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-pico-line 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 (H2O)-O-18 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 x 10(4) M-1 sr(-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-eta(2)-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).