The Hydride‐Ion Affinity of Borenium Cations and Their Propensity to Activate H 2 in Frustrated Lewis Pairs

A range of frustrated Lewis pairs (FLPs) containing borenium cations have been synthesised. The catechol (Cat)‐ligated borenium cation [CatB(P t Bu 3 )] + has a lower hydride‐ion affinity (HIA) than B(C 6 F 5 ) 3 . This resulted in H 2 activation being energetically unfavourable in a FLP with the strong base P t Bu 3 . However, ligand disproportionation of CatBH(P t Bu 3 ) at 100 °C enabled trapping of H 2 activation products. DFT calculations at the M06‐2X/6‐311G(d,p)/PCM (CH 2 Cl 2 ) level revealed that replacing catechol with chlorides significantly increases the chloride‐ion affinity (CIA) and HIA. Dichloro–borenium cations, [Cl 2 B(amine)] + , were calculated to have considerably greater HIA than B(C 6 F 5 ) 3 . Control reactions confirmed that the HIA calculations can be used to successfully predict hydride‐transfer reactivity between borenium cations and neutral boranes. The borenium cations [Y(Cl)B(2,6‐lutidine)] + (Y=Cl or Ph) form FLPs with P(mesityl) 3 that undergo slow deprotonation of an ortho ‐methyl of lutidine at 20 °C to form the four‐membered boracycles [(CH 2 {NC 5 H 3 Me})B(Cl)Y] and [HPMes 3 ] + . When equimolar [Y(Cl)B(2,6‐lutidine)] + /P(mesityl) 3 was heated under H 2 (4 atm), heterolytic cleavage of dihydrogen was competitive with boracycle formation.