Biphenyl derived Schiff-base vanadium(v) complexes with pendant OH-groups—structure, characterization and hydrogen peroxide mediated sulfide oxygenation

Abstract

A series of mononuclear oxovanadium(V) complexes of tridentate Schiff bases HL¹–HL⁴ and H₂L⁵–H₂L⁸ derived from 6-phenylsalicylaldehyde and 6-(2-hydroxyphenyl)salicylaldehyde and four different amines was synthesized. The systematically selected ligands contain aliphatic or aromatic nitrogen, or alkoxy- and phenoxy-oxygen as third donor atom. The complexes were characterised by spectroscopic methods in solution and the solid state. Single-crystal X-ray analyses were performed with VO₂L¹ (1), VO₂L³·½EtOH (3), VO₂L⁴ (4), VO(OiPr)L⁷·iPrOH (7), VO(OiPr)L⁸ (8) and H₂L⁸. For all compounds the vanadium(V) cores contain distorted tetragonal pyramidal geometry around the dioxo- and oxovanadium site at which the N₂O- and NO₂-donor ligands bind equatorially. Complexes 1 and 3 display intramolecular hydrogen bonding of the pendant hydroxyphenyl group to a coordinated oxygen trans to a nitrogen atom and therefore serve as suitable models for the native site of vanadium dependent haloperoxidases. Variable-temperature ¹H NMR spectra revealed significant hydrogen bond interaction in acetonitrile solution. In situ prepared catalysts are active for hydrogen peroxide mediated oxygenation of ethyl phenyl sulfide and showed complete conversion of the substrate to ethyl phenyl sulfoxide, together with small amounts of the corresponding sulfone, as detected by GC/MS after 10 min. The complex of H₂L⁷ turned out to be most efficient while HL¹ and HL² were completely inactive. Catalysis is supported by the pendant OH group in the complex of HL³, the catalyst is twice as active as the complex of HL⁴.