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, K-4{VO[O2CCH2C(O)(CO2)CH2CO2]}(2) . 6H(2)O, was prepared by the reaction of citric acid and metavanadate in a neutral solution. The complex crystallizes in the space group P (1) over bar with unit cell parameters a = 8.474(5) Angstrom, b = 8.902(7) Angstrom, c = 9.596(9) Angstrom, alpha = 71.50(6)degrees, beta = 70.81(9)degrees, gamma = 87.45(2)degrees, V = 647.1 Angstrom(3), 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 approximate to 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 J(xy,x2-y2) is 56 cm(-1). UV-visible, ESCA, and theoretical BVS studies support the proposed molecular and electronic structure of the complex.