acid (2.5 mL, 23
mmol) and water (0.3 mL) were then added to the solution. A solution of
[Rh(TMPP)2(CO)][BF4] (7) (0.0059 g, 4.6 x 10'3 mmol) in 50 uL of CH2C12 was
pipetted into 1 mL of the titania ﬁlm solution. The resulting yellow solution
was ﬁltered through a 0.22-mm Millex-GS millipore ﬁlter to remove
unreacted 7.
B. Flow Rate Determination

A schematic diagram for the apparatus used to estimate CO
concentrations in the ﬁlms is shown in Figure 22. Flow rates of CO and the
argon carrier gas were controlled by adjustment of screw clamps connected to
each line and were estimated by measuring the time required for a soap

bubble to travel a predetermined distance through a glass pipette. The CO

 

 

 

 

137

 

 

 

 

 

 

glass _
pipette
soap
reservoir
Ar _, Fl to IR cell —>
l\|
[J

switch

—>

screw clamp

 

 

O

O

l
:l<—

 

[22

Figure 22. Schematic diagram of the apparatus used to estimate CO
concentrations.

 

 

 

 

138

ﬂow rate was adjusted and measured, ﬁrst in the absence of argon, then the
argon ﬂow rate was adjusted and the combined ﬂow rate for the two gases
was measured. The ﬂow rates for CO and the combined gases were measured
ﬁve times and the average was determined. The % CO concentration was
taken as the inverse of the ﬂow rate ratios.
C. Electrochemical Measurements

Films used for electrochemical studies were prepared as described in
section A. However, in order to make the ﬁlms electrochemically active, 2.0
mg of [Li][CF3SO3] (0.013 mmol) was added to 0.1 mL of the ﬁlm solution as
a supporting electrolyte. The ﬁlms were cast onto a Pt disk electrode using a
cotton swab and dried in a stream of cool air. Cyclic voltammetry
measurements were performed in 0.1 M [Li][CF3803] / H20 at a scan rate of
200 mV/s using a platinum disk working electrode and a Ag/AgCl reference

electrode.

3. Results

Sol-gel derived titania and zirconia composite ﬁlms containing
[Rh(TMPP)2(CO)l[BF4] (7 ) were prepared by addition of a dichloromethane
solution of 7 to titania and zirconia ﬁlm solutions. Thin ﬁlms were fabricated
by either spin casting for 5 minutes or by application to the substrate with a
cotton swab followed by drying in a stream of cool air. Infrared spectroscopic
studies on the ﬁlms were performed by casting the composite zirconia or
titania ﬁlms onto a germanium ATR crystal and placing the crystal in an
liquid ATR cell holder. Initially, a single carbonyl stretch was seen at 1975
cm‘l, which corresponds to the cation species [Rh(TMPP)2(CO)]+ (7)
(v(CO)(3}{2(312 = 1970 cm'l) immobilized in the ﬁlm. Upon exposure of the ﬁlm

to a CO atmosphere, the band at 1975 cm‘1 disappeared and a second, more

 

 

 

 

 

139

 

 

 

 

94 —
J
o \4
O
l:
53 _
E
a 92 - (a)
g 1975 cm-1
5° —1
‘ (b)
‘ 2013 cm'1
90 _ I l 1 l I 17
2000 1800

wavelength cm-1

Figure 23. Infrared spectra of a zirconia composite ﬁlm of
[Rh(TMPP)2(CO)][BF4] (7): (a) in the absence of CO. (b)
exposed to CO.

 

 

 

 

 

 

 

 

 

140

intense, band appeared at 2013 cm'l, signaling the binding of CO to 7 to yield
the dicarbonyl species [Rh(TMPP)2(CO)2][BF4] (6) (V(CO)CH2012 = 2011 cm-1)
(Figure 23). The absence of the 1975 cm'1 band indicates that the conversion
of 7 to 6 within the ﬁlm is quantitative. With regards to sensitivity, the
presence of CO was observable in gas mixtures of CO and Ar containing less
than 0.2% CO, which was the lowest measurable concentration based on our
present method for determining ﬂow rates. In the lower concentration range,
the conversion of 7 to 6 is less than quantitative as evidenced by the
continued presence of the CO band for 7 at 1975 cm'l. Removal of CO from
the system was effected by purging the IR cell with argon for one minute,
whereupon the band at 2013 cm'1 diminished with concomitant reappearance
of the 1975 cm'1 band; this observation is consistent with complete conversion
of [Rh(TMPP)2(CO)2][BF4] (6) back to the parent molecular species
[Rh(TMPP)2(CO)][BF4l (7). Alternatively, the loss of CO from the material
may be achieved by simply exposing the ﬁlm to the atmosphere, although the
process is signiﬁcantly slower under these conditions. As judged by the
invariance of the IR spectra, zirconia and titania composite ﬁlms of
[Rh(TMPP)2(CO)][BF4] (7 ) are selective for CO in the presence of other gases
such as 02, C02, N2, and H2.

In addition to infrared spectroscopy, the reversible addition of CO to
[Rh(TMPP)2(CO)][BF 4] (7 ) within the composite ﬁlms was monitored by
electronic spectroscopy. A zirconia composite ﬁlm containing 7 was spin cast
onto a quartz slide and placed in a 1 cm quartz cuvette capped with a rubber
septum. The cell was purged for 15 min with CO giving rise to an electronic
spectral feature at 435 nm which is comparable to that seen in solution for
[Rh(TMPP)2(CO)2][BF4] (6) (AmapozClz = 438 nm) (Figure 24). Subsequent

removal of CO from the cell by purging with Ar resulted in the disappearance

 

 

 

Abs

 

  
 
 
 
 

 

141

(a) [Rh(FMPP)2(C0)llBF4l

438 run I
(b) [Rh(TMPPh(C0)2][BF4l

 

300

Abs

Figure 24.

 

foo 560 E00

wavelength (nm)

 

 

435nm
H
(b)
r :r‘ :1. w; (a)
I I I I I F r I I r I
400 500

wavelength (nm)

Electronic absorption spectra of (a) [Rh(TMPP)2(CO)][BF4]
and (b) [Rh(TMPP)2(CO)2][BF4] in: (I) CH2C12. (II) a
zirconia composite ﬁlm.

 

 

 

 

142

of the transition at 435 nm, leading to the conclusion that 6 had been
reconverted to the monocarbonyl derivative 7.

Initial electrochemical studies of the sol-gel derived composite ﬁlms
containing [Rh(TMPP)2(CO)][BF4] (7 ) were carried out by casting a zirconia
ﬁlm onto a platinum disk electrode. Unfortunately, these ﬁlms were found to
be electrochemically inactive as no current response was observed. To
circumvent this problem [Li][CF3SO3] was added to the ﬁlm solution as a
supporting electrolyte prior to casting onto the electrode surface. A cyclic
voltammogram of the composite ﬁlm in 0.1 M [Li][CF3SO3]/ H2O exhibited
an essentially featureless voltammogram (Figure 25). Upon purging the
electrochemical cell with CO for a brief period of time (< 30 sec), an
irreversible oxidation wave appears at Ema: + 0.74 V with a chemical return
wave located at EM: + 0.31 V vs. Ag/AgCl. After purging with N2 for several
minutes, the oxidation wave gradually disappears and the original cyclic
voltammogram is obtained. Purging the solution with N2 removes CO from
the system and results in re-formation of 7, in agreement with the reversible
addition of CO that was observed spectroscopically. Films containing only
[Li][CF3S03], when exposed to CO under identical conditions, exhibited no
signiﬁcant change in current response, indicating to us that the observed
signal is due to the encapsulated complex bound to CO. Although the
electrochemical behavior of the carbonyl complexes 7 and 6 incorporated in
these composite ﬁlms differs from the solution behavior, it is apparent that
CO is binding to the rhodium center; more importantly, the process is
reversible. Such behavior is in contrast to the irreversible CO chemistry
reported for conducting polymer ﬁlms containing ferrocenylferraazetine.5 In
order to make electrochemical detection a more viable means of sensing CO

in the present case, the use of a more conducting matrix is in order. One

 

 

 

 

 

 

 

143

(a)

 

 

Volts vs. Ag/AgCl

Figure 25. Cyclic voltammograms of a zirconia composite ﬁlm
containing [Rh(TMPP)2(CO)][BF4] (7) and LiCFgSO3 cast
onto a platinum disk electrode: (a) under a N2 atmosphere.
(b) after purging the cell for 30 seconds with CO. (c)-(e) after
ﬂushing the CO saturated cell with N2 for 2, 5 and 10 min.

 

 

 

 

 

144

potential solution is the incorporation of [Rh(TMPP)2(CO)][BF4] (7 ) into solid
ionic conducting polymer films such as MEEP (poly[bis(2-(2-
methoxy)ethoxy)phosphazene]).6

4. Discussion

These studies demonstrate that the molecular cationic complex
[Rh(TMPP)2(CO)]1+ (7) reversibly binds CO within a glassy polymer matrix
to form the dicarbonyl species [Rh(TMPP)2(CO)2]1+ (6). The facile nature of
the chemistry is a direct consequence of an exceedingly labile metal-ether
bond, which facilitates substitution reactions even in the solid state.
Furthermore, the metal-ether interaction appears to be selective towards CO
as other atmospheric gases do not react. Another advantage of this system is
that the substrate binds directly to the metal center. As a result, the addition
of CO conveniently gives rise to dramatic changes in the spectroscopic and
redox properties of the rhodium cation, commensurate with coordination of a
strong niacceptor to the metal center. Because of the transparent
characteristcs of the sol-gel derived ﬁlms, these spectral and electrochemical
changes are readily detected.

In spite of these attributes, a number of concerns must be addressed
before the application of these materials as sensing devices can be realized;
these include long term stability of material, conditions under which the ﬁlms
will operate, detection limits and response times. Nevertheless, on the basis
of these preliminary results with titania and zirconia ﬁlms incorporating
[Rh(TMPP)2(CO)][BF 4] (7 ), the development of other CO sensing ﬁlms using

molecular composites of ether-phosphine complexes appears attractive.

 

 

 

 

 

145

List of References

(a) Brinker, C. J .; Scherer, G. Sol-Gel Science: The Physics and
Chemistry of Sol-Gel Processing; Academic Press: San Diego, 1989. (b)
Sanchez, C.; Livage, J. New J. Chem. 1990,14, 513.

(a) Dulebohn, J. 1.; Van Vlierberge, B.; Berglund. K. A.; Lessard, R. B.;
Yu, J .; Nocera, D. G. Mat. Res. Soc. Symp. Proc. 1990, 180, 733. (b)
Lessard, R. B.; Wallace, M. M.; Oertling, W. A.; Chang, C. K.;
Berglund. K. A.; Nocera, D. G. Mat. Res. Soc. Symp. Proc. 1989, 155,
109. (c) Lessard, R. B.; Berglund. K. A.; Nocera, D. G. Mat. Res. Soc.
Symp. Proc. 1989, 155, 119. (d) Newsham, M. D.; Cerreta, M. K.;
Berglund. K. A.; Nocera, D. G. Mat. Res. Soc. Symp. Proc. 1988, 121,
627.

For other recent examples see: (a) Kuselman, I.; Kuyavskaya, B. I.;
Lev, O. Analytica Chimica Acta 1992, 256, 65. (b) Dunn, B.; Zink, J. I.
J. Mater. Chem. 199], 1, 903. (c) McKiernan, J. M.; Yamanaka, S. A.;
Knobbe, E.; Pouxviel, J. C.; Parveneh, S.; Dunn, B.; Zink, J. I. J. Inorg.
Organomet. Polym. 1991,], 87. (d) Schwok, A.; Avnir, D.; Ottolenghi,
M. J. Am. Chem. Soc. 199], 113, 3984 and references therein. (e)
Haruvy, Y.; Webber, S. E. Chem. Mater. 1991, 3, 501 and references
therein. (f) Zusman, R.; Rottman, C.; Ottolenghi, M.; Avnir, D. J. Non-
cryst. Solids 1990, 122, 107. (h) McKiernan, J. M.; Yamanaka, S. A.;
Dunn, B.; Zink, J. I. J. Phys. Chem. 1990, 94, 5652. (i) McKiernan, J.
M.; Pouxviel, J. C.; Dunn, B.; Zink, J. I. J. Phys. Chem. 1989, 93, 2129.
(j) Pouxviel, J. C.; Dunn, B.; Zink, J. I. J. Phys. Chem. 1989, 93, 2134.
(k) Durm, B.; Knobbe, E.; McKiernan, J. M.; Pouxviel, J. C.; Zink, J. 1.
Mat. Res. Soc. Symp. Proc. 1988, 121, 331.

(a) Ellerby, L. M.; Nishida, C. R.; Nishida, F.; Yamanaka, S. A.; Dunn,
B.; Selverstone-Valentine, J .; Zink, J. 1. Science 1992, 255, 1113. (b)
Yamanaka, S. A.; Nishida, F.; Ellerby, L. M.; Nishida, C. R.; Dunn, B.;
Selverstone-Valentine, J .; Zink, J. 1. Chem. Mater. 1992,4, 495. (c)
Slama-Schwok, A.; Ottolenghi, M.; Avnir, D. Nature 1992,355, 240.

(a) Mirkin, C. A.; Wrighton, M. S. J. Am. Chem. Soc. 1990, 112, 8596.
(b) Mirkin, C. A.; Valentine, J. R.; Ofer, D.; Hickman, J. J .; Wrighton,
M. S. In Biosensors and Chemical Sensors: Optimizing Performance
Through Polymeric Materials, ACS Symposium Series 487 ; Edelman,
P. G.; Wang, J. Eds., American Chemical Society: Washington, D. C.,
1992, chapter 17, p. 218.

(a) Allcock, H. R.; Austin, P. E.; Neenan, T. X.; Sisko, J. T.; Blonsky, P.
M.; Shriver, D. F. Macromolecules 1986, 19, 1508. (b) Blonsky, P. M.;
Shriver, D. F.; Austin, P. E.; Allcock, H. R. Solid State Ionics 1986, 18,
258. (c) Blonsky, P. M.; Shriver, D. F.; Austin, P. E.; Allcock, H. R. J.
Am. Chem. Soc. 1984,106, 6854. (d) Austin, P. E.; Riding, G. J.;
Allcock, H. R. Macromolecules 1983,16, 719.

 

 

 

 

 

CHAPTER VI

CHEMISTRY OF [Rh11(n3-TMPP)2] [BF4]2 WITH ISOCYANIDE
LIGANDS

146

 

 

 

 

147

1. Introduction

A survey of the literature reveals that most of the documented
research involving mononuclear rhodium complexes has focused on the +1
and +3 oxidation states, due, in large part, to their demonstrated
involvement in homogeneous catalytic processes.1 Only a limited number of
reports have addressed the coordination chemistry of paramagnetic Rh(II),2
and even fewer have focused on mononuclear organometallic complexes of
Rh(II).3 The unusual stability of 3 has presented us with a rare opportunity
to investigate these elusive classes of compounds.4 In light of the recent
reports by Wayland and co—workers of carbon monoxide and methane
activation by Rh(II) metalloradicals,5 this is a particularly attractive area of
research. In Chapter IV, we demonstrated that [Rh(n3-TMPP)2][BF4]2 (3)
reacts reversibly with CO through a pathway that involves the formation of
Rh(I) carbonyl and Rh(III) intermediates. From the chemistry of 3 with CO
it was apparent to us that the primary factor promoting the redox chemistry
was the it-accepting ability of the incoming ligand. Intrigued'by this unusual
chemistry, we set out to explore the reactivity of [Rh(n3-TMPP)2]2+ (3) with
other n-acceptors with the goal of understanding the factors that inﬂuence
the stability of Rh(II) complexes. Herein, we describe the synthesis and full
characterization of a series of novel square planar Rh(II) complexes stabilized
by isocyanides and phosphines.
2. Experimental
A. Synthesis

All reactions were carried out under an argon atmosphere by the use of
standard Schlenk-line techniques unless otherwise stated. The starting
materials tris(2,4,6-trimethoxyphenyl)phosphine (1 ) and [Rh(n3-
TMPP)2][BF4]2 (3) were prepared as described in chapters II and III

 

 

 

 

 

 

 

148

respectively. The reagents tert-butyl isocyanide, iso-propyl isocyanide, n-
butyl isocyanide, cyclohexyl isocyanide and cobaltocene were purchased from
Strem Chemicals and used without further puriﬁcation. Methyl isocyanide
was prepared according to a literature procedure.6

(1) Preparation of [Rh(TMPP)2(CNBut)2][BF4]2 (9)

An amount of ButNC (34 uL, 0.300 mmol) was carefully syringed into a
rapidly stirring solution of [Rh(n3-TMPP)2][BF4]2 (3) (0.200 g, 0.150 mmol) in
CH2C12 (3 mL). The solution color immediately changed from purple to
black/purple. After stirring for 10 min, the solution was layered with 20 mL
of THF. After diffusion had occurred, dark purple crystals separated from
the solution; these were collected by suction ﬁltration in air, washed with
THF (3 x 5 mL) and Et20 (2 x 5 mL), and subsequently dried under vacuum;
yield, 0.213 g (90%). In an alternative work-up procedure, the solution was
ﬁltered in air after 10 min of reaction and diluted with CH2C12 (20 mL)
followed by THF (8 mL). Reduction of the solution volume to 3 - 5 mL on a
rotary evaporator produces a purple crystalline product upon standing
overmght. Anal. Calcd for C64H74F8P2013N2B2Rh: C, 50.98; H, 4.95. Found:
C, 50.45; H, 5.88. IR (CH2C12, cm'l): v(CEN), 2200 vs; (Nujol mull, CsI, cm-l):
v(CEN), 2198 vs; other, 1595 vs, 1578 vs, 1411 s, 1332 s, 1294 w, 1228 s, 1207
s, 1185 w, 1160 s, 1122 s, 1087 s, 1054 s, 1026 s, 950 m, 917 m, 815 m, 675 w,
640 w, 536 w, 522 w, 479 m, 442 w. Electronic absorption spectrum (CHZCIZ)
kmax, nm (e): 819 nm (1770), 546 (630), 319 (22,400), 257 (56,500). 1H NMR
and 31P NMR (CD2C12) 8 ppm: not observed. Cyclic voltammogram (0.1 M
TBABF4/CH2C12, vs Ag/AgCl): E1/2(red) = - 0.04 V.

(2) Preparation of [Rh(TMPP)2(CNPri)2][BF4]2 (10)

Dropwise addition of a solution of PriNC (13.6 uL, 0.150 mmol) in 7 mL

of CH2C12 to a solution of [Rh(n3-TMPP)2][BF4]2 (3) (0.100 g,, 0.075 mmol) in

O

 

 

 

 

149

3 mL of CH2C12 produced a black/purple solution over a period of 10 min.
The reaction was stirred for an additional 10 min, after which time 5 mL of
CH2012 and 10 mL of THF were added. The solution was concentrated to a
volume between 5 - 8 mL on a rotary evaporator and ﬁltered through Celite.
Upon standing overnight, a crop of purple crystals formed; these were
isolated and washed with copious amounts of THF and EtZO; yield 0.053 g
(48%). Anal. Calcd for C62H80F8P2013N2B2Rh: C, 50.32; H, 5.45. Found: C,
49.54; H, 5.50. IR (CH2C12, cm‘l): v(CEN), 2211 vs; (Nujol mull, CsI, cm‘l):
v(CEN), 2209 vs; other, 1595 vs, 1577 vs, 1411 s, 1331 s, 1297 w, 1231 s, 1206
s, 1184 w, 1161 s, 1125 s, 1088 s, 1054 s, 1025 s, 949 m, 917 m, 812 m, 478 m.
Electronic absorption spectrum (CH2C12) Amax, nm (a): 843 nm (1847), 558
(645), 318 (20,685), 258 (57,933). Cyclic voltammogram (0.1 M
TBABF4/CH2C12, vs Ag/AgCl): E1]2(red) = + 0.01 V.

(3) Reaction of [Rh(n3-TMPP)2] [BF4]2 (3) with other isocyanides

(i) Methyl isocyanide: (a) one equivalent. In a typical reaction, MeNC
(2 uL) was added to a solution of [Rh(n3-TMPP)2][BF4]2 (3) (0.050 g, 0.037
mmol) in 5 mL of CH2CIZ. The solution immediately turned from purple to
red with the concominant precipitation of a small quantity of red solid (see
below). An IR monitoring study of the solution reveals the immediate
formation of several CEN stretching vibrations; v(CEN) cm'1: 2239 s, 2248 s,
2267 m, 2176. After 10 min, an additional band appeared at 2150 cm'1.

(b) five equivalents. To a solution of [Rh(n 3-TMPP)2][BF4]2 (3) (0.100 g,
0.75 mmol) in 5 mL of CH2C12 was added 5 equivalents of MeNC (20 uL, 3.75
mmol). A red ﬂocculent solid precipitated from the red/orange solution. The
resulting suspension was stirred for 5 min, after which time the solid was
collected by ﬁltration, washed with a minimal amount of CH2C12 (< 5 mL)

and THF ( 2 x 5 mL) and dried under vacuum; yield: 0.43 g. IR (Nujol, CsI)

 

 

 

 

 

 

 

 

150

cm'1: v(CEN) 2238 s; [BF4]' 1050 br. 1H NMR (CD3CN) 8 ppm: 3.41 (s, 12H),
3.57(s, 18H, o-OCH3), 3.83 (s, 9H, p-OCH3), 6.21 (t, lJP-H = 2.1 Hz, 6H, m-H).
31P NMR (CD3CN) 5 ppm: - 39.5 (t, J = 42.7 Hz). Cyclic voltammogram (0.1
M TBABF4/CH2C12, vs Ag/AgCl): Ep,c = - 0.46 V, Ep,c = - 1.71 V.

(ii) Cyclohexyl isocyanide. To a solution of [Rh(n 3-TMPP)2][BF4]2 (3)
(0.050 g, 0.037 mmol) in 5 mL of CH2C12 was added 2 equivalents of CNCy
(9.3 uL, 0.075 mmol). The purple solution immediately turned dark purple
and then ﬁnally black. The IR spectrum measured within the ﬁrst minute
exhibited three bands; v(CEN): 2245 vw, 2210 m, 2173 m. Over a period of
several hours, the low energy band continued to grow in as the solution color
became pale.

