Mechanism and design of homogeneous heterobimetallic early metal-aluminum alkyl catalysts for ethylene trimerization and polymerization

Abstract

This thesis describes an experimental study of the coordination chemistry of several early-metal based homogeneous catalysts for ethylene trimerization and polymerization. The first part of the thesis discusses a mechanistic study of chromium catalyzed ethylene trimerization and tetramerization based on the established NCy(PPh2)2 [PNP], NH(CH 2CH2SCy)2 [SNS], and pyrrole ligands. Through the isolation and characterization of catalytic resting states, many missing pieces to the mechanistic puzzle have been found, which aids the analysis of critical relationships between the chromium electronic state, geometry, nuclearity, and charge and the observed catalyst activity and selectivity. Some contributions include the first direct evidence for the involvement of Cr(I), Cr(II) and Cr(III) species in catalysis, 1-hexene selectivity enhancement via ligand-directed cocatalyst hemilability, and a well-defined single-component ethylene trimerization catalyst. The second part of the thesis explores olefin polymerization catalysis with pyrrole complexes of vanadium and titanium. The pyrrole ligand was found to enhance alkyl transfer between the transition metal and aluminum centres via its hemilability, which enhances the activation of normally kinetically inert metal electronic configurations. In addition, the alkyl shuttling allows the isolation of well-defined thermally stable complexes which form highly active single-site single-component polymerization catalysts at high temperatures.