Template metal-directed

Acceleration of the Template- and Metal-Directed Polycondensation of Nucleoside-5 -phosphoric Imidazolides by Acylation... 

The ligand reaction step may occur either with the template metal still intact or may take place after removal of the metal ion from the ring. As already mentioned, many of the Schiff-base macrocycles are unstable in the absence of a coordinated metal ion. However, for such systems, it has often been possible to hydrogenate the coordinated imine functions directly. The resulting saturated ligands will not be subject to the hydrolytic degradation which occurs for the imine precursors in the absence of their metal ion. 

Another fascinating approach to catenanes via self-assembly involves metal-coordination which templates or directs the assembly of catenane frameworks. After brief discussions on the recent examples of metal-containing catenanes and related topologically interesting molecules (Sections 4.2.1-4.2.3), the focus will be on the self-assembly of Pd(II)-linked catenanes (Sections 4.2.4 and 4.2.5). 

Probably one of the commonest reactions encountered in the template synthesis of macrocycles is the formation of imine C=N bonds from amines and carbonyl compounds. We have seen in the preceding chapters that co-ordination to a metal ion may be used to control the reactivity of the amine, the carbonyl or the imine. If we now consider that the metal ion may also play a conformational role in arranging the reactants in the correct orientation for cyclisation, it is clear that a limitless range of ligands can be prepared by metal-directed reactions of dicarbonyls with diamines. The Tt-acceptor imine functionality is also attractive to the co-ordination chemist as it gives rise to strong-field ligands which may have novel properties. All of the above renders imine formation a particularly useful tool in the arsenal of preparative co-ordination chemists. Some typical examples of the templated formation of imine macrocycles are presented in Fig. 6-12. 

In terms of template reactions, this combination of kinetic and thermodynamic stability usually means that the metal ion remains co-ordinated to the macrocyclic ligand and the isolation of the metal complex of the macrocycle provides strong circumstantial evidence for the existence of a metal-directed process. This is particularly easy to establish if the incorporation of the metal ion into the macrocyclic ligand can be shown to be slower than the metal-directed formation reaction. 

For a number of years the research interests of the Beer group have covered many areas of macrocychc coordination and supramolecular chemistry. This article reviews our latest results of current research by focusing on three major sections sensing of cations and anions anion templated assembly of pseudorotaxanes and rotaxanes, and metal-directed self-assembly using the dithiocaibamate ligand. 

Among the many desirable properties that are obtained with sol-gel formed metal oxide materials, it is perhaps the hydrophilic/hydrophobic duality of the matrix that allows the successful imprinting of molecular species. In the formation of the gel, templates will direct the placement of siloxane and silanol groups to complementarily interact with various hydrophobic and hydrophilic sites on the template. Figure 8.4 shows a possible form of an imprinted site for propyl orange in a silica matrix. Various aspects of molecular imprinting, such as these concepts, will be explored in the following sections. 

Some important metal oxide materials that have used molecular and supramole-cular templates to direct structure formation are the zeolites and related semi-crystalline aluminosilicates. In this section we shall discuss the use of ammonium cations that direct formation of microporous zeolites and finish with some of the possibilities that exist with the use of surfactant systems and molecular aggregates to create mesoporous structure. Excellent books and reviews are suggested for additional reading into the detailed description of the art [58-60]. The intention of this section is to briefly introduce this area and describe the types of materials being produced using various imprinting techniques in metal oxide materials. 

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