The mechanism of the chain-growth of ammoniaborane: A classic Lewis pairs catalysed by a Frustrated Lewis Pairs

Ammonia borane (AB) is an outstanding hydrogen-storage material, which has been gained many attentions. It can be regenerated from polyborazylene based on the experimental study, but the uncontrollable B–N dehydro-coupling in the polymerization from AB to borazine is still a problem for the continuous application of AB. Frustrated Lewis Pairs (FLPs) is a landmark discovery to dissociate small molecules by activating strong nonpolar bonds reversibly, which is recently applied to directly polymerize three molecular AB to a cyclotriborazylene (CTB, B₃N₃H₁₂, which transforms into borazine after releasing 3 equiv. H₂). Here we analyse how a dimethylxanthene-derived FLPs catalyse AB to CTB. The cyclization experiences three steps by our calculation: the connection between FLPs and AB (1), the growth of B–N chain (2) and an intermolecular dehydrogenation between terminal N-H^δ+ and ^δ−H–B (3), which gives us a clue of a possible reversibility of the dehydrogenation of AB and broadens the applicability of this concept in the relevant catalyst design.