Mechanistic studies of ammonia borane dehydrogenation

Ammonia borane (NH3BH3, AB) has received extensive attention as a potential hydrogen storage medium, however hydrogen release mechanisms from AB are not well understood. AB follows different reaction routes if the dehydrogenation occurs in solvent or solid state, but a comparative study for AB dehydrogenation in these two states is not available. In this work, a detailed study of AB dehydrogenation mechanism in diglyme and solid state is presented, and a comprehensive reaction network for both cases is proposed. The experimental and DFT results suggest that two main reaction pathways occur; one involves cyclization of monomers which results in faster dehydrogenation at lower temperature, while the other involves propagation to acyclic intermediates which requires higher temperature to carry out the cyclization step. AB dehydrogenation in solid state was experimentally found to be initiated by B–N bond cleavage and not by direct dehydrogenation, which agrees with high level CCSD(T)/MP2 calculations reported previously. It was found that diglyme plays a significant role in hindering B–N bond cleavage of AB which facilitates the cyclization pathway. In solid state, experiments including labeled AB (ND3BH3) mapped out the source of hydrogen (from hydridic or protonic ends), and a clear difference in the degree of dehydrogenation from the two ends is demonstrated.

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► Hydrogen release from ammonia borane occurs by two main reaction pathways.
► Monomers can generate cyclic or acyclic products.
► Cyclization provides higher H2 yield with faster kinetics at lower temperature.
► Formation of acyclic intermediates results in slower H2 release kinetics.