3-D molecular stars with covalent axial bonding

In designing three-dimensional (3-D) molecular stars, it is very difficult to enhance the molecular rigidity through forming the covalent bonds between the axial and equatorial groups because corresponding axial groups will generally break the delocalized pi bond over equatorial frameworks and thus break their star-like arrangement. In this work, exemplified by designing the 3-D stars Be-2 (c) Be5E5+ (E = Au, Cl, Br, I) with three delocalized sigma bonds and delocalized pi bond over the central Be-2 (c) Be-5 moiety, we propose that the desired covalent bonding can be achieved by forming the delocalized sigma bond(s) and delocalized pi bond(s) simultaneously between the axial groups and equatorial framework. The covalency and rigidity of axial bonding can be demonstrated by the total Wiberg bond indices of 1.46-1.65 for axial Be atoms and ultrashort Be-Be distances of 1.834-1.841 angstrom, respectively. Beneficial also from the sigma and pi double aromaticity, these mono-cationic 3-D molecular stars are dynamically viable global energy minima with well-defined electronic structures, as reflected by wide HOMO-LUMO gaps (4.68-5.06 eV) and low electron affinities (4.70-4.82 eV), so they are the promising targets in the gas phase generation, mass-separation, and spectroscopic characterization.