End-On and Side-On Peroxo Derivatives of Non-Heme Iron Complexes with Pentadentate Ligands: Models for Putative Intermediates in Biological Iron/Dioxygen Chemistry

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• 作者：Gerard Roelfes ; Vladislav Vrajmasu ; Kui Chen ; Raymond Y. N. Ho ; Jan-Uwe Rohde ; Charon Zondervan ; Rene M. la Crois ; Ebe P. Schudde ; Martin Lutz ; Anthony L. Spek ; Ronald Hage ; Ben L. Feringa ; Eckard M&u
• 刊名：Inorganic Chemistry
• 出版年：2003
• 出版时间：April 21, 2003
• 年：2003
• 卷：42
• 期：8
• 页码：2639 - 2653
• 全文大小：262K
• 年卷期：v.42,no.8(April 21, 2003)
• ISSN：1520-510X

文摘

Mononuclear iron(III) species with end-on and side-on peroxide have been proposed or identified in the catalyticcycles of the antitumor drug bleomycin and a variety of enzymes, such as cytochrome P450 and Rieske dioxygenases.Only recently have biomimetic analogues of such reactive species been generated and characterized at lowtemperatures. We report the synthesis and characterization of a series of iron(II) complexes with pentadentate N5ligands that react with H₂O₂ to generate transient low-spin Fe^III-OOH intermediates. These intermediates havelow-spin iron(III) centers exhibiting hydroperoxo-to-iron(III) charge-transfer bands in the 500-600-nm region. Theirresonance Raman frequencies, _O-O, near 800 cm^-1 are significantly lower than those observed for high-spincounterparts. The hydroperoxo-to-iron(III) charge-transfer transition blue-shifts and the _O-O of the Fe-OOH unitdecreases as the N5 ligand becomes more electron donating. Thus, increasing electron density at the low-spinFe(III) center weakens the O-O bond, in accord with conclusions drawn from published DFT calculations. Theparent [(N4Py)Fe^III(¹-OOH)]²⁺ (1a) ion in this series (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine)can be converted to its conjugate base, which is demonstrated to be a high-spin iron(III) complex with a side-onperoxo ligand, [(N4Py)Fe^III(²-O₂)]⁺ (1b). A detailed analysis of 1a and 1b by EPR and Mössbauer spectroscopyprovides insights into their electronic properties. The orientation of the observed ⁵⁷Fe A-tensor of 1a can be explainedwith the frequently employed Griffith model provided the rhombic component of the ligand field, determined by thedisposition of the hydroperoxo ligand, is 45 rotated relative to the octahedral field. EXAFS studies of 1a and 1breveal the first metrical details of the iron-peroxo units in this family of complexes: [(N4Py)Fe^III(¹-OOH)]²⁺ has anFe-O bond of 1.76 Å, while [(N4Py)Fe^III(²-O₂)]⁺ has two Fe-O bonds of 1.93 Å, values which are in very goodagreement with results obtained from DFT calculations.