Mild Dehydrogenation of Ammonia Borane Complexed with Aluminum Borohydride

Ammonia borane is a promising hydrogen store. However, its dehydrogenation is stepwise, nonreversible, and accompanied by formation of undesirable byproducts. We report on a new Al­(BH4)3·NH3BH3 complex containing 17.7 wt % hydrogen, which undergoes a two-step thermal decomposition below 100 °C. The combination of volumetric, gravimetric, crystallographic, and nuclear magnetic resonance studies shows that both in the solid state and in toluene solutions, the Al-coordinated NH3BH3 already releases two H2 molecules per Al at 70 °C. Contrary to that of the pristine ammonia borane, this process is endothermic, suggesting a possibility for direct rehydrogenation. The dehydrogenation of Al­(BH4)3·NH3BH3 contrasts with the complete destruction of alkali and alkaline earth metal borohydride complexes with ammonia borane in the first decomposition step. Other Al-based Lewis acids, less challenging with respect to the stability and safety than Al­(BH4)3, may be good agents for supporting the reversible dehydrogenation of NH3BH3 under mild conditions.