(iii) n-Butyl isocyanide. In a typical reaction, two equivalents of CNBun
(7.8 uL, 0.075 mmol) were added to a purple solution of [Rh(n3-
TMPP)2][BF4]2 (3) in 5 mL of CH2Cl2. The solution immediately became
dark brown. After 30 min, the solution IR spectrum showed several CEN
stretching vibrations; v(CEN); 2292 w, 2255 vs, 2220 w, 2160. The solvent
was removed under reduced pressure and the resulting residue was
redissolved in THF (5 mL), ﬁltered in air to remove unreacted 3 and ﬁnally
pumped to a residue.

(4) Reactions of [Rh(TMPP)2(CNBut)2] [BF4]2 (9)

(i) Cobaltocene: Preparation of [Rh(TMPP)2(CNBut)2][BF4] (11). A
solution containing [Rh(TMPP)2(CNBut)2][BF4]2 (9) (0.100 g, 0.066 mmol)
and 1 equivalent of Cp2Co ( 0.013 g, 0.066 mmol) in 5 mL of CH2C12 was
stirred at r. t. for 30 min. The yellow solution was evaporated to dryness, and
the resulting residue was redissolved in THF (10 mL). The solution was

ﬁltered through a Celite plug to remove undissolved [Cp2Co][BF4] and

reduced in volume to 5 mL. Diethyl ether (15 mL) was added and the

 

m

151
solution was again ﬁltered. The solvent was then removed under vacuum
and the yellow residue was taken up in 5 mL of CH2C12. To this solution, 35
mL of diethyl ether was added creating a supersaturated solution from which
a yellow crystalline solid separated. The crystals were collected by ﬁltration
and washed with diethyl ether (2 x 5 mL); yield, 0.060 g (64 %). IR (CH2C12,
cm‘l): v(CEN), 2118 vs; (Nujol mull, CsI, cm'l): v(CEN), 2198 vs; other, 1595
vs, 1578 vs, 1411 s, 1332 s, 1294 w, 1228 s, 1207 s, 1185 w, 1160 s, 1122 s,
1087 s, 1054 s, 1026 s, 950 m, 917 m, 815 m, 675 w, 640 w, 536 w, 522 w, 479 4
m, 442 w. Electronic absorption spectrum (CH2C12) km”, nm: 395. 1H NMR
(CD2C12, 25°C) 5 ppm: -C(CH3)3 0.86 (s, 9H), 0.88 (s, 9H); ~0CH3 3.07 (s, 6H),
3.16 (s, 6H), 3.30 (s, 6H), 3.37 (s, 6H), 3.53 (s, 6H), 3.70 (s, 6H), 3.73 (s, 6H),
3.77 (s, 6H), 3.83 (s, 3H), 3.85 (s, 3H); meta-H 5.88 (br, 8H), 6.02 (br, 2H), 6.12
(br, 2H). 31P NMR (CDZClz, 25°C) 8 ppm: PA, -19.1 (d, 1JRh_p = 129.7 Hz);
PB , -20.2 (d, 1JRh_p = 129.7 Hz). Cyclic voltammogram (0.1 M
TBABF4/CH2C12, vs Ag/AgCl): El/2(ox) = - 0.04 V.
(ii) TMPP. A mixture of [Rh(TMPP)2(CNBut)2][BF4]2 (9) (0.020 g, 0.013
mmol) and TMPP (0.007 g, 0.013 mmol) was dissolved in approx. 0.7 mL of
CD3CN. The resulting purple solution was transferred to a NMR tube. No
reaction had occurred after 12 h at r.t. as evidenced by 1H NMR spectroscopy.
(iii) tert-Butyl isocyanide. A solution of [Rh(TMPP)2(CNBut)2][BF4]2 (9)
(0.050 g, 0.033 mmol) in 5 mL of CH2C12 was treated with 3.8 uL of CNBut
(0.033 mmol) and stirred at r.t. After 10 min, the solution color had gradually
changed from purple to black/green and eventually to green. After 12 h an
infrared spectrum of the solution was obtained; IR (CH2C12) cm'l: 2167 s,
2118 m, 2230 w. An analogous 31P NMR study in CD3CN revealed the
presence of unligated TMPP and [TMPP-CH3]+ in addition to a new species;
31P NMR (CD3CN) 8 ppm: - 27.5 (d, lJRh-P = 77.8 Hz)

 

 

 

 

152

(iv) Carbon monoxide. An amount of [Rh(TMPP)2(CNBut)2][BF4]2 (9)
(0.025, 0.017 mmol) was dissolved in 5 mL of CH2C12. The solution was
purged with CO for 30 min. An infrared spectrum of the solution showed that
no reaction had occurred under these conditions.
B. X-ray Crystallography

The structures of [Rh(TMPP)2(CNBut)2][BPh4]2 and
[Rh(TMPP)2(CNBut)2][BF4] were determined by application of general
procedures which have been fully described elsewhere.7 Geometric and
intensity data were collected on a Rigaku AFC6S diffractometer with
graphite monochromated MoKoc (it-a = 0.71069 A) radiation and were
corrected for Lorentz and polarization effects. Important crystallographic
data are summarized in Table 13. All calculations were performed with the
use of VAX computers on a cluster network within the Department of
Chemistry at Michigan State University by using the Texsan software
package of the Molecular Structure Corporation.8
(1) [Rh(TMPP)2(CNBut)2][BPh4]2 (9)
(i) Data Collection and Reduction. We were unable to obtain suitable
single crystals of [Rh(TMPP)2(CNBu")2]2+ as a [BF4]‘ salt for X-ray
diffraction studies. However, metathesis of [BF4]' with KIBPh4] in acetone
followed by slow diffusion of Eth yielded large dark purple crystals of
[Rh(TMPP)2(CNBut)2][BPh4]2. A block-shaped crystal of approximate
dimensions 0.26 x 0.26 x 0.29 mm3 was selected and secured to the tip of a
glass ﬁber with epoxy cement. Cell constants were obtained from a least-
squares ﬁt of 25 centered reﬂections in the range 15 < 20 < 20° and were
consistent with a monoclinic cell. Intensity data were gathered at 23 i 1°C in

the range 4 S 29 S 47° using the (1)-scan technique. Reﬂections with I < 106(1)

 

 

 

 

 

Table 13. Summary of crystallographic data for [RhH(TMPP)2(CNBut)2]

 

[BPh4]2 (9a).

Formula RhP20180112H124B2
Formula weight 1972.69
Space group P21/c

a, A 26.810(7)
b, A 14.076(2)
c, A 27.809(6)
a, deg 90

[3, deg 101.35(2)
7, deg 90

V, A3 10,289(4)
Z 4

dcalc, g/cm3 1.273

)1 (Mo Kat), cm'1 2.57
Temperature, °C 23:2 °C
Ra 0.056
wa 0.071
Quality-of-ﬁt indicatorc 3.80

 

312:: | lFol- chll/ztrol

bRw = [2w( I F0 I— IFc l)2/>:w To I211/2; w = 1/02( lFo l)

CQuality-of-ﬁt = [2w( IFo | — ch DZ/(Nobs-Nparametersﬂ 10-

 

 

 

 

 

 

154

were rescanned a maximum of three times and the counts were accumulated
to ensure good counting statistics. Three representative reﬂections were

monitored at regular intervals and exhibited a 1.5% loss in intensity. A

linear correction factor was applied to the data to account for the observed

decay in intensity. After equivalent data were averaged, there remained

9109 data with F02 2 36(Fo)2 which were used in the structure solution and

reﬁnement.

(ii) Structure Solution and Refinement. The position of the rhodium~
atom was found by direct methods.9 The remaining non-hydrogen atoms

were located and reﬁned through successive least-squares cycles and

difference Fourier maps. After isotropic convergence, an empirical absorption

correction was applied using the program DIFABS.10 Thermal parameters

for all non-hydrogen atoms were then reﬁned anisotropically with the

exception of the three methyl carbons C(62), C(63) and C(64) on one of the

isocyanide ligands; these were treated isotropically. Hydrogen atoms were

calculated at ﬁxed positions and were not reﬁned. Anisotropic reﬁnement of
1219 parameters converged to give R = 0.056 and R“. = 0.071. The quality-of-

ﬁt index was 3.80 and the largest shift/esd = 0.32.

3. Results
A. Synthesis and Spectroscopic Characterization of
[Rh(TMPP)2(CNR)2] [BF4]2 (R = But, Pri).

Dichloromethane solutions of [Rh(n3-TMPP)2][BF4]2 (3) react smoothly
with two equivalents of CNR (R = But, Pri) to yield dark purple solutions that
exhibit a characteristic v(C_=.N ) band at 2200 cm'1 for CNBut and 2211 cm'1
for CNPri. Both values appear at a higher energy than that of the free

isocyanide which is indicative of a higher valent metal complex in which the

 

 

 

 

 

 

 

 

  

_r_‘-7" '
~. .4 ._.

 

PM“: ,. :‘rrv'm'l

155

isocyanides act primarily as a-donors.11 Reaction with only one equivalent of
isocyanide results in a 1:1 mixture of product and starting material,
consistent with the formulation of the stable product as a bis adduct of (3). In
both cases, concentration of the reaction solution followed by addition of THF
produces a dark purple microcrystalline solid. The products,
[Rh(TMPP)2(CNBut)2][BF4]2 (9) and [Rh(TMPP)2(CNPri)2][BF4]2 (10), were
isolated in 90% and 48% yield, respectively. In spite of the highly reactive
nature of many radical d7 rhodium species, these Rh(II) di-isocyanide
complexes are remarkably air-stable both in the solid state and in solution.
The electronic spectra of [Rh(TMPP)2(CNBut)2][BF4]2 (9) and
[Rh(TMPP)2(CNPri)2][BF4]2 (10) both exhibit two prominent low energy
bands in the visible region at 819 nm (e = 1770 M‘lcm'l) and 546 nm (e = 630
M‘lcm'l) for 9 and at 843 nm (e = 1847 M' 1cm‘l) and 558 nm (e = 645 M'
1cm'l) for 10; these transitions give rise to the characteristic dark purple
color exhibited by these paramagnetic complexes.

Although 9 and 10 are stable with respect to oxygen and moisture, both
complexes readily react with additional isocyanide to give a mixture of
diamagnetic products. NMR spectral measurements of CD3CN solutions of
[Rh(TMPP)2(CNBut)2][BF4]2 (9 ) in the presence of excess tert-butyl
isocyanide showed that TMPP dissociates during the reaction to form a new
Rh(TMPP) species. Based on the symmetrical appearance of the proton
resonances, this compound is likely a Rh(I) monophosphine complex with the
general formula [Rh(TMPP)(CNBut)x]1+ (x = 2 or 3). Interestingly, even
though 9 and 10 react readily with excess isocyanide, methylene chloride
solutions of 9 and 10 are inert towards reaction with CO at 1 atm and r.t.
Furthermore, unlike many TMPP complexes, [Rh(TMPP)2(CNBut)2][BF4]2 (9)

does not undergo nucleophilic attack by free TMPP to yield dealkylated

 

.._~. -4

 

 

 

156

complexes. Such behavior suggests that the pendent methoxy groups of
TMPP are not tightly bonded to the metal center and that the coordination
geometry about rhodium is four coordinate. Indeed, this is the case as
evidenced by X—ray crystallography .

Thus far, only tert-butyl and iso-propyl isocyanide ligands have been to
produce stable Rh(II) isocyanide adducts of [Rh(n3-TMPP)2][BF4]2. Reactions
of 3 with other isocyanides, such as cyclohexyl or n-butyl isocyanide, result in
the initial formation of dark colored solutions that eventually decompose to
form a mixture of isocyanide products as evidenced by infrared spectroscopy.
The exact nature of these products has not yet been elucidated. However, by
analogy with the chemistry of 9 and 10 with excess isocyanide, it is likely that
a Rh(II) complex is formed initially, but is subject to facile isocyanide
dissociation. The liberated isocyanide in turn reacts with the other Rh(II)
isocyanide complexes. Clearly, more evidence is required to support this
hypothesis and to formulate the products.

More intriguing than the reactions with cyclohexyl and n-butyl
isocyanide, however, is the chemistry of [Rh(n 3-TMPP)2][BF4]2 (3) with
methyl isocyanide. Unlike the reactions of 3 with longer chain isocyanides in
which the products remain in solution, addition of methyl isocyanide to
methylene chloride solutions of 3 results in the immediate formation of a red
precipitate. This solid is insoluble in most common organic solvents except
for acetonitrile, in which the compound exhibits only limited solubility.
Infrared spectroscopy reveals a single CEN stretching vibration at 2238 cm1
and a broad band at 1060 cm'1 indicating that complex is cationic. The solid
is diamagnetic as evidenced by 1H and 31P spectroscopy. The 1H spectrum of
the solid exhibits one triplet resonance representing all meta-protons of the

phosphine; the observed coupling pattern for the meta-protons indicates a

 

 

 

 

157

trans-arrangement of two phosphines about the metal. The 31P NMR
spectrum of the solid in CD3CN exhibits two idependent resonances that
nearly overlap each other. Each resonance is split into a doublet, presumably
due to Rh-P coupling, but there no evidence for the presence of P-P coupling.
Such a situation might arise if the complex existed in solution as a 1:1 ratio of
two independent geometric conformers in which the trans phosphorus nuclei
of each conformer are magnetically equivalent.12 Further work is needed to
determine the identity of this product.

B. Magnetic and EPR Spectroscopic Properties of

[Rh(TMPP)2(CNR)2][BF4]2 (R = But, Pri).

The paramagnetism of [Rh(TMPP)2(CNBut)2][BF4]2 (9) was probed by
several spectroscopic and magnetic techniques. The 1H and 31P NMR
spectra of [Rh(TMPP)2(CNR)2][BF4]2 are broad and essentially featureless,
consistent with the formulation of 9 and 10 as paramagnetic species. Solid
state and solution magnetic susceptibility studies conﬁrmed the presence of
an S = 1/2 ground state for the isocyanide complexes. A solid state magnetic
susceptibility measurement of 9 at 299 K led to a ueff value of 2.04 B.M. A
diamagnetic correction of -880 x 10'6 cgs was applied based on -24 x 10'6 cgs
for Rh2+, -39 x 10-6 cgs for [BF4]': -330.8 x 10-6 cgs for TMPP and -58.5 x 10-
6 cgs for CNBut.13 Variable temperature magnetic susceptibility
measurements for 9 were made over the temperature range of 5 - 380 K at a
ﬁeld strength of 500 G; Consistent with a simple paramagnet, the sample
displayed Curie-Weiss behavior over this temperature range with 9 = -3.35
K. Solution susceptibility studies by the Evans method yielded a ueff of 2.20
B.M. at 293 K, also consistent with the presence of an S = 1/2 ground state.14

The paramagnetism of the samples was further examined by EPR

spectroscopy. The solid-state EPR spectrum of a polycrystalline sample of 9

158
at 100 K (Figure 26) shows an axial signal with g = 2.45 and g1] = 1.96 with
hyperﬁne“ coupling to 103Rh (1:1/2) in the g1] region (A11 = 62 G). The EPR
spectrum in a 1:1 Me-THF/CH2C12 glass at 100 K exhibits a signal similar to
that observed in the solid state except the signal has become rhombic and the
gx and gy regions show hyperﬁne coupling to 103Rh as well (Figure 27).
Although Rh hyperﬁne coupling in this region is rarely observed, it has been
reported by Wilkinson et al. for the organometallic species Rh(2,4,6-
Pri3CGH2)2(tht)2.15 [Rh(TMPP)2(CNPri)2][BF4]2 (10) exhibits an EPR
spectrum similar to that observed for 9 except that the gx and gy tensors are
nearly coincidental as shown in Figure 28. The rhombic signal with gx ~ gy >

gz, observed for both 9 and 10, is characteristic of a (1,2 ground state.

C. Redox Chemistry of [Rh(TMPP)2(CNR)2] [BF412 (R = But, Pri)

A cyclic voltammogram of [Rh(TMPP)2(CNBut)2][BF4]2 in 0.1 M
TBABF4-CH2C12 shows a reversible couple at Em: -0.04 V vs Ag/AgCl,
corresponding to a one-electron reduction to Rh(I). Not surprisingly, this
process is shifted to more positive potentials relative to that of [Rh(n3-
TMPP)2][BF4]2 (3), due to the electron-withdrawing effect of the n-acceptor
ligands.16 Comp0und 9 can be chemically reduced in the presence of
cobaltocene to give the yellow Rh(I) complex [Rh(TMPP)2(CNBut)2][BF4] (11).
Examination of the cyclic voltammogram for 11 shows that it is identical to
that of 9, except that the redox process at E1/2 = -0.04 V corresponds to an
oxidation of the compound. The infrared spectrum of 11 shows a strong band
v(CEN) = 2118 cm'l, shifted to lower energy than the corresponding stretch in
9 due to increased n-back-bonding upon reduction from Rh(II) to Rh(I).
Although the complex is expected to exhibit a simple trans-nl-phosphine

 

 

159

 

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1918 2334 2750 3167 3583 3998
GAUSS

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solid-state at 100 K.

 

 

 

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162

geometry on the basis of steric considerations, the 1H NMR spectrum of 11
shows an unsymmetrical pattern. A likely explanation for this observation is
that steric hindrance caused by the CNBut groups restricts rotation of the
phosphines about the M-P bond thereby creating syn and anti conformations
for the two TMPP ligands. The different orientations of the phosphines for
the two conformers are illustrated below; in both the syn and anti
conformations the phosphorus nuclei are equivalent, but the spatial
arrangement of the three arene rings causes two of the rings to be different

from the third. The 31P NMR spectrum of 11 in CD§12 provides further

   

Kr!

syn anti

evidence for the presence of two conformers in solution. The compound
exhibits two independent A2X spin systems, 8 = - 19.1 ppm (d, 1JRh_p = 129.7
Hz) and 5 = - 20.2 ppm (d, 1JRh_p = 129.7 Hz) in a 1:1 ratio resonating at
nearly the same frequency. NMR experiments performed in CD3CN reveal
that the ratio of conformers was unchanged. Warming the sample to 75°C in
CD3CN caused both the proton and phosphorus resonances to broaden but
neither completely coalesced. At this temperature, however, decomposition of
11 was also observed. The appearance of different conformers in solution as a

result of restricted rotation about the metal-phosphorus bond has been

163
reported for other square planar Rh(I) and Ir(I) complexes ligated by
sterically encumbering phosphines.12 However, in contrast to the solution
behavior of 11, these complexes were found to undergo interconversions at
much lower temperatures; typically, the barrier to rotation about the metal-
phosphine bond was found to be low in energy and different conformers were
observed only at temperatures less than -30° C. Apparently the combined
size of both TMPP and the t-butylisocyanide ligand limits rotation about the

metal-phosphorus bond well above room temperature.

D. X-ray Crystal Structure of [Rh(TMPP)2(CNBut)2] [BPh4]

The identity of (9) as [Rh(TMPP)2(CNBut)2][BF4]2 was conﬁrmed by an
X-ray crystallographic study. Large single crystals of
[Rh(TMPP)2(CNBut)2][BPh4]2 were obtained by metathesis of
[Rh(TMPP)2(CNBut)2][BF4]2 (9) with K[BPh4] in acetone followed by slow
diffusion of EtZO. An ORTEP drawing of the molecular cation
[Rh(TMPP)2(CNBut)2]2+ is shown in Figure 29. Selected bond distances and
angles are listed in Table 14. The cation contains mutually trans phosphine
and isocyanide ligands arranged in a slightly distorted square plane, as
evidenced by the angles C(55)-Rh(1)-C(60) = 179.0 (3)° and P(1)-Rh(1)-P(2) =
168.78 (8)°. Coordination of the tert-butyl isocyanide ligands has effected a
cis-to-trans isomerization from 3 to 9 similar to that observed in the
formation of [Rh(TMPP)2(CO)2][BF4], with one major exception; namely the
oxidation state has remained +2 in this case rather than reducing to +1 as in
the CO compound. Unlike [Rh(n3-TMPP)2][BF4]2 (3), in which the rhodium
atom is pseudo-octahedral with two metal-phosphorus and four metal-ether
interactions, 9 is bound to TMPP only through the phosphorus lone pair,

leading to a four-coordinate geometry. The shortest Rh-O distances

 

 

 

 

 

O 053

Figure 29.

 

C35

   

ORTEP representation of the molecular cation
[Rh(TMPP)2(CNBut)2]2+ in structure (9). Thermal ellipsoids
are shown at the 40% probability level. Carbon and oxygen
atoms of the phosphine ligand are shown as small spheres
of arbitrary size for clarity.

 

165

Table 14. Selected bond distances (A) and angles (deg) for
[RhH(TMPP)2(CNBut)2][BPh4]2 (9a).

 

 

 

 

Atom 1 Atom 2 bond distance

Rh(l) P(1) 2.364(2)

Rh(l) P(2) 2.380(2)

Rh(l) C(55) 1.990(9)

Rh( 1) C(60) 1.986(8)

Atom 1 Atom 2 Atom 3 bond angles
P(1) Rh(l) P(2) 168.78(8)
P(1) Rh(l) C(55) 87.9(2)
P(1) Rh(l) C(60) ' 92.6(2)
P(2) Rh(l) C(55) 92.2(2)
P(2) Rh(l) C(60) 87.1(2)
C(55) Rh(l) C(60) 179.0(3)
Rh(l) P(1) C(1) 114.1(3)
Rh(l) P(1) C(10) 119.8(3)
Rh(l) P(1) C(19) 101.0(3)

 

 

 

166

along the axial direction in 9 are Rh(1)-O(1) = 2.851 (5)A and Rh(1)-O( 13) =
2.909 (6)A ; these are well outside the expected range for covalent bonding.
The trans Rh-P bond distances of 2.364 (2)A and 2.380 (2)A are longer than
those found in the parent Rh(II) complex, and are indicative of the stronger

trans effect exerted by the phosphine as compared to that of an ether donor.

4. Discussion

At the time of this work, [Rh(TMPP)2(CNBut)2][BF4]2 represented the
ﬁrst mononuclear organometallic «Rh(II) complex sufﬁciently stable to be
allow for full spectroscopic and crystallographic characterization. Unlike the
reactions of [Rh(n3-TMPP)2][BF4]2 with CO, alkyl isocyanide reactions of 3
forms the stable adducts with the general formula [Rh(TMPP)2(CNR)2]2+ (R
= But, Pri). The stability of these paramagnetic species is manifested in their
relative redox properties. As was demonstrated by cyclic voltammetry, the
Rh(II)/Rh(I) couple for 9 and 10 falls at a potential less positive than the
Rh(II)/Rh(III) couple for the parent complex, [Rh('r)3-TMPP)2]2+ (3), however,
and as a result, [Rh(TMPP)2(CNR)2]2+ (R = But, Pri) is stable with respect to
spontaneous reduction to Rh(I) in the presence of [Rh(n3-TMPP)2]2+. This is
in sharp contrast to the situation in the analogous [Rh(n3-TMPP)2]2+/ carbon
monoxide chemistry.

