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THESIS Mifligan State
University

 

This is to certify that the

thesis entitled

Nuclear Magnetic Resonance Studies
of Some Organometallic Molecules

presented by
William David Vernon

has been accepted towards fulfillment
of the requirements for

Ph.D

. degree in Chemistry

WM

Major professor

 

 

 

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Michigan State
University i

 

  

This is to certify that the
thesis entitled
Nuclear Magnetic Resonance Studies
of Some Organometallic Molecules

presented by

William David Vernon

has been accepted towards fulfillment
of the requirements for

Ph. D. degree in Chemistry

WM

Major professor

 

 

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ABSTRACT

NUCLEAR MAGNETIC RESONANCE STUDIES
OF SOME ORGANOMETALLIC MOLECULES

By

William David Vernon

Temperature-dependent carbon-13 and proton magnetic
resonance studies of solutions of ten dinuclear nS-dienyl
ruthenium, iron, and nickel carbonyls have revealed that
the rate of carbonyl interconversion and isomerization
in these complexes is a function of both the size and the
inductive character of the nS-dienyl ligand. The rate
determining factor in the isomerization and carbonyl inter-
conversion of the iron complexes was found to be rota-
tion about the iron-iron bond, whereas bridging carbonyl
opening to form nonbridged rotamers is the rate determin-
ing step for interconversions seen in the ruthenium com-
plexes. The difference in rates of interconversion be-
tween homologous iron and ruthenium complexes may be
attributed to multiple metal-metal bond orders in the
ruthenium complexes.

Temperature dependent carbon-l3 nmr studies of
some dinuclear cobalt carbonyl complexes have revealed
that these complexes are stereochemically nonrigid, and

that the carbonyl interconversions observed do not proceed

 

 

 

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William David Vernon

via the well-known Cotton-Roberts carbonyl interchange
mechanism. By modeling several conceivable mechanisms,

it has been determined that carbonyl rearrangement in
complexes with one non-carbonyl bridging ligand is
effected through a concerted bridge-opening,bridge-closing
process.

Similar temperature dependent carbon-l3 nmr
studies of some five-coordinate organotricarbonyl-iron and
-ruthenium complexes indicate that these complexes are
fluxional, and that it is necessary to observe the nmr
spectra of some of these complexes at very low temperatures
in order to fully describe the rearrangement processes.

Studies of the carbon-l3 T1 relaxation times of
benzene and cyclohexane with added E£i§(acetylacetonato)
chromium(III) and -iron(III) demonstrate that the iron
relaxation reagent is more efficient than the chromium
relaxation reagent. It was also noted that benzene car-
bon Tl relaxation times are decreased more rapidly than
those of cyclohexane.

It has been demonstrated that selenium-77 magnetic
resonance measurements are easily obtained, and that the
use of E£i§(acetylacetonato)chromium(III) as a shiftless
relaxation reagent enhances observation of selenium-77
nmr signals of compounds with long selenium-77 T1 re-
laxation times. Measurement of selenium-77 Tl relaxa-

tion times as a function of temperature and measurement

William David Vernon

of nuclear Overhauser enhancements have established that
the dipole-dipole relaxation mechanism is of little
importance in selenium-77 nmr. It was ascertained that
the spin-rotation mechanism dominates Tl relaxation in
dimethylselenide. Studies of similar tellurium com-
plexes by using tellurium nmr techniques were not as

successful as the selenium studies.

NUCLEAR MAGNETIC RESONANCE STUDIES
OF SOME ORGANOMETALLIC MOLECULES

By

William David Vernon

A DISSERTATION
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of
DOCTOR OF PHILOSOPHY
Department of Chemistry

1975

 

 

 

 

 

 

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ACKNOWLEDGEMENTS

There are many people to whom I owe a debt of
gratitude for their help in my educational experience. I
wish to express my thanks to Dr. Otto A. Gansow for his
guidance of my research activities for the last five
years. He took me in as a senior at Rice to do under-
graduate research, he strongly encouraged me to make the
move to Michigan State, and he has been neither over-
bearing nor lax in his direction of my research. I
would also like to thank the powers-that-be at Rice Uni-
versity and at Michigan State University for making my
transition in midstream an easy one.

For my synthetic background I would like to express
my appreciation to Dr. Donald A. Schexnayder and Dr.
Aaron R. Burke. The sound inert atmosphere techniques
which they imparted to me have made my projects much less
frustrating. I gratefully acknowledge the influence of
my high school chemistry teacher, Mr. Alvin F. Moudy, who
continually encouraged me to the point of providing
supplementary materials to expand my knowledge and who has
maintained an interest in my progress since those days.

I must also mention my appreciation to Mr. Jim

Grannan at Rice and to Mr. Wayne Burkhardt and Mr. Frank

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Bennis at Michigan State for their efforts in keeping the
Bruker in operating condition. Thanks to Dr. Thomas A.
Hardy, formerly of this chemistry department, now at
Stauffer Chemical Co., for his help and concern during
our trips to New York. Thanks also to Ms. Diane Mitchell,
who did the artwork for this dissertation, and to Mrs.
Lee Burkhardt for her meticulous typing.

I wish to extend special appreciation to my wife,
Susan, whose love and understanding have always been
there, even though at times neither of us realized it.
May the years get better! And finally, I express my most
profound thanks to my parents. Since my earliest re-
collections they have encouraged my curiosity and my
attempts to learn. They have backed me economically when
necessary, but more importantly, they have backed me with
intangibles that I can never repay. It is to my parents

that I dedicate this work.

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TABLE OF CONTENTS

Page
INTRODUCTION . . . . . . . . . . . . . . . . . . . . 1
Chapter
1 CONCEPTS IN NMR . . . . . . . . . . . . . . 5
The NMR Experiment . . . . . . . . . . 5
The Nuclear Overhauser Effect . . . . . . . 23
T1 Relaxation Mechanisms . . . . . . . . . . 30
Chemical Shift Theory . . . . . . . . . . . 42
2 A CARBON-l3 AND PROTON MAGNETIC RESONANCE
EXAMINATION OF SOLUTE STRUCTURES, EQUILIBRIA,
AND STRUCTURAL INTERCONVERSIONS IN SOME
DINUCLEAR nS-DIENYL RUTHENIUM, IRON, AND
NICKEL CARBONYLS . . . . . . . . . . 52
Background . . . . . . . . . . . . . . . . . 52
Experimental . . . . 66
Acquisition and Treatment of Spectral Data . 74
Results . . . . . . . . . . . . . . . . . . 81
Discussion . . . . . . . 123
Suggestions for Subsequent Research . . . . 142
3 A CARBON-l3 MAGNETIC RESONANCE EXAMINATION
OF THE FLUXIONAL BEHAVIOR OF SOME FIVE-
COORDINATE ORGANO-IRON AND -RUTHENIUM
CARBONYL COMPLEXES . . . . . . . . . . . . . 145
Background . . . . . . . . . . . . . . . . . 145
Experimental . . . . . . . . . . . . . . . . 150
Results . . . . . . . . . . . . . . . . . . 151
Discussion . . . . . . . . . . . . . . . . . 159
4 SELENIUM-77 NMR,MEASUREMENT OF T1 RELAXATION
TIMES AND ELUCIDATION OF T1 RELAXATION
MECHANISMS OF THE SELENIUM-77 NUCLEUS IN SOME
ORGANOSELENIUM COMPOUNDS. THE UTILITY OF
TRIS(ACETYLACETONATO)CHROMIUM(III) As A RE-
EAXATION REAGENT IN SELENIUM- 77 NMR . . . . 163

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Chapter Page
4 Background . . . . . . . . . . . . . . . . . 163
Experimental . . . . . . . . . . . . . . 168

Results and Discussion . . . . . . . . . . . 172

5 THE EFFECT OF TRIS(ACETYLACETONATO)CHROMIUM

(III) ON CARBON:I§ T1 RELAXATION TIMES . . . 193
Background . . . . . . . . . . . . . . . . . 193
Experimental . . . . . . . . . . . . . . . . 196
Results and Discussion . . . . . . . . . . . 197

6 DINUCLEAR COBALT CARBONYLS IN SOLUTION. A
CARBON-l3 MAGNETIC RESONANCE EXAMINATION OF
THEIR STRUCTURES, EQUILIBRIA, AND STRUCTURAL

INTERCONVERSIONS . . . . . . . . . . . 205
Background . . . . . . . . . . . . . . . . . 205
Experimental . . . . . . . . . . . . . . . . 217
Results . . . . . . . . . . . . . . . . . . 219
Discussion . . . . . . . 256
Suggestions for Subsequent Research . . . . 277
REFERENCES . . . . . . . . . . . . . . . . . . . . . 280

 

 

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12.

13.

LIST OF TABLES

Page
Physical Data for the Previously Unreported
5

Complex, [(n -C9H11)Ru(CO)2]2 (VII) . . . . 70
Chemical Shift and Rate Constant Data for
[(nS-C5H5)Fe(CO)2]2 (I) . . . . . . . . . . 83
Chemical Shift and Rate Constant Data for
[(nS-CH3C5H4)Fe(CO)2]2 (II) . . . . . . . . 85
Chemical Shift and Rate Constant Data for
[(nS-C9H11)Fe(CO)2]2 (III) . . . . . . . . . 87
Chemical Shift and Rate Constant Data for
[(nS-(C9H7)Fe(CO)2]2 (IC) . . . . . . . . . 89
Chemical Shift, Rate Constant Data, and
Isomer Distribution for [(nS-C H )Ru(CO) ]

S 5 2 2
(V)

. . . . . . . . . . 91
Chemical Shift and Rate Constant Data for
[(nS-CH3CSH4)Ru(CO)2]2 (VI) . . . . . . . . 94
Chemical Shift and Rate Constant Data for
[(n5-09H11)Ru(00)212 (VII) . . . . . . . . . 96
Chemical Shift and Rate Constant Data for
[(n5-09H7)Ru(CO)2]2 (VIII) . . . . . . . . . 98
Chemical Shift and Rate Constant Data for
(nS-C5H5)2FeNi(CO)3 (IX) . . . . . . . . . .100
Chemical Shift Data for [(nS-C H )Ni(CO)]

5 5 2
(X) . . . . . . . . . . . . . . .102
Thermodynamic Parameters for Cis-Trans
Equilibria . . . . . . . . . . . . . . . . -116
Activation Parameters for Bridge-Terminal
Carbonyl Interchange Processes . . . . . . ~ll8

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15.

16.

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19.

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24.

Mass Spectral Data for the Ruthenium
Complexes (V)-(VIII) . . . .

Structural and Activation Energy Data for

Some Valence Orbital Isoelectronic

nS-dienyl Metal Carbonyls

77Se T1 Data for (C6HSSe)2 with Added

Cr(acac)3

77
Se Tl Data for C6H58eH .

77
Data for (CH3)ZSe

Se T1

Chemical Shift and Linewidth Data for
Co (CO) (XIII) . . . . . .

2 8
Chemical Shift and Linewidth Data for
C02(CO)7(CAH202) . . . .
Chemical Shift and Linewidth Data for
C02(CO)7 Ge(C6H5)2 (XV) .

Chemical Shift and Linewidth Data for
C02(CO)6 DPPM (XVI) . . . .

Chemical Shift Data for [C02(CO)7]2 DPPA

(XVII)

Page

- 120

- 134

. 174
. 180
. 188

- 222

225

228

231

234

Chemical Shift Data for (nS-C5H5)Fe(CO)2Co(CO)4

(XVIII) in Various Solvents

vii

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LIST OF FIGURES

Figure Page
1 Energy versus field strength diagram for
a spin-% nucleus . . . . . . . . . . . . . . 8
2 Transitions and Boltzmann population dis-
tributions for a spin I = % nucleus . . . . 10
3 Precession of a magnetic moment, i, about
a magnetic field HO . . . . . . . . . . . . 14
4 (A) A pulse which rotates the magnetization

vector an angle 8 onto the XY plane; (B) loss
of coherency by the magnetization vector; the
component vectors "fan out" in the XY plane 18

5 Energy level diagram for the interaction of
two spin-k nuclei. The W's represent transi-
tion probabilities . . . . . . . . . . . . . 24

6 Diagram of two nuclei I and S showing their
relationship to each other and to the magnetic
field, which is along the X axis . . . . . . 33

7 Representation of a T1 experiment. (A) A 180°
pulse is applied; (B) the system is allowed
(to relax for a period I; (C) a 90°pulse is
applied; (D) the magnetization vector returns
to equilibrium . . . . . . . . . . . . . . . 4o

8 Description of a terminal M-CO bond,(I)
illustrates the overlap of a filled carbon
0 orbital with an empty metal d-orbital; (II)
represents the overlap of a filled metal d—

orbital with empty n“ orbitals of co . . . . 46
9 The crystal structure of trans:

[(n5-c5H5)Fe(c0)2]2 (I) . . . . . . . . . . 53
10 Postualted structures in the Manning three

structure hypothesis for solution equilibria

of [(n5-05H5)Fe(C0)2]2 (I) . . . . . . . . . 58

viii

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Figure Page
11 Schematic view of the carbonyl interchange
predicted by Cotton for complexes with
bridging and terminal carbonyls . . . . . . . 61
12 A mechanism for cis-trans isomerization and

carbonyl interchange in dimetallic

(n5-dienyl) metal carbonyls and related com-

plexes . . . . . . . . . . . . . . . . . . . 64
13 The [(nS-dieny1)M(CO)2]2 complexes utilized in

the study of solution dynamics outlined in

Chapter 2 . . . . . . . . . . . . . . . . . 80
14 Variable temperature carbon-l3 nmr spectrum

of the carbonyl region of

[(nS—CSHSFe(CO)2]2 (I) . . . . . . . . . . . 80
15 Variable temperature carbon-13 nmr spectrum

of the carbonyl region of

[(nS-CH3C5H4)Fe(CO)2]2 (II) . . . . . . . . 84
16 Variable temperature carbon-l3 nmr spectrum of

the carbonyl region of

5

[(n -C9H11)Fe(CO)2]2 (III) . . . . . . . . . 86
17 Variable temperature carbon—l3 nmr spectrum of

the carbonyl region of

5

[(n -C9H7)Fe(CO)2]2 (IV) . . . . . . . . . . 88
18 Variable temperature carbon-13 nmr spectrum

of the carbonyl region of

[(nS-C5H5)Ru(CO)2]2 (V); the highest field peak

is CS2 . . . . . . . . . . . . . . . . . . . 90
19 Variable temperature carbon-l3 nmr spectrum of

the carbonyl region of

[(nS-CH3C5H4)Ru(CO)2]2 (VI); the highest field

peak is CS2 . . . . . . . . . . . . . . . . 93
20 Variable temperature carbon—13 nmr spectrum of

the carbonyl region of

[(ns-C9H11)Ru(CO)2]2 (VII); the highest field

peak is C82 . . . . . . . . . . . . . . . . 95

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21

22

23

24

25

26

27

28

29

30

Page
Variable temperature carbon-13 nmr spectrum
of the carbonyl region of
[(n5-09H7)Ru(CO)2]2 (VIII); the highest field
peak is CS2 . . . . . . . . . . . . . . . . 97
Variable temperature carbon-13 nmr spectrum
of the carbonyl region of
(nS-C5H5)2FeNi(CO)3 (IX); the higher field
peak is CS2 . . . . . . . . . . . . . . . . 99
Variable temperature carbon-13 nmr spectrum
of the carbonyl region of
[(n5-05H5)Ni(00)12 (X); the higher field
peak is C82 . . . . . . . . . . . . . . . . 101

A more realistic depiction of the solution

structure of (nS-dienyl) metal carbonyls and
related complexes . . . . . . . . . . . . . 106

Plot of chemical shift vs. temperature for
[(nS-C5H5)Ru(CO)2]2 (V) in various solvents 109

Temperature dependent proton nmr spectrum of
[(nS-C9H7)Ru(CO)2]2 (VIII). Spectra (a)-(c),
50% CDZClz-CS2 solution; (d) CD2C12 solution 111

Description of a bridging M-CO bond.

(a) illustratesthe overlap of a filled CO
o-orbital with two empty metal o-orbitals;
(b) illustrates the back donation ~rom two
filled metal d-orbitals to empty n CO
orbitals . . . . . . . . . . . . 127
Schematic diagram of potential energy vs.
reaction coordinate for structural inter-

conversion in [(nS-C5H5)Fe(CO)2]2 (I) . . . 129

Schematic diagram of potential energy vs.
reaction coordinate for structural inter-

conversion in [(nS-C5H5)Ru(CO)2]2 IV) . . . 132

Some mechanisms which might effect carbonyl
interchange in singly-bridged complexes.

(a) unbridged intermediate; (b) triply-

bridged intermediate; (c) a concerted

mechanism . . . . . . . . . . . . . . . . . 139

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Figure Page
31 Structure of the ligand 2- -vinylpheny1(di-
pheny1)phosphine (SP) . . . . . 149
32 Geometrical isomers of the (SP)M(CO)3

complexes; (a)-(c) trigonal bipyramidal
structures; (d)-(f) square pyramidal
structures . . . . . . . . . . . . . . . . 149

33 Variable temperature carbon—13 nmr spectrum
of the carbonyl region of cycloheptatrienetri-
carbonyliron(0); the highest field peak is
C82 . . . . . . . . . . . . . . . . . . . . 152

34 Variable temperature carbon-l3 nmr spectrum
of the ring carbon region of cyclo-
heptatrienetricarbonyliron(O); the highest
field peak is TMS . . . . . . . . . . . . . 153

 

35 Variable temperature carbon—l3 nmr spectrum
of the carbonyl region of norbornadienetri—
carbonyliron(0); the high field peak is C32 . 154

36 Variable temperature carbon-13 nmr spectrum
of the ring carbon region of norbornadienetri-
carbonyliron(0); the highest field peak is TMS 155

37 Variable temperature carbon—13 nmr spectrum

of the carbonyl region of (SP)Fe(CO)3 . . . . 156
38 Variable temperature carbon- 13 nmr spectrum

of the carbonyl region of (SP)Ru(CO)3;. the

highest field peak is CS2 . . . . . . 157
39 Figurative representation of the Berry

mechanism . . . . . . . . . . . . . . . . . . 161
40 Plot of T1 vs. concentration of Cr(acac)3 for

the selenium-77 nucleus in (CGHSSe)2 . . . . 173
41 Plot of (T1)'1 vs. concentration of Cr(acac)3

for the selenium-77 nucleus in (C6HSSe)2 . . 176
42 (a) Selenium-77 nmr spectrum of C6HSSeH with

no proton decoupling; (b) spectrum with proton
decoupling; (b) spectrum with proton decoupl-
ing and in the presence of Cr(acac)3; spectrum

with proton decoupling, no Cr(acac)3 present;
note that all the integrals are approximately
equal . . . . . . . . . . . . . . . . . . . . 179

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43

44

45

46

47

48

49

50

51

.52

53

Plot of T1 vs. temperature for the

selenium-77 nucleus in 06HSSeH .

(a) Free-induction decay (FID) of the

selenium-77 nucleus in (CH3)ZSe; (b) the

Fourier transform of this FID; (c) the

selenium-77 spectrum of (CH ) Se obtained
3 2

by Lardon . . . . . . . . . . . . . . .

(a) Selenium-77 nmr spectrum of (CH3)ZSe

with no proton decoupling; (b) spectrum with
proton decoupling and in the presence of
Cr(acac)3; (c) spectrum with proton de-

coupling, no Cr(acac)3 present; note that
all the integrals are approximately equal

Plot of T1 vs. temperature for the
selenium-77 nucleus in (CH3)ZSe

(a) Free-induction decay of the tellurium-
123 nucleus in (CH3)2Te; (b) the Fourier
transform of this FID

Plot of T1 vs. concentration of Cr(acac)3

for the carbon-13 nuclei in cyclohexane

Plot of T1 vs. concentration of Cr(acac)3
for the carbon-13 nuclei in benzene

Plot of (T1)"1 vs. concentration of
Cr(acac)3 for the carbon-13 nuclei in

cyclohexane

Plot of (T1)-1 vs. concentration of
Cr(acac) for the carbon-l3 nuclei in
benzene .

Plot of (T1)'1 vs. concentration of
Fe(acac)3 for the carbon-13 nuclei in
cyclohexane.

Plot of (T1).1 vs. concentration of Fe(acac)3

for the carbon-l3 nuclei in benzene

xii

Page

181

184

187

189

191

199

200

201

202

203

204

 

 

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.0. s .u. .1, I. t s I. Add V .) H1
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But

 

 

Figure

54

55
56

57
58

6O

61

62

63

64

65

66

Several structures postulated for C02(CO)8
(XIII) . . . . . . . . . . . . . . . . . .

Crystal structure of C02(CO)7(C4H202) (XIV)

Solution equilibrium postulated by Noack and
Bor for C02(CO)8 (XIII)

Crystal structure of C02(CO)8 (XIII)

The dinuclear cobalt carbonyl complexes
utilized in the study of solution dynamics
outlined in Chapter 6

Variable temperature carbon-13 nmr spectrum
of C02(CO)8 (XIII); the higher field peak is
C82 . . . . . . . . . . . . . . . . . . . .
Variable temperature carbon-13 nmr spectrum
of the carbonyl region of C02(CO)7(C4H202)
(XIV); the highest field peak is CS2 .
Variable temperature carbon-l3 nmr spectrum
of the carbonyl region of C02(CO)7Ge(C6H5)2
(XV); the highest field peak is CS2

Carbon-13 nmr spectrum of the carbonyl region
of C02(CO)7Ge(C6H5)2 (XV) at -164°C

Variable temperature carbon-13 nmr spectrum
of the carbonyl region of
C02(CO)6[(C6H5)2PCH2P(C6H5)2] (XVI); the
highest field peak is CS2

Carbon-13 nmr spectrum of the carbonyl region
of C02(C0)6[(C6H5)2PCH2P(C6H5)2] (XVI) at
-l64°C . . . . . . .

Variable temperature carbon-13 nmr spectrum

of the carbonyl region of
[C02(C0)7]2[(C6H5)2PCCP(C6H5)2] (XVII); the

highest field peak is CS2

Variable temperature carbon-l3 nmr spectrum
of the carbonyl region of

(nS-CSHS)Fe(CO)ZCO(CO)4 (XVIII)

xiii

Page

206
209

211
213

216

221

224

226

227

229

230

233

236

fi

.p'

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no

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2.. . . 2. .e 3 .AJ

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udJ ADJ

 

 

Figure

67

68
69

70

71

72

73

74

75

76

77

”Ambient temperature carbon-l3 nmr spectrum

of the carbonyl region of
(US-C5H5)C0(CO)2 (XIX); the high field peak
is CS2 . . . . . . . . . . . . . . . . . . .

Probable structure of C02(C0)7Ge(C6H5)2 (XV)

Computer-generated carbon—l3 nmr spectra of
C02(CO)7Ge(C6H5)2 (XV) at -102°C; (a) peaks

in 4:3 ratio; (b) peaks in 5:2 ratio;
(c) measured spectrum

Possible isomers of
C02(CO)6[(C6H5)2PCH2P(C6H5)2] (XVI)

Possible isomers of
[C02(C0)7]2[(C6H5)2PCCP(C6H5)2] (XVII)

Plot of 6C0 vs. temperature for
(HS-C5H5)Fe(CO)2Co(CO)4 (XVIII) in a variety

of solvents

Schematic depiction of Mechanism I. All
seven carbonyls are scrambled in a polytopal
rearrangement process through a trigonal
bipyramidal intermediate .

Schematic depiction of Mechanism II. Two
pairs of terminal carbonyls and the bridging
carbonyl are scrambled .

Schematic depiction of Mechanism III.
Concerted opneing of the bridging carbonyl
and closing of one terminal carbonyl
scrambles five of the carbonyls

The measured carbon-13 nmr spectrum of
C02(CO)7(C4H202) (XIV) together with

computer—generated spectra based on
Mechanism II (left) and Mechanism III
(right). . . . .

Alternate view of the rearrangement
effected by Mechanism III

xiv

Page

238
241

244

246

246

251

263

267

269

272

274

Hi

 

 

 

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INTRODUCTION

This dissertation combines topics in two widely
separated areas of chemistry, both of which have only
recently come into prominence. In 1946 Bloch et a1.1
first observed the nuclear magnetic resonance phenomenon,
and, in 1951, ferrocene was first synthesized,2 an event
which marked the beginning of modern organometallic
chemistry. By the late 1950's, nmr instrumentation was
sufficiently sophisticated so that researchers could
obtain routine proton nmr spectra, and a few organo-
metallic researchers, most notably Wilkinson, utilized
proton magnetic resonance in their studies. The advent
of a routine fluorine-19 nmr spectrometer also created an
interest in fluorocarbon-transition metal complexes during

the late 1950's.3

Studies of organometallic complexes by
using proton and fluorine nmr techniques became common-
place during the next decade. Unfortunately, proton and
fluorine nmr studies eventually became limited in scope
because, in most complexes, these nuclei are usually of
peripheral interest. For example, if one were studying
the nature of a metal-carbon bond by using proton nmr, the
spectrum obtained transmits information which has been

attenuated through at least one extra bond. Only recently,

1

 

 

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has instrumentation been developed which measures nmr
spectra of carbon and other more pertinent nuclei.

The first carbon-13 nmr signal was observed in
1957 by Lauterbur.4 During the ensuing ten years, carbon-l3
nmr was only a laboratory curiosity available to but a few
specialists who focused their attention primarily on
carbon-l3 nmr spectra of organic compounds After 1968,
when Fourier transform techniques made the acquisition of
carbon-13 nmr spectra feasible on a routine basis, studies
could be undertaken on organometallic complexes. Since
1968 the number of papers dealing with carbon-l3 nmr
studies of organometallic complexes has increased
dramatically.5

Meanwhile, other remarkable improvements of nmr
instrumentation had been brought about. In the last year
it has become possible to modify some existing nmr
spectrometers in a manner such that, with minimal effort,
the spectrometer may be tuned to any one of a wide range
of radio frequencies and therefore may be used to detect
nmr signals from any of a host of different elements.
Equipped with such an instrument, the researcher can
measure nmr spectra of several different nuclei within a
complex. The experimental work which forms the backbone
of this dissertation is based on applications of such a
”multinuclear" magnetic resonance spectrometer to prob—

lems of interest in organometallic chemistry.

 

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The arrangement of the topics of this dissertation
is as follows: a theoretical section is included initially
in order to acquaint the reader with the basic principles
of pulsed and carrier wave magnetic resonance, and of the
theory of the nuclear Overhauser effect, Tl relaxation
measurements and mechanisms, and chemical shifts. The
experiments performed and data interpretation follows next.
This portion has been divided into several chapters
according to the several organometallic systems studied.
The first chapter describes a variable temperature carbon-13
and proton nmr study of solute structures, equilibria, and
structural interconversions of [(nS-C5H5)Fe(CO)2]2 and
some of its derivatives. Following this part of the
dissertation is a short chapter which oulines a carbon-13
nmr study of some fluxional five-coordinate organotri-
carbonyliron(0) complexes. The next chapter describes
a series of selenium-77 magnetic resonance studies of
some organoselenium compounds and as well attempts to
duplicate these studies on similar tellurium compounds by
using tellurium magnetic resonance measurements. That
chapter is essentially a physical chemical study of the
selenium-77 nucleus in various environments. The next
chapter reports a series of experiments designed to in-
vestigate the mechanism by which "shiftless" relaxation
reagents interact with organic substrates to effect a
reduction in the T1 relaxation times of constituent car-

bon atoms. The last chapter presents an examination of

 

 

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C02(CO)8 and some of its derivatives in solution
similar to the study conducted in Chapter 2. The solu-
tion behavior of the parent complexes in Chapters 2 and
6 has provided the fuel for a controversy for the past
twenty years. These chapters will indicate how several
techniques which we developed7 have allowed us to sort
out the solution dynamics of these complexes as well as

others now in the literature.

 

 

 

 

 

 

 

 

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CHAPTER 1

CONCEPTS IN NMR

The NMR Experiment

 

To explain the origin of the hyperfine structure
observed in optical atomic spectra, Pauli8 hypothesized
that some atomic nuclei possess an intrinsic magnetic
moment, the magnitude of which is a function of the
charge, spin, and angular momentum of the nucleus.~ If
the spin quantum number of a nucleus is denoted by I,
then the nucleus will have (21 + 1) states, all degenerate
in the absence of an external magnetic field, the angular
momentum component of which will have the values
I, I - 1,..., -I along any direction. Therefore, as a
consequence of spin and charge, a magnetic moment u may
be defined for any atomic nucleus. The moment may be

classically viewed as a vector parallel to the angular

momentum vector and is defined as

fi= YNh 1. (l)

where YN is the magnetogyric ratio with dimensions

-l.sec-l. If the nucleus is treated as a

radians-gauss
spinning spherical particle with uniform distribution of

mass and charge on the surface, a magnetic moment arises

5

 

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which imparts to h the value

fiN = gN(efi/2MC)I = gNBNI’ (2)

where gN is the nuclear g factor, e and M are the
charge and mass of the proton, respectively, c is the
velocity of light, and 8N is the nuclear magneton.

When a nucleus with nonzero spin is placed in an
external magnetic field, the field and the magnetic moment

u interact; this interaction is described by the

Hamiltonian

H . (3)

Or, if the direction of the magnetic field is defined as

the z axis,
H = -Yfi H12, (4)

where H is a uniform external magnetic field. For a
nucleus of spin I, there are (21 + 1) energy levels
separated by a field-dependent energy. This splitting of
energy levels in the presence of a magnetic field is
termed the nuclear Zeeman effect.

The basic nmr experiment involves transitions
between nuclear Zeeman levels. Transitions are induced
by irradiating a macroscopic ensemble of nuclei in a
static external magnetic field with photons of a frequency

v which satisfy the Bohr relation

E = hv = uHO/I; or v = uBNHO/hl, (5)

 

 

 

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7
where u is the scalar magnetic moment, u = gNI; this
implies
v = gNBNH/h . (6)

In terms of the magnetogyric ratio,
v = AE/h = yHo/Zn (Hz). (7)

It is observed that in a given magnetic field Ho’ tran-
sitions between atomic energy levels are possible only at
discrete frequencies, with the magnetogyric ratio or the
magnetic moment of the nucleus being the primary factor
determining the resonance frequency. Figure 1 illustrates
the dependence of the energy difference between Zeeman
levels on both the magnitude of y and of Ho; I = %

is this case.

Since the classical nmr experiment involves
observation of energy absorption by a collection of nuclei
as transitions are induced between two energy levels,
some concern must be directed towards the relative popula-
tions of the two energy levels. As depicted in Figure l,
the nuclei in the lower energy level are described by a
spin a and those in the upper level by a spin 8. When
a macroscopic ensemble of nuclear spins is at thermal
equilibrium at a temperature T, there are slightly more
spins in the a state than in the 8 state. The ratio
of the two spin populations is given by the Boltzmann

factor exp(gNBNHO/kT), or

 

 

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Figure 1. Energy vs. field strength diagram for a spin

I = a nucleus

 

 

 

 

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Na = %N(l + gNBNHO/ZkT) and N8 = %N(l - gNBNHO/2kT).(8)

In practical terms, for carbon nuclei in an external
magnetic field of ga. 20 kilogauss at room temperature,
Na/NB is on the order of 1.00000345, or, we may rather
say that the excess population is 9a. 3.5 ppm.

The transitions which are induced by radiofre-
quency irradiation of a collection of nuclei are illustrated
in Figure 2. Let W018 and Wad denote the probabilities
per unit time for a given nucleus to undergo an upward
or a downward transition by some mechanism. These
quantities are not equal, since at equilibrium the number

of a + 8 transitions must equal the number of B + a

transitions, or

NowaB NBWBd ' (9)

By combining Equations 8 and 9 we may write
de/de = 1 + gNBNHO/kT . (10)

If n denotes the excess number of a spins, then a
transition 8 + 6 increases n by 2; thus we can write

the differential equation

dn/dt = ZNBWBG - ZNawaB . (11)
By letting W = (waB + wBa)/2’ the equation becomes
dn/dt = -2W(n - neq), (12)
where
neq = (Na + NB)gNBNHO/kT . (l3)

10

e-gNflNHO /2kT

 

 

 

e+gNBNHO/2kT

Figure 2. Transitions and Boltzmann population distribu-
tion for a spin I = % nucleus

 

 

 

 

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11

Equation 12 may then be rewritten

dn/dt -(n - ne )/T1, (14)

q

in which

Tl

1/2W. (15)
The solution to Equation 12 is then seen to be

(n - neq) = (n - neq)oexp(-t/Tl). (16)

The factor T1’ with dimensions of time, is designated as
the spin-lattice relaxation time. It represents the time
during which the difference between the excess population
and its equilibrium value is reduced by the factor l/e.
The spin—lattice relaxation time is also a measure of

the rate at which the spin system comes into thermal
equilibrium with its surroundings, also called "the
lattice"; or alternatively, it is a nonradiative transi—
tion between two energy levels. There are several
mechanisms by which spin-lattice relaxation can occur; the
nature of these processes will be discussed in a succeeding
section of this dissertation.

An important practical aspect of nmr, implicit in
the concept of the spin-lattice relaxation time,is satura-
tion. Saturation occurs when the rate of absorption of
energy exceeds the rate of the return to equilibrium;
hence, the excess population of the a spins is reduced
and therefore so is the probability of additional energy
absorption. The consequences is that no nmr signal is

observed when a spin system is saturated.

