Iron(III) complex of a crown ether-porphyrin conjugate and reversible binding of superoxide to its iron(II) form

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作者：
Dürr, Katharina [1 ,2 ]
Macpherson, Brendan P. [1 ]
Warratz, Ralf [3 ]
Hampel, Frank [2 ]
Tuczek, Felix [3 ]
Helmreich, Matthias [2 ]
Jux, Norbert [2 ]
Ivanović-Burmazović, Ivana [1 ]
机构：
[1] Institute of Inorganic Chemistry, University of Erlangen-Nürnberg, Egerlandstrasse 1, 91058 Erlangen, Germany
[2] Institute of Organic Chemistry, University of Erlangen-Nürnberg, Henkestrasse 42, 91054 Erlangen, Germany
[3] Institute of Inorganic Chemistry, Christian-Albrechts-University Kiel, Otto Hahn Plan 6/7, 24098 Kiel, Germany
来源：
Journal of the American Chemical Society | 2007年 / 129卷 / 14期
关键词：
The synthesis and characterization of the Fe(III) complex of a novel crown ether-porphyrin conjugate, 52-N-(4-aza-18-crown-6)methyl-5 4,104,154,204-tetra-tert-butyl- 56-methyl-5,10,15,20-tetraphenylporphyrin (H2Porph), as well as the corresponding hydroxo, dimeric, Fe(II), and peroxo species are reported. The crystal structure of [FeIII(Porph)Cl]&middotH 3O+&middotFeCl4-&middotC 6H6&middotEtOH is also reported. [FeIII(Porph) (DMSO)2]+ and K[FeIII(Porph)(O2 2-)] are high-spin species (Mössbauer data: δ = 0.38 mm s-1, ΔEq = 0.83 mm s-1 and δ = 0.41 mm s-1, ΔEq = 0.51 mm s-1, respectively), whereas in a solution of reduced [FeIII(Porph)-(DMSO) 2]+ complex the low-spin [FeII(Porph)(DMSO) 2] (δ = 0.44 mm s-1, ΔEq = 1.32 mm s-1) and high-spin [FeII(Porph)(DMSO)] (δ = 1.27 mm s-1 ΔEq = 3.13 mm s-1) iron(II) species are observed. The reaction of [FeIII(Porph)(DMSO)2] + with KO2 in DMSO has been investigated. The first reaction step, involving reduction to [FeII(Porph)(DMSO) 2], was not investigated in detail because of parallel formation of an Fe-(III)-hydroxo species. The kinetics and thermodynamics of the second reaction step, reversible binding of superoxide to the Fe(II) complex and formation of an Fe(III)-peroxo species, were studied in detail (by stopped-flow time-resolved UV/vis measurements in DMSO at 25°C), resulting in k on = 36 500 ± 500 M-1 s-1, k off = 0.21 ± 0.01 s-1 (direct measurements using an acid as a superoxide scavenger), and KO2- = (1.7 ± 0.2) × 105 (superoxide binding constant kinetically obtained as kon/koff), (1.4 ± 0.1) × 105, and (9.0 ± 0.1) × 104 M-1 (thermodynamically obtained in the absence and in the presence of 0.1 M NBu4PF 6, respectively). Temperature-dependent kinetic measurements for kon (-40 to 25°C in 3:7 DMSO/CH3CN mixture) yielded the activation parameters ΔH‡ = 61.2 ± 0.9 kJ mol-1 and ΔS‡ = +48 ± 3 J K -1 mol-1. The observed reversible binding of superoxide to the metal center and the obtained kinetic and thermodynamic parameters are unique. The finding that fine-tuning of the proton concentration can cause the Fe(III)-peroxo species to release O2- and form an Fe(II) species is of biological interest, since this process might occur under very specific physiological conditions. © 2007 American Chemical Society;
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页码：4217 / 4228
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