PHOTOCHEMICAL STUDIES OF ALKYLAMMONIUM MOLYBDATES .9. STRUCTURE OF DIAMAGNETIC BLUE SPECIES INVOLVED IN THE PHOTOREDOX REACTION OF [MO7O24]6-

Ultraviolet irradiation of aqueous solutions containing a high concentration (greater-than-or-equal-to 10 mmol dm-3) of [Mo7O24]6- in the presence of MeOH results in the formation of a diamagnetic blue species identified as two-electron reduced [Mo14O46]10-. The blue species was isolated as [NMe4]2[NH4]8[Mo14O46]. 8H2O. Its diffuse reflectance spectrum shows bands at lambda-max almost-equal-to 760 and lambda-sh almost-equal-to 630 nm and is similar to the absorption spectrum of the photolyte containing two paramagnetic species of <g> = 1.93 and 1.91. A single-crystal X-ray structural analysis of the blue species [triclinic, space group P1BAR, a = 17.989(3), b = 19.023(2), c = 12.765(2) angstrom, alpha = 96.23(1), beta = 104.16(1), gamma = 119.51(1)-degrees, Z = 2, R = 0.064 for 9887 independent data with back slash > 3-sigma(back slash)] shows that the anion has a di-mu-oxo-bis(heptamolybdate) framework, [(Mo7O23)2]10-, with approximate C2v symmetry and that the added electrons are located over the central two sets of two MoO6 octahedra linked through an axial mu-oxo oxygen atom. Extended-Huckel calculations on a neutral fragment model MoV2MoVI2O8(OH)6(H2O)4 indicate that the t2g d orbitals of the four molybdenum atoms can undergo little mixing with the orbitals of the axial oxygen atoms and comprise most (95%) of the highest occupied molecular orbital. The mechanism of formation of the diamagnetic blue species in aqueous solution is discussed.