- Intermolecular versus intramolecular electron-/atom- (Cl{radical dot}) transfer in heme-iron and copper pyridylalkylamine complexes
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While outer-sphere electron-transfer reactions come with a firm experimental and theoretical basis, less is known about the redox reactions occurring by atom-transfer. In the present study, relevant reactions occur upon mixing, (F8)FeIII(Cl) (F8 is tetrakis(2,6-difluorophenyl)porphyrinate) with a series of [(L)CuI]+ complexes (L are tripodal tetradentate pyridylalkylamine ligands varying in effective chelate ring sizes: TMPA, PMEA, PMAP) to form (F8)FeII and [(L)CuII(Cl)]+. The electron-/atom- (Cl{radical dot}) transfer process is characterized by electrochemical measurements as well as UV-Vis, 1H NMR, and EPR spectroscopies. Stopped-flow UV-Vis spectroscopy in THF indicate the following relative rates (kobs) [CuI(pmea)]+ > [CuI(pmap)]+ > [CuI(tmpa)(thf)]+. However, the redox potentials as related to ΔG{ring operator} for the reaction, [E1/2(oxidant) - E1/2(reductant)], predict the trend [CuI(tmpa)(CH3CN)]+ > [CuI(pmea)]+ > [CuI(pmap)]+. A detailed description of CuI-to-CuII structural changes is provided and these likely influence the observed [(L)CuI]+/(F8)FeIII(Cl) reaction rates. Analogous chemistry is described for a heme-copper system utilizing a heterobinucleating ligand (6L), which is comprised of a TMPA like moiety tethered at the 6-position of one of the pyridyl donors to a F8-like heme. Kinetic/mechanistic insights were obtained from transient absorption spectroscopic monitoring in CH3CN following photoejection of carbon monoxide from [(6L)FeII(CO)?CuII(Cl)].
- Fry, H. Christopher,Lucas, Heather R.,Zakharov, Lev N.,Rheingold, Arnold L.,Meyer, Gerald J.,Karlin, Kenneth D.
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p. 1100 - 1115
(2008/10/09)
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- (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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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: C55H36ClF8-FeN4O; a = 13.6517(2) A, b = 13.6475(2) A, 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) A, b = 11.7738(2) A, c = 19.3875(2), α = 90°, β = 108.6999(6)°, γ = 90°; monoclinic, C2/c, Z = 4; Fe-N4(av) = 2.002 A, N-Fe-N (all) = 90.0°). Close metal-arene contacts are also observed at 3.11-3.15 A. 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 Jd/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.
- Ghiladi,Kretzer,Guzei,Rheingold,Neuhold,Hatwell,Zuberbuehler,Karlin
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p. 5754 - 5767
(2008/10/08)
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- Oxo- and hydroxo-bridged heme-copper assemblies formed from acid-base or metal-dioxygen chemistry
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The iron-copper dinuclear active site in heme-copper oxidases (e.g., cyctochrome c oxidase) has spurred the development of the inorganic chemistry of bridged heme-copper complexes, including species possessing (porphyrinate)FeIII-O(H)-CuII
- Kopf, Marie-Aude,Neuhold, Yorck-Michael,Zuberbühler, Andreas D.,Karlin, Kenneth D.
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p. 3093 - 3102
(2008/10/08)
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