Electrocatalytic four-electron reductions of O₂ to H ₂O with cytochrome c oxidase model compounds

Three different complexes, heme-Cu ([(⁶L)Fe^IICu ^I]⁺ (1), ⁶L=a binucleating ligand having a heme and covalently tethered copper binding tris(2-pyridyl)methylamine tetradentate moiety), heme complex ((⁶L)Fe^II (4), (with "empty" tethered chelate)), and the "parent" iron-porphyrinate ((F₈TPP)Fe^II (5), F ₈TPP=tetrakis(2,6-difluorophenyl)porphyrinate) were employed for the electrocatalytic reduction of O₂. Complexes 1 and 4 reduce O ₂ to water (four-electron reduction) with good efficiency (74 and 59%, respectively), but complex 5 exhibited only an ∼ 20% efficiency, thus primarily the two-electron reduction to hydrogen peroxide occurred. The results of the present electrochemical O₂-reduction studies and the previous studies have elucidated the O₂-binding nature of these three complexes, indicating the formation of quite stable Fe^III-(O ₂^2-)-Cu^II (peroxo) or Fe^III-O ₂^- (superoxo) species. In line with the thinking of other researchers, the fact that both 1 and 4 can well stabilize Fe ^III-O₂^- superoxo species may suggest that the formation of the latter is a key to the pathway favoring four-electron reduction of dioxygen to water.