Small-molecule activation chemistry catalyzed by proton-coupled electron transfer

Abstract

Proton-coupled electron transfer (PCET) is the basic mechanism for bioenergetic conversion. Consummate examples include water oxidation in photosynthesis and oxygen reduction in respiration. Despite the importance of PCET in such catalytic bond-making and bond-breaking reactions, the underlying mechanisms of coupled proton and electron transport in these processes is not well understood. To address mechanistic issues surrounding the role of PCET in catalytic chemical transformations, we have begun characterizing PCET events at a molecular level in a broad spectrum of small-molecule activation reactions for the first time. Two distinct structural scaffolds have been elaborated to study the PCET chemistry of 0-0 bond forming and cleaving reactions. The first consists of platforms containing two redox sites linked face-to-face by a rigid xanthene (DPX) or dibenzofuran (DPD) spacer - Pacman porphyrins. A comparative structural study demonstrates that DPD has the unprecedented ability to open and close its binding pocket by a vertical distance of over 4 A upon substrate binding, providing the first direct observation of the Pacman effect in a single cofacial platform. Moreover, efficient oxygen-activation chemistry is preserved when such cofacial motifs exhibit a large range of vertical motion; for example, dicobalt(II) complexes of both DPX and DPD are effective electrocatalysts for the direct four- electron reduction of oxygen to water despite their ca. 4 A difference in metal-metal distances.

(cont.) The second scaffold consists of acid-base and redox functionalities affixed to a xanthene (HPX) or dibenzofuran (HPD) scaffold - Hangman porphyrins. HPX selectively encapsulates water between its acid-base and redox sites by hydrogen bonding, affording a minimalist model for the cytochrome P450 heme water channel assemblies. Comparative reactivity studies for the catalase-like disproportionation of hydrogen peroxide and the epoxidation of olefins by HPX and HPD platforms bearing acid and ester pendants reveal that the introduction of a proton-transfer network properly oriented to a redox-active platform can orchestrate catalytic 0-0 bond activation. For the catalase and epoxidation reaction types, a marked reactivity enhancement is observed for the xanthene-bridged platform with a pendant carboxylic acid, establishing that this approach can yield superior catalysts to analogs that do not control both proton and electron currencies.