Desolvation and dehydrogenation of solvated magnesium salts of dodecahydrododecaborate: Relationship between structure and thermal decomposition

Attempts to synthesize solvent-free MgB₁₂H₁₂ by heating various solvated forms (H₂O, NH₃, and CH ₃OH) of the salt failed because of the competition between desolvation and dehydrogenation. This competition has been studied by thermogravimetric analysis (TGA) and temperature-programmed desorption (TPD). Products were characterized by IR, solution- and solid-state NMR spectroscopy, elemental analysis, and single-crystal or powder X-ray diffraction analysis. For hydrated salts, thermal decomposition proceeded in three stages, loss of water to form first hexahydrated then trihydrated, and finally loss of water and hydrogen to form polyhydroxylated complexes. For partially ammoniated salts, two stages of thermal decomposition were observed as ammonia and hydrogen were released with weight loss first of 14% and then 5.5%. Thermal decomposition of methanolated salts proceeded through a single step with a total weight loss of 32% with the release of methanol, methane, and hydrogen. All the gaseous products of thermal decomposition were characterized by using mass spectrometry. Residual solid materials were characterized by solid-state ¹¹B magic-angle spinning (MAS)NMR spectroscopy and X-ray powder diffraction analysis by which the molecular structures of hexahydrated and trihydrated complexes were solved. Both hydrogen and dihydrogen bonds were observed in structures of [Mg(H₂O)₆B₁₂H₁₂]η6H ₂O and [Mg(CH₃OH)₆B₁₂H ₁₂]η6CH₃OH, which were determined by single-crystal X-ray diffraction analysis. The structural factors influencing thermal decomposition behavior are identified and discussed. The dependence of dehydrogenation on the formation of dihydrogen bonds may be an important consideration in the design of solid-state hydrogen storage materials.