Theoretical AM1 study of acidity of porphyrins, azaporphyrins and porphyrazines

Abstract

The structure-acidity relationship in the series of non-substituted, meso- or β-substituted and β,β-annulated porphyrins and porpyrazines have been studied using the AM1 method with unrestricted Hartree-Fock basis set. With this purpose, heats of formation have been determined for the geometry optimized structures of the free base macrocycles and corresponding monoanions and dianions formed by deprotonation. Calculated first deprotonation enthalpy values show correlation with available experimental p K1avalues and can be used for prediction of acidity. For porphyrazines bearing electron-withdrawing substituents or π-deficient annulated heteroarens the dianions have lower heats of formation than the corresponding neutral species and such porphyrazines are easily deprotonated upon dissolution in basic solvents (pyridine, DMF). For porphyrazines with annulated 5-member heteroarens it is predicted that deprotonation of peripheral NH groups should occur more easily than deprotonation of the internal NH groups. The influence of different types of annulation of 5- and 6-membered heteroarens to the porphyrazine core on the stability of the macrocyclic system and its acidity are also discussed.