Weakly distorted 8-quinolinolato iron(III) complexes as effective catalysts for oxygenation of organic compounds by hydrogen peroxide

•Two heteroleptic 8-quinolinolato iron(III) complexes were synthesized.•The heteroleptic complexes showed a superiority for the catalyzed-oxygenation of hydrocarbons.•This predominance was due to a structural distortion effect of both.•A free radical mechanism for the catalytic system was proposed.

This paper first discloses that two heteroleptic 8-quinolinolato iron(III) complexes (Qa1Qb2FeIII, Qa2Qb1FeIII) could be synthesized conveniently via the coordination of FeCl2·6H2O with 2 equivalents of 5,7-dichloro-8-hydroxyquinoline (Qb) or 5-chloro-8-hydroxyquinoline (Qa) under N2 and then 1 equivalent of Qa or Qb under air. In comparison with the two homoleptic counterparts (Qa3FeIII and Qb3FeIII), the proposed heteroleptic Q3FeIII complexes possessed similar coordination features to the Qb3FeIII one but showed similar catalysis performances to the Qa3FeIII one in the oxygenation of cyclohexane to cyclohexanol and cyclohexanone by hydrogen peroxide (H2O2) in acetonitrile. More importantly, both heteroleptic Q3FeIII complexes showed a better accelerating effect on this reaction and provided a slightly higher conversion than the Qa3FeIII and especially Qb3FeIII ones. Furthermore, this predominance in catalytic activity was more strikingly apparent upon both-catalyzed oxygenations of benzene, toluene, ethylbenzene or thioanisole by H2O2. This should be due to a structurally distorted effect of the heteroleptic Q3FeIII complexes that is induced by the different in ligand environment, as supported by DFT B3LYP/6-311G (d) calculation. Based the present reaction and UV–vis spectral characterization results, a free radical mechanism for the present catalysis system was proposed.

Graphical abstractTwo heteroleptic 8-quinolinolato iron (III) (Qa1Qb2FeIII, Qa2Qb1FeIII) complexes designed by us were found to show a higher activity for the selective oxidation of organic compounds by hydrogen peroxide compared to the corresponding homoleptic ones, which should be due to their weakly distorted effect.Figure optionsDownload full-size imageDownload high-quality image (153 K)Download as PowerPoint slide