The oxygen reduction reaction (ORR) plays important roles in both biological energy conversion and next-generation energy technologies. Development of earth-abundant molecular mimics of metalloprotein active sites will provide an understanding of the structure-function relationship that controls O2 activation, reduction, and ORR selectivity at well-defined metal active sites, which remains an important question. We report a bioinspired non-heme Fe complex with a tripodal [N3O]– ligand framework (Fe(PMG)(Cl)2) that is electrocatalytically active toward dioxygen reduction with acetic acid as a proton source in acetonitrile solution. Under electrochemical and chemical conditions, Fe(PMG)(Cl)2 selectively produces water via a 2+2 mechanism, where H2O2 is generated as a discrete intermediate species before further reduction to two equivalents of H2O. Mechanistic studies support a catalytic cycle for dioxygen reduction where an off-cycle peroxo dimer species is the resting state of the catalyst. Spectroscopic analysis of the reduced complex FeII(PMG)Cl shows the stoichiometric formation of an Fe(III)-hydroxide species following exposure to H2O2. Electrochemical studies, spectrochemical experiments, and DFT calculations suggest that the carboxylate moiety of the ligand is sensitive to hydrogen-bonding interactions with the acetic acid proton donor upon reduction from Fe(III)/(II), favoring chloride loss trans to the tris-alkyl amine moiety of the ligand framework. These results offer insight into the structure-function relationship of mononuclear non-heme Fe active sites in metalloproteins and how added charge is distributed and how proton donors are poised during reactivity with dioxygen.

Electrocatalytic reduction of dioxygen to water by a bioinspired non-heme iron complex

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ACS Fall 2022

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