Magneto-Structural Correlation Studies of A Ferromagnetically Coupled Dinuclear Vanadium(IV) Complex. Single-Crystal EPR Study

The dimer potassium dioxo(citrato)vanadate(IV)-6-water. K4{VO[O2CCH2C(O)(CO2)CH 2CO2]}2·6H2O, was prepared by the reaction of citric acid and metavanadate in a neutral solution. The complex crystallizes in the space group P1̄ with unit cell parameters a = 8.474(5) Å, b = 8.902(7) Å, c = 9.596(9) Å, α = 71.50(6)°, β= 70.81(9)°, ̊ = 87.45(2)°, V = 647.l Å3, and Z = 1. The dimeric anion contains a centrosymmetric planar four-membered V2O2 ring with the bridging oxygens derived from the hydroxyl groups. The configuration of the anion is anti-coplanar. A frozen solution EPR spectrum with zero-field splitting and 15 line hyperfine patterns on the parallel and perpendicular features characteristic of two equivalent vanadium atoms confirms the presence of a spin triplet. This is further supported by the presence of half-field "forbidden transition" at g ≈ 4. A broad line with half-field forbidden transitions in the powder EPR spectra at X,Q-bands of the neat sample and at different temperatures shows the presence of a spin triplet ground state and a very weak intermolecular dipolar interaction. From anisotropic exchange contributions to D the magnitude of the derived exchange integral Jxy,x2-y2 is 56 cm-1. UV-visible, ESCA, and theoretical BVS studies support the proposed molecular and electronic structure of the complex.