Edge-condensation to access a crystalline planar icosagen cation with σ-aromaticity

Parallel to the hexagonal C₆ motif in carbon materials (such as graphene and carbon nanotubes) and polyaromatic hydrocarbons, three-membered boron and aluminum rings are the smallest key units seen in their clusters and materials (such as borophene). Herein, we report an edge-condensation protocol of a B₃ ring to render a cationic icosagen species. A single-crystal X-ray diffraction analysis and computational studies reveal that the planar B₃Al₂ core consists of three consecutive three-center two-electron bonds. Remarkably, the icosagen ring exhibits global σ-aromaticity. The reactions with a cationic gallium and a copper complex afford a dicationic B₃Al₂ species assimilating a Ga atom by coordination with two borafluorene ligands and a dinuclear copper complex bearing a η²-dianionic diborane ligand, respectively, demonstrating the π-donor property as well as the capability of transferring the dianionic diborane fragment.