Unusual Water Oxidation Mechanism via a Redox-Active Copper Polypyridyl Complex

To improve Cu-based water oxidation (WO) catalysts, a proper mechanistic understanding of these systems is required. In contrast to other metals, high-oxidation-state metal–oxo species are unlikely intermediates in Cu-catalyzed WO because π donation from the oxo ligand to the Cu center is difficult due to the high number of d electrons of Cu^II and Cu^III. As a consequence, an alternative WO mechanism must take place instead of the typical water nucleophilic attack and the inter- or intramolecular radical–oxo coupling pathways, which were previously proposed for Ru-based catalysts. [Cu^II(HL)(OTf)₂] [HL = Hbbpya = N,N-bis(2,2′-bipyrid-6-yl)amine)] was investigated as a WO catalyst bearing the redox-active HL ligand. The Cu catalyst was found to be active as a WO catalyst at pH 11.5, at which the deprotonated complex [Cu^II(L^–)(H₂O)]⁺ is the predominant species in solution. The overall WO mechanism was found to be initiated by two proton-coupled electron-transfer steps. Kinetically, a first-order dependence in the catalyst, a zeroth-order dependence in the phosphate buffer, a kinetic isotope effect of 1.0, a ΔH^⧧ value of 4.49 kcal·mol^–1, a ΔS^⧧ value of −42.6 cal·mol^–1·K^–1, and a ΔG^⧧ value of 17.2 kcal·mol^–1 were found. A computational study supported the formation of a Cu–oxyl intermediate, [Cu^II(L^•)(O^•)(H₂O)]⁺. From this intermediate onward, formation of the O–O bond proceeds via a single-electron transfer from an approaching hydroxide ion to the ligand. Throughout the mechanism, the Cu^II center is proposed to be redox-inactive.