Methane Activation by B₅ ⁺ Toward a CB₅H₂ ⁺ Isomer Containing a Planar Tetracoordinate Carbon Atom

Planar tetracoordinate carbon (ptC) motifs have been identified predominantly in the global-minimum structures of gas-phase clusters generated through laser ablation, limiting access to alternative kinetically persistent structures. Here, gas-phase reactions of mass-selected B₅ ⁺ clusters with methane at room temperature yield CB₅H₂ ⁺ through H₂ elimination, as established by time-of-flight mass spectrometry, isotope labeling, and kinetic measurements. Electronic-structure calculations identify a low-lying ptC-containing isomer accessible from the nascent dehydrogenation product through a multistep rearrangement pathway. Microcanonical analysis indicates that a fraction of the nascent dehydrogenation product population can overcome the highest barrier before collisional deactivation. Once collisionally stabilized, this species is predicted to be kinetically persistent under the experimental conditions. Its bonding pattern combines localized C-B interactions with multicenter σ and π delocalization across the CB₅ framework. These results establish CB₅H₂ ⁺ formation and provide computational support for access to a low-lying ptC-containing system.