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 cadm ium-thiocyanate coordination solids with cationic crown-ether-metal complex es as spacers, controllers, and templates. Specifically, depending upon the size, shape, symmetry, and charge of the cationic host-guest complexes, th is highly interesting series of coordination solids gives rise to a wide va riety of structures, ranging from one-dimensional (1-D) single-chain struct ures 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 s olids exhibit highly anisotropic physical properties such as nonlinear opti cal (NLO) behavior. The arrangement and/or alignment of these polymeric cad mium-thiocyanates can be controlled and/or induced by the host-guest comple xes. In this regard, for 1-D single- or double-chain cadmium-thiocyanate co ordination solids, the cations serve as the spacer/controller, dictating th e crystal structure and symmetry, thereby giving rise to desirable properti es 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 coordinat ion 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 ani sotropic 'template' for the formation of various layered structures with hi ghly interesting motifs. It is hoped that the inorganic polymers with organ ic spacers (IPOS) concept will not only lead to new and interesting materia ls but also to the development of new strategies in materials fabrication a t the molecular engineering level and the discovery of new patterns of crys tallization at the crystal engineering level. Furthermore, the general stra tegies and structural principles developed here can also be extended to oth er IPOS systems. (C) 1999 Elsevier Science S.A. All rights reserved.