Electrocatalytic O_2-Reduction by Synthetic Cytochrome c Oxidase Mimics: Identification of a 'Bridging Peroxo' Intermediate Involved in Facile 4e~-/4H~+ O_2-Reduction

摘要

A synthetic heme-Cu CcO model complex shows selective and highly efficient electrocatalytic 4e~-/4H~+ O_2-reduction to H_2O with a large catalytic rate (＞10~5 M~(-1) s~(-1)). While the heme-Cu model (FeCu) shows almost exclusive 4e~-/4H~+ reduction of O_2 to H_2O (detected using ring disk electrochemistry and rotating ring disk electrochemistry), when imidazole is bound to the heme (Fe(Im)Cu), this same selective O_2-reduction to water occurs only under slow electron fluxes. Surface enhanced resonance Raman spectroscopy coupled to dynamic electrochemistry data suggests the formation of a bridging peroxide intermediate during O_2-reduction by both complexes under steady state reaction conditions, indicating that O-O bond heterolysis is likely to be the rate-determining step (RDS) at the mass transfer limited region. The O-O vibrational frequencies at 819 cm~(-1) in ~(16)O_2 (759 cm~(-1) in ~(18)O_2) for the FeCu complex and at 847 cm~(-1) (786 cm~(-1)) for the Fe(Im)Cu complex, indicate the formation of side-on and end-on bridging Fe-peroxo-Cu intermediates, respectively, during O_2-reduction in an aqueous environment. These data suggest that side-on bridging peroxide intermediates are involved in fast and selective O_2-reduction in these synthetic complexes. The greater amount of H_2O_2 production by the imidazole bound complex under fast electron transfer is due to 1e~-/1H~+ O_2-reduction by the distal Cu where O_2 binding to the water bound low spin Fe~Ⅱ complex is inhibited.