title: Iron(III) complex of a crown ether-porphyrin conjugate and reversible binding of superoxide to its Iron(II) form.
abstract: The synthesis and characterization of the Fe(III) complex of a novel crown ether-porphyrin conjugate, 52-N-(4-aza-18-crown-6)methyl-54,104,154,204-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].H3O+.FeCl4-.C6H6.EtOH is also reported. [FeIII(Porph)(DMSO)2]+ and K[FeIII(Porph)(O22-)] are high-spin species (Mössbauer data: delta = 0.38 mm s(-1), DeltaEq = 0.83 mm s(-1) and delta = 0.41 mm s(-1), DeltaEq = 0.51 mm s(-1), respectively), whereas in a solution of reduced [FeIII(Porph)(DMSO)2]+ complex the low-spin [FeII(Porph)(DMSO)2] (delta = 0.44 mm s(-1), DeltaEq = 1.32 mm s(-1)) and high-spin [FeII(Porph)(DMSO)] (delta = 1.27 mm s(-1), DeltaEq = 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 degrees C), resulting in kon = 36 500 +/- 500 M(-1) s(-1), koff = 0.21 +/- 0.01 s(-1) (direct measurements using an acid as a superoxide scavenger), and KO2- = (1.7 +/- 0.2) x 10(5) (superoxide binding constant kinetically obtained as kon/koff), (1.4 +/- 0.1) x 10(5), and (9.0 +/- 0.1) x 10(4) M(-1) (thermodynamically obtained in the absence and in the presence of 0.1 M NBu4PF6, respectively). Temperature-dependent kinetic measurements for kon (-40 to 25 degrees C in 3:7 DMSO/CH3CN mixture) yielded the activation parameters DeltaH = 61.2 +/- 0.9 kJ mol(-1) and DeltaS = +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.
authors: Institute of Inorganic Chemistry, University of Erlangen-Nürnberg, Egerlandstrasse 1, 91058 Erlangen, Germany.
authors: Katharina
authors: K
authors: Dürr
authors: Brendan P
authors: BP
authors: Macpherson
authors: Ralf
authors: R
authors: Warratz
authors: Frank
authors: F
authors: Hampel
authors: Felix
authors: F
authors: Tuczek
authors: Matthias
authors: M
authors: Helmreich
authors: Norbert
authors: N
authors: Jux
authors: Ivana
authors: I
authors: Ivanović-Burmazović
mesh_terms: D043844:Crown Ethers / Q000737:chemistry*; D004563:Electrochemistry; D005290:Ferric Compounds / Q000737:chemistry*; D005296:Ferrous Compounds / Q000737:chemistry*; D007700:Kinetics; D008958:Models, Molecular; D015394:Molecular Structure; D011166:Porphyrins / Q000138:chemical synthesis / Q000737:chemistry*; D013057:Spectrum Analysis; D013481:Superoxides / Q000737:chemistry*; D013816:Thermodynamics
chemical_list: D043844:Crown Ethers; D005290:Ferric Compounds; D005296:Ferrous Compounds; D011166:Porphyrins; D013481:Superoxides