Mechanisms of Homogeneous Activation of Hydrocarbons by Transition Metal Complexes

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Metal compounds are used extensively as catalysts for facilitating structural conversions of hydrocarbons, as required for reforming petroleum distillates to produce high octane gasoline, and for the formation of carbon-carbon bonds, as is necessary in the utilization of coal for the synthesis of useful organic compounds. The goal of this project is to provide a more thorough understanding of such catalytic processes, with a focus on transition metal catalysis of the formation or cleavage of carbon-carbon or carbon-hydrogen chemical bonds. Mechanistic studies of reactions of hydrocarbons in the coordination sphere of soluble complexes of transition metals will be undertaken. The target systems have been chosen to provide well-defined examples of reactions which form or break C-H and C-C bonds. Reversible alkene insertions into metal carbon bonds, intramolecular gamma-C-H activations, and intermolecular C-H activations provide the focus for the work. Metal oxidation state, steric bulk and electronic effects of ancillary ligands, alkyl group structure, charge and chelate ring size will be investigated as variables in a detailed look at the mechanisms of representative reactions. Systems to be studiesd include complexes of Pt, Pd, Ti and Zr containing a deuterium-labeled 2,2-dimethyl-5-pentyl ligand which shows reversible alkene insertion by specific deuterium site exchange. Gamma-C-H activation will be investigated with a variety of deuterium-labeled square-planar iridium(I) alkyl complexes. Intra- and intermolecular C-H activation will be studied in a series of octahedral Os(II) and Ir(III) phosphine and phosphite alkyl hydride complexes. The possibility of dictating a preference for intermolecular activation of alkanes by the synthesis of cyclometallation-resistant ligands will be investigated. In all cases a thorough examination of mechanisms will be made using kinetics, deuterium site labeling, kinetic isotope effects, and the effect of substrate structure on reactivity.