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

Abstract

Xanthene-backbone FLPs featuring secondary borane functions -B(ArX)H (where ArX= C6F5(ArF) or C6Cl5(ArCl)) have been targeted through reactions of the dihydroboranes Me2S·BArXH2with [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-1thermodynamically, 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-Bpbonding. This proton transfer can be shown to be reversible in the presence of excess ammonia, depending on the nature of the B-bound ArXgroup. It is viable in the case of C6F5substituents, but is prevented by the more sterically encumbering and secondary donor-stabilising capabilities of the C6Cl5substituent.