Mild Dehydrogenation of Ammonia Borane Complexed with Aluminum Borohydride

ABSTRACT: 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