Polar intermetallic compounds as catalysts for hydrogenation reactions: Synthesis, structures, bonding, and catalytic properties of Ca _1-xSr_xNi₄Sn₂ (#=0.0, 0.5, 1.0) and catalytic properties of Ni₃Sn and Ni₃Sn₂

The potential of polar intermetallic compounds to catalyze hydrogenation reactions was evaluated. The novel compounds CaNi₄Sn₂, SrNi₄Sn₂, and Ca_0.5Sr_0.5Ni ₄Sn₂ were tested as unsupported alloys in the liquid-phase hydrogenation of citral. Depending on the reaction conditions, conversions of up to 21.0% (253 K and 9.0 MPa hydrogen pressure) were reached. The binary compounds Ni₃Sn and Ni₃Sn₂ were also tested in citral hydrogenation under the same conditions. These materials gave conversions of up to 37.5%. The product mixtures contained mainly geraniol, nerol, citronellal, and citronellol. The isotypic stannides CaNi₄Sn ₂, Ca_0.5Sr_0.5Ni₄Sn₂, and SrNi₄Sn₂ were obtained by melting mixtures of the elements in an arc-furnace under an argon atmosphere. Single crystals were synthesized in tantalum ampoules using special temperature modes. The novel structures were established by single-crystal X-ray diffraction. They crystallize in the tetragonal space group I4/mcm with parameters: a = 7.6991(7), c = 7.8150(8)Å, wR2 = 0.034, 162 F² values, 14 variable parameters for CaNi₄Sn₂; a = 7.7936(2), c = 7.7816(3)Å, wR2 = 0.052, 193 F² values, 15 variable parameters for Ca _0.5Sr_0.5Ni₄Sn₂; and a = 7.8916(4), c = 7.7485(5) Å, wR2 = 0.071, 208 F² values, 14 variable parameters for SrNi₄Sn₂. The Ca_1-xSr _xNi₄Sn₂ (x = 0.0, 0.5, 1.0) structures can be represented as a stuffed variant of the CuAl₂ type by the formal insertion of one-dimensional infinite Ni-cluster chains [Ni₄] into the Ca(Sr)Sn₂ substructure. The Ni and Sn atoms form a three-dimensional infinite [Ni₄Sn₂] network in which the Ca or Sr atoms fill distorted octagonal channels. The densities of states obtained from TB-LMTO-ASA calculations show metallic character for both compounds.