Catalytic oxidation of aromatic hydrocarbons by mono-oxido- alkoxidovanadium(V) complexes of ONNO donor ethylenediaminebis(phenolate) ligands

Two oxidovanadium(V) complexes, [V^VO(L¹)(OMe)] (1) and [V^VO(L²)(OMe)] (2), with ONNO donor ethylenediamine-bis(phenolate) ligands (H2L¹ and H2L²) have been readily synthesized by the reaction between the ligand precursors and VOSO₄·5H₂O in MeOH, and characterized by physico-chemical techniques and single crystal X-ray diffraction studies. Both complexes 1 and 2 are hexa-coordinated with a pseudo-octahedral geometry in an N₂O₄ coordination environment and are found to catalyze the oxidation of toluene to benzoic acid and isomers of xylene to the corresponding hydroxy acids, with turnover numbers (TON) over 200, except for o-xylene. On changing the ligand fragments and coordination geometry around the metal centre there is an improvement in the catalytic efficiency, selectivity and also TON of the reaction as compared with the previously reported systems. The reactions were monitored using ⁵¹V NMR spectroscopy. Based on mass spectra analysis and ⁵¹V NMR studies, the mechanism of the catalytic process has been proposed, employing the formation of oxido-hydroperoxido/hydroxido-peroxido [V(ONNO)(O)(OOH)] ⇔ [V(OH)(ONNO)(O₂)] intermediates.