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

Over the past few decades, considerable attention has beendevoted to explore the electronic interactions of various sub-stituents with 3D aromatic carboranes. In the case of sub-stituents possessing available p-orbitals, conjugation betweenthe π-system and the σ -framework of the carborane is possi-ble via negative hyperconjugation, where electron density froma filled π-orbital overlaps with an adjacent σ * orbital. How-ever, this is inherently a local effect, which was further bolsteredby 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 possibil-ity that 3D aromatic carboranes may function as σ -conjugatedbridges, enabling electronic communication between spatiallyseparated π-systems. Herein, we investigated the potential con-jugation of various substituents with or through carboranesystems to better understand the nature of the possible inter-action. We demonstrated that significant delocalization throughthe carborane cluster can be achieved only in the case of radicalsystems; however, this type of interaction is not a unique fea-ture of carboranes, as similar behavior can also be observed insaturated hydrocarbons