Synthesis, formation mechanism, and dehydrogenation properties of the long-sought Mg(NH2BH3)2 compound

Abstract

The synthesis of magnesium amidoborane, Mg(NH₂BH₃)₂ (MgAB), has been attracting considerable interest since the recognition of the potential of metal amidoboranes as promising hydrogen storage media. But so far, all the efforts for synthesizing Mg(NH₂BH₃)₂ using mechanochemical or wet chemistry methods have been frustrated. In this paper, we report a successful synthesis of MgAB using ammonia borane (AB) and magnesium hydride (MgH₂) or magnesium (Mg) powder as starting materials. It was found that the post-milled 2AB/MgH₂ and 2AB/Mg mixtures undergo solid-phase reactions under mild temperatures (≤70 °C), resulting in the formation of a new crystalline phase. A combination of X-ray diffraction (XRD), scanning electron microscope (SEM), Fourier transformation infrared (FTIR), and solid-state ¹¹B MAS NMR characterizations, in conjunction with weight loss monitoring and the designed metathesis experiment, strongly indicates that the new crystalline phase is the long-sought MgAB. Our study using the solid-state ¹¹B NMR technique further revealed that the formation of MgAB involves a reaction mechanism that is distinct from other metal amidoboranes. MgH₂ or Mg may actually react with the mobile phase of AB (AB*), rather than with the starting normal AB. The examination of its properties revealed that MgAB is a stable compound at room temperature and can release ∼10 wt% H₂ of high purity at temperature below 300 °C, suggesting that it is a promising hydrogen storage material.