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)(HL 1 )(OH 2 )] 2+ ( 1 Aq ) and [Ru(trpy)(L 2 -κ-N 2 O)] ( 2 ) [where trpy = 2,2':6',2″-terpyridine, HL 1 = 2-(2-pyridyl)benzimidazole, H 2 L 2 = 2-(pyridin-2-yl)-1 H -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)(H 2 L 2 -κ-N 2 )(OH 2 )] 2+ ( 2 Aq ), having a dangling carboxylic group. This exchange phenomenon has been followed by IR, 1 H NMR, and UV-vis spectroscopic techniques. Electrochemical analyses of 1 Aq and 2 Aq (Pourbaix diagram) suggest the generation of a formal Ru V ═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 Ru V ═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 18 O-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 Ru V ═O unit. The present work provides experimental evidence of the uncommon functionality of the carboxylic group, the oxide relay, in molecular water oxidation chemistry.