Synthesis and pH-variable ultracentrifugation molecular weight measurements of the dimeric, Ti–O–Ti bridged anhydride form of a novel di-TiIV-1,2-substituted α-Keggin polyoxotungstate. Molecular structure of the [(α-1,2-PW10Ti2O39)2]10− polyoxoanion

Abstract

The preparation and characterization of a Keggin-type, novel di-Ti^IV-1,2-substituted polyoxotungstate are described. The dimeric, Ti–O–Ti bridged anhydride form of the di-Ti^IV-1,2-substituted α-Keggin polyoxotungstate, K₁₀[α,α-P₂W₂₀Ti₄O₇₈]·12H₂O 1, was unexpectedly found in the varied molar-ratio reactions of tri-lacunary precursor Na₉[A-PW₉O₃₄]·19H₂O with Ti(SO₄)₂ in aqueous solution. Although this compound was first found as a minor product in the preparation of the dimeric, tri-Ti^IV-1,2,3-substituted species, K₁₀H₂[α,α-P₂W₁₈Ti₆O₇₇]·17H₂O 3, it was successfully prepared as a main product in this work and structurally characterized. Compound 1, as analytically pure, homogeneous colorless needle crystals, was obtained as a major product in 29.2% yield (2.7 g scale) from recrystallization under acidic conditions (at pH 2.2) of the 1 ∶ 2 molar-ratio reaction product. X-Ray structure analysis revealed that the molecular structure of 1 consisted of a dimeric anhydride formed by two Ti–O–Ti bonds linking two [α-1,2-PW₁₀Ti₂O₄₀]⁷⁻ Keggin units. Interestingly, ultracentrifugation molecular weight (MW) measurements of this compound in aqueous solution showed the pH-dependent interconversion between monomer [α-1,2-PW₁₀Ti₂O₄₀]⁷⁻2 and dimer [α,α-P₂W₂₀Ti₄O₇₈]¹⁰⁻1; this compound was present as the monomer under less acidic conditions (pH 7.8), while it was in dimeric form under more acidic conditions (pH 1.0 and 2.2). Characterization of 1 was also accomplished by complete elemental analyses, TG/DTA, FTIR and solution (³¹P and ¹⁸³W) NMR spectroscopy.