Homo- and Heterobimetallic Complexes: Synthesis and Investigation of Metal and Ligand Combinations for Catalysis

Abstract

This thesis describes the synthesis of homo- and heterobimetallic complexes and their use as catalysts for the formation of heterocycles. The bimetallic complexes were designed to discover what structural properties would provide the greatest enhancement to the catalytic rate of a reaction. A series of new homobimetallic complexes were synthesised that contained two bis(pyrazol-1-yl)methane (bpm) ligand groups immobilised on an aromatic scaffold that varied the distance between the bpm units and the degree of conformational freedom within the bimetallic complex. These complexes, of the type [Rh2(CO)4(LX)][BArF4]2 (X = xanthene (2.8), dibenzofuran (2.9), ferrocene (2.10)), hexane (2.11), heptane (2.12)) and [Rh2(COD)2(LX)][BArF4]2 (X = hexane (2.14), heptane (2.15)), were fully characterised and solid-state structures of 2.8, 2.10 and 2.14 were determined using X-ray crystallography. The catalytic activity of these complexes was investigated for the dihydroalkoxylation reaction. It was discovered that a higher degree of conformational freedom within the complexes reduced their effectiveness as catalysts due to the potential for greater separation between the metal centres. In an attempt to correlate the activity of the catalysts with the metal-metal separation computational modelling of the three dimensional structures of 2.8, 2.9, 2.22-2.24 were performed. These calculations indicated that the metal-metal distance was in fact poorly defined, even for complexes containing a rigid aromatic scaffold, and no direct correlation with the catalytic activity of the complexes could be found. A series of homo- and heterobimetallic complexes containing Ir(I), Ir(III), Rh(I), Au(I) and Ru(II) were also prepared using heteroditopic ligands that combined a bpm ligand group with and N-heterocyclic carbene ligand group. The solid-state structures of several complexes (3.3d, 3.4, 3.7, 3.21 and 3.24) were determined using X-ray crystallography. These complexes were investigated as catalysts for the tandem dihydroalkoxylation and hydroamination/hydrosilylation reactions. The effectiveness of the catalysts was found to be subtly dependent on the position of the two metals within the heteroditopic ligand scaffold which was clearly indicative of an interaction between the two complex fragments during the catalytic cycles.