Critical Hammett Electron-Donating Ability of SubstituentGroups for Efficient Water Oxidation Catalysis by Mononuclear RutheniumAquo Complexes

[Ru(Rtpy)(bpy)(H2O)]2+ (1R; bpy = 2,2′-bipyridine, and Rtpy = 2,2′:6′,2″-terpyridine derivatives) complexes with a variety of 4′-substituent groups on Rtpy were synthesized and characterized to reveal the effects of substituents on their structures, physicochemical properties, and catalytic activities for water oxidation. The geometric structures of 1R are not considerably influenced by the electron-donating ability of the 4′-substituent groups on Rtpy. Similar multistep proton-coupled electron transfer reactions were observed for 1R, and the redox potentials for each oxidation step tended to decrease with an increase in the electron-donating ability of the substituent, which is explained by the increased electron density on the Ru center by electron-donating groups, stabilizing the positive charge that builds up upon oxidation. This is consistent with the red-shift of the absorption bands around 480 nm assigned to the metal-to-ligand charge transfer transition for 1R due to the increased d orbital energy...