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

Abstract

The first crystal structure of a 3 d -metal borohydride is presented. Solvent-free homoleptic manganese borohydride Mn(BH 4 ) 2 forms at ambient conditions in ball-milled mixtures of alkali metal borohydrides and MnCl 2 . It crystallizes in the trigonal crystal system with the space group symmetry P 3 1 12 and is stable from 90 to 450 K, where the compound melts. Thermal expansion of Mn(BH 4 ) 2 between 90 and 400 K is highly anisotropic and strongly nonuniform. The structure of Mn(BH 4 ) 2 shows interesting similarity to α-Mg(BH 4 ) 2: the two structures are made of similar layers L with the composition M 4 (BH 4 ) 10 per cell. The layers are stacked along the c -axis, and rotated by 120° by the 3 1 axis in Mn(BH 4 ) 2 and by 60° by the 6 1 axis in α-Mg(BH 4 ) 2 . Three identical layers are stacked along one unit cell vector c in Mn(BH 4 ) 2, while six layers are stacked in α-Mg(BH 4 ) 2 . In Mn(BH 4 ) 2 the layers L are connected directly, and share atoms. In α-Mg(BH 4 ) 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(BH 4 ) 2, the structure of Mn(BH 4 ) 2 is not densely packed and contains isolated voids with the estimated volume of 21 Å 3 each, which occupy in total 6% of the space. The resemblance between Mn(BH 4 ) 2 and α-Mg(BH 4 ) 2 is also reflected in their Raman and infrared spectra.