We have seen that subtle electronic changes in the n-acceptor ligand
dramatically inﬂuence the stability of the Rh(II) complex. A case in point is
the chemistry of 3 with methyl isocyanide, which is sterically and
electronically very similar to CO. No stable Rh(II) adducts were isolated;
instead, the reaction mixture contained several diamagnetic species, most
likely Rh(I) and Rh(III) by analogy to the CO chemistry. By contrast, the
tBuNC and iPrNC reactions proceed to yield only Rh(II) products.

167

Surprisingly, reactions of 3 with other isocyanide ligands that are sterically
and electronically similar to tert-butyl and iso-propyl isocyanide appear to
initially form Rh(II) complexes, but these adducts are not stable over time,
probably because of pathways involving isocyanide dissociation followed by
subsequent reaction with the already formed Rh(II) isocyanide complexes.
Clearly, there is a subtle interplay between the steric and electronic
properties of the isocyanide ligands that affects the overall stability of these
paramagnetic Rh(II) isocyanide species. While it is not possible to draw any
ﬁrm conclusions about the inﬂuence of the different R groups on the
isocyanide chemistry of 3, the results suggest that steric factors as well as
electronic factors play a role in the stabilization of these mononuclear Rh(II)
complexes.

The isolation of [Rh(TMPP)2(CNR)2]2+ (R = But, Pri) allows for a rare
opportunity to examine the chemistry of a stable organometallic radical
system. One particular application is the generation of stable Rh(II)
carbenes. The dinuclear Rh(II) complex Rh2(OAc)4 is known to promote the
cyclopropanation of oleﬁns by organic diazo compounds (eq 18), a highly
useful organic process.17 Although they have not been observed directly,
Doyle and co-workers have implicated the intermediacy of Rh(II)-carbene

species in these transformations.18 Rh(I) and Rh(III) carbenes are readily
Y Y
R20N2 + han’n(OAC)4 + =/ —» RR + N2 (18)

prepared by the addition of primary amines across the CEN of coordinated
isocyanides (eq 19).19 By analogy, reaction of [Rh(TMPP)2(CNR)2]2+ (R = But,

Pri) should yield a Rh(II) carbene complex. The reaction is ligand based so no

 

 

 

 

 

168

formal redox chemistry takes place at the metal center; both isocyanides and
Fisher carbenes are considered to be neutral two electron donors, therefore
the formal oxidation state of rhodium will remain +2. Additional
stabilization may be provided by the variable electron donating capabilities of

TMPP and its demonstrated ability to adjust to the electronic

 

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requirements of the metal center. One possible drawback to the formation of
carbene complexes of 3, however, is the additional steric congestion that
would result from addition of an amine to the system. To help alleviate this
problem, smaller amines will be required to minimize steric interactions.
Alternatively, the steric strain may be relieved through isocyanide
dissociation. The formation of stable Rh(II) carbene complexes is just one of

the many potential applications for these systems.

169
List of References

1. (a) Hughes, R. P. in Comprehensive Organometallic Chemistry;
Wilkinson, G.; Stone, F. G. A.; Abel, E. W., eds.; Pergamon: New York,
1982; vol. 5, p 228. (b) Homogenous Catalysis with Metal Phosphine
Complexes; Pignolet, L. H., ed.; Plenum: New York, 1983. (c) Parshall,
G. W. Homogenous Catalysis; Wiley-Interscience: New York, 1980; p
89. (d) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry;
University Science Books: Mill Valley, Ca. 1987; p 523.

2. For example see (a) Felthouse, T. R. Prog. Inorg. Chem. 1982, 29, 73
and references therein. (b) Rawle, S. C.; Yagbasan, R.; Prout, K.;
Cooper, S. R. J. Am. Chem. Soc. 1987, 109, 6181. (c) Blake, A. J.;
Gould, R. D.; Holder, A. J .; Hyde, T. I.; Schrtider, M. J. Chem. Soc.,
Dalton Trans. 1988, 1861. ((1) Cooper, S. R.; Rawle, S. C.; Yagbasan,
R.; Watkin, D. J. J. Am. Chem. Soc. 1991, 113, 1600. (e)
Pneumatikakis, G.; Psaroulis, P. Inorg. Chim. Acta 1980, 46, 97. (f)
Pandey, K. K.; Nehete, D. T.; Sharma, R. B. Polyhedron 1990, 9, 2013.
(g) Anderson, J. E.; Gregor, T. P. Inorg. Chem. 1989, 28, 3905. (h)
Bianchini, C.; Meli, A.; Laschi, F.; Vizza, F.; Zanello, P. Inorg. Chem.
1989,28, 227.

3. (a) Fischer, E. O.; Lindner, H. H. J. Organomet. Chem. 1964, 1, 307.
(b) Fischer, E. O.; Wawersik, H. J. Organomet. Chem. 1966, 5, 559. (0)
Keller, H. J .; Wawersik, H. J. Organomet. Chem. 1967,8, 185. (d)
Dessy, R. E.; King, R. B.; Waldrop, M. J. Am. Chem. Soc. 1966,88,
5112. (e) Dessy, R. E.; Kornmann, R.; Smith, C.; Hayter, R. J. Am.
Chem. Soc. 1968, 90, 2001. (f) Bianchini, C.; Laschi, F.; Ottaviani, F.;
Peruzzini, M.; Zanello, P. Organometallics 1988, 7, 1660. (g) Bianchini,
C.; Laschi, F.; Meli, A.; Peruzzini, M.; Zanello, P.; Frediani, P.
Organometallics 1988, 7, 2575. (h) Bianchini, C.; Laschi, F.;
Ottaviani, F.; Peruzzini, M.; Zanello, P.; Zanobini, F. Organometallics
1989,8, 893. (i) Pilloni, G.; Schiavon, G.; Zotti, L.; Zeechin, S. J.
Organomet. Chem. 1977,134, 305.

4. Other references concerning the reactivity of mononuclear Rh(II)
compounds (a) Vleck, A. Inorg. Chim. Acta 1980, 43, 35. (b) Holah, D.
G.; Hughes, A. N.; Hui, B. C. Can. J. Chem. 1975, 53, 3669. (c)
Valentini, G.; Braca, G.; Sbrana, G.; Colligiani, A. Inorg. Chim. Acta
1983, 69, 215. (d) Valentini, G.; Braca, G.; Sbrana, G.; Colligiani, A.
Inorg. Chim. Acta 1983, 69, 221.

5. (a) Wayland, B. B.; Sherry, A. E.; Cofﬁn, V. L. J. Chem. Soc., Chem.
Commun. 1989, 662. (b) Wayland, B. B.; Sherry, A. E. J. Am. Chem.
Soc. 1989, 111, 5010. (c) Wayland, B. B.; Sherry, A. E.; Poszmik, G.;
Bunn, A. G. J. Am. Chem. Soc. 1992, 114, 1673. (d) Wayland, B. B.; Ba,
8.; Sherry, A. E. J. Am. Chem. Soc. 1991, 113, 5305. (e) Sherry, A. E.;
Wayland, B. B. J. Am. Chem. Soc. 1990, 112, 1259. (f) Poszmik, G;

10.

11.

12.

13.

14.

15.

16.

17.

 

170

Wayland, B. B. presented at the 203rd National meeting of the
American Chemical Society, San Francisco, CA, April 5-10, 1992; paper
INOR 67.

(a) Schuster, R. E.; Scott, J. E.; Casanova, J. Org. Synth. 1966, 46, 75.
(b) Casanova, J .; Schuster, R. E.; Werner, N. D. J. Chem. Soc. 1963,
4280.

(a) Bino, A.; Cotton, F. A.; Fanwick, P. E. Inorg. Chem. 1979, 18, 3558.
(b) Cotton, F. A.; Frenz, B. A.; Deganello, G.; Shaver, A. J. Organomet.
Chem. 1973, 50, 227.

TEXSAN - TEXRAY Structure Analysis Package, Molecular Structure
Corporation, 1985.

(a) SHELXS-86: Sheldrick, G. M. in Crystallographic Computing 3;
Sheldrick, G. M.; Goddard, R. Eds; Oxford University Press, 1984; p.
175. (b) DIRDIF: Direct Methods for Difference Structure, An
Automatic Procedure for Phase Extension; Reﬁnement of Difference
Structure Factors. Beurskens, R. T. Technical Report 1984.

Walker, N.; Stuart, D. Acta. Cryst. 1983,A39, 158.

Nakamoto, K. Infrared and Raman Spectra of Inorganic and
Coordination Compounds,3rd ed; Wiley: New York, 1978, p. 269.

For instance see (a) Bushweller, C. H.; Hoogasian, 8.; English, A. D.;
Miller, S. J.; Lourandos. M. Z. Inorg. Chem. 1981, 20, 3448. (b)
Empsall, H. D.; Hyde, E. M.; Mentzer, E.; Shaw, B. L. J. Chem. Soc.,
Dalton Trans. 1977, 2285. (c) Mann, B. E.; Masters, C.; Shaw, B. L.;
Stainbank, R. E. J. Chem. Soc., Chem. Commun. 1971, 1103.

O'Connor, C. J. Prog. Inorg. Chem. 1982,29, 205.

(a) Evans, D. F. J. Chem. Soc. 1959, 2003. (b) Deutsch, J. L.; Poling S.
M. J. Chem. Educ. 1969,46, 167.

Hay-Motherwell, R. S.; Koschmieder, S. U.; Wilkinson, G.; Hussain-
Bates, B.; Hursthouse, M. B. J. Chem. Soc., Dalton Trans. 1991, 2821.

For the affect of n-acceptor ligands on metal redox potentials see (a)
Treichel, P. M.; Mueh, H. J.; Bursten, B. E. Isr. J. Chem. 1976/77, 15,
(b) Dunbar, K. R.; Walton, R. A. Inorg. Chim. Acta 1984,87, 185.

(a) Doyle, M. P. Chem. Rev. 1986,86, 919. (b) Maas, G. Top. Curr.
Chem. 1987,137, 75. (c) Doyle, M. P. In Catalysis of Organic
Reactions; Augustine, R. L. Ed.; Marcel Dekker: New York, 1985. (d)
Doyle, M. P.; Bagheri, V.; Wandless, T. J .; Harri, N. K.; Brinker, D. A.;
Eagle, C. T.; Loh, K.-L. J. Am. Chem. Soc. 1990, 112, 1906. (e)

18.

19.

 

171

Collman, J. P.; Hegedus, L. 8.; Norton, J. R.; Finke, R. G. Principles
and Applications of Organotransition Metal Chemistry; University
Science Books: Mill Valley, Ca. 1987; p 800.

(a) Doyle, M. P. Acc. Chem. Res. 1986,19, 348. (b) Doyle, M. P.; Dorow,
R. L.; Buhro, W. E.; Grifﬁn, J. H.; Tamblyn, W. H.; Trudell, M. L.
Organometallics 1984,3, 44. (c) Doyle, M. P.; Grifﬁn, J. H.; Bagheri,
V.; Dorow, R. L. Organometallics 1984, 3, 53. (d) Doyle, M. P.; Dorow,
R. L.; Tamblyn, W. H. J. Org. Chem. 1982,47, 4059.

Branson, P. R.; Green, M. J. Chem. Soc., Dalton Trans. 1972, 1303.

 

 

CHAPTER VII

CHEMISTRY OF Rh(II) AND Rh(III) COMPLEXES LIGATED BY
PHOSPHINO-PHENOXIDE LIGANDS

172

 

173

1. Introduction

Conventional wisdom holds that the hard oxygen donor atom of
alkoxides are incompatible with the softer late transition metals; as a result
the potentially rich area of late transition metal alkoxide chemistry went
relatively unexplored for many years.1 Only recently, have researchers
demonstrated that late transition metal-oxygen bonds are not as weak as
once believed, and in fact, are fairly robust when compared to the metal-
carbon bonds of the analogous alkyl complexes.2 These ﬁndings have
stimulated a resurgence in the study of platinum group alkoxides, much of
which has focused on the reactivity of the M-OR bond.3 In light of this, we
were intrigued by the relative ease with which dealkylation reactions of
[Rh(TMPP)2]n+ (n = 2, 3) occur to form a rhodium phenoxide bond. We are
primarily interested in the ramiﬁcations that such a ligand has on the
reactivity of these phosphino-phenoxide complexes.

Previous work by Shaw and co-workers has shown the ability of mixed
phosphine and phenoxide donors to impart unusual thermodynamic stability
to paramagnetic Rh(II) and Ir(II) complexes.4 Work in our own laboratories
has shown that the phenoxide derivative of TMPP, TMPP-O, has been useful
for stabilizing a variety of ﬁrst-row phosphino-phenoxide complexes.5
Furthermore, the hard anionic phenoxide donor has been found to act as a
nucleating agent resulting in the formation of homo- and hetero-bimetallic
and polynuclear complexes.5i 6 Herein is described the formation and
reactivity of a number of unusual mononuclear Rh(II) and Rh(III) complexes
stabilized by the phosphino-phenoxide ligand, TMPP-O.

2. Experimental _

All reactions were carried out under an argon atmosphere by the use of

standard Schlenk-line techniques unless otherwise stated. The starting

 

 

 

174

materials TMPP (1), [Rh(n3-TMPP)2][BF4]2 (3), and ax-[RhHI(n3-TMPP)(T]3-
TMPP-0)][BF4]2 (5) were prepared as described in Chapters II and III. The
salt [szFe][BF4] was prepared by oxidation of szFe with hydroﬂuoroboric
acid in the presence of p-benzoquinone.7 The reagents Cp2Co (Strem),
tricyclohexyl phosphine (Strem), tetra-n—butyl ammonium iodide (Aldrich),
tetra-n—butyl ammonium cyanide (Aldrich), and HBF4-Et20 (Aldrich) were
purchased from commercial sources and used without further puriﬁcation.

A. Preparation of eq-[RhH(TMPP)(TMPP-O)][BF4] (12)

(1) Reactions of [Rh(n3-TMPP)2][BF4]2 (3) with nucleophiles

(i) TMPP. Quantities of [Rh(TMPP)2][BF4]2 (0.300 g, 0.224 mmol)
and‘TMPP (0.119 g, 0.224 mmol) were dissolved in 5 mL of CH3CN. The
solution was stirred for 1 h at r.t. during which time the solution color
changed from purple to red. After 1 h the solvent was removed under
vacuum and the resulting dark red residue was dissolved in 20 mL of THF.
An additional amount of hexanes (~4 mL) was added to precipitate any
remaining [TMPP-CH3+][BF4'] from the solution. The red solution was
separated from the phosphonium salt by ﬁltration through a Celite plug and
then reduced in volume to produce a red solid that was washed with 10 mL of
EtzO and dried in vacuo; yield: 0.137 g (49%). Cyclic voltammogram (0.1 M
TBABF4/CH2C12, vs Ag/AgCl): Elm”) = - 0.02 V.

(ii) Tetra-n-butyl ammonium cyanide. A solution of
[Rh(TMPP)2][BF4]2 (3) (0.050 g, 0.037 mmol) and tetra-n-butyl ammonium
cyanide (0.010 g, 0.037 mmol) in 3 mL of CH2C12 was stirred at -15°C for one
hour. After observing no change in the solution color, the reaction bath was
removed and the reaction was stirred at r.t. for 1 h, which resulted in a
gradual color change of the solution from purple to red. The solvent was

evaporated and the resulting solid was washed with 10 mL of diethyl ether

 

 

 

 

 

175

and dried under vacuum. A 1H NMR spectrum (CD3CN) revealed resonances
attributable to: tetra-n-butyl ammonium cation, 0.96 (triplet), 1.34 (sextet),
1.56 (mult), 3.06 (mult); [TMPP-CH3]+, 3.54 (s), 3.84 (s), 6.21 (d), 2.47 (d);
[TMPP-CH2C1]+, 3.58 (s), 3.85 (s), 6.23 (d), 4.89 (d) and a paramagnetic
species that was identiﬁed as eq-[RhH(TMPP)(TMPP-O)][BF4l (12) by EPR.

(iii) Tricyclohexyl phosphine. A purple solution of [Rh(n3-
TMPP)2][BF4]2 (3) (0.050 g, 0.037 mmol) and tricyclohexyl phosphine (0.011
g, 0.037 mmol) in 5 mL of CH2C12 was stirred overnight to give a red solution.
The solvent was removed under vacuum to yield a red solid which was
subsequently washed with diethyl ether (2 x 5 mL) and dried under reduced
pressure. 31P NMR (CD3CN) 8 ppm: - 2.2 (s), + 8.6 (s), + 35.1 (s), + 35.5 (s), +
36.3 (s), + 49.7 (s). 1H NMR spectroscopy also revealed that a complex
mixture of diamagnetic and paramagnetic species were present.
(2) Reduction of ax-[Rh111(n3-TMPP)(T13'TMPP-0)][BF4]2 (5)

A mixture of ax-[Rh111(n3-TMPP)(T13-TMPP-0)l[BF4]2 (5) (0.050 g,
0.038 mmol) and Cp2Co (0.007 g, 0.038 mmol) was dissolved in 5 mL of
acetone, which caused an immediate reaction as signiﬁed by the color change
from orange to dark red. After 30 min, the solvent was evaporated and the
residue was redissolved in 5 mL of THF and ﬁltered through a Celite plug to
remove [Cp2C0][BF4]. Toluene was added and the solution was cooled to give
a red solid. The solvent was decanted from the product which was then
washed with EtzO and dried in vacuo; yield: 0.038 g (81%).
B. Reaction of [Rh(n3-TMPP)2][BF4]2 (3) with excess TMPP

A mixture of [Rh(n3-TMPP)2][BF4]2 (3) (0.025 g, 0.019 mmol) and
TMPP (0.020 g, 0.038 mmol) was dissolved in ~0.7 mL of CD3CN and
transferred to a NMR tube. Within 10 min, the solution had changed from

purple to dark red. A 1H NMR spectrum recorded after 30 min indicates that

 

 

176

a 1:1 mixture of [TMPP-CH3]+ and TMPP was present in solution. A 31P
NMR spectrum of the solution after 24 h revealed resonances for a previously
unidentiﬁed species; 31P NMR (CD3CN) 8 ppm: + 43.7 (dd, 1JRh,p = 149.7 Hz,
2Jp_p = 19.8 Hz), + 22.9 (dd, 1JRh_p = 132.8 Hz, 2Jp_p = 19.8 Hz). In addition
a phosphorus-containing compound that did not exhibit Rh—P coupling was
observed at 8: +8.8 ppm (s). Integration showed that over 94 % of the TMPP
had been converted to [TMPP-CH3]+. After standing for an additional 48 h,
the solution color eventually became yellow. The 31P NMR spectrum of the
yellow solution was unchanged from above except for the appearance of eq-
[Rh(n3-TMPP-O)2][BF4] (15) in a 1:4 ratio with the unsymmetrical TMPP
complex.
C. Oxidation of eq-[RhH(TMPP)(TMPP-O)] [BF4] (12): Synthesis of eq-
[RhHI(n3-TMPP)(n3-TMPP-O)] [BF4]2 (13)

A solution of eq-[RhIH(n3-TMPP)(T)3-TMPP-0)][BF4]2 (5) (0.050 g,
0.040 mmol) and [Cnge][BF4] (0.011 g, 0.040 mmol) in 5 mL of CH3CN was
stirred at -15°C for 10 min The solvent was evaporated, washed several
times with Et20 (4 x 10 mL) to remove the ferrocene by-product, and dried in
vacuo; yield 0.035 g (65%) 1H NMR (CD3CN) 8 ppm: -OCH3, 2.74 (s, 3H),
2.78 (s, 3H), 3.45 (s, 3H), 3.51 (s, 3H), 3.547 (s, 6H), 3.55 (s, 3H), 3.56 (s, 3H),
3.78 (s, 3H), 3.80 (s, 3H), 3.81 (s, 6H), 3.83 (s, 3H), 3.90 (s, 3H), 3.91 (s, 3H),
4.15 (s, 3H), 4.69 (s, 3H); m-H, 5.76 (dd, 1H), 5.81 (m, 2H), 5.99 (m, 2H), 6.18
(dd, 2H), 6.21 (dd, 1H), 6.36 (m, 3H), 6.68 (dd, 1H). 31P NMR (CD3CN) 8
ppm: + 47.3 (dd, 1JRh_p = 108.8 Hz, 2Jp_p = 13.8 Hz), + 22.5 (dd, lJRh_P = 98.8
Hz, 2Jp_p = 13.8 Hz).
D. Reaction of eq-[Rh“(TMPP)(TMPP-0)][BF4] (12) with co

(1) Infrared study. A solution of eq-[RhH(TMPP)(TMPP-O)llBF4]
(12) (0.025 g, 0.020 mmol) in 5 mL of CH2C12 was gently purged with CO to

 

 

 

177
give a green solution. An infrared spectrum taken on an aliquot showed
bands at 2186 (w), 2136 (w/br), 2085 (w), 2068 (m), 1990 (m) cm'l. After 15
min of additional purging, the solution color became yellow/orange and the IR
spectrum showed prominent features at 2085 (m), 2068 (s) and 1990 (s) cm‘l.
The solution was then purged with argon for 10 min, resulting in a color
change of the solution to bright orange. A ﬁnal aliquot was removed at this
stage and its spectrum showed the three bands seen earlier, but at ~ 50% of

their previous intensity.
(2) NMR study. A solution of eq-[RhH(TMPP)(TMPP-O)l[BF 4] (12) in

~ 0.7 mL of CD3CN was exposed brieﬂy to an atmosphere of CO, which
caused an immediate color change from red to yellow. A 31P NMR spectrum
of the solution obtained within 15 min revealed a mixture of identiﬁable
diamagnetic products including [TMPP-CH3]+, ax,eq-[Rh(n3-TMPP-O)2][BF4]
(14), ax,ax-[Rh(n3-TMPP-O)2][BF4] (1 5) in addition to two unidentiﬁed
species. 31P NMR (CD3CN) 8 ppm: product A, + 0.5 (d, 1JRh_p = 132.8 Hz);
product B, + 40.9 (dd, 1JRh_P = 137.3 Hz, 2Jp_p = 16.7 Hz), + 24.5 (dd, 1JRh_P
= 122.7 Hz, 2Jp_p = 16.7 Hz).
E. Dealkylation of ax-[Rhm(n3-TNIPP)(T]3-TMPP-O)l[BF4]2 (5):
Formation of ax,eq- and ax,ax-[Rh(n3-TMPP-O)2] [BF4] (14) and (15)
(1) TMPP
(i) Bulk reaction. A solution of ax—[Rhm(n3-TMPP)(T)3-TMPP-
0)][BF412 (5) (0.050 g, 0.038 mmol) and TMPP (1) (0.020 g, 0.038 mmol) in 5
mL of CH3CN was reﬂuxed for 36 h and then evaporated to a yellow solid. In
addition to containing [TMPP-CH3]+, the product contains a mixture of ax,ax
and ax,eq isomers of [Rh(n3-TMPP-O)2][BF4] in a ax,eq / ax,ax ratio of
approximately 41 : 59 as evidenced by 1H and 31P NMR spectroscopy. 31P
NMR spectral data for ax,eq -[Rh(n3-TMPP-O)2][BF4] (14) (CD3CN, 8 ppm): +

 

 

178

47.1 (dd, IJth = 137.3 Hz, 2Jp_p = 16.7 Hz), + 19.9 (dd, lJRh-p = 122.9 Hz,
2Jp_p = 16.7 Hz). 31P NMR spectral data for ax,ax-[Rh(n3-TMPP-O)2][BF4]
(15) (CD3CN, 8 ppm): + 35.7 (d, 1JRh_p = 152.5 Hz). 1H NMR for ax,ax--
[Rh(n3-TMPP-O)2][BF4] (15) (CD3CN, 8 ppm): -OCH3, 3.01 (s, 6H), 3.27 (s,
6H), 3.39 (s, 6H), 3.43 (s, 6H), 3.64 (s, 6H), 3.78 (s, 6H), 3.85 (s, 6H), 4.14 (s,
6H); m-H, 5.48 (dd, 2H), 5.53 (dd, 2H), 5.77 (dd, 2H), 5.82 (dd, 2H), 6.20 (dd,
2H), 6.58 (dd, 2H).

