Hydrogen bonding in metalloporphyrins. Mechanistic study of the reactions of (tetraphenylporphinato)iron(III) azide with imidazole and N-methylimidazole

Abstract

The reaction of Fe(TPP)N3 with imidazole (Him) and (V-methylimidazole (Melm) 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 Melm 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' = *11 11/2 -* s/2, and this is manifested in loss of CFSE (large AH*) and a AS* about 15 cal deg"1 mol"1 more positive than those found for analogous metalloporphyrin reactions that do not feature a spin change.

Positive Charge on Porphyrin Ligand and Nature of Metal Center Define Basic Physicochemical Properties of Cationic Manganese and Iron Porphyrins in Aqueous Solution

“…0.007 s –1 at very high concentration of the ligand (see Figure S5B). Although it is rather unusual behavior, a few examples can be found in the literature . Such ligand concentration dependence of k obs2 indicates that the binding of the second imidazole molecule must be preceded by an equilibrium that is not dependent on the [Im] and represents the rate-determining step for the binding of the second Im molecule (see the SI for the applied analysis procedure and detailed discussion of the second binding reaction).…”

Reactivity of chloro(phthalocyaninato)iron(III) in pyridine–water

“…Jt is well known that the phosphine acts as an efficient oxygentrapping agent towards the structurally similar oxoporphyrinatoiron(rv), giving OPPh, and the corresponding porphyrin-Also the kinetic effects of added PPh, may be reconciled with the suggested mechanism. Thus, the type B experiments might be explained on the basis of equations (1 7H20) for the first phosphine-dependent process and equation (21) for the second, phosphine-independent reaction.…”

“…This means that, in the presence of relatively large amounts of PPh,, the oxidised product obtained during the first step would be [Fe(pc)(py)(OPPh,)]. Hence, the second step would correspond to reaction (21), in turn to be identified with the phosphine-independent reaction of type B experiments, and, since the latter has a rate constant very close to that of the first step, the whole process could well appear as occurring as a single kinetic step.14 This interpretation is also consistent with the observed shift towards 100% yield of [Fe(pc)(py),] when the reaction is carried out in the presence of PPh,.…”

Structural characterization of the μ-nitrido complex {[Fe(OEP)]2N}

“…All other chemicals were used as received from Aldrich or Fisher. [Fe(OEP)Cl] and [Fe(OEP)(N 3 )] were synthesized by literature methods [7, 8]. UV-vis spectra were recorded on a Perkin-Elmer Lambda 19 spectrometer and IR spectra on a Perkin-Elmer model 883 as KBr pellets.…”