O2 activation and aromatic hydroxylation performed by diiron complexes

Article abstract of DOI:10.1021/ja981123a

Chemical models of active sites of diiron oxo proteins have been synthesized. The polydendate ligands are EDTA derivatives which provide a balanced supply of nitrogen atoms and carboxylate groups together with an oxidizable phenyl moiety, thus mimicking both the iron coordination in methane monooxygenase and a nearby substrate site. All the diferric complexes have been characterized in solution by ESI-MS, optical absorption, and in some cases by ¹H NMR. In the case of the ligand L1 [L1 = (N,N'-bis(3,4,5- trimethoxybenzyl)ethylenediamine N,N'-diacetic acid)], the X-ray structure of the corresponding iron complex has been determined, revealing an original tetranuclear unit, Fe₄O₂(L1)₄·10H₂O, issued from the dimerization of two [Fe₂O(L1)₂] units linked by carboxylate bridges. In a solution containing water or acetate, the tetranuclear complex decomposed into dinuclear complexes, which proved to be able to react with hydrogen peroxide or dioxygen in the presence of ascorbate. The final product was a mononuclear complex identified as [Fe(III)L'1(H₂O)] with L'1 resulting from the quantitative hydroxylation of L1. The complex and the oxidized ligand were characterized by EPR, NMR, and UV-vis spectroscopies and by mass spectrometry. Labeling experiments showed that with both H₂O₂ or O₂ and ascorbate, the incorporated oxygen came from the oxidant exclusively. This reaction mimicks the transformation of a tyrosine residue, brought into proximity of the active center of Ribonucleotide reductase of Escherichia coli by site-directed mutagenesis, into 3,4-dihydroxyphenylalanine.