Borotungstate Polyoxometalates: Multinuclear NMR Structural Characterization and Conversions in Solutions

The unique heteropolyanion [H3BW13O46]8(-) (BW13), previously suggested on the basis of indirect evidence, and protonated lacunary heteropolyanion [HBW11O39](8-) (BW11) have been identified in aqueous solutions at pH 5-7.5 from NMR spectra. The pattern of tungsten - tungsten connectivities based on the analysis of the (2)Jw-o-w coupling satellites in the W-183 NMR spectrum of BW11, containing six peaks of relative intensities similar to 2:2:2:1:2:2, indicates that the latter is the alpha isomer. The O-17 NMR spectrum confirms the protonated state of the polyanion with the proton delocalized on two out of four terminal O atoms surrounding the tungsten vacancy. The W-183 NMR spectrum of BW13 contains seven peaks of relative intensities similar to 2:1:2:2:2:2:2 with additional large couplings due to the connectivity between BW11 and [W2O7](2-) fragments. According to the O-17 NMR spectrum, two protons of [BW13O46H3](8-) are delocalized on the two terminal trans O atoms of the dimeric fragment while the third one is linked to its bridging O atom. The conversions of BW11 and BW13 in solution were followed by using W-183 NMR spectra at a "fingerprint' level. In the PH range from similar to 7.5 to 6, BW11 transforms to BW13, transforming further to [BW12O40](5-) (BW12) and [B3W39O132Hn](n-21) (B3W39) in different ratios. Conversion of BW13 to BW12 proceeds through an intermediate complex of suggested composition [BW11O39 center dot WO2](7-). At high acidity (pH similar to 0), B3W39 gradually decomposes into tungstic acid, BW12 and H3BO3. Polyanion BW12 persists in the pH range similar to 07-7.5.