Catalytic Dehydrogenation of (Di)Amine-Boranes with a Geometrically Constrained Phosphine-Borane Lewis Pair

The o-phenylene bridged phosphine-borane iPr2P(o-C6H4)B(Fxyl)2 2 was prepared. Despite ring strain, it adopts a closed form, as substantiated by NMR, XRD, and DFT analyses. However, the corresponding open form is only slightly higher in energy. The dormant Lewis pair 2 proved to efficiently catalyze the dehydrogenation of a variety of amine- and diamine-boranes under mild conditions. The corresponding phosphonium-borate iPr2PH(o-C6H4)BH(Fxyl)2 3 was authenticated as a key intermediate of these dehydrogenation reactions. The propensity of 3 to release H2 plays a major role in the catalytic turnover.