NaSc(BH4)4: A Novel Scandium-Based Borohydride

A new alkaline transition-metal borohydride, NaSc(BH4)(4), is presented. The compound has been studied using a combination of in situ synchrotron radiation powder X-ray diffraction, thermal analysis, and vibrational and NMR spectroscopy. NaSc(BH4)(4) forms at ambient conditions in ball-milled mixtures of sodium borohydride and ScCl3. A new tertiary chloride Na3ScCl6 (P2(1)/n, a = 6.7375(3) angstrom, b = 7.1567(3) angstrom, c = 9,9316(5) angstrom, beta = 90.491(3)degrees, V = 478.87(4) angstrom(3)), isostructural to Na3TiCl6, was identified as an additional phase in all samples. This indicates that the formation of NaSc(BH4)(4) differs from a simple metathesis reaction, and the highest scandium borohydride yield (22 wt %) was obtained with a reactant ratio of ScCl3/NaBH4 of 1:2. NaSc(BH4)(4) crystallizes in the orthorhombic crystal system with the space group symmetry Cmcm (a = 8.170(2) angstrom, b = 11.875(3) angstrom, c = 9.018(2) angstrom, V = 874.9(3) angstrom(3)). The Structure of NaSc(BH4)(4) consists of isolated homoleptic scandium tetraborohydride anions, [Sc(BH4)(4)](-), located inside slightly distorted trigonal Na-6 prisms (each second prism is empty, triangular angles of 55.5 and 69.1 degrees). The experimental results show that each Sc3+ is tetrahedrally Surrounded by four BH4 tetrahedra with a 12-fold coordination of H to Sc, while Na+ is surrounded by six BH4 tetrahedra in a quite regular octahedral coordination with a (6 + 12)-fold coordination of H to Na. The packing of Na+ cations and [Sc(BH)(4))(4)](-) anions in NaSc(BH4)(4) is a deformation variant of the hexagonal NiAs structure type. NaSc(BH4)(4) is stable from RT up to similar to 410 K, Where the compound melts and then releases hydrogen in two rapidly occurring steps between 440 and 490 K and 495 and 540 K. Thermal expansion of NaSc(BH4)(4) between RT and 408 K is anisotropic, and lattice parameter b shows strong anomaly close to the melting temperature.