The First Crystallographic and Spectroscopic Characterization
of a 3<i>d</i>-Metal Borohydride: Mn(BH<sub>4</sub>)<sub>2</sub>
Radovan Černý
Nicolas Penin
Hans Hagemann
Yaroslav Filinchuk
10.1021/jp9015883.s003
https://acs.figshare.com/articles/journal_contribution/The_First_Crystallographic_and_Spectroscopic_Characterization_of_a_3_i_d_i_Metal_Borohydride_Mn_BH_sub_4_sub_sub_2_sub_/2855392
The first crystal structure of a 3<i>d</i>-metal borohydride
is presented. Solvent-free homoleptic manganese borohydride Mn(BH<sub>4</sub>)<sub>2</sub> forms at ambient conditions in ball-milled mixtures
of alkali metal borohydrides and MnCl<sub>2</sub>. It crystallizes
in the trigonal crystal system with the space group symmetry <i>P</i>3<sub>1</sub>12 and is stable from 90 to 450 K, where the
compound melts. Thermal expansion of Mn(BH<sub>4</sub>)<sub>2</sub> between 90 and 400 K is highly anisotropic and strongly nonuniform.
The structure of Mn(BH<sub>4</sub>)<sub>2</sub> shows interesting
similarity to α-Mg(BH<sub>4</sub>)<sub>2</sub>: the two structures
are made of similar layers L with the composition <i>M</i><sub>4</sub>(BH<sub>4</sub>)<sub>10</sub> per cell. The layers are
stacked along the <i>c</i>-axis, and rotated by 120°
by the 3<sub>1</sub> axis in Mn(BH<sub>4</sub>)<sub>2</sub> and by
60° by the 6<sub>1</sub> axis in α-Mg(BH<sub>4</sub>)<sub>2</sub>. Three identical layers are stacked along one unit cell vector <i>c</i> in Mn(BH<sub>4</sub>)<sub>2</sub>, while six layers are
stacked in α-Mg(BH<sub>4</sub>)<sub>2</sub>. In Mn(BH<sub>4</sub>)<sub>2</sub> the layers L are connected directly, and share atoms.
In α-Mg(BH<sub>4</sub>)<sub>2</sub> 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<sub>4</sub>)<sub>2</sub>, the structure of Mn(BH<sub>4</sub>)<sub>2</sub> is not densely
packed and contains isolated voids with the estimated volume of 21
Å<sup>3</sup> each, which occupy in total 6% of the space. The
resemblance between Mn(BH<sub>4</sub>)<sub>2</sub> and α-Mg(BH<sub>4</sub>)<sub>2</sub> is also reflected in their Raman and infrared
spectra.
2009-05-21 00:00:00
alkali metal borohydrides
layers L
space group symmetry P 3112
chiral space groups
Mg
Mn
Å3
unit cell vector c