Group 13 five- and six-membered rings with rare 2π aromaticity

According to Hückel's rule, cyclic species are aromatic if they have 4n + 2 (n = 0, 1, 2, etc.) π electrons. However, large aromatic rings (atom number > 4) with minimum 2π electrons (i.e., n = 0) are rather rare because of the structural instability stemming from the deficient π electrons compared to the ring size. To date, the largest 2π-aromatic ring is a five-membered Ga5 core reported in a recent experiment. Herein, density functional theory calculations predicted seven inverse-sandwich Na2(MH)5 and half-sandwich Ca(MH)5 or Ca(MH)6 (M = Al, Ga or In) structures. They all have planar central five- or six-membered Al/Ga/In rings, rather negative binding energies and large HOMO–LUMO gaps. Their dianionic metal rings exhibit obvious aromatic characters and appreciable diatropic ring currents due to the good delocalization of 2π electrons donated by the Na/Ca metals. Interestingly, they also have novel collective bonding with the Na/Ca atoms interacting with both the metal ring and surrounding H atoms. These metalloaromatic rings not only greatly enrich the precious family of 2π aromatics, but also increase the maximum ring atom number from five to six, thus paving the way for Hückel-type 2π-aromatic large rings.