Corrole NH Tautomers: Spectral Features and Individual Protonation

Protonation of a free-base meso-pyrimidinyl-substituted AB2-corrole (H3AB2) in ethanol solution by stepwise addition of sulfuric acid has been studied in the temperature range from 293 to 333 K. The formation rate of protonated species was found to depend profoundly on the temperature at which the titration was undertaken. Two steps in the titration curve were identified at temperatures around 293–298 K, whereas one-step formation of protonated species was found to occur at temperatures above 308 K. The protonation product was the same in both cases, i.e., H4AB2+ corrole, protonated at the macrocycle core nitrogen atoms. The two steps in the protonation kinetics at lower temperatures were attributed to protonation of individual tautomers of the free-base H3AB2 corrole. To the best of our knowledge, this is the first well-illustrated (spectrophotometric) observation of individual properties of corrole NH tautomers in fluid solution. Concomitant increase in the NH tautomerization rate with increasing temperature is proposed to account for the one-step protonation. Evidences for the role of individual corrole NH tautomers in the protonation process as well as their optical features are discussed based on spectroscopic results and simulation data.