 

 

 

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12

The classical nmr experiment involves placing a
sample containing the appropriate nucleus in a strong,
fixed magnetic field (typically on the order 103 to 104
gauss) and irradiating it with radiofrequency photons by
slowly sweeping the frequency. Observation of the fre-
quencies at which nuclear spin transitions occur is the
object of the experiment. In practice, however, the
magnetic field is usually swept at a constant radio-
frequency. This continuous wave (cw) technique is
satisfactory for observation of nuclei such as the proton,
lithium-7, fluorine-l9, or phosphorus-31. These nuclei
possess large magnetogyric ratios and/or are relatively
abundant. Unfortunately, most other nuclei with spin
I 2 % exhibit neither attribute, and so special techniques
must be employed in order to observe their nmr absorp-
tions.

Hahn9 experimentally observed the effect of a
single radiofrequency (rf) pulse applied to a spin system
as suggested by Bloch, gt 31.1 The pulse experiment is
basically quite different from the cw experiment, since
it depends upon the behavior of the spin system monitored
just after the application of a single rf pulse, rather
than its being observed during the continuous application
of low-level rf energy. Since pulse techniques are used
almost exclusively in sophisticated nmr instrumentation,
the following discussion will focus on the theory of pulse

techniques.

 

 

 

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“ ‘4

-.

-

. .-

" v“
I Q...
o. .

~u-a.‘

’ H
“'b "'h

m. ..H V].
" an
- .
4,. P»
.7-.. :1” 3
u-v"“v ;
.v'J‘ ‘
.-
I.
.. '
p
Ll
A

 

 

.

.

'c.

.

.w z - __
f"!
if.

:‘u. -

. ~-_;

I ‘>

._ *L’

l

'l

to

r -
(n

 

13

To understand the processes that occur during a
pulsed nmr experiment, it is necessary to begin with a
classical mechanical treatment and then to proceed to the
Bloch equations. From classical mechanics, it is known
that the torque exerted on a magnetic moment by a magnetic
field H0 at some angle 0 induces the moment to precess
about the direction of the field with a frequency given

by the Larmor equation:
v0 = -YHO/2n (Hz), (17)

Energy absorption occurs only at the resonant condition
v0 = Vrf. This situation is pictured in Figure 3. The
magnetic vector component of the applied rf field rotates
in the xy plane perpendicular to H0 at the Larmor frequency.
Upon absorption of energy from H1, the net magnetic moment
vector is tipped to a different value of 0, but the value
of v0 remains constant.

During an nmr experiment the behavior of a large
number of nuclear spins is observed. For this reason it
is necessary in our analysis to treat the sample system
as an ensemble of nuclear spins. As depicted in Figure 3,
the z axis is defined as the direction parallel to the
applied, fixed magnetic field, HO. Oriented along this
axis is the net macroscopic magnetization vector M which
is the resultant vector of the excess spins a aligned in
the direction of Ho‘ The rate of change of the angular

momemtum p of a spinning nucleus is proportional to the

 

 

 

 

 

 

14

 

 

Figure 3. Precession of a magnetic moment, u, about a
magnetic field HQ

 

 

 

. ‘
v' . -
\ .-
v"'
. .-
V '
. .
4‘-"
C“
—
,O'. >
- -' I
s "‘
. u-~--"
'-
~- 9.»...
- ,.,
...~ .~. -.
...>.. ..
‘ A
..—-A- -
F I n "
he. _
, ..
-..-- ,‘c. .
.
._v.
. r-
u-_
..
., .
._~ 3
.-
I "

 

\

a
-
t

n"
(I)

( 1'
(‘D

 

15

cross product D x H, or the torque, exerted on the

nuclear magnetic moment by the field H, or
dp/dt = D x H . (18)
Since fi = yp, it is also true that

dm/dt = Y(dP/dt) = Yfi x fi; (19)

~

or, since M = Zfi,

~

dM/dt = VM x h . (20)

The term H in the preceeding equations includes both the
static field HO and the magnetic vector of the applied

field H1. The components of H are therefore

HX = H1 cos wt
Hy = -H1 cos wt (21)
H = H ,

z o

where m is the angular frequency of the vector. By
combining these equations with Equation 20, the time de-

‘pendence of the components of M may be calculated:

dMX/dt = y(MyHO + MZHl Sin wt)
dMy/dt = y(MzHl cos wt - MxHo) (22)
sz/dt = -y(MXH1 sin wt + MyHl cos wt)

The relaxation times T1 and T2 are next introduced into
the equations. The return of Mx and My to the equilibrium
values of 0 and M2 to its equilibrium value MO is assumed
to be a first-order process. T2 is the time constant

describing the process in the x and y components of M and

 

Iv!

..,

'v

on.

0..

(l‘

 

 

.n'

a
4
‘

.p-

lt'

a

 

 

 

16

T1 is the time constant describing this process in the 2
component. The Bloch equations in final form are then

seen to be:

dMX/dt y(MyHO + MZH1 sin wt) - Mx/TZ

dMy/dt = y(MZHl cos mt - MxHo) - My/T2 (23)
dMZ/dt = -y(Mle sin mt + MyH1 cos wt) - (Mz - Mo)/T1’

T1 is called the longitudinal relaxation time and is
entirely equivalent to the spin-lattice relaxation time
described previously. T2 is termed the transverse or
spin-spin relaxation time.

The implications of the Bloch equations are more
easily understood if the system of equations is transformed
into a rotating frame of reference. This reference frame
is simply one that rotates about H0 in the same direction
in which the nuclear moments precess. In a rotating

reference frame, Equation 20 becomes

~

(dM/dt)rot = yM x H - a x M, (24)

where a defines the angular frequency of rotation by
its magnitude and the axis of rotation by its direction.

Equation 24 then becomes

“v

(ail/dorot = Vfi x t + it x m/y = yM x (a + m/y>, (25)

in which the term (H + a/y) represents an effective field

Heff’ Therefore in the rotating frame of reference

(dM/dt)rot = yM x Heff. (26)

17

It is thus seen that the net magnetization M precesses
about Heff in the rotating frame of reference. In the
rotating frame, the time dependence of the 2 component of

M is given by

sz/dt = -YH1My - (M - MO)/T1. (27)

2
It is now possible to describe fully the behavior of
the magnetization vector during a pulse experiment.

When an ensemble of nuclear spins comes under the
influence of a laboratory magnetic field HO along the z
axis and an applied, pulsed magnetic field Hl perpen-
dicular to H0 (and therefore in the xy plane), both H1
and the rotating frame of reference rotate at an angular
1

velocity of m rad-sec- In the rotating frame the

effective magnetic field is

H = Ho + m/Y + H (28)

eff 1 ’
which at resonance simplifies to Heff = H1. Hence, in

the rotating frame of reference M, precesses about the z
axis at a Larmor frequency VH1. (See Figure 4A.) The
vector M precesses through an angle 0 during a time tp

given by
0 = H t . 29
Ylp (7
When H1 is removed, the nuclear moments begin to lose
energy to their surroundings and proceed to equilibrium

with a time constant Tl' When the moment is tipped onto

the xy plane, processes also occur which cause the nuclei

 

Figure 4.

 

 

 

A B

(A) A pulse which rotates the magnetization

vector an angle onto the XY plane; (B) loss
of coherency by the magnetization vector; the
component vectors "fan out" in the XY plane

 

 

 

 

 

.—
" \
a

‘
Hi

 

 

19

to exchange energy, and the vector moments thus begin to
spread out, or lose coherency, as shown in Figure 43.
Since H0 is not usually homogeneous, nuclei in different
portions of the sample experience different values of HO
and hence precess at slightly lower or higher frequencies
than does the rotating frame of reference. In this case,

~"

My proceeds to equilibrium with a decay constant TE.
When processes other than field inhomogeneity cause My

to return to equilibrium the time constant is denoted T2.
It should be noted here that the linewidth of a resonance

is dependent upon the magnitude of T2 the width at half-

height of a Lorentzian-shaped resonance is given by
V% = l/nTZ. (30)

In practice it is usual to study a spin system
which, at a fixed value of HO, contains chemically non-
equivalent nuclei, each with a different precession fre-
quency. In a pulsed nmr experiment, instrumentation is
utilized which generates an envelope of rf power approximat-
ing very closely the positive half of a square wave. The
rapid switching necessary to generate such pulses generates
Fourier components over the range v0 : 1/tp, where v0
is the monochromatic applied frequency of the instrument
and tp is the length of the applied pulse. Every frequency
component in the range v0 i l/tp is present, and any

nuclear spin which precesses at a frequency included

within this frequency range is excited by the appropriate

 

 

 

. I..—

o ‘-..b
0‘-
,..—
.- ~
- ...—
-
...v ‘
-o-‘
.v

.v'-" A
v .-
..o-v- "’

-._..- ..
it
....v _
.
'vu... -.
‘
-.- -.‘
.-

.
-vv--v‘. .

---p-. _-
.
..‘-_
n ‘ ",
.o-..,,.‘. w.
'~A '
.-."-"':‘
"rv..~‘
-
v. n
- \
" rev
' Q
.. o
\ xv- .—
— ,4
...-Io‘- .
‘n. rc-
. v4
n_."‘ H
...' .
.V‘u "
u .,
"Jh.
in
A
2. ~ .
a
, ,_"_
,—
M"§_ “‘\
‘55.
.-
L vu.‘
ru~ l A
H x
I “v
P' .
“A
i.: 'v
- "b!
.l
-, -
.VA'..I
..
u. H) v
5“
I
.
5V.
-

 

 

20

frequency. A short rf pulse is therefore equivalent to
the simultaneous application of a wide range of radio-
frequencies to the spin system.

With the aid of Figure 4A, we will consider the
effect of a pulse on a spin system. After a pulse of
duration tp’ the magnetization vector M is tipped to the
angle described in Equation 29. After the pulse, the
vector M lies somewhere in the xy plane, and a signal is
induced in the detector coil in the xy plane which is
proportional in strength to the magnitude of Mxy' The
signal produced is a free induction signal, since thenuclei
responsible for generating it precess without applied
rf. As transverse relaxation occurs, the free induction
signal decays as an exponential with time constant T3,

‘I

where T; is the effective T2. Its value is determined
from Equation 30, where T: replaces the T2 term and v%
includes line broadening from field inhomogeneities. If
the pulse is not precisely centered on the Larmor fre-
quency, or if there are several chemically-different
nuclei present in the sample, the free induction decay
(FID) is not a pure exponential function but instead is a
more complex waveform exhibiting interference effects
identical to the "ringing" observed in conventional cw
nmr.

The free induction decay observed is actually an

10

nmr spectrum in the time domain. It has been shown that

 

 

 

—. 4-

.-
-—-‘o..

'-

u

a“

o
‘c

'-.
7 0-
.“DE if.
C

_U
: .--
.'
.--"‘ ~
‘w". '-
,_ .
.v“
b.‘ .A
.3: u» ‘
','--vv- ‘\
' .
‘0'... 5
. .
..-,

y‘-..

(I)

t
'A‘

t...

.
q
\— a

‘e._'

i

 

 

 

21

the time domain spectrum and the more useful frequency
domain spectrum are Fourier transforms of one another,

the Fourier transform pair being defined as

F(m) = I f(t)exp(-iwt)dt
-oo (31)
f(t) = I (1/2n)F(m)eXp(imt)dm
Modern computers, by using the modified Cooley-Tukey
algorithm,11 can effect the Fourier transform in a matter

of seconds. The resulting frequency domain spectrum may
then be analyzed in the conventional manner.

The power and utility of the application of pulse
techniques to the measurement of nmr spectra can now be
made clear. By using conventional cw techniques, a
spectrum is obtained in a matter of minutes by slowly
sweeping through the entire frequency range over which the
nuclei contained in the sample may possibly precess. The
pulse techniques (hereafter referred to as Fourier Trans-
form NMR, or FTNMR), have the effect of examining the
entire spectral width in a few hundred microseconds, and
the FID is usually monitored for several hundred milli-
seconds. liconventionaltime delay for use between pulses
is perhaps one second, so in one minute a spectrum may be
scanned sixty times, each scan fully equivalent to one cw
scan. Each FID is coherently added to the preceeding
FID's and the coherently—added FID's are stored in a

computer. Sensitivity enhancement can be achieved by

 

 

 

 

uguv ~.~

 

 

 

.ne’ "7
. a
...b -‘15
r _- 'n‘
| ‘
._ ...t
, l
-.-a V‘ R
u \
..... .- 3
..- -
—-\.
‘.-~¥ .-
.
.. .-’.. ...
.F‘
. h- b.

 

 

22

employing a large number of transients per spectrum,

since the signal-to-noise ratio increases as /N, where
N is the number of transients. Likewise, a savings in
time is achieved; the use of FTNMR generally results in
a time-saved factor of 10 to 100 as compared to the use

of conventional techniques.

.7 V...—

s“

 

 

Linn-LI;

SW

run-v,

 

ii... ”We;

 

 

.
.
- I'-
.-—
..
' '5 c.
,...4
.
- - -:V‘ ‘
.
coo—J.
-u -
....V ‘

4 ..._ A-
.-
ncuubb up

--~v--.—

— v
o-vb-o-r-h

7“ W". A
*7- -~.- ,,
‘O‘ as."
F .
"' -..

 

~..._,_‘

“1-
." ts.-- .
-..
‘au _
..
-~“'“C
‘ .
l-u
§
.. ‘
s
_ .
C u
"h’“
h“ -
u-.. ‘
.-~
_
. ~V~VI .
t»‘.
I. ‘
._-
- \..V~.
“'5 L.
‘ \L
.-
.‘--\,.~
"“t,» H
-‘D

 

U)

 

 

23

The Nuclear Overhauser Effect

 

The nuclear Overhauser effect is defined as the
change in integrated intensity of the nmr absorption of a
nuclear spin system as the result of the concurrent
saturation of another resonance. This effect was pre—
dicted by Overhauser,12 who originally applied it to
electron spin resonance. His calculations indicated that
saturation of an electronic resonance should increase the
amount of nuclear polarization and, consequently, increase
the magnitude of the associated nuclear resonance. The

13 then further predic-

effect was subsequently observed,
tions were made that a similar effect should be manifest
in nmr.

The nuclear Overhauser effect is commonly seen in
molecules containing two different species,both with spin
l/2. The most frequently encountered instance is in
carbon-l3 nmr, in cases in which the saturation of the
energy levels of protons coupled to carbon atoms is
observed to enhance the intensity of the carbon reso-

nanCes. The energy level diagram for a two-spin system

(both spins = %) is shown in Figure 5, where it has been

assumed that the two nuclear species are loosely J-coupled.

If the spins are labelled I and S, we find that the

energy levels are

 

 

H

 

 

 

 

24

 

2 an W2 W3
\\\
W11 \WO\\
3.130
[M
4. " 33

Figure 5. Energy level diagram for the interaction of
two spin % nuclei. The W's represent
transition probabilities.

T‘“ aim-g3!
- a

 

 

 

 

 

hr“ -.'-'. . "

 

 

 

.
.l

.

O

.1
‘ D
I r‘
:I .»
.-

. I I. .
.v“"‘. ‘
‘ a
,....-O.

.
I-I. ""

vI-An v“ ‘

O ‘ I
v-.-. -.
‘ ‘
I‘.._..‘

(I)

 

 

‘b
1"
",.,Y-
5‘:

 

25

level 1 spin I is a, spin S is a-aa
level 2 spin I is a, spin S is B-oB (32)
level 3 spin I is B, spin S is a-Ba
level 4 spin I is 8, spin 8 is 8—88

The Wh terms in the figure represent the four transition
probabilities which describe the spin lattice relaxa-
tion times. Wi represents the single quantum transition
probability that spin I will undergo the transition
a + B or B + a while spin S remains unchanged; W?
represents the analogous transition for spin S; W2 re—
presents the two quanta transition probability for
simultaneous relaxation of two spins in the same direction,
do + BB or B8 + dd; and W0 represents the zero quantum
transition probability for a mutual flipping of the spins,
dB + Bo or Ba + dB. The equilibrium populations of the
levels are denoted PO, where P? = K exp(-Ei/kT), (Ei is the
energy of level I) and K is the normalization constant.

The approach of Pi to equilibrium is given by the
equation

_ _ o _ _ 0

Now the longitudinal magnetization M2 is related to the
average value of the nuclear spin operator IZ by the

relation
MZ - Nyfi<lz> , (34)

where N is the number of spins per unit volume. Since the

case in which the spins form a loosely coupled system is

, r
»

 

 

 

.
, .,
- .-
--'-.
‘ ...
q 5 .
.
q
l
-fi._--_a
‘w... -‘ _
'In-
‘ D
"On.
.
a.;.
'O
~~
'IA
.
DA.
I
1‘
A.
.‘~

 

’.'.-..1

«‘:
‘1
_‘ ~
‘,
J

26

of interest, the energy levels may be viewed simply as
eigenfunctions of I2 and consequently it is possible to
discuss the magnetization of each single spin. The

magnetization of a spin k is given by
= i
<Iz<k>> i MkPi . (35)

i
where Mk is the quantum number Iz(k) appropriate to

level 1; and the summation is carried out over all energy
levels (there are four such energy levels in a two spin =
% system). From this equation it is found for the two

spin-% system that

<IZ> = %P1 + %P2 - gP3 - %P4
and (36)

From Equation 33 we obtain the result

dP
l _ o 0
EE‘ ‘ ‘(le + w13 + W14)(P1 ‘ P1) + w12(P2 ‘ P2)

0 O
+ W13(P3 - P3) + 1.114(1)4 - P4) , (37)

where the other quantities P2, P3, and P4 yield similar

results. By combining Equations 35 and 36 we observe that

d<I >
z

_ o _ _ 0
dt ‘ ‘(w13 + W14)(P1 ‘ P1) (”24 + w23032 P

2)
(38)

. o . O
+ (”13 + w23)“)3 ' P3) + (”14 + W24)(P4 ‘1 4)'

.‘vo-

 

 

 

.3,

1..

.Al‘

\..‘

27

This equation may be simplified in the following manner:

. . . . _ _ I
in the loose coupling limit W13 — W24 — W1, W12

w23 = O, and W14 = W2. With these modifications,

equilibrium value of IZ is
= L 0 L O _ L 0 - L 0
IO 2P1 + 2P2 2P3 2P4 ,

which enables us to simplify Equation 38 to

d<I >

z _ I _ - 0
dt — -2Wl[<Iz> - IO] - WOL(P2 - P

2) ’ (P3

 

O O .
' w2[(Pl ' Pl) ‘ (P4 ' P4)J

S

= w = W1,

34
the

(39)

O
- p3)3

(40)

By applying Equation 36 to Equation 38 we find that

 

 

<Iz> + (Sz> = P1 - P4
and
<IZ> - <Sz> = P2 - P3;
hence
d<IZ>
dt = -OI[<IZ> - IoJ - OISE<SZ> - So]
and
d<SZ>
dt = -pS[<Sz> - So] - OSIRIZ> - IoJ ’
where
_ I = II
pI 2W1 + W0 + W2 l/Tl ,
_ S . _ SS
pS — 2W1 + NO + W2 - l/Tl ,
. _ _ IS _ SI
and 018 - OSI — l/T1 — l/T1

(41)

(42)

(43)

28

The 0 terms are relaxation rates and the 0 terms are
cross relaxation terms. ~'The cross relaxation terms will
be nonzero if and only if a relaxation mechanism is
present which couples spins I and S. The three spin—
lattice relaxation mechanisms which couple spins I and S
are the dipole-dipole relaxation mechanism and the two
scalar relaxation mechanisms, mechanisms which will be
discussed in depth in an ensuing section of this disserta-
tion. The cross relaxation terms 018 and 081 are the
terms that describe the nuclear Overhauser effect. If
the spin S is saturated so that <SZ> = 0, it is observed
that the integrated intensity of spin I, <IZ>, is

enhanced by the fraction
nI(S) = (<Iz> — 10mO <44)

compared to the equilibrium intensity, IO. At the steady

state

<IZ> = IO + OISSO/QI (45)
and the fractional enhancement becomes

nI(S) = GISSO/OIIO . (46)
The fractional form

nI(S) = ySoISS(S + 1)/yI I1(1 + 1) (47)

is derived from the relations IOdI(I + l)YI and
SOaS(S + l)yS. If S and I are heteronuclear with spin %,

and if spin I relaxes solely through dipole—dipole coupling

 

 

 

v‘

.v

..‘ .

-- Q
I \
A..-u

0-,.

in»..-

(It

(

 

 

29

to S, then

OIS/pI = 1(1 + 1)/25(s + 1) , (48)

which leads to the expression for the fractional enhance-

ment of spin I
nI(S) = yS/zyl . (49)

This expression represents the maximum enhancement of a
signal. The total maximum nuclear Overhauser enhance-

ment is given by (l + n).

 

Uh

"-
.
-.

 

 

\lrufl»li., Nb... unyrstlib E a.“

 

30

T1 Relaxation Mechanisms

 

The preceeding discussion of the nuclear Overhauser
effect demonstrates the intimate relationship between the
NOE and the nuclear spin-lattice relaxation time T1, as
defined by the cross relaxation terms of Equation 43.

The details of the several T1 relaxation mechanisms will
now be considered in this section.

When a nuclear magnetic dipole is in the presence

of a local magnetic field H it couples to the field with

L,

an energy

E = -u - HL . (50)

The nucleus under consideration is usually a component of
a molecule in motion, such as in a fluid state, so the
local magnetic fields present will fluctuate in time such

that HL is seen as an average value:

<HL(X)> = O . (51)

However, molecular motions, such as translational and
rotational motions (but not vibrational motions) randomly
produce fluctuations in the value of H1. A "typical
molecule" in a non-viscous liquid remains on the average
about 10‘12 seconds in one state of motion, after which
it undergoes a collision which changes its state of motion.
The motion of the molecule will therefore be composed of

12

frequency components ranging from O to 10 Hz. The fre-

quency component at the Larmor frequency of the nucleus

31

under observation will induce transitions between the
nuclear energy levels and therefore cause relaxation.
The result is that fluctuations in H1 induce a transfer
of energy between the spins and the molecular motions
(the lattice) and thermal equilibrium is attained.

The equation

1/Tl = (2/3)y2<H >2Tc/(1 + ngi (52)

L

relates T1 to the mean—square average of the local mag-
netic field and to a molecular correlation time TC which
characterizes the motion which induces the relaxation
process.14 All relaxation processes will be described by
a similar equation; that is, an equation consisting of a
correlation time characterizing the motion of the molecule
and a multiplier consisting of universal constants and

molecular parameters. The general form of the equation

which describes any relaxation process is then seen to be

= EET (53)

_ 2
l/Tl — R C

1
Any mechanism whereby fluctuating magnetic fields are
induced in the vicinity of a nucleus is a possible spin—
lattice relaxation mechanism. The five processes which
can link nuclei to the lattice are l) the magnetic dipole—
dipole interaction; 2) the electric quadrupole inter-
action; 3) the chemical shift anisotropy interaction;

4) the scalar-coupling interaction; and 5) the spin

rotation interaction.15

 

_ a

.h.

 

 

32

The dipole-dipole interaction is the most commonly
encountered relaxation mechanism in carbon—l3 nmr. It
arises in the following manner: if two nuclei I and S
are located in a stationary molecule in a magnetic field,
nucleus I will be influenced by a total magnetic field Ht
which has components HO from the laboratory magnetic
field and H10C from the magnetic moment “8 of nucleus S.
Now two magnetic dipoles separated by a distance r couple

with an energy
E a u - u /r3‘ (54)
dd 1 2’
hence the energy is dependent upon both the separation

and the relative orientation of the nuclear dipoles. We

then write

_ 2 2
Hloc ’ i YS/rIS(3 C03 013 ‘1)
_ 3 2-
- YSISh/rIS(3cos o - l), (55)
where O and rIS are as defined in Figure 6. If the
molecule is in motion, 918 becomes time dependent and the

average value of the local field goes to zero:

2n n 2

I [ (3cos O - l)sinOdOdo = O . (56)
o o

The motion of the molecule provides a means for nuclei I

and S to move about in space and thus causes H to

loc

fluctuate in time. This fluctuation of Hloc induces the

relaxation process. The expression which fully describes

33

c6
915

Figure 6. Diagram of two nuclei I and S showing their
relationship to each other and to the magnetic
field along the X axis

 

 

34

the dipole-dipole relaxation rate is seen to be16
_ DD _ 2 2 2 6
l/T1 — Rl — (4/3)YIYSS(S + l)h TC/r . (57)

This interaction is therefore related to molecular

geometry. It is difficult, however, to define a molecular

correlation time for the dipole-dipole relaxation mechanism.

The rotational correlation time for the intramolecular
dipole—dipole interaction, identical to that of the
quadrupolar interaction, is, for a small molecule in a
nonviscous liquid, 10.10 sec > TC > 10'12 sec.16

One final note concerning this mechanism is that
is is not absolutely necessary for nuclei I and S to be
in the same molecule. There exists an intermolecular
dipole-dipole interaction as well as an intramolecular
dipole-dipole interaction. The correlation times of the
two instances differ, but otherwise the treatment is
essentially the same.

Any nucleus with spin I > % has an electric
quadrupole moment, denoted eQ. The interaction of the
nuclear quadrupole moment with the electric field gradient,
eq, produced by the surrounding electrons provides a
very efficient mechanism for coupling the nucleus to the
rotational motions of the molecule. The quadrupolar

relaxation rate is given by17

1/Tl = RQ =
(58)

(3/40)[(21 + 3)12(21 - 1)](1 + n2/3)(e2Qq/h)rc.

 

 

35

where n is the asymmetry parameter of the field gradient
and the term (eZQq/h) is the nuclear quadrupole coupling
constant, the quantity which usually determines the
magnitude of RQ. As stated above, the rotational correla-
tion time describing this interaction is in the range
10-10 to 10'12 seconds. There can be no quadrupolar re-
laxation in a nucleus with spin-%.

A nucleus in a magnetic field experiences a field

H10C given by the expression Hlo = Ho(l — o), where o

c
is termed the shielding factor. The interaction of the
field and the shielding factor is represented by the

Hamiltonian

”GS = -th - g - I (59)

where g is the chemical shift tensor. The magnitude

of the shielding at the nucleus is dependent upon the
orientation of the molecule in the magnetic field. Through
rapid molecular motion of the molecule in solution the

fluctuations are averaged to give an average value of g:

g = l/3(oXX + Oyy + 022) . (60)

However, if 0 , o , and o are not equal, which
xx yy 22
implies that the chemical shift tensor 9 is anisotropic,
this anisotropy may create a relaxation mechanism.
If m and Cl represent the shielding along
and perpendicular to the symmetry axis of a molecule, the

relaxation rate due to chemical shift anisotropy is seen

to be

36

C3 = (2/15)y2H§(m - oi)2T . (61)

A peculiar aspect of relaxation via the chemical shift
anisotropy interaction is that the relaxation rate is
field dependent. The carbonyl carbon in acetic acid is
known to relax gig this mechanism,18 and recent work has
shown that the chemical shift anisotropy spin-lattice
relaxation mechanism contributes to the relaxation of
the lead-207 nucleus.19'20

When two nuclei I and S are ”J-coupled” the energy

is represented as

ESC = JI . S, (62)

which yields a Hamiltonian of the form

HSC = hI - A - S. (63)

In examining the relaxation of nucleus I due to fluctua-
tions in the field Hloc’ it must be considered that HO
can fluctuate in time in one of two ways; one in which S
is time dependent or the other in which A is time de-
pendent. The first instance is referred to as scalar
relaxation of the second kind; this interaction occurs
when I is relaxed by scalar coupling modulated by the
rapid relaxation of S. This type of interaction is en-
countered in carbon-l3 nmr when carbon nuclei are bonded
to transition metal nuclei which possess large quadrupole

moments, such as cobalt-59, manganese-55, or rhenium-187.

The relaxation time of the directly-bonded carbon nucleus

 

_.
. o
.A""

,.
’_,_,.-.
' 1“

_.-...o- -

_..u-¢I

l
p
O
.
r
(1

v... -

... -v ‘
s 4 4
u-v...‘ I

 

~- an. .
\‘ ~7-
ow- .-~&
‘y-A-~-

"‘4
rut-v.»

 

guy‘... ...h
~ ‘ I-

v...

w v
,1 ‘~—/.
t I
p,, .

,1
b“)" 1".
- ‘ v.

-u..(

u]: ‘2‘- .

”N...“ -
.

Pl
a, ‘..
a.

‘-

u.,_

A

.
5“}.3

.
n. ‘
J "r
‘ 0-
..,~_

.

.
'-

." '
‘va . ..
cw»

1'Y\

 

 

(I)
fr‘

 

37

is modulated by the very rapid, quadrupolar-dominated
relaxation rate of the transition metal nucleus. In the
instance of scalar relaxation of the second kind, no
multiplet splitting of S on I will be observed. When the
operator A is time-dependent, the interaction is termed
scalar relaxation of the first kind. This interaction

is observed when the relaxation of I is modulated by a
process such as chemical exchange.

The relaxation rate due to scalar relaxation is

1/1:l = RS = (2A2/3)S(S + 1)(TC/1 + (m1 - w 212 , (64)

S)
where A represents the spin-spin coupling constant and
TC represents the relaxation time Is of nucleus S in the
case of scalar relaxation of the second kind and Te, the
exchange time, in the instance of scalar relaxation of
the first kind. It has recently been shown that scalar
relaxation of the second kind governs the relaxation rate
of the tin-119 nucleus through scalar coupling to the
halogens in the compounds SnCla, SnBrA, and Sn14.21
The interaction of a nuclear magnetic moment with
the motion of a molecular magnetic moment which arises
from the fact that the electron density in a molecule does
not rigidly follow the rapidly tumbling nuclear framework
provides a mechanism by which nuclear spin energy can be

transferred to the lattice. In a system in which a single

electron revolves about a nucleus at a radius R, the

 

38

rotational frequency V is given by the eXpression

v = hJ/2nI (65)

for a molecule with moment of inertia I in rotational
level J. The electronic motion generates current
i = (e/c)V which induces a magnetic moment

J. 22 (66)

II?

= i(nR2) = (eh/2ch)J “N

“J
The magnetic moment induced by the electronic motion
produces a local magnetic field at the nucleus on the
order of uNJ/R3. As the molecule undergoes collisions
which change the direction and magnitude of its angular
momentum, fluctuations in the local magnetic field at

the nucleus are produced. The relaxation rate due to

the spin-rotation interaction is

SR

1/Tl = R = (2 IkT/h2)CZISR (67)

where C is the average value of the spin rotation tensor

and TSR is the spin-rotation correlation time. This
relaxation mechanism is unique in that Tl relaxation

times which result from its being the dominant relaxation
mechanism exhibit an inverse dependence upon temperature.

It is noteworthy also that freely-rotating groups may

induce relaxation via the spin-rotation relaxation
mechanism. Such is apparently the case in methyl iodide;

in which the relaxation time of the carbon atom is dominated

by the spin-rotation mechanism.23

39

The measurement of T1 relaxation times is best
accomplished through utilization of pulse techniques.
The most commonly employed technique involves pulsing the
sample with a (180°, T, 90°, delay) sequence. The 180°
pulse inverts the magnetization Mz along the z axis, as
demonstrated in Figure 7A. As spin-lattice (or more
appropriate to this discussion, longitudinal) relaxation
occurs, MZ decays from the value -MO through the origin
to its equilibrium value Mo' If, however, at a time I
after the 180° pulse, a 90° pulse is applied along the x
axis (Fig. 7C), the magnetization vector M2 is rotated
into the y axis and a free induction signal is produced.
Nuclear magnetic resonance spectrometers are designed such
that the detector coil is aligned along the y axis, so
that only signals oriented in the xy plane can be de-
tected. The magnitude of the signal produced is propor-
tional to the value of M2 at a time T; if a different
value of T is then used in the sequence, the T1 re-
laxation time can then be calculated. The Bloch equa-

tions describe the decay of M2 back to equilibrium as
dMZ/dt = -(M2 - MO)/Tl, (68)
which yields upon integration

MZ = Mo(l - 2exp(-t/Tl)). (69)

In practice, the quantity ln(l - A/AO) is plotted

against T, where A is the intensity of the resonance

40

A

Figure 7. Representation of a T1 experiment.

(A) A 1800 pulse is applied

(B) The system is allowed to relax for a
period T

(C) A 900 pulse is applied

(D) The magnetization vector returns to
equilibrium

H0 H0 Ho Ho
\ ,’ I
Y II \\
x .. ‘
z
B C D

 

42

Chemical Shift Theory

 

This section includes a discussion of chemical
shift theory of nuclei other than protons, with particular
attention being directed toward the carbon-l3 nucleus.

A survey of the factors affecting the chemical shifts of
carbonyl carbons bonded to metals and to organometallic
moieties follows. i

The first important theoretical interpretation of
chemical shifts observed for nuclei in molecules was pre-
sented in a series of four papers by Ramsey.24 Saika and

Slichterzs

simplified the expressions obtained by Ramsey
by approximating the total shielding constant 0 for a

particular nucleus as the sum of three terms:
_ t
o — 0d + Up + o (70)

where Od’ the diamagnetic contribution, is the Lamb
term26 for the nucleus of interest. The paramagnetic

term op is a correction arising from the molecular
environment and involves the mixing of ground and

excited electronic states by the applied magnetic field.
This term represents the contribution from magnetic fields
induced by the orbital motion of the valence electrons.
The contributions from all the other atoms in the molecule
are designated by 0', a term which overall contributes
very little to the total shielding because the inter-

action falls off as l/r3.

