Nickel and palladium complexes of Di-N-Heterocyclic carbenes

This thesis is concerned with the synthesis of two di-N-heterocyclic carbene ligands, and their reactivity towards nickel and palladium precursors in order to synthesise new organometallic pre-catalysts for a number of important reactions. The catalytic properties of a new rhodium C60 compound are also investigated, with relevance to the evaluation of C60 as a hydrogen storage medium.
Chapter 1 reviews the preparation and chemistry of N-heterocyclic carbenes, emphasising their organometallic reactivity and role as ancillary ligands in homogeneous transition metal catalysis. An overview of relevant nickel and palladium catalysis is also presented, including olefin polymerisation, olefm / CO copolymerisation and Heck coupling reactions.
Chapter 2 describes the synthesis and characterisation of the dicarbene ligands tBuCCmeth and tBuCCeth and their reactions with various nickel precursors in attempts to prepare chelating dicarbene nickel cis-dihalide complexes. The synthesis, characterisation and chemical reactivity of the cations [Ni(tBuCCmeth)2]2+, [(tBuCCmeth)NiCl(PMe3)] + and [(tBuCCeth)NiCl(PMe3)]+ is detailed, and their X-ray structures are compared. The unsuccessful preparation of [Ni(tBuCCeth)2] 2+ and the relative stability of the monocations with respect to dicarbene substitution is discussed and attributed to steric factors.
Chapter 3 describes the synthesis, characterisation, reactivity and catalytic studies of simple nickel and palladium cis-dimethyl complexes of the chelating dicarbene ligands. Variable temperature 1H NMR spectroscopy showed contrasting rates of thermal hydrocarbon elimination from Ni(tBuCCmeth)Me2 and Ni(tBuCCeth)Me2, which has previously been observed for chelating bis-phosphine analogues with various P,P' linkages (CH2)n . These observations further corroborate the analogy between dicarbene and 6/s-phosphine ligands. It was demonstrated that the compounds Pd(tBuCCmeth)Me2and Pd(tBuCCeth)Me2 are effective pre-catalysts for the Heck coupling of 4-bromoanisole and n-butyl acrylate. In addition cations of the type [(tBuCCmeth/eth)PdMe(L)]+ (L = pyridine, THF) which are relevant to olefin / CO copolymerisation were prepared. The X-ray structures of M(tBuCCeth)Me2 (M = Ni, Pd) are discussed as well as the synthesis and structural characterisation of [(|Li-tBu CC meth){Ni(PMe3)Me2}2].
Chapter 4 presents a brief introduction to some relevant C60 chemistry and to the concept of hydrogen storage. The synthesis and characterisation of a new rhodium Ceo compound is described. The compound catalysed the hydrogenation and hydroformylation of simple alkenes as well as the hydrogenation of C60 to C60H36 . The recovery of hydrogen gas from C60H36 was investigated in order to evaluate C60 as a hydrogen storage medium.
Chapter 5 outlines the experimental details for the synthesis, characterisation, reactions and catalytic studies of the new compounds described in the preceding three chapters.
Chapter 6 presents the characterising data for the new compounds described in chapters 2 and 3.
Appendices contain details of the crystallographic data for the eight structurally characterised compounds described in chapters 2 and 3.