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 ac etonitrile, followed by the addition of [(n-C2H9)(4)N]OH, leads to cyc lic voltammograms showing two initial one-electron redox features (Del ta 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 conve niently prepared by comproportionation of alpha 0 and deprotonated alp ha-[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 demonstra ted 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 al pha 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 molybd enum sites, but that there is substantial delocalization (or rapid hop ping) to the mirror-related molybdenum atom. The electronic absorption spectrum shows intervalence transitions at 9710 and 12190cm(-1). Inte rmolecular electron transfer between (alpha 0, alpha I and alpha II is slow on the NMR timescale. In acetonitrile solution the P-31 NMR spec tra of diprotonated and unprotonated (alpha II consist of single narro w lines consistent with effective D-3h symmetry for the reduced Dawson structure. However the monoprotonated anion has a two-line spectrum t hat undergoes coalescence as the temperature is raised. This is attrib uted to a slow intramolecular exchange (k similar to 1 s(-1)) of the p roton 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).