O₂ Activation and Aromatic Hydroxylation Performed by Diiron Complexes

Abstract

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.

Iron(III) Contrast Agent Candidates for MRI: a Survey of the Structure–Effect Relationship in the Last 15 ­Years of Studies

“…However, a surprising observation was made by Fontecave -three-carbon-chained EDTA derivatives had a very poor affinity towards iron(III). [76] Therefore, more bulky, hydrophobic systems are used, maintaining the two-carbon alkyl chains disabling water interaction in the first coordination sphere. This reflects the relaxivity of the CAs, so the most attractive approach to achieving high r 1 values as allowed in the case of gadolinium(III) complexes is not accepted in that of iron.…”

Low‐Molecular‐Weight Analogues of the Soluble Methane Monooxygenase (sMMO): From the Structural Mimicking of Resting States and Intermediates to Functional Models

“…O incorporation was observed. [58] The authors thus showed, that the incorporated oxygen atoms exclusively arose from the added oxidizing reagent, not from the bridging oxido ligand. On the basis of todays knowledge this experiment permits a further conclusion: A high-valent diiron(IV) species with a terminal oxido ligand is most likely not the oxidizing species.…”

Iron‐Promoted ortho‐ and/or ipso‐Hydroxylation of Benzoic Acids with H2O2

“…Complexes 1 and 2 exhibit similar reactivity patterns, but 1 generates a more powerful oxidant than 2. Recently, efficient and selective intramolecular aromatic hydroxylations were reported with mononuclear [28,29] or dinuclear [30][31][32][33][34] iron complexes in which the aromatic ring is forced into close proximity of the iron center by a covalent linkage to the supporting polydentate ligand. The need to independently prepare the compounds containing both reactive iron center and the aromatic substrate, however, limits the applicability of these systems.…”