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, M2 Mg(NH2 BH3 )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 MNH2 BH3 ⋅NH3 BH3 adducts as key intermediate phases which further reacted with MgH2 , giving M2 Mg(NH2 BH3 )4 as final products. The intermediates partially take a competitive pathway toward the oligomeric M(BH3 NH2 BH2 NH2 BH3 ) phases. The crystal structure of the novel bimetallic amidoborane Li2 Mg(NH2 BH3 )4 was solved from high-resolution powder diffraction data and showed an analogous metal coordination to Na2 Mg(NH2 BH3 )4 , but a significantly different crystal packing. Li2 Mg(NH2 BH3 )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.