Structural and Resonance Raman Studies of an Oxygen-Evolving Catalyst. Crystal Structure of [(bpy)<sub>2</sub>(H<sub>2</sub>O)Ru<sup>III</sup>ORu<sup>IV</sup>(OH)(bpy)<sub>2</sub>](ClO<sub>4</sub>)<sub>4</sub>
作者:Jon R. Schoonover、JinFeng Ni、Lee Roecker、Peter S. White、Thomas J. Meyer
DOI:10.1021/ic960348o
日期:1996.1.1
The oxidized form of the blue dimer water oxidation catalyst [(bpy)(2)(H2O)(RuORuIV)-O-III(OH)(bpy)(2)](ClO4)(4) (bpy is 2,2'-bipyridine) has been characterized structurally by X-ray crystallography. Comparisons with [(bpy)(2)(H2O)Ru-III- ORuIII(OH2)(bpy)(2)](ClO4)(4) . 2H(2)O, [(bpy)(2)(ClRuORuCl)-O-III-Cl-IV(bpy)(2)](ClO4)(3) . H2O, and [(tpy)(bpy)(OsOOsIV)-O-III(bpy)(tpy)]Na(ClO4)(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 mu-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 H2O-Ru-O bridge-angle. These changes are discussed in the context of the structural requirements for O ... O coupling and Oz 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-OH2 or Ru-OH stretches; bands at low energy appear to originate from modes that are highly mixed.
Insights into Decomposition Pathways and Fate of Ru(bpy)<sub>3</sub><sup>2+</sup> during Photocatalytic Water Oxidation with S<sub>2</sub>O<sub>8</sub><sup>2–</sup> as Sacrificial Electron Acceptor
The most widely accepted system for homogeneous photocatalytic water oxidation process consists of a water oxidation catalyst, Ru-II(bpy)(3)(2+) as a photopump, and S2O82- as the sacrificial electron acceptor. However, this system is far less than ideal because Ru-II(bpy)(3)(2+) undergoes very rapid decomposition and as a result the process stops before all of the S2O82- is consumed. In this regard its decomposition pathways and the fate of Ru-II(bpy)(3)(2+) should be elucidated to design more efficient photocatalytic water oxidation systems. We found that two pathways exist for decomposition of Ru-II(bpy)(3)(2+) in the light-Ru-II(bpy)(3)2+-S2O82- system. The first is the formation of OH center dot radicals at pH >6 through oxidation of OH- by Ru-III(bpy)(3)(3+) in the dark, which attack the bpy ligand of Ru-II(bpy)(3)(2+). This is a minor, dark decomposition pathway. During irradiation not only Ru-II(bpy)(3)(2+) but also Ru-III(bpy)(3)(3+) becomes photoexcited and the photoexcited Ru-III(bpy)(3)(3+) reacts with S2O82- to produce an intermediate which decomposes into catalytically active Ru mu-oxo dimers when the intermediate concentration is low or into catalytically inactive oligomeric Ru mu-oxo species when the intermediate concentration is high. This is the major, light-induced decomposition pathway. When the Ru-II(bpy)(3)(2+) concentration is low, the light-Ru-II(bpy)(3)(2+)-S2O82- system produces O-2 even in the absence of any added catalysts through the O-2 -producing dark pathway. When the Ru-II(bpy)(3)(2+) concentration is high, the system does not produce O-2 because the overall rate for the light-induced decomposition pathway is much faster than that of the O-2-producing dark pathway.