(25a) Ab Initio Thermal Dehydrogenation Mechanisms for Ammonia Borane in Solvent

The reactivity of ammonia-borane (AB) dimers is investigated using ab initio simulations to determine the most active pathways leading to H₂ evolution. Elucidation of the detailed dehydrogenation mechanisms of AB in solvent provides insight into how to avoid unwanted decomposition of AB in solution. We show that in ethereal solvent the hydrogen-bonded AB dimer reacts via two energetically favorable pathways, one leading to its ion-pair isomer, the diammoniate of diborane (DADB), and the other directly to H₂ formation. The latter of these pathways has not been previously reported, but is the lowest barrier pathway in vacuum and highly competitive in solvent. Both of these pathways lead to the formation of NH₂BH₂, H₂, NH₃ and BH₃ and thus provide a source of NH₂BH₂, which oligomerizes into the experimentally observed aminoborane intermediates. Furthermore, NH₃ and BH₃ catalyses are also considered as competing pathways for AB dehydrogenation. To determine the effects of dynamics on the product distribution, quasi-classical dynamics simulations are performed from the transition states for important reactions in this chemistry. These simulations lead to different products than those predicted by the standard intrinsic reaction coordinate computation of the minimum energy pathway. This highlights the potential prediction of incorrect products when using 0K gas phase minimum energy pathways in any simulation, including those using sophisticated ab initio methods, that does not account for solvation and dynamics effects. Here we show that solvent and dynamics effects are of fundamental significance in determining the mechanism of uncatalyzed AB dehydrogenation.