Coordination polymers synthetic approaches

The synthesis of polycatenanes requires, like the synthesis of catenanes, the preorientation of the macrocycle precursors into a favorable geometry before cycliza-tion (Scheme 4) [5], This pre-orientation is commonly achieved via a template, resulting from rc-donor-acceptor interactions, hydrogen-bonding, and coordination bonds [1-3, 5, 41], The use of a template in catenane synthesis is the subject of Chapters 4 and 6-8 and will not be treated further in this section. The aim of this section is to present the state of the art of the various synthetic approaches leading to the polycatenane polymers and networks. 

In conclusion, with this synthetic approach a wide range of redox-active polymers can be obtained where factors such as the nature of the polymer backbone (e.g use of copolymers), the loading of the metal center, and the nature and coordination sphere of the metal center can be controlled systematically. This has been used to great effect in the design of electrochemically driven chemical and biochemical sensors. 

Figure 2.4.4 View of a porous cadmium coordination polymer, Cd(N03)2[2,4 -(l,4-phenylene)bispyridine] °° [50], Such materials are a particular target for coordination polymer chemists and require rational, designed, synthetic approaches.

In addition to polystyrene, several other polymers have been provided with the hypercrosslinked structure, and, in addition to the postcrosslinking of preformed polymeric chains, other synthetic approaches to hypercrosslinked open networks have been developed, including the direct polymerization and polycondensation of appropriate monomers or co-monomers. The recently developed metal-organic frameworks and covalent organic frameworks constitute three-dimensional coordination and element-organic polymers with an unusually h%h free volume they fit into the new and rapidly growing class of hypercrosslinked network materials as well. 

Self-assembly is the most useful synthetic method. This is because (i) it permits the realization of a wide variety of structures from simple building blocks of metal ions and organic ligands, (ii) it allows an easy and rational modification of organic ligands, (hi) several types of interactions such as coordination bonds, hydrogen bonds, tt-tt interactions, CH-tt interactions, M—bonds, and van der Waals interactions can be incorporated and exploited, and (iv) there is the possibility of reaction control by temperature, pH, solvent, etc. For coordination polymers, recrystallization is unavailable due to their insolubility in most of solvents. Therefore, a new synthetic approach has been developed vide infra). 

An example of a nontrivial polyrotaxane formed using a similar synthetic approach is shown in Figure 21(b). In this case, rotaxane ligands bridge cadmium ions into 2D (4,4) sheets, and the wheels (in this case dibenzo-24-crown-8) can no longer be separated from the polymeric network. Indeed, while the previous example contained only rotaxane motifs, in this nontrivial example one can also define catenane-type interactions between the wheels and M4L4 rings formed by the coordination polymer. 

Finally, in Chapter 10, M. Andruh and C. Ruiz-Perez discuss synthetic approaches to the crystal engineering of coordination polymers. They address important questions concerning architecture and packing arrangements i.e., cavities or channels, because such empty spaces may be filled by host anions, solvent molecules, uncoordinated ligand molecules—and interpenetration in which voids associated with one framework are occupied by one or more independent frameworks. 

For more practical purposes, therefore, one should take recourse to metal particles as produced by other means, in particular on supports or in matrices. The advantage is the availability of macroscopic amounts of sample the disadvantage is that interaction with the supporting medium must be assessed. A great variety of synthetic methods exists, of which we can mention only a few. Metal clusters can be produced by aerosol techniques, by vapor deposition, by condensation in rare-gas matrices, by chemical reactions in various supports, e.g. zeolites, SiOi, AI2O3, or polymer matrices. Many different metal-nonmetal composites, such as the ceramic metals (cermets) have been obtained with metal particles with sizes varying from nanometers upward. In alternative approaches, metal particles are stabilized by chemical coordination with ligand molecules, as in metal colloids and metal cluster compounds. 

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