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.