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

Two oxidovanadium(V) complexes, [VVO(L1)(OMe)] (1) and [VVO(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.

Graphical abstractThe synthesis and characterization of two oxovanadium(V) complexes of ONNO donor amine-bis(phenolate) ligands were carried out by combined single-crystal X-ray diffraction, NMR spectroscopy and DFT calculations. These complexes were found to catalyze the oxidation of aromatic hydrocarbons through an oxohydroperoxo intermediate.Figure optionsDownload full-size imageDownload as PowerPoint slide