Thermodynamically self-assembled catenanes

Illustrated in Fig. 2 is one of the most famous examples of a metal-mediated, thermodynamically self-assembled catenane. The reaction of 1 (M = Pd) with the angled bipyridyl ligand 2 was found by Fujita and coworkers to lead to the formation of cycle 3, along with the interlocked catenane The nature of the self-assembly process... 

Catenanes have attracted considerable current interest particularly owing to their recently explored potential as molecular-scale devices. The incorporation of a coordination bond into catenane frameworks makes it possible to realize a quick molecular motion, which is somewhat similar to that of well-known magic rings under thermodynamic conditions a catenane framework rapidly arises from two preformed molecular rings. In the following sections, we describe the self-assembly, chemical manipulation, and characterization of several self-assembled catenanes. 

Two self-assembly techniques are predominantly employed in the preparation of catenanes "thermodynamic" self-assembly and self-assembly with covalent "modification."... 

Thermodynamic self-assembly involves the establishment of a dynamic equilibrium between the reagents and products in which catenane structures are energetically highly favored. If such systems are kinetically labile, the products obtained in greatest proportions will be those that are also the most thermodynamically stable that is. catenane structures. In such processes, thermodynamic influences drive the cyclization and the interlocking steps required in the formation of catenanes. Different types of... 

Fig. 2 Thermodynamic self-assembly Formation of a [2]-catenane. mediated by coordinative and other noncovalent interactions.

While the simultaneous use of several different classes of noncovalent interactions provides an elegant means of preparing a catenane. such systems are not always easily designed. Another method of thermodynamically selfassembling catenanes involves using two or more metal-mediated interactions to carry out the cyclization and interlocking steps. Illustrated in Figs. 3 and 4 are examples of thermodynamic self-assembly of this type. 

Fig. 7 Thermodynamic self-assembly of multiring catenanes— Molecular necklaces. Formation of a [4]MN, mediated by co-ordinative and other noncovalent interactions.

Fig. 8 Thermodynamic self-assembly of a polycatenane. Formation of a linear poly[2]catenane containing covalent, topological, and...

Self-Assembling Capsules, p. 123 i Self-Assembling Catenanes, p. 1240 Self-Assembly in Biochemistry, p. 1257 Self-Assembly Definition and Kinetic and Thermodynamic Considerations, p. 1248 Self-Assembly Terminology, p. 1263 The Template Effect, p. 149S... 

The remarkable thermodynamic stability of catenane 28 overcame a combination problem which should arise in self-assembly from a larger set of components. At least, formation of three component rings 32-34 is possible because their thermodynamic stability is comparable. Nevertheless, catenane 28 self-assembled as a sole product. This result shows that, having an ideal van der Waals separation (3.5 A) in its framework, only rectangular box 34 can be stabilized by filling its cavity with another copy of itself. Actually, the efficient aromatic stacking of four... 

Interestingly, the self-assembly of catenanes was also observed from a three-component system. Three components, la, 2, and 34, self-assembled in water in a 2 1 1 stoichiometry to give catenane 35 in very high quantitative yield of 94% (Scheme 12). The structure of 35 was conformed by X-ray crystallography (Figure 8), ESI-MS, and NMR. It is noteworthy that the thermodynamic stability of 35 overcomes the combination problem which arises in the self-assembly of larger sets of components. At least, the formation of the three component macrocycles 3a, 36, and 37 is possible because their thermodynamic stability is comparable with that of 35. Further, the formation of more flexible and expanded catenane 40 was also observed when the components la, 38, and 39 were combined in water in 2 1 1 ratio (Scheme 13). The structure of 34 was deduced from an ESI-MS study. 

The system depicted in Fig. 4 illustrates another noteworthy feature of catenane self-assembly the thermodynamically most stable moiety, which would arguably... 

Catenanes and Other Interlocked Molecules, p. 206 Hydrophobic Effect, p. 673 Molecular Squares, Boxes, and Cubes, p. 909 Rotaxanes and Pseudorotaxanes, p. 1194 Selectivity Thermodynamic and Kinetic, p. 1225 Self-Assembling Capsules, p. 123/... 

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