A new family of metal borohydride ammonia borane complexes: Synthesis, structures, and hydrogen storage properties

We report the first two examples of borohydride ammonia borane complexes: Li2(BH4)2NH3BH3 and Ca(BH4)2(NH3BH3)2. Their structures are successfully determined using a combination of X-ray diffraction and first-principles calculations. Both structures are composed of alternating layers of borohydride and ammonia borane. Examination of bond lengths indicates that this arrangement is stabilized via dihydrogen bonding between ammonia borane and their surrounding BH4−, and the interactions between ammonia borane ligands and cations. Our experimental results show that more than 10 wt% and 11 wt% hydrogen can be released from Li2(BH4)2NH3BH3 and Ca(BH4)2(NH3BH3)2, respectively. Negligible ammonia was detected compared to ammonia borane and its ammidoborane derivatives. Further improvements are needed to reduce borazine emission. Cycling studies show that decomposed Li2(BH4)2NH3BH3 and Ca(BH4)2(NH3BH3)2 can be partially hydrogenated under hydrogen pressures at high temperatures.