Heterocyclic carbene complexes : reaction chemistry and catalytic applications

This thesis describes the synthesis of a broad range of heterocyclic carbene complexes of transition metals, their fundamental reaction chemistry and their use as catalysts in a number of reactions. Mechanistic studies have been performed in order to gain insight into likely catalytic cycles and the decomposition processes of the complexes. Pd complexes of donor-functionalised heterocyclic carbene ligands have been synthesised through a route involving carbene transfer from Aglcarbene precursors. A variety of complexes containing 0 and N donor-functionalised carbenes have been prepared and tested as catalysts for Heck-type and polymerisation reactions. The complexes are highly active for Heck and Suzuki couplings, but do not catalyse chain growth reactions such as ethylene polymerisation or CO-ethylene copolymerisation. Zerovalent carbene complexes of Pd of formula Pd(tmiy)2(alkene) (tmiy = 1,3,4,5- tetramethylimidazolin-2-ylidene, alkene = maleic anhydride, tetracyanoethylene) have been synthesised. Spectroscopic studies on the complexes provide strong evidence of the almost purely donor nature of the carbene ligand. Reaction with a number of substrates, including aryl halides, leads to Pd11 complexes through oxidative addition. The zerovalent Ni complex Ni(tmiy)2 has also been produced in situ and found to undergo oxidative addition with organohalides and dihalides. Halide abstraction from Pd(tmiy)2(4-nitrophenyl)I in the presence of n-butylacrylate leads to migratory insertion of the olefin and elimination of the Heck coupling product. Under stoichiometric reaction conditions hydrocarbyl-imidazolium ions are also produced as major byproducts, indicating that carbene elimination as the hydrocarbyl-imidazolium salt is a probable route to catalyst deactivation. Additionally, the reaction of CO with [PdMe(tmiy)(µ-C1)]2 has been studied, and the migratory insertion of CO into the PdMe bond, followed by rapid decomposition by elimination of an acyl-imidazolium cation is reported. This observation provides an explanation for the lack of activity of the Pd carbene complexes in chain growth reactions. Elimination of the imidazolium ion from hydrocarbyl Pd and Ni carbene complexes was considered a potentially serious route to catalyst deactivation. In order to elucidate the mechanism of the reaction a number of new methyl Pdri complexes of the form [PdMe(tmiy)L2]BF4 (L = cyclooctadiene, methyldiphenylphosphine, triphenylphosphite, triphenylphosphine) were prepared and their decomposition kinetics studied. The complexes decompose via elimination of pentamethylimidazolium and generation of Pd° following first order kinetics. The kinetic investigations combined with density functional studies show that the complexes decompose via a mechanism of concerted reductive elimination. An understanding of the mechanism of the reaction led to the prediction of methods by which it can be impeded. Density functional studies of Ni, Pd and Pt complexes show that the energetics of the reaction can be changed such that oxidative addition of imidazolium to M ° is energetically favourable. This is demonstrated experimentally by the synthesis of hydrido Pt carbene complexes, via oxidative addition of azolium salts to Pt °. The results suggest that conducting reactions in ionic liquids of imidazolium salts will be a promising method of stabilising M-carbene complexes against decomposition. Consistent with this proposal, Ni complexes of the form NiI2(carbene)2 have been synthesised (and crystal structures obtained) and shown to be very active and stable catalysts for olefin dimerisation when immobilised in ionic liquids. A variety of carbenes and imidazolium salts with two flanking donor groups were prepared and their coordination behaviour investigated. Coordination of a dipicolyl carbene ligand to Pd was found to increase the stability of the complex (to reductive elimination) relative to other cationic methyl Pd carbene complexes. The ligands were tested in conjunction with the metals Ti, V, Cr and Pd as catalysts for ethylene polymerisation, and several combinations were found to give moderately active catalysts.