Carbocation, diradical, and superelectrophile in one molecule?

The pentafluorophenyl cation (C₆F₅⁺) breaks these rules with a borderline “crazy” reactivity.

Phenyl cations (C₆H₅⁺) usually possess a closed-shell singlet ground state with a triplet state about 1 eV higher in energy. Their reactivity depends on spin: the singlet is an indiscriminate electrophile, even attacking nitrogen (N₂) or solvents, while the triplet is chemoselective and preferentially reacts with π nucleophiles such as alkynes and aromatics. However, triplet states decay within a few picoseconds, making their reactivity nearly impossible to study.

Perfluorination inverts the usual electronic ordering compared to the parent phenyl cation. The resulting pentafluorophenyl cation is so electrophilic that it can even extract electrons from noble gases such as xenon, one reason why it evades detection in condensed phases. In this study, we report the first observation of C₆F₅⁺ in the gas phase, produced from thermally generated pentafluorophenyl radicals using vacuum-ultraviolet synchrotron radiation. Ionization of the radical yields the cation with an adiabatic ionization energy of 9.84 ± 0.02 eV, as measured by our isomer-selective spectroscopy. High-level ab initio calculations combined with synchrotron-based experiments reveal that perfluorination twists the electronic structure: the cation is best described as a diradical with an open-shell singlet (¹A₂) ground state, while the triplet (³A₂) lies only 1.5 ± 0.4 kcal mol⁻¹ (0.07 eV) higher. This explains its borderline “crazy” reactivity, reflected in a ~40 kcal mol⁻¹ higher hydride affinity than the parent phenyl cation. This discovery challenges how we think about carbocations and may open the door to designing new superelectrophiles in synthesis.

We also investigated the high-temperature reactivity of the radical, which undergoes fluorine detachment leading to complete destruction of the aromatic ring. Together, these results provide novel insights into the reactivity of elusive intermediates and deliver valuable benchmarks for computational chemistry as well as input for the development of thermochemical databases.