- Reaction of Sterically Hindered Imidazolate Complexes with Iron Porphyrins
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Reaction of sterically hindered copper(II) or Ni(II) imidazolate complexes derived from imidazole-2-carbaldehyde with iron porphyrins results in the formation of monoadducts which are dinuclear complexes.The spin states of the iron atoms in some of the ad
- Brewer, Cynthia T.,Brewer, Greg
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- Replacement of imidazoles by azide at an iron(III) porphyrin center: Part 1 replacement of N-methylimidazole in bis-(N-methylimidazole) tetraphenylporphyrinato iron(III)
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Addition of tetra-n-butylammonium azide to acetone solutions of the tetraphenylporphyrinato iron(III) complex [Fe(TPP)(MeIm)2] +N-3 formed in situ from the reaction of N-methylimidazole and [Fe(TPP)N3] afforded equilibrium amounts of [Fe(TPP)(MeIm)2]+ and [Fe(TPP)(MeIm)N3]. Equilibrium experiments made using a range of known concentrations of added tetra-n-butylammonium azide and N-methylimidazole using stopped-flow apparatus gave an estimate of about 50 for the equilibrium constant for the formation of [Fe(TPP)(MeIm)N3] from the addition of azide to [Fe(TPP)(MeIm) 2]+. Kinetic studies indicated that the substitution of azide ion by N-methylimidazole is a dissociative process, and the results were interpreted using a stationary state approach in which [Fe(TPP)(MeIm)] + was the transient intermediate that discriminated in favor of the reaction with azide as opposed to the reaction with N-methylimidazole by about a factor of two. Loss of N-methylimidazole from [Fe(TPP)(MeIm)2] + is at least 50 times faster than that from the product [Fe(TPP)(MeIm)N3]. Using calculated values of free azide concentrations from experimentally determined ion-pair formation constants led to success in rationalizing results obtained under different conditions.
- Jones, John G.,McKnight, John,Twigg, Martyn V.
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experimental part
p. 2766 - 2778
(2010/12/25)
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- Hydrogen bonding in metalloporphyrins. Mechanistic study of the reactions of (tetraphenylporphinato)iron(III) azide with imidazole and N-methylimidazole
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The reaction of Fe(TPP)N3 with imidazole (HIm) and N-methylimidazole (MeIm) has been studied in acetone and dichloromethane. Kinetic measurements at room temperature as well as low-temperature spectroscopic, conductivity, and electrochemical studies were used to fully characterize the intermediate complex Fe(TPP)(RIm)N3 as six-coordinate and low spin. This complex reacts further to give Fe(TPP)(RIm)2+N3-. The rate-limiting step in the overall reaction is azide ionization from Fe(TPP)(RIm)N3 to give the high-spin Fe(TPP)(RIm)+N3-. The activation free energy of this step is ca. 3 kcal lower with HIm compared to that with MeIm because of hydrogen bonding to the departing azide ion in the transition state; this acceleration via hydrogen bonding is an entropic effect. A detailed comparison of M(Por-)X systems is presented for M = Fe and Co, Por = TPP, PPIX, and PPIXDME, and X = F-, Cl-, Br-, and N3-. The importance of spin changes on the kinetics and thermodynamics of intermediate and product formation is quantified. Hydrogen-bonding effects are found to have a greater influence on the kinetics than on the thermodynamics. The spin change for the reaction Fe(TPP)(RIm)N3 → Fe(TPP)(RIm)+N3- is S = 1/2 → 5/2, and this is manifested in loss of CFSE (large ΔH?) and a ΔS? about 15 cal deg-1 mol-1 more positive than those found for analogous metalloporphyrin reactions that do not feature a spin change.
- Byers,Cossham,Edwards,Gordon,Jones,Kenny,Mahmood,McKnight,Sweigart,Tondreau,Wright
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p. 4767 - 4774
(2008/10/08)
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