Spectroscopic and magnetic investigation of the K5[PMo2VW9O40]· 24H2O heteropolyoxometalate and its monoprotonated form †

The K5[PMo2VW9O40]·24H2O heteropolyoxometalate complex was synthesized and investigated by means of spectroscopic and magnetic susceptibility measurements. The co-ordination of the vanadium(IV) ion to the monovacant α-[PMo2W9O39]7− heteropolyanion results in the shift of the bicentric νasym(MOa) and νasym(MOd) and tricentric νasym(M–Ob,c–M) vibrations (M = W or Mo) in the 700–1100 cm−1 region of the IR spectrum of the complex compared to those of the ligand. Two low wavenumber bands in VIS spectrum were assigned to 2B2 → 2E (11310 cm−1) and 2B2 → 2B1 (13710 cm−1) transitions for VIV in an axial C4v environment. The two shoulders at 16150 and 19220 cm−1 correspond to VIV→MoVI,WVI and MoV→MoVI,WVI transitions, respectively. The UV transitions (38480 and 47680 cm−1) correspond to charge transfer bands. The simulation of the powder EPR spectrum of the sample obtained at room temperature indicates the presence, in the same amounts, of one-electron-reduced species (g∥ = 1.922, g⊥ = 1.972, A∥ = 181 G, A⊥ = 63 G) and a monoprotonated two-electron-reduced species with mixed valence VIV, MoV and MoVI (giso = 1.972, ΔBiso(p-p) = 450 G). The magnetic parameters (μeff = 2.11 μB, θ = −59 K) of the sample and the effective spin S′ = 0.84 estimated for the two-electron-reduced species suggest the presence of one antiferromagnetic coupling between the VIV and the MoV–MoVI pair.