Transition metal containing decatungstosilicate dimer [M(H2O)(2)(gamma-SiW10O35)(2)](10-) (M = Mn2+, Co2+, Ni2+)

The new, monometal substituted silicotungstates [Mn(H2O)(2)(gamma-SiW10O35)(2)](10-) (1), [Co(H2O)(2)(gamma-SiW10O35)(2)](10-) (2) and [Ni(H2O)(2)(gamma-SiW10O35)(2)](10-) (3) have been synthesized and isolated as the potassium salts K-10[Mn(H2O)(2)(gamma-SiW10O35)(2)]center dot 8.25H(2)O (K-1), K-10[Co(H2O)(2)(gamma-SiW10O35)(2)]center dot 8.25H(2)O (K-2) and K-10[Ni(H2O)(2)(gamma-SiW10O35)(2)]center dot 13.5H(2)O (K-3), which have been characterized by IR spectroscopy, single crystal X-ray diffraction, elemental analysis and cyclic voltammetry. Polyanions 1-3 are composed of two (gamma-SiW10O36) units fused on one side via two W-O-W' bridges and on the other side by an octahedrally coordinated trans-MO4(OH2)(2) transition metal fragment, resulting in a structure with C-2v point group symmetry. Anions 1-3 were synthesized by reaction of the dilacunary precursor [gamma-SiW10O36](8-) with Mn2+, Co2+ and Ni2+ ions, respectively, in 1 M KCl solution at pH 4.5. The electrochemical properties of 1-3 were studied by cyclic voltammetry and controlled potential coulometry in a pH 5 buffer medium. The waves associated with the W-centers are compared with each other and with those of the parent lacunary precursor [gamma-SiW10O36](8-) in the same medium. They appear to be dominated by the acid-base properties of the intermediate reduced species. A facile merging of the waves for 3 is observed while those for 1 and 2 remain split. Controlled potential coulometry of the single wave of 3 or the combined waves of 1 and 2 is accompanied by catalysis of the hydrogen evolution reaction. No redox activity was detected for the Ni2+ center in 3, whereas the Co2+ center in 2 shows a one-electron redox process. The two-electron, chemically reversible process of the Mn2+ center in 1 is accompanied by a film deposition on the electrode surface.