Modelling fundamental arene-borane contacts: spontaneous formation of a dibromoborenium cation driven by interaction between a borane Lewis acid and an arene pi system

Reaction of 2,6-dimesityl pyridine (L-py) with BBr3 leads to the spontaneous formation of the trigonal dibromoborenium cation [L-py center dot BBr2](+) via bromide ejection. Systematic structural and computational studies, and the reactivity displayed by a closely related N-heterocyclic carbene (NHC) donor, reveal the role played by arene-borane interactions in this chemistry. [L-py center dot BBr2](+) features a structurally characterized (albeit weak) electrostatic interaction between the borane Lewis acid and flanking arene pi systems.