(ii) NMR reaction. A mixture of ax-[RhHI(n3-TMPP)(n3-TMPP-
0)][BF4]2 (5) ( 0.015 g, 0.011 mmol) and TMPP (0.006 g, 0.011 mmol) was
dissolved in ~ 0.7 mL of CD3CN and transferred to a NMR tube. Within 5
min, the solution color had changed from orange to yellow. A 31P NMR
spectrum recorded after 20 min showed resonances corresponding to a 60 : 40
mixture of ax,eq-[Rh(n3-TMPP-O)2][BF4] (1 4) and ax,ax-[Rh(n3-TMPP-
O)2][BF4] (15) respectively. After 12 h, the ax,eq to ax,ax isomer ratio was 55
: 45.

(2) Tetra-n-butyl ammonium iodide

Quantities of ax-[RhHI(n3-TMPP)(n3-TMPP-0)][BF4Jg (5) (0.015 g,
0.011 mmol) and tetra-n-butylammonium iodide (0.004 g, 0.011 mmol) were
dissolved in ~ 0.07 mL of d6-acetone. The solution was transferred to a NMR
tube in order to monitor the reaction progress by 1H and 31P NMR
spectroscopy. Conversion of 5 to ax,ax-[Rhm3-TMPP-O)2][BF4] (15) was
found to be quantitative as no ax,eq isomer was observed by 31P NMR
spectroscopy. The presence of CH31 (8 = 2.16 ppm) was detected by 1H NMR.
F. Reactions of ax-[Rh111(n3-TMPP)(n3-TMPP-O)][BF4]2 (5)

(l) HBF4°Et20

To a solution of ax-[RhIH(n3-TMPP)(n3-TMPP-O)][BF4J2 (5) (0.015 g,

0.011 mmol) in ~ 0.07 mL of CD3CN in a NMR tube was added 2 drops of

 

 

179

HBF4-Et20. The tube was shaken for several minutes after which time a 31P
NMR spectrum was measured. 31P NMR (CD3CN) 8 ppm: product A (major),
+ 36.2 (d, lJRh_P = 129.7 Hz); product B (minor), + 36.3 (dd, 1JRh_P = 111.4
Hz, 2Jp_p = 13.7 Hz), + 33.5 (dd, 1JRh_p = 133.6 Hz, 2Jp_p = 13.7 Hz). The
relative amounts of both species remained unchanged after 48 h.
(2) CO

A solution of ax-[Rh111(n3-TMPP)(T13—TMPP-0)][BF4]2 (5) (0.025 g,
0.019 mmol) in 10 mL of CH2C12 was exposed to CO at r.t. for 30 min. The
solution color remained unchanged and an infrared spectrum of the solution
showed no CO stretching vibrations.
(3) H2

A solution of ax-[RhHI(n3-TMPP)(113-TMPP-O)][BF4]2 (5) (0.050 g,
0.038 mmol) in 10 mL of CH3CN was purged at r.t. with H2. For the ﬁrst
hour, additional CH3CN was added periodically to maintain a constant
solution volume. After 1 h the solution was allowed to evaporate under a
constant stream of H2 and the resulting solid was washed with diethyl ether
(5 mL) and dried under reduced pressure. A 1H NMR spectrum of the solid in
CD3CN revealed resonances corresponding only to unreacted 5.
(4) MeOH

To a solution of ax-[Rh1H(n3-TMPP)(T]3-TMPP-O)][BF4]2 (5) (0.015 g,
0.011 mmol) in ~ 0.07 mL of CD3CN in a NMR tube was added ~0.1 mL of
MeOH. A 1H NMR spectrum measured after 36 h revealed that no reaction

had occurred.

 

 

 

180

3. Results
A. Synthesis and Characterization of eq-[RhH(TMPP)(TMPP-
0)][BF4l (12)

One equivalent of TMPP readily reacts with [Rh(n3-TMPP)2][BF4]2 (3)
to produce red solutions of an air sensitive paramagnetic species. Monitoring
the progress of the reaction by 1H NMR spectroscopy reveals that the cation
[TMPP-CH3]+ is the only diamagnetic species present in solution. Based on
this observation, we propose that 3 undergoes demethylation in the presence
of TMPP to yield the paramagnetic Rh(II) complex eq-[RhII(TMPP)(TMPP-
0)][BF4] (12). Isolated yields of 12 have been generally low (~ 50%) due to
difficulties encountered in separating the methylphosphonium by-product.
As a result, samples of 12 prepared by this route are invariably contaminated
with minor amounts of [TMPP-CH3][BF4] as veriﬁed by 1H NMR
spectroscopy. The compound [Rh(n 3-TMPP)2][BF4]2 (3 ) has also been
observed to react with other potential nucleophiles to. produce 12, but the
reactions are typically less selective, and result in the formation of other
diamagnetic species.

Not unexpectedly, the 1H NMR spectrum of 12 is essentially
featureless, exhibiting only broad resonances between 3 and 5 ppm. A 31P
NMR signal was not observed for 12, further supporting its assignment as a
paramagnetic species. Unfortunately, an accurate variable temperature
magnetic susceptibility measurement could not be obtained due to the
presence of [TMPP-CH3][BF4] in solid samples of l 2. However the
paramagnetism of eq—[RhII(TMPP)(TMPP—O)][BF4] (12) was veriﬁed by EPR
spectroscopy. An EPR spectrum of 12 at 100 K in a 1:1 CH2012Me-THF glass
displays a rhombic signal similar to that observed for [Rh(n3-TMPP)2][BF4]2
(3) with gx = 2.26, gy = 2.32, and g2: 1.99 ( Figure 30). The gz region of the

 

 

 

181

 

 

 

 

l L L l

2635 2844 3053 3261 3470 3579
Field (GAUSS)

 

Figure 30. EPR spectrum of eq-[RhH(TMPP)(TMPP-O)][BF4] (12) at 100
K in a 1:1 CH2Cl2/Me-THF glass.

 

 

 

 

182

spectrum, however, exhibits hyperﬁne coupling to two different types of
nuclei (ARh = 29 G, Ap = 8 G), thus creating a doublet of doublets splitting
pattern in the gz tensor region. The larger of the hyperﬁne coupling
constants results from 103Rh coupling in agreement with the values reported
for other Rh(II) systems. Therefore, the weaker coupling constant is likely
due to interaction of the unpaired spin with one of the phosphorus nuclei.
The magnitude of the phosphorus hyperﬁne coupling suggests that this
phosphorus nucleus is not located directly along the dz2 axis, but nonetheless,
can sufﬁciently interact with the unpaired electron to produce the observed

coupling pattern.
B. Redox behavior of eq-[RhH(TMPP)(TN[PP-O)] [BF4] (12)

The cyclic voltammogram of 12 exhibits a reversible oxidation at E1/2 =
- 0.02 V vs Ag/AgCl. Not surprisingly, the Rh(II)/Rh(III) couple for 12
appears at a much less positive potential than that observed for [Rh(n 3-
TMPP)2][BF4]2 (3) (El/2(ox) = + 0.46 W. The increased ease of oxidation for 12
reﬂects the superior basicity of the newly formed phenoxide ligand Rh-O(R)
compared to that of the neutral ether donor in the original compound Rh-
OMe(R). Although the two cations, 5 and 12, contain the same ligand set,
their electrochemical behavior is quite different. A comparison of the cyclic
voltammograms for the Rh(II) complex eq-[RhII(TMPP)(TMPP-0)][BF4] (12)
and the Rh(III) complex ax-[RhHI(1‘]3-TMPP)(113-TMPP-0)]2+ (5) is presented
in Figure 31. An examination of the two cyclic voltammograms reveals that
the reversible oxidation couple of [RhII(TMPP)(TMPP-0)]1+ (12) coincides
with the chemical return wave of ax-[RhIH(n3-TMPP)(n3-TMPP-0)]2+ (5).
This result may be interpreted as follows; reduction of ax-[Rhm(n3-
TMPP)(113-TMPP-O)]2+ (5) produces a transient d7 Rh(II) species, presumably
ax-[RhII(TMPP)(TMPP-0)][BF4] (12a), that immediately undergoes a rapid

 

 

 

183

[RhIIIm3-TMPP)(n3-TMPP-0)]2+

Ep,¢= .030 v

Ep,a= -0.02 V

[RhII(TMPP)(TMPP-O)l1+
E1/2,ox= -0.02 V

I T I I I l l | l I I l

+1.0 +0.5 0.0 -0.5 -l.0

 

Volts vs Ag/AgCl

Figure 31. Cyclic voltammograms of ax-[Rhm(n3-TMPP)(n3-TMPP-
0)][BF4]2 (5) and eq-[RhH(TMPP)(TMPP-0)][BF4] (12) in 0.1
M TBABF4/CH2C12.

 

 

 

 

184

structural rearrangement to a more electronically favorable conformation. In
support of this hypothesis is the chemical reduction of ax-[Rhm(n3-TMPP)(n3-
TMPP-0)][BF4J2 which yields a dark red product in good yield that is
spectroscopically identical to 5. Logic dictates that two different structural
isomers are possible for compound 12 (shown as a and 1) below). Isomer (a)
is formed by rearrangement of the phosphorus donors from a cis to a trans
orientation. A second possibility concerns the isomerization of the phenoxide
ligand from being cis to both phosphorus nuclei, as in the structure of 5, to
becoming trans to one of the phosphorus nuclei, while the phosphorus nuclei
remain cis to each other. Unfortunately the paramagnetism of 12 precludes
us from gleaning any structural information about this species by NMR
spectroscopy. However, the fact that the newly formed compound 12 exhibits
a chemically reversible oxidation (ipa/ipc ~ 1) is quite interesting. The
reversibility of this couple suggests that it should be possible oxidize 12 to a
diamagnetic Rh(III) species (13) without effecting a major structural
rearrangement. Consequently, it is possible to infer the structure of
[RhH(TMPP)(TMPP-0)][BF4] (12) by investigating the NMR spectrum of the
diamagnetic Rh(III) complex 13, since, the two complexes are expected on the
basis of cyclic voltammetry be structurally similar. Indeed,
[RhII(TMPP)(TMPP-0)][BF4] (l 2) is rapidly oxidized in the presence of
[Cp2Fe][BF4] to yield a red/brown product 13. As anticipated, the NMR
spectral features of 13 vary signiﬁcantly from those observed for the related
compound, ax-[RhHI(n3-TMPP)(11 3-TMPP-0)][BF412. The 31P NMR spectrum
of 13 shows the presence of two magnetically inequivalent phosphorus nuclei
that couple to both Rh centers and to each other resulting in an ABX spin
system; 8 ppm: + 47.3 (dd, 1JRh-pA = 108.8 Hz, ZJPA_PB = 13.8 Hz), + 22.5 (dd,

1J Rh_pB = 98.8 Hz, 2JpB_pA = 13.8 Hz). Just as was observed for 5, the small

 

 

 

 

185

magnitude of 2JpB_PA indicates a cis orientation of the two phosphorus nuclei.

However, unlike the 31P NMR spectrum for 5, the chemical shifts and Rh-P
coupling constants for the two nuclei are quite disparate. This is consistent
with the phenoxide group bonded trans to the phosphorus atom of TMPP and
resides in the equatorial plane deﬁned by the two phosphorus atoms. Such a
bonding arrangement would produce two distinct electronic environments for
the phosphorus nuclei stemming from the different trans inﬂuences caused
by an anionic phenoxide donor versus a neutral ether donor. Therefore on the
basis of the 31P NMR spectral data for 13 and the observed electrochemical
relationship between complexes 5 12 a n d 13, the structure of
[RhH(TMPP)(TMPP-0)]1+ (12) is predicted to be a cis arrangement of
phosphines with the phenoxide group of the dealkylated phosphine in an
equatorial position.

Interestingly, when eq-[Rhmm3-TMPP)(n3-TMPP-0)]2+ (13) is allowed
to stand in solution, the complex rearranges slowly, over a period of weeks,
to form [RhHI(n3-TMPP)(n3-TMPP-O)]2+ (5). The process, however, is
complicated as evidenced by the formation of the bis-demethylated phosphine
complex ax, ax-[Rh(n3-TMPP-O)2][BF4] (1 5). The pathway for the
isomerization is not understood and merits further investigation.

C. Reactivity of ax-[RhIH(n3-TMPP)(T]3-TMPP-O)] [BF4l2 (5)

The complex [Rhm(n3-TMPP)(T13-TMPP-0)][BF4J2 like [Rh(n3-
TMPP)2][BF4]2 (3) and [Rh(n3-TMPP)2][BF4]3 (4) is also susceptible to
nucleophilic attack. Compound 5 reacts swiftly with TMPP in acetonitrile to
produce yellow solutions of [Rh(n3-TMPP-0)2][BF4]. 31P NMR data reveal
that two geometric isomers are present in solution; an unsymmetrical
complex, in which the two phosphorus nuclei are inequivalent, located at 8 =

47.1 ppm (dd, lJRh_P = 137.3 Hz, 2Jp_p = 16.7 Hz) and 5 = + 19.9 ppm (dd,

186

1JRh_p = 122.9 Hz, 2Jp_p = 16.7 Hz), and a species, with a resonance at 8 = +
35.7 ppm (d, 1JRh_p = 152.5 Hz), in which both phosphine ligands are
magnetically equivalent. The formation of both isomers of [Rh(n3-TMPP-
0)2][BF4] results from non-speciﬁc nucleophilic attack taking place at
different metal-ether positions in the structure of 5. Logically there are two
possible sites for this second dealkylation reaction to occur, either in the
equatorial plane, trans to the neutral TMPP ligand or in the axial position,
trans to the phenoxide group (Figure 32). Assuming that phosphine
rearrangement does not occur, demethylation of the methoxy group that is
trans to phosphorus is predicted to result in the formation of two
magnetically distinct phosphine ligands that should exhibit two ABX spin
systems at chemical shifts similar to those observed for the unsymmetrical
product. On the other hand, dealkylation of the methoxy group trans to the
phenoxide ligand should create two chemically equivalent phosphine ligands.
Based on these arguments, the unsymmetrical product is assigned the
formula ax,eq-[RhIH(n3-TMPP-0)2][BF4] (l4) and the product with equivalent
phosphino-phenoxide ligands is designated ax,ax-[Rh(n3-TMPP-0)2][BF4]
(15). Although both species are present in solution, the relative amounts of
isomers l4 and 15 appear to be sensitive to the reaction conditions;
dealkylation reactions performed at higher temperature give a greater
proportion of the ax,ax isomer 15. Moreover, there is apparently slow
conversion of 14 to 15in solution over extended periods of time as evidenced
by NMR spectroscopy.

The susceptibility of 5 to nucleophilic attack is further demonstrated
by its reaction with free iodide. ax-[Rhm(n3-TMPP)(1]3-TMPP-0)][BF4]2
readily reacts with one equivalent of [(Bu-n)4N][I] to yield exclusively the

ax,ax isomer 15. The resulting by-product of the reaction, CH3I, is easily

 

187
1
Me Me _I +
[:0 <-> Oj
[PM-l RhPIII]
2+
Me Me —I
MeO) ax,eq-[RhmmM -mp-0),]1+
LO(\T‘ (14>
[ﬂu RhHI P) NUC:
NEE/0’ ;h|)/Me 1+
M Me —]
III _ _ - 2+ 0 0 0)
ax-[Rh (n3 'I'MPP)(T]3 TMPP 0)]
(5)

Figure 32.

ax.ax-[Rhm(n)3-TI\/IPP-0)]1+
(15)

Possible dealkylation products resulting from nucleophilic
attack on ax-[RhHI(n3-TMPP)(113-TMPP-0)][BF4]2 (5).

 

 

188
detected by 1H NMR spectroscopy. Nucleophilic attack on a coordinated
methoxy group is expected to be influenced by a variety of factors including
steric hindrance of the ancillary ligands, electrophilicity of the -OCH3 moiety
and basicity of the attacking nucleophile. In light of this, it is not surprising
that minor variations in reaction conditions lead to different relative amounts
of the isomers 14 and 15. This concept is illustrated by the fact that
demethylation of 5 with a weaker nucleophile, such as iodide, produces 15

exclusively.
The phenoxide group of ax-[Rhm(n3-TMPP)(n3-TMPP-0)][BF4]2 is

easily protonated in the presence of excess HBF4. However, the expected
product [Rhm(n3-TMPP)(n3-TMPP-0H)][BF4]3 appears to be present in only
minor amounts as evidenced by 31P NMR spectroscopy. The 31P NMR
spectrum of the major product exhibits a doublet at 5 = + 36.2 ppm (1JRh_p =
129.7 Hz); this is indicative of two chemically equivalent phosphines. One
possible explanation is that upon protonation of 5 to form [RhHI(n3-
TMPP)(n3-TMPP-0H)][BF4]3, a dealkylation reaction occurs similar to the
one that transforms [Rh(n3-TMPP)2][BF4]3 (4) to 5. By analogy with the
dealkylation of 4, such a process is not unreasonable, if one considers the
high positive charge concentrated on the Rh atom. The absence of anionic
ligands, which would tend to offset this charge, creates a highly electrophilic
metal center that may activate another coordinated methoxy group.
Following demethylation, the newly formed phenoxide ligand is quickly
protonated to form [Rh(n3-TMPP-OH)2][BF4]3. A axial orientation of the
phenol groups would result in an arrangement where both phosphino-phenol
groups are chemically equivalent. Clearly, the preceding arguments
represent just one of several possible scenarios and further work is needed to

either prove or disprove the hypothesis.

   

189
D. Chemistry of eq-[RhH(TMPP)(TMPP-0)][BF4] (12) with CO

One of our primary goals in preparing a demethylated analog of
[Rh(n3-TMPP)2][BF4]2 (3) was to modify the reactivity of the system by
reducing the ﬂein'bility of the ligand set. We argued that dealkylation of the
pendent methoxy group would result in the formation of a less labile
phenoxide interaction, thereby constraining the coordination environment
about the metal. Based on the recent fascinating work of Wayland et al. on
Rh(II) porphyrins, we anticipated that by introducing a more rigid ligand
framework, we may inhibit disproportionation and therefore isolate a Rh(II)
carbonyl species. The presence of an anionic phenoxide ligand is also
expected to help stabilize the Rh(II) oxidation state with respect to Rh(I) and
further deter disproportionation.

Moderate purging of a methylene chloride solution of eq-
[RhH(TMPP)(TMPP-0)][BF4] (12) with CO results in an immediate change in
the solution color from red to dark green. An infrared spectrum measured
within the ﬁrst 10 minutes exhibits a set of moderate intensity bands at
2084, 2068, and 1990 cm'l. Moreover, two weak, higher energy bands appear
at 2186 and 2136 cm'l. Upon continued purging of CO, the green color
quickly converts to yellow/orange. An infrared spectrum of the solution at
this stage reveals an absence of the two higher energy bands and an increase
in the intensity of the three lower energy absorptions. Removal of the CO
atmosphere by purging with argon causes a reduction in the bands at 2084,
2068, and 1990 cm'l, but the original red solution color never reappears
indicating that the overall addition of CO to eq-[RhH(TMPP)(TMPP-0)l[BF4]
(12) is not a reversible process.

Intrigued by the possibility that the higher energy intermediate bands
could be Rh(II) carbonyl adducts, the reaction was monitored by EPR

190

spectroscopy in order to detect the formation of paramagnetic intermediates.
A frozen solution of 12 dissolved in a 1:1 mixture of CH2Cl2/Me-THF in an
EPR tube was exposed to CO; the sample was gently warmed, and upon
reaction was quickly refrozen. The EPR spectrum of the frozen solution at
100 K (Figure 33) revealed the presence of a new paramagnetic species (g1 ~
2.31, g2 ~ 2.27, g3 ~ 1.98, A3 ~ 14.6 G) in addition to unreacted 12. A similar
experiment performed with labeled 13CO, resulted in an EPR spectrum that
showed complete conversion to a new Rh(II) species (g1 ~ 2.31, g2 ~ 2.27, g3 ~ .
1.99, A3 ~ 48 G). The lack of signiﬁcant 13CO coupling to any of the g tensors
indicates there is little electron density residing on the CO ligand and that
the radical is primarily metal-based. This behavior is in contrast to the
signiﬁcant delocalization of electron density between the Rh and CO moiety
observed for the Rh(II)-porphyrin-carbonyl complexes.8 In fact it is exactly
the delocalization that induces the carbon coupling reactions in these
systems. Unfortunately, the ability of Rh(II)-TMPP complexes to effect
similar coupling reactions as those observed for the porphyrin systems
appears to be limited. 1

Although EPR spectroscopy provided evidence for the formation of
transient Rh(II) CO adducts, it was evident that this was followed by a
disproportionation pathway. In order to identify the key participants in this
pathway, the progress of the reaction was also monitored by 31P NMR
spectroscopy. The 31P NMR spectrum of a solution of eq-[RhH(TMPP)(TMPP-
0)][BF4] (12) in CD30N exposed to CO for one minute reveals that a complex
mixture of diamagnetic species is formed. Careful inspection reveals that two
of the products are a mixture of ax,eq and ax,ax isomers of [Rhmm3-TMPP-
0)2][BF4], characterized in the previously mentioned dealkylation chemistry.

