Dangling Carboxylic Group That Participates in O–O Bond Formation Reaction to Promote Water Oxidation Catalyzed by a Ruthenium Complex: Experimental Evidence of an Oxide Relay Pathway

Two mononuclear ruthenium­(II) complexes of the types [Ru­(trpy)­(HL1)­(OH2)]2+ (1 Aq ) and [Ru­(trpy)­(L2-κ-N2O)] (2) [where trpy = 2,2′:6′,2″-terpyridine, HL1 = 2-(2-pyridyl)­benzimidazole, H2L2 = 2-(pyridin-2-yl)-1H-benzo­[d]­imidazole-4-carboxylic acid] have been synthesized and thoroughly characterized by analytical and spectroscopic [UV–vis, NMR, high-resolution mass spectrometry, and IR] techniques. Complex 1 Aq has been further characterized by X-ray crystallography. In an acidic aqueous medium (pH 1), complex 2 undergoes carboxylate/water exchange readily to form an aqua-ligated complex, [Ru­(trpy)­(H2L2-κ-N2)­(OH2)]2+ (2 Aq ), having a dangling carboxylic group. This exchange phenomenon has been followed by IR, 1H NMR, and UV–vis spectroscopic techniques. Electrochemical analyses of 1 Aq and 2 Aq (Pourbaix diagram) suggest the generation of a formal RuVO species that can potentially promote the oxidation of water. A comparative study of the water oxidation activity catalyzed by 1 Aq and 2 Aq is reported here to see the effect of a dangling carboxylic group in the catalytic performance. Complex 2 Aq shows an enormously higher rate of reaction than 1 Aq . The pendant carboxylic group in 2 Aq participates in an intramolecular O–O bond formation reaction with the reactive formal RuVO unit to form a percarboxylate intermediate and provides an electron-deficient carbon center where water nucleophilic attack takes place. The isotope labeling experiment using 18O-labeled water verifies the attack of water at the carbon center of the carboxylic group rather than a direct attack at the oxo of the formal RuVO unit. The present work provides experimental evidence of the uncommon functionality of the carboxylic group, the oxide relay, in molecular water oxidation chemistry.