Catalytic Reduction of Dioxygen to Water by a Bioinspired Non-Heme Iron Complex via a 2+2 Mechanism

Abstract

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 Fe^II(PMG)Cl shows the stoichiometric formation of an Fe(III)-hydroxide species following exposure to H₂O₂; no catalytic activity for H₂O₂ disproportionation is observed, although the complex is electrochemically active for H₂O₂ reduction to H₂O. 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 <i>trans</i> to the tris-alkyl amine moiety of the ligand framework. These results offer insight into how mononuclear non-heme Fe active sites in metalloproteins distribute added charge and poise proton donors during reactions with dioxygen.