Structure and Electronic Configurations of the Intermediates of Water Oxidation in Blue Ruthenium Dimer Catalysis

Abstract

Catalytic O₂ evolution with <i>cis</i>,<i>cis</i>-[(bpy)₂(H₂O)­Ru^IIIORu^III(OH₂)­(bpy)₂]⁴⁺ (bpy is 2,2-bipyridine), the so-called blue dimer, the first designed water oxidation catalyst, was monitored by UV–vis, EPR, and X-ray absorption spectroscopy (XAS) with ms time resolution. Two processes were identified, one of which occurs on a time scale of 100 ms to a few seconds and results in oxidation of the catalyst with the formation of an intermediate, here termed [3,4]′. A slower process occurring on the time scale of minutes results in the decay of this intermediate and O₂ evolution. Spectroscopic data suggest that within the fast process there is a short-lived transient intermediate, which is a precursor of [3,4]′. When excess oxidant was used, a highly oxidized form of the blue dimer [4,5] was spectroscopically resolved within the time frame of the fast process. Its structure and electronic state were confirmed by EPR and XAS. As reported earlier, the [3,4]′ intermediate likely results from reaction of [4,5] with water. While it is generated under strongly oxidizing conditions, it does not display oxidation of the Ru centers past [3,4] according to EPR and XAS. EXAFS analysis demonstrates a considerably modified ligand environment in [3,4]′. Raman measurements confirmed the presence of the O–O fragment by detecting a new vibration band in [3,4]′ that undergoes a 46 cm^–1 shift to lower energy upon ¹⁶O/¹⁸O exchange. Under the conditions of the experiment at pH 1, the [3,4]′ intermediate is the catalytic steady state form of the blue dimer catalyst, suggesting that its oxidation is the rate-limiting step