Anion‐Dependent Reactivity of Mono‐ and Dinuclear Boron Cations

The dinuclear bis(N‐heterocyclic carbene) borane adduct 2 rapidly reacts with tritylium salts at room temperature but the outcome is strongly impacted by the respective counter‐ion. Using tritylium tetrakis(perfluoro‐tert‐butoxy)aluminate affords – depending on the solvent – either the bis(boronium) ion 4 or the hydride‐bridged dication 5. In case of tritylium hexafluorophosphate, however, H/F exchange occurs between boron and phosphorus yielding the dinuclear BF3 adduct 3 along with phosphorus dihydride trifluoride. H/F exchange also takes place when using the mononuclear N‐heterocyclic carbene BH3 adduct 6 and hence provides a facile route to PH2F3, which is usually synthesized in more complex reaction sequences regularly involving toxic hydrogen fluoride. DFT calculations shed light on the H/F exchange between the borenium ion and the [PF6]‐ counter‐ion and the computed mechanism features only small barriers in line with the experimental observations.