Zinc ion coordination to porphyrins with different architecture of peripheral substitution has been studied in acetonitrile solution at room temperature. Considering the metalloporphyrin formation rate as a function of the overall (for two deprotonation steps) acidity constant pK a1,2 value the hysteresis relationship has been found for the first time. It is suggested that the hysteresis is due to an inversion of the rate limiting steps in the transition state: one branch is formed due to higher rate of the proton removal which is ultimately followed by the metal ion binding, whereas the metal ion binding rate is much higher compared to the proton removal for the other. Thermodynamic analysis of the metal ion coordination reaction in terms of formation of transition state gave rise to conclusion that the enthalpy-entropy compensation takes place. This effect is also observed for the first time, and the origin of the correlation between the enthalpy ΔH ≠ and entropy ΔS ≠ of activation of the transition state was proposed due to the substitution electronic effect.
