Synthesis and structure of carboxyl-bonded oxovanadium(V) complexes incorporating α-amino acid salicylaldiminates and quinolin-8-olate

Abstract

The complexes [VO(L¹)(hquin)] and [VO(L²)(hquin)] incorporating N-salicylidene-glycinate and -L-phenylalaninate respectively have been synthesised in excellent yields from [V^IVO(L)(H₂O)] and quinolin-8-ol (Hhquin) in air which acts as the oxidant. The crystal structure of [VO(L²)(hquin)] revealed tridentate ONO and bidentate ON binding by [L²]^2– and hquin^– respectively. The V–O (carboxylate) bond length is longer than V–O (phenolate) by ≈0.1 Å. The CD spectrum of [VO(L²)(hquin)] in the visible region revealed the composite nature of the phenolato→vanadium charge-transfer band. The absolute configuration of [VO(L²)(hquin)] is CS both in the crystalline state and in solution. In CDCl₃ solution the ¹H NMR spectrum of the complex shows that the amino acid side chain has the same conformation as that in the crystal lattice. The complexes display the quasi-reversible one-electron couple [VO(L)(hquin)]–[VO(L)(hquin)]^– near 0.0 V vs. saturated calomel electrode. Electrogenerated solutions of [VO(L)(hquin)]^– are EPR-active corresponding to a d_xy¹ configuration.