Ammine Lanthanum and Cerium Borohydrides, M(BH4)(3 center dot)nNH(3); 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)center dot nNH(3), 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 P2(1)2(1)2) built from cationic complexes, [M(NH3)(6)(BH4)(2)](+), and are charge balanced by BH4-. The structure of Ce(BH4)(3)center dot 5NH(3) is orthorhombic (space group C222(1)) 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)center dot 4NH(3) are monoclinic (space group C2), built from neutral molecular complexes of [M(NH3)(4)(BH4)(3)]. The new compositions, M(BH4)(3)center dot 3NH(3) (M = La, Ce), among ammine metal borohydrides, are orthorhombic (space group Pna2(1)), containing molecular complexes of [M(NH3)(3)(BH4)(3)]. A revised structural model for A(BH4)(3)center dot 5NH(3) (A = Y, Gd, Dy) is presented, and the previously reported composition A(BH4)(3)center dot 4NH(3) (A = Y, La, Gd, Dy) is proposed in fact to be M(BH4)(3) center dot 3NH(3) 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)center dot 3NH(3) (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)center dot nNH(3) and M(BH4)(3).