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.
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