- Weak coordination of H2S to the solid-state ferrous porphyrin complexes with diatomic molecules. Characterization of 6-coordinate adducts at low temperature
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The interaction of hydrogen sulfide (H2S) with ferrous porphyrin five coordinate complexes bearing CO and O2 (L) was studied at low temperatures in sublimated porphyrin layers using electronic and infrared absorption spectroscopy. Upon addition of H2S to a cryostat containing Fe(II)(TPP)(L) (TPP is meso-tetraphenyl-porphyrinato dianion) at low temperatures, the spectral changes were observed that are consistent with H2S binding in axial position trans to the diatomic ligand. Density functional theory (DFT) computational analysis also supports formation of the six-coordinate adducts. These complexes are stable only at very low temperature and dissociate upon warming. The stretching frequencies of diatomic ligands in the six-coordinate adducts were recorded and compared with those, known for the other thiol and thioether ligands.
- Martirosyan, Garik G.,Hovhannisyan, Astghik A.,Hovhannisyan, Gohar S.,Iretskii, Alexei V.,Kurtikyan, Tigran S.
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- Matrix Isolation Infrared Spectra of Dioxygen Adducts of Iron(II) Porphyrins and Related Compounds
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The O2 stretching bands of the following dioxygen adducts were located in Ar matrices at ca. 15 K: Fe(TPP)O2 (1195 and 1106 cm-1), Fe(OEP)O2 (1190 and 1104 cm-1), Fe(Pc)O2 (1207 cm-1), and Fe(salen)O2 (1106 cm-1).The first two compounds form two isomeric dioxygen adducts whose O2 stretching frequencies are ca. 1190 (isomer I) and ca. 1105 cm-1 (isomer II).Isotope scrambling experiments (16O2 + 16O18O + 18O2) were carried out to elucidate the mode of coordination of dioxygen in these adducts.Neither isomer showed splitting of the 16O18O stertching band under the experimental conditions employed.Normal coordinate calculations predict that splitting should occur if dioxygen coordinates to the metal in the end-on fashion with a FeOO angle larger than 130 deg C.We propose that isomer I prossesses end-on geometry with the FeOO angle smaller than 130 deg C whereas isomer II assumes symmetric side-on geometry.These structures are consistent with the observed relative intensities of the O2 stretching bands, the proposed electronic ground states, and general spectra-structure relationships in a series of base-free dioxygen adducts of iron(II) chelates.
- Watanabe, T.,Ama, T.,Nakamoto, K.
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- Matrix Isolation Infrared Spectra of Carbonyl Complexyes of Iron(II) Tetraphenylporphyrin
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The IR spectra of condensation products of Fe(TPP) with CO/Ar exhibit six ν(CO) bands which are assigned to CO-H2O (2148/2100), monomeric CO (2135/2088), Fe(TPP)(CO)2 (2036/1997), Fe(TPP)(CO))2 (2004/1961), Fe(TPP)(CO)(H2O) (1988/1944), and Fe(TPP)(CO) (1
- Kuroi, T.,Oshio, H.,Nakamoto, K.
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- Mechanism of Catalytic O2 Reduction by Iron Tetraphenylporphyrin
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The catalytic reduction of O2 to H2O is important for energy transduction in both synthetic and natural systems. Herein, we report a kinetic and thermochemical study of the oxygen reduction reaction (ORR) catalyzed by iron tetraphenylporphyrin (Fe(TPP)) in N,N′-dimethylformamide using decamethylferrocene as a soluble reductant and para-toluenesulfonic acid (pTsOH) as the proton source. This work identifies and characterizes catalytic intermediates and their thermochemistry, providing a detailed mechanistic understanding of the system. Specifically, reduction of the ferric porphyrin, [FeIII(TPP)]+ , forms the ferrous porphyrin, FeII(TPP), which binds O2 reversibly to form the ferricsuperoxide porphyrin complex, FeIII(TPP)(O2 ?-). The temperature dependence of both the electron transfer and O2 binding equilibrium constants has been determined. Kinetic studies over a range of concentrations and temperatures show that the catalyst resting state changes during the course of each catalytic run, necessitating the use of global kinetic modeling to extract rate constants and kinetic barriers. The rate-determining step in oxygen reduction is the protonation of FeIII(TPP)(O2 ?-) by pTsOH, which proceeds with a substantial kinetic barrier. Computational studies indicate that this barrier for proton transfer arises from an unfavorable preassociation of the proton donor with the superoxide adduct and a transition state that requires significant desolvation of the proton donor. Together, these results are the first example of oxygen reduction by iron tetraphenylporphyrin where the pre-equilibria among ferric, ferrous, and ferric-superoxide intermediates have been quantified under catalytic conditions. This work gives a generalizable model for the mechanism of iron porphyrin-catalyzed ORR and provides an unusually complete mechanistic study of an ORR reaction. More broadly, this study also highlights the kinetic challenges for proton transfer to catalytic intermediates in organic media.
- Pegis, Michael L.,Martin, Daniel J.,Wise, Catherine F.,Brezny, Anna C.,Johnson, Samantha I.,Johnson, Lewis E.,Kumar, Neeraj,Raugei, Simone,Mayer, James M.
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supporting information
p. 8315 - 8326
(2019/06/04)
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- Hexacoordinate oxy-globin models Fe(Por)(NH3)(O2) react with NO to form only the nitrato analogs Fe(Por)(NH3) (η1-ONO2), even at ~100 K
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The oxy-globin models Fe(Por)(NH3)(O2), prepared by sequential reactions of O2 (18O2) and NH 3 with thin porous layers of FeII(Por), react with NO (15NO) at 80-100 K t
- Kurtikyan, Tigran S.,Ford, Peter C.
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p. 8570 - 8572
(2010/12/20)
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