Catalytic Water Oxidation By Novel Mono-and Dinuclear Ruthenium Catalysts

In recent years, conversion of solar energy into clean, renewable hydrogen has become a hot research topic due to energy crisis and environmental pollution that are confronted with human beings. With respect to these crucial issues, a potential and promising way is to develop devices that can convert solar energy into storable and clean fuels. Photochemical cells (PCEs) which split water through sunlight into hydrogen and oxygen have exhibited great potentials in this area. Water oxidation, a complicated reaction involving transfer of four protons and four electrons as well as the formation of O-O bond, is currently considered a major bottleneck in assembling PCEs. Therefore, finding an effective catalyst capable of oxidizing water to dioxygene and its assembly to PCEs is reckoned to be a sustainable solution. This paper focuses on the design of effective water oxidation catalysts (WOCs).Five mononuclear ruthenium complexes Ru(bda)L2 [bda= 2,2'-bipyridine-6,6'-dicarboxylic; L= 4-methylpyridine, Rul; 4,4'-bipyridine, Ru2; 4-(trifluoromethyl)pyridine, Ru3; 4-chloropyridine, Ru4; 4-tert-butylpyridine, Ru5] with low overpotentials for water oxidation were synthesized and characterized. Their electrochemic propertities under acidic or neutral conditions were systemically studied by means of cyclic voltammetry. Their catalytic activities towards water oxidation using [Ce(NH4)2(NO3)6] (CeIV) as an oxidant in aqueous solutions at pH 1.0 were investigated. Results show that complex Ru2 bearing 4,4'-bipyridine axial ligands is one of the most effective water oxidation catalysts yet reported, exhibiting a remarkable activity of 4319 turnover number (TON). It was found that the more electron-withdrawing group at the para position of axial pyridine lignad leads to a higher activity for the ruthenium complex in water oxidation.To elucidate the mechanism of water oxidation catalyzed by mononuclear catalysts Rul-5, we designed and perapared three dinuclear catalysts (μ-L)[Ru(bda)(4-Me-py)]2 (L= 1,4-bis(N-imidazolyl)butane, Ru9; 1,3-bis(N-imidazolylmethyl)benzene, Ru10; 1,3-bis(4-pyridyl)propane, Rull).Also, a two-step synthetic procedure was successfully developed to prepare ruthenium complexes Rul-5. The overall yields are superior to previous methods. This provides an easy and convenient way to prepare similar complexes.