PNNL

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)]–, 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 containing isomers of C6H12. Spectroscopic investigations of adducts of 1-hexene and cyclohexane indicate the same product structure with an open C6 chain. Computational investigations indicate that low lying transition states are present which enable a ring opening reaction of cyclohexane, when binding to [B12Br11]–. Overall, this study provides a comprehensive mechanistic description of the unusual direct functionalization reactions of alkanes by the special anion [B12Br11]–.