Synthesis, structures and thermal decomposition of ammine M_xB₁₂H₁₂ complexes (M = Li, Na, Ca)

A series of ammine metal-dodecahydro-closo-dodecaboranes, M_xB₁₂H₁₂·nNH₃ (M = Li, Na, Ca) were synthesized and their structural and thermal properties studied with in situ time-resolved synchrotron radiation powder X-ray diffraction, thermal analysis, Fourier transformed infrared spectroscopy, and temperature-programmed photographic analysis. The synthesized compounds, Li₂B₁₂H₁₂·7NH₃, Na₂B₁₂H₁₂·4NH₃ and CaB₁₂H₁₂·6NH₃, contain high amounts of NH₃, 43.3, 26.6 and 35.9 wt% NH₃, respectively, which can be released and absorbed reversibly at moderate conditions without decomposition, thereby making the closo-boranes favorable 'host' materials for ammonia or indirect hydrogen storage in the solid state. In this work, fifteen new ammine metal dodecahydro-closo-dodecaborane compounds are observed by powder X-ray diffraction, of which six are structurally characterized, Li₂B₁₂H₁₂·4NH₃, Li₂B₁₂H₁₂·2NH₃, Na₂B₁₂H₁₂·4NH₃, Na₂B₁₂H₁₂·2NH₃, CaB₁₂H₁₂·4NH₃ and CaB₁₂H₁₂·3NH₃. Li₂B₁₂H₁₂·4NH₃ and Na₂B₁₂H₁₂·4NH₃ are isostructural and monoclinic (P2₁/n) whereas Na₂B₁₂H₁₂·2NH₃ and CaB₁₂H₁₂·3NH₃ are both trigonal with space groups P3m1 and R3c, respectively. Generally, coordination between the metal and the icosahedral closo-borane anion is diverse and includes point sharing, edge sharing, or face sharing, while coordination of ammonia always occurs via the lone pair on nitrogen to the metal. Furthermore, a liquid intermediate is observed during heating of Li₂B₁₂H₁₂·7NH₃. This work provides deeper insight into the structural, physical, and chemical properties related to thermal decomposition and possible ammonia and hydrogen storage.