Five- and six-coordinate vanadium(V) complexes with tridentate Schiff base ligands derived from S(+)-isoleucinol: Synthesis, characterization and catalytic activity in the oxidation of sulfides and olefins

Abstract

A series of vanadium(V) complexes with chiral tridentate Schiff base ligands, obtained by a single condensation of S(+)-isoleucinol with salicylaldehyde and its derivatives, and also six-coordinate complexes with additional monoanionic bidentate benzohydroxamate co-ligand, were prepared. The complexes were characterized by elemental analysis and by their IR, CD, UV–Vis, one- (1H, 51V) and two-dimensional (COSY, gHSQC and NOESY) NMR spectra. The X-ray analysis of the complex, (benzohydroxamato-κ2O,O′){S(+)-2-[(1-oxido-3-methylpentyl)iminomethyl]phenolato-κ3N,O,O′}oxidovanadium(V), 1b, revealed a distorted octahedral VO(ONO)(OO) geometry. The five-coordinate vanadium(V) complexes have ability to catalyze the oxidation of sulfides [PhSR (R=Me, Bz)] using aqueous 30% H2O2 in good yields and enantiomeric excesses. Catalytic activity of these complexes were also tested in the oxidation of styrene and cyclohexene, using aqueous 30% H2O2 or tert-butyl hydroperoxide (TBHP) as oxidant.