STABILIZATION AND MAGNETIC-RESONANCE CHARACTERIZATION OF THE ONE-ELECTRON HETEROPOLY BLUE DERIVATIVE OF THE MOLYBDOPHOSPHATE [P2MO18O62](6-) - SLOW INTRAMOLECULAR PROTON-EXCHANGE OF THE 2-ELECTRON BLUE IN ACETONITRILE SOLUTION

Reduction of the heteropolyanion alpha-[P2Mo18O62](6-) (alpha 0) in acetonitrile, followed by the addition of [(n-C2H9)(4)N]OH, leads to cyclic voltammograms showing two initial one-electron redox features (Delta E=150 mV) and the possibility of generating solutions that contain the paramagnetic anion alpha-[P2Mo18O62](7-) (alpha I), the existence of which had earlier been doubted. Solutions of alpha I are most conveniently prepared by comproportionation of alpha 0 and deprotonated alpha-[P2Mo18O62](8-) (alpha II) in acetonitrile solution. Such solutions are ESR-active at room temperature (g similar to 1.94; linewidth 80 G) and yield a single P-31 NMR resonance at +13.4 ppm (linewidth 220 Hz). The transient existence of alpha I in aqueous solution is demonstrated by the observation of the P-31 NMR signal in a rapidly-alkalinized solution of alpha II. Based on comparison of the ESR parameters of alpha I (g perpendicular to, 1.945; g parallel to, 1.965) with those for the two isomers of alpha-[(P2W17MoO62)-O-v](7-), it is concluded that the reduction of alpha 0 has occurred at one of the equatorial molybdenum sites, but that there is substantial delocalization (or rapid hopping) to the mirror-related molybdenum atom. The electronic absorption spectrum shows intervalence transitions at 9710 and 12190cm(-1). Intermolecular electron transfer between (alpha 0, alpha I and alpha II is slow on the NMR timescale. In acetonitrile solution the P-31 NMR spectra of diprotonated and unprotonated (alpha II consist of single narrow lines consistent with effective D-3h symmetry for the reduced Dawson structure. However the monoprotonated anion has a two-line spectrum that undergoes coalescence as the temperature is raised. This is attributed to a slow intramolecular exchange (k similar to 1 s(-1)) of the proton between O(---Mo-v) atoms on either side of the equatorial plane of the Dawson structure. Based on the spectral changes between 297 and 333 K, the activation energy for this process is 5.9 kcal mel(-1).