B.» r .-

 

.A‘

i
_..~"

,'-.--1

‘ 4
.n- "'

"Av-v
r
x._ ‘

u

U
.-‘ -
. \
.ns. .1

 

)9—1 ..

‘9'...’

"L..-

.__._‘

.
‘vx-

.OV\..

 

 

,A_- -
.4
"-.s.
.
ll
~r-
0—.
o
’u ,
u"
o
.
'5
1
v.
.
'9
-'VP
s.~ ..
.5,“
.

 

,V.
1‘?-
‘

 

 

43

The paramagnetic term was predicted by Saika and
Slichter to dominate fluorine chemical shifts in all
instances except in the completely ionic F- species.
Griffith and Orgel27 derived an expression which predicted
very large paramagnetic shifts for the cobalt—59 nucleus,
and subsequently this was demonstrated to be true. Some-
what later, Karplus and Pople28 concluded that the primary
contribution to the carbon-l3 chemical shift originates
from changes in the "local paramagnetic" term op.

29

Pople had earlier calculated the formula for the tensor

0AA, the local paramagnetic contribution to the shielding

of a carbon atom A, to be

2 2
AA e h l -3
0 = - —-2-—2 —— <r > Z Q (71)
where
= 4 36 P + P + P

QAB / AB( xAxB yAyB ZAZB)

- 2/3 (P P + P P + P P )
yAyB ZAZB ZAZB XAXB XAXB YAYB

+ 2/3 (P P + P + P

ZX X PX)
B AyB YA B

yAzB szB A

The summation Z is carried over all atoms other than A;
B

<r >2p is the mean inverse cube of the distance from

the nucleus for the carbon 2p orbital, and the PW terms
are elements of the change density and bond-order matrix

in the unperturbed molecule:

OCC.

P = 2 z

uv i CipCiv’ (72)

44

with the summation being taken over all occupied molecular
orbitals, and the subscripts xA, XB’ yA, yB, ZA’ and 2B
corresponding to the various 2p atomic orbitals of atoms

A and B. The AE term is the excitation energy between

the carbon ground state and the lowest lying paramagnetic

-3

state of the molecule. The factor <r > correlates

2p
the electron density around the carbon atom to the size

of the 2p orbital. As electron density is acquired by the

atom, the 2p orbitals may expand and the <r.3>2p term

decreases in magnitude and hence the magnitude of 0AA
likewise decreases. Added electron density to the carbon
2p orbitals therefore results in a net increase in the
total shielding and hence an upfield shift.

The total variation of the screening constant due

to changes in the 0d and 0' components were estimated

30

by Pople to be no greater than 20 ppm. These terms were

also demonstrated to be of negligible importance in the

31 32

shielding tensor of cobalt-59 and manganese-55 , two

nuclei which also exhibit very large chemical shift ranges.

Stothers and Lauterbur33

surveyed over 150

organic carbonyl compounds in order to understand the factors
influencing the carbon chemical shifts of carbonyls. These
workers concluded that the chemical shift can be rational-
ized almost entirely in terms of the shielding effect of

the electron density around the carbon nuclei. Some of

the afore-discussed quantitative concepts concerning carbon

45

chemical shifts were then applied by Maciel34 to the

chemical shifts of organic carbonyls. Maciel concluded
that carbonyl chemical shifts correlate well with the
polarity of the carbonyl w-bond; he observed that varia-
tions in the w-bond order were sufficient to explain the
observed shielding.

Gansow and co-workers first established that,
for metal bonded carbonyls, the observed chemical shifts
are dependent on the extent of carbonyl back-bonding. In
investigations of a series of complexes of the type
(n5-05H5)Fe(C0)2X,35
CN‘; and LM(CO)536, where M = Cr, Mo, or w, and L =

where X = alkyl, aryl, halide, or

phosphine, arsine, stibine, or amine, a linear correla-
tion was observed between the carbonyl chemical shifts
and the Cotton-Kraihanzel carbonyl stretching force con-
stants of the carbonyl stretching frequencies. The data
indicate that in the first series of complexes, when X is
an electron-donating group, the carbonyl resonance is
shifted further downfield than when X is an electron-
‘withdrawing group. The rationale for these observations
'may be understood by considering the nature of the metal-
carbonyl bond. As illustrated in Figure 8, a forward
coordinate o-bond is formed when the filled carbon
o—orbital overlaps with an empty metal d-orbital and a
dative metal-ligand n-bond is formed when a filled metal

*
d-orbital overlaps with the empty n antibonding molecular

46

 

In
9

LQMG + @Dcso: —» MGM c

 

 

 

. .Qc-OQ Q $.60
II %' -

.9.
% Q Q M5 Q

Figure 8. Description of a M-CO bond. (1) illustrates
the overlap of a filled carbon 0 orbital with
an empty metal d- orbital; (II) represents the
overlap of a filled metal d-orbital with empty
n* orbitals of co

I I
l‘>

'
‘.p

__.-J‘

(I-

9- an

cox-Al

..,u-—.

g .-
nil-‘4

 

 

 

. 1
.. b.
s” u.
t u
.. u.
s“ -.
r
‘ 1
., ‘
A
b..- .
‘ x
a.
A c'
on v.
. a
an.“
D-.~Ll

n
u.,“-‘
n

-a-..‘

.
‘Ann.

"cu“.

,,
Al”?
5“ ‘

 

 

 

47

orbital of the carbon monoxide. As electron density is
released into the n* molecular orbitals of the CO by the
metal, the n* orbital becomes more populated with
electron density and the C-0 bond order (and hence the
C-0 stretching force constant) decreases. From Equa-
tion 71 we find that downfield shifts of carbonyl re-
sonances (deshielding) with increasing electron density
in the n* orbitals of the CO group can arise from changes
in the QAB terms, the <r-3>2p term, or the AE term.

A series of complexes (carbene)Cr(CO)5 and
(carbene)W(CO)5 was studied by Todd 33 al.,37 who obtained
an excellent correlation between the carbonyl chemical
shifts and the k2 stretching force constant. These
authors stated that as the metal-to-carbonyl n back-
bonding decreases, the QAB term (for the M-C bond) should
decrease; this would mean a smaller negative value for
the QAB term and therefore a higher chemical shift for the
carbonyl carbon. In a subsequent study by Bodner and Todd38
of a series of (n-arene)Cr(CO)3 complexes, a linear
correlation was found between the carbonyl chemical shifts
and the infrared stretching frequencies for the two carbonyl
stretching modes. These authors concluded that the
carbonyl chemical shift is dependent upon the extent of
n back-bonding from the metal to the n* molecular
orbitals of the carbonyl. Furthermore, they excluded the
<r'3>2p term in Equation 71 as the dominant factor in the

op tensor by demonstrating that the carbonyls of

48

(n-C H NH2)Cr(CO)3 and (n-C H COZCH3)Cr(CO)3 are the most

6 5 6 5
deshielded and most shielded, respectively; the opposite
would be observed were <r-3>2p the dominant term.
Bodner and Todd also noted, and quite correctly so, that

35

Gansow gt gt. had neglected the negative sign of the

op screening tensor in their studies, but these authors
incorrectly continue that Gansow gt gl.utilized this in-
correct equation to predict a net shielding of the carbonyl
resonance as the substituents become increasingly electron
donating. The findings of Gansow gt gt., which antedate
any contributions from Todd or Bodner to this particular
area, are in perfect agreement with the findings of Todd

and Bodner.37’38

Finally, Bodner and Todd discussed the
contributions of the AE term and the QAB term with respect
to the total shielding tensor. These authors note that
changes in either term would result in the deshielding
of the carbonyl carbon with increasing electron density at
the transition metal.

In studies of a similar scope Braterman, Randall

39

et al. obtained the carbon-13 nmr spectra of a number of

(CO)6_nMLn complexes, where M = Mo, W; L = alkylphosphine,
alkylphosphite, ttg(diphenylphosphino)ethane,
ttg(diphenylphosphino)methane; n = 1,2. These authors
observed a correlation between the carbonyl chemical shift

and the lowest energy ultraviolet transition which in-

volves a charge transfer from the metal to the carbonyl

. 'A
.. .-~
. V
' v.
.. "
o
.—

 

.vn-.

-.v-.

 

El" '- fl H'uAink'

m
r

l.

..
“my-

u...

1...;- I

_ .
.="~
..,..

6.1

(l‘

(1‘
. a
a .4

(1'
(I)

 

 

 

 

49

L

wk molecular orbital. They argue that the QAB term should
be neglected because the symmetry of an MCO group is
cylindrical (such is the case for the QAB term in
acetylenes) and that therefore the AE term dominates the
op screening tensor. It was also argued that for LM(CO)5
complexes the observed order for the carbonyl chemical
shifts (6 increasing as L = C0 < P(OR)$ < PR3 < amine <
carbene) correlates with the increasing charge donor
ability (o-bonding ability) of the ligands. However, the
ordering of the ligands in this manner also reflects the
n-acceptor abilities of these ligands.

Other communications of the general topic carbonyl
chemical shifts are those which report experimental data
but do not analyze the results to any significant degree.
Carbon-l3 nmr studies of some gtg(RuClZ(CO)2L2) complexesao
and of some ttggg square planar RhCl(CO)(PR3)241 complexes
both include plots of the linear relationship between the
carbonyl chemical shifts and the Cotton-Kraihanzel
stretching force constants.

The chemical shift tensors in CO, Ni(CO)4, and
Fe(CO)5 have been rigorously analyzed in a recent paper
by Mahnke, Sheline, and Spiess.“2 By using theoretical
estimates, they calculated the diamagnetic screening term
od for free CO and the transition metal carbonyls and
from these calculations they maintain that the small dif-
ference in screening between CO and Ni(CO)4 (~12.5 ppm43)

can be completely accounted for by this term. These

50

authors then calculated the significance of the para-
magnetic screening term op for Fe(CO)5 by using spin
rotation constants which were calculated from the results

44’45 It was found that the in-

of T1 relaxation studies.
creased size of op, which results from increased

M-CO n back-bonding, is solely responsible for the
observed deshielded carbonyl chemical shift. Since the
screening tensor Gav in Fe(CO)5 is considered to be axial,
the relation Gav = l/3(oli + 2ol) arises. For this series
of molecules the of term was found to vary to an in-
significant degree, but the 0? term varied sizably for
the axial and equatorial carbonyls of Fe(CO)5. That of

is negligible is a consequence of the pseudolinear be-
havior of the metal-bound CO. The only energy transition
affecting the paramagnetic screening contribution is a
transition from the ground state 2+ to a state with
n-symmetry, and free CO has only one such state. The

wk state of free CO is stabilized by addition of electron
density from a transition metal, so the AE term in Equa-
tion 71 will decrease as compared to free CO, and there—
fore 02v will increase, thus inducing a downfield shift.
The inescapable conclusion is that the chemical shifts of
carbonyls reflect the nature of the n back-bonding from
the metal; the more back-bonding, the greater the

observed downfield chemical shift. Hence these authors

believe the AE term is the major contributor to the

observed chemical shifts.

51

It is our position that in terms of molecular
orbital theory, a decrease in the amount of w back-bonding
is synergic with an increase in the average excitation
energy AE. That these two effects are synergic would
seemingly imply that they are inseparable.

From this discussion it is clear that no definitive
model has as yet been put forth to account adequately for
carbonyl chemical shifts in terms of only one of the
components of the op tensor. It is most likely that
further attempts to correlate chemical shifts with only
one of the components will likewise be doomed; it would
seem that within each series of complexes studied each of
the terms should be weighted in a manner to accord accurate
descriptions and predictions of the chemical shifts. Much

work remains to be done in this particular area.

CHAPTER 2

A CARBON-l3 AND PROTON MAGNETIC RESONANCE EXAMINATION OF
SOLUTE STRUCTURES, EQUILIBRIA, AND STRUCTURAL

INTERCONVERSIONS IN SOME DINUCLEAR nS-DIENYL RUTHENIUM,
IRON, AND NICKEL CARBONYLS

Background

 

46 the structure and

Since its synthesis in 1955,
behavior of [(nS-C5H5)Fe(CO)2]2 (I) and other similar
complexes in solution have been the subject of over twenty
scientific papers. Cotton, Wilkinson, and Liehr47 pro—
posed a solution structure that contains both bridging
and non-bridging carbonyl groups, the bridging carbonyls
being functionally similar to ketone carbonyls. The in-
frared study these authors undertook, convinced them
that, in solution, (I) consists of a centrosymmetric
structure with one bridging- and three terminal-type

48 obtained the

carbonyl groups. Three years later Mills
structure of I in the solid state by using X-ray
crystallographic techniques. This work established that
the molecules of I possess two bridging and two terminal
carbonyl groups with all five carbon atoms of the
cyclopentadienyl rings apparently being bonded to the iron

atoms, since all Fe-C distances are identical. The iron

atoms were found to be formally bonded through an electron

52

 

 

 

53

 

Figure 9. The crystal structure of trans

x;
L(n“-C5H5)Fe(CO)2]2 (I)

54

pair and the cyclopentadienyl rings are in a trans con-
figuration. The two bridging carbonyl carbon atoms and
the two iron atoms form a planar four-membered ring. This
structure is shown in Figure 9. Bryan and Greene49 re-
determined the structure in 1970.

After Mills' announcement of the crystal structure
of (I), Cotton gt gl.50 re-investigated the solution
structure by using infrared and Raman spectroscopy and
discovered that the data obtained were inconsistent
with the existence of centrosymmetric structures in
solution. These authors considered but rejected the
possibility that more than one molecular species was pre-
sent in solution.

Noack51

subsequently investigated the temperature
dependent infrared spectrum of (I) in a variety of sol-
vents. He observed a solvent dependence of infrared
absorptions in the carbonyl region, but observed no changes
in the spectrum with temperature. He proposed that in
solution (I) loses its center of symmetry and becomes

so distorted that it exhibits CS symmetry. He rationalized
the proposal by citing the observed dipole moment of I

in benzene, which had recently been measured as 3.10

Debeye.52

Noack gt gt.53

, in an ensuing paper, detailed a
thorough spectroscopic examination of (I) and its

ruthenium homolog [(nS-C5H5)Ru(CO)2]2 (V) at temperatures

55

between ~100°C and +100°C. These results led these
authors to propose that both complexes exist in solution
in an equilibrium mixture of two structurally isomeric
molecular configurations. At low temperatures a con-
figuration with two bridging and two terminal carbonyl
groups was found to be more stable whereas at higher
temperatures an open form with four terminal carbonyls was
proposed to exist. The bridged structure was found to be
the dominant structure of (I) at room temperature, but (V)
exhibited both isomers in approximately equal proportions
at room temperature. Furthermore, these workers assigned
a predominantly cis configuration to the isomers, which
would explain the observed lack of a center of symmetry
and the non-zero dipole moment. The tendency of the
molecule to adOpt the cis configuration in solution was
ascribed to stabilization of the metal-metal bond by

o- and/or n-contributions. Complex (V) had been
synthesized in 196254 and its dipole moment in solution

55 de-

was measured as 2.75 i 0.12 Debeye. Mills and Nice
termined the crystal structure of (V), and demonstrated
that it is similar to (I), except that the metal-metal
bond of (V) is 0.20 A longer.

By relating infrared absorption intensities of
solutions of (I) and (V) in the carbonyl region to the
temperature at which the absorptions were recorded,

56

Noack obtained an estimate of the enthalpy and entropy

56

differences between the two postulated solution structures.
For (I) he calculated: AH = 4 kcal/mole, AS = 3 cal/mole-
degree. At +30°C the open isomer comprises 0.6% of the
equilibrium mixture in nonane. For (V) he obtained the
results AH = 1.56 kcal/mole, AS = 5.5 cal/mole-degree.

In carbon disulfide solution at +30°C the open isomer was
suggested to comprise 55% of the equilibrium mixture of
isomers.

The structures of (I) and (V) in non-polar sol-
vents were then investigated by Cotton and Yagupsky,57
who utilized ir techniques. These workers concluded that
solution (I) exists in a carbonyl—bridged dimeric config-
uration in which the two cyclopentadienyl rings lie on one
side of the plane formed by the two iron atoms and the
two bridging carbonyl groups, and the terminal carbonyl
groups lie on the other side of the plane. It was
suggested that this structure and the centrosymmetric
structure observed in the solid state are interconverted
upon dissolution gig a non-bridged intermediate, a tautomer
in which rotation about the metal-metal bond can occur.
Moreover the conclusions of Noack regarding the behavior
of (V) in solution were substantiated, and Cotton and
Yagupsky proposed that the non-bridged isomer present to a
large extent in solutions of (V) is the structure of the
intermediate through which the cis and trans form of (I)

interconvert.

.1

57

Manning58 next investigated infrared absorptions
of the carbonyl region of (I) in solution and found the
absorptions to be markedly solvent-dependent. He pro-
posed that, in solution, cis, trans, and non-bridged
species are all present, and that their relative abun-
dances are dependent upon solvent polarity. He concluded
that in solutions of (I) the more polar cis species
dominates the equilibrium and that there are only
negligible amounts of the non-bridged isomer present.
(See Figure 10.) The solvent-dependence of the dipole
moment can also be explained by Manning's interpretation,
to wit: the polar cis isomer is more stable in the more
polar solvents, and hence the magnitude of the observed
dipole moment increases as the solvent polarity in-
creases.

An infrared spectroscopic study of two similar
iron complexes [(nS-CH3C5H4)Fe(CO)2]2 (II) and
[(nS-C9H7)Fe(CO)2]2 (IV) was then performed by Manning

and McArdle.59

This investigation revealed that these
complexes also exist in solution as mixtures of cis and
trans isomers, together with small amounts of the non-
bridged species. Shortly thereafter these workers studied
the infrared spectra of the homologous ruthenium complexes
in solution60 and interpreted the results in terms of a

complex equilibrium involving cis and trans, bridged and

non-bridged tautomers. The bridged to non-bridged tautomer

58

 

n/
\

0“\ 3%
\no
/

Figure 10. Postulated structures in the Manning three
structure hypothesis for solution equilibria

of [(n5 - C 5H 5)Fe(CO)2] 2(1 1))

59

ratio was observed to be dependent on ring substituent,
solvent polarity, and temperature. The proportion of non-
bridged species was observed to increase with increasing
temperature and decrease with increasing solvent polarity.
No evidence was found for a non-bridged species at any
temperature or in any solvent for [(nS-C9H7)Ru(CO)2]2
(VIII).

In 1969 Greene gt gi.6l’62

reported both the
isolation and crystal structure of the cis isomer of (I).
It may be formed by recrystallizing the material obtained
from any synthetic route from a polar solvent such as
dichloromethane at -78°C. These workers also observed
the pmr spectrum of the cis isomer of (I); a single
resonance at 4.78 ppm for the cyclopentadienyl protons
was observed at room temperature. It began to be resolved
at -49°C and appeared to be completely resolved into two
resonances at -56°C. The two resolved resonances were
observed to be of unequal intensity, suggesting that the
cis-trans equilibrium postulated by Manning was sub-
stantially correct.

In 1969 Cotton gt gi.63 published "A definitive
identification of the structures of dicyclopentadienyldi-
iron tetracarbonyl in solution." These workers carried
out a pmr study of (I) in various solvents and verified
that, in solution, two isomeric species exist which
rapidly interconvert above gg. -40°C. The relative

intensities of the two resonances obtained below -40°C

 

 

 

'- .
. .-
' '_‘V
I
.... q
.
-

 

w

I

 

 

60

were seen to be solvent dependent. Cotton gt gi.
postulated that bridged-nonbridged carbonyl interconver-
sions of the type depicted in Figure 11 occur, and provide
a mechanism by which the cyclopentadienyl rings of (I)

may be interconverted between the cis and trans con-
figurations. These workers undertook asimilar study of

(V) as well,64

but the pmr spectrum at -100°C still showed

a narrow singlet. A computer-simulated study of the pmr
spectrum of (I) indicated that the activation enthalpy

for the cis-trans interconversion process is gg. 12 kcal/mole.
In our view the observations of Cotton gt gi.63’64
concerning the existence of the bridged-nonbridge carbonyl
interconversion process fall short of actually proving
that it occurs. Certainly there was a good case made

for such an hypothesis, but it was not until techniques
developed in our laboratory were applied to the problem
that this structural interconversion process was demon-
strated.7 The carbon-13 nmr spectrum of (I) dissolved

in dichloromethane reveals at +55°C a single resonance
50.2 ppm downfield from the resonance of carbon disulfide,
the internal reference preferred at the time for metal
carbonyl spectra. At -90°C, two discrete resonances are
detected, one at 18.1 ppm, the other at 82.3 ppm, a
separation of nearly 1200 Hz. When the temperature is
raised to -73°C a broad resonance appears approximately

midway between the two low-temperature resonances. This

new resonance sharpens to a maximum at -59°C, then it begins

61

(II)
M——M :15 M_——M
\c/ \CO
ll
(3

Figure 11. Schematic view of the carbonyl interchange

predicted by Cotton63 for complexes with
bridging and terminal carbonyls

62

to broaden concurrently with the other two resonances.
All resonances are broadened into the baseline noise at
-12°C. Then, a single resonance at 50.2 ppm appears and
sharpens as the temperature is raised further. The car-
bon-l3 nmr spectrum of the cyclopentadienyl rings, found
106.7 ppm upfield from carbon disulfide, exhibits features
similar to that of the temperature-dependent pmr spectrum.
An interpretation fully consistent with Manning's
three-structure hypothesis and with the pmr and carbon-l3
nmr observations can now be put forward. Both cis-trans
interconversion and bridge-terminal carbonyl interchange
must occur in the temperature range above -35°C, since
only one carbonyl resonance and one cyclopentadienyl
resonance are observed. The cis and trans isomers both
undergo bridge-terminal carbonyl interchange, but with
different activation energies and hence at different rates.
As the temperature is raised from -90°C, where no carbonyl
interconversion occurs, the trans tautomer, less stable
in the polar dichloromethane, begins to undergo carbonyl
interconversion whereas the cis tautomer remains intact.
At -35°C the cis tautomer begins to undergo bridge-terminal
carbonyl interconversion; this temperature is also the one
at which cis-trans isomerization of the cyclopentadienyl
rings commences. Carbon-l3 nmr is thus the only technique,
available at present, that is capable of providing data
adequate to the task of solving the solution behavior of

(I) and its homologs.

63

A year after we published the above data, a

65 (and sometime

mechanism was proposed by Cotton gt gi.
later by Roberts gt gi,66)that explains the rate dif-
ferences observed for carbonyl interconversions of the cis
and trans isomers of (I) and of some related complexes.

The salient points of this mechanism, illustrated in

Figure 12, now follow. Two bridging carbonyl groups open
simultaneously, each moving to a terminal position on a
different metal atom. Accompanying this step is what-

ever degree of twist or deformation is necessary for the
open species to pass intotfluamost easily accessible,
staggered rotamers 12A, 12B, 12C. Since rotamer 12A has
C2h symmetry, bridge opening renders the four carbonyls
homotopic and random pairwise closing of two ggti

carbonyls completes carbonyl interchange. However, inter-
conversion of the cis isomer 24A requires, in addition,
rotations of 120° about the metal-metal bond. These are
diagrammed as (a) 24A212B212C224A or (b) 24A212BZl2A2
12CZ24A. Pathway (b) has two advantages. It avoids a
transition state in which the bulkier organic ligands are
eclipsed and it provides a mechanism by which cis-trans
isomerization and carbonyl interconversion are accomplished
in concert. Roberts gt gi.66 performed a computer-simulated
lineshape analysis of the temperature-dependent carbon-l3
spectrum of the carbonyl region of (I) and calculated the
activation energy for cis-trans isomerization to be

11.7 i 1.0 kcal/mole.

64

 

mmmeQEoo pmumamu paw makconHMU HmquAamcmwp- CV owHHMDmEHp CH

m
mwcmnoumucw Hzconumo paw cowummfiumEOmH mcmuuamwo How Emwcmnowa < .NH muswwm

<§ 1: ‘.’ % 3N

65

The possibility of other mechanisms which des-
cribe carbonyl interchange processes has been discussed

by Cotton. Mechanisms such as a one-for-one carbonyl
65

interchange, or interconversion via a triply-bridged

72 have been discussed to some extent. These

intermediate
variations of the Cotton-Roberts mechanism have found

little application thus far.

7 1....‘5 if

Magnetic resonance studies of complexes similar
in structure to (I), but with various phosphine, phosphite,
or isocyanide substituents in place of carbonyl groups
have also been reported. The mechanism outlined above was

based partially on Cotton's results from a proton nmr

study of the di(methylisocyanide) derivative of (I).65

The methylisocyanide ligands were seen to interchange in
the same manner as the carbonyl groups. An X-ray
crystallographic study of this complex revealed that the

isocyanide ligands occupy the bridging positions in the

67

solid state. Studies of other isocyanide complexes

reveal that they all undergo ligand scrambling.68’69

Similar nmr studies were conducted on the triethylphosphite66

7O derivatives of (I). These

and triphenylphosphite
complexes, too, were shown to be fluxional, and in the
solid state, the latter complex was seen to have two car-

71 The Cotton-Roberts mechanism has even

bonyl bridges.
been shown to be applicable to the ligand scrambling pro-
cess in the dimethylgermanium-bridged derivative of (I),

l(OS-CSH5)2F62(CO)3Ge(CH3)2].72

66

Experimental

 

All preparative procedures and manipulations
were carried out under an atmosphere of prepurified
nitrogen. Solvents were deoxygenated prior to use. All
chromatographic procedures were performed by using Woelm
neutral alumina, activity grade I (ICN Pharmaceuticals).

[(nS-C5H5)Fe(CO)2]2 (I). A modified version of the

 

method suggested by King and Stone73 was used by synthesize
complex (I). A mixture of 1.5 ml Fe(CO)5 (Ventron Corp.),
1.5 ml heptane, and 15 m1 dicyclopentadiene was refluxed
until cessation of gas evolution. The reaction mixture

was allowed to slowly cool to room temperature, after

which the precipitate which formed was filtered and re-
crystallized from dichloromethane-pentane.

[(nS-CH3C5H4)Fe(CO)2]2 (II). A similar method was used

 

to synthesize this complex. A mixture of 1.5 ml Fe(CO)5,

1.5 ml heptane, and 15 ml purified74

di(methylcyclo-
pentadiene) (Aldrich Chemicals) was refluxed until
evolution of gas ceased. The reddish-brown reaction
mixture was cooled to -15°C overnight and the resulting
precipitated material was filtered and washed with pentane.
The crude material (80% based on Fe(CO)5) was chromato-
graphed by using dichloromethane eluant, and, after
solvent evaporation on a rotary evaporator, the material
was recrystallized from chloroform-pentane. [Calculated
for C16H14Fe204: C: 50.3%, H: 3.8%; found: C: 50.1%,

H: 3.8%.]

67

[(nS-C9H7)Fe(CO)2]2 (IV). The indenyl complex (IV) was

 

similarly synthesized by refluxing 1.5 ml Fe(C0)5, 1.5 ml
heptane, and 15 ml distilled indene (Aldrich Chemicals)
until gas evolution stopped (gg. 6 hours). The resulting
reddish-brown solution was cooled slowly to room tempera-
ture and then poured into 60 ml of cold (0°) pentane.

The resulting precipitate (representing 63% yield based
on Fe(CO)5) was purified in a manner identical to that
described for (II) above. [Calculated for C22H14Fe204:
C: 58.2%, H: 3.1%; found: C: 57.5%, H: 3.5%.]

[(nS-C9H11)Fe(CO)2]2 (III). The tetrahydroindenyl complex

 

(III) was obtained by catalytic hydrogenation of an
ethanolic solution of (IV) over Adams' catalyst at

atmospheric pressure.75

This complex was purified by
recrystallization from ethanol.

[(nS-C5H5)Ru(CO)2]2 (V). Four different methods were

 

utilized for the synthesis of (V) during the course of
this research. Initially, the method of Fischer and
Vogler76 was used to prepare the material. Anhydrous
RuI3 was converted to the uncharacterized polymeric
[Ru(CO)212]n by heating at 220°C in a stream of carbon

77 This material

monoxide until iodine ceased to evolve.
was treated with NaCSH5 in tetrahydrofuran and the product
was recovered from the reaction mixture. A second method

involved carbonylation of RuCl3 under 10 atmospheres

of carbon monoxide pressure, and reaction of the resulting

[Ru(CO)3C12]2 with NaC5 5.78 A recently-reported synthesis

68~

of this material from dicyclopentadiene and Ru3(CO)12
in heptane proved to be the most facile route to this

79 The fourth method was developed in our

complex.
laboratory in order to circumvent the laborious steps
required in the first two methods and the use of the
extremely expensive Ru3(C0)12 required in the third
method. The first step of our synthesis is formation of
the uncharacterized chlorocarbonylruthenium complex,
Ru(CO)nClm, by refluxing a 2-ethoxyethanol solution of
hydrated RuCl3 under a stream of carbon monoxide.80
To the yellow solution is added a tetrahydrofuran solution
of NaCSH5 (200% excess based on RuCl3). The mixture is
stirred at 80°C eight hours. After solvent removal at

3 mm, the residue is extracted with dichloromethane and
the concentrated extracts are chromatographed by using
dichloromethane eluant. Rotary evaporation of the sol-
vent leaves a residue which is sublimed at 110° (0.3 mm).
We believe that this method is superior to the three
established procedures if both cost and time are con-
sidered. A procedure for the rather inexpensive synthesis
of Ru3(C0)12 gig the reduction of the chlorocarbonyl-
ruthenium species with zinc has recently appeared,80 but
our method utilizes the chlorocarbonylruthenium species

directly; there is no need to form the Ru3(C0)12.

[(nS-CH3C5H4)Ru(CO)2]2 (VI). This complex was synthesized

 

from in a manner identical to that described for (V) by

heating the chlorocarbonylruthenium complex with

 

 

 

 

 

..
.‘O s
c
. —
.-
n
.u
.— 4
o
...

\ o

h .u..\

.. .
-

 

‘0...

~1-

(I)

,
‘-v-\.
H.“ it";

n
‘ ,
v , n
,
' I

 

 

 

69

Na(CH3C5H4). Purification of this material was effected
in a manner identical to that described for (V) above.
[Calculated for C16H14Ru204: C: 40.51%, H: 2.91%; found:
C: 40.75%, H: 2.95%.]

[(nS-C9H11)Ru(CO)2]2 (VIII). The method of Humphries

 

and Knox79 for the preparation of (V) was adapted for the
synthesis of (VIII). A mixture of 1.0 g Ru3(CO)12 and

10 ml distilled indene in 50 ml heptane was refluxed 24
hours without exclusion of air. The hot solution was
filtered and the precipitate was placed in a sublimation
apparatus at 110° (0.3 mm) for four hours in order to re-
move any unreacted Ru3(CO)12. The residue was then
chromatographed by using 50% dichloromethane-50% pentane
to elute the orange band. The solvent was removed and the
product was recrystallized from dichloromethane-pentane.
[Calculated for C22H14Ru04: C: 48.35%, H: 2.56%; found:
C: 48.02%, H: 2.44%.]

[(nS-C9H11)Ru(CO)2]2 (VII). The final ruthenium complex

 

was synthesized gig the catalytic hydrogeneration of
(VIII) in tetrahydrofuran over PtO2 at atmospheric pres-
sure. The resulting solution was filtered, the solvent
removed, and the residue recrystallized from
dichloromethane-pentane. Since this complex is as yet un-
reported in the literature, its properties are shown in
Table 1. [Calculated for C22H22Ru204: C: 47.65%, H:

3.97%; found: C: 47.30%, H: 3.86%.]

 

70

Table 1. Physical Data for the Previously Unreported
Complex [(n5-09H11)Ru(CO)2]2 (VII)a’b

Melting Point: 123-4°C

6(c0)°, cm-lz 1986(17.9); 1954(15.4); 1835(2.1);
1809(10.0)
lH nmr,d 6: 4.90 (area 1); 4.88 (area 2);

2.56 (area 4); 1.87 (area 4)

a for carbon-l3 nmr data, see Table 8.
b for mass spectral data, see Table 14.
C in tetrahydrofuran

d in CD013

71

[(nS-CSH5)2FeNi(CO)3] (IX). The mixed metal complex (IX)

 

was prepared from nickelocene81 and Fe(CO)5 and was

purified according to the method of Tilney-Bassett.82

[(nS-CSHSNi(CO)]2 (X). The nickel dimer (X) was purchased

 

from Ventron Corporation and was purified by sublimation.

Two other complexes were synthesized for eventual
use in this research, but for reasons to be given were not
utilized. Their synthesis follow:

[(nS-(CH3)5C5)Fe(CO)2]2. The permethylated iron dimer

 

was synthesized from Fe2(CO)9 (Ventron Corp.) and
5-acetyl-l,2,3,4,5-pentamethylcyclopentadiene (from

hexamethylbicyclo 0.2.0 hexadiene and m-chloroperoxybenzoic

acid)83 according to the method of King and Efraty.83’84

We were unable to enrich this material with 13CD, as
will be elaborated in the next section.

