Water Oxidation by a Mononuclear Ruthenium Catalyst: Characterization of the Intermediates

A detailed characterization of intermediates in water oxidation catalyzed by a mononuclear Ru polypyridyl complex [Ru II–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 electrospray ionization mass spectrometry (ESI-MS) with H2 18O labeling experiments and theoretical calculations. The results reveal a number of intriguing properties of intermediates such as [Ru IVO]2+ and [Ru IV–OO]2+. At pH > 2.9, two consecutive proton-coupled one-electron steps take place at the potential of the [Ru III–OH]2+/[Ru II–OH2]2+ couple, which is equal to or higher than the potential of the [Ru IVO]2+/[Ru III–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 k 1 = 2 × 104 M–1 s–1. While pH-independent oxidation of [Ru IVO]2+ takes place at 1420 mV vs NHE, bulk electrolysis of [Ru II–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 indicates that this species has an O–O bond. This species required an additional oxidation to liberate an oxygen molecule, and without any additional oxidant it completely decomposed slowly to form [Ru II–OOH]+ over 2 weeks. While there remains some conflicting evidence, we have assigned this species as 1[Ru IV–η2-OO]2+ based on our electrochemical, spectroscopic, and theoretical observations alongside a previously reported analysis by T. J. Meyer’s group (J. Am. Chem. Soc. 2010, 132, 1545–1557).