Synthesis and electrochemical behavior of [SiW11O39RuIII(H2O)](5-) and its oxo-bridged dimeric complex [(SiW11O39RuORuSiW11O39)-O-IV-Si-III](11-)

Caesium salts of mono-ruthenium(III) substituted Keggin-type silicopolytungstate ([SiW11O39RuIII(H2O)](5-) (1)) and its oxo-bridged dimeric species ([{SiW11O39RuIV/III}(2)O](11-) (8)) were prepared and characterized by elemental analysis, infrared-spectroscopy, UV-Vis spectroscopy, and cyclic voltammetry for the first time. Both complexes could be isolated in pure form by hydrothermal reaction of K-8[SiW11O39] and Ru( acac) 3 followed by precipitation with CsCl. At low concentration (16 mM of Ru and [SiW11O39], respectively), only the monomeric complex was obtained, whereas increasing the concentration of both reagents produced the dimeric species as a side product. Cyclic voltammetry showed that 1 was reversibly reduced to the aquaruthenium(II) derivative, oxidized to the oxoruthenium(IV), hydroxyruthenium(IV), and aquaruthenium(IV) complex depending on the pH of the solution, and further oxidized to the oxoruthenium(V) complex. The pK(a) value of the aquaruthenium(IV) and the hydroxyruthenium(IV) complex was estimated to be ca. 1.8 and 3.3, respectively. The dimeric complex (8) could be reversibly oxidized to the Ru(IV)-O-Ru(IV) derivative, reduced to the Ru(III)-O-Ru(III) complex and further to the Ru(III)-O-Ru(II) complex. The Ru(III)-O-Ru(II) complex disproportionates to the monomeric complex.