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

A series of ammine metal-dodecahydro-closo-dodecaboranes, MBH·nNH (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, LiBH·7NH, NaBH·4NH and CaBH·6NH, contain high amounts of NH, 43.3, 26.6 and 35.9 wt% NH, 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, LiBH·4NH, LiBH·2NH, NaBH·4NH, NaBH·2NH, CaBH·4NH and CaBH·3NH. LiBH·4NH and NaBH·4NH are isostructural and monoclinic (P2/n) whereas NaBH·2NH and CaBH·3NH are both trigonal with space groups P3[combining macron]m1 and R3[combining macron]c, 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 LiBH·7NH. This work provides deeper insight into the structural, physical, and chemical properties related to thermal decomposition and possible ammonia and hydrogen storage.