Electrocatalytic O2‑Reduction by Synthetic Cytochrome c Oxidase Mimics: Identification of a “Bridging Peroxo” Intermediate Involved in Facile 4e/4H+ O2‑Reduction

A synthetic heme–Cu CcO model complex shows selective and highly efficient electrocatalytic 4e/4H+ O2-reduction to H2O with a large catalytic rate (>105 M–1 s–1). While the heme-Cu model (FeCu) shows almost exclusive 4e/4H+ reduction of O2 to H2O (detected using ring disk electrochemistry and rotating ring disk electrochemistry), when imidazole is bound to the heme (Fe­(Im)­Cu), this same selective O2-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 O2-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 16O2 (759 cm–1 in 18O2) 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 O2-reduction in an aqueous environment. These data suggest that side-on bridging peroxide intermediates are involved in fast and selective O2-reduction in these synthetic complexes. The greater amount of H2O2 production by the imidazole bound complex under fast electron transfer is due to 1e/1H+ O2-reduction by the distal Cu where O2 binding to the water bound low spin FeII complex is inhibited.