8-Quinolinolato iron(III)-catalyzed oxygenation of cyclohexane with hydrogen peroxide under heating or visible light irradiation

A series of hexadentate binding 8-quinolinolato iron(III) (Q(3)Fe(III) (1a-1e)) complexes were synthesized conveniently via the coordination of FeCl3 center dot 6H(2)O with 8-hydroxyquinoline (a) or its derivatives (b-e). These Q(3)Fe(III) complexes were found to be active for the oxygenation of cyclohexane to cyclohexanol and cyclohexanone by aqueous hydrogen peroxide (H2O2) in acetonitrile under heating conditions. Among the Q(3)Fe(III) complexes examined, the 5-chlorinated 1b and the unsubstituted 1a showed a good accelerating effect on this oxygenation due to their good ability to activate H2O2, as supported by DFT B3LYP/6-311G (d) calculation and TGA characterization. Notably, the above-described catalysis reactions were accelerated to some extent under visible light irradiation, providing an equivalent to a slightly higher cyclohexane conversion with a similar product distribution compared to the corresponding thermal catalysis reactions. Moreover, this light-accelerated effect on cyclohexane oxygenation was more apparent upon the dihalogenated Q(3)Fe(III) complexes 1c-1e, these having a relatively small difference of LUMO and HOMO energies. The improved effect of light irradiation on substrate conversion also became more prominent in the Q(3)Fe(III)-catalyzed oxygenation of other substrates like benzene, toluene, ethylbenzene and thioanisole with H2O2. Based on the UV-vis spectral characterization, a free radical mechanism for the present catalysis oxygenation system was proposed. (C) 2013 Elsevier B.V. All rights reserved.