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Transcript
Inorganic Reaction Mechanisms at the Molecular Level Oxford, January 4-6, 2009 Low Oxidation State Catalysts for Oxidation Reactions: Overcoming the Seeming Dilemma and Realizing the AdvantagesTitle Roy Periana Scripps Energy Laboratories, The Scripps Research Institute, Scripps Florida, 130 Scripps Way #3A1, Jupiter, Florida 33458, USA [email protected] Oxidation catalysts are typically based on the highest oxidation states of a particular metal complex, e.g., OsVIIIO4, RuVIIIO4, MnVIIO4-, etc. This seems consistent since the strongest “oxidants” should most efficiently catalyze “oxidation” reactions. However, this reasoning only holds if the rate determining steps in the reaction mechanisms involve reactions, such as hydrogen atom abstraction or outer-sphere electron transfer reactions and others that correlate with redox potentials. However, since these reactions typically involve free-radicals or transition states with radical character, these types of reactions are not amenable to selective generation of primary oxidation products such as alcohols from alkanes. For these types of oxidation reactions, due to the low homolytic bond strength of the CH bonds of the alcohol product compared to the starting alkane, transition states or intermediate with radical character must be minimized or the reaction selectivity will be poor. In this presentation, we will show that, counter intuitively, lower oxidation state catalysts are preferred for catalyzing the conversion of alkanes to alcohols. This is because such catalysts are most effective at catalyzing coordination reactions that operate by heterolytic CH bond cleavage. However, a seeming dilemma to this strategy of using lower oxidation state metal complexes to catalyze oxidation reactions is the thermodynamic instability of these catalysts in the presence of oxidants required for high yield conversion of the alkanes to alcohols. We will present strategies to overcoming this seeming dilemma. 1