Synthesis and characterization of tri-titanium(iv)-1,2,3-substituted α-Keggin polyoxotungstates with heteroatoms P and Si. Crystal structure of the dimeric, Ti–O–Ti bridged anhydride form K10H2[α,α-P2W18Ti6O77]·17H2O and confirmation of dimeric forms in aqueous solution by ultracentrifugation molecular weight measurements

Abstract

The preparation and characterization of the α-Keggin-type, novel tri-Ti^IV-1,2,3-substituted polyoxotungstates with heteroatoms P and Si are described. The molecular structure of K₁₀H₂[α,α-P₂W₁₈Ti₆O₇₇]·17H₂O 1, obtained as a main product (30.4% yield) from the 1 ∶ 3 molar ratio reaction of tri-lacunary precursor Na₉[A-PW₉O₃₄]·19H₂O with Ti(SO₄)₂·4H₂O in aqueous solution, was determined by X-ray structure analysis. It had the dimeric, anhydride structure formed by three Ti–O–Ti bonds linking two Keggin [α-1,2,3-PW₉Ti₃O₄₀]⁹⁻ units. From the filtrate after 1 was filtered off, a different compound 2 was obtained as a minor product. A related α-Keggin tri-Ti^IV-1,2,3-substituted polyoxotungstate with heteroatom Si, the dimeric anhydride form K₁₀H₄[α,α-Si₂W₁₈Ti₆O₇₇]·15H₂O 3, was obtained as a main product (31.1% yield) from a 1 ∶ 3 molar ratio reaction of Na₁₀[A-α-SiW₉O₃₄]·15H₂O with Ti(SO₄)₂. Ultracentrifugation molecular weight measurements under less acidic conditions revealed that 1 and 3 were present as dimers in aqueous solution. Characterization of 1 and 3 was also accomplished by complete elemental analyses, TG/DTA, FTIR and solution (³¹P and ¹⁸³W) NMR spectroscopies.