Magnetic properties of vanadium(IV)-based extended systems: [(VO) ₃(μ-PO₄)₂(2,2′-bpy)(μ-OH ₂)]·1/3H₂O and (VO)₂H₄P ₂O₉

The magnetic properties of [(VO)₃(μ-PO₄) ₂(2,2′-bpy)(μ-OH₂)]1/3H₂O (1) and (VO)₂H₄P₂O₉ (2), a tubular and a layered vanadium(IV) phosphates containing triply oxido bridged V^IV dimers, are analyzed considering the Bleaney-Bowers S = 1/2 dimer model. In compound 1 the presence of an additional V^IV connected with the V^IV dimers through μ_1,2-PO₄^3- bridges is described with a Curie-Weiss type correction. This model reproduces the magnetic properties of compound 1 with g = 1.956, J_dim = -102.1 cm^-1, θ = -0.4 cm^-1 and Nα = 278 × 10^-6 emu mol^-1. In compound 2, the presence of a small percentage of paramagnetic impurity has to be considered to account for the divergence of χ_m at low temperatures. This simple model reproduces the magnetic data of compound 2 with g = 1.99, J = -62.4 cm ^-1 and a 1.2% of monomeric impurity. The moderate antiferromagnetic coupling found in the triply oxido bridges V^IV dimers is justified from the structural parameters of the bridge. These studies confirm that the coupling through -O-P-O- bridges is antiferromagnetic and relatively weak, as well as previous magneto-structural correlations in this kind of oxido bridges. DFT calculations on a dinuclear fragment model for the two systems gave the following values of J_calc = -72.4 cm^-1 for 1 and J _calc = -5.2 cm^-1 for 2. These values reproduce the antiferromagnetic nature of the superexchange interactions between the V ^IV centers.