Initial steps for the thermal decomposition of alkaline-earth metal amidoboranes: a cluster approximation

A DFT study of thermal decomposition mechanisms of [M(NH₂BH₃)₂]₄ clusters with M = Mg, Ca, and Sr is presented. Multi-step reaction pathways leading to elimination of the first H₂ molecule are explored at the M06/TZVP level of theory. For all studied M, the clusters adopt similar structures and exhibit similar transformations along the reaction pathways. Their activation energies decrease in the order Mg < Ca ≤ Sr. Four metal atoms in the cluster form a rigid planar construction that is found to be nearly unchanged during all transformations. Cleavage of the B-H bond in the environment of alkaline-earth metal atoms leads to the "capture" of the released H atom by neighboring metal atoms with the formation of a M₃H moiety. While the activation energies for the cleavage of H^δ- can be as low as 14.3, 22.6 and 23.3 kcal mol^-1 for M = Mg, Ca and Sr, respectively, barriers for the subsequent cleavage of H^δ+via destruction of the M₃H moiety are about twice larger.