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 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 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-·²-OO]²⁺ based on our electrochemical, spectroscopic, and theoretical observations alongside a previously reported analysis by T. J. Meyer group (J. Am. Chem. Soc. 2010, 132, 1545-1557).