Oxidation of cyclohexane by transition-metal complexes with biomimetic ligands

Abstract

This work reports the catalytic activity, in homogeneous phase, of transition-metal complexes of the first-row (V(IV), Mn(III), Fe(III) Co(III) and Cu(II)) with biomimetic Schiff base ligands with N2O2 coordination sphere, as well as an N4 (Fe(II)), in the room-temperature oxidation of cyclohexane using environmentally benign reagents: hydrogen peroxide (30wt%) as the oxygen source and acetonitrile as the solvent. Nitric acid is also used as promoter of the oxidation reaction. The structure of the ligands is confirmed by FTIR, 1H NMR and high-resolution ESI mass spectrometry. The corresponding transition metal complexes are characterized by elemental analysis, high resolution ESI mass spectrometry, FTIR and UV–vis. Cyclohexanone and cyclohexanol are the main products of the oxidation of cyclohexane, obtained when the following complexes are used as homogeneous catalysts in only 1mol% based on the substrate: VO(IV), Fe(III) and Cu(II) complexes with the N2O2 Schiff base, new Fe(II) complex with the Schiff base with N4 coordination sphere and commercial [VO(acac)2] with O4 coordination sphere. The Fe(III) complex with N2O2 Schiff base ligand ([Fe(salhd)Cl]) is the homogeneous catalyst with highest activity, which could be further enhanced by the addition of methyl electron donating groups to the N2O2 Schiff base aldehyde fragment (reaching 46% oxygenate yields and 45 turnover numbers). Cyclooctane and n-hexane could also be oxidized to the corresponding ketones and alcohols with higher turnover numbers than cyclohexane by the Fe(III) complex with N2O2 Schiff base ligand.