Borotungstate Polyoxometalates: Multinuclear NMR Structural Characterization and Conversions in Solutions

The unique heteropolyanion [H3BW13O46]8– (BW 13 ), previously suggested on the basis of indirect evidence, and protonated lacunary heteropolyanion [HBW11O39]8– (BW 11 ) 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 J W–O–W coupling satellites in the 183W NMR spectrum of BW11, containing six peaks of relative intensities ∼2:2:2:1:2:2, indicates that the latter is the α isomer. The 17O 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 183W NMR spectrum of BW 13 contains seven peaks of relative intensities ∼2:1:2:2:2:2:2 with additional large couplings due to the connectivity between BW 11 and [W2O7]2– fragments. According to the 17O 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 BW 11 and BW 13 in solution were followed by using 183W NMR spectra at a “fingerprint” level. In the pH range from ∼7.5 to 6, BW 11 transforms to BW 13 , transforming further to [BW12O40]5– (BW 12 ) and [B3W39O132H n ] n−21 (B 3 W 39 ) in different ratios. Conversion of BW 13 to BW 12 proceeds through an intermediate complex of suggested composition [BW11O39·WO2]7–. At high acidity (pH ∼ 0), B 3 W 39 gradually decomposes into tungstic acid, BW 12 and H3BO3. Polyanion BW 12 persists in the pH range ∼0–7.5.