New vanadium-(IV) and -(V) analogues of Amavadin‡

Abstract

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 V^V–V^IV redox couple in H₂O (E_½ = +0.43 V, vs. saturated calomel electrode), this oxidation potential is considerably lower in organic solvents (e.g. Me₂SO, E_½ = –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 V^V–V^IV redox couple in CH₂Cl₂ (E_½ = –0.09 V).