New vanadium-(IV) and -(V) analogues of Amavadin‡
The new pro-ligand meso-2,2′-(hydroxyimino)dibutyric acid (R,S-H3hidba) has been synthesised from hydroxylammonium chloride and 2-bromobutyric acid. Reaction of H3hidba with [VO(acac)2] (acac = acetylacetonate) yielded the complex [V(R,S-hidba)2]2– 1, which was crystallised in the presence of Ca2+ ions from H2O as blue tabular crystals. X-Ray crystallography confirmed the same distinctive eight-co-ordinate structure of the complex anion as identified for Amavadin, the form in which vanadium(IV) is bound in Amanita muscaria mushrooms. The crystal structure of 1 contains a network of linked Ca and V centres where the asymmetric unit consists of a Ca2V2 box-like configuration. The Ca and V metal ions are bridged to one another via a series of unidentate and bidentate carboxylate groups from H3hidba, extending throughout the lattice framework. The novel interaction between Ca2+ ions and Amavadin-style complexes has been further illustrated in the crystal structure of [Ca(H2O)5][V(hida)2]·H2O (H3hida = N-hydroxyiminodiacetic acid) 2. The unit cell packing arrangement observed for 2 differs from 1, comprising helical chains formed by alternate Ca and V units linked only by unidentate carboxylate groups. Cyclic voltammetric studies of 1 exhibited a reversible VV–VIV redox couple in H2O (E½ = +0.43 V, vs. saturated calomel electrode), this oxidation potential is considerably lower in organic solvents (e.g. Me2SO, E½ = –0.07 V). The chemical oxidation of 1 in aqueous medium by ammonium ceric nitrate produced a dark red solution which was transferred into CH2Cl2 using [PPh4]Br. From this solution [PPh4][V(R,S-hidba)2] 3 was isolated and studied using 1H, 13C and 51V NMR spectroscopy. The cyclic voltammogram of 3 also displays a reversible VV–VIV redox couple in CH2Cl2 (E½ = –0.09 V).