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

Gao, Yanhong and Szathmári, Balázs and Buzsáki, Dániel and Kelemen, Zsolt (2025) Reassessing the Possibility of π–σ–π Full Electron Delocalization Through 3D Aromatic Carboranes. CHEMISTRY-A EUROPEAN JOURNAL, 31 (41). No.-e202501806. ISSN 0947-6539

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Abstract

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

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