The hydride-ion affinity of borenium cations and their propensity to activate H2 in frustrated Lewis pairs

Ewan R. Clark, Alessandro Del Grosso, Michael J. Ingleson*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


A range of frustrated Lewis pairs (FLPs) containing borenium cations have been synthesised. The catechol (Cat)-ligated borenium cation [CatB(PtBu 3)]+ has a lower hydride-ion affinity (HIA) than B(C 6F5)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 °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. Dichloro-borenium cations, [Cl 2B(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 °C to form the four-membered boracycles [(CH2{NC 5H3Me})B(Cl)Y] and [HPMes3]+. When equimolar [Y(Cl)B(2,6-lutidine)]+/P(mesityl)3 was heated under H2 (4 atm), heterolytic cleavage of dihydrogen was competitive with boracycle formation. DFT and experimental studies indicated that [Cl 2B(2,6-lutidine)]+ has a greater hydride-ion affinity (HIA) than B(C6F5)3. Heating the frustrated Lewis pair formed from [Cl2B(2,6-lutidine)]+ and P(mesityl)3 under H2 resulted in activation of H 2 and deprotonation of lutidine to form the boracycle [(CH 2{NC5H3Me})BCl2] (see scheme).

Original languageEnglish
Pages (from-to)2462-2466
Number of pages5
JournalChemistry - A European Journal
Issue number7
Publication statusPublished - 11 Feb 2013


  • borenium cations
  • boron
  • density functional calculations
  • frustrated Lewis pairs
  • hydrides


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