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Water Oxidation by a Mononuclear Ruthenium Catalyst: Direct Pathway via Ru(IV)=O without forming Ru(V)=O

A detailed characterization of intermediates in water oxidation catalyzed by a mononuclear Ru polypyridyl complex [RuII–OH2]2+ (Ru = Ru complex with one 4-t-butyl-2,6-di-1',8'-(naphthyrid-2'-yl)-pyridine ligand and two 4-picoline ligands) has been carried out using electrochemistry, UV-vis and resonance Raman spectroscopy, pulse radiolysis, stopped flow, and ESI-MS with H218O labeling experiments, and theoretical calculations. The results reveal a number of intriguing properties of intermediates such as [RuIV=O]2+ and [RuIV–OO]2+. At pH > 2.9, two consecutive proton-coupled one-electron steps take place at the potential of the [RuIII–OH]2+/[RuII–OH2]2+ couple, which is equal to or higher than the potential of the [RuIV=O]2+/[RuIII–OH]2+ couple (i.e., the observation of a two-electron oxidation in cyclic voltammetry). At pH 1, the rate constant of the first one-electron oxidation by Ce(IV) is k1 = 2 × 104 M-1 s-1. While pH-independent oxidation of [RuIV=O]2+ takes place at 1420 mV vs NHE, bulk electrolysis of [RuII–OH2]2+ at 1260 mV vs NHE at pH 1 (0.1 M triflic acid) and 1150 mV at pH 6 (10 mM sodium phosphate) yielded a red colored solution with a Coulomb count corresponding to a net four-electron oxidation. ESI-MS with labeling experiments clearly indicate that this species has an O–O bond. The formation of [RuIII–OOH]2+ can proceed via formation of [RuV=O]3+ followed by nucleophilic attack by a water molecule at pH < 1, however this pathway cannot account for the product formation at pH 6. A proposed alternative pathway is the reaction of [RuIV=O]2+ with a water molecule accompanied by the concomitant removal of an electron and a proton (“direct pathway”). The direct pathway becomes predominant at higher pH in underpotential bulk electrolysis experiments and in the onset of catalytic current in background-subtracted CV scans as a function of pH.

J. Am. Chem. Soc. 2011, 133, 14649-14665, DOI: 10.1021/ja203249e.