Download Inorganic Reaction Mechanisms at the Molecular Level Oxford

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.
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