Model oxygen-evolving center composed of polymer membrane and dimer ruthenium complex

The in situ spectrocyclic voltammetric investigations of the dimeric ruthenium complex used for water oxidation, [(bpy)2(H2O)Ru-O-Ru(H2O)(bpy)2]4+ (H2O-RuIII-RuIII-OH2), were carried out in a homogeneous aqueous solution and in a Nafion membrane under different pH conditions. The in situ absorption spectra recorded for the dimer show that the dimer H2O-RuIII-RuIII-OH2 complex underwent reactions initially to give the detectable H2O-RuIII-RuIV-OH and H2O-RuIII-RuIV-OH2 complexes, and at higher positive potentials, this oxidized dimer underwent further oxidation to produce a presumably higher oxidation state RuV-RuV complex. Since this RuV-RuV complex is reduced rapidly by water molecules to H2O-RuIII-RuIV-OH2, it could not be detected by absorption spectrum. Independent of the pH conditions and homogeneous solution/Nafion membrane systems, the dimer RuIII-RuIV was detected at higher potentials, suggesting that the dimer complex acts as a three-electron oxidation catalyst. However, in the Nafion membrane system it was suggested that the dimer complex may act as a four-electron oxidation catalyst. While the dimer complex was stable under oxidation conditions, the reduction of the dimer RuIII-RuIII to RuII-RuII led to decomposition, yielding the monomeric cis-[Ru(bpy)2(H2O)2]2+