Cubic Aromaticity: Fact or Fiction?

Although silicon clusters (Sin) exhibit a diversity of 3-dimensional shapes, a cubic form has not been found to be stable in their small size of 8 Si atoms. While the cubane C8H8 is wellknown for chemists, the octasilacubane Si8H8 possessing a cubic structure has never been synthesized. Using quantum chemical computations we find that doping can lead to stable cubic silicon clusters such as Be@Si8, B@Si8, C@Si8... Analyses of phenomenological shell model (PSM) combined with partial electron localizability indicators (pELI-D) and atomic charges reveal three conditions for the existence of a cubic cluster, namely a full occupancy of electronic shells (34 electrons), the presence of a positive charge at the center, and a new type of spherical aromaticity. The aromaticity in these cubic systems stems from eight delocalizing electrons distributed on the outward sp lobes of Si atoms. The clusters of the heavier congeners Ge and Sn also have a comparable behaviour. Although the 8-electron cube systems feature a distinct type of spherical aromatic character, the question of interest is as to whether a “cubic aromaticity” is a fact of life.