Coordination polymers building block approach

Coordination Polymer Design Approaches 2.4.3.1 The Building-block Methodology... 

Approaches to growing crystals of coordination polymers must be found and these may take two major directions. The first involves the judicious choice of building-blocks. Coordination polymer systems that involve the formation of kinetically inert coordinate bonds rarely, if ever, form highly crystalline material. In simplistic terms the reason behind this is that once a bond has been formed in building a coordination polymer, if that bond is inert, it is difficult for that bond to be broken under conventional crystallisation conditions. Thus, mistakes in the coordination polymer formation cannot be readily corrected, typically leading to small crystallites or microcrystalline material. 

These approaches to crystal growth rely upon slow introduction of the component building-blocks, however, another approach is possible. The key issue in coordination polymer crystal growth is to slow the crystal formation process. This can be achieved by providing greater solubility of the building-blocks, or possibly small oligomers of... 

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). 

A simpler but equally effective approach has been employed by Ward and coworkers [25] in the preparation of coordination polymers using luminescent anionic complexes. Here transition metals with emissive MLCT states act as effective sensMsers for lanthanide emission in the NIR [25]. In this case cyanide groups were used as bridging units starting from stable Ru " complexes and simple Ln salts. Examples include [Ru(Bipy)(CN)4] [26,27], [Ru(Phen) (CN)4] - [28], [Ru(Bpym)(CN)4] -, [Ru(CN)4]2([i-Bpym) 4- [29], [Ru(Hat)(CN)4] - [25], [Ru(CN)4]3(p"-Hat) -, [ Ru(CN)4 2([i -Hat)]4-, [Cr(CN)6]"-, and [Co(CN)6] - [30]. The advantage of this method is that the building blocks are already kinetically stable in solution and the solid structure is dictated by the coordination number adopted by the lanthanide ion (Fig. 9.5). 

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