Anion‐Dependent Reactivity of Mono‐ and Dinuclear Boron Cations

• Published in: Chemistry : a European journal 2023-11, Vol.29 (65), p.e202302544-n/a
• Main Authors: Röther, Alexander, Farmer, James C., Portwich, Flavio L., Görls, Helmar, Kretschmer, Robert
• Format: Article
• Language: English
• Subjects: bond activation; Boron; Carbenes; cation; Cations; Chemistry; Dihydrides; Hydrogen fluoride; Lewis base; Phosphorus; Room temperature
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.202302544

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. If NHC‐stabilized borenium ions [NHC−BH2]+ and [PF6]− take the same bus, they will get off neutralized and with a noticeable change, i. e., as NHC−BF3 and PH2F3. However, if tetrakis(perfluoro‐tert‐butoxy)aluminate is the second passenger, the anion remains untouched and a bis(boronium) or a bis(borenium) ion can enjoy the journey.