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 [W10O32](6-) (2) is reported here. Additionally, full experimental details are given for the acquisition of both high signal-to-noise and integrable natural-abundance O-17 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 O-17 and W-183 NMR spectra of 2 both indicate retention of the overall structure and symmetry (D-4h) of the oxidized d(0) precursor complex [W10O32](4-) (1), and both spectra are consistent with delocalization of the two d electrons on the NMR time scale. The relative O-17 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 O-17 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. W-183 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 alpha-[P2W18O62](8-) also is reported to be on the equatorial sites. Both 2 and alpha-[P2W18O62](8-) are the only known diamagnetic W-V-site ''pure-addenda'' (isometal) polyoxometalates which exhibit an asymmetric d-electron distribution over the metal sites.