NAl₄X₄ ⁺ (X = S, Se, Te): Clusters with a planar tetracoordinate nitrogen and significantly improved stability

The design of clusters featuring non-classical planar hypercoordinate atoms (phAs) often depends on the delocalized multicenter bonds involving reactive electron-deficient elements, which both destabilize the clusters and lead to difficulty in achieving the phA arrangement for electronegative elements such as nitrogen due to their preference for localized bonds. In this work, we computationally designed a series of aluminum chalcogenide clusters NAl₄X₄ ⁺ (X = S, Se, Te) with a desired planar tetracoordinate nitrogen and meaningfully improved chemical stability, as evidenced by the wide gaps (6.51-7.23 eV) between their highest occupied molecular orbitals and lowest unoccupied molecular orbitals, high molecular rigidity (dynamically stable up to 1500 K), and exclusively low global energy minima nature (their isomers locate at least 51.2 kcal/mol higher). Remarkably, these clusters are stabilized by peripheral chalcogen atoms, which not only sterically protect the NAl₄ core moiety but also electronically compensate for the electron-deficient aluminum atoms via X → Al π back bonds, meeting the description of our recently proposed "electron-compensation" strategy.