Synthesis of organometallic catalysts that oxidize water and hydrogenate substrates: elucidation of structure/function relationships

Joseph DePasquale

doi:10.17918/etd-4273

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 ¹H NMR studies indicated that arene lability increases in the order hexamethylbenzene < cymene < benzene, and this lability is directly correlated with catalytic activity. The catalysis appears to be homogeneous, and a mechanism invoking arene loss is proposed. The S enantiomers of ester functionalized NHC ligand precursors were synthesized from L-valine. Coordination of these NHC ligands to Ru(II) resulted in the isolation of a monodentate complex (with only NHC coordination) and a bidentate complex, which has carboxylate coordination (from in situ hydrolysis of the ester) in addition to NHC coordination. The evidence shows that both Ru complexes are racemic, but with alternate synthetic methods perhaps racemization during complexation of ligand precursors to metals could be avoided. These ruthenium complexes also serve as catalysts for ketone transfer hydrogenation. Highly active iridium precatalysts for water oxidation that are supported by recently designed dihydroxybipyridine (dhbp) ligands are reported. These ligands can readily be protonated/deprotonated 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.

Synthesis of organometallic catalysts that oxidize water and hydrogenate substrates: elucidation of structure/function relationships

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Synthesis of organometallic catalysts that oxidize water and hydrogenate substrates: elucidation of structure/function relationships

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