Electronic effects of redox-active ligands on ruthenium-catalyzed water oxidation

The process of water oxidation presents a significant challenge in the development of artificial pho-tosynthetic systems. This complexity arises from the necessity of charge accumulation, involving four electrons and four protons, and O-O bond formation. The strategy of using redox-active ligands in conjunction with metals is recognized as an effective approach for managing this charge accumu-lation process, attracting considerable attention. However, the detailed mechanisms by which the electronic effect of the redox-active ligands affect the reactivity of the catalytic centers remain am-biguous. In this study, the electronic effect of a series of mononuclear Ru complexes furnished with redox-active ligands ([(LRN5-)RuIII-OH]+, R = OMe, 3a; Me, 3b; H, 3c; F, 3d; CF3, 3e) was examined on water oxidation. A correlation was observed between redox potentials and substituent constants (sigma para), indicating that different successive redox pairs are influenced by electron effects of varying intensities. Particularly, the ligand-centered oxidation (E{[(LN5-)+center dot RuIV=O]2+/[(LN5-)RuIV=O]+}) shows a greater dependence than the metal-centered PCET oxidations, E(RuIII-OH/RuII-OH2) and E(RuIV=O/RuIII-OH). The critical intermediate, [(LN5-)+center dot RuIV=O]2+, formed through ligand-centered oxidation, triggers O-O bond formation via its reaction with water. The rate constants of this crucial step can be effectively modulated by the substituents of the ligand. This study provides intricate insights into the role of the redox-active ligand in regulating the water oxidation process and further substantiates the effectiveness of the synergistic interaction of redox ligands and metals in control-ling the multi-electron catalytic process.(c) 2023, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.