Electrocatalytic O<sub>2</sub>‑Reduction by Synthetic Cytochrome <i>c</i> Oxidase Mimics: Identification of a “Bridging Peroxo” Intermediate Involved in Facile 4e<sup>–</sup>/4H<sup>+</sup> O<sub>2</sub>‑Reduction

A synthetic heme–Cu C<i>c</i>O model complex shows selective and highly efficient electrocatalytic 4e<sup>–</sup>/4H<sup>+</sup> O<sub>2</sub>-reduction to H<sub>2</sub>O with a large catalytic rate (>10<sup>5</sup> M<sup>–1</sup> s<sup>–1</sup>). While the heme-Cu model (FeCu) shows almost exclusive 4e<sup>–</sup>/4H<sup>+</sup> reduction of O<sub>2</sub> to H<sub>2</sub>O (detected using ring disk electrochemistry and rotating ring disk electrochemistry), when imidazole is bound to the heme (Fe­(Im)­Cu), this same selective O<sub>2</sub>-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<sub>2</sub>-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<sup>–1</sup> in <sup>16</sup>O<sub>2</sub> (759 cm<sup>–1</sup> in <sup>18</sup>O<sub>2</sub>) for the FeCu complex and at 847 cm<sup>–1</sup> (786 cm<sup>–1</sup>) for the Fe­(Im)Cu complex, indicate the formation of <i>side-on</i> and <i>end-on</i> bridging Fe-peroxo-Cu intermediates, respectively, during O<sub>2</sub>-reduction in an aqueous environment. These data suggest that <i>side-on</i> bridging peroxide intermediates are involved in fast and selective O<sub>2</sub>-reduction in these synthetic complexes. The greater amount of H<sub>2</sub>O<sub>2</sub> production by the imidazole bound complex under fast electron transfer is due to 1e<sup>–</sup>/1H<sup>+</sup> O<sub>2</sub>-reduction by the distal Cu where O<sub>2</sub> binding to the water bound low spin Fe<sup>II</sup> complex is inhibited.