Alpha and beta isomers of tetrahafnium(iv) containing decatungstosilicates, [Hf4(OH)6(CH3COO)2(x-SiW10O37)2]12− (x = α, β)

Abstract

The two dimeric, Hf-containing tungstosilicates [Hf₄(OH)₆(CH₃COO)₂(α-SiW₁₀O₃₇)₂]¹²⁻ (1) and [Hf₄(OH)₆(CH₃COO)₂(β-SiW₁₀O₃₇)₂]¹²⁻ (2) have been synthesized by reacting HfCl₄ with [γ-SiW₁₀O₃₆]⁸⁻ in potassium acetate solutions at pH = 3.6 and 4.8, respectively. Polyanions 1 and 2 comprise a tetrahafnium(IV) assembly {Hf₄(OH)₆(CH₃COO)₂} sandwiched between two dilacunary [α-SiW₁₀O₃₆]⁸⁻ (in 1) or {β-SiW₁₀} (in 2) Keggin type decatungstosilicates. This implies that the starting precursor {γ-SiW₁₀} isomerizes in situ to [β-SiW₁₀O₃₇]¹⁰⁻ and then to [α-SiW₁₀O₃₇]¹⁰⁻ by controlled variation of pH and temperature. The hydrated potassium salts of 1 and 2 were investigated in the solid state by single-crystal XRD, FT-IR, thermogravimetric and elemental analyses, and in solution by UV-Vis and ¹⁸³W NMR spectroscopy, and electrochemistry.