Artificial Photosynthesis

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 [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 ESI-MS with H₂¹⁸O 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 k₁ = 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 indicate that this species has an O–O bond. The formation of [Ru^III–OOH]²⁺ can proceed via formation of [Ru^V=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 [Ru^IV=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.