Synthesis, characterization and crystal structure of cyclic vanadate complexes with monosaccharide derivatives having a free adjacent diol system

Abstract

A new approach to the study of reactions between vanadate and monosaccharide derivatives by introducing them into an acetonitrile medium has been elaborated. Based on this approach, a crystalline product of the reaction of [NBuⁿ₄][H₂VO₄] with methyl O-4,6-benzylidene-α-D-mannopyranoside (H₂L¹) has been obtained, [NBuⁿ₄]₂[(VO₂L¹)₂], and its structure has been determined by X-ray crystallography. The anion comprises a dinuclear V₂(µ-O)₂ centre and two deprotonated H₂L¹ molecules. Each vanadium atom is bound to the deprotonated vicinal cis-diol of one mannopyranoside ring resulting in a five-membered ring cyclic complex. The geometry of the two five-co-ordinated vanadium(V) atoms is intermediate between a square pyramid and a trigonal bipyramid. A complex with a similar formula [NBuⁿ₄]₂[(VO₂L³)₂]{H₂L³= 5′-O-[(p-methoxyphenyl)diphenylmethyl]uridine}, has also been prepared and spectroscopically characterized. Multinuclear NMR studies and conductivity measurements in acetonitrile provided strong evidence that the dimeric structure of both complexes is maintained in solution. Vanadium-51 NMR titration studies in acetonitrile solutions showed that the chelation ability follows the order L³ > L¹ > L²(H₂L²= methyl O-4,6-benzylidene-α-D-glucopyranoside). Molecular modelling suggested that the O^–⋯ O^– distance and dihedral angle between the adjacent hydroxyl groups in a sugar ring are the determining factors for vanadium chelation.