Synthesis and structures of niobium(V) and tantalum(V) analogues of Amavadin: [M(hida)2]- (M = NbV or TaV, H3hida = hydroxyimino-diacetic acid) and [Nb(R,R-hidpa)2]- [H3hidpa = 2,2′-(hydroxyimino)dipropioni c acid]

Abstract

Reactions of M(OEt) ₅ (M = Nb or Ta) with N-hydroxyiminodiacetic acid (H ₃ hida) yielded the complexes [M(hida) ₂ ] ^- (M = Nb 1 or Ta 2). Single-crystal X-ray diffraction revealed the same novel eight-co-ordinated structure identified for Amavadin, the form in which vanadium(IV) is bound in Amanita muscaria mushrooms. This type of structure gives rise to chirality at the metal, where 1 and 2 consist of an equimolar mixture of the Δ and Λ helical forms in the crystal lattice. The complexes [Nb(R,R-hidpa) ₂ ] ^- 3a and [Nb(R,S-hidpa) ₂ ] ^- 3b [H ₃ hidpa = 2,2′-(hydroxyimino)dipropionic acid] were synthesized via the route used for 1 and 2, and X-ray crystallography confirmed the presence of the same Amavadin-style geometry. Cyclic voltammetry in CH ₂ Cl ₂ showed that at 254 K 1 and 3a exhibit quasi-reversible reductions at E _½ = -1.22 and -1.35 V (vs. saturated calomel electrode), respectively, whereas 2 displays an irreversible reduction (-1.92 V).