Reassessing the Possibility of π–σ–π Full Electron Delocalization Through 3D Aromatic Carboranes

Over the past few decades, considerable attention has been devoted to explore the electronic interactions of various substituents with 3D aromatic carboranes. In the case of substituents possessing available p-orbitals, conjugation between the π-system and the σ-framework of the carborane is possible via negative hyperconjugation, where electron density from a filled π-orbital overlaps with an adjacent σ* orbital. However, this is inherently a local effect, which was further bolstered by the fact that aromatic conjugation between planar (2D) π-systems and the 3D aromatic carborane cage does not exist. Nevertheless, recent studies have raised the intriguing possibility that 3D aromatic carboranes may function as σ-conjugated bridges, enabling electronic communication between spatially separated π-systems. Herein, we investigated the potential conjugation of various substituents with or through carborane systems to better understand the nature of the possible interaction. We demonstrated that significant delocalization through the carborane cluster can be achieved only in the case of radical systems; however, this type of interaction is not a unique feature of carboranes, as similar behavior can also be observed in saturated hydrocarbons.