Dehydrogenation mechanisms and thermodynamics of MNH2BH3 (M = Li, Na) metal amidoboranes as predicted from first principles

Abstract

Electronic structure calculations have been used to determine and compare the thermodynamics of H₂ release from ammonia borane (NH₃BH₃), lithium amidoborane (LiNH₂BH₃), and sodium amidoborane (NaNH₂BH₃). Using two types of exchange correlation functional we show that in the gas-phase the metal amidoboranes have much higher energies of complexation than ammonia borane, meaning that for the former compounds the B–N bond does not break upon dehydrogenation. Thermodynamically however, both the binding energy for H₂ release and the activation energy for dehydrogenation are much lower for NH₃BH₃ than for the metal amidoboranes, in contrast to experimental results. We reconcile this by also investigating the effects of dimer complexation (2×NH₃BH₃, 2×LiNH₂BH₃) on the dehydrogenation properties. As previously described in the literature the minimum energy pathway for H₂ release from the 2×NH₃BH₃ complex involves the formation of a diammoniate of diborane complex ([BH₄]⁻[NH₃BH₂NH₃]⁺). A new mechanism is found for dehydrogenation from the 2×LiNH₂BH₃ dimer that involves the formation of an analogous dibroane complex ([BH₄]⁻[LiNH₂BH₂LiNH₂]⁺), intriguingly it is lower in energy than the original dimer (by 0.13 eV at ambient temperatures). Additionally, this pathway allows almost thermoneutral release of H₂ from the lithium amidoboranes at room temperature, and has an activation barrier that is lower in energy than for ammonia borane, in contrast to other theoretical research. The transition state for single and dimer lithium amidoborane demonstrates that the light metal atom plays a significant role in acting as a carrier for hydrogen transport during the dehydrogenation process via the formation of a Li–H complex. We posit that it is this mechanism which is responsible, in condensed molecular systems, for the improved dehydrogenation thermodynamics of metal amidoboranes.