Photochemical studies of the alkylammonium molybdates. Part 6. Photoreducible octahedron site of [Mo7O24]6– as determined by electron spin resonance

Abstract

The Mo^V centre formed by u.v.-irradiation of a single crystal of [NH₃Pr]₆[Mo₇O₂₄]·3H₂O has been studied by e.s.r. at room temperature. The e.s.r. spectra are attributed to a localized octahedral Mo^VO₅(OH) site, resulting from transfer of a hydrogen bonding proton from a [NH₃Pr]⁺ nitrogen to a bridging oxygen atom. The principal values of the g, A_Mo, and A_H tensors are g₁= 1.895, g₂= 1.925, g₃= 1.937, A_Mo(1)= 2.34 × 10^–3, A_Mo(2)= 4.20 × 10^–3, A_Mo(3)= 6.99 × 10^–3, A_H(1)= 8.15 × 10^–4, A_H(2)= 9.14 × 10^–4, and A_H(3)= 12.5 × 10^–4 cm^–1. Analysis of the e.s.r. parameters indicates the direct spin polarization between the paramagnetic electron orbital of molybdenum and the hydrogen orbital. From the direction of the largest principal value for the A_H tensor, which is approximately parallel to the H(O)⋯Mo^V direction, the paramagnetic site in seven MoO₆ octahedra is determined in correlation with the X-ray crystal structure data. It is proposed that Mo^V formation occurs at an end of three octahedra in a line in the central horizontal level of the [Mo₇O₂₄]^6– configuration, where the molybdenum atom has a long Mo–O bond (2.5 Å).