Macrocycles interlocked

For instance, complex 246+ exchanges up to a total of nine electrons. On reduction, it shows two monoelectronic and one bielectronic processes involving the bipyridinium units, and three monoelectronic processes concerning the bpy moieties (Fig. 13.20). On oxidation, two monoelectronic processes are observed the first one, being reversible, is assigned to the oxidation of the metal center and the second one, not fully reversible, to the oxidation of the alongside DMN unit of the macrocycle interlocked with the cyclophane. 

Controlled rotation of the molecular rings has also been achieved in catenanes composed of three interlocked macrocycles. For example, catenane 19H26+ (Fig. 21) is made up of two identical dioxybenzene-based macrocycles interlocked with a cyclophane containing two bipyridinium and two ammonium units [87],... 

The templates used by Stoddart [39a, b,c] in the syntheses of catenanes and rotaxanes use r- r interactions to enhance both cyclization and interweaving. The template directed cyclization (Scheme 1-9) results in rotaxane formation if the ends of the template 33 are made so bulky that it cannot escape from the macrocycle, and in catenane formation if the two ends of the template are joined to form a macrocycle interlocked with the new macrocycle. The template is an integral part of the final product [39a,d]. Cucurbituril 48 can act as both a linear template and a topological template in the formation of rotaxane, as discussed later (Scheme 1-20). 

In macrocycle 3,1201 the two electron-acceptor units, a bipyridinium and a bis(pyridinium)ethy-lene, are different and therefore are reduced at different potentials, as expected on the basis of their electron-acceptor ability (Figure 5). When this macrocycle is interlocked with 1/5DN38C10 in the cate nane 4, the bipyridinium unit occupies the inside position and therefore it becomes more difficult to reduce compared with the bis(pyridinium)ethylene one since it experiences a stronger CT interaction. As a consequence, the first reduction of the bis(pyri-dinium)ethylene unit becomes the first reduction process of the whole system and therefore is displaced toward less negative potentials with respect to same process in the free macrocycle 3, in which such a process follows the first reduction of the other unit. 

In the following sections, examples of hydrogen-bonding templates for the synthesis of macrocycles, cages, interlocked species, helicates and for the photochemical reaction of olefins will be discussed. The use of hydrogen-bonding templates in dynamic combinatorial libraries will also be presented. 

Squaraines 17b and 17c have terminal acetylene residues, which allowed to convert the squaraine dyes and tetralactam macrocycles into permanently interlocked rotaxane structures using copper-catalyzed and copper-free cycloaddition reactions with bulky stopper groups [58]. 

Ceo-TTF molecules, interlocked molecules (catenanes, rotaxanes), donor-acceptor macrocycles, cage molecules, etc. (Jeppesen et al, 2004). It is beyond the scope of this book to review such developments and I appeal to the curiosity of researchers really not familiar with such macromolecules to see how big the molecules can become. 

As already said, catenanes are minimally composed of two interlocked rings. If it is arranged during the template-directed synthesis to have two identical units, that is, recognition sites, located within two different macrocycles, then the resulting... 

Interestingly, the second reduction of the two bipyridinium units splits in the case of catenane 144+ (Fig. 13.15) obtained by interlocking cyclophane 124+ with a symmetric macrocycle containing two electron donor dimethoxynaphthalene (DMN)... 

Electrochemical techniques can also give interesting information in the case of catenanes of higher complexity, as shown by the results obtained by investigating a series of catenanes made of up to seven interlocked rings.20 The three basic components of these catenanes are the tetracationic cyclophanes 124+ and 154+, and macrocycle 16 containing three electron donor DMN units (Fig. 13.16). For space reasons, only the electrochemical behavior of catenanes 174 1, 188+, 194+, and 204+ (Fig. 13.17), compared to those of their molecular components, is reported. 

Starting with catenane 174+, obtained by interlocking macrocycle 16 with only one cyclophane 124+, it is found, in agreement with these expectations, that the two bipyridinium units of 124+ undergo their first reduction in separated processes that are cathodically shifted with respect to the free cyclophane (Fig. 13.17a). Comparison... 

Controlled rotation of the molecular rings has also been achieved in catenanes composed of three interlocked macrocycles. For example, catenane 42H26+ (Fig. 13.37) is made up of two identical macrocycles 2 interlocked with a cyclophane containing two bipyridinium and two ammonium units.44 Because of the type of the macrocycles used, the stable coconformation of 42H26+ is that in which the two rings surround the bipyridinium units (Fig. 13.37a, state 0). Upon addition of one electron in each of the bipyridinium units, the two macrocycles move on the ammonium stations (Fig. 13.37b, state 1) and move back to the original position when the bipyridinium units are reoxidized. 

Figure 13.37 Redox controlled movements of the ring components in catenane 42H composed of three interlocked macrocycles. These motions are obtained upon reduction-oxidation of the bipyridinium units of the cyclophane.

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