Dihydrogen Activation by B(p-C6F4H)3 and Phosphines

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(C6F5)(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), Et3P (5), i-Pr-2(H)P (6), Ph3P (7), (3,5-M Me2C6H3)(3)P (8), t-Bu-2(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-C6F4H)(3)] (11) and [Cy3PH][HB(p-C6F4H)(3)] (12). The sterically demanding phosphines R3P (R = t-Bu, i-Pr, 2,4,6-Me3C6H2, and 2-MeC6H4), R2R'P (R, R' = t-Bu, i-Pr and t-Bu, Cy), and (Ph2CH)R2P (R = t-Bu (22), i-Pr (23), Cy (24), and C5H9 (25)) show no evidence of adduct formation with 1, but react with 142 to give the corresponding salts [R3PH][HB(p-C6F4H)(3)] (13-16), [R2R'PH][HB(p-C6F4H)(3)] (17-20), and [(Ph2CH)R2PH][HB(p-C6F4H)(3)] (26-29), respectively. The [t-Bu2PH2](+) species 11 is stable only under an H-2 overpressure, while the [t-Bu3PH](+) salt 13 cleanly extrudes isobutene and H-2 at elevated temperatures to give 9. The [(2-MeC6H4)(3)PH](+) derivative 16 readily liberates H-2 under vacuum, thus providing a metal-free system capable of reversible H, uptake and release. Crystallographic data for compounds 1, 5, 8, 9, 10, 13, and 26 are reported.