Non-heme iron polyazadentate complexes as catalysts for aromatic hydroxylation by H2O2: Particular efficiency of tetrakis(2-pyridylmethyl)ethylenediamine–iron(II) complexes

Two iron(II) complexes with ligands related to TPEN, N,N,N′,N′-tetrakis-(2-pyridylmethyl)ethane-1,2-diamine, were prepared, characterized and compared as catalysts for the hydroxylation of aromatic compounds by H2O2. The (TPEN)FeII complex gave the best yields ever reported for such iron polyazadentate catalysts with yields up to 86, 59, 59 and 38% for the hydroxylation of anisole, benzene, chlorobenzene and ethylbenzene, respectively.Experiments using H218O2 and H218O clearly showed that the oxygen atom incorporated in the aromatic substrates almost exclusively comes from H2O2. Preliminary experiments on the effects of O2 and spin-traps on the reactions suggested a minor contribution of uncontrolled radicals such as OH. From the presently available data, a tentative mechanism based on the hydroxylation of substrates by a FeIIIOOH intermediate or, more probably, by a species resulting from the O–O bond cleavage of this intermediate, such as [FeIVO + OH], is proposed.

Graphical abstractTwo Fe(II) complexes with hexaazadentate ligands were prepared, characterized and compared as catalysts for the hydroxylation of aromatic substrates. One of these complexes gave the best yields ever reported for such coordination complexes.Figure optionsDownload full-size imageDownload as PowerPoint slide