Alpha and beta isomers of tetrahafnium(IV) containing decatungstosilicates, [Hf-4(OH)(6)(CH3COO)(2)(x-SiW10O37)(2)](12-) (x = alpha, beta)

The two dimeric, Hf-containing tungstosilicates [Hf-4(OH)(6)(CH3COO)(2)(alpha-SiW10O37)(2)](12)- (1) and [Hf-4(OH)(6)(CH3COO)(2)(beta-SiW10O37)(2)](12-) (2) have been synthesized by reacting HfCl4 with [gamma-SiW10O36](8-) in potassium acetate solutions at pH = 3.6 and 4.8, respectively. Polyanions 1 and 2 comprise a tetrahafnium(IV) assembly {Hf-4(OH)(6)(CH3COO)(2)} sandwiched between two dilacunary [alpha-SiW10O36](8-) (in 1) or {beta-SiW10} (in 2) Keggin type decatungstosilicates. This implies that the starting precursor {gamma-SiW10} isomerizes in situ to [beta-SiW10O37](10-) and then to [alpha-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 W-183 NMR spectroscopy, and electrochemistry.