Carbohydrate Binding to VO3+. Sugar Vanadate Esters Incorporating l-Amino Acid Schiff Bases as Coligands

Abstract

The glycosides methyl-2,6-dimethoxy-β-d-galactopyranoside (β-d-H2Me3GP), methyl-4,6-dimethoxy-α-d-mannopyranoside (α-d-H2Me3MP), and methyl-5-methoxy-β-d-ribofuranoside (β-d-H2Me2RF) have been synthesized, and their reaction with VO(l-Asal)(OMe)(OHMe) in dichloromethane has afforded esters of the type VO(β-d-HMe3GP)(l-Asal), VO(α-d-HMe3MP)(l-Asal), and VO(β-d-HMe2RF)(l-Asal) as dark-colored solids (red in solution). Here, l-Asal2- is the deprotonated salicylaldimine of l-alanine (A = a), l-valine (A = v), and l-phenylalanine (A = p). The X-ray structures of VO(β-d-HMe3GP)(l-vsal)·H2O and VO(α-d-HMe3MP)(l-psal)·H2O have revealed five-membered (O,O)-chelation by monoionized carbohydrates, the undissociated hydroxyl group lying trans to the oxo oxygen atom. In the carbohydrate frame, the alkoxidic oxygen atom is axial in the former ester and equatorial in the latter. The V−O(alkoxidic) and V−O(alcoholic) distances are, respectively, ∼1.80 and ∼2.30 Å. The ONO coordinating tridentate salicylaldimine ligand is folded (by ∼35°) along a C−N bond. The chiral configuration of the metal site corresponds exclusively to the endo disposition of the VO and the amino acid C−R (R = Me, CHMe2, CH2Ph) bonds. In VO(β-d-HMe3GP)(l-vsal)·H2O two ester molecules constitute the asymmetric unit and these along with the two water molecules form a macrocyclic supramolecule (diameter, ∼6.1 Å) held by hydrogen bonds involving alcohol and an OMe function as well as water (O···O distance, 2.59−2.86 Å). On the other hand, in VO(α-d-HMe3MP)(l-psal)·H2O the water molecule bridges two symmetry-related ester molecules via alkoxide···water and alcohol···water hydrogen bonds forming an infinite chain structure (O···O lengths, 2.65 and 2.85 Å). The molecular structures observed in the solid state are preserved in solution (1H and 51V NMR). No isomerization is detectable either at the metal site or at the anomeric carbon atom, and the V−O(alkoxidic) and V−O(alcoholic) sites and the metal−carbohydrate binding remain in tact. The VO(β-d-HMe2RF)(l-Asal) species did not afford single crystals but NMR results are consistent with (O,O)-chelation by the ribose fragment, the alkoxidic carbon being C3. Crystal data are as follows. VO(β-d-HMe3GP)(l-vsal)·H2O:  chemical formula, C21H32NO11V; crystal system, orthorhombic; space group, P212121; a = 13.146(7) Å, b = 15.142(5) Å, c = 25.631(9) Å; Z = 8. VO(α-d-HMe3MP)(l-psal)·H2O:  chemical formula, C25H32NO11V; crystal system, monoclinic; space group, P21; a = 13.645(4) Å, b = 7.022(2) Å, c = 15.500(4) Å; β = 113.98(2)°; Z = 2.