[(nS-CH3C5H4)2FeNi(CO)3]. A synthesis of the

 

methylcyclopentadienyl homolog of the iron-nickel dimer,
(IX), was also attempted. Reaction of the green liquid
1, l'-dimethylnickelocene with Fe(CO)5 in refluxing
benzene yielded a brown reaction mixture. Following the
procedure of Tilney-Bassett outlined for (IX),82 the
mixture was chromatographed on a 24 in. alumina column
and four bands developed. A green band eluted rapidly;
it proved to be unreacted 1,1'-dimethylnickelocene. A
pink band followed which was demonstrated by an infrared

spectrum to be the l,1'-dimethyl homolog of the dinickel

dimer (X). The third band, brown in color, was isolated

1"?

 

v

 

 

.n—v-a

.y—r .
.

 

vs~‘-

-v~-~

o... .

.FH,
bvd.

Aq‘a

"vu

V‘qn
1" '

v...‘v

 

 

 

72

and treated as the desired product. The final band eluted
was reddish-brown in color and was demonstrated to be
complex (II). The brown material isolated from the

third band proved to be so air-sensitive that neither a
satisfactory analysis nor a pmr spectrum could be obtained.
Inasmuch as the methylated iron and nickel complexes were
obtained as byproducts, it is virtually certain that the
brown product was indeed the methyl homolog. During the
course of this synthesis, a new scheme for obtaining
l,l'-dimethylnickelocene was used. This product was
obtained by refluxing a tetrahydrofuran solution of

NaCH3C H4 and Ni(NH3)6C1281 overnight. The solvent was

5
removed, the residue dissolved in dichloromethane and
filtered, and the concentrated solution was chromato-
graphed. The green band which rapidly developed was
eluted, the solvent removed, and the green liquid remain-
ing was utilized in the synthetic scheme.

l3CO Enrichment. Complexes (I), (II), and (IV) were

 

synthesized from Fe(CO)5 which had been enriched to gg.

33% in 13C0 by stirring for two hours a mixture of 1.5 m1
heptane and 1.5 ml Fe(CO)5 in an atmosphere of 90% carbon-l3
enriched carbon monoxide (Mound Laboratories, Monsanto)

(250 ml at 27°C) together with 10% palladium on activated

charcoal.85

After two such enrichment processes the
mixture was filtered and the heptane-Fe(CO)5 mixture was
used in the synthesis. Previous attempts to remove

pentane from a pentane-Fe(CO)5 solution through distillation

 

 

- . . .. s :15...—

 

 

73

yielded a green product, indicating that the Fe(C0)5

was being converted to Fe3(CO)12. Heptane was chosen be-
cause its boiling point nearly matches that of Fe(CO)5.
Neither (II) nor (IV) could be induced to exchange with
13CO directly, either in the presence or absence of the
catalyst or at elevated temperatures; only decomposition
products were obtained. Attempts to enrich the permethylated
iron dimer directly yielded brown solutions which exhibited
infrared spectra with no bridging carbonyls. Complex

(III) was synthesized fron enriched (IV).

Complex (V) was enriched by stirring a dichloro-
methane solution of the complex in an atmosphere of en-
riched carbon monoxide for two weeks at ambient tempera-
ture and laboratory (fluorescent lamp) illumination.
Sublimation of the enriched material effected purification.
Complexes (VI), (VII), and (VIII) were enriched in a
similar manner; however, the time was reduced to 72 hours
and the enrichment was performed in the dark. Extensive
decomposition of the products resulted when the enrich-
ment process was performed in the presence of light.

Complex (IX) was enriched in an identical manner
to that described for (V) above. Chromatography of the
enriched product yielded traces of (I), nickelocene, and
Ni(CO)4 as well as the enriched complex. Complex (X) was
enriched by stirring a dichloromethane solution overnight

in an atmosphere of enriched l3CO.86

74

Acquisition and Treatment of Spectral Data

 

Proton nmr spectra were obtained by using either
a Varian A56/60 or HA-lOO spectrometer. Temperatures
were measured by inserting a copper-constantan thermo-
couple directly into a solvent-filled tube in the nmr
probe and reading the temperature from a Doric Trendicator
400 digital output. Samples were allowed fifteen minutes
of equilibration time. Infrared spectra were recorded
by use of a Perkin—Elmer 457 spectrophotometer; both
nujol mull and solution media were used, and polystyrene
film was used to calibrate the spectra. Mass spectra
were measured by an Hitachi RMU-6 Spectrometer by Mrs.
R.L. Guile of this department.

Carbon-13 nmr spectra were measured by using a
Bruker HFX-lO spectrometer modified for multinuclear

measurements as described by Traficante gt gi.6 and

equipped with a Nicolet 1083 computer with 12K of memory,

a Diablo disk memory unit, and a Nicolet 293 I/0 controller.

The spectra were obtained at a frequency of 22.63 MHz.
Temperature calibration was achieved by reading values
from athermometer inserted directly into a solvent—filled
sample tube in the nmr probe. Temperatures were observed
to vary no more than : 1°C.
All samples were prepared under an atmosphere of

prepurified nitrogen by using suitably degassed solvents.
Samples to be used in the temperature range above +9°C

were prepared by using dibromomethane or toluene with 20%

 

‘ ;

...

 

 

F1

«la

6..

 

75

l,2-difluorotetrachloroethane (PCR, Incorporated) added

to serve as a fluorine-l9 lock. Samples studied in the
temperature range +9°C to -l30°C were prepared by using
dichlorofluoromethane (Genetron 21, Matheson Gas Products),
a compound which serves as both fluorine-l9 lock and sol-
vent. Spectra obtained below -l30°C employed samples dis-
solved in gg. 50-50 mixtures of dichlorofluoromethane and
difluorochloromethane (Freon 22, Matheson Gas Products),
with the former compound serving as a lock. All samples
were 0.03 M in ttig(acetylacetonato)chromium (III) (re-
crystallized from benzene-pentane). This reagent serves
to reduce the lengthy carbonyl carbon-l3 Tl relaxation

times.7’87

Samples prepared for use in the temperature
range -l40°C to +70°C contained in addition 5% carbon
disulfide, present as an internal chemical shift reference.
All carbonyl chemical shifts are reported in ppm downfield
from TMS, with the conversion factor being éobserved =
6 + 192.4.

CS
2 The spectra in this section were measured over a

5000 Hz spectral width. Optimum signal-to-noise was
obtained by using between 1024 and 2048 transients per
spectrum, and by multiplication of the resulting free
induction decay by an exponential function with a negative
exponent. The resulting artificial line broadening
(.2 to .4 Hz) was kept to a minimum.

Computer-simulated spectra were generated by using

88

the lineshape simulation program EXCHSYS. This program

 

,- 11'

 

 

 

76

simulates nmr spectra by performing the calculation

  
 

 

F ~ ~ ~ -«
~d1-Kll/r KlZ/T Kl3/I... l
R /T -a -R /T R /T... l
1(0))(1 Re(11,12,13,..) 21 2 22 23
K31/T K32/T -a3-K33/I.. l
L. \ J

  

 

where I(w) is the intensity of a resonance at angular

frequency w, IR is the intensity of resonance k,

dk = i(wk - w) + l/Tzk, where wk

of the kth

intensity is to be calculated, and T2
k

relaxation characterizing the linewidth of the k
resonance in the absence of exchange; Rij are the

elements of the exchange matrix R which specifies

is the angular frequency
resonance, w is the frequency at which the

is the transverse
th

matrix
the

is

transfer of magnetization among the sites, and T
the mean pre-exchange lifetime.
The input parameters for this calculation are the

following: the frequencies and linewidths of the resonances
in the slow—exchange limit, their relative intensities,

the intensities of the resonances as exchange occurs,

values of the mean pre-exchange lifetime (I) describing

the rate of interchange, and the exchange matrix that
specifies the details of the mechanism by which the
magnetization is transferred among sites. Each chemically
nonequivalent set of atoms is designated as a site.

Superposition of calculated and observed spectra taken in

 

77

the range between the slow- and fast—exchange limits per-
mits estimates of the mean pre-exchange lifetime I,
and therefore the first—order rate constant k = l/I. An
Arrhenius plot of the calculated rate constants yielded a
value for the activation energy Ea. The other activation
parameters were obtained from an Eyring plot of the rate
constant data. A plot of ln(k/T) versus l/T yields a
straight line with slope -AH#/R and intercept

ASE/R + ln(k/h). A calculation performed in this manner
insures that AHI and A35 are temperature independent.
From the relation AG76 = AHI - TASI, the quantity AG§986
may be calculated. The thermodynamic parameters for the
equilibrium between the bridged and nonbridged isomers

of the complex [(nS-C5H5)Ru(CO)2]2 (V) were determined
from the temperature dependence of the equilibrium con-
stant measured from chemical shift changes as a function
of temperature. The thermodynamic parameters describing
the equilibrium between the cis and trans isomers of the
iron complexes (I)-(IV) were determined from the tempera-
ture dependence of the relative populations of the two
isomers. The relative population of each isomer was
obtained either by electronic integration of the spectra
or by cutting out the peaks and weighing them. A plot

of ln K versus l/T, where K = [trans]/[cis], yields a
straight line with slope -AH/R and intercept AS/R. The
values of AG298° may be calculated from the AH and AS

values as described above.

"”‘”3

78

The exchange matrix which was used to describe
the carbonyl site exchange in the trans isomers of complexes

(I)-(IV), and for the complexes (V)-(IX), is given by the

general formula

_§_
l-X 1-x

where the fractional populations of site A and site B are

l-X and X, or I = (I 18) = (1-X,X). The matrix used

A!
to describe the three-site exchange process observed in
the cis isomers of complexes (I)-(IV) is given by the

general formula

I

 

 

'91 0 T .SC
' T + .50 T + .DC
R = .ST -T .5T ,
T 5C -1 0

T + .5c T 4 .SC

where C represents the relative population of the cis
isomer and T the relative population of the trans isomer
(T = 1 - C).

In order to simulate the temperature dependent
chemical shift of the ruthenium complex (V), an exchange
matrix was constructed to accurately reflect the relative
populations of the bridged and nonbridged isomers. At
-l3l°C a bridging carbonyl resonance was detected at 60.0
ppm and a terminal carbonyl resonance was detected at

6.42 ppm (with respect to carbon disulfide). The relative

 

 

'r'V' ‘

'1‘ 1’.“-

 

 

'
. .nen
'pr
..ov"‘"

,..~.-A-v-
.«

.v-~.b~“

,n
'14- DD

.44.. '

...... .. _
’i r- .
5.6.1.“...

I
"vnfiww

VO-y'gen
.

(I)

i‘ us.

An.

v.55.

~
C!

.-
h“

’nn

u...

—

 

 

 

79

population of the nonbridged species was calculated from

the relation 6 33.21(l-x) + 6.42x, where x re-

observed =
presents the fraction of nonbridged isomer and the quantity
33.21 is the average of the bridging and terminal carbonyl
resonances. These calculations incorporate these assump-
tions: 1) the bridging and terminal carbonyl chemical
shifts are taken as temperature independent except for
exchange effects and 2) the chemical shift of the terminal
carbonyls in the nonbridged isomer is identical to that

of the terminal carbonyls in the bridged structure. The
exchange matrix was then constructed by defining IA as the
relative population of the nonbridged species and IB as

the relative population of the bridged species. The

matrix thus constructed yielded simulated spectra which

are congruent with the observed spectra.

80

 

——M
\ /

Ill M=Fe
VII M=Ru
8
/ .
/;Q\\C”hh
0C 0

IX

0
M/——C\M/CO
C)c/ \g/fl‘cuz,
II M = Fe
VI M =Ru
o
fl/8\M/C
\ /
IV M = Fe
VIII M =Ru
fl 8
./ \ .
hh;:;;;flflu
o g
X

Figure 13. The [(nS-dienyl)M(CO)2]2 complexes utilized

in the study of solution dynamics outlined in

Chapter 2

81

Results

Proton and carbon-l3 nmr spectra of gg. 0.2 M
solutions of the ten dimetallic nS-dienylmetal carbonyls
(see Figure 13) utilized in the present study have been
obtained at numerous sample temperatures between -160°
and 100°C. The wide separations of chemical shifts and
the facile, line broadening chemical exchange processes
observed for these metal-bonded carbonyls required, for
resonance detection within a reasonable amount of time,
the preparation of 1300 enriched compounds and the use of
several different solvents. The spectra obtained in this
study are displayed in Figures 14 through 23, with the
computer simulated Spectra shown in the appropriate place.
Tabulations of carbonyl chemical shifts measured and sol-
vents employed at various temperatures are provided in
Tables 2 through 11.

In every instance, the more downfield of the two
or more signals detected at low temperatures were assigned
as bridged carbonyl resonances. Higher sample temperatures
resulted ultimately in broadening of all carbonyl
resonances, followed by coalescence, except for the easily
decomposed indenyl complexes (III), (IV), into a single
averaged line which for the iron systems and for (VIII)
remained chemical shift invariant with temperature. In
contrast, the averaged resonances of the ruthenium com-

plexes (V)-(VII) were seen to move upfield as sample

‘40]

32

 

 

55'

37'

 

DWI..-”

22°

8‘

0'

qz'um#—*.pH~h—unuu-'\up

W
I

 

 

JWL...

22° W

 

 

 

 

 

 

 

 

_52. HL‘LL “‘V‘Jfé.“ :“T‘V £438.

Figure 14. Variable temperature carbon-13 nmr spectrum
of the carbonyl region of

[(nS-C5H5)Fe(CO)212 (I)

 

 

83

Table 2. Chemical Shift and Rate Constant Data for

 

 

T (0C) saverage abridge aterminal K(best fit)

+55° 239.4a

+39° 239.4a

+21° 239.1a

-20° 241.4b 272.5a 210.3C 133e

-32° 241.7b 272.8C 210.0C 409

-40° 241.9b 273.2C 210.0C 206

-51° 242.1b 273.5C 210.0C 4 x 105f,6.06e

-60° 242.2b 273.9C 210.1c 2.5 x 10Sf

-70° 242.4b 274.1C 210.2C 1.0 x 10f

-80° 274.4C 210.2C 4 x 1o4f

-90° 274.7C 210.2C

-110° 275.3C 210.3C

~126° 275.9d 210.4d

a average of all carbonyls of both isomers

b average of trans isomer

C bridge, terminal carbonyl resonance of cis isomer

d probably bridge, terminal carbonyl resonances of both
isomers

e rate constant for cis isomer carbonyl interchange

f

rate constant for trans isomer carbonyl interchange

74’

.. 4W «W11..-

-LW

 

 

-u°

A I I 4WD-..
(.11....

Figure 15. Variable temperature carbon-l3 nmr spectrum
of the carbonyl region of

5
[(n -CH3C5HA)Fe(CO)2]2 (II)

 

 

 

 

 

 

 

 

 

 

 

 

85

Table 3. Chemical Shift and Rate Constant Data for

5
[(n -CH3C5H4)Fe(CO)2]2 (II)

 

 

T (°C) 5.2119128? 112%? aterminal K(best fit)

+74° 240.4a

+59° 240.6a

+48° 240.6a

+36° 240.6a 267°

-14° 242.4b 274.2C 210.5C 123°

-24° 242.6b 274.7C 210.6c 40°

-38° 242.8b 275.0C 210.6c 4 x 105f, 13 33°
-48° 243.0b 275.0C 210.7C 2 x 105f I
-57° 243.2b 275.6c 21o.7° 8 x 1o4f

—69° 243.4b 276.0° 210.8C 24,390f

-81° 276.3C 210.8C

-90° 276.5C 210.9C

-102° 277.1c 210.1°

-117° 277.2d 211.0d

-125° 277.4d 211.1d

a

average of all carbonyls of all isomers
average of trans isomer
bridge, terminal carbonyl resonance of cis isomer

probably bridge, terminal carbonyl resonances of
both isomers

rate constant for cis isomer carbonyl interchange

rate constant for trans isomer carbonyl interchange

C3

 

-34‘

 

 

 

 

 

 

1111.11

425°

441'

Figure 16. Variable temperature carbon-13 nmr spectrum
of the carbonyl region of

[(05'C9H11)F€(C0)2]2 (III)

87

Table 4. Chemical Shift and Rate Constant Data for
5
[(n -C9H11)F€(CO)2]2 (III)

abridge

274.
275.
276.
276.
276.
276.
277.
277.
277.
277.
278.
278.
278.
278.

T (°C) (Saverage
+1° 243.1a
-7° 242.8a
—14° 243.6a
-24° 243.8a
-34° 243.9a
-43° 243.9a
-52° 244.0a
-61°
-72°
-85°
-96°
-110°
-125°
-l41°
a average of trans isomer
b
C

both isomers
d
e

211.
211.
211.
211.
211.
211.
211.
211.
211.
211.
211.
211.
211.
211.

6terminal

K(best fit)

 

200d

74d

15d

4 x 105°, 8d

2.5 x 105°

1.6 x 105°
5 x 104°

2 x 1046

bridge, terminal carbonyl resonances of cis isomer

probably bridge, terminal carbonyl resonances of

rate constant for cis isomer carbonyl interchange

rate constant for trans isomer carbonyl interchange

‘w'hfifihflhfiwnuhuuaflmyghu

-40.

3¢”ilflflfll‘flpbluflduu&*pu “'4F*HF“*“'"***'*VA'*'
:::WW 111.31...
Mt

 

 

::W 1......

-l'.
‘20. -402'

Figure 17. Variable temperature carbon-13 nmr spectrum
of the carbonyl region of

[(nS-C9H7)Fe(CO)212 (IV)

 

 

89

Chemical Shift and Rate Constant Data for

Table 5.
[(n5-69H7)Fe(co)2]2
T (°C) saverage °gggggg
+26° 240.3a 267.1b
+23° 240.3a 267.2b
+19° 240.0a 267.1b
+15° 240.1a 267.1b
+11° 240.0a 267.2b
+9° 240.0a 267.4b
+5° 240.0a 267.4b
-2° 240.7a 268.7b
-11° 240.8a 268.9b
~20° 240.8a 269.3b
-300 269.5b
-400 269.9b
-490 270.1b
-6o° 270.6b
-70° 271.0b,272.5C
-86° 271.3b,272.8C
-102° 271.6b,273.2C
a
b average of trans isomer

(IV)

5terminal

 

210.

210.
210.
210.
210.
210.

210.
210.
210.
210.
210.
210.
210.
210.
210.
210
210.

3b,211.5C

.2b,211.6C

3b,211.6C

K(best fit)

 

400d

200d

114d

61d

40d
18.5b

2.5 x 105°

1.5 x 105°

6.67 x 104°

2.5 x 104°

40

bridge, terminal carbonyl resonances of cis isomer

bridge, terminal carbonyl resonances of trans isomer

rate constant for cis isomer carbonyl interchange

rate constant for trans isomer carbonyl interchange

 

 

v‘ w —— “82.

1.1.1
1.1.1

9.
4*

 

 

—33' ”Li
-50. __v L... -‘AL—v—v—L

Figure 18. Variable temperature carbon—l3 nmr spectrum
of the carbonyl region of

[(ns-CSHS)Ru(CO)212 (V); the highest field
peak is CS2

kquf

 

 

 

 

91

OH x N No.0 de.mNN .Nw-

 

 

 

mow x m.H $6.0 om.mmm omen
moa x m.q No.0 om.mmm com:
00H $O.NH o¢.NNN owmu

pm.mNN 66m-

pH.NNN damn

00H x mm.m Nm.ma om.HNN com:
po.HNN Oman

NOH $5.0N 0H.omm on:

pm.omm oH+

50H x N Nm.mm ow.wHN oom+
pq.mam 0NN+

pq.wHN omm+

Nm.mm nm.mmm oom+

pH.NHN 0Hm+

pm.mam 6Hm+

NH.Ho nm.mom owm+

Nm.m© nm.wom oom+

Auflm ummnvx mHmCHEumuw mmmpflpno mp660udmu pmwpflunucoc N we wum>m© Auov H

m
A>V «.mxoovsmxmzmo-ch.
pom cofludnfluumwm umEOmH paw .mUmQ ucmumcou mumm umanm HmoHEono .o manme

92

 

. .i.
“14: . HIIIIIIIIII

COHumadono pom uxmu mom

 

 

m

mumEomH Loon mo mmucmCOmmu Hzconpmo HmcHEHou .mwpflun zanmnoua m

coHusHOm mawuuflcoumom CH p

conusaom Namzo an d

COHDDHOm owSHOD SH 3

mmocmcomww Ham mo maxconumo Ham mo mwmpm>m m

m.me m.NmN coma:

ono w.me q.mmm damau

m.mma m.mmH m.HmN owHHu

Auww ummflvm HmaEHmuw mm wwuflm mMmEousmu Umwpwuflucoc N mm mum>mm Aoov H

m

A.ecoev e dfiede

 

WWW» _,,.

 

Wk» 43°

-40'

40?
‘46.

 

3» WM 111.-....

Figure 19. Variable temperature carbon-l3 nmr spectrum
of the carbonyl region of
5 . .
[(n -CH3C5H4)Ru(CO)2]2 (VI), the highest

field peak is CS2

 

 

Table 7.

T (°C)

-23°
-28°
-33°
—40°
-46°
-54°
-98°
-105°
-121°

94

Chemical Shift and Rate Constant Data for

[(nD-CH3C5H4)Ru(CO)2]2 (VI)

Egveragea 6bridgeb Sterminalb K(best fit)

 

 

224.8 1.11 x 106
224.9 7.7 x 105
225.1 4.2 x 105
225 2 2 x 105
225.3 1.33 x 105
225.5 8.33 x 10°

254.5 200

255.1

255.6

average of all carbonyls of all resonances

b

probably bridge, terminal isomers of both isomers

\x)
L3

 

58" WM W 4°

 

 

1...).

10' -60°
6.5’ 1- 402°
Figure 20.

Variable temperature carbon-13 nmr spectrum
of the carbonyl region of

5 . .
[(0 -C9H11)Ru(CO)2]2 (VII), the highest

field peak is CS2

 

 

 

Table 8.

T (°C)

+100°

+71°
+58°
+55°
+41°
+35°
+28°
+20°
+17°
+10°

+6.5°

+4°
-52°
-60°
-71°
-85°
-95°

-ll6°

toluene solution

b

C

chloroform solution

CHFC12 solution

96

Chemical Shift and Rate Constant Data for

5

K(best fit)

 

5averaged 6bridgee aterminale

220.7a
221.0a
222.4b
221.5a
222.7b 2 x 107
221.73
221.6b
222.0a
223.3b 4 x 105
223.4b 2 x 105
223.5b 1.33 x 105
223.8b 8 x 104
250.7 199.7 100
251.1 200.0 40
251.6 200.1 5.5
252.1 200.1
252.7 200.2
252.9 200.2
d

average of bridge,
terminal carbonyls
resonances of all isomers

probably bridge, terminal
carbonyl resonances of
all isomers

97

WM

58' W

:w'uw-uun~1unw~wqun
20'-~u~qu»~eu.._...1__

.._....I.

-500

-55'

~65.

 

 

1...]. 1.4.1

1.-...-

Figure 21. Variable temperature carhon-l3 nmr spectrum

of the carbonyl region of

[(nS-C9H7)Ru(CO)2]2 (VIII); the highest

field peak is CS2

 

 

 

 

Table 9.

1? (°C)

89°
68°
56°
35°
20°
-46°
-55°
-65°
-75°
-97°
-lll°

average of all carbonyls of both isomers

98

Chemical Shift and Rate Constant Data for

[(nS—C9H7)Ru(CO)2]2 (VIII)

6average
.5

219

219.
220.
220.
220.

WNNCD

247.
247.
248.
248.
249.
250.

itridgeb

I-‘-I-\\II—'\IU1

6terminalb

 

198.
198.
198.
198.
198.
198.

DDb-I-‘b

UI'

K(best fit)

 

33 x 107

3.8 x 107

5.25 x 107

8.4 x 105

2.6 x 105

55.6
13.33

probably bridge, terminal carbonyl resonances of
both isomers

99

 

 

 

 

 

 

 

 

 

.141 I)-..

 

IP‘igure 22. Variable temperature carbon—l3 nmr spectrum
of the carbonyl region of

(05-C5H5)FeNi(CO)3 (IX); the higher field
peak is C32

 

Table 10.
[(nS-CSH
T (°C) §average
~14° 237.8
~34° 238.2
-43° 238.5
-57° 238.7
-910
-102°
-117°
-130°

Chemical Shift
FeNi(CO)3]

100

6bridge

254.2
254.5
254.8
254.9

(IX)

6terminal

 

209.
209.
209.
209.

and Rate Constant Data for

K(best fit)

106

1.33 x 105
6.67 x 10°
1.67 x 104
200

40

 

.1“:

 

 

25' "J

 

1.....1
L wW-ve-

 

-..- J

Figure 23.

101

 

 

 

 

 

 

 

L..1 71......»

Variable temperature carbon-l3 nmr spectrum
of the carbonyl region of

[(05-C5H5)Ni(c0)12 (X); the higher field
peak is CS2

 

102

Table 11. Chemical Shift Data for [(nS-csasmimonz (X)

T (°C) 6

+25° 225.5

0° 226.2
-38° 226.8
-77° 226.1
—84° 227.4

-98° 227.8

 

 

 

 

 

 

103

Itemperatures were raised. This chemical shift dependence
was utilized for complex (V) to determine thermodynamic
parameters for the equilibrium between bridged and non-
bridged tautomers present in solution. Similarly, the
integrated intensities of carbonyl resonance lines
measured as a function of temperature for (I)-(IV) were
employed to obtain the thermodynamic parameters for the
cis-trans equilibrium between structural isomers of the
form 24A224B. Analyses of nmr lineshapes were performed
to obtain rate constants and activation parameters for the
structural interconversions detected.

Carbon-l3 nmr spectra measured in this study for
the iron carbonyls (I)—(IV) are reminiscent of those pre—
viously communicated7 by us for (I) and, as such, provide
evidence for two separate carbonyl exchange processes.
the most facile of which can be viewed in Figure 15 for
the methylcyclopentadienyliron complex (II) as occurring
between -125° and —35°C, in Figure 16 for the tetra-
hydroindenyliron complex (III) as occurring between -110
and ~24°C, and in Figure 17 for the indenyliron complex
(IV) as occurring between -102° and -9°C. This low tempera-
ture process is clearly seen for (IV) in Figure 17 to
average two equally intense bridged (273.2 ppm) and terminal
(211.6 ppm) carbonyl resonances. Studies at even very low
temperatures, gg. -l60°, failed to resolve similar,

Se . -
parate bridged and terminal resonances for complexes

t‘_

104

(I)-(III), although careful examination of the signals
observed for these complexes at -125°C reveals that they
are asymmetric. Activation energy data reported below
leave little doubt that, at the temperatures utilized in
this study, separate signals should, in principle, be
seen. It is likely that viscosity broadening, which be—
low -120°C limits resolution to gg. 1.0 ppm, as measured
from the carbon disulfide reference line, prevents their

detection.

The source of the lower temperature exchange pro-

cess is easily understood. Cotton gt gi,63'64

have re-
ported that it is possible to alter the ratio of the two
separated cyclopentadienyl resonances seen for (I) at low
temperatures by increasing solvent polarity, and have argued
that the more polar cis form 24A would reasonably be
expected to be more stable in the more polar medium.
Similarly, if less polar solvents than dichloromethane or
dichlorofluoromethane are used to obtain carbon-13 nmr
spectra of the carbonyl region of (I)-(IV), the intensity
of the low temperature averaged carbonyl resonance in-
creases in favor of the other as yet stationary carbonyl
signals. The lower temperature carbonyl averaging process
is therefore assigned to trans isomer bridge-terminal
carbonyl interchange.

The higher temperature exchange process, which
occurs for (II), (III), (IV) above -l4°, -10°, and +1l°,

respectively, results in the broadening and coalescing

 

“‘W'H‘Ln Isl: .

 

Figure 24.

105

A more realistic depiction of the solution

structures of (nS-dienyl) metal carbonyls
and related complexes

 

107

of all carbonyl resonances. Above 40°C, solutions of the
indenyl complexes decompose; however, spectra of the two
remaining iron systems taken at and above this temperature
display the growth of an averaged, chemical shift invariant
resonance found for (II) at 240.2 ppm. Concurrent with
this process, both the proton and carbon-l3 signals of
the cyclopentadienyl ligands of (I) are bbserved to broaden
and coalesce into a sharp line. Similar, though not so
straightforward data are obtained for the more complex
ligand spectra of compounds (II)-(IV). This set of
spectral observations can be handily accounted for by
postulating that, at the higher sample temperatures,
simultaneous cis-trans isomerization and cis isomer
carbonyl interconversion become energetically allowed.65’66
Spectra recorded for the ruthenium complexes (V)-
(VIII) may be compared with those obtained for the
homologous iron complexes. Low temperature spectra again
evidence the presence of both bridged and terminal car-
bonyls, which, as illustrated for these complexes in
Figures 18-21, are seen to broaden and coalesce at higher
temperatures into a single sharp line, thereby providing
evidence for only one bridge-terminal carbonyl inter-
change process. Yet another feature of the high tempera-
ture carbonyl spectra of the cyclopentadienylruthenium
complex (V) is of particular interest. As sample tempera-
tures are raised, the bridge-terminal averaged carbonyl

resonance shifts monotonically upfield towards the terminal

108

carbonyl chemical shift region, without appreciable line
broadening, from the averaged position which is a shift
from 223.2 ppm at -50° (dichlorofluoromethane) to 208.7
ppm at 96°C (toluene). The chemical shift of the averaged
signal was also seen to vary with solvent polarity at a
given temperature. The graph in Figure 25 illustrates
the behavior of the chemical shift of the averaged
resonance of (V) as a function of both temperature and
solvent. Only relatively small shifts of the averaged
resonance are observed for complexes (VI), (VII) and none
from (VIII). The marked chemical shifts seen for (V) at
various temperatures and.invarious solvents indicate that
we are able to detect the population redistribution be-
tween open and bridged isomers of the form 24E,F¢24G,H
as has been postulated from ir spectral studies.56
Concurrent, temperature dependent proton nmr
studies have been performed for the several ruthenium
complexes (V), (VI), (VIII). As reported previously,65
even at —110°C only one sharp pmr signal may be observed
for the cyclopentadienyl carbonyl (V) and, under similar
conditions, we could detect only one methyl proton and
carbon resonance for complex (VI). However, pmr spectra
of the indenyl complex (VIII) are more informative. The
spectrum in Figure 26a is typical of those obtained above

-30°C in a variety of polar solvents, such as dichloro-

fluoromethane, dichloromethane-d2, or acetone-d6. Spectral

109

A>vm_mxoo.emxm:mu -

rilr .. . . 1:.

CV_

. mucm>aom mooflum> CH
pow musumpdeou .m> DMHLm HmoHEoLo we uon

.mN deemed

 

 

 

m
86:.
oo. om om ow 0m 0 ow - o? om u om -
_ d _ q _ _ _ _ _ _
.I I. DON
..L../.T IT - ed
1 I... 9m
1 1 cum
I 1 mum
2:26.34 I
.N c2350 0
1 3028.4 1 0mm
_ [h b I]; _ _ _ _ _ _

 

 

 

(same/(0)039

-‘m"air IF h"

 

1‘“ I

E‘s;

Figure 26.

110

Temperature dependent proton nmr spectrum of
[(nS-C9H7)Ru(CO)2]2 (VIII). Spectra (a)-(c),
50% CDZClZ-CS2 solution; (d) CD2C12 solution

111

(0) I2’

\mmm
K180

IIZb

(15)-54°

 

trans

cis

 

 

bnn
U»...

....\x .on
v.5
“to

 

 

.j

112

features detected are easily assigned by reference to those

89 for the analogous (ms-indenyl)2Ru. The cyclo-

reported
pentadienyl ring protons form an AZB pattern, JAB = 0.5 Hz,
(vA-vB) = 11 Hz, while one relatively broad, slightly
structured resonance is observed for the protons on the
aromatic six-membered ring. In less polar solvents such

as 50% carbon disulfide-50% chloroform-d the J

1' AB/AVAB
ratio becomes gg. 1.0, giving rise to a complex multiplet.
At -54°C, in polar media, (see Figure 26b) both the
aromatic and cyclopentadienyl resonances are coalesced
into two indistinct multiplets. Below -70°C, the aromatic‘
resonances are narrowed somewhat into one relatively sharp
line at 732 Hz and a broader line centered at 670 Hz.
Simultaneously, the AZB multiplet is converted into a well
defined, unsymmetrical five line pattern, as illustrated
in Figure 26c,d. The character of the low temperature pmr
spectra is strongly influenced by solvent polarity. In
dichloromethane-d2 solution, the integrated intensity
ratio of the two aromatic resonances is 1.58:1, whereas
when the solution is diluted to 25% carbon disulfide, that
ratio becomes 1.22:1. The intensity of the most upfield
cyclopentadienyl line may also be observed to increase
markedly upon addition of carbon disulfide. This signal
becomes the dominant spectral feature of the cyclopenta-
dienyl ring proton region at -70°C when the least polar
solvent available to us, namely 50% carbon disulfide-50%

chloroform-d1, is used.

113

Two interpretations of the temperature dependent
pmr data were initially considered: a) as temperature is
lowered, the rate of cis-trans isomerization is decreased
and separate resonances characteristic for ligands on each
isomer are seen, or b) rotation of the indenyl ligand
about the ligand-metal bond slows and this in some complex
manner alters the pmr spectra.

