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

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Abstract

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 (as exemplified by [(18C6)K][Cd(SCN)3] and [(18C6)2Na2(H2O)2]1/2[Cd(SCN)3]), to 1-D double-chain structures (as observed in [(15C5)2K][Cd2(SCN)5] and [(15C5)2Na2(H2O)2]1/2[Cd2(SCN)5]), to two-dimensional (2-D) layered structures (as observed in [(12C4)2Cd][Cd2(SCN)6]and [(12C4)2Cd][Cd3(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 (as observed in [(18C6)K][Cd(SCN)3]). In the case of 2-D cadmium-thiocyanate coordination solids, the cationic complex such as [(12C4)2Cd]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.

Introduction

One way to take advantage of the superior qualities of both inorganic, organic, and polymer materials is to combine them in the same compound. Recently, we developed a new class of hybrid materials based on inorganic polymers with organic spacers/controllers (IPOS) systems [1], [2]. In particular, we have designed, synthesized, and structurally characterized a new series of hybrid crystalline materials of the general formula [H–G][M–L], where the cation [H–G] is a host (H)–guest (G) complex [3], [4] and the anion [M–L] is a metal (M)–ligand (L) polymer, as depicted in Chart 1. These low-dimensional coordination solids exhibit anisotropic physical properties such as NLO behavior [5], [6], [7]. Our systematic structural studies on this series of coordination solids consisting of crown-ether–metal complexes and the anionic cadmium-thiocyanate polymers [8], [9], [10], [11], [12], [13], [14] led to a wide variety of structures, ranging from one-dimensional (1-D) single-chain structures (as exemplified by [(18C6)K][Cd(SCN)3] [1a] and [(18C6)2Na2(H2O)2]1/2[Cd(SCN)3] [1a]), to 1-D double-chain structures (as observed in [(15C5)2K][Cd2(SCN)5] [1c] and [(15C5)2Na2(H2O)2]1/2[Cd2(SCN)5] [1c]), to two-dimensional (2-D) layered structures (as observed in [(12C4)2Cd][Cd2(SCN)6] [2a,b] and [(12C4)2Cd][Cd3(SCN)8] [2b]), depending upon the size, shape, symmetry, and charge of the cationic host–guest complexes. The arrangement and/or alignments 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 NLO behavior (as observed in [(18C6)K][Cd(SCN)3]). In the case of 2-D cadmium-thiocyanate coordination solids, the cationic complex such as [(12C4)2Cd]2+ dication 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 unprecedented motifs [2].

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. It is hoped that the IPOS concept will lead to novel strategies in materials fabrication at the molecular engineering level as well as 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.

Section snippets

Inorganic polymers with organic spacers (IPOS) systems

The IPOS concept [1], [2] combines the advantageous properties of inorganic, organic, and polymeric materials. It offers an excellent opportunity to synthesize isolated inorganic polymers of low dimensions (1- or 2-D) interspersed in an ‘organic medium’. This particular IPOS series, as exemplified by the above-mentioned compounds, allows concomitant but separate molecular and crystal engineering [15], [16], [17], [18], [19], [20], [21] (i.e. design of molecular structure and crystal packing in

One-dimensional single-chain anionic cadmium-thiocyanate structures with cationic spacers/controllers

1-D anionic cadmium thiocyanate coordination solids are rather rare. Only a few examples of 1-D [Cd(SCN)3] and [Cd(SCN)4]2− chain structures are known [1], [10], [11]. Fig. 2 depicts the structure of a representative anionic polymeric cadmium-thiocyanate chain observed in a number of 1-D cadmium-thiocyanate and crown-ether–metal coordination solids [1b]. It can be seen that each Cd atom is octahedrally coordinated with three S and three N atoms (in fac configuration, i.e. N atoms are trans to

One-dimensional double-chain anionic cadmium-thiocyanate structures with cationic spacers/controllers: structures of [(15C5)2Na2(H2O)2]1/2[Cd2(SCN)5] and [(15C5)2K][Cd2(SCN)5]

