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)(6)(2+) hexamminemagnesium cations and Mg(NH2BH3)(4)(2-) 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 H-2 upon heating to 300 degrees C in a closed system. A combination of XRD, Fourier transformation infrared spectroscopy (FTIR), and solid-state B-11 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).