Synthetic Considerations and Approaches

The requirement for single crystals is perhaps more of a challenge considering the highly insoluble nature of coordination polymers. It is also notable that recrystallisation, often used for molecular systems, is not as relevant for coordination polymer systems, as not only are coordination polymers typically highly insoluble but even if they are dissolved in aggressive solvents, such as DM SO or DMF, the polymer structure will break up and any coordination polymer that reforms may bear no resemblance to the product from the original reaction. 

Why is making single crystals of a coordination polymer a challenge Many coordination polymer reactions precipitate a product rapidly from solution. Although it is possible to encounter systems that give single crystals of sufficient 

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. 

It is not insignificant that a large proportion of reported coordination polymers contains highly labile metal centers, in particular d10 centers Cu(I) [54], Ag(I) [55], Zn(II) [51] and Cd(II) [42], but also d9 Cu(II) [38] and d7 Co(II) [56] cations and increasingly with lanthanide cations, Ln(III) [7]. Examples of second or third row transition metal based coordination polymers, with the exception of Ag(I) and Cd(II), are extremely rare amongst reported examples. 

The second consideration when attempting to prepare crystals of coordination polymers is the crystallisation condition used. As stated above, recrystallisation approaches are not usually appropriate for the growth of coordination polymer systems, due to solubility reasons. Another method of crystallisation often used for molecular systems, that of vapor diffusion of an anti-solvent into solutions of the product, is also rarely possible for coordination polymer systems. As with traditional recrystallisation this is due to the insolubility of the product material and consequently to the difficulty of obtaining solutions of either the coordination polymer or its constituent parts. 

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