Synthesis, Crystal Structures and Thermal Properties of Ammine Barium Borohydrides

Abstract

Ammine metal borohydrides show large compositional and structural diversity, and have been proposed as candidates for solid-state ammonia and hydrogen storage as well as fast cationic conductors. Here, we report the synthesis method of ammine barium borohydrides, Ba(BH4)2·xNH3 (x = 1, 2). The two new compounds were investigated with time-resolved temperature-varied in situ synchrotron radiation powder X-ray diffraction, thermal analysis, infrared spectroscopy and photographic analysis. The compound Ba(BH4)2·2NH3 crystallizes in an orthorhombic unit cell with space group symmetry Pnc2, and is isostructural to Sr(BH4)2·2NH3, forming octahedral [Ba(NH3)2(BH4)4] complexes, which are connected into a two-dimensional layered structure, where the layers are interconnected by dihydrogen bonds, N–Hδ+⋯−δH–B. A new structure type is observed for Ba(BH4)2·NH3, which crystallizes in an orthorhombic unit cell with space group symmetry P212121, forming a three-dimensional framework structure of [Ba(NH3)(BH4)6] complexes. The structure is built from distorted hexagonal chains, where NH3 groups form dihydrogen bonds to the nearby BH4−-groups within the chain. Ba(BH4)2·2NH3 is unstable at room temperature and releases NH3 in two subsequent endothermic reactions with maxima at 49 and 117 °C, eventually reforming Ba(BH4)2. We demonstrate that the thermal stability and composition of the gas release for the ammine alkaline earth metal borohydrides can be correlated to the charge density of the metal cation, but are also influenced by other effects.

Trends in the Series of Ammine Rare-Earth-Metal Borohydrides: Relating Structural and Thermal Properties

“…The compounds RE­(BH 4 ) 2 ·2NH 3 (RE 2+ = Sm, Eu) crystallize in the orthorhombic space group Pnc 2 (Figure ) and are isostructural with M­(BH 4 ) 2 ·2NH 3 (M 2+ = Sr, Ba). , In the structure of RE­(BH 4 ) 2 ·2NH 3 , one unique RE 2+ position is coordinated by four bridging BH 4 – groups and two NH 3 molecules, providing corner-shared bridging [RE­(NH 3 ) 2 (BH 4 ) 4 ] octahedral complexes in a trans geometry, forming two-dimensional layers in the bc plane, which are stacked in the sequence AAA along the a axis. The BH 4 – groups coordinate to one RE 2+ via face sharing (κ 3 ) and to one via edge sharing (κ 2 ); thus, CN­(RE 2+ ) = 12.…”

“… T dec (peak temperature from DSC) as a function of V (M 2+ ) for the series of M­(BH 4 ) 2 · x NH 3 (M = Ca, Sr, Ba, Eu, Sm, Yb). , …”

“…The majority of metal borohydrides react exothermically with ammonia, resulting in ammine metal borohydrides, M­(BH 4 ) n · x NH 3 . Among the alkali-metal borohydrides, only LiBH 4 reacts with NH 3 at room temperature, ,, while ammines have been reported for all of the alkaline-earth-metal borohydrides, M­(BH 4 ) 2 · x NH 3 (M = Mg, Ca, Sr, Ba), except for the poisonous Be­(BH 4 ) 2 , with x values in the range x = 1–6. ,− The thermal stability decreases down the group, i.e. the thermal stability decreases as Mg > Ca > Sr > Ba, mainly due to the increasing ionic radii and decreasing charge density of the cation.…”

Hydrogen storage in liquid hydrogen carriers: recent activities and new trends

“…The NH 3 /BH 4 ratio has also been suggested to be important, where a ratio of 1 or lower should result in hydrogen evolution. However, in the case of the stable metal borohydrides, NH 3 is released in an open system, independent of the ratio, as observed for M(BH 4 ) n •xNH 3 (M=Li, Ca, Sr, Ba, Sm, Eu, and Yb) [143,158,159,[165][166][167]169]. Instead, the thermal stability of the ammine metal borohydrides and the composition of the released gas can be correlated to the M-N bond strength, which roughly correlates to the charge density of the cation and the thermal stability of the metal borohydrides [138,143,166].…”

Beyond Hydrogen Storage—Metal Hydrides as Multifunctional Materials for Energy Storage and Conversion

“…Gattia et al showcase the effectiveness of inexpensive energy storage materials in pellet form over long cycling periods thereby demonstrating the highly practical nature of these systems for potential use in the future. Jakob B. Grinderslev et al describe in their paper, "Synthesis, Crystal Structures and Thermal Properties of Ammine Barium Borohydrides", candidates for solid-state NH 3 and H 2 storage as well as fast cationic conductors [25]. The authors focus on the synthesis method of two ammine barium borohydrides, Ba(BH 4 ) 2 •xNH 3 (x = 1, 2), and investigate those by time-resolved temperature-varied in situ synchrotron radiation powder X-ray diffraction, thermal analysis, infrared spectroscopy, and photographic analysis.…”