In addition, two unidentiﬁed species are also present in solution, a doublet at

191

 

 

 

l

 

l

2552.1 2859.85 3077.52 3285.38 3493.14 3700.
Field (GAUSS)

 

Figure 33. EPR spectrum of a frozen 1:1 CH2C12/Me-THF solution of eq-
[Rhn(TMPP)(TMPP-0)][BF4] (l2) exposed to CO.

192

5 = + 0.5 ppm (lJRh_p = 132.8 Hz) and an unsymmetrical species
characterized by two ABX resonances at 8 = + 40.9 ppm (1JRh_p = 137.3 Hz,
2Jp_p = 16.7 Hz) and 6 = + 24.5 ppm (lJRh_P = 122.7 Hz, 2Jp_p = 16.7 Hz). The
presence of [Rh(n3-TMPP-0)2][BF4] implies that certain reactions must
participate in the overall reaction pathway. The formation of [Rh(n3-TMPP-
0)2]+ necessitates an initial oxidation of 12 to 13 By analogy to the CO
chemistry of [Rh(n3-TMPP)2][BF4]2 (3 ), oxidation of 12 to 13 could,
presumably, arise from reaction between 12 and a CO adduct of 12 Such an
event is not unreasonable considering the accessible nature of the
Rh(II)/Rh(III) redox couple for 12 (El/2(ox) = - 0.02 V). The presence of [Rh(n3-
TMPP-0)2]1+ also implies that, during the course of the reaction, TMPP
dissociates from one of the products and acts as a nucleophile in the
dealkylation of [RhIII(n3-TMPP)(T]3-TMPP-0)]2+. This conclusion is
supported by the presence of a signiﬁcant amount of [TMPP-CH3]+ in the

NMR spectrum of the reaction.

4. Discussion

The results presented in this chapter demonstrate the susceptibility of
RhHJH-TMPP complexes to dealkylation of a coordinated methoxy group.
The thermodynamic driving force behind this reaction is undoubtedly the
high positive charge on the metal center coupled with activated methyl
groups. Although TMPP has been shown to be a highly basic 2 e' donor
phosphine ligand with the lone pair, when the ligand is participating in an 113
bonding mode it is a relatively poor 6 e' donor due to the weak donating
ability of the pendent ether groups. Consequently, the neutral phosphine is
unable to effectively counterbalance the high positive charge on the metal

center, and as a result, the coordinated methoxy groups become activated

 

 

193

towards dealkylation. As the metal oxidation state increases, and hence the
overall charge on the complex, dealkylation becomes increasingly more
thermodynamically favorable. This point is nicely demonstrated by a
comparison of the relatively stability of the [RhH(n3-TMPP)2]2+ (3) and
[RhIH(n3-TMPP)2]3+ (4) complexes towards dealkylation; compound 4 readily
demethylates in the presence of very weak nucleophiles such as [BF4]', while
3 generally requires strong nucleophiles. The Rh(III) oxidation state appears
to be highly susceptible, as evidenced by the observation that ax-[Rhm(n3-
TMPP)(n3-TMPP-0)][BF4J2 will undergo a second dealkylation in the
presence of free iodide.

The introduction of a phenoxide donor into the metal coordination
sphere sets up the possibility of preparing structural isomers based on the
relative positions of the phosphine and phenoxide groups. As a result, there
are two primary structural conformations, assuming a cis-phosphorus donor
atom geometry is maintained; the phenoxide group may either be equatoiral
or axial position with respect to the equatorial plane containing the
phosphorus atoms. The favored structural conformation is highly dependent
on the metal oxidation state. The relationship between the various isomers of
the dealkylated and non-dealkylated complexes 3, 4, 5, 12 and l3is depicted
in Figure 34. An important underlying principle that is pervasive in the
chemistry of these phosphine complexes is the difference in substitutional
lability between the +2 and +3 oxidation states of rhodium. The Rh(III)
oxidation state, being a (16 metal ion, is relatively substitutionally inert and
therefore any structural rearrangements that may occur will be very slow or
kinetically inhibited. In contrast, paramagnetic Rh(II) complexes are
substitutionally labile and as a result, the distorted RhH-TMPP complexes

are less structurally rigid. This lack of structural rigidity is demonstrated by

 

 

194

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195

the facile axial to equatorial isomerization of [RhH(TMPP)(TMPP-OlllBF4]
(12), while the analogous rearrangement for the Rh(III) d6 complex is
exceedingly slow.

Curiously, in each of the complexes described in this chapter, the
phosphine lie cis to each other rather than in the sterically more logical trans
orientation. Evidently, the preference for the cis geometry is electronic and
not kinetic in origin. Presumably, the presence of the weak ether donors
requires that the phosphine ligands remain cis in order to maximize electron
donation to the relatively electron-poor metal center. Several observations
support this conclusion. Previously, we noted that [Rh(n3-TMPP)2][BF4]2 (3)
was present as a cis-phosphine conformation and that it could not be
thermally converted to the trans orientation. One could conceivably argue
that the steric bulk of the ligands prevents isomerization to the steric favored
conformation and that the cis orientation of the phosphines resulted from the
original attack of the phosphine on [Rh2(MeCN)10][BF4]4. This seems
unlikely in light of the facile rearrangement of ax-[Rhm(n3-TMPP)(n3-TMPP-
0)]2+ (5) upon reduction to the corresponding d7 complex
[RhII(TMPP)(TMPP-0)]1+ (12). The substitutionally labile nature of the d7
complexes allows the phenoxide group to readily change its relationship
(axial/equatorial) to the neutral phosphine, yet in compound 3 the
phosphorus atoms remain cis to each other. Moreover, introduction of strong
donor ligands such as CO and CNR results in an immediate structural
rearrangement to a trans phosphine geometry as exhibited by complexes 6 -
l 1. This point is dramatically illustrated by the oxidation of
[RhI(TMPP)2(CO)]1+ (7) to [RhH(113-TMPP)2]2+ (3). Although the phosphines
are trans in the initial Rh(I)CO complex, oxidation followed by CO

dissociation results in phosphine isomerization to the thermodynamically

 

 

196

favored cis orientation. These observations clearly support that the
disposition of the phosphine ligands in these Rh(II) and Rh(III) complexes is
not kinetic in origin but instead thermodynamic.

A ﬁnal point regarding the dealkylation of TMPP to form the anionic
phosphino-phenoxide needs to be addressed, namely, the effect that
dealkylation has on the chemistry of these complexes. An important
consequence of demethylation is that it limits the ﬂexibility of the ligand by
anchoring it at two positions rather than only one. Furthermore, the
formation of a relatively non-labile metal-phenoxide bonds reduces the
number of potential coordination sites for an incoming substrate.
Nonetheless, these phenoxide groups do not occupy all of the coordination
sites. The presence of two or three labile metal ether interactions still leaves
open the possibility for further chemistry. Ironically, the formation of
phenoxide donors may potentially induce higher lability in the remaining
coordinated methoxy groups by providing more electron density at the metal.
This in effect will reduce the electrophilic character of the metal and decrease
the need for electron donation by the pendent ether groups and subsequently
making them more labile. This will be particularly true for higher valent
metal centers. Consequently, [RhIH(T]3-TMPP)(T]3-TMPP-0)]2+ (5) and
[Rh111(n3-TMPP-0)2]1+ (14, 15) may prove to be more reactive towards
addition of small substrates to the metal center. Comprehensive studies of
these systems are needed to establish the use of phosphino-alkoxide groups

as ancillary ligands for late transition metal complexes.

 

 

 

197
List of References

1. (a) Mehrotra, R. C.; Agarwal, S. K.; Singh, Y. P. Coord. Chem. Rev.
1985, 68, 101. (b) Bryndza, H. E.; Tam, W. Chem. Rev. 1988,88, 1163.

2. (a) Bryndza, H. E.; Calabrese, J. C.; Marsi, M.; Roe, D. C.; Tam, W.;
Bercaw, J. E. J. Am. Chem. Soc. 1986, 108, 4805. (b) Bryndza, H. E.;
Fong, L. K.; Paciello, R. A.; Tam, W.; Bercaw, J. E. J. Am. Chem. Soc.
1987, 109, 1444. (c) Bryndza, H. E.; Domeille, P. J.; Tam. W.; Fong, L.
K.; Paciello, R. A.; Bercaw, J. E. Polyhedron 1988, 7, 1441. (d)
Hartwig, J. F.; Andersen, R. A.; Bergman, R. G. Organometallics 1991,
10, 1875.

3. For leading references see (a) Loren. S. D.; Campion, B. K.; Heyn, R.
W.; 'I‘illey, T. D.; Bursten, B. E.; Luth, K. W. J. Am. Chem. Soc. 1989,
111, 4712 and references therein. (b) Glueck, D. S.; Newman-Winslow,
L. J .; Bergman, R. G. Organometallics 1991, 10, 1462. (c) Hartwig, J.
F.; Andersen, R. A.; Bergman, R. G. J. Organomet. Chem. 1990, 394,
417. (d) Kegley, S. E.; Schaverian, C. J.; Freudenberger, J. H.;
Bergman, R. G. J. Am. Chem. Soc. 1987, 109, 6563.

4. (a) Empsall, H. D.; Hyde, E. M.; Jones, C. E.; Shaw, B. L. J. Chem.
Soc., Dalton Trans. 1974, 1980. (b) Mason, R.; Thomas, K. M.;
Empsall, H. D.; Fletcher, S. R.; Heys, P. N.; Hyde, E. M. Jones, C. E;
Shaw, B. L. J. Chem. Soc., Chem. Comm. 1974, 612. (c) Empsall, H.
D.; Hyde, E. M.; Shaw, R. L. J. Chem. Soc., Dalton Trans. 1975, 1690.
(d) Empsall, H. D.; Heys, P. N.; McDonald, W. 8.; Norton, M. C.; Shaw,
B. L. J. Chem. Soc., Dalton Trans. 1978, 1119.

5. (a) Dunbar, K. R.; Quillevéré, A. Inorg. Chem. submitted for
publication. (b) Quillevéré, A. Ph. D. Dissertation, Michigan State
University, 1992.

6. (a) Chen, S. J .; Dunbar, K. R. Inorg. Chem. 1991, 30, 2018. (b) Chen, S.
J.; Dunbar, K. R. Inorg. Chem. 1990, 29, 588. (c) Dunbar, K. R.;
Saharan, V.; Matonic, J. M. manuscript in preparation.

7. Gray, H. B.; Hendrickson, D. N.; Sohn, Y. S. Inorg. Chem. 197], 10,
1559.

8. (a) Wayland, B. B.; Sherry, A. E. J. Am. Chem. Soc. 1989, 111, 5010.
(b) Wayland, B. B.; Sherry, A. E.; Poszmik, G.; Bunn, A. G. J. Am.
Chem. Soc. 1992, 114, 1673.

 

CHAPTER VIII

CHEMISTRY OF TRIS(2,4,6-TRIlVIETHOXYPHENYL)PHOSPHINE
WITH RHODIUM (I) AND IRIDIUM (I) OLEFIN COMPLEXES

198

 

 

199

1. Introduction

In chapter III, we demonstrated that it was possible to prepare stable
Rh(II) TMPP complexes from the solvated dinuclear precursor
[Rh2(MeCN)10][BF4]4. In spite of this success, we were interested in
synthesizing complexes of the general formula [Rh(TMPP)2]n+ (n: 1, 2, 3)
from more conventional starting materials. The development of such
synthetic strategies is particularly important for the isolation of homoleptic
Ir/TMPP complexes, since the analogous solvated dinuclear Ir complex is
unknown. Others have established that Rh(II) and Ir(II) phosphine
complexes can be prepared by reaction of M(III) trihalides with excess
phosphine in alcohol.1 These conditions, however, are unsuitable for the
preparation of [M(TMPP)2]n+ (n: 1, 2, 3), because of the susceptibility of
these species to dealkylation. An alternative approach is to begin with
partially solvated Rh(I) and Ir(I) oleﬁn complexes which are prepared in situ
by halide abstraction from the corresponding halide bridged dinuclear metal
oleﬁn complexes.28L Such systems have proven to be good precursors for a
variety of group 8 metal phosphine complexes.2 We set out to adapt this
methodology to the preparation of Rh(I) and Ir(I) complexes of type
[MI(TMPP)2]1+. This chapter describes our efforts to prepare homoleptic
TMPP complexes using Rh(I) and Ir(I) oleﬁn complexes. In addition, the
synthesis and characterization of the iridium dicarbonyl complex

[Ir(TMPP)2(CO)2][BF4] is presented.

2. Experimental
A. Synthesis
All reactions were carried out under an argon atmosphere by the use of

standard Schlenk-line techniques unless otherwise stated. TMPP (1) was

 

 

 

200

prepared as described in chapter II. The starting material, [Ir(cod)Cl]2, was
prepared from IrCl3 - xHZO using a standard literature procedure.3
[Rh(CZH4)2Cl]2 was prepared according to a literature procedure.4
[Rh(cod)Cl]2 (Strem) and NOBF4 (Strem) were obtained from commercial
sources and used as received. The oxidizing agent, [(p-BrCGH4)N][BF4], was
prepared by oxidation of the amine, (p-BrC6H4)N, with I2 in the presence of
Ag“ following a modiﬁcation of the procedure described by Bell et al.5
(1) Preparation of [Rh(cod)(n2-TMPP)I[BF4] (16)

To a mixture of [Rh(cod)Cl]2 (0.200 g, 0.406 mmol) and AgBF4 (0.158 g,
0.812 mmol) was added 5 mL of THF. The resulting yellow suspension was
stirred for 5 min before being ﬁltered through a Celite plug into a 100 mL 3-
necked ﬂask equipped with an addition funnel containing a solution of TMPP
(0.432 g, 0.811 mmol) in 15 mL of THF. The yellow solution was then cooled
to approximately - 40°C with a dry ice / MeCN slush. The THF solution of
TMPP was carefully added dropwise to the chilled solution over a period of 30
min. The reaction was stirred for an additional hour at -40°C, during which
time a yellow solid separated from solution. Additional product was obtained
from the solution by slow addition of diethyl ether (50 mL) while the solution
was being stirred. The cold suspension was ﬁltered in air and the yellow
solid was washed with several portions of diethyl ether (4 x 10 mL) and dried
under reduced pressure for 1-2 h; yield 0.574 g (85%). Anal. Calcd for
C35H4509F4BPRh: C, 50.62; H, 5.46. Found: C, 50.16; H, 5.58. 1H NMR
(CD2C12) 8, ppm: 3.68 (s, 36H, o-OCH3), 3.84 (s, 18H, p-OCH3), 6.14 (d, 4Jp_H
= 3.6 Hz, 12H, m-H), 1.80 (br, cod), 2.36 (br, cod). 31P NMR (Cchlzl 8, ppm:
+ 1.4 (d, 1JRh-p = 137.3 Hz). Cyclic voltammogram (0.2 M TBABF4 / CHzClg,
vs Ag/AgCl): E1(p,a) = + 0.60 V, E2(p,a) = + 1.16 V, Ep,c = + 0.92 V. Mass
spectrum (FAB, 3-nitrobenzyl alcohol) m/z: 742 (Rh(cod)(n2-TMPP)+).

 

 

 

 

 

 

201
(2) Preparation of Rh(cod)(n2-TMPP-0) (17)

A solution of [Rh(cod)(n2-TMPP)][BF4] (16) (0.200 g, 0.240 mmol) and
TMPP (0.128 g, 0.240 mmol) in 5 mL of MeCN was stirred at r. t. for 12 h.
The solvent was removed under vacuum to yield a pale yellow residue. The
yellow product was extracted by addition of 10 mL of Et20 and a minimal
amount of THF (an amount sufﬁcient to dissolve the yellow solid completely).
The solution was ﬁltered to remove the [TMPP-CH3][BF4] by-product. The
solvent was then removed under vacuum to produce a yellow powder; yield, '
0.142 g (81%). 1H NMR (CD2C12) 8, ppm: -OCH3, 3.35 (s, 3H), 3.55 (s, 12H),
3.68 (s, 3H), 3.80 (s, 6H); m-H, 5.45 (t, 1H), 5.72 (dd, 1H), 6.04 (d, 4Jp_H = 3.3
Hz, 4H); cod, 1.86 (br), 2.38 (br), 3.38 (br), 4.91 (br). 31P NMR (CDzClz) 8,
ppm: + 8.0 (d, lJRh-P = 157.2 Hz). Mass spectrum (FAB, ONPO) m/z: 728
(Rh(cod)(n2-TMPP-0)+).

(3) Reaction of [Rh(C2H4)2Cl]2 with TMPP

A solution of TMPP (0.548 g, 1.03 mmol) in 10 mL of MeCN was added
dropwise to a cold suspension (0°C) of [Rh(C2H4)2Cl]2 (0.100 g, 0.257 mmol)
and KBF4 (0.064 g, 0.514 mmol). After the addition of the phosphine was
complete, a slight vacuum was applied periodically to induce loss of ethylene.
The reaction was allowed to warm slowly to r. t. After 2 h of stirring, the
solvent was removed under reduced pressure. The resulting residue was
dissolved in CH2012 (10 mL) and ﬁltered through a Celite plug to remove
undissolved KCl. The ﬁltrate was evaporated to a solid under reduced
pressure which was washed with diethyl ether (2 x 10 mL) and dried in
vacuo. 1H and 31P NMR spectroscopy showed that the solid was comprised of
a mixture of [TMPP-CH3]+ and [TMPP-CH2C1]+ together with a Rh species
similar to the species observed upon reduction of [Rh(n3-TMPP)2][BF4]2 (3)

with cobaltocene.

 

 

 

202

(4) Preparation of Ir(cod)(n2-TMPP-0) ( 18)

To a solution of [Ir(cod)Cl]2 (0.150 g, 0.223 mmol) and KBF4 (0.056 g,
0.446 mol) at 0°C was added dropwise to a solution of TMPP (0.475 g, 0.892
mmol) in 10 mL of MeCN. The solution color gradually converted from yellow
to orange as the reaction was allowed to warm to r. t. After stirring for 2 h,
the solvent was removed under vacuum to yield an orange residue. The solid
was taken up with 10 mL of CH2Cl2 and ﬁltered through a Celite plug to
remove the KC] by-product. The Celite plug was further washed with 5 mL of
CH2012 to ensure complete transfer of the product. The volume of the
solution was reduced to ~ 5 mL, and diethyl ether (20 mL) was slowly added
while the solution was being stirred. The resulting yellow/white precipitate
was removed by ﬁltration and the orange ﬁltrate was concentrated to
approximately 5 mL. To the orange solution was added 20 mL of hexanes.
The volume was again concentrated to 3-5 mL resulting in the formation an
orange precipitate. The solid was ﬁltered in air, washed with hexanes (2 x 10
mL), and dried under reduced pressure; yield, 0.302 g (83%). 1H NMR
(CD3CN) 8, ppm: -OCH3, 3.36 (s, 3H), 3.49 (s, 12H), 3.68 (s, 3H), 3.79 (s, 6H);
m-H, 5.55 (t, 1H), 5.77 (dd, 1H), 6.09 (d, 4JP-H = 3.6 Hz, 4H).

(5) Preparation of [Ir(TMPP)2(CO)2] [BF4] (19)

A 100 mL 3-necked ﬂask, equipped with an addition funnel, gas inlet
and a septum, was charged with [Ir(cod)Cl]2 (0.200 g 0.30 mmol), NaBF4
(0.065 g, 0.60 mmol) and MeCN (5 mL). The resulting solution was cooled to
0°C and gently purged with CO. A solution of TMPP (0.634 g, 1.20 mmol) in
10 mL of MeCN was added dropwise to the CO saturated solution over a
period of ten minutes. After the addition was complete, the CO purge was
discontinued, and the reaction was stirred under a CO atmosphere at 0°C.

After 2 h, the solvent was removed under vacuum. The resulting residue was

 

 

 

 

 

 

203

dissolved in 5 mL of CH2C12 and ﬁltered through a Celite plug. The Celite
was further washed with 5 mL of CHzClZ. The orange solution was
concentrated to 4-6 mL and 35 mL of Et20 was added slowly. The resulting
bright orange precipitate was ﬁltered in air; washed with 4 x 10 mL of diethyl
ether and dried in vacuo; yield 0.666 g (80%). Anal. Calcd for
C57H68020F4BC12P2Rh: C, 46.10; H, 4.62. Found: C, 46.75; H, 4.88. IR
(CH2C12) cm'1: v(CO), 1993 (vs), 1946 (w). IR (Nujol, CsI) cm'l: v(CO), 1987
(s), 1942 (m). 1H NMR (CD3CN) 8, ppm: 3.38 (s, 36H, o-OCH3), 3.77 (s, 18H,
p-OCH3), 6.06 (t, 4Jp_H = 2.0 Hz, 12H, m-H). 31P NMR (CD013) 8, ppm: - 39.0
(s). Electronic absorption spectrum (CH2C12) kmax, nm (e): 547 (449), 490
(2300), 455 (sh) 386 (4510) 315 (sh). Cyclic voltammogram (0.1 M TBABF4 /
CH2C12, vs Ag/AgCl): Ep,a = + 0.82 V, E1(p,c) = + 0.46 V, E2(p,c) = - 0.81 V.
(6) Reaction of TMPP with [Ir(TMPP)2(CO)2] [BF4] (19)

A quantity of [Ir(TMPP)2(CO)2][BF4] (19) (0.100 g, 0.071 mmol) and TMPP
(0.038 g, 0.071 mmol) was dissolved in 5 mL of MeCN. The orange solution
was stirred at r. t. for 12 h. An infrared spectrum of the solution revealed
that no reaction had occurred during this period. A condenser was connected
to the ﬂask and the solution was reﬂuxed for 24 h during which time the
solution color turned from orange to yellow. An IR spectrum of the solution
was obtained after this period of time: IR (CH3CN) cm‘1: v(CO), 2051 (s),
1973 (s),1899 (w). The reaction was reﬂuxed for an additional 24 h, after
which time a small aliquot was removed and an IR spectrum was measured:
IR (CH3CN) cm'1: v(CO), 1998 (w, br) 2018 (w, br). The solvent was then
removed under vacuum and the solid was redissolved in a 2:1 mixture of THF
and diethyl ether. The yellow solution was ﬁltered and pumped to dryness.
The resulting solid was washed with diethyl ether (2 x 5 mL) and dried in

 

 

204

vacuo. The 1H NMR spectrum of the solid in CD3CN revealed the presence
of [TMPP-CH3]+, but no other resonances were easily discernible.

