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

• Published in: International journal of hydrogen energy 2018-03, Vol.43 (9), p.4177-4185
• Main Authors: Wang, Kun, Cheng, Long-jiu, Zhang, Jian-guo, Yu, Xue-bin
• Format: Article
• Language: English
• Subjects: Density functional theory; Frustrated Lewis Pairs (FLPs); Hydrogen-storage material; Metal-free hydrogenation; Reaction kinetic study
• ISSN: 0360-3199; 1879-3487
• DOI: 10.1016/j.ijhydene.2018.01.002

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 BN 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 BN chain (2) and an intermolecular dehydrogenation between terminal N-Hδ+ and δ−HB (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. •We obtain the cyclization pathway from AB to CTB catalysed by a metal-free FLPs.•The activation of the terminal H(N) of each BN chain leads to a continuous BN chain-growth.•It is a synergetic step of heterolytic cleavage of BH bond in AB in the activation.•The cyclization to form CTB is the only pathway after three BN chain growth steps.•The energy barrier of BH activation is a function of FLP's structure.