5,20-Bis(ethoxycarbonyl)-Substituted Antiaromatic [28]Hexaphyrin and Its Bis-NiII and Bis-CuII Complexes

5,20-Bis(ethoxycarbonyl)-[28]hexaphyrin was synthesized by acid catalyzed cross-condensation of meso-diaryl-substituted tripyrrane and ethyl 2-oxoacetate followed by subsequent oxidation. This hexaphyrin was found to be a stable 28 pi-antiaromatic compound with a dumbbell-like conformation. Upon oxidization with PbO2, this [28]hexaphyrin was converted into an aromatic [26]hexaphyrin with a rectangular shape bearing two ester groups at the edge side. The [28]hexaphyrin can incorporate two Ni-II or Cu-II metals by using the ester carbonyl groups and three pyrrolic nitrogen atoms to give bis-Ni-II and bis-Cu-II complexes with essentially the same dumbbell-like structure. The antiaromatic properties of the [28]hexaphyrin and its metal complexes have been well characterized.