The First Crystallographic and Spectroscopic Characterization of a 3d-Metal Borohydride: Mn(BH4)2

The first crystal structure of a 3d-metal borohydride is presented. Solvent-free homoleptic manganese borohydride Mn(BH4)2 forms at ambient conditions in ball-milled mixtures of alkali metal borohydrides and MnCl2. It crystallizes in the trigonal crystal system with the space group symmetry P3112 and is stable from 90 to 450 K, where the compound melts. Thermal expansion of Mn(BH4)2 between 90 and 400 K is highly anisotropic and strongly nonuniform. The structure of Mn(BH4)2 shows interesting similarity to α-Mg(BH4)2: the two structures are made of similar layers L with the composition M4(BH4)10 per cell. The layers are stacked along the c-axis, and rotated by 120° by the 31 axis in Mn(BH4)2 and by 60° by the 61 axis in α-Mg(BH4)2. Three identical layers are stacked along one unit cell vector c in Mn(BH4)2, while six layers are stacked in α-Mg(BH4)2. In Mn(BH4)2 the layers L are connected directly, and share atoms. In α-Mg(BH4)2 the layers L are intercalated by a thin layer L′, which contains one Mg atom per layer per cell. The layer L is chiral, and both borohydrides crystallize in chiral space groups. Similar to α-Mg(BH4)2, the structure of Mn(BH4)2 is not densely packed and contains isolated voids with the estimated volume of 21 Å3 each, which occupy in total 6% of the space. The resemblance between Mn(BH4)2 and α-Mg(BH4)2 is also reflected in their Raman and infrared spectra.