Synthesis and structure of [(DB24C8)Na][Cd(SCN)(3)]. Formation of a novel linear Cd center dot center dot center dot Cd center dot center dot center dot 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 (-)](infinity) 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 degrees. In addition, the [(DB24C8)Na](+) monocation adopt s a new structural type-a coiled structure-for the combination of crown eth er DB24C8 and alkali metal Na+. The title compound crystallizes in a monocl inic unit cell of C2/e space group symmetry with lattice parameters a = 16. 110(8) Angstrom, b = 20.380(5) Angstrom, 11.01(1) Angstrom, beta = 119.87(3 )degrees, and Z = 4. The arrangement of the [Cd(SCN)(3)(-)](infinity) chain s in the crystal lattice in the title compound is approximately hexagonal, creating triangular channels which are filled with [(DB23C8)Na](+) monocati ons. It was previously reasoned by us that the coiled [(DB24C8)Na](+) monoc ation, which lacks inversion or mirror symmetries, should enhance the tende ncy for the formation of the noncentrosymmetric space group of the title cr ystal, making it a potential second-order nonlinear optical crystal. Intere stingly, however: the title compound crystallizes in a centrosymmetric spac e group (C2/c) and gives rise to no second harmonic generation (SHG). Previ ously known [Cd(SCN)(3)(-)](infinity) chains adopt fac-CdN3S3 coordination and a zigzag Cd chain configuration with a Cd . . . Cd . . Cd angle of 165 degrees. The zigzag chains can align in either parallel or antiparallel fas hion, resulting in efficient or no SHG effects, respectively. The linear Cd . . . Cd . . . Cd chain configuration observed in the title compound, on t he other hand, makes it indistinguishable between parallel and antiparallel alignments. It is concluded that, to ensure the formation of noncentrosymm etric space groups, it is necessary to employ optically pure chiral cations as spacers and/or controllers. Furthermore, to enhance the nonlinear optic al responses, [Cd(SCN)(3)(-)](infinity) chains with fac-CdN3S3 coordination and parallel alignments of the zigzag Cd chains should be used.