Two new ruthenium(ii) complexes bearing dissociable protons, [Ru(trpy)(H 2bim)Cl]PF 6 (1) and [Ru(trpy)(H 2bim)(OH 2)](PF 6) 2 (2) (H 2bim = 2,2′-biimidazole and trpy = 2,2′:6′, 2′′-terpyridine), were synthesized and characterized, where the H 2bim and M-OH 2 moieties are expected to serve as proton-dissociation sites. Single crystal X-ray diffraction analyses revealed that the H 2bim and M-OH 2 moieties act as proton donors in intermolecular hydrogen bonds. Two pK a values of 2 (pK a1 = 9.0 and pK a2 = 11.3) were spectrophotometrically determined, where the first proton dissociation is assigned to that from H 2bim and the second is from M-OH 2. This assignment was supported by the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations for two sets of conjugated bases, [Ru(trpy)(Hbim)(OH 2)] + and [Ru(trpy)(H 2bim)(OH)] + for the first proton dissociation, and [Ru(trpy)(Hbim)(OH)] + and [Ru(trpy)(bim)(OH 2)] + for the second dissociation. Electrochemical studies in aqueous solutions under various pH conditions afforded the Pourbaix diagram (potential versus pH diagram) of 2, where the pK a values found from the diagram agree well with those determined spectrophotometrically. It was also found that 2 demonstrates four-step proton-coupled electron transfer (PCET) reactions to give the four-electron oxidized species, [Ru IV(trpy)(bim)(O)] 2+, without electrostatic charge buildup during the reactions. The multiple PCET ability of 2 would be applicable to various multi-electron oxidation reactions. Catalysis of electrochemical water oxidation was indeed evaluated in the initial attempt to demonstrate multi-electron oxidation reactions, revealing that the water oxidation potential for 2 is lower than that for other ruthenium catalysts, [Ru(trpy)(bpy)(OH 2)] 2+, [Ru(trpy)(bpm)(OH 2)] 2+ and [Ru(tmtacn)(bpy)(OH 2)] 2+ (bpy = 2,2′-bipyridine, bpm = 2,2′-bipyrimidine, and tmtacn = 1,4,7-trimethyl-1,4,7-triazacyclononane), which are known to be active catalysts for water oxidation.
All Science Journal Classification (ASJC) codes
- Inorganic Chemistry