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 2JW–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 BW13 contains seven peaks of relative intensities ∼2:1:2:2:2:2:2 with additional large couplings due to the connectivity between BW11 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. T...