Oxidation-induced σ-aromaticity in halogenated cycloalkanes

In this study, we investigated a series of dicationic halogenated cycloalkanes with the general formula C_nH_nX_n²⁺ (n = 4, 5, 6, and 7, and X = Br and I), where halogen substituents are positioned on each carbon atom, forming a cyclic structure. To assess potential σ-aromaticity within the halogen atom rings, we employed magnetically induced current density (MICD) calculations, the electron density of delocalized bonds (EDDB) and aromatic stabilization energy (ASE), as an energetic descriptor. Our results revealed significant σ-electron delocalization throughout the halogen atom rings, consistently supported by all applied aromaticity indices. This way, we showed that nonaromatic, neutral halogenated cycloalkanes can be converted into σ-aromatic dicationic species through conventional oxidation procedures. Furthermore, we demonstrated that this σ-electron delocalization closely resembles that observed in the prototypical double aromatic system C₆I₆²⁺. Interestingly, the observed σ-aromaticity found in the studied dications does not always conform to Hückel's rule. These results can be rationalized in the context of selection rules governing virtual transitions between occupied and unoccupied molecular orbitals.