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Crystal Engineering in [(12C4)2Na][Cd(SCN)3]:  First Example of an Anionic Cadmium Thiocyanate Coordination Solid with a Sandwich [(12C4)2Na]+ Cation as Spacer/Controller, Resulting in a Hexagonal Arrangement of Antiparallel Zigzag [Cd(SCN)3-] Chains

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posted on 24.06.1998 by Hong Zhang, Ximin Wang, Kecong Zhang, Boon K. Teo
This paper reports the synthesis, characterization, and structure of [(12C4)2Na][Cd(SCN)3] (3). The title compound 3 represents the first example of an anionic cadmium thiocyanate coordination solid in which a sandwich [(12C4)2Na]+ monocation serves as the spacer/controller, giving rise to a hexagonal array of antiparallel infinite zigzag [Cd(SCN)3-] chains with the cations occupying the triangular channels. The title compound 3 crystallizes in a monoclinic unit cell of P21/n space group symmetry with lattice parameters of 13.204(2) Å, 10.692(2) Å, 21.036(1) Å, 95.520(8)°, and Z = 4. A detailed comparison of the title structure (3) with that of [(18C6)2Na2(H2O)2]1/2[Cd(SCN)3] (1) and [(18C6)K][Cd(SCN)3] (2) revealed that the arrangement and the alignment of the infinite anionic [Cd(SCN)3-] zigzag chains are dictated by the dimensions and symmetry, respectively, of the cations. The infinite anionic [Cd(SCN)3-] chains in 1 and 2 form an approximate tetragonal array, creating square channels which are filled with the dimeric [(18C6)2Na2(H2O)2]2+ (in 1) or with two monomeric [(18C6)K]+ (in 2) cations while the infinite anionic [Cd(SCN)3-] chains in 3 adopt an approximate hexagonal arrangement with the triangular channels filled by the smaller [(12C4)2Na]+ cations (the dimension effect). The relative alignment (either parallel or antiparallel) of the anionic chains is dictated by the symmetry or approximate symmetry of the cations. Thus, both the dimeric cation in 1 and the sandwich cation in 3 give rise to antiparallel alignment of the anionic [Cd(SCN)3-] chains with the centrosymmetric space group P21/n whereas the disklike cation in 2 gives rise to parallel alignment of [Cd(SCN)3-] chains with noncentrosymmetric space group Cmc21 (the symmetry effect).