Theoretical study on the mechanism of water oxidation catalyzed by a mononuclear copper complex: important roles of a redox non-innocent ligand and HPO42− anion

Abstract

The water oxidation reaction is the bottleneck problem of the artificial photosynthetic system. In this work, the mechanism of water oxidation catalyzed by a mononuclear copper complex in alkaline conditions was studied by density functional calculations. Firstly, a water molecule coordinating with the copper center of the complex (Cu^II, 1) generates Cu^II–H₂O (2). 2 undergoes two proton-coupled electron transfer processes to produce intermediate (4). The oxidation process occurs mainly on the ligand moiety, and 4 (˙L–Cu^II–O˙) can be described as a Cu^II center interacting with a ligand radical antiferromagnetically and an oxyl radical ferromagnetically. 4 is the active species that can trigger O–O bond formation via the water nucleophilic attack mechanism. This process occurs in a step-wise manner. The attacking water transfers one of the protons to the HPO₄²⁻ coupled with an electron transfer to the ligand radical, which generates a transient OH˙ interacting with the oxyl radical and H₂PO₄⁻. Then the O–O bond is formed through the direct coupling of the oxo radical and the OH radical. The triplet di-oxygen could be released after two oxidation processes. According to the Gibbs free energy diagram, the O–O bond formation was suggested to be the rate-limiting step with a calculated total barrier of 19.5 kcal mol⁻¹. More importantly, the copper complex catalyzing water oxidation with the help of a redox non-innocent ligand and HPO₄²⁻ was emphasized.