Synthesis of oxalato-bridged (Oxo)vanadium(IV) dimers using L-ascorbic acid as oxalate precursor: structure and magnetism of two systems

The reaction of an aqueous solution of NH4VO3 with L-ascorbic acid and Et4NBr has afforded [Et4N]2[(VO)2(C2O4)3-(H2O)2]·4H2O (3). A similar reaction, in which malonic acid is also added, has furnished [Et4N]2[(VO)2(C2O4)(C3H2O4)2(H2O)2]·2H2O (4). In these syntheses the oxalate ion is derived from L-ascorbic acid. In both the complexes the dinuclear anion is centrosymmetric. In 3 the metal atoms are displaced from the plane of the bridging oxalate anion by 0.05 Å, the corresponding displacement in 4 being 0.17 Å. The terminal oxalato chelate ring in 3 is roughly planar but the malonato chelate ring in 4 has a distorted boat conformation. The V-O distances in 4 are systematically shorter than the corresponding distances in 3 by 0.02-0.06 Å. The V···V distance within the dimer is 5.616(2) Å in 3 and 5.529(2) Å in 4. The two uncoordinated water molecules in the asymmetric unit of 3 constitute a hydrogen-bonded dimer and the lattice consists of a supramolecular anionic assembly held by five different types of O···O hydrogen bonds. The anionic assembly in 4 involves three types of O···O hydrogen bonds. The Et4N+ cations reside between the anionic layers. The magnetic susceptibility data of both 3 and 4 fit excellently with the Heisenberg dimer model in the temperature interval 2-300 K. The species are antiferromagnetic and the J values are -6.50 cm-1 for 3 and -6.25 cm-1 for 4. The two opposing factors - V-O(bridge) bond length and the displacement of the metal atom from the oxalate plane - appear to have more or less balanced each other resulting in virtually equal J values for the two complexes