A flat carborane with multiple aromaticity beyond Wade-Mingos' rules

It is widely known that the skeletal structure of clusters reflects the number of skeletal bonding electron pairs involved, which is called the polyhedral skeletal electron pair theory (PSEPT) or Wade and Mingos rules. While recent computational studies propose that the increase of skeletal electrons of polyhedral clusters leads to the flat structure beyond the PSEPT, little experimental evidence has been demonstrated. Herein, we report the synthesis of a C₂B₄R₄ carborane 2 featuring a flat ribbon-like structure. The C₂B₄ core of 2 bearing 16 skeletal electrons in the singlet-ground state defies both the [4n + 2] Hückel's rule and Baird's rule. Nevertheless, the delocalization of those electrons simultaneously induces two independent π- and two independent σ-aromatic ring currents, rendering quadruple aromaticity.