In-Situ and Real-time Monitoring of Mechanochemical Preparation of Li2 Mg(NH2 BH3 )4 and Na2 Mg(NH2 BH3 )4 and Their Thermal Dehydrogenation

For the first time, in-situ monitoring of uninterrupted mechanochemical synthesis of two bimetallic amidoboranes, M2Mg(NH2BH3)4 (M = Li, Na), by means of Raman spectroscopy has been applied. This approach allowed real-time observation of key intermediate phases and a straightforward follow-up of the reaction course. Detailed analysis of time-dependent spectra revealed a two-step mechanism through MNH2BH3.NH3BH3 adducts as key intermediate phases which further reacted with MgH2, giving M2Mg(NH2BH3)4 as final products. The intermediates partially take a competitive pathway toward the oligomeric M(BH3NH2BH2NH2BH3) phases. The crystal structure of the novel bimetallic amidoborane Li2Mg(NH2BH3)4 was solved from high-resolution powder diffraction data and showed an analogous metal coordination as in Na2Mg(NH2BH3)4, but a significantly different crystal packing. Li2Mg(NH2BH3)4 thermally dehydrogenates releasing highly pure H2 in the amount of 7 wt% and at a lower temperature then its sodium analogue making it significantly more viable for practical applications.