The Hydride-Ion Affinity of Borenium Cations and Their Propensity to Activate H2 in Frustrated Lewis Pairs

A range of frustrated Lewis pairs (FLPs) containing borenium cations have been synthesised. The catechol (Cat)-ligated borenium cation [CatB(PtBu3)]+ has a lower hydride-ion affinity (HIA) than B(C6F5)3. This resulted in H2 activation being energetically unfavourable in a FLP with the strong base PtBu3. However, ligand disproportionation of CatBH(PtBu3) at 100 degrees C enabled trapping of H2 activation products. DFT calculations at the M06-2X/6-311G(d,p)/PCM (CH2Cl2) level revealed that replacing catechol with chlorides significantly increases the chloride-ion affinity (CIA) and HIA. Dichloroborenium cations, [Cl2B(amine)]+, were calculated to have considerably greater HIA than B(C6F5)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 degrees C to form the four-membered boracycles [(CH2{NC5H3Me})B(Cl)Y] and [HPMes3]+. When equimolar [Y(Cl)B(2,6-lutidine)]+/P(mesityl)3 was heated under H2 (4atm), heterolytic cleavage of dihydrogen was competitive with boracycle formation.