Oxovanadium(IV) complexes of N-D-gluconylamino acids

Abstract

The complex formation between five N-D-gluconylamino acids (the derivatives of glycine, α-L-alanine, β-alanine, L-serine and L-methionine) and V^IVO ion was studied in aqueous media by means of combined pH-metric, CD, UV-Vis and EPR spectroscopic methods. The equilibrium study (25 °C, I = 0.2 mol dm⁻³ (KCl)) revealed the formation of various mono- and di-nuclear complexes, deprotonated to extents depending on the pH. The EPR spectra confirmed the formation of EPR-inactive complexes involving strong antiferromagnetic coupling between the metal centres in the pH interval 5–10. A dinuclear species with the composition [(VO)₂L₂H₋₄]²⁻ predominates at pH ≈ 5 in each system; the changes in shape of the CD spectra strongly suggested the deprotonation and hence coordination of the amide nitrogen in this species. The CD results provided evidence of the metal ion-promoted deprotonation and coordination of the C(2)–OH group too under these conditions. On increase of the pH further alcoholic OH groups are deprotonated, but the amide group remains in the first coordination sphere both in the dinuclear species [(VO)₂L₂H₋₅]³⁻ and [(VO)₂L₂H₋₆]⁴⁻ and in the monomeric complex [(VO)LH₋₄]³⁻. No mononuclear bis complexes were observed, even at a high excess of ligand.