Halogenated C5H5X (X = Cl, Br, and I) heterocycles: unprecedented halogen-containing Lewis acids bridging halobenzenes and halonium cations.

While the halogen bond is a well-established concept in contemporary chemistry, hypervalent halogen bond donors represent a relatively uncharted territory for exploration. Here, an unusual X-ray structure of 2,7-dinitro-10-p-nitrophenyl-9-iodonia-10H-anthracene thiocyanate serves as a spark for studying halogenated heterocyclic C5H5X (X = Cl, Br, and I) compounds, which may arise as a novel group of σ-hole donors in halogen bonds. This collection of systems exhibits an intriguing intramolecular bonding structure, characterized by two resonant dative C-X bonds. Such structures share common traits with two other cyclic groups: haloarenes and halonium cations. They exhibit neutrality analogous to C6H5X halobenzenes; however, the halogen atom participates in two C-X bonds, as observed in the [H5C6-X-C6H5]+ halonium cations, resulting in the formation of two σ-holes at the halogen atom. These cations are therefore promising Lewis acids, distinguished by σ-hole maxima within the range of 15-24 kcal mol-1. Halogenated heterocycles can form stable complexes with ammonia, a prototypical Lewis base, showing CCSD(T) interaction energies in the range of -4.4 to -2.6 kcal mol-1, positioning them between halobenzenes and halonium cation dimers. Our results set a challenge for researchers in experimental chemistry to synthesize novel compounds that can be potent σ-hole donors.