Photochemistry of polyoxovanadates. Part 1. Formation of the anion-encapsulated polyoxovanadate [V15O36(CO3)]7– and electron-spin polarization of α-hydroxyalkyl radicals in the presence of alcohols

Abstract

Prolonged photolysis of aqueous solutions containing [V₄O₁₂]^4– and MeOH at pH 9 (pH adjusted by use of K₂CO₃) led to the formation of K₅H₂[V₁₅O₃₆(CO₃)]·1 4.5H₂O. A single-crystal X-ray structural analysis of dark green crystals of K₅H₂[V₁₅O₃₆(CO₃)]·14.5H₂O [triclinic, space group Pa= 12.361 (4), b= 18.149(5), c= 11.415(4)Å, α= 93.44(3), β= 107.76(3), γ= 93.54(3)°, Z= 2, R= 0.081 for 4182 independent data with I > 5σ(I)] showed that the pentadecavanadate encapsulates CO₃^2– with approximate D_3h symmetry and formally contains eight V^IV and seven V^V centres. The co-ordination geometries at the vanadium centres in the anion consist of twelve (V^IV, V^V) VO₅ square pyramids and three V^IVO₆ octahedra. Each of the VO₆ octahedra co-ordinates the CO₃^2– oxygen atoms with V–O bond lengths 2.2616(8)–2.3710(7)Å. The photochemical encapsulation of other anions such as Cl^–, Br^–, NO₃^– or PO₃^3– in the [V₁₅O₃₆]^5– spherical cluster shell is observed in a similar way, which is applicable to the photofixation of CO₂. The photoinduced electron transfer to the O → V ligand-to-metal charge-transfer (l.m.c.t.) excited state of [V₄O₁₂]^4– from alcohols such as MeOH, EtOH or PrⁱOH was investigated with chemically induced dynamic electron polarization (CIDEP) with 100 ns time resolution. The formation and decay of α-hydroxyalkyl radicals, following pulsed-laser excitation of the polyoxometalate was also observed for [Mo₇O₂₄]^6– and [W₁₀O₃₂]^4–. Observation of an emissive CIDEP ESR pattern for the α-hydroxyalkyl radicals revealed that the reaction precursor is the excited triplet state of the O → M (M = V, Mo or W) l.m.c.t. which undergoes hydrogen abstraction from alcohols to yield the one-protonated reduced polyoxometalate (for example [V₄O₁₂H]^4–) and the α-hydroxyalkyl radical. The absence of distortion in the ESR polarization of α-hydroxyalkyl radicals and the external magnetic field effect indicated that the interaction between [V₄O₁₂H]^4– and α-hydroxyalkyl radicals is extremely weak in water.