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[O(2)CCH(2)C(O)(CO(2))CH(2)CO(2)]}(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&onemacr; with unit cell parameters a = 8.474(5) Å, b = 8.902(7) Å, c = 9.596(9) Å, alpha = 71.50(6) degrees, beta = 70.81(9) degrees, gamma = 87.45(2) degrees, V = 647.1 Å(3), and Z = 1. The dimeric anion contains a centrosymmetric planar four-membered V(2)O(2) 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 approximately 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)()(,)(x)()()2(-)(y)()()2 is 56 cm(-)(1). UV-visible, ESCA, and theoretical BVS studies support the proposed molecular and electronic structure of the complex.