Synthesis of Organometallic Catalysts that Oxidize Water and Hydrogenate Substrates

Abstract

The ability to acquire and transfer hydrogen catalytically is fundamental to the development of clean and sustainable fuels. N-heterocyclic carbene (NHC) based ruthenium complexes were synthesized and studied as catalysts for the transfer hydrogenation of ketones. Variations in the catalyst structure were investigated for their impact on hydrogenation. Catalyst attributes included bis- or mono-NHC ligands, pendant ether groups, and arene ligands of varied bulk and donor strength. Variable temperature 1 H NMR studies indicated that arene lability increases in the order hexamethylbenzene in situ to alter the electronic properties at the metal in a switchable and reversible manner. Comparison of initial rates at pH 3-6 with Ir(dhbp) complexes and complexes with bipyridine ligands that lack protic groups, showed that rate enhancement with dhbp complexes occurs at high pH due to ligand deprotonation rather than the pH alone accelerating water oxidation. Thus, the protic groups in dhbp improve the catalytic activity by tuning the complexes electronic properties upon deprotonation, although preliminary studies suggest a mechanism involving O - groups shuttling protons may also contribute to some extent. Mechanistic studies show that the rate law is first-order in iridium precatalyst, and all evidence indicates that catalysis is homogeneous. Base-free transfer hydrogenation and the direct hydrogenation of ketones and carbon dioxide were also investigated.%%%%Ph.D., Inorganic Chemistry – Drexel University, 2013