ELECTRONIC COMMUNICATIONS MEDIATED BY METAL CLUSTERS AND πCONJUGATED SYSTEMS WU JIANGUO A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMISTRY NATIONAL UNIVERSITY OF SINGAPORE 2003 i 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. ii TABLE OF CONTENTS Acknowledgement ii Table of contents iii Abbreviations vi Summary viii CHAPTER ONE Introduction 1.1 Mixed-Valence compounds 1.2 Classification of MV complexes 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 1H and 31P NMR of complexes and 39 2.2.3 Spectroscopic and Electrochemical Properties 42 iii 2.3 Conclusion 55 2.4 Experimental Section 55 CHAPTER THREE Electronic Communication Mediated By PtI-PtI σ-Bond 3.1 Introduction 61 3.2 Results 65 3.2.1 Structure of 65 3.2.2 1H and 31P NMR of complexes 67 3.2.3 Electrochemistry of 69 3.2.4 Electronic absorption spectrum of 3+ 74 3.2.5 Structures of and 77 3.2.6 1H and 31P NMR of complexes and 81 3.2.7 Electrochemistry of and 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 iv 4.1 Introduction 103 4.2 Objectives 104 4.3 Results and Discussion 108 4.3.1 Syntheses and Structures 108 4.3.2 1H and 31P NMR of complexes , and 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 Metalligand 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 v ABBREVIATIONS A acceptor ac2 two electron acetylene Bu butyl Ca About Calcd. calculated CV Cyclic voltammetry or voltammogram D Donor dppe 1,2-bis(diphenylphosphino)ethane dppm di(phenylphosphino)methane e electron transfer Et ethyl F Faraday constant Fc ferrocenyl, C5H5FeC5H4 G gram(s) h hour(s) HOMO highest occupied molecular orbital IR infrared IVCT intervalence charge transfer LUMO lowest unoccupied molecular orbital m multiple M metal vi Me methyl MV mixed valence MLCT metal-to-ligand charge transfer N Avogadro’s number NMR Nuclear Magnetic Resonance NIR Near infrared R Alkyl or gas constant T temperature UV-vis Ultra-violet visible vii 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 [Ag3(µ-dppm)3(µ3-η1-C≡CFc)]•2CF3SO3 (1•2CF3SO3) and [Cu3(µ-dppm)3(µ3-η1-C≡C-Fc)2]•PF6 (2•PF6) which are composed of trimetallic AgI3 and CuI3 cores bridged by three dppm and capped with one or two ferrocenylacetylides, respectively. Cyclic voltmmogram (CV) of 2•PF6 shows two reversible Fc-oxidations separated by 110 ± 14 mV. This gives a comproportionation constant Kc of 77 ± 30. The stability of the mixed-valence 2+ is attributed to the reduction of Coulombic repulsion and electron delocalization mediated by the Cu3 cluster is insignificant. The second system consists of the binuclear [Pt2(dppm)2(C≡C-Fc)2] (3) and the trinuclear A-frames [Pt2(µ-AuX)(dppm)2(C≡CFc)2] [X = Cl. (4); = Br (5)]. Complex 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 indicates substantial electronic communications mediated by the metal-metal bond: two quasi-reversible oneelectron Fc oxidations separated by 267 ± 10 mV is shown in the CV and this translates into a Kc of 3.3 ± 1.5 × 104. The UV-vis-NIR absorption spectrum of shows an intervalence-charge-transfer band at 11300 ± 50 cm-1 (εmax = 610 ± 10 M-1 cm-1), and viii electronic coupling parameter HAB is estimated to be 190 ± 20 cm –1 . The electronic communications are shut down in the Pt2Au complexes as the CVs of the compounds and show an unresolved oxidation wave for the ferrocenyl groups. To probe the interaction between the bimetallic M2 (M = Pt and Pd) and acetylene or vinylidene, complexes [Pt2(dppm)2(µ-η1:η1-HC=CFc)Cl2] (6), [Pd2(dppm)2(µ-η1:η1HC=CFc)Cl2] (7) and [Pd2(dppm)2(µ2-η1-C═CH-Fc)Cl2] (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. ix . Vinylidene v 4 .1 Introduction 10 3 4.2 Objectives 10 4 4.3 Results and Discussion 10 8 4.3 .1 Syntheses and Structures 10 8 4.3.2 1 H and 31 P NMR of complexes 6 , 7 and 8 11 5 4.3.3 Electrochemistry. 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 11 300 ± 50 cm -1 (ε max = 610 ± 10 M -1 cm -1 ), and ix electronic coupling. 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