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

Abstract

The activation of H₂ using Mes₃P–AlX₃ (X = Cl, Br, I) FLPs was investigated. Heating a 1 : 2 mixture of Mes₃P–AlI₃ under H₂ afforded [Mes₃PH][H(AlI₃)₂] 1. The alane Al(p-C₆F₄H)₃2 was prepared and a 1 : 2 solution of (otol)₃P and 2 under H₂ generated the salt [(otol)₃PH][(μ-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 Mes₃P and exposure to H₂ resulted in the formation of [Mes₃PH][(μ-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)₃PH][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.