Mechanism of water oxidation catalyzed by vitamin B_(12):Redox non-innocent nature of corrin ligand and crucial role of phosphate

摘要

Vitamin B_(12)(macrocyclic cobalamin)has been recently reported to be capable of electrochemically catalyzing water oxidation in a neutral phosphate buffer solution.In this work,density functional calculations were employed to elucidate the water oxidation mechanism catalyzed by vitamin B_(12).The calculations showed that the catalytic cycle starts from the L·-Co^(Ⅱ)-OH_(2)complex 1.A proton-coupled electron transfer process then leads to the formation of a L^(·)-Co^(Ⅲ)-OH complex 2,followed by another proton-coupled electron transfer event to afford a corrin ligand radical cation intermediate 3(L^(·)-Co^(Ⅲ)-O^(·)).The redox non-innocent nature of the corrin ligand plays an essential role in the oxidation process.3 is capable of triggering the O-O bond formation via a water nucleophilic attack mechanism,in which a hydrophosphate dianion functions as a base to accept a proton from the water nucleophile.A dioxygen molecule is released after the oxidation of the Co^(Ⅲ)-OOH intermediate.The rate-determining step was calculated to be the O-O bond formation with a total barrier of 16.5 kcal/mol.While the use of water molecules as the proton acceptor was found to be less feasible for the O-O bond formation,with a barrier of 31.2 kcal/mol,further highlighting the crucial of phosphate in water oxidation.