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, V^VO(Asal)(Hpd) and V^VO(Asal)(H₂pt), have been synthesized in excellent yields by reacting V^IVO(Asal)(H₂O) with propane-1,3-diol (H₂pd) and glycerol (H₃pt), respectively, in methanol. Here Asal^2- 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 Asal^2- 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.