Electronic communications mediated by metal clusters and pi conjugated systems 1

i
ELECTRONIC COMMUNICATIONS
MEDIATED BY METAL CLUSTERS AND π-
CONJUGATED SYSTEMS




WU JIANGUO







A THESIS SUBMITTED FOR
THE DEGREE OF DOCTOR OF PHILOSOPHY
DEPARTMENT OF CHEMISTRY
NATIONAL UNIVERSITY OF SINGAPORE

2003




ii
Acknowledgement


I would first like to express my hearty gratitude to my supervisor Dr. John Yip

Hon Kay for his invaluable guidance, critical comments and unswerving support
throughout the course of this project. I am grateful for the valuable synthesis and
analysis skills he has imparted to me.
Particular thanks are due to Professor Wong Kwork-Yin and his students Yeung
Kin-wai, Ho Kam Piu, Pun Christine So Ngan for measurement of Cyclic Voltammogram
and Differential pulse voltammogram in Hong Kong Polytechnic University. Also, a
large debt of gratitude is owed to Professor Vittal J.J., Professor Koh Lip Lin, Ms Tan
Geok Kheng for carrying out X-ray crystal structure determination. Acknowledgement
is also made to the staff working in Nuclear Magnet Resonance Laboratory, Element
Analysis Laboratory, Mass Spectroscopy Laboratory and other facility resources for their
kind assistance and cooperation.
Many thanks are extended to following persons: Chen Min, Ren Feng, Sebastian
Muthu, Janardhana Prabharathy, Lin Ronger, Xu Huan, Zhang Ke , Hu Jian and other
members in Dr. Yip’s group for their help and cooperation.
I wish to acknowledge the National University of Singapore for the financial
support.
Finally, I am very grateful to my parents and my wife for their continuous help
and encouragement.



iii
TABLE OF CONTENTS


Acknowledgement ii
Table of contents iii
Abbreviations vi
Summary viii

CHAPTER ONE
Introduction
1.1 Mixed-Valence compounds 1
1.2 Classification of MV complexes 3
1.3 Stability of Mixed-Valence complexes 10
1.4 Importance of bridge in mediating electronic communication 14
1.5 Objectives 26

CHAPTER TWO
Syntheses, Structures and Electrochemistry of Polynuclear Cu(I) and
Ag(I) Complexes Bearing Ferrocenyl Groups

2.1 Introduction 31
2.2 Results and Discussion 33
2.2.1 Syntheses and Structures 33
2.2.2
1
H and
31
P NMR of complexes 1 and 2 39
2.2.3 Spectroscopic and Electrochemical Properties 42
iv
2.3 Conclusion 55
2.4 Experimental Section 55

CHAPTER THREE
Electronic Communication Mediated By Pt
I
-Pt
I

σ-Bond
3.1 Introduction 61
3.2 Results 65
3.2.1 Structure of 3 65
3.2.2
1
H and
31
P NMR of complexes 3 67
3.2.3 Electrochemistry of 3 69
3.2.4 Electronic absorption spectrum of 3
+
74
3.2.5 Structures of 4 and 5 77
3.2.6
1
H and
31
P NMR of complexes 4 and 5 81
3.2.7 Electrochemistry of 4 and 5 83
3.3 Discussion 90
3.3.1 Electronic communication through C2-Pt-Pt-C2 linkage 90
3.3.2 Shut down of electronic communication in 4
+
and 5
+
95
3.4 Conclusions 97
3.5 Experimental Section 97

CHAPTER FOUR
Interactions Between Binuclear Pt and Pd Centers and Acetylene and
Vinylidene

v
4.1 Introduction 103
4.2 Objectives 104
4.3 Results and Discussion 108
4.3.1 Syntheses and Structures 108
4.3.2
1
H and
31
P NMR of complexes 6 , 7 and 8 115
4.3.3 Electrochemistry of the Binuclear Pt and Pd Compounds 119
4.3.4 Implications of the electrochemistry of the Complexes on the Nature of Metal-
ligand Interactions 126
4.4 Conclusion 134
4.5 Experimental Section 134

