title: Solution behavior of iron(III) and iron(II) porphyrins in DMSO and reaction with superoxide. Effect of neighboring positive charge on thermodynamics, kinetics and nature of iron-(su)peroxo product.
abstract: The solution behavior of iron(III) and iron(II) complexes of 5(4),10(4),15(4),20(4)-tetra-tert-butyl-5,10,15,20-tetraphenylporphyrin (H(2)tBuTPP) and the reaction with superoxide (KO(2)) in DMSO have been studied in detail. Applying temperature and pressure dependent NMR studies, the thermodynamics of the low-spin/high-spin equilibrium between bis- and mono-DMSO Fe(II) forms have been quantified (K(DMSO) = 0.082 ± 0.002 at 298.2 K, ΔH° = +36 ± 1 kJ mol(-1), ΔS° = +101 ± 4 J K(-1) mol(-1), ΔV° = +16 ± 2 cm(3) mol(-1)). This is a key activation step for substitution and inner-sphere electron transfer. The superoxide binding constant to the iron(II) form of the studied porphyrin complex was found to be (9 ± 0.5) × 10(3) M(-1), and does not change significantly in the presence of the externally added crown ether in DMSO (11 ± 4) × 10(3) M(-1). The rate constants for the superoxide binding (k(on) = (1.30 ± 0.01) × 10(5) M(-1) s(-1)) and release (k(off) = 11.6 ± 0.7 s(-1)) are not affected by the presence of the external crown ether in solution. The resulting iron(II)-superoxide adduct has been characterized (mass spectrometry, EPR, high-pressure UV/Vis spectroscopy) and upon controlled addition of a proton source it regenerates the starting iron(II) complex. Based on DFT calculations, the reaction product without neighboring positive charge has iron(II)-superoxo character in both high-spin side-on and low-spin end-on forms. The results are compared to those obtained for the analogous complex with covalently attached crown ether, and more general conclusions regarding the spin-state equilibrium of iron(II) porphyrins, their reaction with superoxide and the electronic structure of the product species are drawn.
authors: Department of Chemistry and Pharmacy, University of Erlangen-Nürnberg, Egerlandstr. 1, 91058, Erlangen, Germany.
authors: K
authors: K
authors: Duerr
authors: O
authors: O
authors: Troeppner
authors: J
authors: J
authors: Olah
authors: J
authors: J
authors: Li
authors: A
authors: A
authors: Zahl
authors: T
authors: T
authors: Drewello
authors: N
authors: N
authors: Jux
authors: J N
authors: JN
authors: Harvey
authors: I
authors: I
authors: Ivanović-Burmazović
mesh_terms: D056831:Coordination Complexes / Q000138:chemical synthesis / Q000737:chemistry*; D004121:Dimethyl Sulfoxide / Q000737:chemistry*; D004563:Electrochemistry; D004579:Electron Transport; D005290:Ferric Compounds / Q000737:chemistry*; D005296:Ferrous Compounds / Q000737:chemistry*; D006418:Heme / Q000737:chemistry; D007700:Kinetics; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D015394:Molecular Structure; D011166:Porphyrins / Q000737:chemistry*; D011522:Protons; D012995:Solubility; D021241:Spectrometry, Mass, Electrospray Ionization; D013481:Superoxides / Q000737:chemistry*; D013816:Thermodynamics
chemical_list: D056831:Coordination Complexes; D005290:Ferric Compounds; D005296:Ferrous Compounds; D008024:Ligands; D011166:Porphyrins; D011522:Protons; D013481:Superoxides; D006418:Heme; D004121:Dimethyl Sulfoxide