A simple and efficient approach to synthesize amidoborane ammoniates: case study for Mg(NH2BH3)2(NH3)3 with unusual coordination structure

Abstract

Metal amidoborane ammoniates are a new type of promising hydrogen storage material consisting of metal cation, [NH₂BH₃]⁻ anionic unit and NH₃ ligand. Herein, we report a new reactive ball milling approach for preparation of magnesium amidoborane ammoniate, Mg(NH₂BH₃)₂(NH₃)₃. Our study found that mechanically milling the NH₃BH₃–MgH₂ mixture in a 2 : 1 molar ratio under NH₃ atmosphere can readily produce Mg(NH₂BH₃)₂(NH₃)₃. Its crystal structure was successfully determined by a combination of X-ray diffraction (XRD) analysis and first-principles calculations. This compound possesses a novel ordered structure with alternating layers of Mg(NH₃)₆²⁺ hexamminemagnesium cations and Mg(NH₂BH₃)₄²⁻ complex anions, in which Mg²⁺ exhibits both VI and IV coordinations. Property measurements found that Mg(NH₂BH₃)₂(NH₃)₃ can release 7 equivalents (10.6 wt%) of H₂ upon heating to 300 °C in a closed system. A combination of XRD, Fourier transformation infrared spectroscopy (FTIR), and solid-state ¹¹B MAS NMR techniques has been employed to characterize Mg(NH₂BH₃)₂(NH₃)₃ and its dehydrogenation product. A series of control experiments have also been conducted to gain insight into the formation mechanism of Mg(NH₂BH₃)₂(NH₃)₃.