SUPRAMOLECULAR ENTROPY GAINS - SIMPLE AND SUPERSTRUCTURED IRON PORPHYRINS

The, determination of the enthalpy-energy balance of the reaction Fe(I)- --> Fe("O")2- in simple and superstructured iron porphyrins makes apparent that entropy factors play a crucial role in the modulation of the reactivity by the molecular superstructures. In the case of the ether-linked structures, the decrease of reactivity mainly derives from the rigidification of the molecular superstructure in the reaction. With secondary amide-containing structures, the increase in reactivity derives essentially from a decrease of the enthalpy-entropy compensation: solvent-change interactions are substituted by NHCO dipole charge interactions, and the resulting increase in entropy arises from the fact that the NHCO dipoles fluctuate much less than solvent molecules. The increase in reactivity brought about by the NHCO-containing superstructures offers a remarkable example of a system where entropy expenses allowed during the synthesis of the molecular superstructures result in entropy gains in the reactivity of the active molecular center.