Molecular design and crystal engineering of a new series of inorganic polymers separated by organic spacers: Structures of [(18C6)K][Cd(SCN)(3)] and [(18C6)(2)Na-2(H2O)(2)](1/2)[Cd(SCN)(3)]

This paper reports the synthesis and structures of two key members, [(18C6)K][Cd(SCN)(3)] (1) and [(18C6)(2)Na-2(H2O)(2)](1/2)[Cd(SCN)(3)] (2), of a new inorganic polymers with organic spacers (IPOS) series with the general formula [H-G][M-L], where the organic cation [H-G](q+) is a host (H)-guest (G) complex such as [(crown ether)-(alkali metal)](+) and the inorganic anion [M-L](q-) is a metal (M)-ligand (L) coordination polymer such as [Cd(SCN)(3)(-)]infinity. 1 and 2 were prepared from 18C6, MSCN, and CdSO4 where M = K+ (for 1) and Na+ (for 2), respectively. 1 crystallizes in an orthorhombic unit cell of Cmc2(1) space group symmetry with lattice parameters a = 14.734(3) Angstrom, b = 15.304(2) Angstrom, c = 10.625(3) Angstrom; V = 2396(1) Angstrom(3), and Z = 4. The structure was refined to R(1) = 2.3% for 878 independent reflections (2 theta less than or equal to 46 degrees) with I > 3 sigma. 2 crystallizes in a monoclinic unit cell of P2(1)/n space group symmetry with lattice parameters a = 10.277(3) Angstrom, b = 10.915(5) Angstrom, c = 22.596(7) Angstrom, beta = 111.48(5)degrees; V = 2359(5) Angstrom(3), and Z = 4. The structure was refined to R(1) = 3.7% for 2415 independent reflections (2 theta less than or equal to 46 degrees) with I > 3 sigma. These crystals exhibit interesting physical properties Such as nonlinear optical effects (NLO). The novel features of this particular IPOS series, as exemplified by the title compounds, as tailorable crystalline materials are (1) the anions form isolated polymeric one-dimensional zig-zag chains of [Cd(SCN)(3)(-)]infinity with extended pi-conjugation system within the polymeric -[M-S-C=N]-M ... chain and high polarizabilities (which are important for NLO effects); and (2) the cationic host-guest complex [(18C6)M](+) serves as a spacer/controller of the crystal structure and crystal symmetry. The most important characteristic of the IPOS system reported herein is that it allows concomitant but separate molecular and crystal engineering (i.e., design of molecular structures and crystal packings in two independent controls). A detailed analysis of the structures of 1 and 2 revealed parallel and antiparallel arrangements, respectively, of the infinite anionic [Cd(SCN)(3)(-)]infinity polymeric zig-zag chains; such arrangements are presumably dictated by the monomeric and dimeric host-guest [(18C6)M]+ cations. General design strategies, in terms of symmetry control, as well as molecular and crystal engineering, in terms of electronic and stereochemical controls, are discussed. Furthermore, the structures of the title crystals may be considered as ''crystal host-guest clathrates'' in which the infinite anionic polymeric [Cd(SCN)(3)(-)]infinity chains form an approximate square motif, creating square channels which are filled by ''molecular host-guest complexes'' such as [(18C6)K](+) (in 1) or [(18C6)(2)Na-2-(H2O)(2)](2+) (in 2).