Planar tetracoordinate beryllium in σ-aromatic Li4Be and Na4Be clusters: A missing member in first-octal row planar tetracoordinate family

While planar tetracoordinate (pt) centers have been extensively explored from carbon to other octal-row elements or their heavier analogs, their counterparts involving alkali (A) and alkaline-earth metals (Ae) remain elusive due to the large atomic radius and absence of p orbitals. In this work, we found six hitherto unknown anionic ptA (A4A-) and neutral ptAe (A4Ae) centers through an extensive exploration of potential energy surfaces. The D4h-symmetry ptBe structures in Li4Be and Na4Be emerge as the lowest-energy configurations, and all the other ptA/ptAe structures are higher in energy or saddle points. The global-minimum ptBe structure can be described as Be- with a 2s12px12py1 electronic configuration, forming three σ electron sharing interactions with quartet Li4+/Na4+ motifs. The delocalized σ orbitals contribute to σ aromaticity, thereby enhancing the overall stability of these intriguing title ptBe species. Furthermore, these ptBe systems can be encapsulated within the [n]cycloparaphenylene nanoloop (n = 7, 8) thermochemically spontaneously, without any disturbance in planarity in the ptBe moiety, where the systems get stabilized by a predominant electrostatic interaction between Li4/Na4 and the nanoloop.