Kinetics and Thermodynamics of the Reactions of Two Iron (III) Porphyrins with Imidazole and 1-Methylimidazole1 in Dimethyl Sulfoxide

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The reactions of tetraphenylporphinatoiron (III) chloride and hemin chloride with imidazole and 1-methylimidazole have been studied in dimethyl sulfoxide at μ = 0.04 M (NaNO3). Stability constants have been determined for the reactions SFeP + 2L ⇌ L2FeP + S over a range of temperatures. There was no evidence for appreciable concentrations of LFeP in any of the systems studied. For FeTPP+: ΔH° = -10.7 kcal/mol, ΔS° = -13.8 eu for imidazole and ΔH° = -10.2 kcal/mol, ΔS° = -15.4 eu for 1-methylimidazole. For hemin: ΔH° = -8.0 kcal/mol, ΔS° = -4.6 eu for imidazole and ΔH° = -9.2 kcal/ mol, ΔS° = -10.9 eu for 1-methylimidazole. Although the thermodynamic parameters are quite similar for each of the metalloporphyrins with a given ligand, the kinetic features differ markedly. The hemin reactions are faster than those of FeTPP+ and while τ-1 = kf [L]2 + kr for FeTPP+ with both ligands at all temperatures studied, for hemin the inverse relaxation time varies as [L]2 at low concentrations of ligand only. At higher concentrations, the reaction approaches a first-order dependence on ligand. It is suggested that this more complicated kinetic profile for hemin reflects an alternative pathway to the formation of the diliganded adduct to the one applicable to the other iron (III) porphyrins thus far studied. For hemin, we suggest an activated complex in which the second ligand interacts with the iron atom before the spin state change has occurred. © 1978, American Chemical Society. All rights reserved.

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Journal of the American Chemical Society

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