Synthesis and Structure of [(DB24C8)Na][Cd(SCN)₃]. Formation of a Novel Linear Cd···Cd···Cd Chain with a mer-CdN₃S₃ Coordination Configuration and a New Coiled [(DB24C8)Na]⁺ Cation

We report herein a novel coordination solid, [(DB24C8)Na][Cd(SCN)₃] (6) (DB24C8 denotes dibenzo-24-crown-8), which exhibits a new type of [Cd(SCN)₃^-]_∞ chain with two unusual stereochemical characteristics:  (1) a mer-CdN₃S₃ coordination and (2) a linear Cd chain with a Cd···Cd···Cd angle of 180°. In addition, the [(DB24C8)Na]⁺ monocation adopts a new structural typea coiled structurefor the combination of crown ether DB24C8 and alkali metal Na⁺. The title compound crystallizes in a monoclinic unit cell of C2/c space group symmetry with lattice parameters a = 16.110(8) Å, b = 20.380(5) Å, c = 11.01(1) Å, β = 119.87(3)°, and Z = 4. The arrangement of the [Cd(SCN)₃^-]_∞ chains in the crystal lattice in the title compound is approximately hexagonal, creating triangular channels which are filled with [(DB24C8)Na]⁺ monocations. It was previously reasoned by us that the coiled [(DB24C8)Na]⁺ monocation, which lacks inversion or mirror symmetries, should enhance the tendency for the formation of the noncentrosymmetric space group of the title crystal, making it a potential second-order nonlinear optical crystal. Interestingly, however, the title compound crystallizes in a centrosymmetric space group (C2/c) and gives rise to no second harmonic generation (SHG). Previously known [Cd(SCN)₃^-]_∞ chains adopt fac-CdN₃S₃ coordination and a zigzag Cd chain configuration with a Cd···Cd···Cd angle of 165°. The zigzag chains can align in either parallel or antiparallel fashion, resulting in efficient or no SHG effects, respectively. The linear Cd···Cd···Cd chain configuration observed in the title compound, on the other hand, makes it indistinguishable between parallel and antiparallel alignments. It is concluded that, to ensure the formation of noncentrosymmetric space groups, it is necessary to employ optically pure chiral cations as spacers and/or controllers. Furthermore, to enhance the nonlinear optical responses, [Cd(SCN)₃^-]_∞ chains with fac-CdN₃S₃ coordination and parallel alignments of the zigzag Cd chains should be used.