Dihydrogen activation by B(p-C6F4H)3 and phosphines

Article abstract of DOI:10.1021/om100563m

The Grignard reagent (p-C₆F₄H)MgCl was used to prepare (Et₂O)B(p-C₆F₄H)₃ (2) and the free borane B(p-C₆F₄H)₃ (1). On the basis of Lewis acidity tests, the borane 1 exhibits ca. 95% of the acidity of B(C ₆F₅)₃. 1 forms classical adducts with ethers and phosphines of the general formula (L)B(p-C₆F₄H) ₃ (L = Et₂O (2), THF (3), Me₃P (4), Et ₃P (5), i-Pr₂(H)P (6), Ph₃P (7), (3,5-Me ₂C₆H₃)₃P (8), t-Bu₂(H)P (9), and Cy₃P (10)). Compounds 9 and 10 react with 4 atm of H ₂ to give the salts [t-Bu₂PH₂][HB(p-C ₆F₄H)₃] (11) and [Cy₃PH][HB(p-C ₆F₄H)₃] (12). The sterically demanding phosphines R₃P (R = t-Bu, i-Pr, 2,4,6-Me₃C ₆H₂, and 2-MeC₆H₄), R ₂R′P (R,R′ = t-Bu, i-Pr and t-Bu, Cy), and (Ph ₂CH)R₂P (R = t-Bu (22), i-Pr (23), Cy (24), and C ₅H₉ (25)) show no evidence of adduct formation with 1, but react with H₂ to give the corresponding salts [R₃PH] [HB(p-C₆F₄H)₃] (13-16), [R₂R′ PH][HB(p-C₆F₄H)₃] (17-20), and [(Ph ₂CH)R₂PH][HB(p-C₆F₄H)₃] (26-29), respectively. The [t-Bu₂PH₂]⁺ species 11 is stable only under an H₂ overpressure, while the [t-Bu ₃PH]⁺ salt 13 cleanly extrudes isobutene and H₂ at elevated temperatures to give 9. The [(2-MeC₆H₄) ₃PH]⁺ derivative 16 readily liberates H₂ under vacuum, thus providing a metal-free system capable of reversible H₂ uptake and release. Crystallographic data for compounds 1, 5, 8, 9, 10, 13, and 26 are reported.