Theoretical Study on the Possibility of Using Frustrated Lewis Pairs as Bifunctional Metal-Free Dehydrogenation Catalysts of Ammonia Borane

Density functional theory calculations have been carried out to study the possibility of using "frustrated Lewis pairs" (FLPs) as bifunctional metal-free dehydrogenation catalysts of ammonia-borane, NH3BH3 (AB), according to the following points: (1) some FLPs react with H-2 at low temperature and the hydrogenated products release H-2 at high temperature; (2) some FLPs hydrogenate imines by taking the protic (N-bound) and hydridic (B-bound) H atoms of AB. In this work, the N/B pair (N-PMTN CH2C6H4B(C6F5)(2)) is chosen to explore the possibility that FLPs release H-2 from AB. Two steps are involved in the proposed mechanism: first, AB is dehydrogenated by the N/B pair to form the NH/BH compound and NH2BH2 at low temperature, and then H-2 is released from the NH/BH compound at 110 degrees C and the N/B pair regenerates. Several competitive reaction channels have been considered, The calculational results show that the dehydrogenation of AB by the N/B pair has the lowest free-energy barrier. On the basis of the experimental N/B pair, a new dehydrogenation catalyst of AB has been designed, which can release H-2 from AB under milder conditions than the former. This work may be helpful the experimental chemists to broaden the application of FLPs and to design new dehydrogenation catalysts of AB.