Catalytic Four-Electron Reduction of Oxygen to Water by a Molecular Cobalt Complex Consisting of a Proton Exchanging Site at the Secondary Coordination Sphere

Controlling the selectivity of 4e^–/4H⁺ reduction of oxygen over 2e^–/2H⁺ reduction is a key challenge in making efficient catalysts for fuel cell cathodes. A tyrosine residue poised over the active site of cytochrome c oxidase (CcO) has been demonstrated to control the hydrogen atom transfer reactions and cleavage of the O–O bond of a Fe–O–O–Cu moiety to yield water. In a couple of small-molecule iron complexes supported by porphyrin derivatives, it was shown that the presence of protonation sites at the secondary coordination sphere plays an important role in directing the selectivity and rate of the oxygen reduction reaction (ORR). In this study, we designed and synthesized a mononuclear Co^III complex (1) of a bis-pyridine-bis-oxime ligand where the oxime site can participate in reversible proton exchange reactions. Electrocatalytic ORR of 1 was investigated in aqueous buffer solutions and acetonitrile containing trifluoroacetic acid as the proton source. We observed that in a 0.1 M phosphate buffer solution (PBS), 1 is selective for 4e^–/4H⁺ reduction of O₂ at pH 4, and the selectivity decreases with increasing the buffer medium’s pH, producing ca. 75% H₂O at pH 7. However, in a 0.1 M acetate buffer solution (ABS), 1 remained highly selective for the cleavage of the O–O bond to produce H₂O at pH 4 and pH 7. The overpotential (η) of H₂O formation (ca. 0.8–0.65 V) decreased proportionally with increasing pH in PBS and ABS. In acetonitrile, 1 remained highly selective for 4e^–/4H⁺ reduction for electrocatalytic and chemical ORR. An overpotential of 760 mV was estimated for H₂O production in acetonitrile. Kinetic analysis suggests the first-order dependence of catalyst concentration on the reaction rate at 25 °C. However, the formation of a peroxo-bridged dinuclear cobalt(III) complex was noted as a reaction intermediate in the ORR pathway in acetonitrile at −40 °C. We conjecture that the oxime scaffold of the ligand works as a proton exchanging site and assists in the proton-coupled electron transfer (PCET) reactivity to cleave the O–O bond in the acidic buffer solutions and acetonitrile, further corroborated by theoretical studies. Density functional theory (DFT) calculation suggests that the acetate ion works as a mediator at pH 7.0 for transferring a proton from the oxime scaffold to the distal oxygen of the Co^III(OOH) intermediate, responsible for high selectivity toward 4e^–/4H⁺ reduction of O₂.