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

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 alpha-Keggin polyoxotungstate, K-10 [alpha,alpha-P2W20Ti4O78].12H(2)O 1, was unexpectedly found in the varied molar-ratio reactions of tri-lacunary precursor Na-9 [ A-PW9O34].19H(2)O 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 [alpha,alpha-P2W18Ti6O77].17H(2)O 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 [alpha-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 [alpha-1,2-PW10Ti2O40](7-) 2 and dimer [alpha,alpha-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 (P-31 and W-183) NMR spectroscopy.