Possibility (b) can be safely discarded on several
grounds. First, activation energies for rotations of

symmetrically-bonded nS-CS ring moieties are typically

very small,90 on the order of l kcal for ferrocene.91

The temperature required for coalescence of the cyclo-
pentadienyl ring pmr signals 11 Hz apart for (VIII)
was -54°C, which, by assuming a reasonable pre-exponential
frequency factor leads to an estimated Ea < 10 kcal, a
figure far in excess of that required for this type of
ring rotation. (A more complete analysis will be given
below.) Furthermore, ligand pmr data obtained under con-
ditions of slow ligand rotation are expected to be complex
A2B2’ AZXZ’ gtg. and ABC, ABX, gtg, spectra which should
exhibit solvent independent, integral intensity ratios of
separated resonances. Our observations are entirely at
variance with these expectations.

We must account for the observed pmr spectra in
terms of possibility (a). It is well established that

solvent polarity affects the equilibrium ratio of cis-trans

114

. 63,64
isomers.

We are therefore led to assign the sharp
aromatic ring signal to the cis isomer, and correspondingly,
the broad multiplet which increases in intensity in non-
polar media is attributed to those protons of the trans
isomer. The intense, most upfield cyclopentadienyl ring
resonance is the only signal to similarly increase its
intensity in solutions containing carbon disulfide. It
must therefore be assigned as an A3 signal arising from
all three protons on the trans isomer five membered ring;
the AZB multiplet, the A2 portion of which is clearly
observed at 574, 577 Hz in Figure 26c, remains for those
protons of the cis isomer. These assignments can be tested
for internal consistency by examining the relative inte-
grated intensities of the several sets of well-separated
spectral lines in Figure 26c. The cis-trans isomer ratio
of 1.22:1 obtained from integration of the aromatic ring
proton data may be used to predict an intensity ratio of
1.69 for the two separated five-membered ring multiplets
if indeed the cis B resonance is overlapped by the intense
trans A3 line. The measured ratio of upfieldzdownfield
multiplets is 1.62, in good agreement with our signal
assignments.

A temperature-dependent cmr study of the iron-
nickel system evidences only one bridged-terminal carbonyl
interchange process, as seen in the spectra in Figure 22.
As illustrated by the spectra in Figure 23, no such car-

bonyl interchange was detected for the related nickel-

I".

nu. _‘-___
1
I

115

nickel dimer (X) in the carbon-13 nmr study, nor did a
pmr study of this system reveal any dynamic behavior.

Finally, we note a temperature dependence of the
chemical shifts of the bridging and terminal carbonyl
resonances after the slow-exchange limit is reached. As
can be seen by examination of Tables 2-11, some of the
chemical shifts of the carbonyls, especially the bridging
carbonyls, are quite large; for instance, the bridging
carbonyl resonance of complex (II) shifts 3.2 ppm down-
field in going from -l4°C, where it was first detected, to
-125°C. Concurrently, the terminal carbonyl resonance is
shifted downfield only 0.6 ppm. The resonance of the
ruthenium complexes were not subject to such sizeable
temperature shifts, but this is probably because these
resonances were not detected until very low temperatures
were reached, and consequently their behavior could be
monitored over only a small temperature range.

Integrated signal intensities of the widely
separated resonances ascribed to the cis and trans isomers
for the iron systems (I)-(IV) were compared as described
in the experimental section to determine cisZtrans equilib-
rium constants. The thermodynamic parameters obtained
are exhibited in Table 12. The accuracy of the reported
data is somewhat limited due to the small temperature range
over which suitable spectral information could be obtained.
Overlap between Freon lock and ligand carbon resonances

precluded their use in similar studies. The data reported

116

 

Table 12. Thermodynamic Parameters for Cis-Trans
Equilibria
a b a
COMPOUND AH AS AG298
I 1 3 i 0 5 1.0 i 0 5 l 0 i 0.3
II 2.4 i 0 2 2 2 t 0 2 l 8 i 0.2
III 2.5 i 0 5 1 7 i 0 6 1 7 i 0.3
IV 3.5 i 0.1 3.3 i 0.1 2 5 i 0.1
a kcal/mole.
b

811.

117

are consistent with those previously determined for (I)

64 with the differences attributable to

from pmr studies,
the fact that our experiments were performed in somewhat
polar Freon solvents.

For the equilibrium between the bridged and non—
bridged isomers of the ruthenium complex (V), the follow-
ing thermodynamic parameters were obtained: AH =
2.6 kcal/mole, AS = 6.9 e.u., and AG298° = 0.5 kcal/mole.
To justify our assumption of the terminal carbonyl
chemical shift value, we note that resonance positions
measured for the other terminal ruthenium carbonyls are
very near to those assumed for the non-bridged structures.
Additionally, terminal carbonyl chemical shifts reported
for several other organoruthenium carbonyls in general do
not differ markedly from our assumed chemical shift.5
Attempts to analyze the bridged Z nonbridged equilibria of
the ruthenium systems (VI)-(VIII) were unfortunately
thwarted by the small temperature dependence of their
bridge-terminal averaged resonances.

Lineshape analysis of the temperature-dependent
cmr spectra for the nine fluxional complexes allowed
computation of activation parameters as described pre-
viously in the experimental section. The activation param-
eters acquired in the manner described are tabulated in
Table 13.

Computer analysis of the pmr spectra of complex

(VIII) proved unrewarding because of both the complexity

118

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119

of the spectra and the small chemical shift differences
measured between the spectral lines. It is possible,
fortunately, to extract some information from the
coalescence temperature (gg. -54°C) of the sets of
cyclopentadienyl resonances in order to test whether cis-
trans isomerization and bridge-terminal carbonyl inter-
change occur at similar rates. By assuming the Arrhenius
frequency factor measured for carbonyl interchange, an
activation energy may be calculated for the cis-trans
interconversion. Since K = 11 Hz = exp(26.6:1.5)exp(Ea/RT)
at coalescence, we can calculate Ea (cisitrans) to be
gg. 14.9i0.4 kcal/mole, in excellent agreement with
E8 = 14.5i0.5 kcal/mole measured for bridge-terminal
carbonyl interchange.

The results of a mass spectral investigation of
the ruthenium complexes V-VIII are tabulated in Table 14.
Naturally-occurring ruthenium is comprised of seven
isotopes, with the mass number and the percent natural
abundance of each isotope as follows: 96 (5.51%);
98 (1.87%); 99 (12.72%), 100 (12.62%); 101 (17.07%);
102 (31.61%), and 104 (15.85%). Since each molecule of
these complexes contains two ruthenium atoms, ruthenium-
containing fragments exhibit complex mass spectra. The
numbers quoted in the table are with respect to the isotOpe

of mass 101, since the atomic weight of ruthenium is

101.07.

120

 

Table 14. Mass Spectral Data for the Ruthenium Complexes
(V) — (VIII)
Complex (m/e); (relative abundance)
V 446(32); 418(20); 390(8); 362(8);
334(100); 206(36); 280(20); 231(4)
v: 474(24); 446(21>; 418(20); 388(73).
360(100); 332(14); 306(12); 280(12);
259(6)
VII 552(13); 496(18); 468(19); 434(100);
406(13); 331(11)
VIII 546(14); 462(19); 434(100); 406(20);
380(20); 331(8)

121

For complex (V), a molecular ion appears at m/e 444,
and is seen to lose four carbon monoxide groups in a
stepwise manner to yield the most intense peak at m/e 334.
Then stepwise loss of two fragments with m/e 26 is the
next major feature observed. The fragments of m/e 26 are
probably acetylene, or C2H2 moieties. A very small peak
due to ruthenocene (m/e 231) was also detected.

The mass spectrum of complex (VI) exhibits a
molecular ion at m/e 474. Stepwise loss of two carbon
monoxide groups is followed by loss of an m/e 30 fragment.
The most intense ion m/e 360 appears after loss of the
final carbon monoxide group. Following the appearance of
the most intense ion, successive loss of two m/e 30
fragments is again observed. Only a very small peak due
to l,l'-dimethylruthenocene (m/e 259) is observed.

A molecular ion is also observed in the mass
spectrum of complex (VII), with m/e 552. Simultaneous
loss of two carbon monoxide groups results in the
appearance of an ion at m/e 496. Then a mass loss of
m/e 34 follows. The most intense ion then makes an
appearance at m/e 434; this ion exhibits the same m/e
as that observed in complex VIII. However, the appearance
of the most intense ion of complex VII is followed by loss
of the final carbon monoxide group to yield a peak with
m/e 406. A small peak at m/e 331, corresponding again

to (C9H7)2Ru+, is also observed.

122

A molecular ion at m/e 546 is observed for complex
(VIII), which is followed by an ion at m/e 462, indicat-
ing that three carbon monoxide groups are lost simul-
taneously. Following the loss of the final carbon monoxide
group, the most intense ion appears at m/e 434. Successive
loss of two CzHé groups from the indenyl ring is then the
final important step. A small amount of (C9H7)2Ru+ is
detected at m/e 331.

The single most important feature of the mass
spectra of all four ruthenium complexes is that the
‘majority of the ions observed contain two ruthenium atoms,
an observation which is in sharp contrast to the mass
spectrum of the diiron complex I, in which no ions con-
taining two iron atoms are observed. This reflects the
relative strength of the metal-metal bonds in the two dif-
ferent homologous series. That the integrity of the
ruthenium—ruthenium bond is maintained under conditions of
a mass spectrum experiment indicates that it is consider-

ably stronger than the iron—iron counterpart.92

123

Discussion

 

The results of this investigation clearly demon-
strate that structural interconversions in solution of
dinuclear nS-dienyliron and -ruthenium carbonyls are very
general and facile processes. Both cis-trans isomeriza-
tion and intramolecular bridge-terminal carbonyl inter-
change could be examined by carbon-13 and/or proton nmr
techniques. The trans structure of the iron systems (I)-
(IV) is seen to interconvert carbonyls at different rates
and with very different activation energies than the
corresponding cis isomers. Carbonyl interchange for the
cis isomer is always slower than for trans and, moreover,
is always accompanied by simultaneous cis-trans structural
interconversion. In contrast, concurrent proton and
carbon-l3 nmr studies of the ruthenium homologs (V)-(VIII)
show that in these systems both cis and trans isomers
simultaneously undergo the isomerization 24E224F and inter-
change bridged and terminal carbonyls. The sizeable tem-
perature dependent upfield chemical shifts detected for
the bridge-terminal averaged carbonyl resonances of (V)-
(VII) suggest that non-bridged structures such as 24G,H
are appreciably more stable in solution than for the
corresponding iron system tautomers.

These spectral results offer strong confirmatory
evidence for the nmr interpretations we have previously
presented7 for (I) and, as well, support the mechanistic

explanation of these data recently put forth by Cotton

 

124

65

et gi. and Roberts gt gi.,66 and outlined in the

introduction to Chapter 2 and in Figure 12. In order to
test the general applicability of this mechanism, we per-
formed experiments designed to answer the following
questions: i) Can the (presumably steric) barrier to
rotation be altered by changing the nS-dienyl ligand?

ii) Does changing the inductive character of the ligand
have a measurable and understandable effect on structural
interchange rates? iii) Will the mechanistic considera-
tions apply to structural interconversions of homologous
ruthenium systems?

To answer these questions, the dinuclear molecules
(I)-(VIII) were chosen for study since they offered
systematic variation of two properties, extent of alkyl
substitution and ligand size. The portion of space which
must be subtended by the rapidly rotating metal-bonded n5-
dienyl moiety increased from (I) + (IV) and alkyl sub-
stitution similarly increases the electron donating pro-
perties of the cyclopentadienyl ring for (I) + (III).

Activation parameters recorded in Table 13
demonstrate that decreasing the bulkiness of the ligand
has a remarkable lowering effect on the free energies
(A6298°) and enthalpies (AHI) of activation measured for
cis isomer carbonyl interconversions in the iron system.
Parallel observations of temperature dependent pmr spectra
for (I)-(IV) leave little doubt that this process is

accompanied by'andintegral to cis-trans isomerization.

 

 

h___._
I u

125

Activation energies for the two processes in (I) agree to

64’66’93 This evidence is

within a few tenths of a kcal.
strongly supportive of the proposed mechanism in that, at
least for (III), (IV) and probably for (I), (II), the rate
determining step for cis—trans interconversion must be
attributed to rotation over the steric barrier in the
Cotton-Roberts mechanism. It is also interesting to note

a

that the large excursions in AH with ligand size are
accompanied by parallel changes in activation entropy
(A85). Inspection of molecular models for cis-(IV) reveals
that the two ligands may not freely rotate against each
other. The phenyl ring protons in the most eclipsed con-
former would overlap by gg. 1.3 A, and indeed, would still
interact strongly in the rotamers in which the ligands

are gauche. Passage to the trans rotamer would allow

free ligand rotation and could account for the increased
entropy. The pmr spectrum of the trans bridged isomer of
(IV) and (VIII) does in fact contain a collapsed A3 signal
for all three protons on the cyclopentadienyl ring, in-
dicating that they experience similar, averaged environ-
ments.

The large variation in activation parameters is not,
however, seen for iron system trans isomer carbonyl inter-
conversions. Instead, AHI and A85 increase in the series
(I) + (III) as alkyl substituents are added to the five-
membered ring. This can be understood in terms of a

picture developed by Braterman for bonding in these

126

complexes.94 Bridged metal carbonyl bonds are thought of
as having, in addition to a metal-metal bond, two three
center components, as illustrated in Figure 27. One, 27a,
is formed by overlap of the filled ligand 0 orbital

with two empty metal 0 orbitals and the second, 27b, is
constructed from back donation from two filled metal non-
bonding g orbitals to empty ligand 0* orbitals. The three
center bonding concept for bridging carbonyl systems

gains credence in light of the statement by Cotton that
"bridging carbonyl groups never occur unless the bridged

metal atoms are formally bonded to each other."95

Electron
donating alkyl substitutents on the cyclopentadienyl ring
would tend to increase its electron density which might
then be transported to the metal carbonyl dative 0 bond,
since carbonyl 0* orbitals tend to act as electron sinks.
This would make bridge-breaking more difficult and could
be the source of the small increase in AHI seen for car-
bonyl interconversion of trans (I)-(III).

As discussed on pages 45-51 of this dissertation,
the chemical shifts of metal-bonded carbonyls are quite
sensitive to the inductive properties of the other metal-
bonded ligands. The carbon-l3 nmr data evidence downfield
shifts for both bridged and terminal carbonyls in the
sequence (I) + (III), (V) + (VII), and an upfield shift
is observed for the indenyl derivatives. The latter shift

96

is not unexpected since it is known that the indenyl

127

[no

9
4?,
O

 

Figure 27. Description of a bridging M~C0 bond.
(a) illustrates the overlap of a filled CO
0 orbital with two empty metal 0 orbitals;
(b) illustrates the back donation from two
filled metal g-orbitals to empty 0“ CO

orbitals

128

ligand is slightly electron withdrawing. Furthermore, the
chemical shift of the decomposition product of the
decamethyliron dimer, [(nS—(CH3)5C5)Fe(CO)2]2, almost
certainly the chloride, (ms-(CH3)5C5)Fe(CO)2C1, is found at
214.1 ppm, which is the lowest field terminal iron carbonyl
observed in this laboratory. The systems in this study

do exhibit carbonyl chemical shifts that reflect the in-
ductive ability of the cyclopentadienyl ligand. The
observed shifts for (I)-(III) are consistent with our
interpretation of the effects of alkyl substitution on
rates of carbonyl interconversion.

By using the thermodynamic data obtained for the
observed structural interconversions, a schematic repre-
sentation of enthalpy versus reaction coordinate may be
constructed for the iron system (Figure 28). Our work
provides clarification of the several already published,

differing portrayals63’64’93’97

of these processes in
that AH3, the barrier to rotation of non-bridged conformers,
is shown to be sizable and to determine the rates of cis-
trans isomerization for (III), (IV), and probably (I),
(II). The figure is rigorously correct only for (I).

The enthalpy difference AH2 between the bridged
and non-bridged isomers was obtained for (I) and (V) by
Noack56 from ir studies. Manning and coworkers reported
that ir bands assigned to non-bridged species of (I) and
(V) may also be observed for (II), (VI), but that in-

58-60,98

tensities are sharply reduced. No such bands could

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pow mum50p0ooo c00uomou .m> >mrmce 0m00cm0oa mo Empwmmc 000mEm£om .wm murm0m

 

 

 

 

 

 

129

 

 

 

 

 

130

be seen for (IV) or (VIII). Our ir results also follow
this pattern, as only barely detectable non-bridged isomer
absorptions were seen for complex (VII), and these absorp-
tions are solvent polarity dependent.

It is apparent that alkyl substitution on or
aromatic fusion with the cyclopentadienyl ring decreases
the stability of non-bridged conformers. This indicates
that AHZ values increase somewhat in the sequences
(1) + (IV) and (V) + (VIII). Alkyl substitution in the
iron systems certainly increases both the enthalpy for the
cis-trans equilibrium (AHI) and the barrier to trans
isomer carbonyl interconversion (AH6). At least a portion
of the increase in AH6 can probably be attributed to the
greater AHZ, since AH4 values are estimated to be < 0.2
kcal/mole.64 Barriers to bridge opening are assumed to be
identical in both isomers in Figure 28.

Results of our nmr investigations of the nS-dienyl-
ruthenium carbonyls reveal a striking difference in their
solution behavior as compared to the iron systems in that
only one bridge-terminal carbonyl interconversion may be
observed in the carbon-13 nmr spectra. Concurrent, tempera-
ture dependent proton nmr studies of (VIII) demonstrate
that bridge-terminal carbonyl interconversion and cis-
trans isomerization occur simultaneously. The presence in
solution of sizeable quantities of non—bridged isomers for
(V)-(VII) again suggests these species as intermediates

in the structural interconversion processes and renders

131

probable the mechanistic pathways described above.

Comparison of activation parameters for cis-trans
isomerization (cis data for the iron systems and the
ruthenium parameters in Table 13) shows that for the
ruthenium systems, the bulkier ligands change AH]6 and
A85 appreciably, but not nearly so dramatically as for
iron. Increased ASI values still indicate some release
of steric compression in the transition state, but the
barrier to rotation of non-bridged tautomers is markedly
reduced. The increase of AHI in the sequence (V) + (VII)
in fact parallels similar values for trans isomer car-
bonyl interconversions of the iron homologs. Alkyl sub-
stitution effects on rate for (I)-(III) parallel (V)-(VII).
The above evidence and the observed simultaneous isomeriza-
tion and carbonyl interconversion suggest strongly that
bridge Opening to non-bridged rotamers is the rate de-
termining step for the ruthenium system structural inter-
conversions.

A reaction coordinate diagram may be constructed
to portray the energetics of structural interconversions
for (V)-(VIII). The actual thermodynamic values repre-
sented in Figure 29 are those for (V) and include our
measured value for AHZ. While precise values for AH1
could not be obtained, proton nmr spectra for (VII) and
(VIII) leave little doublt that both cis and trans isomers
are present in about the same quantities and have the

same solvent polarity dependencies as measured for the

 

 

 

 

 

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133

homologous iron systems. The exact barrier to rotation
AH5 cannot at present be determined, but for simultaneous
isomerization and carbonyl interconversion to occur, it
must certainly be smaller than the enthalpies required
for bridge opening in both isomers. For purposes of the
diagram, equal activation enthalpies for opening cis and
trans isomers were assumed.

Now that solution structural interconversion studies
have been performed for a number of isoelectronic
(ms-dienyl)2M2(L)2(L')2 complexes (M = Cr, L = L' = NO),

(M = Mn, L = NO, L' = C0), (M = Ru, Fe, L = L' = C0), as.
well as for numerous iron systems in which one carbonyl

was replaced by an isocyanide or a phosphite, it seems
profitable to consider more carefully the exact nature of
the barrier to rotation of the non-bridged conformers by
examining the structural and activation energy data in

Table 15. The tabulated Ea values for [(n5C5H5)Mn(CO)(NO)]2
(XI) and [(nSC5H5)Cr(NO)212 (XII) were obtained by a more
detailed consideration of previously reported nmr data93
in terms of the Cotton-Roberts mechanism. In their
analysis of the structural interconversions in solutions
of these complexes, Marks gt gi. proposed that direct cis-
trans isomerization could be effected directly through the
non-bridged intermediate obtained by concerted, pairwise
bridge opening of either isomer. Rather, rotation about

the M-M bond is also required, as has been outlined above

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135

for carbonyl interconversions. Similar considerations
apply to the pmr data for complex (XII).

Several factors which determine the magnitude of
the barrier to rotation of the non—bridged conformers can
easily be extracted from Table 15 and Figures 28 and 29.
Both the enthalpy difference between bridged and non-
bridged structures (AHZ) and the size of the steric
barrier (included in AHS) are important. The AH2 value
for complex (I) is nearly twice that of (V), which partially
accounts for the decreased rotational barrier observed
for the ruthenium systems.

The barrier to rotation AH3 was measured, and was
seen to increase from 7.1 to 28.8 kcal/mole upon substitu-
tion of indenyl for the cyclopentadienyl ligand in the
iron system. Probably only a small portion of this dif-
ference can be attributed to AHZ, so there is at least an
SE- 15 kcal/mole increase in the steric barrier. Yet, in
the ruthenium complexes, upon similar substitution, less
than 7 kcal total increase in E8 is observed, and a portion
of this, at least 1-2 kcal, must be due to the larger AHZ.
This marked difference between the iron and ruthenium
systems seems surprising at first glance because the change
in M-M bond distances from complex (I) to complex (V) is
only 0.2 A. It may be that the Ru-Ru bond is sub—
stantially longer in (VIII), but this seems highly un-
likely. At present, its crystal structure has not been

determined. Alternatively, the steric barrier may simply

 

l

4

.1
| 'I ' 7‘
a.»

 

136

be unusually sensitive to even small changes in M-M bond
distances. The M-M bond distances for the non-bridged
complexes [(nS-C5H5)M(CO)3]2, M = Cr, Mo, W, decrease as
Cr(3.281 A) > Mo(3.235 A) > W(3.222 A); yet, changes in
A0398, sizeable Cr(12.l) < Mo(lS.O) < W(16.2).99 Since
bond distances measured for both cis and trans (I) are
identical to within experimental error, it is unlikely
that non-bonded repulsion interactions used to explain
the above Group VIB carbonyl data can be important for
(I)—(VIII).

Steric considerations alone certainly cannot
account for the increased barrier to rotation measured
for (XII) > (XI) > (I). Bond distances are measured to
increase regularly with decreasing metal atomic number,
leading us to expect a corresponding, regular decrease in
the steric barrier. Increases in strength of metal-
nitrosyl versus metal—carbonyl bonds could, as suggested
by Marks gt £1.93 account for larger Ea values for trans
isomer interconversions, but cannot account for the in-
creased rotational barrier, assuming no change in mechanism.
Rather, we must suspect that AHZ increases substantially
in order (I) < (XI) < (XII). Unfortunately, no AHZ data
are available for the latter two complexes. The very
large changes in AHZ which would be required to explain
the Ea data in Table 15 lead us to speculate that yet
another factor may be of importance. The ability of the

first row transition metals to form metal-metal dn-d7T

137

bonds is expected to increase with decreasing nuclear
charge for isoelectronic systems. The order of dn-dTr
bond strength predicted is Cr > Mn > Fe, in the order of
decrease in rotational Ea values. An inspection of the d
orbitals in the non-bridged conformer reveals that dn—dTT
overlap is indeed possible for (I), (XI), and (XII). No
evaluation of the possible importance of directional M-M
bonds is now possible, if, indeed they even exist. No
independent studies relevant to this speculation are
evident to us. Further work is necessary to test the
validity of this speculation.

The iron-nickel dimer (IX) undergoes carbonyl
interconversion. In this case, proton nmr spectra are,
however, unchanged with temperature, in agreement with ir

98 which indicate the presence of only the trans

studies
isomer in solution. In this case, a mechanism involving
concerted bridge making and breaking is required to effect
carbonyl interchange, rather than the Cotton—Roberts
mechanism, since the absence of cis-trans isomerization
precludes a bridge opened structure and interconversion of
carbonyls via the pathways depicted for the other di—
metallic carbonyls examined here.

A mechanism was proposed by Cotton et al.65 to
account for the carbonyl interconversion of the singly-
bridged complex (nS-C5H5)2Rh2(C0)3 . As depicted in
Figure 30b, this mechanism involves passage of the

structure through a triply-bridged intermediate. The

11*:

 

”*1

Figure 30.

138

Some mechanisms which might effect carbonyl
interchange in single—bridged complexes.
(a)

unbridged intermediate;

bridged intermediate;

(b) triply-
(c) a concerted mechanism

 

 

 

139

 

 

 

 

140

mechanism preferred by Lewis gt gt.100

for a phosphite-
substituted derivative of the rhodium complex is given
in Figure 30c; this mechanism involves the concerted
opening of the bridging carbonyl and closing of one of
the terminal carbonyls. The synchronous mechanism seems
to be more believable. From our data we cannot deduce
the mechanism by which carbonyl interchange is effected
in the iron-nickel dimer but we can note that the activa-
tion energy and the other activation parameters for the
process are of the same order as those which proceed ztg
the Cotton—Roberts mechanism.

Finally, a short statement on the chemical shift
dependence of the slow-exchange resonances for these
complexes would seem in order here. Two experimental
observations which eliminate a few speculations are a) the
shift is observed both in the presence and the absence of
the shiftless relaxation reagent, tttg(acetylacetonato)
chromium(III) ; and b) the shift is observed in the non-
fluxional nickel-nickel dimer (X). It is knownlOI’102
that bridging carbonyl ligands are better Lewis bases
than their terminal counterparts. The downfield shifts
of both the bridging and terminal carbonyl resonances,
with the bridging carbonyl resonance being shifted to a
greater degree, can then be rationalized by postulating

that as the temperature is lowered, solvent-ligand inter-

actions become stronger. The solvent molecules interact

rm- -— \ .- g1
8

141

more strongly with the bridging carbonyls than with the

terminal carbonyls because of their basicity. At higher

temperatures, the interaction becomes less important be-

cause the carbonyl groups begin to scramble.

142

Suggestions for Subsequent Research

 

Some of the developments in the course of this
study suggest that its scope be extended. This section
outlines some thoughts about further developments along
the line of this research project.

A carbon-13 and pmr study of the permethylated
iron dimer (a carbon-l3 nmr study was attempted during the
course of this research) would certainly be in order.
From the reported solution ir of this complex,64 it is
apparent that the trans structure is maintained in solu-
tion. This observation, together with the fact that the.
permethylated cyclopentadienyl ring is extremely bulky,
should make a study of this molecule quite interesting.
Since the complex itself does not undergo exchange with

13CO, it would be necessary to synthesize it using

free
enriched Fe2(CO)9, which in turn would have to be
synthesized from enriched Fe(CO)5. Since this latter

103 this compound

reaction can give yields as high as 90%,
would not be difficult to synthesize.

Another complex of immediate interest is the
complex (nS-CSHA-CH(N(CH3)2)z-nS-C5H4)Fe2(CO)4, which we
have christened "the tied-back dimer." The crystal

104 reveals a molecule similar

structure of this complex
in structure to the cis isomer of (I); the molecule is

locked into the cis configuration by a two-carbon bridge
which links the two cyclopentadienyl rings. A carbon-13

nmr study of the carbonyl region of this complex would

143

prove interesting because, if indeed the carbonyl groups
were observed to be scrambling, a modification of the
Cotton-Roberts mechanism would be in order.

In this study we utilized only one complex with
an electron withdrawing ligand, the indenyl ligand. The
perchloroiron dimer, which has not as yet been reported
in the literature, would be of considerable interest,
since the pentachlorocyclopentadienyl ligands would be
extremely electron withdrawing. A possible route to this
complex is the following: Reduction of hexachlorocyclo-
pentadiene with stannous chloride affords l,2,3,4,5—

105 which upon treatment with

pentachlorocyclopentadiene,
thallium ethoxide at -78° in pentane yields the white pro-
duct thallium pentachlorocyclopentadienide. Reaction of
this material with Nil2 in acetone yields the green

106

perchloronickelocene. By following the method of

Tilney-Bassett for the synthesis of the iron-nickel dimer

82 it may be

(IX) by reaction of nickelocene with Fe(CO)5,
possible to obtain quantities of the desired product plus
quantities of the perchlorinated iron-nickel and nickel-
nickel dimers through an analogous route by using

perchloronickelocene and Fe(CO)5.

Investigations of the possible fluxional behavior

of adducts of (I) with Lewis acids such as triethylaluminum

are feasible. Reaction of complex (I) with excess

A12(CH2CH3)3 yields a red solution from which crystals of

144

I(US-C5H5)Fe(CO)2]2-2Al(C2HS)3 may be isolated.101 The
ir spectrum of this complex shows a decrease of 112 cm-1
101

in the stretching frequency of the bridging carbonyl.
Of interest here is whether the carbonyl groups can still
interchange when a Lewis acid is coordinated to the
bridgir@;carbonyls, and whether cis-trans isomerization
can<occur. If complexes of this type are found to be
static: in solution, the chemical shifts of the coordinated
carborryls might provide information concerning the extent
to wfuich.the electronic distribution of the carbonyl carbon
is peflrturbed when a Lewis acid is coordinated to the oxygen.
The relaxation times of bridging and terminal
carborryls are at present unknown. By utilizing a highly
enriCIIed sample of the indenyliron complex (IV), it should
be qui.te easy to obtain Tl relaxation times for the
carborryls. Use of the indenyl dimer precludes the use of
verYIOW temperatures in order to reach the slow—exchange
limit,, An interesting extension would be to determine the
T1 rEIJixation time of the interconverting carbonyls at
highex- temperatures. Should it prove feasible to obtain
carborgyl Tl relaxation times, the studies could be
furtheq: augmented with investigations of the T1 relaxa-

tion tiJnes of carbonyls coordinated to Lewis acids.

 

CHAPTER 3
A CARBON-13 MAGNETIC RESONANCE EXAMINATION OF THE

FLUXIONAL BEHAVIOR OF SOME FIVE-COORDINATE
ORGANO-IRON AND -RUTHENIUM CARBONYL COMPLEXES

Background

 

Pentacoordination is observed in transition metal
complexes in which the transition metal is formally a (18
system. X—ray diffraction studies of the pentacoordinated
transition metal complexes completed to date indicate they
have structures which are either trigonal bipyramidal or
square pyramidal, or a distorted structure related to one
of these idealized geometries. Stereochemical nonrigidity
is a characteristic of pentacoordinate compounds of ele-
ments in group VA, such as P(V) compounds. Numerous in-
vestigations of pentacoordinate compound of group VA
elements have provided evidence from which a general re-
arrangement mechanism has been established.lO7’108
Similar intramolecular rearrangements are likely to be
common in the transition metal complexes as well, although
they have received only limited attention.

An early carbon-l3 nmr study of Fe(CO)5 showed only

one resonance,43’109

rather than the expected two re-
sonances of 3:2 ratio, as would be expected from the

trigonal bipyramidal geometry. A single signal can arise

145

146

from either a coincidental identical chemical shift value
for the axial and equatorial carbonyls, or a rapid ex-
change of the carbonyls between the two nonequivalent
sites. The latter is almost certainly the case for
Fe(CO)5.

Recently a study of a series of Fe(CO)5_x(PF3)x
complexes by using carbon-l3 nmr and infrared spectroscopy

110 From the data obtained-

was completed by Mahnke gt gt.
these workers calculated the chemical shift difference
between the axial and equatorial sites of Fe(CO)5 to be
17.5 i 1.5 ppm. Furthermore, they calculated the axialj
equatorial exchange frequency to be on the order of

1.1 x 1010 sec"1 in the Fe(CO)5 molecule.

The first unambiguous proof that a pentacoordinate
transition metal complex can undergo rapid intramolecular
rearrangement was obtained from a variable temperature
fluorine-l9 nmr study of the molecule CF3Co(CO)3PF3.111
It was demonstrated that all the ligands in this complex
rapidly rearrange even at low temperatures. Since this
discovery, other fluxional pentacoordinate transition
metal complexes have been studied.112

In 1966 two investigations of the carbon-13 nmr

113

spectra of butadienetricarbonyliron(O) and dienetricar-

bonyliron(0)114
tions the authors mentioned in passing that only one

resonance was observed in the carbonyl region of the cmr

spectrum, but neither group of authors speculated as to

complexes were reported. In both communica-

147

the reason for the appearance of one resonance, and not
two, or three, resonances. The structure of these

dienetricarbonyliron-type molecules is based on a square

115

pyramidal geometry, so ideally there are one apical

and two basal CO groups in the molecule. Furthermore,
these molecules have Cl symmetry, so all three carbonyls
are, strictly speaking, nonequivalent. However the carbon
chemical shifts of the two basal carbonyls should be very
similar.

From carbon-l3 nmr investigations of butadienetri-
carbonyliron(0) and cyclohexadienetricarbonyliron(O) systems
and their PF3-substituted derivatives, Clark gt gt.116’ll7
concluded that these molecules are fluxional, the intra-
molecular rearrangement being effected through a rotational

118

motion comparable to the Bailar twist. Giacometti

119

gt gt. then established that the carbonyl groups in

cyclooctatetraenetricarbonyliron(O) exchange at a rate

independent of the ring carbon exchange process.

120

Kruczynski and Takats reported that the single

carbonyl resonance observed in a series of dienetri-
carbonyliron(0) complexes can be resolved into two re-
sonances of 2:1 intensity at low temperatures, thus in—
dicating that molecules of this type are indeed fluxional.
These authors suggested that the carbonyl groups permute
independently from the dien moiety. This suggested

mechanism is similar to that suggested by Clark gt gt.ll6'll7

121 122

and is related to the Berry pseudorotation and turnstile

148

mechanisms which are often invoked to rationalize the
intramolecular rearrangements observed in pentacoordinate
group VA molecules.

