Critical Hammett Electron-Donating Ability of Substituent Groups for Efficient Water Oxidation Catalysis by Mononuclear Ruthenium Aquo 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 level of the Ru center. The turnover frequency (k_O2) of 1R for water oxidation catalysis, however, depended greatly on the Rtpy ligands, varying from 0.05 × 10^–2 to 44 × 10^–2 s^–1 (as the highest k_O2 was observed for R = ethoxy) by a factor of 880. A critical electron-donating ability of the 4′-substituent groups with a narrow range of Hammett constants (σ_p = −0.27 to −0.24) found for the highest k_O2 values is valuable for understanding the great difficulty in the search for efficient water oxidation catalysts. On another front, the k_O2 values increased with a decrease in the redox potentials of Ru^IVO/Ru^VO for 1R, indicating that the potential of formation of Ru^VO species for 1R is crucial for water oxidation catalysis under the employed conditions.