Protonated free-base corroles: acidity, electrochemistry, and spectroelectrochemistry of [(Cor)H4]+, [(Cor)H5]2+, and [(Cor)H6]3+

Protonated meso-substituted free-base macrocycles of the form (Cor)H4]+, (Cor)H5]2+, and (Cor)H6]3+ where Cor is the trianion of a given corrole, were chemically generated from neutral (Cor)H3 in benzonitrile by addition of trifluoroacetic acid (TFA) and characterized as to their relative acidity, electrochemistry, and spectroelectrochemistry. Three types of protonated free-base corroles with different electron-donating or electron-withdrawing substituents at the meso positions of the macrocycle were investigated. One is protonated exclusively at the central nitrogens of the corrole forming (Cor)H4]+ from (Cor)H3, while the second and third types of corroles undergo protonation at one or two meso pyridyl substituents prior to protonation of the central nitrogens and give as the final products (Cor)H5]2+ and (Cor)H6]3+, respectively. Altogether the relative deprotonation constants (pKa) for 10 different corroles were determined in benzonitrile and analyzed with respect to the molecular structure and/or type of substituents on the three meso positions of the macrocycle. Mechanisms for oxidation and reduction of the protonated corroles are proposed in light of the electrochemical and spectroelectrochemical data.