A Family of (N-Salicylidene-α-amino acidato)vanadate Esters Incorporating Chelated Propane-1,3-diol and Glycerol: Synthesis, Structure, and Reaction

Abstract

The brown colored title complexes, VVO(Asal)(Hpd) and VVO(Asal)(H2pt), have been synthesized in excellent yields by reacting VIVO(Asal)(H2O) with propane-1,3-diol (H2pd) and glycerol (H3pt), respectively, in methanol. Here Asal2- is the deprotonated salicylaldimine of glycine (A = g), l-alanine (A = a), l-valine (A = v), or l-phenylalanine (A = p). The complexes have relatively low oxovanadium(V)−oxovanadium(IV) reduction potentials (−0.2 V vs SCE in dimethyl sulfoxide). The X-ray structures of VO(gsal)(Hpd) and VO(gsal)(H2pt) have revealed distorted octahedral VO5N coordination. Six-membered and five-membered O,O chelation occur for Hpd- and H2pt-, respectively, an undissociated alcohol function lying trans to the oxo oxygen atom. The tridentate salicylaldimine ligand spans meridionally and has a folded structure consisting of two planar parts intersecting along a C−N bond. The V−O(alkoxide) bond in the complexes is ∼0.6 Å shorter than the V−O(alcohol) bond. The alkoxidic chelate ring partially hydrolyzes in moist solventsthe six-membered ring in VO(Asal)(Hpd) more easily than the five-membered ring in VO(Asal)(H2pt). The hydrolysis is suppressed in the presence of the relevant free alcohol. The 51V NMR chemical shifts differ by ∼20 ppm between VO(Asal)(Hpd) and VO(Asal)(H2pt) and are diagnostic of the alkoxidic chelate ring size. Species with chiral Asal2- ligands display diastereoisomeric equilibria in solution, and the equilibrium constants K = [endo]/[exo] have been determined from the 51V NMR signal intensity. The trend in K values is l-alanine < l-phenylalanine < l-valine (for given alkoxidic chelation) and Hpd- < H2Pt- (for given amino acid residue). These trends are consistent with size effects. Crystal data are as follows. VO(gsal)(Hpd):  chemical formula, C12H14NO6V; crystal system, monoclinic; space group, P21/n; a = 9.229(5), b = 12.655(6), c = 11.478(4) Å; β = 99.61(4)°; Z = 4. VO(gsal)(H2pt):  chemical formula, C12H13NO7V; crystal system, monoclinic; space group, P21/c; a = 10.962(6), b = 9.544(4), c = 13.323(5) Å; β = 102.67(4)°; Z = 4.