Photochemical studies of alkylammonium molybdates. Part 9. Structure of dimagnetic blue species involved in the photoredox reaction of [Mo7O24]6–

Abstract

Ultraviolet irradiation of aqueous solutions containing a high concentration (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 λ_max≈ 760 and λ_sh≈ 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 P, a= 17.989(3), b= 19.023(2), c= 12.765(2)Å, α= 96.23(1), β= 104.16(1), γ= 119.51(1)°, Z= 2, R= 0.064 for 9887 independent data with I > 3σ(I)] shows that the anion has a di-µ-oxo-bis(heptamolybdate) framework, [(Mo₇O₂₃)₂]^10–, with approximate C_2v symmetry and that the added electrons are located over the central two sets of two MoO₆ octahedra linked through an axial µ-oxo oxygen atom. Extended-Hückel calculations on a neutral fragment model Mo^v₂Mo^VI₂O₈(OH)₆(H₂O)₄ indicate that the t_2g 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.