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, (Me 4 N)[Co III (L 1 )] ( 1 ) and (Me 4 N)[Co III (L 2 )] ( 2 ), are synthesized and assessed through a range of characterization 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 Co III (L 2- )(OH) species, which, followed by a proton-coupled electron transfer reaction, generates a Co IV (L 2- )(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 Co III (L 2- )(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 Co III (OH) complex supported by the one-electron oxidized ligand, [Co III (L 3- )(OH)] - , a formal Co IV (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.