SHORT COMMUNICATION DOI: 10.1002/ejic.200800014 Theoretical Investigation on the Mechanism of Oxygen Atom Transfer between Two Non-Heme Iron Centres Sam P. de Visser,* [a] Yong-Min Lee, [b] and Wonwoo Nam* [b] Keywords: Bioinorganic chemistry / Enzyme models / Non-heme iron enzymes / Density functional calculations / Oxygen Density functional theory calculations are presented on the oxygen atom transfer reaction between two non-heme iron centres: One contains Bn–tpen [N-benzyl-N,N',N'-tris(2-pyr- idylmethyl)ethane-1,2-diamine], whereas the other contains N4Py [N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methyl- amine]. The calculations show that the (Bn–tpen)Fe–O– Fe(N4Py) complex is a stable entity but considerably higher in energy than isolated species. However, a mechanism of oxygen atom transfer from one non-heme iron centre to the other will proceed via this oxido-bridged intermediate. This Introduction Oxido-bridged diiron complexes are found in the active sites of iron-containing enzymes that bind and activate di- oxygen, such as ribonucleotide reductase (RNR) and meth- ane monoxygenase (MMO). [1] Synthetic iron complexes have been developed to understand the chemical and physi- cal properties of the oxido-bridged diiron active sites. In particular, biomimetics with oxido-bridged diiron units have been known for quite some time in heme and non- heme iron systems; the (μ-oxido)diiron(III) complexes of heme and non-heme ligands are thermally stable and are well characterized with various spectroscopic techniques. [2] Recently, Collins and co-workers reported the isolation and characterization of a (μ-oxido)diiron(IV) complex formed in the reaction of a non-heme iron(III) complex and O 2 . [3] It has been shown very recently that non-heme oxido- iron(IV) complexes transfer their oxygen atom to other non-heme iron(II) complexes, probably by the formation of a(μ-oxido)diiron(III) species [Equation (1)]. [4] The com- plete intermetal oxygen atom transfer between the oxido- iron(IV) and iron(II) complexes and the failure of the isola- [a] The Manchester Interdisciplinary Biocentre and the School of Chemical Engineering and Analytical Science, The University of Manchester 131 Princess Street, Manchester, M1 7DN, United Kingdom Fax: +44-1613065201 E-mail: sam.devisser@manchester.ac.uk [b] Department of Chemistry, Division of Nano Sciences, and Cen- tre for Biomimetic Systems, Ewha Womans University, Seoul 120-750, Korea Fax: +82-232774441 E-mail: wwnam@ewha.ac.kr Supporting information for this article is available on the WWW under http://www.eurjic.org or from the author. Eur. J. Inorg. Chem. 2008, 1027–1030 © 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1027 oxido-bridged complex has both iron atoms in oxidation state III so that in the process of the formation of the complex, an electron transfer from the Fe II centre to the Fe IV (O) centre has taken place. Nevertheless, both metal atoms have dif- ferent orbital and spin-density occupation. A large solvent effect on the reaction barriers is obtained, indicating that the reaction proceeds only in very polar environments. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) tion of the oxido-bridged diiron(III) intermediate imply that the latter species is in a thermally unstable, high energy state, which is different from what used to be reported pre- viously. [2] The oxygen atom transfer reaction was also found to depend on the oxidizing power of the oxidoiron(IV) complexes in oxygenation reactions; [(Bn–tpen)Fe IV =O] 2+  [(N4Py)Fe IV =O] 2+  [(TMC)Fe IV =O] 2+ . [4–6] Apart from the oxygen atom transfer reactions between two non-heme iron complexes, nitrogen atom transfer between manganese complexes bearing different macrocycles have also been re- ported. [7] (1) Although transfer of the oxygen atom formally happens in hydroxylation reactions, [8] it is still remarkable that it happens between different catalysts. In order to elucidate the mechanism by which non-heme oxidoiron(IV) com- plexes transfer their oxygen atom to other non-heme iron(II) systems, we present here the first density functional theory studies into a non-heme (μ-oxido)diiron(III) com- plex and its dissociation patterns into the respective ox- idoiron(IV) and iron(II) complexes. Results and Discussion We chose two non-heme iron complexes bearing pentaco- ordinate ligands, Bn–tpen and N4Py (see Supporting Infor-