Ionic liquids vs conventional solvents: A comparative study in the selective catalytic oxidations promoted by oxovanadium(IV) complexes

Abstract

Two oxovanadium(IV) complexes containing 4-acyl-5-pyrazolonate-κ2-O,O' bidentate ligands with different chain lengths in the acyl moiety, namely HQC6 (complex I) and HQC17 (complex II), have been synthetized and full characterized, to study their catalytic activity toward the mild and selective oxidation of olefins or model organosulphur substrates, promoted by H2O2 or tert-butyl hydroperoxide (TBHP). The influence due to the type of solvent, likewise ionic liquids (ILs) or conventional medium, was analysed. H2O2 has proven less efficient, in comparison to TBHP. Quantum-chemical calculations have shown that, the key catalytic species involved and, consequently, the actual mechanism might be slightly different according to the oxidant used, i.e. the peroxo VO(QMe)(O2) adduct in the case of H2O2 and the tert-butylperoxo VO(QMe)2(OOt-Bu) adduct in the case of TBHP. Preliminary calculations suggested that, in ionic liquids, the VO(QMe)2 complex might reveal relatively unstable, hence qualitatively explaining the moderate efficiency observed in these media.