Haloniumionins

Haloniumionins are a recently characterized class of halogenated nitrogen-containing ions that combine the electrophilic halonium core (such as bromonium or iodonium) with a pendant amine functionality. First reported in 2018 during a survey of ionospheric plasma, they have since been isolated in laboratory syntheses and detected in trace atmospheric samples.

Structurally, a haloniumionin features a three‑atom halonium ring bridged by a halogen atom (Cl, Br, I) and two carbon atoms, with an exocyclic nitrogen atom bonded to one of the ring carbons. Resonance between the halonium cation and the adjacent amino group affords significant charge delocalization, which stabilizes the species and reduces its reactivity relative to simple halonium ions. Synthesis typically proceeds by reacting a haloalkane with a primary or secondary amine in the presence of an oxidant such as m‑CPBA or trifluoroacetic acid, which facilitates halonium formation and concurrent N‑substitution.

The ion shows a characteristic C–X–C bond angle of approximately 90°, and the halogen insertion alters the electron density of the nitrogen, rendering it moderately basic. Haloniumionins display high stability in polar aprotic solvents and can act as mild halogenating agents in organic transformations, providing a controlled transfer of the halogen to electron-rich substrates. Because of the nitrogen component, these ions can also serve as electrophilic coupling partners in cross‑coupling reactions, opening new pathways for the synthesis of heterocycles.

Industrial interest focuses on their potential as safer fluorinating reagents, particularly for the introduction of trifluoromethyl groups in pharmaceutical intermediates. In atmospheric chemistry, haloniumionins are presumed to contribute to the formation of ozone‑depleting compounds under specific photochemical conditions. Current research aims to characterize their kinetic behavior, expand the halogen repertoire beyond iodine, and evaluate their environmental persistence.