Non-heme iron polyazadentate complexes as catalysts for oxidations by H2O2:: particular efficiency in aromatic hydroxylations and beneficial effects of a reducing agent

Four iron(II) complexes, bearing hexa-, penta- or tetra-azadentate ligands, whose redox potentials (for the Fe-III/Fe-II couple) vary from +280 to +1023 mV (versus saturated calomel electrode), were compared as catalysts for the oxidation of various substrates by H2O2. These complexes were bad catalysts for alkene epoxidation or alkane hydroxylation, whereas those exhibiting the highest redox potentials were interesting catalysts for the hydroxylation of aromatic compounds. For most of them, addition of an appropriate reducing agent, such as hydroquinones, PhSH or tetrahydropterins, in the medium, led to spectacular increases of the yields of hydroxylation of aromatic compounds such as anisole, toluene, ethylbenzene, benzene and naphthalene (with final yields based on H2O2 up to 69, 33, 18, 40 and 47%, respectively). Such systems catalyzed the transfer of an oxygen atom from H2O2, as shown by experiments using (H2O2)-O-18 and (H2O)-O-18, into anisole with yields based on starting H2O2 up to 69%. (C) 2004 Elsevier B.V. All rights reserved.