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]^-.