The first examples of double-chain [Cd2(SCN)5] coordination solids are observed in [(15C5)2Na2(H2O)2]1/2[Cd2(SCN)5] (6) [1c] and [(15C5)2K][Cd2(SCN)5] (7) [1c] (Fig. 6) [1f]. In both structures, the infinite anionic cadmium-thiocyanate structures consist of 1-D polymeric chains with the building blocks of dicadmium-thiocyanate complex, [Cd2(SCN)5], which may be referred to as double-chain structures, as portrayed in Fig. 8. Cd atoms are all octahedrally coordinated with 3N and 3S from four

Two-dimensional layered anionic cadmium-thiocyanate structures with cationic templates

To the best of our knowledge, there is only one previous example of 2-D anionic cadmium-thiocyanate layered structure, namely, RbCd(SCN)3 [13], though layered structures are known for other mixed-metal thiocyanate compounds (e.g. CoHg2(SCN)6 · C6H6) [14]. In the following sections, we shall describe the structures of two 2-D anionic cadmium-thiocyanate coordinate solids recently synthesized in our laboratory.

Three-dimensional metal-thiocyanate structures

3-D anionic cadmium-thiocyanate structures are rare. Most known metal thiocyanate compounds with 3-D structures contain other metals or a combination of metals. We shall describe a few examples here.

Design criteria for nonlinear optical materials

We shall now consider the design criteria and the novel features of the IPOS systems as NLO materials [5], [6], [7]. The design of new materials may be conceptually divided into two steps: molecular engineering wherein the electronic properties of the molecule are optimized and crystal engineering wherein crystallization in certain symmetry pattern is achieved [15], [16], [17], [18], [19], [20], [21]. For organic NLO materials [7], the commonly adopted molecular design rules are (1) highly

Crystal engineering: dimension (size and shape), symmetry, and template effects

Table 1 summarizes the results of crystal engineering for 1-D single-chain [Cd(SCN)3] and double-chain [Cd2(SCN)5] coordination solids with crown-ether–alkali-metal cations. The same results are represented diagramatically in Chart 10. We shall discuss each of the dominant effects—the geometry, symmetry, and template effects—in the following sections.

From molecular host–guest complexes to crystal host–guest clathrates

In the 1-D cadmium-thiocyanate system, the infinite [Cd(SCN)3] zigzag chains create certain pattern of channels which are filled by the host–guest cationic molecules. Thus, the square channels created by the tetragonal arrangement of the [Cd(SCN)3] chains in [(18C6)M][Cd(SCN)3] (M+=K+ (1), Na+ (2)) [1a] are filled by the [(18C6)K]+ (in 1) or [(18C6)2Na2(H2O)2]2+ (in 2) cations, respectively. Similarly, the triangular channels created by the hexagonal array of [Cd(SCN)3] chains in 3 [1b]

Advantageous properties of cadmium-thiocyanate systems as nonlinear optical crystals

The distinguishing features of the IPOS system, exemplified by the cadmium-thiocyanate coordination solids, as tailorable crystalline materials are:

  • 1.

    the anions form isolated (well-separated) polymeric 1-D single chains such as [Cd(SCN)3], 1-D double chains such as [Cd2(SCN)5]∞, or 2-D networks such as [Cd2(SCN)62−] and [Cd3(SCN)82−],

  • 2.

    the cationic host–guest complex serve as a spacer/controller of the crystal structure and crystal symmetry,

  • 3.

    the extended π-conjugation system within the

Concluding remarks and future prospects

General design strategies, in terms of symmetry control, as well as molecular and crystal engineering, in terms of electronic and stereochemical controls for NLO responses, have been formulated for the class of IPOS compounds involving 1- and 2-D cadmium-thiocyanate polymers. In particular, the crystal engineering principles developed here allows us to predict the spatial arrangement of the anionic cadmium-thiocyanate chains (geometry effect) and the relative alignment of the zigzag chains

Acknowledgements

The authors would like to thank the National Science Foundation (USA) for financial support of this research. They would also like to thank H. Zhu, W. Xiao, and H. Zang for their participation in this research.

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