ic0105866_si_001.cif (35.72 kB)

(F8TPP)FeII/O2 Reactivity Studies {F8TPP = Tetrakis(2,6-difluorophenyl)porphyrinate(2−)}:  Spectroscopic (UV−Visible and NMR) and Kinetic Study of Solvent-Dependent (Fe/O2 = 1:1 or 2:1) Reversible O2-Reduction and Ferryl Formation

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posted on 10.10.2001, 00:00 by Reza A. Ghiladi, Ryan M. Kretzer, Ilia Guzei, Arnold L. Rheingold, Yorck-Michael Neuhold, Karen R. Hatwell, Andreas D. Zuberbühler, Kenneth D. Karlin
In this report, we describe in detail the O2-binding chemistry of the metalloporphyrin (F8TPP)FeII (1). This complex was synthesized from aqueous dithionite reduction of (F8TPP)FeIII-Cl (X-ray structure reported:  C55H36ClF8FeN4O; a = 13.6517(2) Å, b = 13.6475(2) Å, c = 26.3896(4), α = 90°, β = 89.9776(4)°, γ = 90°; monoclinic, P2(1)/c, Z = 4). Complex 1 crystallizes from toluene/heptane solvent system as a bis(toluene) solvate, (F8TPP)FeII·(C7H8)2, with ferrous ion in the porphyrin plane (C58H36F8FeN4; a = 20.9177(2) Å, b = 11.7738(2) Å, c = 19.3875(2), α = 90°, β = 108.6999(6)°, γ = 90°; monoclinic, C2/c, Z = 4; Fe−N4(av) = 2.002 Å; N−Fe−N (all) = 90.0°). Close metal−arene contacts are also observed at 3.11−3.15 Å. Upon oxygenation of 1 at 193 K in coordinating solvents, UV−visible and 2H and 19F NMR spectroscopies revealed the presence of a reversibly formed dioxygen adduct, formulated as the heme-superoxo complex (S)(F8TPP)FeIII-(O2-) (2) (S = solvent) [(i) tetrahydrofuran (THF) solvent:  UV−visible, 416 (Soret), 536 nm; 2H NMR:  δpyrrole 8.9 ppm; (ii) EtCN solvent:  UV−visible, 414 (Soret), 536 nm; (iii) acetone solvent:  UV−visible, 416 (Soret), 537 nm; 2H NMR:  δpyrrole 8.9 ppm]. Dioxygen-uptake manometry (THF, 193 K) revealed an O2:1 oxygenation stoichiometry of 1.02:1, consistent with the heme-superoxo formulation of 2. Stopped-flow UV−visible spectrophotometry studies of the (F8TPP)FeII (1)/O2 reaction in EtCN and THF solvents were able to provide kinetic and thermodynamic insight into the reversible formation of 2 [(i) EtCN:  ΔH° = −40 ± 5 kJ/mol; ΔS° = −105 ± 23 J/(K mol); k1 = (5.57 ± 0.04) × 103 M-1 s-1 (183 K); ΔH = 38.6 ± 0.2 kJ/mol; ΔS = 42 ± 1 J/(K mol); (ii) THF:  ΔH° = −37.5 ± 0.4 kJ/mol; ΔS° = −109 ± 2 J/(K mol)}. The (F8TPP)FeII (1)/O2 reaction was also examined at reduced temperatures in noncoordinating solvents (toluene, CH2Cl2), where UV−visible and 2H and 19F NMR spectroscopies also revealed the presence of a reversibly formed adduct, formulated as the peroxo-bridged dinuclear complex [(F8TPP)FeIII]2-(O22-) (3) [CH2Cl2:  UV−visible, 414 (Soret), 535 nm; 2H NMR, δpyrrole 17.5 ppm]. Dioxygen-uptake spectrophotometric titrations revealed a stoichiometry of 2 (F8TPP)FeII (1) per O2 upon full formation of 3. Addition of a nitrogenous base, 4-(dimethylamino)pyridine, to a cold solution of 3 in dichloromethane gave rapid formation of the iron(IV)-oxo ferryl species (DMAP)(F8TPP)FeIVO (4), based upon UV−visible [417 (Soret), 541 nm] and 2H NMR (δpyrrole = 3.5 ppm) spectroscopic characterization. These detailed investigations into the O2-adducts and “ferryl” species formed from (F8TPP)FeII (1) may be potentially important for a full understanding of our ongoing heme-copper oxidase model studies, which employ 1 or similar “tethered” (i.e., covalently attached Cu-chelate) porphyrin analogues in heme/Cu heterobinuclear systems.

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