Catenanes - Molecular Daisy Chains

Although the formation of helical complexes described above demonstrates the subtle and elegant control that is possible over the conformation of co-ordinated ligands, we have not yet extended these effects to control of the reactivity. 

Consider a difunctional molecular thread, with reactive sites at each end. These two reactive sites can react with some other difunctionalised molecule to give a cyclic structure (Fig. 7-37). This is exactly the type of process that we discussed at the beginning of Chapter 6. 

Now let us consider what happens if two such molecular threads containing didentate metal-binding domains are twisted into a helical arrangement after co-ordination to a tetrahedral metal centre. Reaction with the difunctional reagent could proceed in several ways. For example, the result could be the formation of a [2+2] macrocyclic complex as a result of the difunctional reagent linking together the two molecular threads (Fig. 7-38). 

7 The Three-Dimensional Template Effect, Supramolecular Chemistry and Molecular Topology 

When the copper complex of 7.62 reacts with ICH2(CH2OCH2)4CH2l in the presence of base, an intramolecular cyclisation occurs to form the macrocyclic ether 7.63. However, because of the arrangement of the starting ligands about the copper(i) centre, the two macrocycles are interlinked, and the consequence is the formation of the copper(i) complex of the catenand (catenand = catenane ligand) (Fig. 7-41). 

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