Molecular and crystal engineering of a new class of inorganic cadmium-thiocyanate polymers with host-guest complexes as organic spacers, controllers, and templates

This review provides an account of the structures of polymeric anionic cadmium-thiocyanate coordination solids with cationic crown-ether-metal complexes as spacers, controllers, and templates. Specifically, depending upon the size, shape, symmetry, and charge of the cationic host-guest complexes, this highly interesting series of coordination solids gives rise to a wide variety of structures, ranging from one-dimensional (1-D) single-chain structures las exemplified by [(18C6)K][Cd(SCN)(3)] and [(18C6)(2)Na-2(H2O)(2)](1/2)[Cd(SCN)(3)]), to 1-D double-chain structures las observed in [(15C5)(2)K][Cd-2(SCN)(5)] and [(15C5)(2-) Na-2(H2O)(2)](1/2)[Cd-2(SCN)(5)]), to two-dimensional (2-D) layered structures las observed in [(12C4)(2)Cd][Cd-2(SCN)(6)]and [(12C4)(2)Cd][Cd-3(SCN)(8)]). These low-dimensional coordination solids exhibit highly anisotropic physical properties such as nonlinear optical (NLO) behavior. The arrangement and/or alignment of these polymeric cadmium-thiocyanates can be controlled and/or induced by the host-guest complexes. In this regard, for 1-D single- or double-chain cadmium-thiocyanate coordination solids, the cations serve as the spacer/controller, dictating the crystal structure and symmetry, thereby giving rise to desirable properties of these crystals such as nonlinear optical (NLO) behavior las observed in [(18C6)K][Cd(SCN)(3)]). In the case of 2-D cadmium-thiocyanate coordination solids, the cationic complex such as [(12C4)(2)Cd](2+) serves not only as the 'spacer' and 'controller' of the crystal packing, but also as an anisotropic 'template' for the formation of various layered structures with highly interesting motifs. It is hoped that the inorganic polymers with organic spacers (IPOS) concept will not only lead to new and interesting materials but also to the development of new strategies in materials fabrication at the molecular engineering level and the discovery of new patterns of crystallization at the crystal engineering level. Furthermore, the general strategies and structural principles developed here can also be extended to other IPOS systems. (C) 1999 Elsevier Science S.A. All rights reserved.