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.