A variable temperature carbon-13 nmr study of
1,3,5—cycloheptatrienetricarbonylmolybdenum(O) and

123 found that

-chromium(0) undertaken by Kreiter and Lang
the carbonyl groups undergo rearrangement at a rate in-
dependent of the ring carbon rearrangement. No hypothesis
was proposed to explain this observation.

Variable temperature carbon-13 nmr studies of
olefintetracarbonyliron(O) complexes by two groups of

workerslzzi’125

have confirmed that these molecules undergo
intramolecular rearrangement ytg a concerted coupling of
rotational motion of the olefin and Berry pseudorotation.
A subsequent study of the homologous olefintetracarbonyl-
ruthenium(0) complexes indicates that these complexes

too are fluxional, although the activation energies are
slightly higher than in the homologous iron complexes.

We have undertaken an investigation of the fluxional
behavior of several five-coordinate metal carbonyl com-
plexes by using variable temperature carbon-l3 nmr tech-
niques. The complexes studied are cycloheptatrienetri-
carbonyliron(0) and norbornadienetricarbonyliron(O), as
well as the related complexes (SP)M(CO)3, where M is Fe
or Ru and SP (illustrated in Figure 31) is 2—vinylpheny1
(diphenyl)ph05phine. The details of this investigation

follow.

 

149

 

 

 

Figure 31. Structure of the ligand 2-vinylphenyl-
(diphenyl)phosphine (SP)

(3

= P c
030 N (4

oc I P oc T [I P “I" to

C C C

C) C) C)

(a) (b) (c)

 

_ O
0c \ 0c P 0c P
“ M W
C I
c 0‘3 o C)c //

(d) (e) (f)
Figure 32. Geometrical isomers of the (SP)M(CO)3 complexes;

(a)-(c) trigonal bipyramidal structures;
(d)-(f) square pyramidal structures

150

Experimental

 

Cycloheptatrienetricarbonyliron(0) was synthesized
from Fe(CO)5 and 1,3,5-cycloheptatriene (Aldrich Chemicals)

126

by the method of Wilkinson. Norbornadienetricarbonyl-

iron(0) was synthesized from Fe(C0)5 and norbornadiene

127

(Aldrich Chemicals) as described by Pettit. The two

(SP)M(CO)3 complexes were generously provided by Professor F’

13 ll

M.A. Bennett. Each complex was enriched with C0 by

stirring gg. 0.4 grams of complex, dissolved in dichloro-

13C-enriched carbon

methane, in an atmosphere of 9 %
monoxide for two weeks. The diene complexes, being liquids,
were redistilled after enrichment. The two SP complexes
were recrystallized. Spectra were obtained in solutions

of dichlorofluoromethane in all instances except (SP)Fe(CO)3,
which was dissolved in dichloromethane. Chemical shifts

of carbonyls are reported in ppm downfield from tetra-

methylsilane as are ring carbon chemical shifts. All

solutions were 0.05 M in tris(acetylacetonato)chromium(III).

151

Results

The variable temperature carbon-13 nmr spectrum
of the carbonyl region of l,3,5-cycloheptatrienetri-
carbonyliron(O) is shown in Figure 33. At 0°C one
resonance is observed at 211.3 ppm. As the temperature
is lowered this resonance broadens and then splits into
two components at -65°C of relative intensity 1:2 at
214.3 and 210.4 ppm. When the temperature is lowered to
-90°C, the resonance attributed by Kruczynski and Takats120
to the basal carbonyl is resolved into two components;
thus all three carbonyls are demonstrated to be unique.,
At this temperature the carbonyl chemical shifts are
214.4, 210.6 and 210.3 ppm. The proton—decoupled variable
temperature carbon-13 nmr spectrum of the ring carbons
of this compound is shown in Figure 34. It can be seen
that the resonances are essentially identical at 0°,

-51°, and -93°C.

The variable temperature carbon-13 nmr spectrum
of the carbonyl region of norbornadienetricarbonyliron(0)
is pictured in Figure 35. At -6°C one resonance is seen
at 216.8 ppm. At —125°C a single resonance is found at
217.9 ppm, unchanged except for line broadening due to
solution viscosity. It is certain that the broadening is
due to viscosity broadening, and not to exchange broaden-
ing, since the carbon disulfide resonance is broadened to

the same extent.

 

 

 

 

(H,

IAAHHMfiHkbar’
”LflflhflflflJwbzr

 

 

 

 

 

 

I

  

 

 

 

 

45' Mk v “40.

Figure 33. Variable temperature carbon-13 nmr spectrum
of the carbonyl region of cycloheptatrienetri-
carbonyliron(0); the highest field peak is CS2

k~

mZH we xmea namww umocwflc ore moncoufiazcopumowuumcmflpumodecoHowo wo
conmu Cocumo wcflu msu mo Enuuomam MES mauconhmo musumumaEmu wHLmHL¢> .qM mpnwwm

.ne-

iii-jig...

 

 

 

 

 

 

 

 

 

 

 

 

154

 

 

 

 

 

 

WWI-

 

 

em

 

 

 

 

 

 

 

J

W

422°

 

Figure 35. Variable temperature carbon-l3 nmr spectrum of
the carbonyl region of norbornadienetri-
carbonyliron(0); the high field peak is CS2

 

. J
« .\

 

 

 

 

 

—fl'

 

 

-93’ “W ' "ABET”
l

 

 

 

it...

Figure 36. Variable temperature carbon-13 nmr spectrum of
the ring carbon region of norbornadienetri-

carbonyliron(0); the highest field peak is TMS

.520

 

 

 

 

 

k...

-46'

Figure 37. Variable temperature carbon-l3 nmr spectrum
of the carbonyl region of (SP)Fe(CO)3

 

157

 

 

 

 

 

 

 

.30. ' .5.

 

 

-50. h 402.

Figure 38. Variable temperature carbon- 13 nmr spectrum
of the carbonyl region of (SP)Ru(CO)3 the
highest field peak is CS2

‘13‘*“b.’-_._-.- _
“ Fl

 

158

The spectra of the two SP complexes are shown in
Figures 37 and 38. The spectra are not of high quality,
but it is evident that from a single resonance at room
temperature the spectrum becomes more complicated as the

temperature is lowered.

159

Discussion

 

The carbon-l3 nmr spectrum of 1,3,5-cycloheptatriene-
tricarbonyliron(0) as a function of temperature was re-
ported while we were in the process of obtaining similar
spectra.120 However these authors were content to end
their experiment at the point at which the basal carbonyl
resonance was not resolved into two separate resonances.
We believed that our nmr system could achieve sufficient
resolution to separate the basal carbonyl resonances if
indeed the basal carbonyls are nonequivalent. Our con-
fidence was justified, as the spectra plainly illustrate.

We are unable to speculate any further concerning
the mechanism of carbonyl rearrangement without con-
current rearrangement of the ring carbons. It appears as
if the mechanism involving simultaneous rotation of the
carbonyl groups and the diene moiety, accompanied by small
bending motions of the diene moiety, can neither be
challenged nor fortified by our data. We have merely
demonstrated that the three carbonyl groups within this
complex are nonequivalent, and that in the future chemists'
investigations of similar complexes should include studies
at much lower temperatures than have previously been
used.

The norbornadienetricarbonyliron(O) complex is
believed to possess the square pyramidal geometry featured
in the previously-discussed complex and yet, even at

-122°C, only a single relatively narrow resonance is

fir—“7‘ nth-fl

160

observed. In the light of the results obtained from our
studies of the cycloheptatriene complex and from pre-
vious studies performed by other workers, we can only
conclude that this molecule is extremely fluxional even
at very low temperatures. The norbornadiene ligand is a
1,4-diene; it can conceivably span the axial-equatorial
sites of a trigonal bipyramidal intermediate (the inter-
mediate invoked in a Berry-type mechanism) more easily 1“
than can the 1,3-diene moiety of 1,3,5—cycloheptatriene. 1_
Since this complex is exceedingly more fluxional (”more
fluxional" in the sense of being characterized by a much
lower activation energy) than the cycloheptatriene complex,
our data indicate that a Berry-type mechanism is perhaps
the mechanism of choice. This mechanism is pictured in
Figure 39.

The 2-vinylphenyl(diphenyl)phosphine ligand is
bidentate; it has been shown to coordinate to the central
metal atom through the tricoordinate phosphorus atom and

128 The

through the olefin moiety on the styrene ring.
iron and ruthenium complexes of interest can conceivably
exhibit trigonal bipyramidal or square pyramidal geometries,

128 indicate that the structure

but solution ir studies
is trigonal bipyramidal. Figure 32 depicts the three
geometrical isomers which in principle can exist for these
complexes. Bennett gt gt.128 suggested that isomer

32a,b, or c is the structure of the molecule, but they

reported that cis and trans proton-phosphorus coupling

161

(D D
{Zig‘ —- B
A. | C. <I-—-' A"- C
‘5
E
Figure 39. Figurative representation of the Berry

mechanism

#2:

162

constants are identical, as shown by a pmr investigation
of the two complexes. This observation should have
implied to earlier researchers that these complexes are
stereochemically nonrigid.

The cmr spectra of the carbonyl regions of these
two complexes do indicate that a rearrangement process
takes place between ambient temperatures and gg. -100°C.
Unfortunately, the complexity of the spectra plus their
overall poor quality ruled out any attempts to deduce
mechanisms by which these complexes rearrange and to make
assignments in regard to which isomers are present in

solution in the slow-exchange limit.

 

CHAPTER 4

SELENIUM-77 NMR. MEASUREMENT OF T1 RELAXATION TIMES
AND ELUCIDATION OF Tl RELAXATION MECHANISMS OF THE

SELENIUM-77 NUCLEUS IN SOME ORGANOSELENIUM COMPOUNDS.
THE UTILITY OF TRIS(ACETYLACETONATO)CHROMIUM(III)
AS A RELAXATION REAGENT IN SELENIUM-77 NMR

Background

 

The selenium-77 isotope has a nuclear spin of +g
and it comprises 7.58% of naturally-occurring selenium.
By considering its natural abundance and the magnitude of
its nuclear magnetic moment, u = +0.534 nuclear magnetons,
the sensitivity for the nmr experiment may be calculated
at 3.09 relative to the nmr sensitivity of carbon-l3 at
a constant magnetic field. Yet, few investigations of
selenium-containing compounds by using selenium-77 nmr
have been reported.

The first important selenium-77 nmr data were col-

129

lected by Birchall gt gt., who measured the selenium nmr

spectra of twenty-six halo— and oxyhaloselenium compounds
as well as various selenium oxides and oxyacids. The
chemical shift range of selenium proved to be quite large:
SeOBr2 was found 80.3 ppm downfield from SeOClZ and

HZSe was found 1704.5 ppm upfield from SeOClZ. Selenium-

fluorine coupling constants of 600 to 1600 Hz were

163

164

observed in these complexes whereas selenium-proton

1

coupling constants were found to be much smaller; J

Se-H
for HZSe, the largest selenium-proton coupling constant
observed, was measured to be 62 i 1.5 Hz.

The first application of selenium-77 nmr to

130 who

organoselenium chemistry was made by Lardon,
measured the selenium chemical shifts of some cymmetric
dialkyl- and diphenylselenides and diselenides. Since
Lardon used cw techniques to obtain the spectra, neat
liquids or melts were employed. The chemical shifts
observed ranged from 822 ppm for diphenyldiselenide to.
1282 ppm for dimethylselenide (upfield from external
H28e03 (aq.)). Large differences in chemical shift were
observed between selenides and diselenides having the
same organic substituents, with the diselenide resonance
being shifted downfield compared to the resonance of

the corresponding selenide. Lardon explained the large
downfield shifts observed for selenium in terms of the
domination of the screening tensor by the paramagnetic
term, a rationalization not unlike that given for other
heavy nuclei. He also correlated chemical shift data of
these compounds with their known electronic excitation
energies En - E0. The mean electronic transition energy
is lower for phenylselenide than for alkylselenides, and
these in turn are higher than the mean electronic transi-
tion energy of diselenides. Hence the prediction that the

phenyl selenide resonance should appear upfield from those

166

Hammett's o-constants or the two-parameter equations of

Swain and Lupton.133

Halogen-substituted selenophenes
exhibited chemical shifts which correlated well with
Pauling's electronegativity values; likewise a correla-

tion in the values of the selenium-proton coupling con—

stants was noted in these compounds: JSe-H increases

5
with increasing electronegativity whereas JSe-H and p
3 F
JSe-H decrease with increasing electronegativity. These f
4 3
authors were also able to correlated the selenium-77 W

chemical shifts of these compounds with the carbon-l3
chemical shifts in some similarly-substituted benzenes.
This close correlation suggested that selenium-77 chemical
shifts are dominated by the paramagnetic term of the
screening tensor in much the same manner as those of
carbon-13.

Upon completion of the multinuclear modifications
of the Bruker HFX-lO, it was decided that investigations
of less well-known and therefore less well-studied nuclei
would be in order. Knowledge of the nmr sensitivity
of the selenium-77 nucleus as well as the availability of
several selenium compounds within this department prompted
our choice of selenium. This investigation was approached
initially as a demonstration of the utility of the re-
laxation reagent tttg(acetylacetonato)chromium(III) in
obtaining nmr spectra for nuclei other than carbon-13.

Our study soon became more ambitious when it became apparent

that the selenium-77 nucleus exhibits many interesting

165

of alkylselenides and that the dimethyldiselenide re-
sonance should appear further downfield than the other
diselenide resonances is borne out by experiment. Lardon
took these observations to be substantial evidence that
the AE term in Equation 71 dominates the paramagnetic
screening tensor in selenium. The first-order selenium-
proton coupling constants in these complexes range from
10 to 15 Hz and the second-order coupling constants range
from 2 to 9 Hz. 1

131 studied the effect

In a subsequent paper Lardon
of para substitution on the smr spectra of some diphenyl-

diselenides, XPhSeSePhX, where X = NH 0CH3, H, Cl, Br,

2’
or N02. The g priori prediction that the selenium
nucleus should be more shielded as the electron density

on the carbon to which it is bonded decreases was proven
to be inaccurate. Likewise, an attempted correlation be—

tween the selenium-77 chemical shifts and the electronic

transition energies of these complexes failed to develop.

However Lardon stated that since the spectra were obtained

from melts, then perhaps the susceptibility of the dif-
ferent compounds is the origin of the aberrative shifts.
Lardon also utilized selenium-77 nmr to study thermally-
induced rearrangements in dibenzyldiselenide.

A series of 2-substituted selenophenes were re-

132

cently studied by Gronowitz gt gt. and no correlation

could be found between selenium-77 chemical shifts and

166

Hammett's o-constants or the two-parameter equations of

Swain and Lupton.133

Halogen-substituted selenophenes
exhibited chemical shifts which correlated well with
Pauling's electronegativity values; likewise a correla-

tion in the values of the selenium-proton coupling con-

stants was noted in these compounds: JSe-H increases

5
with increasing electronegativity whereas JSe-H and
3
JSe-H decrease with increasing electronegativity. These

4
authors were also able to correlated the selenium-77

chemical shifts of these compounds with the carbon—l3
chemical shifts in some similarly-substituted benzenes.
This close correlation suggested that selenium-77 chemical
shifts are dominated by the paramagnetic term of the
screening tensor in much the same manner as those of
carbon-l3.

Upon completion of the multinuclear modifications
of the Bruker HFX—lO, it was decided that investigations
of less well-known and therefore less well-studied nuclei
would be in order. Knowledge of the nmr sensitivity
of the selenium-77 nucleus as well as the availability of
several selenium compounds within this department prompted
our choice of selenium. This investigation was approached
initially as a demonstration of the utility of the re-
laxation reagent tttg(acetylacetonato)chromium(III) in
obtaining nmr spectra for nuclei other than carbon—l3.

Our study soon became more ambitious when it became apparent

that the selenium-77 nucleus exhibits many interesting

 

167

properties which we were able to observe. A similar study
was attempted by using compounds of the more metallic
congener of selenium, tellurium. However, due to severe
decomposition of the tellurium-containing samples and, in
most cases, our inability to even detect a tellurium
magnetic resonance signal, this experiment was deferred
for the present. W
The remaining portion of Chapter 4 details the
results of.a study of the selenium-77 nucleus in several
organoselenium compounds, as well as our attempts to
measure tellurium magnetic resonance signals is similar
tellurium compounds. Investigations of the nuclear
magnetic resonance properties of these nuclei were under-
taken in a very conservative manner —— we gauged relaxa-
tion times, measured Overhauser enhancements, and recorded
chemical shifts in a manner such that our work might
serve as a basic starting point for subsequent investiga-

tions which utilize magnetic resonance studies of these

nuclei.

168

Experimental

 

Chemical shift and coupling constant measurements,
nuclear Overhauser enhancement measurements, and T1 re-
laxation time measurements were made by using a broad-band
nmr insert tuned to 17.17 Mz for selenium-77 or a carbon-l3
insert detuned to 17.17 Mhz. It was ascertained that the
carbon-13 insert could deliver more power to the sample fl
and hence the width of the pulse necessary to flip the
magnetization vector 180° was shorter in comparison than F“
a similar pulse transmitted by the wide-band insert. Our
T1 measurements were performed using the (l80°-T-9O°)
sequence. In a typical experiment, thirty T values were
utilized, with the initial I value, the final I value, and
a randomly selected T value being equal to T, which was
chosen to be five times T1' The pulse width required
to flip the spins 180° was determined by pulsing a sample
of diphenyldiselenide heavily doped with tttg(acetyl-
acetonato)chromium(III) at the exact resonant frequency.
The value of the pulse width was then varied until the
exponential FID had zero intensity. A 90° pulse is exactly
half the value of a 180° pulse. The thirty FID's acquired
during each T1 experiment were Fourier transformed under
identical conditions, the frequency domain spectra were
plotted, the intensities of the resonances were measured,
and the T1 value was calculated from the slope of the

graph of ln(l - A/AO) versus T. Nuclear Overhauser

 

169

effects on signal intensity were measured in a three part
experiment: 1) spectra were obtained without proton de-
coupling; 2) this experiment was repeated with the proton
decoupler on; and 3) experiment (2) was repeated with the
proton decoupler switched on and with sufficient
tris(acetylacetonato)chromium(III) present to make the
solution gg. 0.1 M in the relaxation reagent. Addition

of tttg(acetylacetonato)chromium(III) has been demonstrated

133,134 The

to undermine the nuclear Overhauser effect.
signals obtained from all three experiments were then
integrated by the 1083 computer, and the integrals were
compared to determine the amount of signal enhancement.

Diphenyldiselenide was purchased from Eastman
Organic Chemicals and used without further purification.
Measurements were made on a 1.5 M solution of diphenyldi-
selenide in 80% carbon disulfide-20% hexafluorobenzene
(PCR, Incorporated). tttg(acety1acetonato)chromium(III)
was added in carefully weighed amounts to 2.5 m1 of the
solution. The broadband insert was used in the
diphenyldiselenide experiment and the 180° pulse width
was 105 usec. Fifteen pulses for each I value were
necessary to achieve a good signal-to-noise ratio.

Benzeneselenol was purchased from Strem Chemicals
and was trap-to-trap distilled immediately prior to use
in order to free it of any diphenyldiselenide formed by
air oxidation. Data were acquired by using a sample

comprised of 80% benzeneselenol and 20% hexafluorobenzene.

 

170

The detuned carbon-l3 insert was utilized in this experi-
ment and the l80°pulse width was shortened to 40 usec.
Twelve scans per T value were used in the T1 experiments.

Dimethylselenide was purchased from Strem Chemicals
or synthesized by the method of Bird and Challenger135
and was distilled trap-to—trap immediately before use in
order to free it of paramagnetic materials which severely
broaden the nmr signal. A sample consisting of 50%
dimethylselenide, 30% dichloromethane, and 20% hexafluoro-
benzene was used in all experiments. Again, the detuned
carbon-13 insert was used and the 180° pulse width was.
measured as 40 usec. Eight scans per T value yielded a
sufficiently good signal-to noise ratio to calculate T1
values.

Attempts were made to observe tellurium-123
resonances at 23.59 MHz and tellurium-125 resonances at
28.43 MHz; (where both isotopes are spin I = -%) the
broad-band insert was used in both instances. A sample
of dimethyltelluride (Strem Chemicals) was prepared in
the same manner as was the dimethylselenide sample; it
proved to be extremely air sensitive, easily forming a
paramagnetic material which severely broadened the nmr
signal. Telluric acid, tellurium tetrachloride, tellurium
tetrabromide, and tellurium tetraiodide (all purchased
from Strem Chemicals) also servedaussources of tellurium
for this experiment. Samples of the latter three com-

pounds dissolved in carbon tetrachloride or the first

171

dissolved in water failed to produce tellurium nmr signals,
either in the presence or in the absence of relaxation
reagents. Several unsuccessful attempts were made to
observe both the tellurium-123 and tellurium-125 re-

sonances of these compounds.

 

172

Results and Discussion

 

The T1 relaxation time of the two equivalent
selenium nuclei in diphenyldiselenide was found to be
15.7 seconds. If selenium-77 nmr is to become a routinely-
used technique, we reasoned that a way must be devised to
shorten the long Tl relaxation time which impedes the
efficient acquisition of data. Since it is known that
tttg(acetylacetonato)chromium(III) reduces T1 relaxation
times of carbonyl carbons without broadening the reso—
nances,7’87’136 we investigated the effect of its addi-
tion on selenium Tl relaxation times. The results are‘
given in Table 16.

A plot of T1 versus the concentration of added
tttg(acetylacetonato)chromium(III) is shown in Figure 40.
From this plot it is readily apparent that this relaxa-
tion reagent is very effective in reducing the long
selenium Tl relaxation time, a necessity if FTNMR methods
are to be of general use. This relaxation reagent was
moreover demonstrated to be "shiftless" by the following
experiment: the selenium-77 nmr spectrum of diphenyldi-
selenide and benzeneselenol dissolved in dichloromethane
was measured in the presence and in the absence of
tttg(acety1acetonato)chromium(III). The chemical shift
difference in the absence of added relaxation reagent was
311.5 ppm.and in its presence was 312.8 ppm. With the

spectrum being measured over a 12.5 Khz spectral width.

173

 

T.(sec)

-4

+

l l l i' 11 J r—?-
.04 .06 .08 .IO

[Cr(acacl3]

 

 

 

 

 

Figure 40. Plot of T1 vs. concentration of Cr(acac)3
for the selenium-77 nucleus in (C6HSSe)2

GE?

 

.1;

4 ‘11 .

174

Table 16. T1 Data for (C6H58e)2 with Added

Cr(acac)3

 

[Cr(acac)3] T1(sec)
0 15.7
.0025 10.0
.005 7.7
.0075 5.2
.01 3.5
.025 2.0
.05 1.1
.075 0.7

0.1 0.5

 

175

Bloch gt gt.1 first observed that the addition of
paramagnetic ions to water dramatically reduces the proton
T1 relaxation time. This effect was investigated by

137 who developed a theory

Bloembergen, Purcell, and Pound,
concerning its origin similar to that for dipole-dipole
relaxation. The relaxation time observed is a function
of the diffusional Brownian motion of the water molecules

in the vicinity of the paramagnetic ions. The following

expression was derived for the relaxation rate:

1/T1 = 4n2-2anufiff/kT (73)
where N is the number of paramagnetic ions per cubic
centimeter, “:ff is the effective mean—square magnetic
moment of the electrons on the paramagnetic ion, and n is

137 Figure 41 shows a plot

the viscosity of the solution.
of l/Tl versus the concentration of added tttg(acetylace-
tonato)chromium(III), and the expected straight line is
obtained, since the changing variable is the concentration
of paramagnetic ions Np' If other paramagnetic relaxa—
tion reagents such as tttg(acety1acetonato)gadolinium(III)
or tttg(acetylacetonato)iron(III) were instead used, the
value of piff would be altered, and a similar plot would
probably exhibit the same slope, but the intercepts would
vary.

Benzeneselenol was chosen as the next sample because

the nature of our eXperiment dictated use of a compound

with a proton directly bonded to the selenium nucleus.

176

 

|/ 1'I

 

 

 

l

l 1 l l

.06 .08 .I0
[Cr (acackfl

 

0 .02 .04

Figure 41. Plot of (T1)-l vs. concentration of
Cr(acac)3 for the selenium-77 nucleus in
(C6H58e)2

177

The non-(proton) decoupled spectrum obtained initially

1

revealed that J is 56.12 Hz and 3J is 4.27 Hz.

Se-H Se-H
The maximum nuclear Overhauser enhancement for selenium-77

is calculated (by using Equation 49) to be
n = YH/ZYSe = uH/Zuse = 2.61; (l + n) = 3.61 (74)

However, measurement of the enhancement gained through

the nuclear Overhauser effect at room temperature showed
(1 + n) to be 1.07. The results of this experiment are
shown in Figure 42. This somewhat surprising development
indicates that the familiar dipole-dipole relaxation
mechanism, so common in carbon-13 nmr , does not dominate
the selenium relaxation time in this compound; the
selenium is not relaxed primarily through interactions
with the directly-bonded proton. This led us to perform

a T1 study of the selenium-77 nucleus in benzeneselenol,
and the results are given in Table 17 and pictured
graphically in Figure 43. As sample temperature is in-
creased, the T1 relaxation time increases, a trend which
eliminates from consideration the spin-rotation mechanism.
Until a field-dependent Tl relaxation study is completed,
we can only surmise that the T1 relaxation mechanism is
either chemical shift anisotropy or scalar relaxation, the
T1 relaxation induced by the former being characterized by
a field-dependent Tl relaxation time. Since scalar re-
laxation is usually induced by either chemical exchange

(scalar relaxation of the first kind) or by rapid

 

 

 

 

Figure 42.

178

(a) Selenium-77 nmr spectrum of C6H5SeH

with no proton decoupling; (b) spectrum with
proton decoupling and in the presence of
Cr(acac)3; (c) spectrum with proton de-

coupling, no Cr(acac)3 present; note that

all the integrals are approximately equal

180

 

Table 17. 77Se Tl Data for C6HSSeH
T(°C) Tl(sec) g
5° 1.2 .0776
25° 1.8 .07

55° 2.3 .0000033

181

 

TI (sec)

 

 

 

1.111Lllllll
ID 2030 40 50

T(°C)

 

Figure 1&1 Plot of T1 vs. temperature for the selenium-77

nucleus in C6HrSeH
5

‘ w .ik‘fi

.x‘-
i

182

fluctuations in an adjacent (and usually a spin > %)
nucleus, it would seem most likely that benzeneselenol

is relaxed primarily through the chemical shift anisotrOpy
interaction.

The chemical shift difference between diphenyldi-
selenide and benzeneselenol was found to be 312.8 ppm,
with the diphenyldiselenide resonance being the more
downfield resonance. The C6HSSe- moiety apparently directs
more electron density around the selenium than does a
single proton. The chemical shift of the proton directly
bonded to the selenium in benzeneselenol, measured at

138 is indicative of the fact that there

1.35 ppm from TMS,
is nothing unusual about the electronic properties of the
C6H5Se- moiety.

In order to determine whether or not the absence
of dipole-dipole relaxation in selenium compounds is a
generally-occurring phenomenon we examined the selenium
spectrum of dimethylselenide, which, although it has no
protons directly bonded to the selenium does exhibit

139 An undecoupled spectrum of

proton-selenium coupling.
dimethylselenide was obtained over a spectral width of
1000 Hz; after 900 transients all seven components of
the expected septet could be seen. ZJSe-H is 10.74 Hz.
Figure 44 shows the free-induction decay of the unde-
coupled spectrum, its Fourier transform, and the

selenium-77 nmr spectrum of dimethylselenide obtained by

Lardon. The comparison of Lardon's spectrum, obtained

 

 

 

 

 

Figure 44.

183

(a) Free-induction decay (FID) of the
selenium-77 nucleus in (CH3)28e; (b) the

Fourier transform of this FID; (c) the

selenium-77 spectrum of (CH ) Se obtained
3 2

by Lardon

 

 

 

 

(a)

 

(b)

(C)

185

from a spectrometer equipped with cw instrumentation,
with the spectrum we obtained vividly demonstrates the
superiority of FTNMR techniques.

The nuclear Overhauser experiment was performed
and, once more, the result is that no significant NOE
enhancement was observed at room temperature, where
n = 0.036. The spectra are shown in Figure 45. A tempera-
ture-dependent T1 study was performed, and the results
are given in Table 18 and illustrated in Figure 46. In
the case of dimethylselenide the trend in T1 versus tem-
perature indicates the dominance of the spin-rotation.
relaxation mechanism; as the temperature increases, the
T1 relaxation time decreases. From the microwave spectrum

140 it was shown that there is internal

of dimethylselenide
rotation of the methyl groups as well as overall rotation
of the molecule. The barrier to internal rotation was
determined to be 1500 i 20 cal/mole. Either the internal
rotation of the methyl groups, the overall molecular rota-
tion, or a combination of the two processes is capable of
providing a pathway by which the nuclear spins can return
to equilibrium. Conceivably the latter process is the
most important case, since the microwave spectrum exhibits
a fine structure attributal to the coupling of the two
rotational modes.
Our findings from this series of experiments with

Selenium-77 nmr are 1) the nuclear Overhauser effect is,

in the molecules we have examined, not of major importance,

 

 

Figure 45.

186

(a) Selenium-77 nmr spectrum of (CH3)ZSe

with no proton decoupling; (b) spectrum with
proton decoupling and in the presence of
Cr(acac)3; (c) spectrum with proton de-

coupling, no Cr(acac)3 present; note that

all the integrals are approximately equal

 

188

Table 111 77Se Tl Data for (CH3)ZSe
T (°C) T1(sec)
5° 5.6
18° 5.0
27° 4.5 (n‘= .036)

40° 3.2

 

 

T. (39C)

Figure 46.

 

 

189

 

2'07-1-1il-1-ni‘

IO 20 30 40 50
T(°C)

 

 

 

Plot of T1 vs. temperature for the selenium-77

nucleus in (CH3)ZSe

2.1
i

 

190

since relaxation processes other than the dipolar-dipolar
mechanism dominate the T1 relaxation. 2) Selenium Tl
relaxation times may be quite long; and 3) the use of a
shiftless relaxation reagent such as tttg(acetylace-
tonato)chromium(III) can effectively reduce long selenium
relaxation times. We have demonstrated that the future

of selenium nmr should be viewed optimistically, once
required instrumentation becomes generally available.
Indications are that those chemists interested in selenium
will eventually be able to utilize selenium nmr techniques
in experiments much easier to effect than carbon nmr
experiments, which are at present routine.

Attempts were also made in our laboratory to ob-
serve magnetic resonanee signals of the two spin -%
tellurium nuclei, tellurium—123 and tellurium-125. The
isotope of mass 123 has only 93% of the sensitivity of
carbon-13 at constant field, whereas the isotope of mass
125 has a sensitivity 12.8 times that of carbon-13 at
a constant magnetic field (and is therefore 4.2 times as
sensitive as selenium-77!). However, the only tellurium
magnetic resonance signal we were able to obtain was the
tellurium—123 resonance of dimethyltelluride. Its free-

induction decay and its Fourier transform spectrum are
2

shown in Figure 47. The JTe-H was measured to be 17.08 Hz.

The spectrum shown resulted from 1000 transients, and even

with this number of scans only five of the components of

v

(b)
Figure 47. The tellurium-123 nmr spectrum of (CH3)2Te;

(a) the free-induction decay; (b) the Fourier
transform spectrum of (a)

 

192

the septet were resolved. The extreme air sensitivity,
coupled with the truly unbearable stench of this com-
pound, prompted our abandonment of further experimenta-
tion with this compound. We were unable to find the
tellurium—125 resonance for this compound, and as stated
previously, our attempts to locate tellurium signals of

either isotope in other tellurium compounds were fruit-

less.

 

 

CHAPTER 5

THE EFFECT OF TRIS(ACETYLACETONATO)CHROMIUM ON
CARBON-13 T1 RELAXATION TIMES

Background

 

This chapter deals in a rather limited fashion
with the effect of an added shiftless relaxation reagent
upon the T1 relaxation times of carbon nuclei. Initial
use of tttg(acetylacetonato)chromium(III) in carbon-l3
nmr was focused on its ability to undermine the nuclear
Overhauser enhancement observed when protons coupled to

carbon atoms are decoupled.87'134»141-142

This observed
"short-circuiting” of the nuclear Overhauser effect is
attributed to an electron-nuclear dipole-dipole relaxation
mechanism dominated by the large magnetic moment of theelec-
tron (see Equation 57). The electronic magnetic moment

is gg. 103 times that of the proton. As the paramagnetic
molecules diffuse in solution, their motion induces tre-
mendous fluctuating local magnetic fields which in turn
rapidly relax the excited protons. Because the proton
spins are relaxed as rapidly as they are excited, the
carbon nuclei are not influenced by the energy transi-
tions.

Due to the magnitude of the electronic magnetic

moment, the attenuation of the dipole-dipole relaxation
193

194

process by the factor l/r6 apparently does not dominate

the influence of the unpaired electrons.. In most in-

stances involving organic substrates, all carbon atoms

are relaxed to the same degree by the relaxation reagent.141

A dramatic demonstration of the influence a shiftless

relaxation reagent possesses is seen in the carbon

spectrum of the carbonyl cluster carbide [Rh6(CO)15C]-2.

