3-D aromaticity is not as usual as it may seem

The aromaticity of 3-D closo boranes, zintl ions, and charged fullerenes is widely accepted. On the other hand, several fully p-conjugated macrocycles with puckered or cage-type structures were recently found to exhibit aromatic character according to both experiments and computations. We examine their electronic structures and put them in relation to 3D-aromatic molecules (e.g., closo-boranes) and to 2D-aromatic polycyclic aromatic hydrocarbons. Using qualitative theory combined with quantum chemical calculations, we find that the macrocycles explored hitherto should be described as 2D-aromatic with three-dimensional molecular structures (abbr. 2D-aromatic-in-3D) and not as truly 3D-aromatic. We establish the conditions that have to be fulfilled to classify a compound as 3D-aromatic. Indeed, we show that it is not easy to design new 3D-aromatic compounds. This work was carried out by Dr. Ouissam El Bakouri, Dr. Kjell Jorner, Dr. Rabia Ayub and Prof. Henrik Ottosson from the Ångström Laboratory of the Uppsala University, Prof. Patrick Bultinck from the Department of Chemistry of Chent University and Dr. Dariusz W. Szczepanik and Prof. Miquel Solà from the DiMoCat group of the Institute of Computational Chemistry and Catalysis of the University of Girona

El Bakouri, D. W. Szczepanik, K. Jorner, R. Ayub, P. Bultinck, M. Solà* and H Ottosson*. Three-Dimensional Fully p-Conjugated Macrocycles: When 3D-Aromatic and when 2D-Aromatic-in-3D? J. Am. Chem. Soc., 2022, DOI: 10.1021/jacs.1c13478.

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Many molecules mislabelled as 3D aromatic