Activation of H2 using P/Al based frustrated Lewis pairs and reactions with olefins

The activation of H-2 using Mes(3)P-AlX3 (X = Cl, Br, I) FLPs was investigated. Heating a 1 : 2 mixture of Mes(3)P-AlI3 under H-2 afforded [Mes(3)PH][H(AlI3)(2)] 1. The alane Al(p-C6F4H)(3) 2 was prepared and a 1 : 2 solution of (otol)(3)P and 2 under H-2 generated the salt [(otol)(3)PH][(mu-H)(Al(p-C6F4H)(3))(2)] which could not be isolated. However, the bulky anion salt [Li(OEt2)][FAl(C12F9)(3)] was readily converted to [Et4N][FAl(C12F9)(3)] 3 and subsequently to the hydride salt [Et4N][HAl(C12F9)(3)] 4. Further reaction of 4 with [CPh3][B(C6F5)(4)] generated the free alane, Al(C12F9)(3) which in the presence of Mes(3)P and exposure to H-2 resulted in the formation of [Mes(3)PH][(mu-H)(Al(C12F9)(3))(2)] 5. Exposing a solution of 1 in C6D5Br with ethylene, propylene or cyclohexene followed by quenching with water resulted in liberation of ethane, propane, or cyclohexane, respectively. In contrast, exposure of 5 to an atmosphere of ethylene showed no evidence of reaction. In related chemistry, protonation of [Et4N][EtAl(C6F5)(3)] by the phosphonium salts [R3PH][Al(C6F5)(4)] (R = otol (6), Mes (7), tBu (8)) and [(2,6-Cl2C6H3)(3)PH][AlCl4] (9) was also examined. In the case of the reaction of 6 and 9 ethane was liberated whereas with 7 and 8, no reaction occurred. The implications for transition metal-free hydrogenation catalysis are considered.