Ligand Binding by Metalloporphyrins. I. Thermodynamic Functions of Porphyriniron( II)-Pyridine Complexes

Enthalpy, entropy, and free energy values for the addition of a series of substituted pyridine ligands to a number of porphyriniron(II) dimethyl esters in benzene solution were measured spectrophotometrically. Variations in free energy of reaction with the changes in ligand and metalloporphyrin were not great but changes in enthalpy and entropy of reaction were large. The results obtained best fit a model in which the ligand is bound by both σ bonding from the pyridine nitrogen to the metal atom and π bonding by back-donation from the iron atom to the pyridine. While π bonding becomes more important in the order 4-methyl \u3c 4-vinyl \u3c pyridine \u3c 4-carboxybutyl ester \u3c 4-cyanopyridine, σ bonding is most important for 4-methylpyridine and decreases in importance in the same order, π bonding was not as important for 2,4-diacetyldeuteroporphyriniron(II) dimethyl ester as it was for the meso-, deutero-, and protoporphyrin complexes. Entropies of reaction were large and variable and were well correlated with the enthalpy changes observed. This and the compensatory nature of the entropy change in relation to the enthalpy change gave an isoequilibrium temperature of 340 °K. The close proximity of this to the temperature at which the reactions were studied led to the small changes observed in the free energy of reaction