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 Fe^II complex is inhibited.