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
Jn. Barrows et Mt. Pope, 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, Inorganica Chimica Acta, 213(1-2), 1993, pp. 91-98
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).