(7) Oxidation of [Ir(TMPP)2(CO)2][BF4] (19)

(i) Chemical oxidation with NOBF4. In a typical reaction, a solution of
[Ir(TMPP)2(CO)2][BF4l (19) (0.100 g, 0.071 mmol) and NOBF4 (0.009 g, 0.0.71
mmol) in 5 mL of MeCN was stirred at r. t. In the ﬁrst minute a vacuum was
applied to remove NO(g) or CO that may evolve from the reaction. Within
several minutes, the solution color changed from orange to orange-red. An'
aliquot was removed after 30 min and its IR spectrum was recorded. IR
(CH3CN) cm'1: v(CO), 2079 (s), 2109 (w), 2148 (w), 2029 (vw). The reaction
was stirred overnight, after which time the solution was pump to dryness,
washed with 10 mL of Et20 and dried in vacuo; yield, 0.053 g of crude
product.

(ii) Chemical Oxidation with [(p-BngH4)3N][BF4]. In a typical
reaction, a mixture of [Ir(TMPP)2(CO)2][BF4] (19) (0.100 g, 0.071 mmol) and
[(p-BrC6H4)3N"_I[BF4] (0.041 g, 0.071 mmol) was dissolved in 5 mL of MeCN.
The solution immediately became dark red/orange in color. The reaction was
periodically subjected to a vacuum to help remove evolved CO from the
solution. Within 30 min, the solution color began to lighten and an IR
spectrum of the solution showed the presence of unreacted 19 together with
several other v(CO) bands indicative of a new compound or compounds; IR
(CH3CN) cm'1: v(CO), 2079 (s), 2110 (m), 2148 (m); 19, 1991 (s), 1942 (w).
The reaction was stirred overnight during which time the solution color
became pale green and a pale precipitate had settled out. An aliquot of the
green solution was removed and its IR spectrum was recorded; IR (CH3CN)
cm'l: v(CO), 2079 (vs), 2027 (w), 2148 (vw), 2110 (vw), 2046 (vw), 2009 (w).

The solution was ﬁltered and evaporated to a residue under reduced pressure

205

to yield a pale yellow/green solid. 1H NMR (CD3CN) 8, ppm: -OCH3, 3.46 (s),
3.82 (s); m-H, 6.20 (t, 4JP-H = 2.2 Hz).
(iii) Bulk Electrolysis. Bulk electrolysis of [Ir(TMPP)2(CO)2][BF4] (19)
was performed in a four-compartment electrolysis cell separated by coarse
porosity sintered glass frits (Figure 35). A quantity of [Ir(TMPP)2(CO)2][BF4]
(19) (0.050 g, 0.036 mmol) was added to a degassed CH2C12 solution of 0.05 M
[(Bun)4N][BF4_'| in the working compartment of the cell. The solution was
electrolyzed at a potential of E = + 1.0 V using a Pt gauze worh'ng electrode.
After 1 h, the solution color had changed from orange to yellow. A small
aliquot of the solution was removed and its IR spectrum was recorded. IR
(CH2Clz) cm'lz v(CO), 2075 (m). The solution was then transferred via
syringe into a Schlenk ﬂask under Ar and was carefully layered with Et20 (5
mL). After diffusion was complete, the remaining yellow solution was
decanted away from the [(Bun)4N][BF4] salt that had precipitated. The
solvent was removed under vacuum to give a pale yellow solid. 1H NMR
(CD3CN) 8, ppm: -OCH3, 3.57 (s), 3.86 (s); m-H, 6.19 (t, 4Jp_H = 1.8 Hz).
(8) Reaction of [Ir(TMPP)2(CO)2][BF4] (19) with Iodine

An amount of [Ir(TMPP)2(CO)2][BF4l (19) (0.050 g, 0.036 mmol) in 5
mL of MeCN was stirred with one equivalent of 12 (0.009 g, 0.036 mmol). and
NaBF4 (0.004 g, 0.036). After stirring for 3 h, the solution color became
yellow. A small aliquot was removed and its IR spectrum was recorded: IR
(CH3CN) cm'1:v(CO), 2148 (vw), 2073 (m), 2055 (s). The solution was
pumped to dryness and dissolved in 5 mL of CHZCIZ. The yellow solution was
then ﬁltered through a Celite plug to remove the remaining undissolved
sodium salt. The ﬁltrate was evaporated under reduced pressure and dried

under vacuum.

 

 

 

 

?

 

 

 

 

 

 

 

 

 

Pt gauze Ag/AgC]
electrode reference
Pt wire electrode

 

 

iléi
th.

fnt

L

Figure 35. Schematic drawing of the electrolysis cell used in the
electrochemical oxidation of [IrI(TMPP)2(CO)2]1+ (19).

207

B. X-ray Crystallography

The structure of [Ir(TMPP)2(CO)2][BF4] (19) ° CH2012 was determined
by application of general procedures that have been fully described
elsewhere.6 Geometric and intensity data were collected on a Rigaku AFCGS
diffractometer with graphite-monochromated MoKa (15 = 0.71069 A)
radiation and were corrected for Lorentz and polarization effects. Relevent
crystallographic parameters for 19 are summarized in Table 15. All
calculations were performed with the use of VAX computers on a cluster
network within the Department of Chemistry at Michigan State University
using the Texsan software package of the Molecular Structure Corporation.7
(1) [Ir(TMPP)2(CO)2][BF4] (19) . CH2C12
(i) Data Collection and Reduction. Single crystals of 19 suitable for X-
ray analysis were obtained as a CH2C12 solvate from careful layering of Et20
on a CH2C12 solution of 19. A regular block shaped crystal with approximate
dimensions, 0.23 x 0.34 x 0.15 mm3, was selected and secured onto the tip of a
glass ﬁber with epoxy cement. Least-squares reﬁnement of 24 orientation
reﬂections in the range 20 < 20 < 33° resulted in cell constants consistent
with a triclinic cell. Intensity measurements were performed at 23 i 3°C
using the co-20 scan technique. Reﬂections with I < 106(1) were re-scanned a
maximum of two re-scans and the counts were accumulated to assure good
counting statistics. Routine measurement of three check reﬂections at
regular intervals throughout data collection revealed that the crystal had
experienced 57% loss in diffraction intensity. A linear decay correction was
applied to compensate for the observed loss. In addition, an empirical
absorption correction was applied based on azimuthal scans of 3 reﬂections

with Eulerian angle x near 90° resulting in maximum and

 

 

208

Table 15. Summary of crystallographic data for [IrI(TMPP)2(CO)2][BF4] -

 

CH2012(19)
Formula IrP2020C57H63BF4C12
Formula weight 1592.32
Space group P-1
a, A 13.512(2)
b, A 18.348(3)
c, A. 13.358(2)
or, deg 9726(1)
[3, deg 9055(1)
7, deg 95.o2(1)
V, A3 3272(2)
Z 2
dcalc, g/cm3 1.616
I1 (Mo Ka), cm'1 41.73
Temperature, °C 23i2 °C
Ra 0.051
wa 0.065
Quality-of-ﬁt indicatorc 2.19

 

aR=£ I lFol- IFcIl/ztrol
bRw = [2w< I F0 I— I re I)2/zw IF0 |211/2; w = 1/02( IFo I)
CQuality-of-ﬁt = [zw( IFo I - IFc [)2/(Nobs-Nparameters)]1/2

 

209

minimum transmission factors of 1.00 and 0.79. A total of 10,134 data were
collected in the range of 4 S 20 S 47°. After averaging equivalent reﬂections
(Rmerge = 3.4%), there remained 9652 unique data of which 6165 had F02 2
36(Fo)2.

(ii) Structure Solution and Refinement. The position of the Ir atom was
located directly from a Patterson Fourier synthesis. The remaining non-
hydrogen atoms were located by application of the program DIRDIF followed
by several alternating least-squares cycles and Fourier maps.8 Due to a
disorder resulting from several random orientations, the [BF4]' anion was
modeled as an ideal tetrahedron with ﬁxed bond distance and angles. With
the exception of the [BF4]‘ anion, all non-hydrogen atoms were reﬁned with
anisotropic thermal parameters. Hydrogen atoms were included in the
structure factor calculation as ﬁxed contributors at calculated positions and
were not reﬁned. In the end, ﬁnal least-squares reﬁnement of 731
parameters gave residuals of R = 0.051 and RW = 0.065 and a quality-of-ﬁt
index of 2.19. Not unexpectedly, the highest remaining peak in the ﬁnal
Fourier difference map was 1.19 e'/A3 and was associated with the disordered

[BF4]' anion.

3. Results
A. Synthesis and Characterization of [M(cod)(n2-TMPP)]1+ (M = Rh)
and M(cod)(n2-TMPP-0) (M = Rh, Ir)

One equivalent of TMPP readily reacts with the partially solvated Rh
oleﬁn complex [Rh(cod)(solvent)2]+, which is formed in situ by halide
abstraction using Ag+, to produce [Rh(cod)(n2-TMPP)][BF4] (16) (eq 20). The
[BF4]' salt of the cation is relatively insoluble in THF, and as a result, the

-_——-1

210

M e1+
e
I

/Rh [Cl R/\ A IFHFI TMPP m0

RhClR h/ /Rh‘TH (20)

main
r’ArO Me
Ar

product is isolated as a ﬁne yellow precipitate in high yield from THF.
Although it is anticipated that the TMPP ligand participates in an 112-
bonding mode to the metal, thereby ﬁlling the vacant fourth coordination site,
the 1H NMR spectrum of 16 in CDZCIZ exhibits a highly symmetrical pattern
of resonances in which each phenyl ring is equivalent. Clearly, the phosphine
is participating in a dynamic process that rapidly exchanges each of the non-
coordinated phenyl rings with the coordinated ring. A similar ﬂuxional
process was also noted for (n 3 -TMPP)Mo(CO)3 and [Rh(n 2-
TMPP)(TMPP)2(CO)][BF4] (7).9 Furthermore, the coordinated diene is also
apparently participating in a dynamic process as evidenced by the broad
appearance of the 1H NMR resonances for the cod ligand. The 31P NMR
spectrum exhibits the expected doublet at 8 = + 1.4 ppm ((1, 1J Rh_p = 137.3
Hz) as a result of one phosphorus nucleus experiencing 103Rh-31P coupling.

In an attempt to displace the coordinated oleﬁn to form the homoleptic
Rh(I) TMPP complex analogous to 3, [Rh(cod)(n2-TMPP)][BF4] (16) was
deliberately reacted with a second equivalent of TMPP. However, instead of
substituting the diene, the phosphine behaved as a nucleophile resulting in
the dealkylation of a coordinated methoxy group to give [TMPP-CH3]+ (eq 21)
and the dealkylated complex, Rh(cod)(n2-TMPP-0) (17), which was isolated
from the phosphonium salt by extraction with a mixture of diethyl ether and

a minimal amount of THF. Quite unexpectedly, 17 is relatively soluble in

..-

 

211
Me TMPP M M9
/\ [53 /\ ,0 o
Rh Rh n (21)
\ \
\/ B, \/ R, 0‘
"’1Ar TMPP CH3 * r" . Me
Ar
.AI‘Ar Ar'

diethyl ether; this is presumably a consequence of its charge neutrality.
Crystalline solids may be obtained by careful layering of hexanes onto
CH2012 solutions of 17.

An infrared spectrum of Rh(cod)(n2-TMPP-O) (17) shows the
characteristic bands for coordinated TMPP and conﬁrms the absence of the
[BF4]' anion consistent with the neutral formulation of 17. A FAB mass
spectrum of the compound exhibits a parent ion peak at 728 m/z in accord
with the empirical formula RhPC3409H42. The 1H NMR spectrum of
RhI(cod)(n2-TMPP-O) (1 7) in CD2 C12 reveals a relatively symmetric
resonance pattern. As was observed for the parent compound, the
cyclooctadiene resonances are broad, indicating that a dynamic process is
occurring in solution. The most striking feature of the spectrum is the
appearance of the resonances corresponding to meta-protons on the phenyl
rings of TMPP. The two multiplets that appear at 8 = 5.45 and 5.7 2 ppm are
assigned to the proximal and distal meta-protons of the chelating phenoxide
ring. The observed coupling pattern arises from coupling of these
magnetically distinct resonances to the phosphorus nucleus and to each
other. The meta-protons of the two remaining phenyl rings are magnetically
equivalent and appear as doublets at 8 = 6.04 ppm (4Jp_H = 3.3 Hz).

Evidently, there is still free rotation about the P-C bonds of the non-

212

coordinated phenyl rings, inspite of the fact that one phenyl ring is bonded to
the metal. Of further interest is the 31P NMR spectral properties of 17 in
CD2C12; the 31P NMR spectrum exhibits a doublet at 8 = - 8.0 ppm (1JRh-p =
157.2 Hz), which is shifted upﬁeld relative to 16. Typically the formation of a
ﬁve membered chelate ring involving phosphorus results in a signiﬁcant
downﬁeld shift of the phosphorus resonance.10 Apparently, however, in this
case the chelation affect is more than compensated for by the formation ofa
more strongly donating phenoxide interaction, which increases the electron
density at the metal center, thereby further shielding the phosphorus
nucleus. The net effect is an upﬁeld shift of the phosphorus resonance.

The analogous iridium complex, Ir(cod)(n2-TMPP-O) (18) was prepared
by reaction of [Ir(cod)Cl]2 with excess TMPP in the presence of KBF4.
Although it was not isolated, [Ir(cod)(n2-TMPP)]+ is believed to be formed in
the initial stages of the reaction. This species then undergoes dealkylation in
the presence of additional phosphine to yield 18. COmpound l8 exhibits
essentially the same solubility and stability properties as the rhodium
complex Rh(cod)(n2-TMPP-0) (17). The 1H NMR spectrum of Ir(cod)(n2-
TMPP-O) (18) in CD3CN exhibits three resonances at 8 = 5.55 ppm (t), 5.77
ppm (dd), and 6.09 ppm (d, 4Jp_H = 3.6 Hz) in a ratio of 1 : 1 : 4 corresponding
to the meta-protons of the phenyl rings. Just as was observed for 17, the
spectrum indicates that one ring is tightly bonded to the metal leading to the
observation of proximal and distal resonances for the two meta-protons on
that ring. The remaining two phenyl rings, however, freely rotate about the

P-C bond.

 

 

 

213
B. Synthesis and Spectroscopic Characterization of
[Ir(TMPP)2(CO)2l[BF4] (l9)

[Ir(TMPP)2(CO)2][BF 4] (19) was synthesized in manner similar to the
method used for [Rh(TMPP)2(CO)]+ (eq 22). As in the rhodium chemistry,
coordinated diene may be displaced from the iridium complex by CO in the
presence of TMPP to give [Ir(TMPP)2(CO)2]+ (19). A solution of 4 equivalents
of TMPP in MeCN was added dropwise to a MeCN solution of [Ir(cod)Cl]2 in
the presence of MBF4 (M = Ag+, Na+, or K+). The addition was performed
under an atmosphere of CO at 0°C with yields typically in the range 80-85%.
A drastic reduction in yield was noted when THF was

Me- 03:: 1 1+
F‘Ar'
/1’Cl1/\\)L<;/1NCM: TMPP Me-Ooc-liar-CO (22)
\/Ir\ CllI ry) MeCN \NCMe CO I

Ar' ~ 1:‘Ar'
A1"

used as a solvent instead of MeCN, unlike the synthesis of [Rh(TMPP)2CO]1+
(7 ) which works very well with THF as a solvent. The choice of metathesis
reagent apparently does not effect the yield, as Ag+, Na+, and K+ have
produced comparable yields f0 19.

If the reaction is not purged with CO, the neutral complex
Ir(cod)(TMPP-0) (18) is formed instead of 19 As was observed in the
analogous Rh system, CO is required to displace the cod. Otherwise, the
second equivalent of TMPP acts as a nucleophile and dealkylates the

coordinated phosphine instead of displacing the diene. However, unlike

[Rh(TMPP)2(CO)2]1+ (6), CO does not reversibly dissociate from 19 to form an

 

214

iridium monocarbonyl species. Only under more forcing thermal and
photolytic conditions is the loss of CO apparent. Unfortunately, such
conditions lead to a mixture of intractable products.

The infrared spectrum of [Ir(TMPP)2(CO)2][BF4] (1 9) in CH2 C12
exhibits two carbonyl stretching vibrations at v(CO) = 1993 cm’1 vs and 1946
cm'1 (w). In the solid state these bands shift slightly and change intensity to
v(CO) = 1987 cm'1 (s) and 1942 cm'1 (In). The 1H NMR spectrum of 191s
consistent with a trans disposition of two magnetically equivalent phosphine
ligands with resonances appearing at 8 = 3.38 (s, 36 H, o-OMe), 3.77 (s, 18 H,
p-OMe), 6.06 (t, 4Jp-H=2.0 hz, 12H, m-H). The 31P NMR spectrum of 19
(CDCl3) exhibits a single resonance for both phosphines at 8 = -39 ppm. The
cyclic voltammogram of [Ir(TMPP)2(CO)2][BF4] in 0.1 M TBABF4/CH2C12
shows an irreversible oxidation at Ema: +0.82 V. Coupled with this oxidation
are two chemical waves at E = +0.46 V and -0.81 V.

Of further interest is the observation that [Ir(TMPP)2(CO)2][BF4] (19)
is emissive in the solid state at room temperature. Exposure of solid samples
of 19 to long wave ultraviolet radiation produces a brilliant orange
luminescence. Emission spectra have been previously found for other square
planar Rh(I) and Ir(I) complexes.11 Typically, the emissive state results
from MLCT from the metal dz2 orbital to a low-lying ligand orbital of 1:
symmetry. When the ligands are n-acceptors, such as CO or CN', the MLCT
is interpreted as excitation from the metal dz2 orbital to the 1t* orbital of the
ligand.11g By analogy, we expect a similar charge transfer process to be
responsible for the emissive properties of 19. In contrast to the solid state
behavior, solutions of [Ir(TMPP)2(CO)2][BF4] (19) do not appear to be
luminescent at room temperature. This is in agreement with other square

planar (18 complexes.

   

 

215

C. Crystal Structure of [Ir(TMPP)2(CO)2][BF4] (19)

The identity of the product as [Ir(TMPP)2(CO)2][BF4] was conﬁrmed by
X—ray crystallography. An ORTEP diagram of the molecular cation is
presented in Figure 36. A listing of pertinent bond distance and angles are
found in Table 16. As expected, the complex is square planar with the
phosphines situated trans to each other. Not surprisingly, the phosphine
ligands are monodentate and there are no interacting methoxy groups along
the axial direction (r(Ir-O) > 3.0 A). Although the phosphines are ‘
magnetically equivalent in solution, they are not crystallographically
identical, as the iridium atom does not lie on a crystallographic symmetry
element. The Ir-P distances, 2.338(3) A and 2.345(3) A, are only slightly
longer than those in the analogous Rh complex [Rh(TMPP)2(CO)2][BF4] (6)
(2.327(1) A and 2.332(1) A). There are no distortions evident in the structure,
as the angles between the ligands are nearly ideal. A packing diagram shows
that the square planar Ir cations stack along the z-axis, but the closest
distance between iridium centers is over 10 A. So it appears that the
observed emission properties are associated with the ligands and do not arise
from association of cations in the solid state.

D. Oxidation of [Ir(TMPP)2(CO)2] [BF4]

Bulk electrolysis of [Ir(TMPP)2(CO)2][BF4] in 0.05 M TBABF4 /
CH2C12 at a potential of +1.1 V for 1 hour gave a yellow solution that
exhibited a carbonyl stretch at v(CO) = 2075 cm'1(m). The 1H NMR spectrum
of the solid in CD2C12 showed resonances due to [(nBu)4N]+ in addition to a
new TMPP containing species: 8, ppm; 3.57 (s, broad), 3.86 (s), 6.19 (t). The
triplet at 6.19 is indicative of a trans diphosphine complex and the breadth of
the methoxy resonances suggest that some ﬂuxionality is occurring.

Chemical oxidation of [Ir(TMPP)2(CO)2][BF4] with NOBF4 in MeCN

 

   

 

 

216

C52
C35
n
C C48
011 [:30 Q/C'5>3
017

 

Figure36. ORTEP representation of the molecular cation
[IrI(TMPP)2(CO)2]1+ (19).

 

 

 

 

217

 

 

 

 

Table lSelected bond distances (A) and angles (deg) for
[IrI(TMPP)2(CO)2][BF4] 0 CH2012(19).

Atom 1 Atom 2 bond distance

Ir(l) P(1) 2.338(3)

Ir(l) P(2) 2.345(3)

Ir(l) C(55) 1.94(1)

Ir(l) C(56) 1.89(1)

P(1) C(l) 1.87(1)

P(1) C(10) 184(1)

P(1) C(19) 1.84(1)

Atom 1 Atom 2 Atom 3 bond angles

P(1) Ir(l) P(2) 178.4(1)

P(1) Ir(l) C(55) 90.0(4)

P( 1) Ir(l) C(56) 90.4(4)

P(2) Ir(l) C(55) 88.4(4)

P(2) Ir(l) C(56) 91.2(4)

C(55) Ir(l) C(56) 177.8(6)

Ir(1) P(1) C(l) 100.5(4)

Ir(l) P( 1) C(10) 115.8(4)

Ir(l) P( 1) C(19) 118.8(4)

 

 

 

 

218
produced a red-orange solution. The infrared spectrum of the solution
showed several bands at energies greater than 2000 cm'l; these occur at 2079
cm‘1 (s), 2148 cm‘1 (w), 2109 cm‘1 (w), 2029 cm'1 (vw). Presumably, the
intense band at 2079 cm'1 corresponds to the same product formed in the
bulk electrolysis experiment and is shifted due to solvent effects (MeCN
versus CHzClz).

