James K. Hurst

A durable molecular catalyst for water oxidation has been made from readily available and relatively inexpensive materials.

Earlier ¹⁸O−H₂O labeling studies had indicated that two concurrent pathways may exist for water oxidation catalyzed by [Ru(bpy)₂(OH₂)]₂O⁴⁺, a μ-oxo bridged diruthenium complex known colloquially as the “blue dimer”. Specifically, the distribution of O₂ isotopomers obtained following its generation by the catalytically active form, [Ru(bpy)₂(O)]₂⁴⁺, suggested pathways in which either (1) one O atom was obtained from the terminally coordinated oxo atom and the second from the solvent or (2) both O atoms were obtained from the solvent. Plausible mechanisms have been advanced for the former pathway, but the second is enigmatic. In the present study, experiments are described that eliminate possibilities that the second pathway arises artifactually from rapid water exchange in reactive intermediary oxidation states of the catalyst, by mechanisms involving scrambling of the O₂ that is formed during reaction, or by mechanisms involving participation of the oxidant (Ce⁴⁺ or S₂O₈²⁻). Comparative studies of partitioning between the two pathways made using catalysts containing substituted bipyridine ligands are consistent with a previously proposed pathway that involves noninnocent participation of these ligands.

Higher oxidation states of the μ-oxo bridged ruthenium “blue dimer” ([Ru(bpy)₂(OH₂)]₂O⁴⁺) have been characterized by redox titration measurements, resonance Raman (RR) spectroscopy, EPR spectrometry, and pulse radiolysis. The cumulative results indicate that the progression of accessible oxidation states in acidic media is {3,3} → {3,4} → {4,4} → {5,5}, but changes to {3,3} → {3,4} → {4,5} → {5,5} above pH 2. Although the reaction 2{4,5} + 2H₂O → 2{3,4} + O₂ is thermodynamically favorable, no O₂ was detected during the decay of {4,5} to {3,4}. One-electron oxidation of {3,4} by radiolytically generated sulfate and carbonate radicals allowed determination of the {4,4} optical spectrum in neutral and alkaline media, where it exists only as a short-lived transient species. This spectrum was identical to that previously reported for {4,4} in acidic media; this observation and comparative RR spectra suggest that its molecular formula is [Ru(bpy)₂(OH)]₂O⁴⁺, that is, both Ru atoms contain a coordinated hydroxo ligand. Upon application of an acidic pH jump, electrochemically prepared {4,5} underwent disproportionation to {4,4} and {5,5}, as determined from changes in the EPR spectra of the solutions. These studies clarify the nature of redox transients formed during water oxidation catalysis by the “blue dimer”, thereby providing information that is critical to performing accurate mechanistic analyses.