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·BArXH2 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 p bonding. This proton transfer can be shown to be reversible in the presence of excess ammonia, depending on the nature of the B‐bound ArX 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.