Hunting dimers

Conjugation between 3D and 2D aromaticity: does it really exist? The case of carborane-fused heterocycles

Although several synthesized icosahedral carborane fused 2D π-ring systems are known, and even considerable conjugation has been noted between them in some cases, the phenomenon itself is not fully understood. Based on the results of our computational study, it can be concluded that the 2D aromatic character of the fused (exo) five-membered ring is low, even in cases where significant conjugation was proposed in previous studies. Moreover, the carborane moiety constricts the bonding properties of the exo ring, thus prohibiting or promoting different Lewis resonance structures. These results will shed further light on the design and electronic modulation of new carborane-based materials.

We have demonstrated that carborane fused heterocycles can definitely be considered as non-2D aromatic compounds; therefore, their aromatic character was significantly overestimated in earlier studies.

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.

Global structure of C2B4H4: hypercloso or not

Through an extensive isomeric search utilizing a “skeleton-ligand” cluster-growth strategy, the global minimum of C₂B₄H₄ is found to adopt a ribbon-like structure (01) rather than the previously reported hypercloso structure (04).