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

Abstract

A detailed characterization of intermediates in water oxidation catalyzed by a mononuclear Ru polypyridyl complex [Ru(II)-OH(2)](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 H(2)(18)O 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)-OH(2)](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 × 10(4) 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)-OH(2)](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)-η(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).

Towards a Molecular Level Understanding of the Multi-Electron Catalysis of Water Oxidation on Metal Oxide Surfaces

“…This hydroperoxide forming step (1) [52,53]. Optical and resonance Raman spectra of water oxidation intermediates of a mononuclear Ru polypyridyl complex obtained by electrochemical, stopped-flow and nanosecond pulse radiolysis experiments gave structural and kinetic insights on intermediates supported by theory that provided an early precedent for a single high valent metal oxo attacking H 2 O to form an O-O bond [54]. Very recently, a mononuclear organo Co complex was shown to oxidize water by forming the …”

Intramolecular Proton Transfer Boosts Water Oxidation Catalyzed by a Ru Complex

“…11,12,13 In particular, Ru-aqua complexes with polypyridylic ligands 14,15,16,17 are robust catalysts that allow for the analysis of mechanisms much more accessible than those of first row transition metals with labile ligands. 18,19,20,21,22 When combined with the theoretical analysis, via density functional theory (DFT) calculations, spectroscopic and electrochemical measurements provide detailed information on the nature of reaction intermediates and activation free energies along the catalytic cycle of water oxidation.…”

Hydrogen Bonding Rescues Overpotential in Seven-Coordinated Ru Water Oxidation Catalysts

“…4 Molecular transition metal complexes constitute an excellent platform to examine these factors since significant information based on an arsenal of spectroscopic, electrochemical and analytical techniques can be used together with the valuable complementary information provided by computational studies. 5,6,7,8,9,10,11,12,13,14 The best water oxidation catalysts reported today are based on seven coordinated Ru complexes containing dianionic ligands such as [2,2'-bipyridine]-6,6'-dicarboxylato (bda 2-) 15,16,17 and [2,2':6',2''-terpyridine]-6,6''-dicarboxylato (tda 2-) (see Figure 1 for drawn structures of these ligands). Particularly impressive is the seven coordinate complex [Ru IV (tda--N 3 O)(py)2(O) eq ], 4 IV (O), (the superscript in roman numbers indicates the formal oxidation state of Ru; py is pyridine; the "eq" superscript means equatorial) that is capable of oxidizing water to dioxygen at maximum turnover frequencies (TOFMAX) of 7,700 s -1 and 50,000 s -1 at pH = 7.0 and pH = 10.0 respectively.…”