Exceptional stability of azacalixphyrin and its dianion

Azacalixphyrin is a recently synthesized precursor of potentially highly versatile analogues of porphyrins. Surprisingly, this macrocyclic compound is stable to such an extent that it could be exposed to air for months as a solid or for days in solution without detectable changes. However, no rationalization of this specific property has been established yet as the structure/electronic features of this novel compound are still very much unknown. Here, we present an extensive theoretical study on the stability of azacalixphyrin. We use density functional theory to quantify both its aromaticity and its reactivity, or more precisely its lack thereof, with water. In addition, since we find that neutral azacalixphyrin has the ability to capture easily two electrons, we also discuss the results obtained for its dianion. We show that the azacalixphyrin core is strongly aromatic despite the antiaromatic peripheric rings, and we find that all reactions with water are extremely endergonic, hence explaining the stability of azacalixphyrin. Upon reduction, the aromatic signature is reversed as the center becomes antiaromatic while the antiaromatic character of the peripheric rings is either remarkably reduced or completely annihilated. Accordingly, we find that the reactivity of the dianion with water is considerably inhibited.