Structural Study on Highly Oxidized States of a Water Oxidation Complex [Ru^III(bpy)₂(H₂O)]₂(μ-O) ⁴⁺ by Ruthenium K-Edge X-ray Absorption Fine Structure Spectroscopy

The oxidation-induced structural change of a water-oxidizing diruthenium complex, [(bpy)₂(H₂O)Ru^III(μ-O)Ru ^III(OH₂)(bpy)₂]⁴⁺ (bpy = 2,2′-bipyridine), was investigated by means of X-ray absorption spectroscopy. Ru K-edge XANES (X-ray absorption near-edge structure) spectra from the acidic solution and solid precipitates obtained by oxidation showed that the absorption edge shifts toward higher energy with a preedge feature slightly more enhanced than those of the lower oxidation states. This indicates that the higher oxidation state has a lower symmetry due to shortening of the Ru-O bonds that originated from the water ligands. The EXAFS (extended X-ray absorption fine structure) spectra were similar to those of the lower oxidation states, whose analysis revealed the existence of short Ru-O double bonds and an almost linear Ru-O-Ru angle (169 ± 2°). Ab initio EXAFS simulations for several possible structural models suggest that the dimeric structure is maintained during the water oxidation reaction.