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

The complexes [VO(L1)(hquin)] and [VO(L2)(hquin)] incorporating N-salicylidene-glycinate and -L-phenylalaninate respectively have been synthesised in excellent yields from [VIVO(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 dxy1 configuration.