Synthesis and Characterization of the Mixed-Valence Diamagnetic Two-Electron-Reduced Isopolytungstate [W₁₀O₃₂]⁶⁻. Evidence for an Asymmetric d-Electron Distribution over the Tungsten Sites

The synthesis of the unprotonated two-electron-reduced isopolytungstate [W 10 O 32 ] 6 - ( 2 ) is reported here. Additionally, full experimental details are given for the acquisition of both high signal-to-noise and integrable natural-abundance 17 O NMR spectra of polyoxometalates. Titration of an acetonitrile solution containing 2 with 0.13 M triflic acid is followed by UV−visible/near-IR spectroscopy. The spectra reveal isosbestic points which are consistent with five distinct protonation states, H x 2, where x = 0−4. Similar to a reported mixture of H x 2 species ( x = 1−2), 2 is EPR silent over the temperature range 4−300 K and is effectively diamagnetic. The 17 O and 183 W NMR spectra of 2 both indicate retention of the overall structure and symmetry ( D 4 h ) of the oxidized d 0 precursor complex [W 10 O 32 ] 4 - ( 1 ), and both spectra are consistent with delocalization of the two d electrons on the NMR time scale. The relative 17 O electric field gradients (efg) between 1 and 2 for the same oxygen site ( e.g. O a ( 2 )/ O a ( 1 ) for site a) were deduced from 17 O line width measurements. These data are consistent with an increase in covalency for all three types of W−O−W bonds in 2 relative to 1 . 183 W NMR chemical shift and T 1 measurements indicate that the d electron density in 2 is located primarily on the equatorial tungsten sites. Similarly, the location of the two d electrons in the reduced Wells−Dawson heteropolytungstate α-[P 2 W 18 O 62 ] 8 - also is reported to be on the equatorial sites. Both 2 and α-[P 2 W 18 O 62 ] 8 - are the only known diamagnetic W V -site “pure-addenda” (isometal) polyoxometalates which exhibit an asymmetric d-electron distribution over the metal sites.