All are aromatic: A 3D globally aromatic cage containing five types of 2D aromatic macrocycles

The studies on three-dimensional (3D) aromaticity have been mainly focused on fullerenes, boron-based deltahedrons/clusters, metal clusters, and polyhedral hydrocarbons, but there is very limited research on the fundamental aromaticity rule for 3D fully pi-conjugated molecules. Herein, we report a pi-conjugated molecular cage in which two aromatic porphyrin units are bridged by four thiophene-based arms. Two-electron chemical oxidation leads to a 3D globally aromatic cage with a C-2 symmetry according to X-ray diffraction, NMR, electronic absorption spectra, and theoretical calculations. Detailed magnetic shielding response analysis along different axes reveals that all the possible five types of two-dimensional (2D) macrocycles in the cage skeleton are aromatic and follow the Hiickel rule. The switch from localized aromaticity to global aromaticity upon chemical oxidation is also observed in a tricyclic model compound. This study indicates that 3D global aromaticity in a molecular cage can be explained by 2D Huckel aromaticity of the individual pi-conjugated macrocycles.

Automated summary

Researchers reported a globally aromatic cage, showing a transition from localized aromaticity to global aromaticity through chemical oxidation. The 2D macrocycles in the cage skeleton exhibit aromaticity and follow the Hiickel rule.