Event

Bimetallic Redox Chemistry in Catalysis and Energy Conversion

Abstract

Polynuclear transition metals complexes, embedded in the active sites of metalloenzymes, are responsible for effecting a diverse array of oxidation reactions in nature. For example, aerobic hydroxylation of methane is accomplished at a diiron site in soluble methane monooxygenase. In contrast, synthetic catalyst platforms that take advantage of cooperative metal–metal (M–M) interactions are relatively rare. In this seminar, I will discuss two examples of catalysis in which redox cooperation within binuclear transition metal complexes has been exploited to accomplish challenging bond-forming reactions in both oxidative C–H functionalization as well as in solar energy conversion.

During the past 50 years, Pd-catalyzed C–H functionalization reactions have been developed for the synthesis of a diverse array of C–heteroatom bond-containing products. The critical bond-forming steps in these reactions have historically been proposed to proceed via mononuclear Pd(IV) intermediates. I will discuss evidence that implicates binuclear Pd(III) intermediates in catalysis and show that both metal-centered oxidation and product-forming reductive elimination reactions proceed at binuclear complexes. In addition, the role of M–M redox cooperation in facilitating redox chemistry via M–M bond formation and cleavage will be discussed.

Photochemical HX-splitting, in which a hydrohalic acid (HX) is split to its elemental constituents (H2 and X2), is an attractive approach to solar energy conversion. The efficiencies of extant photocatalysts are limited by X2 elimination chemistry, which is an energetically demanding, multi-electron reaction required for closed HX-splitting solar-to-fuels cycles. The intimate details of X2 photoelimination from dirhodium complexes have been elucidated by transient absorption spectroscopy and photocrystallography experiments. Critical ligand-bridged photointermediates have been identified in X2 elimination reactions. Based on these insights, new HX-splitting photocatalysts, based on ligand-bridged geometries, have been prepared and found to be more active catalysts.