LiSc(BH 4 ) 4 has been prepared by ball milling of LiBH 4 and ScCl 3 . Vibrational spectroscopy indicates the presence of discrete Sc(BH 4 ) 4 -ions. DFT calculations of this isolated complex ion confirm that it is a stable complex, and the calculated vibrational spectra agree well with the experimental ones. The four BH 4 -groups are oriented with a tilted plane of three hydrogen atoms directed to the central Sc ion, resulting in a global 8 + 4 coordination. The crystal structure obtained by high-resolution synchrotron powder diffraction reveals a tetragonal unit cell with a ) 6.076 Å and c ) 12.034 Å (space group P-42c). The local structure of the Sc(BH 4 ) 4 -complex is refined as a distorted form of the theoretical structure. The Li ions are found to be disordered along the z axis.
A new alkaline transition-metal borohydride, NaSc(BH 4 ) 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(BH 4 ) 4 forms at ambient conditions in ball-milled mixtures of sodium borohydride and ScCl 3 . A new ternary chloride Na 3 ScCl 6 (P2 1 /n, a ) 6.7375), isostructural to Na 3 TiCl 6 , was identified as an additional phase in all samples. This indicates that the formation of NaSc(BH 4 ) 4 differs from a simple metathesis reaction, and the highest scandium borohydride yield (22 wt %) was obtained with a reactant ratio of ScCl 3 /NaBH 4 of 1:2. NaSc(BH 4 ) 4 crystallizes in the orthorhombic crystal system with the space group symmetry Cmcm (a ) 8.170(2) Å, b ) 11.875(3) Å, c ) 9.018(2) Å, V ) 874.9(3) Å 3 ). The structure of NaSc(BH 4 ) 4 consists of isolated homoleptic scandium tetraborohydride anions, [Sc(BH 4 ) 4 ] -, located inside slightly distorted trigonal Na 6 prisms (each second prism is empty, triangular angles of 55.5 and 69.1°). The experimental results show that each Sc 3+ is tetrahedrally surrounded by four BH 4 tetrahedra with a 12-fold coordination of H to Sc, while Na + is surrounded by six BH 4 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(BH 4 ) 4 is a deformation variant of the hexagonal NiAs structure type. NaSc(BH 4 ) 4 is stable from RT up 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(BH 4 ) 4 between RT and 408 K is anisotropic, and lattice parameter b shows strong anomaly close to the melting temperature.
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