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

Abstract

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 ν_asym(MO_a) and ν_asym(MO_d) and tricentric ν_asym(M–O_b,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 V^IV in an axial C_4v environment. The two shoulders at 16150 and 19220 cm⁻¹ correspond to V^IV→Mo^VI,W^VI and Mo^V→Mo^VI,W^VI 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 V^IV, Mo^V and Mo^VI (g_iso = 1.972, ΔB_iso(p-p) = 450 G). The magnetic parameters (μ_eff = 2.11 μ_B, 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 V^IV and the Mo^V–Mo^VI pair.