Protonation-Induced Antiaromaticity in Octaaza[8]circulenes: Cyclooctatetraene Scaffolds Constrained with Four Amidine Moieties.

The switching of cyclic π-conjugation pathways using external stimuli is an attractive research topic in the field of organic chemistry. Here, we synthesized C 4h -symmetric octaaza[8]circulenes with four peripherally arranged amidine moieties that exhibit enhanced antiaromaticity upon protonation. Titration experiments with methanesulfonic acid revealed the formation of the tetraprotonated forms of the octaaza[8]circulenes in solution. Single-crystal X-ray diffraction analyses and theoretical calculations indicated that the contribution of the 8π antiaromatic character of the octaaza[8]circulenes is enhanced by the delocalization of charge through the protonation of the pyridine rings.