Modulating Hydrogen Shuttling in Ammonia by Neutral and Cationic Boron-Containing Frustrated Lewis Pairs (FLPs)

Xanthene-backbone FLPs featuring secondary borane functions -B(Ar^X)H (where Ar^X=C₆F₅ (Ar^F) or C₆Cl₅ (Ar^Cl)) have been targeted through reactions of the dihydroboranes Me₂S ⋅ BAr^XH₂ with [4,5-xanth(PR₂)Li]₂ (R=Ph, ⁱPr), and investigated in the synthesis of related cationic systems via hydride abstraction. The reactivity of these systems (both cationic and charge neutral) with ammonia have been probed, with a view to probing the potential for proton shuttling via N-H bond 'activation.' We find that in the case of four-coordinate boron systems (cationic or change neutral), the N-H linkage remains intact, supported by a NH⋅⋅⋅P hydrogen bond which is worth up to 17 kcal mol^-1 thermodynamically, and enabled by planarization of the flexible xanthene scaffold. For cationic three coordinate systems, N-to-P proton transfer is viable, driven by the ability of the boron centre to stabilise the [NH₂]^- conjugate base through N-to-B π bonding. This proton transfer can be shown to be reversible in the presence of excess ammonia, depending on the nature of the B-bound Ar^X group. It is viable in the case of C₆F₅ substituents, but is prevented by the more sterically encumbering and secondary donor-stabilising capabilities of the C₆Cl₅ substituent.