A Bioinspired Nonheme Fe^III-(O₂^2-)-Cu^II Complex with an S_t = 1 Ground State

Cytochrome c oxidase (CcO) is a heme copper oxidase (HCO) that catalyzes the natural reduction of oxygen to water. A profound understanding of some of the elementary steps leading to the intricate 4e^-/4H⁺ reduction of O₂ is presently lacking. A total spin S_t = 1 Fe^III-(O₂^2-)-Cu^II (I) intermediate is proposed to reduce the overpotentials associated with the reductive O-O bond rupture by allowing electron transfer from a tyrosine moiety without the necessity of any spin-surface crossing. Direct evidence of the involvement of I in the CcO catalytic cycle is, however, missing. A number of heme copper peroxido complexes have been prepared as synthetic models of I, but all of them possess the catalytically nonrelevant S_t = 0 ground state resulting from antiferromagnetic coupling between the S = 1/2 Fe^III and Cu^II centers. In a complete nonheme approach, we now report the spectroscopic characterization and reactivity of the Fe^III-(O₂^2-)-Cu^II intermediates 1 and 2, which differ only by a single -CH₃ versus -H substituent on the central amine of the tridentate ligands binding to copper. Complex 1 with an end-on peroxido core and ferromagnetically (S_t = 1) coupled Fe^III and Cu^II centers performs H-bonding-mediated O-O bond cleavage in the presence of phenol to generate oxoiron(IV) and exchange-coupled copper(II) and PhO^• moieties. In contrast, the μ-η²:η¹ peroxido complex 2, with a S_t = 0 ground state, is unreactive toward phenol. Thus, the implications for spin topology contributions to O-O bond cleavage, as proposed for the heme Fe^III-(O₂^2-)-Cu^II intermediate in CcO, can be extended to nonheme chemistry.