Ammine Lanthanum and Cerium Borohydrides, (BH)·NH; Trends in Synthesis, Structures, and Thermal Properties.

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, (BH)·NH, M = La ( = 6, 4, or 3) or Ce ( = 6, 5, 4, or 3). The two compounds with = 6 are isostructural and have new orthorhombic structure types (space group 222) built from cationic complexes, [(NH)(BH)], and are charge balanced by BH. The structure of Ce(BH)·5NH is orthorhombic (space group 222) and is built from cationic complexes, [Ce(NH)(BH)], and charge balanced by BH. These are rare examples of borohydride complexes acting both as a ligand and as a counterion in the same compound. The structures of (BH)·4NH are monoclinic (space group ), built from neutral molecular complexes of [(NH)(BH)]. The new compositions, (BH)·3NH (M = La, Ce), among ammine metal borohydrides, are orthorhombic (space group 2), containing molecular complexes of [(NH)(BH)]. A revised structural model for (BH)·5NH ( = Y, Gd, Dy) is presented, and the previously reported composition (BH)·4NH ( = Y, La, Gd, Dy) is proposed in fact to be (BH)·3NH along with a new structural model. The temperature-dependent structural properties and decomposition are investigated by synchrotron radiation powder X-ray diffraction in vacuum and argon atmosphere and by thermal analysis combined with mass spectrometry. The compounds with = 6, 5, and 4 mainly release ammonia at low temperatures, while hydrogen evolution occurs for (BH)·3NH (M = La, Ce). Gas-release temperatures and gas composition from these compounds depend on the physical conditions and on the relative stability of (BH)·NH and (BH).