Characterization of Reaction Intermediates Involved in the Water Oxidation Reaction of a Molecular Cobalt Complex

Molecular cobalt(III) complexes of bis-amidate-bis-alkoxide ligands, (Me4N)[CoIII(L1)] (1) and (Me4N)[CoIII(L2)] (2), are synthesized and assessed through a range of character-ization techniques. Electrocatalytic water oxidation activity of the Co complexes in a 0.1 M phosphate buffer solution revealed a ligand-centered 2e-/1H+ transfer event at 0.99 V followed by catalytic water oxidation (WO) at an onset overpotential of 450 mV. By contrast, 2 reveals a ligand-based oxidation event at 0.9 V and a WO onset overpotential of 430 mV. Constant potential electrolysis study and rinse test experiments confirm the homogeneous nature of the Co complexes during WO. The mechanistic investigation further shows a pH-dependent change in the reaction pathway. On the one hand, below pH 7.5, two consecutive ligand-based oxidation events result in the formation of a CoIII(L2-)(OH) species, which, followed by a proton-coupled electron transfer reaction, generates a CoIV(L2-)(O) species that undergoes water nucleophilic attack to form the O-O bond. On the other hand, at higher pH, two ligand-based oxidation processes merge together and result in the formation of a CoIII(L2-)(OH) complex, which reacts with OH- to yield the O-O bond. The ligand-coordinated reaction intermediates involved in the WO reaction are thoroughly studied through an array of spectroscopic techniques, including UV-vis absorption spectroscopy, electron paramagnetic resonance, and X-ray absorption spectroscopy. A mononuclear CoIII(OH) complex supported by the one-electron oxidized ligand, [CoIII(L3-)(OH)]-, a formal CoIV(OH) complex, has been characterized, and the compound was shown to participate in the hydroxide rebound reaction, which is a functional mimic of Compound II of Cytochrome P450.

Automated summary

In this study, two molecular cobalt(III) complexes were synthesized and their electrocatalytic water oxidation activities were assessed under different pH conditions. The reaction pathways of these complexes were found to vary depending on the pH, with detailed studies of the ligand-coordinated reaction intermediates highlighting the involvement of a functional mimic of Compound II of Cytochrome P450.