Mechanistic investigation on the formation and dehydrogenation of calcium amidoborane ammoniate

Possessing high H(2) capacities and interesting dehydrogenation behavior, metal amidoborane ammoniates were prepared by reacting Ca(NH(2) )(2) , MgNH, and LiNH(2) with ammonia borane to form Ca(NH(2) BH(3) )(2) ⋅2 NH(3) , Mg(NH(2) BH(3) )(2) ⋅NH(3) , and Li(NH(2) BH(3) )(2) ⋅NH(3) (LiAB⋅NH(3) ). Insight into the mechanisms of amidoborane ammoniate formation and dehydrogenation was obtained by using isotopic labeling techniques. Selective (15) N and (2) H labeling showed that the formation of the ammoniate occurs via the transfer of one H(N) from ammonia borane to the [NH(2) ](-) unit in Ca(NH(2) )(2) giving rise to NH(3) and [NH(2) BH(3) ](-) . Supported by theoretical calculations, it is suggested that the improved dehydrogenation properties of metal amidoborane ammoniates compared to metal amidoboranes are a result of the participation of a strong dihydrogen bond between the NH(3) molecule and [NH(2) BH(3) ](-) . Our study elucidates the reaction pathway involved in the synthesis and dehydrogenation of Ca(NH(2) BH(3) )(2) ⋅2 NH(3) , and clarifies our understanding of the role of NH(3) , that is, it is not only involved in stabilizing the structure, but also in improving the dehydrogenation properties of metal amidoboranes.