Reference 139
List of Publications 152
Appendix I 153
Appendix II 154

Appendix III 161

vi
ABBREVIATIONS



A
ac
2

Bu
Ca
Calcd.
CV
D
dppe
dppm
e
Et
F
Fc
G
h
HOMO
IR

IVCT
LUMO
m
M
acceptor
two electron acetylene
butyl
About
calculated
Cyclic voltammetry or voltammogram
Donor
1,2-bis(diphenylphosphino)ethane
di(phenylphosphino)methane
electron transfer
ethyl
Faraday constant
ferrocenyl, C
5
H
5
FeC
5
H
4

gram(s)
hour(s)
highest occupied molecular orbital
infrared
intervalence charge transfer

lowest unoccupied molecular orbital
multiple
metal
vii
Me
MV
MLCT
N
NMR
NIR
R
T
UV-vis
methyl
mixed valence
metal-to-ligand charge transfer
Avogadro’s number
Nuclear Magnetic Resonance
Near infrared
Alkyl or gas constant
temperature
Ultra-violet visible

viii
Summary


To understand how polymetallic centers and
π
-conjugated systems mediate
electronic communications, three model systems are investigated in this thesis.

The first system contains trinuclear complexes [Ag
3
(
µ
-dppm)
3
(
µ
3
-
η
1
-C≡C-
Fc)]•2CF
3

SO
3
(1•2CF
3
SO
3
) and [Cu
3
(
µ
-dppm)
3
(
µ
3
-
η
1
-C≡C-Fc)
2
]•PF
6
(2•PF
6
) which are
composed of trimetallic Ag
I
3
and Cu
I

3
cores bridged by three dppm and capped with one
or two ferrocenylacetylides, respectively. Cyclic voltmmogram (CV) of 2•PF
6
shows two
reversible Fc-oxidations separated by 110 ± 14 mV. This gives a comproportionation
constant K
c
of 77 ± 30. The stability of the mixed-valence 2
+
is attributed to the reduction
of Coulombic repulsion and electron delocalization mediated by the Cu
3
cluster is
insignificant.

The second system consists of the binuclear [Pt
2
(dppm)
2
(C≡C-Fc)
2
] (3) and the
trinuclear A-frames [Pt
2
(
µ
-AuX)(dppm)
2
(C≡CFc)

2
] [X = Cl. (4); = Br (5)]. Complex 3
contains a linear array of a Pt-Pt
σ
-bond and two ferrocenylacetylides. The X-ray crystal
structure of the complex shows a Pt-Pt bond distance of 2.7023(2) Å and a Fe-Fe
separation of 14.474(2) Å. The cyclic voltammogram (CV) of 3 indicates substantial
electronic communications mediated by the metal-metal bond: two quasi-reversible one-
electron Fc oxidations separated by 267 ± 10 mV is shown in the CV and this translates
into a K
c
of 3.3 ± 1.5 × 10
4
. The UV-vis-NIR absorption spectrum of 3 shows an
intervalence-charge-transfer band at 11300 ± 50 cm
-1
(ε
max
= 610 ± 10 M
-1
cm
-1
), and
ix
electronic coupling parameter H
AB
is estimated to be 190 ± 20 cm
–1
. The electronic
communications are shut down in the Pt

2
Au complexes as the CVs of the compounds 4
and 5 show an unresolved oxidation wave for the ferrocenyl groups.

To probe the interaction between the bimetallic M
2
(M = Pt and Pd) and acetylene
or vinylidene, complexes [Pt
2
(dppm)
2
(
µ
-
η
1
:
η
1
-HC=CFc)Cl
2
] (6), [Pd
2
(dppm)
2
(
µ
-
η
1

:
η
1
-
HC=CFc)Cl
2
] (7) and [Pd
2
(dppm)
2
(
µ
2
-
η
1
-C═CH-Fc)Cl
2
] (8) were synthesized. The
ferrocenyl group in the complex functions as an electrochemical reporter. Our
electrochemical studies show that the metal-ligand
π
-back bonding plays a minor role in
the electronic interactions. The major electronic interaction which determines the electron
density on the ferrocenyl group is the ligand-to-metal
σ
-donation.