Magneto-structural correlation studies of a ferromagnetically coupled dinuclear vanadium(IV) complex. single-crystal EPR study

Abstract

The dimer potassium dioxo(citrato)vanadate(IV)-6-water, K₄{VO[O₂CCH₂C(O)(CO₂)CH₂CO₂]}₂·6H₂O, was prepared by the reaction of citric acid and metavanadate in a neutral solution. The complex crystallizes in the space group P 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)°, γ=87.45(2)°, V=647.1Å 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 ≈ 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.