Oxidation of [Ir(TMPP)2(CO)2][BF4] (19) with the amminium salt [(p-
BrC6H4)3N][BF4J initially gave a deep blue solution that immediately became.
red-orange as the amminium salt was consumed. As in the other oxidation
reactions of 19, a vacuum was applied to help remove evolved CO from the
solution. After a day of continual stirring the solution color changed from
red-orange to green. An infrared spectrum of the green solution revealed the
presence of a strong CO stretch at 2079 cm'l. In addition to this stretch,
several less intense bands were observed between 2009 cm'1 and 2148 cm'l.
An infrared spectrum of the solution taken during the orange stage shows not
only the same bands, but also those corresponding to a signiﬁcant amount of
unreacted 19, which implies that the reaction is not complete until the
solution has become green. The 1H NMR spectrum of the product (CD3CN)
shows three primary resonances at 6 = 3.46 ppm (s), 3.82 ppm (s); m-H, 6.20
ppm (t, 4JP-H = 2.2 Hz). It is worth noting that the IR and NMR
spectroscopic characteristics for the green product correspond to those
observed for the compound prepared by bulk electrolysis and by chemical
oxidation with NO+.

Based on the aforementioned infrared and NMR spectral data, one can
begin to draw several conclusions as to the identity of this oxidized product.
The infrared spectrum of the oxidized product indicates that at least one CO

ligand is still present. This is not surprising considering the large number of

 

219

Ir(III) carbonyl complexes that have been reported.12 The shift of v(CO) to
higher energy is consistent with oxidation of the metal center which results
in a decrease in the degree of n-back bonding present. Based on the sharp
and unshifted natrure of the 1H NMR spectrum, the product is diamagnetic,
which is consistent an Ir(III) species. The triplet resonance observed for the
meta protons indicates that the phosphines are situated trans to each other.
The TMPP ligand resonances are magnetically equivalent and highly
symmetric, consistent with either a monodentate bonding mode or a ﬂuxional
process that exchanges all potentially equivalent protons. A six coordinate

complex, which one would expect if the compound is an Ir(III) species,

Mao/ﬂ Me\O/\\ /\

 

 

/‘ P P
Me- 0 0‘Me Me~ 0 0‘Me
00 Ilr— ’Me 00 Ir—CO
OC Me. O \JO-Me
V K’O‘ VLO‘
Me’ Me Me
cis trans

requires that the phosphines are multidentate assuming no additional
ligands have been added to the coordination sphere. If this is the case, the
molecule must exhibit some fluxional behavior. Indeed, the slightly
broadened appearance of the methoxy resonances indicates that an exchange
process is operative. The infrared data combined with the NMR data suggest
that the observed product is an Ir(III) dicarbonyl species containing either cis
or trans carbonyl groups as shown below. Unfortunately, attempts to isolate

pure crystalline samples have yielded only oily solids.

 

220

4. Discussion

The primary motivation for investigating the chemistry of TMPP with
Rh(I) and Ir(I) oleﬁn complexes was to develop efﬁcient methods for
preparing homoleptic TMPP complexes of rhodium and iridium. Others have
found that compounds of the type [MI(diene)(solvent)2]1+ are excellent
precursors for a number of Rh(I) and Ir(I) phosphine complexes.2 In the
present study, TMPP easily displaces the coordinated solvent molecules to
form [Rh1(cod)(n2-TMPP)]1+ (1 6). Addition of a second equivalent of
phosphine, however, does not result in substitution of the oleﬁn, but instead
dealkylates the coordinated TMPP of 16 to produce Rh(cod)(n2-TMPP-O) (17).
The strong afﬁnity of the metal for the oleﬁn is further evident by the
observation that 17 is inert with respect to substitution with additional
TMPP. Reaction of 17 with excess TMPP under reﬂuxing conditions produced
only an intractable mixture of products. One possible solution to this
problem would be to alter the nature of the coordinated oleﬁn. Replacement

of cod with an alkene may encourage dissociation in the absence of the

chelate effect. Indeed, reaction of [Rh(CzH5)ZCl]2 with excess TMPP in the
presence of KBF4 led to the formation of the same product produced by
cobaltocene reduction of [Rh(n3-TMPP)2][BF4]2 (3). Although an analogous
ethylene complex is not known for Ir, the corresponding cyclooctene complex
[Ir(CgH14)2Cl]2 could be used as a precursor to Ir-TMPP complexes.13
Another approach to synthesizing compounds of the type MI(TMPP)(TMPP-
O) (M = Rh, Ir) is by hydrogenation the coordinated diene of 17 and 18 in a
donating solvent followed by addition of TMPP. This method has proven
effective in the formation of solvated metal phosphine complexes that are

active hydrogenation catalysts.14 A third possible route would be to use a

non-olefin Rh(I) or Ir(I) precursor such as M(acac)2.15 Presumably, the

   

 

 

221
acetylacetonate ligand would be more susceptible to substitution than
cyclooctadiene.

Although oleﬁn-phosphine complexes are not novel, compounds 16 - 18
may be useful as synthetic building blocks for other Rh and Ir TMPP
complexes. Removal of the diene by either simple substitution or through
oleﬁn hydrogenation could lead to the development of a new class of mixed
ligand complexes that incorporate the advantages of TMPP with the added
reactivity provided by other ligand sets. For example, it may be possible to
prepare novel mixed phosphine/thiolate complexes with the general formula,
MIH(n2-TMPP)(112-TMPP-0)(SR)2 (M = Rh, Ir), by oxidative addition of alkyl
disulﬁdes to 17 - 18 in the presence of TMPP. As an additional point, many
Rh and Ir oleﬁn complexes serve as catalyst precursors.14 The combination
of a hard oxygen donor with a soft phosphorus donor may induce unusual
catalytic properties. As a result, the potential application of these rhodium
and iridium systems towards homogenous catalysis is currently under
investigation. 16

Previously, we found that [RhI(TMPP)2(CO)][BF4] (7) may be
chemically oxidized to yield the paramagnetic Rh(II) compound [Rh(n3-
TMPP)2][BF4]2 (3). Oxidation of 7 yield an unstable Rh(II) carbonyl species
that immediately undergoes dissociation. By analogy, we expected that
oxidation of the iridium dicarbonyl complex 19 might yield an unstable Ir(II)
dicarbonyl species that readily dissociates CO to form [IrII(TMPP)2]2+.
Clearly, however, CO remains bonded to the metal as evidenced by IR
spectroscopy. In any event, it is apparent that we may have reached a
limitation of using a containing iridium carbonyl complex as a precursor to

homoleptic Ir-TMPP species. Unlike the analogous chemistry of

 

 

 

 

222

[Rh(TMPP)2(CO)n]1+, CO is more tightly bound to iridium,17 therefore the

isolation of [IrH(n3-TMPP)2]2+ may not be feasible using this approach.

 

10.

 

223

List of References

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197], 3679. (d) Empsall, H. D.; Heys, P. N.; Shaw, B. L. Transition
Met. Chem. 1978, 3, 165. (e) Empsall, H. D.; Hyde, E. M.; Pawson, D.;
Shaw, B. L.; J. Chem. Soc., Dalton Trans. 1977, 1292. (f) Mason, R.;
Thomas, K. M.; Empasall, H. D.; Fletcher, S. R.; Heys, P. N.; Hyde, E.
M. Jones, C. E; Shaw, B. L. J. Chem. Soc., Chem. Comm. 1974, 612. (g)
Empsall, H. D.; Hyde, E. M.; Shaw, R. L. J. Chem. Soc., Dalton Trans.
1975, 1690. (h) Empsall, H. D.; Heys, P. N.; McDonald, W. 8.; Norton,
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(a) Schrock, R. R.; Osborn, J. A. J. Am. Chem. Soc. 197], 93, 2397. (b)
Shapley, J. R. Schrock, R. R. J. Am. Chem. Soc. 1969,91 , 2816. (c)
Green, M.; Kuc, T. A.; Taylor, S. H. J. Chem. Soc. (A) 197], 2334. (d)
Haines, L. M.; Singleton, E. J. Chem. Soc., Dalton Trans. 1972, 1891.
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Herde, J. L.; Lambert, J. C.; Senoﬂ', S. V. Inorg. Synth. 1974, 15, 18.
Cramer, R. Inorg. Synth. 1974,15, 14.

Bell, F. A.; Ledwith, A.; Sherrington, D. C. J. Chem. Soc. (C) 1969,
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(a) Bino, A.; Cotton, F. A.; Fanwick, P. E. Inorg. Chem. 1979, 18, 3558.
(b) Cotton, F. A.; Frenz, B. A.; Deganello, G.; Shaver, A. J. Organomet.
Chem. 1973, 50, 227.

TEXSAN - TEXRAY Structure Analysis Package, Molecular Structure
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DIRDIF: Direct Methods for Difference Structure, An Automatic
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(a) Dunbar, K. R.; Haefner, S. C.; Burzynski, D. J. Organometallics
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Chem. Soc., 1991, 113, 9540. (c) Dunbar, K. R.; Haefner, S. C.;
Swepston, P. N. J. Chem. Soc., Chem. Commun. 199], 460.

(a) Garrou, P. E. Inorg. Chem. 1975, 14, 1435. (b) Garrou, P. E.Chem.
Rev. 198], 81, 229.

 

 

11.

12.

13.

14.

15.
16.
17.

224

(a) Johnson, C. E.; Eisenberg, R.; Evans, T. R.; Burberry, M. S. J. Am.
Chem. Soc. 1983, 105, 1795. (b) Fordyce, W. A.; Crosby, G. A. Inorg.
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Chem. Phys. Lett. 198], 77, 405. (e) Andrews, L. J. Inorg. Chem. 1978,
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Leigh, G. J.; Richards, R. L. in Comprehensive Organometallic
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CHAPTER IX

CONCLUSION

225

 

 

226

The results presented in this dissertation establish the use of
tris(2,4,6-trimethoxyphenyl)phosphine as an excellent supporting ligand for
the stabilization of divalent rhodium species. The mononuclear Rh(II)
complex, [Rh(n3-TMPP)2][BF4]2 (3) was isolated in high yield from a reaction
of the solvated dinuclear Rh(II) species [Rh2(MeCN)10][BF4]4 with TMPP.
[Rh(n3-TMPP)2][BF4]2 (3 ) represents the ﬁrst crystallographically
characterized mononuclear Rh(II) complex. The remarkable stability of this
metalloradical species provided us with an opportunity to study the
chemistry of mononuclear rhodium in an unusual oxidation state. The
combination of both hard and soft donor groups engenders the phosphine
with the ability to stabilize a metal atom in a variety of coordination
geometries and oxidation states. Although considered sterically
encumbering, the ligand is nevertheless quite ﬂexible as evidenced by the
variety of ﬂuxional and isomerization processes in which have been observed
for metal complexes of TMPP. This ﬂexibility has rendered it possible to
access different oxidation states of the rhodium center while maintaining the
same ligand set. Each of the complexes described in this study, exhibit one or
more chemically accessible redox processes. The chemical relationship
between the rhodium-TMPP complexes in this study is illustrated in Figure
37. The degree of methoxy interaction varies according to the rhodium
oxidation states. Rather than serving to enforce a particular geometry on the
metal center, the ligand adjusts to the available coordination sites and the
electronic requirements of the metal center (Figure 38). This ability is
readily apparent when one examines the structural changes in going from
Rh(I) to Rh(III) species. As the metal center is oxidized, the hapticity of the
phosphine ligand increases from n1 to n3 in response to the increased need for

electron density at the metal center.

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A critical point of the chemistry of TMPP was underscored in this
research, viz, that although [Rh(n3-TMPP)2][BF4]2 (3) is coordinatively
saturated, the hemilabile nature of the ether groups allows for facile and
often reversible substitution chemistry. It was found that the incoming
ligand must possess certain electronic requirements as the Rh(II) phosphine
complexes are resistant to attack by moisture or oxygen but are quite
susceptible to n-acids such as CO, CNR and NO. The ability of the ether
groups to participate in such "arm on, arm off ' type of mechanism is critical -
for the future development of these complexes as viable systems for catalytic
or stoichometric transformations of small substrates. The work described in
chapter 5 demonstrates the possibility of exploiting the labile nature of the
pendent methoxy substituents for the development of molecule-based
chemical sensors. The future design of other selective chemical sensors based
on complexes of TMPP appears to be very promising.

The complex chemistry of [Rh(n3-TMPP)2][BF4]2 (3) with carbon
monoxide provided valuable insight into the subtle electronic inﬂuences that
govern the relative stabilities of the +1, +2 and +3 oxidation states. The
reversible interaction of carbon monoxide with [Rh(n3-TMPP)2][BF4]2 (3) was
found to proceed through a series of redox reactions brought about by the
initial disproportionation reaction between 3 and an intermediate Rh(II)
carbonyl species. By reducing the n-acceptor strength of the incoming ligand
through the use of alkyl isocyanide ligands, the redox pathway was shutdown
and stable isocyanide adducts of 3 were isolated. At the time of their
discovery, the complexes [RhH(TMPP)2(CNR)2][BF4]2 (R = But, Pri)
represented the ﬁrst mononuclear organometallic Rh(II) species sufﬁciently

stable enough to be spectroscopically and crystallographically characterized.

 

230

The stability of these complexes provides for a rare opportunity to investigate
the chemistry of an organometallic radical system.

For the purpose of future directions, it should be pointed out that a
potential drawback to the use of TMPP as a supporting ligand is its
documented susceptibility to nucleophilic attack at a coordinated methoxy
substituent. This results in dealkylation of the coordinated ether group and
formation of a metal-phenorn'de bond. The tendency of these systems to
dealkylate is highly dependent on the electrophilicity of the metal center.
This point was illustrated by the relative stabilities of the molecular cations
[RhH(n3-TMPP)2]2+ (3) and [RhIH(n3-TMPP)2]3+ (4) towards dealkylation. As
the electrophilicty of the metal center was increased, the ease of dealkylation
also increased. It is expected that such behavior will be particularly
troublesome for the more electrophilic early transition metals; as a result,
this will necessarily limit the use of the free phophine form of TMPP to lower
valent early and late transition metals.

One of our principlal goals prior to undertaking this chemistry was to
the develop the coordination and organometallic chemistry of odd-electron
systems. The work presented here has demonstrated that TMPP provides the
proper combination of kinetic and thermodynamic stability for the isolation of
mononuclear Rh(II) complexes. Clearly, based on the demonstrated ability of
TMPP to stabilize Rh(II) metalloradicals, extension of this work to other d7
metal systems is plausible. Indeed, recent work in our laboratories has led to
the successful isolation of the Ni(III) complex, [Ni(TMPP-0)2]1+.1
Unfortunately, initial efforts to synthesize analogous Ir(II) species have
proven unsuccessful. This is attributed, in part, to the lack of a suitable

precursor such as a solvated [Ir2]4+ species. Undoubtedly, further work in

this area will ultimately result in the isolation of stable Ir(II)-TMPP systems.

 

 

 

231

As for complexes of Pd, Pt and Ru, nitrile complexes of these metals are
known213’4 and should provide excellent precursors for the synthesis of d8 and
d6 metal TMPP complexes. These intermediates, in turn, may be either
oxidized or reduced to the corresponding paramagnetic d7 species. In related
work, Gladfelter et al. have shown that paramagnetic Ru(I) carbonyl
complexes are accessed by chemical ozddation of zero-valent Ru(PR3)2(CO)3
species (R: phenyl, benzyl, p-tolyl, cyclohexyl).5 Similar d7 carbonyl species
have also been isolated and crystallographically characterized for Re(0).6 By
analogy, the preparation of similar metalloradicals of the general formula
[M(TMPP)2(CO)x]n+ is feasible. These odd-electron carbonyl complexes will
be especially intriguing in light of the rich chemistry already documented for
TMPP complexes of Rh(I), Ir(I) and Mo(0) carbonyl compounds.

 

 

232

List of References
1. Dunbar, K. R.; Quillevéré, A submitted for publication.
2. Thomas, R. R.; Sen, A. Inorg. Synth. 1989, 26, 128.

3. De Renzi, A.; Panunzi, A.; Vitagliano, A.; Paiaro, G. J. Chem. Soc.,
Chem. Commun. 1976, 47 .

4. (a) Rapaport, I.; Helm, L.; Merbach, A. E.; Bernhard, P.; Ludi, A. I norg.
Chem. 1988, 27, 873. (b) Bown, M.; Fontaine, X. L. R.; Greenwood, N.
N.; Kennedy, J. D.; Thornton-Pett, M. J. Chem. Soc., Dalton Trans.
1987, 1169. (c) Kolle, U.; Flunkert, G.; Gérissen, R.; Schmidt, M. U.;
Englert, U. Angew. Chem. Int. Ed. Engl. 1992, 31 , 440.

5. Sherlock, S. J .; Boyd, D. C.; Moasser, Gladfelter, W. L. Inorg. Chem.
199], 30, 3626.

6. (a) Walker, H. W.; Rattinger, G. B.; Belford, R. L.; Brown, T. L.
Organometallics 1983,2, 775. (b) Crocker, L. S.; Heinekey, D. M.;
Schulte, G. K. J. Am. Chem. Soc. 1989,111, 405.

 

APPENDICES

 

 

 

 

APPENDIX A

PHYSICAL MEASUREMENTS

 

 

 

 

APPENDIX A

PHYSICAL MEASUREMENTS

1. Infrared Spectroscopy Infrared spectra were recorded on a Perkin-
Elmer 599 or a Nicolet 740 FT-IR spectrophotometer. Solid state spectra
were measured as Nujol mulls between CsI plates. Solution spectra were
recorded using CaF2 solution cells.

2. NMR Spectroscopy 1H NMR spectra were measured either on a WM
250-MHz Bruker spectrometer with an ASPECT 3000 computer, a Gemini
300-MHz, Varian 300-MHz or Varian 500-MHz spectrometer. Chemical
shifts were referenced relative to the residual proton impurity of the
deuterated solvent: d2-methylene chloride, 5.32 ppm; d3-acetonitrile, 1.93
ppm; de-acetone, 2.04 ppm; dl-chloroform, 7.24. 13'C{1H} NMR spectra were
recorded on a Varian BOO-MHz spectrometer operating at 73.1 MHz and were
referenced relative to the 13C solvent resonance. 31P{1H} NMR spectra were
obtained on a Varian 300-MHz spectrometer operating at 121.4 MHz.
Chemical shifts were referenced relative to an external standard of 85%
H3PO4. Positive chemical shifts were reported downﬁeld relative to H3PO4.
3. EPR Spectroscopy X-band EPR spectra were obtained using a Bruker
ER200D spectrometer. To obtain an accurate measure of g values and line
widths, a Bruker ER035M NMR Gaussmeter and a Hewlett-Packard 5245L
frequency counter (with a 3-12 GHz adapter) were used to measure magnetic
ﬁeld strength and microwave frequency, respectively.

4. Electronic Absorption Spectroscopy Electronic absorption spectra

were measured on a Hitachi U-2000 or a Cary 17 spectrophotometer.

 

 

 

 

 

234

5. Electrochemistry Electrochemical measurements were performed by
using an EG&G Princeton Applied Research Model 362 scanning potentiostat
in conjunction with a BAS Model RXY recorder. Cyclic voltammetry
experiments were carried out at 22+2°C using 0.1 M tetra-n-butylammonium
tetraﬂuoroborate (TBABF4) as a supporting electrolyte, unless noted
otherwise. Measurements were made at a scan rate of 200 mV/s using a
platinum disk working electrode and a Ag/AgCl reference electrode. E1/2
values, determined as (Ep,a+Ep’c)/2, were referenced to a Ag/AgCl electrode
and uncorrected for junction potentials. Under the same experimental
conditions, the szFe / CpZFe+ couple occurred at Em = +0.50 V.

6. Magnetic Susceptibility Variable temperature and ﬁeld magnetic
susceptibility measurements were carried out on a 10 KG BTI
Superconducting Quantum Interference Device (SQUID) at Michigan State
University. Additional solid state magnetic susceptibility measurements
were determined at room temperature by using a J ohnson-Matthey magnetic
susceptibility balance. Solution magnetic susceptibility measurements were
carried out by application of the Evans method on a Bruker 250-MHz or
Varian 300-MHz spectrometer.

7. Mass Spectrometry Fast Atom Bombardment (FAB) mass spectrometry
studies were performed on a JEOL HX double-focusing mass spectrometer
housed at the National Institutes of Health / Michigan State University Mass
Spectrometry Facility.

8. Elemental Analysis Elemental analyses were performed at Galbraith

Laboratories, Inc.

 

 

 

 

APPENDIX B

CHEMISTRY OF TRIS(2,4,6-TRIIVIETHOXYPHENYL)PHOSPHINE
WITH GROUP VI METALS

 

 

 

APPENDIX B

CHEMISTRY OF TRIS(2,4,6-TRIMETHOXYPHENYL)PHOSPHINE
WITH GROUP VI METALS

1. Introduction

As our eventual goal in this research is to establish that TMPP, with
its labile metal-ether interactions, is an excellent ligand for the design of
reactive metal centers, we set out to prepare a complex that would allow us to
probe the small molecule binding properties of a TMPP supported metal.
Based on our newly acquired knowledge of the bonding capabilities of the
ligand, we predicted that it would be possible to prepare compounds of
general formula (TMPP)ML3 for metals that ordinarily exhibit octahedral
structures. A convenient backdrop for our work is the elegant research of
Kubas and co-workers who demonstrated that highly donating, bulky
phosphine ligands stabilize the ﬁve-coordinate complexes M(CO)3(PR3)2
M(=Mo, W; R=CGH11, i-C3H7) which readily add molecular H2 and N 2.1 Since
no examples of four-coordinate complexes of general formula M(CO)3(PR3)
have been reported, we set out to prepare such a monophosphine derivative
with TMPP with the expectation that the product would exhibit unusual
properties and reactivity. Herein, we report the synthesis of the novel
fluxional molecule (n3-TMPP)M0(CO)3 (TMPP = tris(2,4,6-
trimethoxyphenyl)phosphine) from a reaction between (116-C 7H8 )Mo(CO)3 and
TMPP. In addition, the chemistry of TMPP with partially solvated

mononuclear and dinuclear molybdenum halide complexes is also presented.

