Mechanochemical Synthesis and Thermal Dehydrogenation of Novel Calcium-Containing Bimetallic Amidoboranes

Metallic amidoboranes are widely investigated candidates for solid-state hydrogen storage, and much focus shifted recently toward bimetallic amidoboranes. Bimetallic amidoboranes are expected to introduce novel and enhanced physicochemical properties regarding storage and stability. However, these materials are still scarce and mostly grouped around magnesium- or aluminum-containing compounds. We present here a rapid and green mechanochemical solvent-free synthesis of two novel calcium-containing bimetallic amidoboranes, Li2Ca(NH2BH3)(4) and Na2Ca(NH2BH3)(4), from metal hydrides and ammonia borane. The insight into mechanochemical syntheses is provided by the in situ tandem synchrotron X-ray diffraction and thermal monitoring. The in situ data reveal how the choice of alkali metal hydride governs the course of reactions and their thermal profiles. In situ monitoring of thermal dehydrogenation of these materials is conducted by mass spectrometry and infrared spectroscopy, showing how the course of thermal decomposition varies depending on the structure of the amidoborane, resulting however in the same final products. These new hydrogen-rich bimetallic amidoboranes are structurally characterized by high-resolution powder X-ray diffraction, and they both show potential for hydrogen storage applications: high theoretical gravimetric capacities and low desorption temperatures of hydrogen without the significant presence of harmful gases. We also show how the choice of the milling reactor material can be decisive for the efficiency and overall success of the mechanochemical synthetic procedure, which may impact the design of milling syntheses for other thermally labile chemical systems.