Three-Dimensional\nFully π‑Conjugated\nMacrocycles: When 3D-Aromatic and When 2D-Aromatic-in-3D?

Abstract

Several fully π-conjugated\nmacrocycles with puckered or cage-type\nstructures were recently found to exhibit aromatic character according\nto both experiments and computations. We examine their electronic\nstructures and put them in relation to 3D-aromatic molecules (<i>e.g.</i>, <i>closo</i>-boranes) and to 2D-aromatic\npolycyclic aromatic hydrocarbons. Using qualitative theory combined\nwith quantum chemical calculations, we find that the macrocycles explored\nhitherto should be described as 2D-aromatic with three-dimensional\nmolecular structures (abbr. 2D-aromatic-in-3D) and not as truly 3D-aromatic.\n3D-aromatic molecules have highly symmetric structures (or nearly\nso), leading to (at least) triply degenerate molecular orbitals, and\nfor tetrahedral or octahedral molecules, an aromatic closed-shell\nelectronic structure with 6<i>n</i> + 2 electrons. Conversely,\n2D-aromatic-in-3D structures exhibit aromaticity that results from\nthe fulfillment of Hückel’s 4<i>n</i> + 2\nrule for each macrocyclic path, yet their π-electron counts\nare coincidentally 6<i>n</i> + 2 numbers for macrocycles\nwith three tethers of equal lengths. It is notable that 2D-aromatic-in-3D\nmacrocyclic cages can be aromatic with tethers of different lengths, <i>i.e.</i>, with π-electron counts different from 6<i>n</i> + 2, and they are related to naphthalene. Finally, we\nidentify tetrahedral and cubic π-conjugated molecules that fulfill\nthe 6<i>n</i> + 2 rule and exhibit significant electron\ndelocalization. Yet, their properties resemble those of analogous\ncompounds with electron counts that differ from 6<i>n</i> + 2. Thus, despite the fact that these molecules show substantial\nπ-electron delocalization, they cannot be classified as true\n3D-aromatics.