Three-Dimensional Fully pi-Conjugated Macrocycles: When 3D-Aromatic and When 2D-Aromatic-in-3D?

Several fully ?-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. 3D-aromatic molecules have highly symmetric structures (or nearly so), leading to (at least) triply degenerate molecular orbitals, and for tetrahedral or octahedral molecules, an aromatic closed-shell electronic structure with 6n + 2 electrons. Conversely, 2D-aromatic-in-3D structures exhibit aromaticity that results from the fulfillment of Hückel’s 4n + 2 rule for each macrocyclic path, yet their ?-electron counts are coincidentally 6n + 2 numbers for macrocycles with three tethers of equal lengths. It is notable that 2D-aromatic-in-3D macrocyclic cages can be aromatic with tethers of different lengths, i.e., with ?-electron counts different from 6n + 2, and they are related to naphthalene. Finally, we identify tetrahedral and cubic ?-conjugated molecules that fulfill the 6n + 2 rule and exhibit significant electron delocalization. Yet, their properties resemble those of analogous compounds with electron counts that differ from 6n + 2. Thus, despite the fact that these molecules show substantial ?-electron delocalization, they cannot be classified as true 3D-aromatics.

It is a collaboration of Ouissam El Bakouri and Miquel Solà with the groups of Henrik Ottosson (Uppsala University), Patrick Bultinck (Ghent University) and Dariusz W. Szczepanik (Jagiellonian University) and was recently published in Journal of the American Chemical Society:

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

Girona, May 10, 2022
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