235

 

 

 

 

-_—
236
2. Experimental
A. Synthesis

All reactions were carried out under an argon atmosphere by the use of
standard Schlenk-line techniques unless otherwise stated. The phosphine,
tris(2,4,6-trimethoxyphenyl)phosphine (1), was prepared as described in
Chapter II. The starting material, (n6-C7H8)M0(CO)3, was purchased from
Strem and used as received. M02C14(MeCN)4 and MoCl3(THF)3 were
prepared using standard literature procedures.2’3 The tungsten starting
material, (nG-C7H8)W(CO)3, was prepared following the literature procedure
described by Kubas.4
(1) Preparation of (n3-TNIPP)M0(CO)3 (20)

In a typical synthesis, a quantity of (n6-C7H8)Mo(CO)3 (0.300 g, 1.10
mmol) and TMPP (0.587 g, 1.10 mmol) was dissolved in 10 mL of benzene
that had been thoroughly purged with argon in order to remove dissolved
N2(g). The resulting suspension was stirred at r. t. for ca. 12 h. The orange
supernatant was decanted and the yellow precipitate was washed with
benzene (4 x 5 mL). The yellow product was recrystallized by dissolution in
CH2012 (10 mL), followed by slow addition of diethyl ether (10 mL). The
solution was decanted and the yellow crystalline solid was washed with
diethyl ether (3 x 5 mL) and dried in vacuo; yield, 0.624 g (80 %) IR (Nujol,
CsI) cm'l: v(CO), 1914.5 vs. 1775 vs. 1791 vs. IR (CHZCIZ) cm‘l: v(CO), 1921
vs, 1799 vs, 1782 vs. IR (CGH6) cm'1: v(CO), 1930 vs. 1798 s. 1H NMR
(CD2C12, 20°C) 8 ppm: ~OCH3, 3.55 (br), 3.81 (s); m-H, 6.15 (br). 1H NMR
(06D5CD3, -60°C) 6 ppm: o-OCH3, 2.80 (s, 6H), 3.19 (s, 6H), 3.81 (s, 6H); p-
OCH3, 3.33 (s, 6H), 3.44 (s, 3H); m-H, 5.77 (s, 2H), 5.86 (s, 2H), 6.01 (br, 2H).
1H NMR (CDZCI2, -4000) 5 ppm: o-OCH3, 3.49 (s, 12H), 4.36 (s, 6H); p-OCH3,

 

 

 

237

3.77 (s, 6H), 3.82 (s, 3H); m-H, 6.02 (s, 2H), 6.07 (d, 4Jp_H= 3.9 Hz, 2H), 6.15
(dd, 2H). 31P NMR(CD2012, 20°C) 8 ppm: -1.9 (s).
(2) Reaction of TMPP with (n 6-C7H3)W(CO)3

To a mixture of (n6-C7H8)W(CO)3 (0.215 g, 0.597 mmol) and TMPP
(0.318 g, 0.597 mmol) was added 5 mL of diethyl ether. The resulting
suspension was stirred for 12 h at r.t. During this time, a yellow solid
precipitated from the solution. The yellow product was collected by ﬁltration,
washed with benzene (2 x 5 mL) and diethyl ether (4 x 10 mL), and dried
under vacuum; yield of crude precipitate 0.333 g. IR (Nujol, CsI) cm'l: v(CO),
1910 vs. 1769 vs. IR (CHZClg) cm'1:v(CO), 1912 vs, 1789 vs, 1775 vs.
(3) Reaction of TMPP with MoCl3(THF)3
‘ A solution of TMPP (0.382 g, 0.717 mmol) in 10 mL of THF was added
dropwise to a solution of M0C13(THF)3 (0.300 g, 0.717 mmol) in 5 mL of THF
at 0°C. The reaction was stirred for ca. 12 h, during this time the solution
was allowed to slowly warm to r. t. The resulting red solution was ﬁltered
through a Celite plug and then evaporated to yield a red solid. The solid was
washed with diethyl ether (3 x 5 mL) and dried under reduced pressure; yield
of red solid, 0.348 g. 1H NMR (CD3CN) 8 ppm: -OCH3, 3.51 (s), 3.82 (s); m-H,
6.14 (d, 4Jp_H = 5.7 Hz).
(4) Reaction of TMPP with M02014(MeCN)4

(i) 4 equivalents. In a typical reaction, a mixture of M02C14(MeCN)4
(0.250 g, 0.502 mmol) and TMPP (1.07 g, 2.01 mmol) was dissolved in 25 mL
of MeCN at r.t. After 3 h, the solution had become green in color. The
solvent was evaporated under reduced pressure to yield a green solid that
was washed with 20 mL of benzene. The colorless benzene washing was
evaporated to give a white powder (0.41 g) that was shown to be TMPP by 1H

NMR spectroscopy. The green residue was further washed with 10 mL of

 

 

238

diethyl ether and dried under vacuum; yield of crude solid, 0.48 g. A 1H NMR
spectrum of the crude product reveals the presence of only [TMPP-HT:

(ii) 6 equivalents. In a typical reaction, a solution of M02C14(MeCN )4
(0.200 g, 0.402 mmol) and TMPP (1.28 g, 2.40 mmol) in 20 mL of MeCN was
reﬂuxed for 4 days. During this time, the solution color changed from green
to dark brown with no further change throughout the reaction. The solution
was then ﬁltered through a Celite plug and evaporated to a residue. The
brown solid was washed with benzene (2 x 10 mL) and diethyl ether (2 x 10
mL) and dried under vacuum; yield of brown solid, 0.766 g. 1H NMR
spectroscopy showed that the solid was primarily comprised of [TMPP-CH3]+.
B. X-ray Crystallography

The structure of (n3-TMPP)Mo(CO)3 (20) was determined by
application of general procedures that have been fully described elsewhere.5
Geometric and intensity data were collected on a Nicolet P3/F diffractometer
with graphite monochromated MoKoc Ova: 0.7107 3 A) and were corrected for
Lorentz and polarization effects. All calculations were performed on a
VAXSTATION 2000 computer. Data reduction and reﬁnement were
performed using the programs from the Enraf-Nonius Structure
Determination Package (SDP). Crystal parameters and basic information
pertaining to data collection and structure reﬁnement are summarized in

Table 17

(l) (113-TMPP)M0(CO)3 (20) 0 CH2012

(i) Data Collection and Reduction. Crystals of (n3-TMPP)M0(CO)3 (20) -
CH2C12 were obtained from slow diffusion of diethyl ether into a CH2012

solution of 20. A single crystal with approximate dimensions 0.60 x 0.40 x
0.15 mm3 was selected and mounted onto the tip of a glass ﬁber with epoxy

cement. Least-squares reﬁnement of 25 carefully centered reﬂections in the

239

Table 17. Summary of crystallographic data for (n3-TMPP)Mo(CO)3 .
CH2C12 (20).

 

Formula
Formula weight
Space group

a, A

b, A

c, A

or, deg

[3, deg

7, deg

V, A3

Z

dcalc, g/cm3

u (Mo Ka), cm'1
Temperature, °C
Ra

wa

Quality-of-ﬁt indicatorc

MoPC12012031H35
801.92

P2 Va

17. 155(8)

1.2019(4)

16.985(6)

90

9569(3)
90
3485(2)
4

1.493
6.219
22:2 °C
0.069
0.084

2.09

 

aR=Z I lFol' chll/leol

bRw = [M I F0 I— I Fc I)2/2w tro I211/2; w = 110% [PO I)
cQuality-of-ﬁt = [2w( IFO I - IFc[)2/(Nobs-Nparameters)]1’2

 

 

240

range 15 < 20 < 25° revealed that the compound (n3-TMPP)M0(CO)3 -

CH2C12 crystallized in a monoclinic space group P21/a with a = 17 .155(8) A, b
= 12.019(4) A, c = 16.985(6) A, [3 = 9569(3) A, V = 3485(2) A3. Axial
photographs conﬁrmed the monoclinic symmetry of the cell. Intensity
measurement of three representative reﬂections at regular intervals
throughout data collection revealed that a 41.7 % loss in diffraction intensity
had occurred. A linear decay correction was applied to the data using the
program CHORT in SDP. A Nicolet. P3/F diffractometer was used to collect ‘
3845 unique data in the range 4 S 20 S 43° at 22 i 2°C; 2243 data with F02 >

36(F02) were used in the reﬁnement.

(ii) Structure Solution and Refinement. The position of the

molybdenum atom was successfully located by application of the direct
methods program in SHELXS-86. The remaining non-hydrogen atoms were

located through successive least-squares reﬁnements and difference Fourier

maps. After the reﬁnement had successfully converged with isotropic

thermal parameters, an absorption correction based on the program DIFABS

was applied to the data.6 In the end, after anisotropic reﬁnement of 424

parameters, residuals of R = 0.069 and RW = 0.084 were obtained. The
quality-of-ﬁt index was 2.09 and the largest shift/esd was 0.08. A ﬁnal

difference Fourier map revealed that no peak remained over 0.86 e'/A3 in

height.

3. Results
A. Synthesis and Characterization of (113-TMPP)M0(CO)3 (20)

One equivalent of TMPP reacts smoothly with (n3-C7H8)Mo(CO)3 at

room temperature in benzene over the period of 6 hours to produce a yellow

microcrystalline compound formulated as (n3-TMPP)M0(CO)3 (20) in 80%

 

 

241

yield. The product is insoluble in most common solvents except toluene and
benzene in which it is sparingly soluble and dichloromethane in which it is
very soluble but prone to decomposition over long periods of time. Yellow
solutions of 20 are very air sensitive, turning brown immediately upon
exposure to air. The solid and crystalline forms decompose slowly over the

period of several hours.
The solution properties of (n 3-TMPP)Mo(CO)3 attest to its high

reactivity as it easily converts to Mo(CO)3(NCCH3)3 in acetonitrile and is

extremely air sensitive. The 1H NMR spectrum of 20 revealed that an
intramolecular exchange process involving the ortho-methoxy groups is
occurring at room temperature. Variable temperature 1H NMR data were
obtained in d8-toluene and CD2C12 over the range 420°C and -60°C and the
results clearly indicate that all three rings are participating in a low energy
ﬂuxional process (Figure 39). The low temperature limiting spectrum at
-60°C in d8-toluene exhibits eight distinct resonances which integrate in
accordance with the magnetically inequivalent meta, ortho and para groups
observed in the solid state structure. At temperatures above -60°C the
spectral features broaden and gradually collapse in a non-symmetrical
manner due to a dynamic exchange of interacting and non-interacting ortho-
methoxy groups. Concomitantly the para and meta regions broaden and
eventually coalesce at ca -15°C. Similar behavior is observed in CD2012 ,
although in this solvent the low temperature limiting spectrum shows only
one broad resonance for the non-interacting ortho-methoxy groups at 5 = +
3.49 ppm (Figure 40); Attempts to obtain spectra at higher temperatures
were thwarted by the thermal instability of the complex.

 

 

 

 

 

242

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244

B. Molecular Structure of (113-TMPP)Mo(CO)3 (20)

Single crystals of the title compound were grown by slow diffusion of
diethyl ether into a CH2C12 solution of the compound at 0°C. An analysis of
the X-ray data established the identity of the compound and revealed that the
phosphine ligand occupies three positions in the coordination sphere of the
molecule. As the ORTEP diagram in Figure 41 shows that (113-
TMPP)Mo(CO)3 (20) possesses a distorted octahedral geometry with two of
the coordination sites being occupied by oxygens from ortho-methoxy
substituents on two separate phenyl rings. This bonding mode has previously
been noted in the structure of [Rh(n3-TMPP)2][BF4]2 (3 ).'7 The chelation
effect provided by weakly interacting pendant methoxy groups accounts for
the ease of isolation and moderate stability of 20. The distortion of the
coordination geometry about the Mo atom is evidenced most dramatically by
the acute angles P(1)-Mo(1)-O of 71.6(2)° and 74.8(2)° for 0(4) and 0(9)
respectively. These deviations from 90° are a consequence of the formation of
the ﬁve-membered rings Mo-P-C-C-O. All other angles within the molecule
are also non-ideal but to a lesser degree (Table 18). The high degree of
ﬂexibility of the TMPP ligand in achieving the observed bonding mode is
apparent from an examination of the disparate Mo(1)—P(1)-C angles; these
vary from 104.8(3)° for C(4) which is involved in a metallacycle, to 120.8(3)°
for C(22) on the lone free ring. Other metric parameters within the molecule
are typical for carbonyl-phosphine complexes. The Mo-O distances of 2.363(6)
A and 2.337(7) A are long which is not unexpected for metal-ether
interactions. The Mo-C distances are inequivalent (Table 18) with the Mo-
C(2) distance trans to the phosphorus being the longest (1.97 (1) A).

 

245

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\3 C(24) ‘29)
C(28

Figure 41. ORTEP representation of (n3-TMPP)M0(CO)3 (20).

 

246

Table 18. Selected bond distances (A) and angles (deg) for (n3-TMPP)-

 

 

 

 

Mo(CO)3 (20).
Atom 1 Atom 2 bond distance
Mo(1) P(1) 2.476(3)
Mo(1) C(1) 185(1)
Mo(1) C(2) 1.97(1)
Mo(1) C(3) 1.92(1)
Mo(1) 0(4) 2.363(6)
Mo(1) 0(9) 2.337(7)
Atom 1 Atom 2 Atom 3 bond angles
P(1) Mo(1) C(1) 101.3(3)
P(1) Mo(1) C(2) 171.5(4)
P(1) Mo( 1) C(3) 96.6(3)
P(1) Mo(1) 0(4) 71.6(2)
P(1) Mo(1) 0(9) 74.8(2)
C(1) Mo(1) C(2) 86.4(5)
C(1) Mo(1) C(3) 84.3(5)
Mo(1) P(1) C(4) 104.8(3)
Mo(1) P(1) C(22) 120.8(3)

 

 

247

C. Reaction of TMPP with (n6-C7H3)W(CO)3

The oleﬁn complex (nG-C7H8)W(CO)3 reacts with one equivalent of
TMPP in diethyl ether to yield a yellow precipitate similar to that observed
for the analogous molybdenum reaction. Although the solubility of TMPP is
only slight, it is sufﬁcient to allow for complete reaction within several hours.
The solid exhibits solubility properties similar to (n3-TMPP)M0(CO)3 (20) and
appears to be just as air sensitive. An IR spectrum of the crude precipitate
reveals that it is comprised of a mixture of carbonyl-containing products. By
analogy to 20, the bands at v(CO) = 1910 and 1796 cm'1 are tentatively
assigned to the monophosphine adduct (n3—TMPP)W(CO)3. This product has
been independently synthesized by reaction of the proprionitrile complex
W(CO)3(NCEt)3 with TMPP in toluene.8 A 1H NMR spectrum of the
precipitate in d8-toluene also reveals a mixture of products, one of which
exhibits broad features, indicative of a ﬂuxional process occurring in solution.
By analogy to the solution behavior of 20, this species most likely corresponds
to (n3-TMPP)W(CO)3.
D. Reaction of TMPP with Solvated Molybdenum Chloride
Complexes

Reaction of the M0211,II dinuclear complex, M02Cl4(MeCN)4, with
excess TMPP invariably leads to the formation of quaternary phosphonium
salts as evidenced by 1H and 31P NMR spectroscopy. The cations [TMPP-H]+
and [TMPP-CH3]+ were the only detectable diamagnetic species in solution.
The metal species are either paramagnetic and/or do not contain TMPP.
Attempts to isolate metal containing species by repeated fractional
crystallization yielded intractable mixtures of phosphonium salts. Similar
results were observed in the chemistry of TMPP with ﬁrst row metal halide

compounds.9 Through careful variation of the reaction conditions either

 

 

248

[TMPP-H1”r or [TMPP-CH3]+ may be isolated as the sole phosphonium cation

species. When the reaction is performed under mild conditions, [TMPP-H]+ is
the primary product observable by 1H and 31P NMR spectroscopy; under
reﬂuxing conditions formation of [TMPP-CH3]+ is favored. Formation of the
alkyl phosphonium cation suggests that coordination of the phosphine to the
metal occurs and that the coordinated ligand undergoes dealkylation to form
a phosphino-phenoxide derivative of TMPP. This is not surprising
considering the high oxophilicity of molybdenum which would promote
dealkylation reactions. TMPP also reacts with MoCl3(THF)3 under mild
conditions to give a red diamagnetic product that exhibits a symmetrical
pattern of resonances in the 1H NMR spectrum. The diamagnetism of the
sample suggests the complex has dimerized, thereby facilitating

communication between the unpaired electrons.

4. Discussion

The (n3-TMPP)M0(CO)3 (2 0) represents an unusual phosphine
derivative of molybdenum tricarbonyl. A single crystal X-ray study revealed
that the molecule is comprised of a Mo(0) center ligated by three carbonyl
ligands and a TMPP ligand in a facial bonding mode. The metal-ether
interactions from the pendant methoxy groups are quite labile. Variable
temperature 1H NMR spectra in d8-toluene and 0132012 revealed that a low-
energy intramolecular exchange process is occurring. We envision that the
exchange process occurs through the dissociation of one of the coordinated
ether groups to form a ﬁve coordinate trigonal bipyramidal geometry.
Rotation about the P-C bonds followed by association of a previously unbound
methoxy substituent eventually equilibrates each of the phenyl rings. This

process is depicted in Figure 42. The formation of similar ﬁve-coordinate,

 

 

 

 

 

 

 

 

 

 

249

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250

trigonal bipyramidal intermediates has been proposed for isotopic scrambling
in substitution reactions of octahedral metal carbonyl complexes.10

Our future interest in this molecule centers around its reactivity with
small molecules. The solution lability of the metal-ether interactions is
expected to provide the requisite open coordination sites for binding
substrates. Indeed, work by others in our laboratory has shown that (113-
TMPP)Mo(CO)3 (20) readily adds an additional CO ligand to form the tetra
carbonyl species (112-TMPP)M0(CO)4 .10 The addition of a fourth carbonyl
ligand is irreversible, but the tetracarbonyl complex reversibley adds a
second CO ligand to form the pentacarbonyl complex, (11 1-TMPP)Mo(CO)5.
These reactions further exemplify the lability of the metal-ether interactions
in TMPP complexes.

   

 

 

 

 

10.
11.

251
List of References

(a) Kubas, G. J. J. Chem. Soc. Chem. Commun. 1990,29, 0000. (b)
Kubas, G. J.; Ryan, R. R.; Swanson, B. I.; Vergamini, P. J.;
Wasserman, H. J. J. Am. Chem. Soc. 1984, 106, 451. (c) Kubas, G. J .;
Ryan, R. R.; Wrobleski, D. A. J. Am. Chem. Soc. 1986, 108 , 1339. (d)
Wasserman, H. J .; Kubas, G. J .; Ryan, R. R. J. Am. Chem. Soc. 1986,
108, 2294. (e) Kubas, G. J .; Unkefer, C. J .; Swanson, B. 1.; Fukushima,
E. J. Am. Chem. Soc. 1986, 108, 7000. (f) Kubas, G. J .Acc. Chem. Res.
1988,21, 120. (g) Kubas, G. J. Comments Inorg. Chem. 1988, 7, 17.

San Filippo, J .; Sniachoch, H. J .; Grayson, R. L. Inorg. Chem. 1974, 13,
2121.

Dilworth, J. R.; Richards, R. L.Inorg. Synth. 1980, 20, 119.

Kubas, G. J. Inorg. Chem. 1983,22, 692.

Bino, A.; Cotton, F. A.; Fanwick, P. E. Inorg. Chem. 1979, 18, 3558. (b)
Cotton, F. A.; Frenz, B. A.; Deganello, G.; Shaver, A. J. Organomet.
Chem. 1973, 50, 227.

Walker, N.; Stuart, D. Acta. Cryst. 1983,A39, 158.

(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. 199], 113, 9540.

Dunbar, K. R.; Sun, J .-S. unpublished results.

Quillevéré, A. Ph. D. dissertation, Michigan State University, 1992.
Howell, J. A. S.; Burkinshaw, P. M. Chem. Rev. 1983, 83, 557.

Dunbar, K. R.; Sun, J .-S. manuscript in preparation.

   

APPENDIX C

COMPILATION OF 31P NMR SPECTRAL DATA

   

APPENDIX C

Table 19. COMPILATION OF 31P NMR SPECTRAL DATA

 

 

Compound Chemicala J Rh_P J p A-pB
' Shift Hz Hz
(1) TMPP -— 66.3 (s) - -
(2) TMPP=O + 10.8 (s) - -
(3) [RhH(Tl3'TMPP)2][BF4]2 not observed
(4) [Rhm(n3-TMPP)2l[BF4]3 + 37.4 (d)b 107 —

(5)

(6)
(7 )
(8)
(9)
(10)

(11)

(l2)

(l3)

(l4)

(l5)

ax-[Rhm(n3TMPP)(n3-TMPP-0)l[BF4]2 +37.9(dd)b 139 13.7
+31.5(dd)b 140 13.7

[RhI(TMPP)2(CO)2][BF4] — 23.8 (d)b 116 —
[RhI(TMPP)2(CO)][BF4] - 11.1 (d)b 128 —
[RhI(TMPP)2(CO)(CNBut)l[BF4] - 21.8 (d)b 128 —
[RhH(TMPP)2(CNBut)2][BF4]2 not observed
[RhI1(TMPP)2(CNPri)2l[BF4]2 not observed
[RhI(TMPP)2(CNBut)2][BF4] - 19.1 (d)b 129.7 —

-2o.2(d)b 129.7 —

eq-[RhH(TMPP)(TMPP-0)l[BF4l not observed

eq-[RhHI(n3-TMPP)(n3-TMPP-0)][BF4]2 +473 (dd) 108.8 13.8
+22.5 (dd) 98.8 13.8

ax,eq-[Rhm(n3-TMPP-0)2][BF4] +47.1(dd) 137.3 16.7
+19.9 (dd) 122.9 16.7-

ax,ax-[RhIHm3-TMPP-0)2][BF4] +357 ((1) 152.5 -

252

 

 

 

253

 

Compound Chemicala J Rh-P J p A_pB
Shift Hz Hz
(16) [Rh1(cod)(n2-TMPP)][BF4] + 1.4 (d)b 137.8 —
(17) Rh1(cod)(n2-TMPP-0) + 8.0 (d)b 157.2 —
(18) IrI(cod)(n2-TMPP-0) not measured
(19) [IrI(TMPP)2(CO)2][BF4] — 39.0 (s)c — -
(20) (n3-TMPP)M00(CO)3 _ 19 (3)5 _ _

 

a Chemical shift reported in CD3CN relative to 85% H3PO4 unless noted
otherwise.

'0 Chemical shift reported in CDZCIZ.
0 Chemical shift reported in CD013.

 

   

 

 

 

 

 

 

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