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.