Ammine Magnesium Borohydride Complex as a New Material for Hydrogen Storage: Structure and Properties of Mg(BH4)2·2NH3

The ammonia complex of magnesium borohydride Mg(BH4)2·2NH3 (I), which contains 16.0 wt % hydrogen, is a potentially promising material for hydrogen storage. This complex was synthesized by thermal decomposition of a hexaaammine complex Mg(BH4)2·6NH3 (II), which crystallizes in the cubic space group Fm3m with unit cell parameter a = 10.82(1) A and is isostructural to Mg(NH3)6Cl2. We solved the structure of I that crystallizes in the orthorhombic space group Pcab with unit cell parameters a = 17.4872(4) A, b = 9.4132(2) A, c = 8.7304(2) A, and Z = 8. This structure is built from individual pseudotetrahedral molecules Mg(BH4)2·2NH3 containing one bidentate BH4 group and one tridentate BH4 group that pack into a layered crystal structure mediated by N−H···H−B dihydrogen bonds. Complex I decomposes endothermically starting at 150 °C, with a maximum hydrogen release rate at 205 °C, which makes it competitive with ammonia borane BH3NH3 as a hydrogen storage material.