The structure of this cluster compound is a bare carbon

atom surrounded by an octahedron of rhodium atoms, which

are in turn bonded to terminal and bridging carbonyl

groups. The central carbide carbon is buried deeply

within the metal—carbonyl framework, and yet a carbon—13

nmr spectrum obtained from a Fourier transform experiment

was able to be measured in a few thousand pulses when the

solution was doped with tttg(acetylacetonato)chromium(III).143
It has subsequently been discovered, however, that

when intermolecular interactions of any sort are present

between various carbon nuclei and the relaxation reagent,

these carbon nuclei are selectively relaxed at a faster

rate. Levy gtgt.144’145

measured the T1 relaxation times
of carbon nuclei in noninteracting substrates such as
benzene or toluene as well as those in substrates such as
phenol, anisole, or benzonitrile, compounds which possess
groups which can perhaps attract the relaxation reagent.
These workers observed that in various mixtures of these

compounds the noninteracting carbons were indeed affected

to a lesser extent by the presence of the relaxation

 

195

reagent than those carbons in the vicinity of the inter-
acting site. When the interaction between substrate and
relaxation reagent is through hydrogen bonding, the re-
laxation times of the various carbon nuclei are observed
to vary as l/r6. However, there is no angular dependence

142'146 In principle, then, a relaxation reagent

observed.
may be utilized to calculate bond distances and to make
assignments of resonances.

We undertook a study of the carbon T1 relaxation
times in mixtures of benzene and cyclohexane as a function
of added tttg(acetylacetonato)chromium(III) or tttg-
(acetylacetonato)iron(III), in order to determine whether
these molecules might indeed be able to interact weakly
with the relaxation reagent and in order to compare the

relative effectiveness of two different relaxation reagents.

The results of this study follow.

196

Experimental

 

Solutions were prepared from degassed benzene and
cyclohexane in the following manner: aliquots from a
stock solution of 0.1 M relaxation reagent (Cr(acac)3:
Research Organic/Inorganic Chemicals; Fe(acac)3: Ventron
Corp.) in benzene were measured and transferred ytg
pipette and diluted with sufficient benzene, cyclohexane,
and hexafluorobenzene (PCR, Inc.) to form solutions con-
sisting of 40% benzene, 40% cyclohexane, and 20% hexa-
fluorobenzene (by volume) and a known concentration of
relaxation reagent.

T1 relaxation times were measured by the 180°-I-9O°
method described in Chapter 4. Optimum signal-to-noise
was achieved with 4 scans per I value, and a total of
twenty-seven values and three infinity values (i.e.

T = 5 x T1) were used per T1 run. Spectra were recorded
and their intensities were directly measured by hand, and
the T1 calculation was carried out as outlined previously.

The 180° pulse was measured as 70 usec.

 

 

I." I‘alfor. '.-.r‘jq

Ev

 

197

Results and Discussion

 

The results from this series of experiments are
illustrated graphically in Figures 48-53. Plots of T1
versus [Cr(acac)3] for benzene and cyclohexane, Figures
48 and 49, yield curves qualitatively similar to that
observed previously for selenium T1 relaxation times and
plots of UTI versus [Cr(acac)3] and [Fe(acac)3] are
linear, as would be expected. However, the difference
in slope between the iron and chromium relaxation reagent
graphs, Figures 50, 51 and 52, 53, should be noted. As
would be expected from the relative squared ionic
magnetic moments, “:ff’ of the two metal ions, the plot
of 1/T1 against [Fe(acac)3] shows a steeper slope than
the similar plot for Cr(acac)3. It is apparent that
tttg(acetylacetonato)iron(III) is a more efficient re-
laxation reagent than tttg(acety1acetonato)chromium(III).
Were efficiency the only criterion, certainly the iron
relaxation reagent would be used exclusively. However,
the chromium relaxation reagent is preferred in this
laboratory because Cr(III) is kinetically inert, whereas
Fe(III) is not strictly so, and because the presence of
iron oxide on the surface of some samples which con-
tained tttg(acetylacetonato)iron(III) has been noted.

The plots shown in Figures 50 and 51 clearly in-
dicate that the benzene carbon Tl relaxation times are
decreased more rapidly than cyclohexane carbon T1 re-

laxation times in the presence of a relaxation reagent.

198

It is believed that this is a manifestation of a weak
interaction, probably through hydrogen bonding, between
the benzene and the relaxation reagent. The cyclohexane
molecule most likely has only a minimal interaction with
the relaxation reagent, hence it is less—strongly in-
fluenced. Inasmuch as this experiment involved symmetric,
cyclic molecules, no further conclusions can be extracted
from the data, but certainly experiments involving
molecules with many functional groups, some interactive
with the relaxation reagent, other noninteractive, should
prove interesting. Other possible variations of this
experiment include the use of more highly substituted
ligands, such as 3,5-pentanedione or 2,2,6,6-tetramethyl-
3,5-heptanedione, for the synthesis of relaxation reagents.
These bulkier ligands would increase the distance of
closest approach between the paramagnetic metal ion and

the carbon atoms in the substrate.

199

 

 

 

 

 

Figure 48. Plot of T1 vs. concentration of Cr(acac)

for the carbon—13 nuclei in cyclohexane

 

200

 

 

 

 

IL l—H

.03 .04
[Cr (acac)3]

 

Figure 161 Plot of T1 vs. concentration of Cr(acac)3

for the carbon-l3 nuclei in benzene

201

 

be»

In.

 

 

 

l l l l l l J l I

 

.005 .OIO 0|?) .0 20
[Cr(acacl3]

Figure 50. Plot of (T1)'l vs. concentration of
Cr(acac)3 for the carbon-l3 nuclei in

cyclohexane

202

 

 

 

 

 

1 1 J 1 I 1 1 1 1
.005 .OIO .0l5 .020
[Cr(acac)-5,]

Figure EH_ Plot of (T1).1 vs. concentration of

Cr(acac)3 for the carbon—13 nuclei in benzene

203

 

 

 

 

 

I 1 l I I
.5 - -‘
.4
K: .3
.2
.l
l l l l L
.00I .0025 .005 .0075 .0l
[Fe(acoclil

Figure 52. iPlot of (T1)-1 vs. concentration of Fe(acac)3

for the carbon-13 nuclei in cyclohexane

204

 

 

 

 

 

.5
.4
I: .3
2
.2
. I
L l l l J
.OOI .0025 .005 0075 .0l
[Fe(acacla]

Figure 53. 'Plot of (Tl)-1 vs. concentration of Fe(acac)

for the carbon-l3 muclei in benzene

CHAPTER 6
DINUCLEAR COBALT CARBONYLS IN SOLUTION. A CARBON-13

MAGNETIC RESONANCE EXAMINATION OF THEIR STRUCTURES,
EQUILIBRIA, AND STRUCTURAL INTERCONVERSIONS

Background

 

The structure and behavior of C02(CO)8 (XIII) in
solution, as well as its structure in the solid state, have
been the focus of numerous papers in the chemical literature
during the past three decades. In 1947 Anderson147
speculated that the diamagnetism and dimeric nature of
C02(CO)8 might be accounted for in terms of a structure
with bridging carbonyls and a cobalt-cobalt bond. He
based this conjecture on the known structure of Fe2(CO)9,

which had been reported eight years earlier.148

149

Syrkin
and Kyatkina proposed that, from the standpoint of the
electronic distribution within the molecule, there is no
reason to eliminate the structure featuring eight terminal
carbonyl groups and a cobalt-cobalt bond.

The solution structure of C02(CO)8 was subsequently

150 who utilized infrared,

studied by Nyholm gt gt.,
visible, and untraviolet spectrosc0py. The infrared
spectrum clearly evidence the presence of bridging carbonyl

7': .
groups, as did the ketone-like n + n transition observed

in the ultraviolet spectrum. These authors then considered

205

206

 

 

 

 

 

O | o
C%;\. C£D.—av"C£> ,’E:>CQ7”’(L 0"TT—CK)
C-
C)
C 2v C2 h
0 O o O
C: C C C
C) C)
C\ | CO I I
|° I
C: C c; C
C) C) c) c)
D 2h 02h
W\\‘\. :,C:\ ,I’CICK>
AC0
”III’ \\\‘

D2h

Figure 54. Several structures postulated for C02(CO)8
(XIII)

207

the structure to be one with one of the following symmetry
groups: D2h’ C2h’ C2v’ D3h’ and D3d' The infrared spectra
they obtained made the choice of the correct structure a
difficult one, but these authors preferred the D2h
structure. See Figure 54.

Wender gtgt.151

reported the infrared spectrum
of C02(CO)8 in solution and in the gas phase and concurred
that bridging carbonyls are present. These authors con—
sidered but rejected the possibility that in solution two
forms of the molecule were present. They did obtain
direct proof, though, that the bridging to terminal carbonyl,
ratio is 2:6 by treating C02(CO)8 with acetylene. The
spectrum of the resulting compound, C02(CO)6C2H2, in-
dicated that the bridging carbonyl infrared absorptions
had disappeared, while the absorptions corresponding to
the terminal carbonyl groups remained. The molar
absorptivity measured from the infrared absorptions of the
bridging carbonyls of C02(CO)8 matched quite well the
extinction coefficients observed in organic carbonyl
groups. From their data, these authors proposed a
structure based on trigonal bipyramidal coordination about
the cobalt atoms with the cobalt atoms and the bridging
carbonyl groups being coplanar. This structure is shown
in Figure 54.

The structure of a related acetylene complex

C02(CO)7(C4H202) (XIV) (empirical formula C02(CO)9C2H2)

208

was obtained by Mills and Robinson.152 Its characteristic

structural feature is that the two metal atoms and the two
bridging carbons are not coplanar. In the light of that
evidence, these authors suggested that perhaps specula-
tions about the structure of C02(CO)8 should be based on
the absence of two cobalt atoms ceplanar with the two
bridging carbonyls. See Figure 55.

In a study primarily designed to test the resolu-
tion of their infrared Spectrophotometer, Bor and Mark0153’154
investigated some solutions of C02(CO)8. They considered
the structure proposed by Mills and Robinson to be quite
likely correct, but stated that infrared spectroscopy pgt
gg was incapable of distinguishing between differing
structures of C2V symmetry.

By using newly developed calcium fluoride optics,
rather than the conventional sodium chloride optics,

Cotton and Monchamp155

undertook an ir investigation of
solutions of C02(CO)8 and were able to attain superior
resolution as compared to all previous studies. The
observations recorded by Cotton and Monchamp are
essentially identical with those of Bor and Marko: the
results obtained from ir spectrosc0pic studies reflect

the symmetry of the species of interest. As a consequence,
ir studies cannot be used to discern differences in
structure among molecules with identical functional groups

156 157

and identical symmetry. Bor and Noack then reported

the infrared spectrum of C02(CO)8 along with spectra of

209

1‘
—n

3—

/;

II
_\_.<'/
/ \\ o

\

- — — n

o
____n/

?
n
n
o

I;

O
\
n—
I
-\
I
I
/
I
I

O-n~—-_

Figure 55. Crystal structure of CoZ(CO)7(C4H002) (XIV)

210

several other metal carbonyls by using the improved ir
optical system. Bor reported that some of the terminal
carbonyl bands appeared to be doublets under the higher
resolution.

A major breakthrough in interpreting the results
obtained from infrared studies of C02(CO)8 was announced
by Bor in 1963.158 By using the results he obtained from
a high resolution infrared study, and by citing the fact
that C02(CO)8 exchanges its carbonyl groups quite readily

with free 14COlsg’160

he postulated that two configura—
tions of the molecule exist in solution. He based his
hypothesis on the observation that the intensity ratio
of the doublets he had observed earlier is solvent de-

pendent. Shortly thereafter Noack161

presented evidence
obtained from a temperature-dependent infrared study of
C02(CO)8 in pentane. The spectra obtained at room
temperature and at -105°C were conspicuously different.
Bridging carbonyl absorptions were present at both tem-
peratures; however, only three of the terminal carbonyl
absorptions remained with their original intensity at
-105°C. The other terminal carbonyl absorptions were
considerably weakened in intensity. Noack interpreted
the results to indicate that two configurations of C02(CO)8
exist in solution in a temperature-dependent equilibrium.

The low temperature form possesses bridging carbonyls and

the high temperature form possesses only terminal carbonyls,

211

 

 

o o

0“ 8 /(’O I :l;
\

oc—Co< >Co—CO :co—Co o—C
C/ C \ /C

o 0 Co 06’ O

A 8

Figure 56. Solution equilibrium postulated by Noack
and Bor for C02(CO)8 (XIII)

212

as is depicted in Figure 56. He proposed that the high
temperature form was linear and similar in structure to

the isoelectronic [F€2(CO)8]2’.162

Bor163

immediately and independently announced the
same conclusion, based on his analysis of the solvent
dependence of the intensities of some of the terminal
carbonyl infrared absorptions. His conjectures concerning
the structure of the open isomer parallelled those of
Noack.

The temperature dependent changes in intensity of
the terminal carbonyl infrared absorptions of C02(CO)8
in solution were studied in greater detail by Noack.164
From variations of absorption intensities he calculated
that the open structure is less stable than the bridged
structure by AH = 1.3 kcal/mole, and that the entropy
difference between the two isomers is +5 e,u, He also
determined that, at room temperature, a pentane solution
of C02(CO)8 is comprised of 43% A and 57% B, whereas at
~105°C there is present 84% A and 16% B.

During the same period when Bor and Noack were
elucidating the behavior of C02(CO)8 in solution, Klug

165 communicated its solid state structure. It

9.221.
was found to be a distorted Fe2(CO)9 structure exactly
like that predicted by Mills and Robinson; the cobalt

atoms and the two bridging carbonyls are not coplanar.

See Figure 57.

213

 

(XIII)

Crystal structure of C02(CO)8

Figure 57.

214

The cobalt-59 nmr spectrum of C02(CO)8 in solu-

tion has been measured by Lucken gt gt.31

166

and by Haas
and Sheline. Only one very broad signal was observed
(10.41 gauss in benzene, 6.35 gauss in pentane), rather
than two resonances, which would be expected if the two
isomers present are distinct and non-interconverting. It
was stated by Lucken gt gt. that perhaps the resonance
from one of the isomers (and they believed the bridged

form was the likely candidate) was too broad to be seen.

Recent carbon—13 nmr studies of the polynuclear

67 68

cobalt carbonyl C04(CO)121 and the related RhCo3(CO)12l
demonstrate that the carbonyl groups within these molecules
are fluxional in solution in the manner predicted by
Cotton169 in 1966. The fluxional process in the former
compound is stopped on the nmr time scale at -lOO°C and

at -85°C in the latter. The only carbon-l3 nmr study of

a dinuclear cobalt carbonyl complex to date is the study

170
of (H5C6CCC6H5)COZ(CO)6-

a complex which contains no
bridging carbonyls. Even at —125°C a single resonance is
observed in the carbonyl region, indicating that all
terminal carbonyls are undergoing some type of scrambling

171 of some phosphine

process. A carbon-13 nmr study
derivatives of methinyltricobalt enneacarbonyl,
H3CCCo3(CO)8PR3, has established that these cluster com-

pounds exist in solution as mixtures of isomers and that

they too undergo bridge-terminal carbonyl interconversions.

215

From evidence obtained from carbon-13 nmr studies
of polynuclear cobalt carbonyl complexes we concluded that
C02(C0)8’ (XIII) should likewise exhibit bridge-terminal
carbonyl interconversions of the type shown in Figure 11.
In continuing our investigations of bridge-terminal
carbonyl interchange processes in dinuclear metal carbonyl
complexes, we examined the carbon-13 nmr spectrum of
(XIII) and discovered that, even at -164°C, our present
experimental low temperature limit, only a single,
relatively narrow resonance is observed, indicating that
if bridge-terminal carbonyl interconversion does occur,
the process is scarcely affected by this low temperature.
Several complexes structurally-similar to C02(CO)8 were
then utilized in our studies in the hope that the sub-
stituents would slow the rate of carbonyl interconversion
and therefore make it amenable for study by carbon-13
nmr. The bridge—substituted complexes C02(CO)7(C4H202)
(XIV) and C02(CO)7Ge(C6H5)2 (XV) and the terminally-sub-
stituted complexes C02(CO)6DPPM (XVI) (DPPM = bis—
(diphenylphosphino)methane) and [C02(CO)7]2DPPA (XVII)
(DPPA) = bis(diphenylphosphino)acetylene were synthesized
for this purpose. These complexes are pictured in Figure
58. The fluxional behavior of these complexes is observ-
able in the carbon-l3 nmr spectrum, and the slow-exchange
limit could be reached for most of these complexes. The
processes through which carbonyl interconversion is

effected are novel.

216

 

 

 

 

 

 

’9
"fr—i
4’ d2
OCHCKC’O (:o o ‘68 Co
‘\ /’ ‘\../ z” - /’
OC-Co— Co-CO 00— Co Co -CO
0 O O o o
XIV XV
0c 8 (MP 0C 8 Co
\ / \ / \ / \ /
OC—Co o—P 2 OC—Co Co—CO
/ C \ \c/ \
C O C C o P¢2
O O O \
L- C\\\ 42
XVI XVII
0 o
c 0 c
\ /C\ /
OC—Co Fe
/ \8/
OC

Figure 58. The dinuclear cobalt carbonyl complexes
utilized in the study of solution dynamics
outlined in Chapter 6

217

Experimental

 

Dicobalt octacarbonyl(XIII) and C02(CO)7(C4H202)

(XIV) were purchased from Strem Chemicals and were

purified by recrystallization from pentane.

C02(CO)7Ge(C6H5)2 (XV). The procedure of O'Brien gt gt.172

 

was used to synthesize the cobalt-germanium complex from
diphenylgermane (Columbia Organic Chemicals) and C02(CO)8.
The complex was purified by recrystallization from pentane.

C02(CO)6DPPM (XVI). The DPPM complex was synthesized

 

from ttg(diphenylphosphino)methane (Strem Chemicals) and
C02(CO)8 according to the method of Chia and Cullen.173
Purification was achieved through chromatography with
dichloromethane elution and recrystallization from

dichloromethane-pentane.

[CoZ(CO)7]2DPPA (XVII). The synthesis of DPPA complex

 

from C02(CO)8 and bis(diphenylphosphino)acetylene (Strem

Chemicals) was performed according to the procedure

174

established by Ng and Carty. Recrystallization from

dichloromethane-ethanol yielded the product.

(nS-C5H5)Fe(CO)2Co(CO)4 (XVIII). The method of Joshi and

175

 

Pauson was used to prepare the iron-cobalt complex from
(nS-CSHSFe(CO)2C146 and NaCo(CO)4. This very unstable I
material was then recrystallized from dichloromethane-
pentane.

The dense, reddish-brown liquid (nS-C5H5)C0(CO)2
(XIX), similar to bromine in appearance, was purchased

from Ventron Corporation and used without further purifica-

tion.

218

13CO Enrichment. Solutions of (XIII) and of (XIV) in

 

dichloromethane were stirred overnight under an atmosphere
of 90% carbon-l3 enriched carbon monoxide in order to

160 Enrichment of

enrich the complexes to gg. 60% 13CO.
(XV), (XVI), and (XVII) was accomplished by synthesis of
these complexes starting with enriched C02(CO)8. The
unstable complex (XVIII) was enriched under an atmosphere
of enriched carbon monoxide overnight at —15°C. Enrich-
ment of (XIX) was accomplished in the same manner as
described for (XIII) and (XIV).86

All instrumental procedures are identical to those
described in Chapter 2 of this dissertation. Solvents
employed in the temperature dependent nmr studies were
the same as thoseemployed in the study of the iron dimer
complexes described in Chapter 2 unless otherwise indicated.
All carbon-13 nmr samples were 0.03M in tttg(acetylace-
tonato)chromium(III). In retrospect, the addition of
the relaxation reagent to the cobalt samples was un-
necessary, since the carbonyl carbons are relaxed primarily
through interactions with the quadrupolar cobalt nuclei.

Nevertheless, addition of the relaxation reagent does not

alter the spectral results.

219

Results

Carbon—l3 nmr spectra have been obtained for the
six dinuclear cobalt carbonyls (XIII + XIX) over a tem-
perature range +50°C to -164°C. Each of these molecules
is observed to be fluxional and at higher temperatures,
since all bridged and terminal carbonyls are seen to be
completely averaged. For complexes (XIII) and (XVIII),
a single averaged resonance was observed even at the
lowest temperatures, as shown by spectra pictured in
Figures 59 and 66. For complexes (XIV)-(XVII), however,
the Spectra were observed to change markedly with tempera-
ture, as illustrated in Figures 60, 61, 63, and 65. In
the low temperature limit of each set of spectra, carbonyl
resonances were detected in the region 229-241 ppm down-
field of TMS and in the region 193—203 ppm from TMS. The
resonance of complex (XIX), which has only terminal car-
bonyls, was detected at 204.6 ppm. We therefore assigned
the resonances in the downfield spectral regions to the
bridging carbonyls by basing our assignments on the
magnitude of the observed chemical shift of the terminal
carbonyls in (XIX) plus observations that 1) integrated
intensity ratios become meaningful when this assignment
is made, and 2) in other dinuclear metal carbonyl systems
examined thus far, the far downfield resonances may always
be assigned to the bridging carbonyls.

A single carbonyl resonance was measured for

octacarbonyldicobalt in dichlorofluoromethane even down

 

Figure 59.

220

Variable temperature carbon—13 nmr spectrum
of C02(CO)8 (XIII); the higher field peak

is C82

8.
.22.
-57°
~85°
-415’

 

 

 

2221

 

 

3 v1.17 :‘I‘N~
5.45 fr"

. o, 1

n ,q 1'
81’ W '

I” .g-.' "

222

Table 19. Chemical Shift and Linewidth Data for
C02(C0)8 (XIII)

 

T (°C) 6CD 02°
+8° 201.8 43.1
-20° 202.5 27.7
-57° 203.2 12.7
-85° 203.2 8.8
-115° 203.9 8.3
-127° 204.1 7.3

a full-width at half height in Hz

:n‘n— —

 

hr”

223

Figure 60. Variable temperature carbon-13 nmr spectrum
of the carbonyl region of Coz(CO)7(C4H202)

(XIV); the highest field peak is CS2

 

n-13 titsaf

eak is 55

222211

4°

-23°

~33°

 

A k
' -53°

 

 

400°

 

[Ix-4'

Figure 60.

223

Variable temperature carbon-l3 nmr spectrum
of the carbonyl region of C02(CO)7(CAH202)

(XIV); the highest field peak is CS2

1354 a

U)
(*3
Li)

I l

r 7“ w
N' 5.5,...

r‘ \ '1‘".

-O!_H

.V / ~‘-“‘
. "b'

224

~ 26°

40

 

 

 

 

 

v v 400‘

225

Table 20. Chemical Shift and Linewidth Data for
C02(CO)7(C2H402) (XIV)

 

 

 

T(°C) 6average 6bridge 6terminal v%a
+47° 200.4 61.6
+26° 200.3 . 54.9
+4° 200.7 85.4
-5° gg.200.5 140.0
-12° 192.0
-18° 134.0
-20° gg.198.9

-23° gg.l98.0 102.0
-30° gg,196.9 97.1
-37° complex

-42° complex

-44° complex

-52° 193.6b 229.8 197.0

-590 193.5b 230.0 197.2

-64° 193.5b 230.1 197.1

-69° 230.1 197.1,193.5

-78° 230.5 l97.2,l93.6

-89° 230.6 197.2,193.6 3o.5°,25.0,36.6
-100° 230.3 196.7,196.2,193.1 19.5°,ca 18.3
a

full width at half height in Hz

average of four terminal carbonyls

C width of bridging carbonyl resonance

_]02.

M It...» Lt...

.
A‘— ‘ 1,. “W—-';: ‘4‘: *2... .132

-82' m A: —‘;—‘r“~ w___ v 54-145.

Figure 61. Variable temperature carbon—l3 nmr spectrum
of the carbonyl region of C02(CO)7Ge(C6H5)2

(XV); the highest field peak is CS

 

 

 

 

 

 

2

 

 

227

NxmmcoccuerovN

quOHl Um A>xv
00 W0 COHwNM H%COQHQU NEH MO Eflhuuwgm REC MHICODHQU

 

.Nc champs

Table 21.

T (°C)

+4°
-l8°
-38°
-62°
-82°
-93°
-102°
-114°
—122°
-125°
-132°
-145°
-l64°

a

Chemical Shift and Linewidth Data for
(XV)

 

C02(CO)7Ge(C6H5)2

Gaverage abridge

204.5

204.6

204.8

205.0

complex

complex

complex

complex

complex

complex
237.4
236.8

gg.237.0

228

full width at half height in Hz

0

C
N\*‘

terminal

 

200.1
200.1
gg.200.0

 

34.8
21.4
17.1
14.6
28.7

 

K.)
N
\D

 

 

I I
I

 

 

 

4L1

Figure 63. Variable temperature carbon-l3 nmr spectrum
of the carbonyl region of

highest field peak is CS2

 

t i
CC

405'

 

 

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LLLIEII

 

 

t

L 424'

-53'

Figure 65. Variable temperature carbon—13 nmr spectrum
of the carbonyl region of
[C02(CO)7]2[(C6H5)2PCCP(C6H5)2] (XVII); the

highest field peak is C82

Table 23.

T (°C)

+63°
+40°
+24°
+5°
.70
-21°
-36°
-53°
-65°
-80°
-96°
-107°
-124°

a
CHFCl2

234

 

 

Chemical Shift Data for [CoZ(CO)7]2 DPPA
(XVII)a
5average 6bridge éterminal
222.1b 206.2b
213.9C 197.4C
214.0C 197.6C
214.7 197.8
212.8 197.4
211.3 197.5
210.6 197.5
211.1 234.7 197.6
211.3 234.8 197.6
235.4 197.5
236.0 197.6
236.7, 2 2.4 complex
236.9, 2 2.6 206.4, 200.0,
199.2, 197.9,
193.8
solvent unless stated otherwise

Tetrahydrofuran solvent

c
CH2C12

solvent

235

Figure 66. Variable temperature carbon-13 nmr spectrum

of the carbonyl region of (nS-C5H5)Fe(CO)2Co(GD4

(XVIII) in dichlorofluoromethane; the highest
field peak is CS2

 

 

13

91111

91.21 .
1112.90.11
3.5L”.

 

 

 

 

 

 

 

236
I
b51-

 

LI
5L.

 

O A
— w v—

8'
-31'
4o

Figure 66.

235

Variable temperature carbon-13 nmr spectrum

of the carbonyl region of (nS-C5H5)Fe(CO)ZCo(CO)4

(XVIII) in dichlorofluoromethane; the highest
field peak is CS2

 

 

It...
I...

 

 

.111 _

Ml...

 

 

 

 

 

 

.. .AVJ @410
L125.

,WLL I '
A... ....J

 

 

.w. wtg‘tv ‘

237

Table 24. Chemical Shift Data for
I(HS-C5H5)Fe(CO)2Co(CO)4] (XVIII) in Various

 

Solvents
Sgttggt t_t:tt 6C0 (average)
CHFCl2 +8° 211.9
-6° 212.7
-18° 213.0
-31° 213.5
-40° 214.1
-53° 214.8
-63° 215.5
-81° 216.3
-96° 217.5
-110° 217.9
-115° 218.5
-125° 218.6
CH2C12 +8° 212.1
-37° 213.7
-60° 214.7
CHCl3 +8° 212.4
-55° 214.6
C32 +8° 210.0
-30° 210.4
—45° 210.7
—64° 211.2
-87° 211.9
CHZBr2 -46° 213.9
Acetone +8° 212.8
-22° 213.5
-60° 214.7

238

 

 

 

 

 

 

 

Figure 67. Ambient temperature carbon-13 nmr spectrum of
the carbonyl region of (ns-C5H5)C0(CO)2
(XIX) the high field peak is CS2

239

to -164°C. This resonance was observed to undergo a
measurable chemical shift as the sample temperature was
lowered; at 24°C, the signal is located at 201.8 ppm,
whereas, at -130°C, the resonance was observed to have
shifted 2.2 ppm downfield. As tabulated in Table 19, the
linewidth of the carbonyl resonance decreased appreciably
with decreasing temperature.

The crystal structure of (XIV), determined by

152 shown in Figure 57, shows that each

Mills and Robinson,
cobalt atom is bonded to three terminal carbonyl groups
with the Co-Co bond being spanned by one bridging carbonyl
and by the five-membered lactone ring, with the C02(CO)
(lactone) ring being non-planar. The low temperature
carbon-l3 nmr spectra of (XIV) are consistent with this
structure (see Figure 60 and Table 17). Below ~100°C,
three resonances which may be assigned to the three
chemically-different terminal carbonyls are observed at
196.7, 196.2, and 193.1 ppm. A bridging carbonyl reson-
ance, with only 50% of the integrated intensity of each

of the terminal carbonyl resonances, is detected at 230.3
ppm. As the temperature is raised, the two highest field
terminal resonances are observed to broaden, whereas the
third terminal carbonyl resonance apparently does not
undergo any such broadening at this temperature. Con-
currently, the bridged carbonyl resonance broadens and,

at gg. -53°C, the two terminal resonances are averaged

240

and the bridged carbonyl resonance is quite broad. Near
-30°C the remaining unaveraged terminal carbonyl begins
to take part in the averaging processes, and near room
temperatures, only a single resonance is observed at 200.3
ppm, which is the precise weighted average of the four
low temperature resonances. Apparently there are at
least two distinct carbonyl interchange processes de-
tected; there is a high temperature process which averages
all seven carbonyls, and a low temperature process which
averages only four of the terminal carbonyls with the
single bridging carbonyl.

Only a single distinctive feature was observed
in the proton nmr Spectrum of (XIV). Two doublets, with
chemical shifts at 6.01 ppm and 7.48 ppm, and a coupling
constant of 5.0 Hz, were observed at ambient temperature,
and these signals remained virtually unchanged as the
temperature was lowered.

172 and from

From reported ir spectral evidence,
the nmr data reported herein, there is sufficient evidence
that the structure of complex (XV) is that shown in Figure
68, a structure which is analogous to that determined for
(XIV). The second bridging ligand is a diphenylgermanium
moiety, and the four-membered ring is assumed to be non-
plgnar in the complex. The carbon-13 nmr spectra of the
carbonyl region of this complex are shown in Figures 61

and 62. At -l64°C, two resonances of exactly 1:6 intensity

are detected at 200.1 and 236.7 ppm, respectively. Again

 

Figur-

 

 

241

 

Figure 68. Probable str ‘t °
uc urc of C02(L.O)7Ge(C6H5)2 (XV)

the downfi
carbonyl.
able to t1
carbonyl 1
that even
which is .
viscosity
more than
resonance
A
carbonyl
begin int
resonance
the termi
imPOSed -
becomes I
resonanca
terminal
exchange

SPEetruH

 

 

242

the downfield resonance was assigned to the bridging
carbonyl. That there is only one large resonance attribut-
able to the terminal carbonyls implies that the terminal
carbonyl resonances may be coincidental, or, more likely,
that even at -164°C there is some process in operation
which is averaging the terminal carbonyls. Solvent
viscosity at this temperature limits resolution to no

more than gg. 10 Hz, as measured from nmr lock carbon
resonance linewidths.

As temperature is raised to —l32°C, the bridging
carbonyl and four of the terminal carbonyls are seen to
begin interchange. At -125°C, the bridging carbonyl
resonance is completely broadened into the baseline, while
the terminal region consists of a very broad signal super-
imposed upon a sharp resonance. At -ll4°C, when averaging
becomes more rapid, the broadened terminal carbonyl
resonance is seen to shift out from under the remaining
terminal resonance, which now is just beginning to undergo
exchange broadening. The computer simulation of the -102°C
spectrum, Figure 69, demonstrates that the intensity
ratio of the two signals at 206.6, 200.6 ppm is 5:2. As
the temperature is raised further, the two broad lines
broaden further and eventually coalesce at -62°C into a
sharp signal at 205.0 ppm which remains unchanged in
higher temperature spectra. The chemical shift value of
the averaged resonance observed above -62°C is very near
the weighted average of the low temperature limiting re—

sonance S .

243

Figure (El Computer-generated carbon-l3 nmr spectra of
C02(CO)7Ge(C6H5)2 (XV) at -102° C; (a) peaks

in 4:3 ratio; (b) peaks in 5:2 ratio;
(c) measured spectrum

244

E
3.. .

 

 

I I I@

 

 

 

 

:tra 3f

3) 383315

245

In contrast to complexes (XIV), (XV), which are
bridgesubstituted derivatives of C02(CO)8, complex (XVI)
is formed by diphosphine substitution of two terminal
carbonyls. This complex was first synthesized by Chia

173 in order to contrast the structure of a

and Cullen
molecule containing a chelate with a small "bite angle"
to the structures of molecules containing chelating
ligands which had longer chains. As depicted in Figure
70, there are two possible isomers which can be visualized;
one, 70A has the chelate ligand coordinated in a symmetric
fashion. This isomer therefore contains four chemically
distinct carbonyls, with two equivalent bridging carbonyls.
Isomer 70B, on the other hand, places the chelate un-
symmetrically, thus rendering all six carbonyls non-
equivalent. The present carbon-l3 nmr study indicates
that only one isomer, B, is found in solution.

