Chemical and photocatalytic water oxidation by mononuclear Ru catalysts

Abstract

Four mononuclear Ru complexes with different substituents on the para position of the pyridine ligand of Ru(bda)(pic) 2 (H 2 bda = 2,2′-bipyridine-6,6′-dicarboxylic acid; pic = picoline) were synthesized and characterized by 1 H nuclear magnetic resonance or X-ray crystallography. The electrochemical properties of this series of compounds in acidic and neutral conditions were studied by cyclic voltammetry. Their catalytic activity towards water oxidation was investigated using a chemical oxidant ([Ce(NH 4 ) 2 (NO 3 ) 6 ] (Ce IV ) in acidic solution, or driven by visible light in a three-component system containing a photosensitizer ([Ru(bpy) 3 ] 2+ ) and an electron acceptor (S 2 O 8 −2 ). For the chemical water oxidation, complex 1 was found to be the most effective, exhibiting a turnover number (TON) of up to 4000. The pyridine substituent at the 4-position in 1 may be protonated giving an intensive electron-withdrawing effect. Complex 2 bears the most electron-withdrawing trifluoromethyl group under neutral conditions and showed the highest photocatalytic activity with a TON of 270 over 2 h. It was concluded that the more electron-withdrawing substituents led to higher activity towards oxygen evolution for this type of Ru catalysts in the oxidation of water. Mononuclear ruthenium complexes based on the bipyridine-dicarboxylate (bda) ligand were prepared and showed high catalytic efficiencies for chemical and photochemical water oxidation, a key challenge for solar energy conversion into fuels. The more electron-withdrawing substituents on the axial ligands of the catalysts lead to higher activities towards water oxidation.