Binding of saturated and unsaturated C6-hydrocarbons to the electrophilic anion [B12Br11]−: a systematic mechanistic study

Abstract

The highly reactive gaseous ion [B₁₂Br₁₁]⁻ is a metal-free closed-shell anion which spontaneously forms covalent bonds with hydrocarbon molecules, including alkanes. Herein, we systematically investigate the reaction mechanism for binding of [B₁₂Br₁₁]⁻ to the five hexane isomers yielding [B₁₂Br₁₁(C₆H₁₄)]⁻, as well as to cyclohexane and several hexene isomers (yielding [B₁₂Br₁₁(C₆H₁₂)]⁻) using collision-induced dissociation (CID), infrared photodissociation spectroscopy (IRPD) and computational methods. CID of the different [B₁₂Br₁₁(C₆H₁₄)]⁻ ions results in distinct fragmentation patterns dependent on the structure of the hexane isomer. The observed fragmentation reactions provide insights into the addition mechanism of [B₁₂Br₁₁]⁻ to hexane. Based on the observed CID patterns, we identified that either B–C bond formation through heterolytic C–C or C–H bond cleavages or B–H bond formation through heterolytic C–H cleavage occur dependent on the structure of the hexane isomer. Meanwhile, we observe identical CID spectra of adducts originating from isomers of C₆H₁₂. Spectroscopic investigations of adducts of 1-hexene and cyclohexane indicate the same product structure with an open C₆ chain. Computational investigations evidenced that low lying transition states are present, which enable a ring opening reaction of cyclohexane when binding to [B₁₂Br₁₁]⁻.