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

Abstract

The new, monometal substituted silicotungstates [Mn(H₂O)₂(γ-SiW₁₀O₃₅)₂]¹⁰⁻ (1), [Co(H₂O)₂(γ-SiW₁₀O₃₅)₂]¹⁰⁻ (2) and [Ni(H₂O)₂(γ-SiW₁₀O₃₅)₂]¹⁰⁻ (3) have been synthesized and isolated as the potassium salts K₁₀[Mn(H₂O)₂(γ-SiW₁₀O₃₅)₂]·8.25H₂O (K-1), K₁₀[Co(H₂O)₂(γ-SiW₁₀O₃₅)₂]·8.25H₂O (K-2) and K₁₀[Ni(H₂O)₂(γ-SiW₁₀O₃₅)₂]·13.5H₂O (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 (γ-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 [γ-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 [γ-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.