Ammine Lanthanum and Cerium Borohydrides, M(BH₄)₃·nNH₃; Trends in Synthesis, Structures, and Thermal Properties

Abstract

Ammine metal borohydrides show potential for solid-state hydrogen storage and can be tailored toward hydrogen release at low temperatures. Here, we report the synthesis and structural characterization of seven new ammine metal borohydrides, M(BH4)3·nNH3, M = La (n = 6, 4, or 3) or Ce (n = 6, 5, 4, or 3). The two compounds with n = 6 are isostructural and have new orthorhombic structure types (space group P21212) built from cationic complexes, [M(NH3)6(BH4)2]+, and are charge balanced by BH4 –. The structure of Ce­(BH4)3·5NH3 is orthorhombic (space group C2221) and is built from cationic complexes, [Ce­(NH3)5(BH4)2]+, and charge balanced by BH4 –. These are rare examples of borohydride complexes acting both as a ligand and as a counterion in the same compound. The structures of M(BH4)3·4NH3 are monoclinic (space group C2), built from neutral molecular complexes of [M(NH3)4(BH4)3]. The new compositions, M(BH4)3·3NH3 (M = La, Ce), among ammine metal borohydrides, are orthorhombic (space group Pna21), containing molecular complexes of [M(NH3)3(BH4)3]. A revised structural model for A(BH4)3·5NH3 (A = Y, Gd, Dy) is presented, and the previously reported composition A(BH4)3·4NH3 (A = Y, La, Gd, Dy) is proposed in fact to be M(BH4)3·3NH3 along with a new structural model. The temperature-dependent structural properties and decomposition are investigated by in situ synchrotron radiation powder X-ray diffraction in vacuum and argon atmosphere and by thermal analysis combined with mass spectrometry. The compounds with n = 6, 5, and 4 mainly release ammonia at low temperatures, while hydrogen evolution occurs for M(BH4)3·3NH3 (M = La, Ce). Gas-release temperatures and gas composition from these compounds depend on the physical conditions and on the relative stability of M(BH4)3·nNH3 and M(BH4)3.

Synthesis, Crystal Structures and Thermal Properties of Ammine Barium Borohydrides

“…This is due to the close correlation between the thermal stability and the partial pressure of NH 3 , which is also observed for other ammine metal borohydrides, e.g., M(BH 4 ) 2 •xNH 3 (M = Ca 2+ and Sr 2+ ) [25]. This is also observed for RE(BH 4 ) 3 •xNH 3 (RE = La 3+ , Ce 3+ ), where in situ SR-PXD was performed in closed capillaries and in dynamic vacuum, respectively, changing the ammonia desorption temperatures by up to 75 • C [26]. 2+ , Ba 2+ and x = 1, 2) decompose via the release of ammonia, reforming the metal borohydride.…”

“…The new compound formed in sample s2, Ba(BH 4 ) 2 •2NH 3 , is unstable at room temperature, hence the reaction mixture was kept at T = −10 • C during synthesis to avoid decomposition. Furthermore, the reaction between Ba(BH 4 ) 2 and NH 3 was expected to be exothermic, as also observed for other ammine metal borohydrides [18,19,25,26]. Ba(BH 4 ) 2 •2NH 3 gradually releases NH 3 and forms Ba(BH 4 ) 2 •NH 3 if a vacuum is applied for a longer period at −10 • C (>10 min).…”

“…Inorganics 2020, 8, x FOR PEER REVIEW 3 of 13 reaction between Ba(BH4)2 and NH3 was expected to be exothermic, as also observed for other ammine metal borohydrides [18,19,25,26]. Ba(BH4)2•2NH3 gradually releases NH3 and forms Ba(BH4)2•NH3 if a vacuum is applied for a longer period at −10 °C (>10 min).…”

“…The most extensive series of ammine metal borohydrides are observed for Y(BH 4 ) 3 •xNH 3 (x = 1, 2, 3, 5, 6, 7), where the isostructural compounds with RE = Gd 3+ and Dy 3+ were also reported for x = 3, 5, 6 and 7 [18]. Note that the structure of RE(BH 4 ) 3 •5NH 3 was recently revised, while the composition and structure of RE(BH 4 ) 3 •3NH 3 , previously reported as RE(BH 4 ) 3 •4NH 3 , were also revised [26]. The structures of the early rare earths RE(BH 4 ) 3 •xNH 3 (RE = La, Ce) are isostructural to the Y-analogs for x = 5 and 3, while new structures were reported for x = 6 and 4.…”

Neutron Scattering Investigations of the Global and Local Structures of Ammine Yttrium Borohydrides

“…Ammonia derivatives are known for the majority of the metal borohydrides, and the largest series of compositions and polymorphs among ammine metal borohydrides are observed for Y­(BH 4 ) 3 · x NH 3 , with x = 7, 6, 5, 3, 2 (α- and β-polymorph), and 1. , The structures possess an intriguing crystal chemistry dependent on the number of ammonia molecules, varying from structures built from complex ions ( x = 5, 6, and 7) to molecular structures ( x = 3), one-dimensional chains ( x = α-2 and β-2), and structures built from two-dimensional layers ( x = 1). Note that the crystal structure for x = 5 and both the structure and composition for x = 3 were recently revised using a combination of powder X-ray diffraction (PXD), density functional theory (DFT) calculations, 11 B nuclear magnetic resonance (NMR) spectroscopy, and thermal analysis. , …”