At -164°C, the lowest temperature attainable with
the present temperature controller, the spectrum shown
in Figure 64 was observed. In the bridging carbonyl
region, there are two discernable resonances; the re-
sonance at 253 ppm appears to form a doublet, and the re-
sonance at 240.5 ppm appears to be a poorly defined triplet,

2

with J = 8.5 Hz. The separation between the "doublet”

C-P
peaks is nearly 100 Hz. Two broad lines centered at 206,
201 ppm are observed in the terminal region of the

spectrum. Integrated intensities reveal a bridgedzterminal

246

O
C P
P P P CO
V
A B

Figure 70. Possible isomers of
C02(CO)6[(C6H5)2PCH2P(C6H5)2] (XVI)

O
P C
(3C:ZE!E::CI) (3f;:§!gz:CC)
0C CO OC p
A 8
Figure 71. Possible isomers of

[C02(CO)7]2[(C6H5)2PCCP(C6HS)2] (XVII)

247

carbonyl ratio of 1:2. Upon increasing the temperature

to -153°C, the terminal carbonyl resonances are seen to
broaden. At —l38°C, the downfield bridging carbonyl
resonance has broadened beyond the limits of detection,
whereas the upfield bridging carbonyl resonance is main-
tained. The broad, averaged terminal carbonyl resonance
is seen at -125°, -120°C to broaden and shift downfield,
an effect due undoubtedly to a process which averages it
with the bridging carbonyl. The resonances eventually
sharpen until, at -95°C, the bridging carbonyl resonance,
now a well-defined triplet, and the terminal averaged
resonance are seen at 243.6, 213.5 ppm, respectively.

The signal at 213.5 ppm is very near the average position
of the one bridging and two terminal carbonyl sets seen in
the -164°C spectrum. Saddling the large averaged resonance
are two small lines, which, if they are the two outer
components of a doublet of doublets from splitting of

the terminal carbonyl on the phosphorus substituted cobalt.
The large averaged resonance is not precisely between the
two satellite peaks. Spectra obtained at successively
higher temperatures are characterized by the collapse of
the multiplet structures of both resonances, broadening of
the remaining bridge resonance with concurrent broadening
of the averaged resonance, and finally coalescence of the
two resonances into a single resonance which then sharpens
as the temperature is raised. At ambient temperature

the resonance appears at 216.9 ppm, which is, within

248

experimental error, the weighted average chemical shift
of the low temperature spectra.
The structure of complex (XVII) is as yet unknown,

but ir studies174

indicate the presence of both bridging
and terminal carbonyls. The structure pictured in Figure
58 is that suggested by the workers who initially synthesized
the complex. As was the case with complex (XVI), the
possibility of two different isomers must be considered.
The phsophorus atom Of the DPPA ligand may substitute for
a terminal carbonyl to give a symmetric isomer 71A or an
asymmetric isomer 71B, when the steric requirements of
the bulky diphenylphosphine group are neglected. The
present carbon-13 nmr study indicates the presence of only
one isomer, the asymmetric one, 718, and inspection of a
molecular model of this complex reveals that interaction
of the diphenylphosphine moiety with the bridging carbonyls
establishes serious doubt that isomer 71A could be formed.
The spectra of complex (XVII), collected in Figure
65, clearly indicate fluxional behavior. At -124°C, seven
resonances are detected, thus indicating the presence of
only 71B. The bridging carbonyl resonances are seen at
242.6 and 236.9 ppm, and the terminal region consists of
resonances at 206.4, 200.0, 199.2, 197.9, and 193.8 ppm.
The bridging carbonyl resonance at 242.6 ppm and the
terminal resonance at 206.4 ppm are the lowest field
resonances thus far observed for cobalt carbonyls. As

the temperature is raised, the downfield bridge broadens

249

and disappears, and the terminal resonances begin to
broaden and coalesce, until, at -96°C, only two resonances
are seen at 236.0 and 197.6 ppm. The terminal resonance
continues to sharpen, and at -65°C, it appears to be quite
narrow. A new resonance appears at gg. 211 ppm at this
temperature; it begins to sharpen with concurrent broaden-
ing and ultimate disappearance of the bridging carbonyl
resonance, until, at -36°C, only two resonances at 197.5
and 210.6 ppm are detected. As the temperature is in-
creased, the downfield resonance is seen to broaden and
to reappear at 214.7 ppm, whereas the upfield resonance
begins to broaden. At +5°C in dichlorofluoromethane
solvent, the two resonances are found at 197.8 and 214.7
ppm. A change in solvent at this point dramatically
affects the chemical shifts of the carbonyl resonances;
at +24° in dichloromethane, the resonances are shifted up-
field 0.3 ppm, and in tetrahydrofuran at +63°C, the reson-
ances are shifted over 8 ppm upfield. At this temperature
in tetrahydrofuran, the downfield resonance is seen to be
narrow, in contrast to the broad appearance of the up-
field resonance.

Based on the results of a crystal structure
determination of the complex (nS-C5H5)Fe(CO)2Co(CO)3

176

P(CH3)(C6H it can be surmised that the structure

5)2:
of complex (XVIII) is characterized by a nonplanar

Fe(CO)2Co ring, with the cyclopentadienyl ring trans to

250

the puckering. As indicated in the spectra shown in
Figure 66, no resonances due to bridging and terminal
carbonyls were observed at the lowest temperatures, and
moreover, the single averaged resonance observed is still
very narrow at the lowest temperature. The spectra are
characterized by a relatively broad resonance which
exhibits a temperature-dependent chemical shift and a
narrow resonance which (in dichlorofluoromethane) remains
chemical shift invariant at 212.4 ppm. It is not known

at this time whether the narrow resonance is due to a non-
scrambling terminal iron carbonyl in the complex, or an
impurity with a terminal iron carbonyl. Since the low
temperature limit was not reached, the assignment of this
resonance is not at this time of utmost importance. Since
it has been demonstrated by ir studies that this molecule
exists in solution in an equilibrium of the type depicted

177 it was decided to examine the effects of

in Figure 11,
different solvents on the chemical shift of the averaged
carbonyl resonance. The results of this study may be
seen in Table 24. At a temperature of +8°C, it can be
seen that the resonance is shifted upfield in the less
polar carbon disulfide solution, (210.0 ppm), and that it
shifts downfield slightly when more polar solvents are
used. Interestingly enough, there seems to be no

correlation between the downfield shift and solvent

polarity; in chloroform, where e = 4.81, the chemical

251

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252

shift value is 212.4, but in acetone, with e = 20.70, the
resonance appears at 212.8 ppm (6 is the solvent dielec-
tric constant). Figure 72 is a graph of chemical shift
versus temperature for this complex in a variety of solvents.
One distinctive feature of all the cobalt carbonyl
spectra is that, at room temperature, the single, averaged
resonance is very braod, and as the temperature is lowered,
the resonance sharpens somewhat before it commences broaden-
ing due to the slowing down of the carbonyl exchange pro-
cesses.’ Both types of broadening are due to the scalar re-
laxation interaction, with the former referred to as scalar
relaxation of the second kind and the latter as scalar re-
laxation of the first kind. Since the cobalt nucleus is
relaxed extremely efficiently by the quadrapolar relaxation
interaction, it follows that T1 = T2 = TS, where TS is
the correlation time which modulates the relaxation rate
of the cobalt nucleus. The expression which describes the
scalar relaxation interaction interaction in terms of the

relaxation rate of spin I coupled to a spin S is

R = R2 = (AZ/3)S(S + 1)-S + TS/l + (ml - wS)2T§, (75)

where A is the spin-spin coupling constant in units of
radians per second. In this discussion, the spin S re-
fers to the cobalt nucleus (S = 7/2) and spin I refers

to carbon. From Equationfifl) it is apparent that as R;
decreases, the width of the resonance decreases; hence,
as the temperature is lowered, TS, or the relaxation time

of the cobalt nucleus, must be decreasing.

253

By recalling Equation 58, (where I now refers to
the spin of the cobalt nucleus), it is seen that the three
variables which may affect the magnitude of RQ are the
asymmetry parameter, n, the quadrupole coupling constant
(or more precisely, the electric field gradient), or the
molecular correlation time, TC. Since neither the
asymmetry parameter nor the electric field gradient would
be expected to vary significantly with temperature, we
can be confident that the relaxation time decreases with
decreasing temperature as the result of an increasing
molecular correlation time. That the molecular correlation

178

time is temperature-dependent is known, and the de-

pendence is given by

TC = Toexp(Ea/RT), (76)
where E8 is the activation energy of the reorientation
of the molecule under study. Therefore the sharpening
of the averaged resonances with decreasing temperature
arises from the temperature dependence of the quadrupolar
relaxation rate of the cobalt nucleus, which in turn
interacts with the carbon nuclei through the scalar re-
laxation mechanism.

Another remarkable observation in this series of
spectra is that the averaged resonance of complexes (XIV),

(XV) is significantly broader than the averaged resonance

of complexes (XVI), (XVII) at room temperature. The

254

latter two complexes are substituted in terminal posi-
tions whereas the former two complexes are bridge-sub-
stituted. Again we must refer to Equation 58 in order to
determine the relative contributions of the three variables
to the observed linewidth differences. Unfortunately,
the specification of one of the variables as the major
contributor to the linewidth difference is not so straight-
forward. We may first consider the asymmetry parameter
and the electric field gradient contributions. Certainly,
a bridging ligand other than a carbonyl group would perturb
the electric field gradient around each cobalt atom by
virtue of its size and electronegativity characteristics.
Unquestionably, the terminal phosphine substituents would
perturb the electric field of the cobalt atom, but the
averaged resonances observed result from carbonyl carbons
which are under the influence of both a relatively asymmetric
field gradient on the one cobalt atom and a relatively
symmetric field gradient on the other. It is possible that
the narrower averaged resonances result from the fact that
only one of the cobalt atoms, rather than both, exhibits
a relatively large asymmetric field gradient.

However, the effect of the correlation time cannot
be ignored. The bridging ligands in complexes (XIV), (XV)
most likely do not open to form a non-bridged species,
whereas there is nothing to prevent the opening of the
bridging carbonyls in complexes (XVI), (XVII) to form

non-bridged species. The rotational correlation times

255

in the relatively rigid bridged species would be more
pronounced than those in the systems which are capable of
Opening and therefore experiencing more degrees of
freedom. It is tempting to assign the linewidth dif-
ferences between the two types of complexes to the dif-
ferences in correlation times, but to adamantly maintain
that this is the sole contributing factor would be only

a speculation.

 

4‘. .

1... pl‘.\

 

256

Discussion

 

The results of this carbon—l3 nmr investigation of
dicobalt carbonyls in solution demonstrate that carbonyl
interconversions in these complexes are also facile and
general processes. Carbonyl interconversion was con-
clusively demonstrated for the substituted complexes
(XIV)-(XVII), since discrete resonances, assignable to

bridging and terminal carbonyls, were detected at very

 

low temperatures. Although we were unable to reach the
slow-exchange limit of complexes (XIII) and (XVIII), it

is nonetheless certain that the single resonances ob-
served for these complexes at all temperatures result from
a carbonyl averaging process, and not to an accidental
overlap of bridging and terminal carbonyl resonances,
since the bridging resonances in complexes (XIV)-(XVII)
always appeared far downfield (at least 30 ppm) from

the terminal resonances.

Because these complexes are demonstrably fluxional,
we must attempt to explain the process by which structural
interconversions are effected. Fluxional behavior of the
methyl groups has been reported in the cobalt complex
[(Ge(CH3)2)Co(CO)3]2, where the Ge(CH3)2 moieties function

as bridging ligands.179

This phenomenon is explained
non-mechanistically in terms of a "butterfly” motion of
the Ge(CH3)2 groups about the cobalt-cobalt axis which

interconverts the methyl groups. In a mechanistic sense

257

this operation is viewed as a converse Berry mechanism

in which the square pyramidal coordination about each
cobalt atom is converted during the flapping process to

an intermediate with trigonal bipyramidal coordination
about each cobalt atom, with the pseudo—threefold axis
being in the plane of the Co-Ge(CH3)2-Co ring. Such a
mechanism was employed to rationalize the fluxional be-
havior of the iron complex Si(CH3)2FeZ(CO)7, which features

two bridging Si(CH3)2 groups and a bridging carbony1,180

Job and Curtis181

have reported that C02(CO)7Ge(CH3)2
exhibits a single line in the methyl region of the proton
nmr spectrum. These workers have proposed that the inter-
conversion might be accomplished th a bridge-terminal
carbonyl interchange in which the new Co-CO-Co bridge is
formed on the opposite side of the Co-Ge-Co plane. This
process would necessarily average the terminal carbonyl
groups.

A previously investigated complex which would seem
pertinent to our study is [(nS-C5H5)2Fe2(CO)3Ge(CH3)2],72
a complex with a bridging Ge(CH3)2 moiety. However, the
carbonyls in this complex were observed to be static on
the nmr timescale even at +160°C. The Cotton—Roberts
mechanism, and hence the proposal that the molecule
interconverts carbonyls ytg a nonbridged intermediate
possessing a terminally-coordinated dimethylgermylene

moiety, was used to rationalize the fluxional behavior

of the -CH3 groups. The interconversion rate of the

258

carbonyl groups was presumed to be so slow that the car-
bonyl groups appear to be static.

In considering mechanisms by which the carbonyls
of these complexes are interconverted, we must examine
initially the applicability of the Cotton-Roberts mechanism.

65 and Roberts66

As described more fully in Chapter 2, Cotton
independently proposed that complexes similar to
[(nS-C5H5)Fe(CO)2]2 (I) undergo carbonyl interconversion
through pairwise Opening of the bridging carbonyls to form
a non-bridged intermediate, followed by reclosing of the
terminal carbonyls to form bridges. The key point in
determining the applicability of this mechanism is
naturally to ponder the possibility of the non-carbonyl
bridges opening in complexes (XIV) and (XV), and to weigh
the possibility that the phosphorus ligands in complexes
(XVI) and (XVII) can occupy bridging positions.

If it is assumed that the bridging ligands in
complexes (XIV), (XV) can open to form a nonbridged
intermediate, the focus of this must be the geometry
of the intermediate, and then the manner in which it
might rearrange. An intermediate with trigonal bi-
pyramidal geometry, like the open structure in Figure 56,
could, in principle, provide a pathway for a "five—two"
process (five carbonyls exchanging, two stationary).

The apical CO might very well remain unaveraged in an

interconversion process which involves rotation about the

259

cobalt-cobalt bond and reformation of the bridging
ligands. However, a lineshape analysis based on the re-
arrangement of a trigonal bipyramidal intermediate (this
technique is described below) yielded spectra that did
not match the observed spectra closely. Therefore,
rearrangement of carbonyls through a trigonal bipyramidal,
nonbridged intermediate is not the process observed.

The alternate five-coordinate Stereochemical
configuration about the cobalt nuclei in the nonbridged
intermediate is the square pyramidal configuration, with
the cobalt-cobalt bond forming the apical element. There
is no "five-two" process imaginable which could proceed
through such an intermediate.

The possibility that the bridging ligands in the
first two complexes can open to form terminal groups
appears to be remote. A three-coordinate carbon atom in
a lactone ring is without precedent. There is a precedent
for the opening of a germanium bridge to form a nonbridged
intermediate in [(nS-C5H5)2Fe2(CO)3Ge(CH3)2], but this
process was observed at higher temperature; indeed, the
rate of this interconversion process is so slow that even
at +160°, discrete bridging and terminal carbonyl reson—
ances were observed.

Not only are the temperatures at which our observa-
tions were made too low to provide sufficient energy for
the non-carbonyl bridging groups to open, there are other

182

factors to be considered as well. Poe gt gt. studied

261

hindrance. Therefore, appeal to mechanisms other than the
Cotton-Roberts carbonyl interchange mechanism becomes
necessary, since the basic tenet of that mechanism, that
pairwise opening of bridging ligands initiates the
process, cannot be satisfied in these complexes.

Solutions of complexes of formula [(nS—C5H5)M(CO)3]2,
where M is Cr, Mo, or W, yield proton nmr spectra which
indicate that interconversion among trans, cis, and gauche
rotamers is a facile process.99 Since these complexes
have only terminal carbonyls, the stereochemistry about
each metal atom closely matches that of the cobalt atom
to which the bridging carbonyl breaks in complexes (XIV),
(XV). Although Cotton gt gt. discussed the possibility
that carbonyl and cyclopentadienyl rearrangement may pro—
ceed through an intermediate with one or both halves of
the molecule assuming a trigonal bipyramidal configura-
tion, these authors favored a rotational pathway similar
to that described for the rotamers in the Cotton-Roberts

mechanism.99

However, the idea that rearrangement may
proceed through a trigonal bipyramidal intermediate formed
the basis of several carbonyl interconversion mechanisms
considered in this dissertation.

In order to deduce the process(es) by which
carbonyl interchange accomplished in the substituted

dicobalt carbonyl complexes, it was decided to simulate

several series of spectra by modeling several different

262

exchange mechanisms. These results were then compared
to the measured carbon-l3 nmr spectra. The spectra
obtained from complex (XIV) were treated in this manner
initially, because among the several complexes it alone
was resolved at low temperatures into the predicted number
of resonances, and there appeared to be no viscosity
broadening of the resonances at the lower temperatures.
After preliminary simulation of approximately
ten possible rearrangement mechanisms, the list of
possibilities was narrowed to four processes. The basic
figure of the molecule utilized in this scheme of analysis
is depicted in Figure 73A. The black sphere which serves
as a bridging ligand in the figure merely represents the
bridging lactone ring or diphenylgermanium ligands which
are presumed to maintain their bonding to both cobalt
atoms throughout the carbonyl interchange processes.
Henceforth this ligand shall be referred to as the "handle."
The exchange processes to be described can be
characterized by a 4 X 4 exchange matrix based on Figure
73A, or a 7 X 7 exchange matrix which is based on a model
with one set of terminal carbonyls labelled E, F, and G.
The simulated spectra obtained from a 4 X 4 exchange
matrix were practically identical to those obtained
through use of the 7 X 7 exchange matrix. The 4 X 4
exchange matrices were used whenever possible, since the
cost of running a program using a 4 X 4 matrix is only

gg. 30-40% of the cost required to run a program based

263

D B D
CACo/——-—.\C{—A C-\Co/.\C o/f-D
s/ \C/ \o s/ \3/\ \A

Figure 73. Schematic depiction of Mechanism 1. All
seven carbonyls are scrambled in a polytopal
rearrangement process through a trigonal
bipyramidal intermediate.

264

on the corresponding 7 X 7 matrix. However, selected
spectra were matched with both sets of matrices to in-
sure that identical results were obtained.

The exchange matrix used in the calculations was
of the following form:

IA 'B 'C 'D'

   

Carbonyl A is the bridging carbonyl and carbonyl B, C,
and D form the terminal carbonyl set as shown in
Figure 73A. Carbonyl interchange mechanisms which can
conceivably permute the carbonyl groups and the matrices
which describe these mechanisms are now considered.

The first carbonyl interchange mechanism which
was considered, designated as Mechanism I, can be des-
cribed as a polytopal rearrangement which scrambles all
seven carbonyl groups, a mechanism most likely operant at
the higher temperatures. Consider the structure of the
dicobalt carbonyl complexes: in the solid state structure
each cobalt atom is approximately square pyramidal
(neglecting the cobalt-cobalt bond), where terminal
carbonyl D in Figure 73 serves astjmzapical element. As
depicted in Figure 73, the initial step of this inter-
change mechanism is the Opening of the carbonyl bridge,
A, to either side of the molecule; simultaneously, the
cobalt-cobalt bond breaks. The breaking of the metal-

‘metal bond is postulated in order to maintain a five-

265

coordinate intermediate, but more importantly, it insures
that the effective atomic number (EAN) rule is obeyed by
the cobalt atoms; were the metal-metal bond to remain
intact, the cobalt atom to which the bridging carbonyl
opens would be surrounded by 37 electrons and the other
cobalt atom would be complemented with only 35 electrons.
Since the EAN rule is maintained "perhaps 99% of the

time"185

by metal carbonyl systems, it does indeed appear
necessary to postulate the metal-metal bond scission.

The cobalt atom toward which the bridging carbonyl opens
is seen in Figure 73B to be trigonal bipyramidal, with
carbonyls B and C and the handle occupying the equatorial
positions and carbonyls A and D occupying the equatorial
positions of the trigonal bipyramid. This trigonal bi-
pyramidal intermediate can then rearrange in the following
manner: carbonyl A can undergo an axial + equatorial
interchange with B or C, and carbonyl D can interchange
with C or B, processes which yield intermediates 73C or
73E. The axial carbonyl in a position to do so then
closes to form a bridging carbonyl, which can then Open
to either side and continue with the scrambling process.
Concurrent with bridging carbonyl reformation is the re-~
establishment of the cobalt—cobalt bond, which implies
that the cobalt atoms are once again square pyramidal.

Mechanism I may then be described as an inverse

Berry type mechanism, since the scheme square pyramidal +

266

trigonal bipyramidal + square pyramidal describes the
process. The exchange matrix utilized to describe this

process is

.5 -1 0 .5
.5 0 -1 .5
_0 .5 .5 -1

The second process considered is somewhat related
to that effected by Mechanism 1. From the spectra of
complex (11) at -47°C and of complex (III) at -102°C, it
is apparent that only five of the seven carbonyls are
interconverting, whereas two others appear to remain
static. Mechanism II, which effects the interconversion
of four terminal carbonyls with the bridging carbonyl, is
pictured in Figure 74. The initial step is the
simultaneous opening of the bridging carbonyl to one side
and the scission of the cobalt—cobalt bond. In configura-
tion 74H, it can be seen that carbonyls A, C, and D form
the equator of a trigonal bipyramid, with carbonyl B and
the handle forming the apices. Rotation of :120° about
the B-Co-handle axis is then accomplished, and in the pro-
cess carbonyls A, C, and D are scrambled, whereas carbonyl
B retains its identity. Repetition of the process on
either side of the molecule completely scrambles four of
the terminal carbonyls and the bridging carbonyl. This
process would be expected to dominate at lower temperatures,

where rotation about the B-Co-handle axis would require

267

 

-f5Co’//J.\\Cgf:Cl
B/'I\§A/’/’ ‘\
G
D\
°\ 1’ ;c
C— , C ,I’
3/ AZ’N

 

Figure 74. Schematic depicition of Mechanism II. Two
pairs of terminal carbonyls and the bridging
carbonyl are scrambled

268

a minimum amount of energy. The exchange matrix which

describes this process is

-1 0 .5 .5
K = o o o o
.5 o -1 .5

.5 0 .5 -1

Another process which scrambles only five of the
carbonyls is described by Mechanism III in Figure 75.
After bridge opening and cobalt-cobalt bond scission,
terminal carbonyl D can reclose to form a bridging car-
bonyl, a process which rearranges, or "rocks" the other
terminal carbonyls in the following way: in order to
accommodate the closing of carbonyl D to form a bridge,
the molecule must undergo simultaneous deformation. This
deformation, which is more or less a torsional motion,
interchanges terminal carbonyls C and D on the cobalt
atom to which the original bridging carbonyl A did not
open, and it interchanges carbonyls A and C on the other
side; during this rearrangement process, however, neither
terminal carbonyl B experiences a change of environment.
The net effect is the scrambling of the bridging carbonyl
with four of the terminal carbonyls, two from each
cobalt atom. The exchange matrix for this process is

given by

O
0000

O

O

ma2£0£pmo 030 MO 0>Hw
moHnEmwom axconumo HOCHEMOO oco mo mchoHo cam ammonumo mewwcwun
ecu mo wcflcoao couuoocou .HHH Emflcmnooz mo cofluowaoc OHumEmnom .mn ouswflm

9 < U m,< m m m

m / \ / x / < \
clone/\V/oolo II ol 6\ 8 IO III ul oo\ /8 lo

m o M y/o\ /o o\ /o\ /o

270

Finally, a mechanism which would probably proceed
at higher temperatures is one which describes a totally
random carbonyl interchange, and is described using a
4 x 4 matrix of the form

-1 .33 .33 .33
.2 -l .4 .4
.2 .4 -l '.4
.2 .4 .4 -1

In order to fit the computer-generated spectra
to the measured spectra, it was necessary to make several
adjustments in the input parameters of the program. At
the lower temperatures, we perceived no problem with line
broadening due to the cobalt quadrupole moment, since it
is effectively decoupled by the lower temperatures by a
process termed "thermal decoupling.” However, at higher
temperatures (beginning around -50° for complex (II))
there is noticeable broadening due to interaction with the
cobalt quadrupole moment. In order to simulate spectra
in this temperature region and above, it became necessary
to introduce different T2 values into the input parameters
at each temperature.

The most successful computer-simulated spectra
for the low temperature processes considered, together
with the measured spectra, are shown in Figure 76. The
processes described by Mechanisms II and III match the
Ineasured spectra closely at low temperatures. However,
only one mechanism, that which describes the concerted

one-for-one carbonyl interchange process, Mechanism III,

 

 

 

1‘.“ ill-Ls

 

 

Figure 76.

271

The measured carbon-13 nmr spectrum of
C02(CO)7(C4H202) (XIV) together with computer-

generated spectra based on Mechanism II
(left) and Mechanism 111 (right)

 

272

b)SPECTRA OF
Co(CO)7(C4 HZOZXII)

47° C52

26°

0) RANDOM c) CONCERTED
EXCHANGE(5-2) -300 EXCHANGEIS-Z)

-----:§7:J\k -005_ J‘\

3144...;5? 00325 A
w .0025 A

A

A

A

A __J\ -52° .I\J\ _/\.ous

A 03:25 A .Jx... -5go - M $03125 it
A 5.3- III
II
II

.005

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

A .075 -69° 40°75

_. . .1 431 -- I1

_j\ 5 .1 -1; III JL-5 III
230 2|O I90

273

is seen to generate spectra in the region -59°C to -37°C
that closely match the measured spectra. The spectra
generated by the process described by Mechanism II show
too much separation between the terminal carbonyl re-
sonances in this intermediate temperature range to match
the measured spectra.

Figure 77 presents an alternate view of Mechanism
III. It can be seen that five successive applications
of the process that Mechanism III describes completely
averages carbonyls A,C,C',D,D', whereas carbonyls B,B'
remain unaveraged, thus neatly accounting for the observed
Spectra. The calculated value for the activation energy
of the carbonyl interconversion process is roughly
15.5 i 2.5 kcal/mole for complex (XIV). The high tempera-
ture process may be governed by the polytopal rearrange-
ment process, described by Mechanism I, or the totally
random exchange process described by Mechanism IV. It is
impossible at this time to choose between the two, since
the simulated spectra do not discernably differ.

It is assumed that the carbonly interconversion
process in the germanium-bridged complex (XV) is governed
by the same mechanisms. Figure 69 definitely indicates
that a 5-2 process is in operation at the lower tempera-
tures. Because the slow-exchange limit is below -l64°C,
we were unable to accurately simulate spectra for this

complex. By assuming a pre-exponential coefficient of

274

HHH Emwcmzom: zn vmuoommo ucoEmwcmupmmp map mo 3mH> mumcumua< .Nm musmwm

 

 

 

 

 

.o .u a .o a .m < m
ol/oo\o/oo\l< u ol/ou\ /oU\Ju u ul Vubouru
\ /u\ M \ /o\ / \ /o\ /

.m u < o .o
.m/ \.u/ \m .U/ \Q/ \< .m/ \</ \m
.ol 8 ou.I< fl” bl. co 8 lo Mllulr .oloo oulu
ox/-\/u R /o\/._ G\/o\ /o

275

1013

and a coalescence temperature of -lOO°C, we were
able to estimate the activation energy of carbonyl inter-
change to be 23- 7.9 kcal/mole. Substitution of the
germanium bridge for the lactone bridge in this series

of complexes is thus seen to have a marked effect on

the activation energies of carbonyl interchange.

Due to the complexity of the spectra obtained for
complexes (XVI) and (XVII) in the slow-exchange limit,
computer fitting of the data was not possible. There
are qualitative aspects of the spectra which do provide
some information concerning the mechanisms by which car-
bonyl interconversion is effected in these complexes. At
higher temperatures, all the carbonyls are seen to inter-
change in complex (XVI). There are no bridging groups to
prevent bridging carbonyl opening, so interconversion of
the carbonyls may well be effected through a nonbridged
intermediate. At lower temperatures, however, one of the
bridging carbonyls ceases to interchange, a phenomenon
also observed in the DPPA complex (XVII). The remaining
bridging carbonyl and some (or all) the terminal carbonyls
then continue to interchange until very low temperatures
are reached. From the coupling constant information
obtained from the DPPM complex (XVI) at -95°, it is apparent
that the bridging carbonyl cis to the phosphorus atom in
the asymmetric isomer 70B ceases to interchange at the
higher temperature. This assignment is substantiated in

the spectrum obtained at -l64°C, where the downfield

 

 

276

bridging resonance, which appears only at very low tem-
peratures, exhibits a doublet character. The downfield
resonance in the DPPA complex (XVII) likewise shows a
broad, doublet-like shape. The far downfield chemical
shifts of the bridging carbonyl resonances trans to the
phosphorus atoms are not surprising, since phosphorus is

a poorer n-acceptor than CO. The CO accepts more electron

*
density into its n orbitals when it is trans to a

 

phOSphorus atom than when it is trans to a stronger
n-acceptor, such as another CO. The added n* electron
density induces a downfield shift in the resonances. A
mechanism which explains the observed spectra has thus

far been elusive.

Acknowledgement:

 

The author expresses his appreciation to Dr.
Kelly B. Triplett for synthesizing the iron-cobalt com-
plex (XVIII) and to Dr. D.G. Farnum for his many helpful
discussions concerning the rearrangement mechanisms of

the cobalt complexes.

277

Suggestions for Subsequent Research

 

Studies of several other dinuclear cobalt
complexes that would be interesting extensions of the
present investigation. Two complexes closely related to
the bridge-substituted cobalt complexes (XIV) and (XV)
are (acac)ZSnC02(CO)7, prepared by adding NaCo(CO)4

186,187 and

dropwise toaasolution of (acac)ZSnC12,
C2F4C02(CO)7 (CZF4 = p-tetrafluorethylidene), synthesized
from (CO)4Co(C2F4)Co(CO)4 (C2F4= u-tetrafluoroethylene)

by vacuum decarbonylation.188'189

The latter complex
is intriguing in that it yields a simple fluorine-l9 nmr
spectrum, indicating the presence of either one isomer,
or, more likely, rapid interconversion between two isomers.
Studies of these ligand-bridged complexes would indicate
the generality of the concerted carbonyl interchange
mechanism in cobalt complexes, and would help determine
to what extent the bridging ligand controls the rate of
carbonyl interchange.

An important series of terminally-substituted di-
cobalt carbonyl complexes is described by the general for-

190

nula [(diolefin)Co(CO)2]2. The ir spectrum of the 2,3-

dimethylbuta-l,3-diene complex in solution evidences the
presence of the trans isomer only, whereas the norbornadiene
complex probably exists solely as the cis isomer and the

cyclohexa-1,3-diene complex as a mixture of cis and trans

191,192

species. Furthermore, the ir studies suggested

that the Co(CO)2Co ring is planar, rather than puckered,192

 

278

and this was subsequently substantitated when the crystal
structure of the 2,3-dimethylbuta-1,3-diene was determined.193
Preliminary studies in this laboratory of the 2,3-dimethyl-
buta-l,3-diene complex revealed that there is indeed
bridge-terminal carbonyl interconversion occurring in solu-
tion, and that the slow-exchange limit is reached at a
relatively high temperature (93. -70°C). The norbornadiene
complex was shown to be too insoluble below -20°C to be
of practical importance in this study.

Another important series of cobalt complexes is
the complexes of formula [R3ECo(CO)3]2, where E = P or
As and R is an alkyl or aryl substituent. Complexes of
this type have been shown to exist in solution in an
equilibrium consisting of a nonbridged and two bridged

194'195 Other similar complexes are the

196

isomers.

and the novel
97

C02(CO)7ER3 series
[{P(OCH2)3C(CH2CH3)}2Co(CO)2]2.l All of these com-
plexes should exhibit bridge-terminal carbonyl inter-
change, and in cases where E = P, the coupling patterns
should facilitate elucidation of the exchange mechanism.

With the present instrumentation, the studies just
described are certainly feasible, and the results promise
to be interesting. However, if meaningful activation
parameters are to be determined, the broadening effect of
the quadrupolar cobalt nucleus must be eliminated. It

is possible in principle to irradiate the cobalt frequency

with cw radiation and henceto decouple the cobalt from the

 

279

carbon spectrum in a manner exactly like proton decoupling.
There are several problems inherent in such a procedure.
Since the cobalt-59 nucleus is spin-7/2, the transitions
are "smeared" over a fairly broad frequency range, thus
necessitating a radiofrequency cw source which covers
a large range of frequencies. Another difficulty is that
large amounts of power would probably be necessary to
completely decouple the cobalt. And finally, the cobalt
resonance frequency is only 1.4 MHz from that of carbon,
so there might exist problems with mixing frequencies.
Fortunately, pulse techniques may help circumvent
the first two difficulties. A sufficiently narrow, power-
ful pulse could deliver enough power over the prescribed
frequency range to keep the cobalt transitions saturated.
This scheme, in effect, would be akin to the gated de-
coupling experiments already in use. Instrumentation of
this nature is necessary, if studies of complexes con-

taining quadrupolar nuclei are to become more rigorous.

 

If

I“ ‘1': a:
‘ I

 

 

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. ...,!IIIII|u2mm

3 129