Structural and Resonance Raman Studies of an Oxygen-Evolving Catalyst. Crystal Structure of [(bpy)₂(H₂O)Ru^IIIORu^IV(OH)(bpy)₂](ClO₄)₄

Abstract

The oxidized form of the blue dimer water oxidation catalyst [(bpy) 2 (H 2 O)Ru III ORu IV (OH)(bpy) 2 ](ClO 4 ) 4 (bpy is 2,2‘-bipyridine) has been characterized structurally by X-ray crystallography. Comparisons with [(bpy) 2 (H 2 O)Ru III ORu III (OH 2 )(bpy) 2 ](ClO 4 ) 4 ·2H 2 O, [(bpy) 2 ClRu III ORu IV Cl(bpy) 2 ](ClO 4 ) 3 ·H 2 O, and [(tpy)(bpy)Os III OOs IV (bpy)(tpy)]Na(ClO 4 ) 6 ·3H 2 O (tpy is 2,2‘:6‘,2‘‘-terpyridine) reveal that oxidation of Ru III −O−Ru III to Ru III −O−Ru IV results in significant structural changes at the μ-oxo bridge. There is an increase toward linearity along M−O−M, a decrease in the M−O bond distances at the bridge, and an increase in the H 2 O−Ru−O bridge angle. These changes are discussed in the context of the structural requirements for O···O coupling and O 2 evolution in higher oxidation states. Resonance Raman spectra of these and related complexes reveal that internal ligand vibrations as well as overtone and combination bands of an intense, symmetrical M−O−M stretch at 360−400 cm - 1 contribute significantly to the Raman spectra. Additional M−O−M bands are identified near 800 cm - 1 and, tentatively, near 130 cm - 1 . It is not possible to assign bands to Ru−OH 2 or Ru−OH stretches; bands at low energy appear to originate from modes that are highly mixed.