Tetracationic cyclophanes

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. 

Macrocycle 16, containing three equivalent DMN electroactive units, shows three distinct oxidation processes (Fig. 13.17). Such a contrasting behavior between 16 and the tetracationic cyclophanes, in which the two incorporated bipyridinium units undergo simultaneous first and second reductions, can be interpreted considering that, in the cyclophanes, the rigidity of the structure prevents interaction between the two bipyridinium units, whereas the flexible structure of macrocycle 16 allows the three DMN units to approach one another. 

Figure 13.22 The circumrotation of the tetracationic cyclophane component of catenane 254+ can be controlled reversibly by adding-protonating -hexylamine that forms a charge transfer adduct with the diazapyrenium unit of the catenane.

As discussed in Section 13.2.4, when one of the two rings of a catenane carries two different recognition sites, the dynamic processes of one ring with respect to the other can be controlled. In particular, if redox units are incorporated into the catenane structure, there is the possibility of controlling these processes upon electrochemical stimulation. Catenanes that exhibit such a behavior can be seen as electrochemically driven molecular rotors. An example is offered by catenane 384+ (Fig. 13.33a), which incorporates macrocycle 2 and a tetracationic cyclophane comprising one bipyridi-nium and one trans-l,2-bis(4-pyridinium)ethylene unit.19,40... 

In the major isomer, the bipyridinium unit is located inside the cavity of the macrocyclic poly ether and the /7Y//7,v-bis(pyridinium)ethylene unit is positioned alongside, as confirmed by the electrochemical analysis. The cyclic voltammo-gram of the catenane shows four monoelectronic processes that, by a comparison with the data obtained for the free cyclophane, can be attributed as follows the first and third processes to the first and second reductions of the bipyridinium unit, and the second and fourth ones to the first and second reductions of the trans-bis (pyridinium)ethylene unit. The comparison with the tetracationic cyclophane also evidences that all these reductions are shifted toward more negative potential values (Fig. 13.33b). 

The discussion can be restricted to the first and second reduction processes that are of particular interest in this context. The shift of the bipyridinium-based process is in agreement with the catenane coconformation in which the bipyridinium unit is located inside the cavity of the macrocyclic polyether (Fig. 13.33a) because of the CT interactions established with both the electron donor units of the macrocycle, its reduction is more difficult than in the free tetracationic cyclophane. The shift of the trans-1,2-bis(4-pyridinium)ethylene-based reduction indicates that, once the bipyridinium unit is reduced, the CT interaction that stabilize the initial coconformation are destroyed and, thereby, the tetracationic cyclophane circumrotates through the cavity of the macrocyclic polyether moving the tra ,v-bis(pyridinium)ethylene unit inside, as shown by comparison of its reduction potential with that of a catenane model compound.19 The original equilibrium between the two coconformations associated with catenane 384+ is restored upon oxidation of both units back to their dicationic states. 

It is interesting to notice that for a machine-like performance, the presence of an asymmetric ring is a necessary but not sufficient requirement. This statement is clearly demonstrated by the behavior of catenane 394+ made by the same tetracationic cyclophane of 384+ and a macrocycle containing two DMN units.19,40 As in... 

Figure 13.33 (a) The circumrotation of the tetracationic cyclophane component of catenane... 

Catenane 404+ (Fig. 13.35) is another example of a system in which the coconformational motion can be controlled electrochemically.41 It is made of the symmetric tetracationic cyclophane 124+ and a nonsymmetric ring comprising two... 

Figure 13. The self-assembly of [2]pseudorotaxanes incorporating the tetracationic cyclophane 15-4PF6 and the acyclic dioxyarene-based polyethers 16-22.

This [2]catenane is composed of a jt-electron-deficient tetracationic cyclophane interlocked with a Jt-electron-rich macrocyclic polyether. In addition to a mechanical bond, [jt Jt] stacking interactions between the complementary aromatic units, [C-H---0] hydrogen bonds between the a-bipyridinium hydrogen atoms and the poly-ether oxygen atoms, and [C-H---Jt] interactions between the 1,4-dioxybenzene hydrogen atoms and the p-phenylene spacers in the tetracationic cyclophane hold the two macrocyclic components together and control their relative movements in solution. As a result of the asymmetry of the tetracationic cyclophane, two transla-... 

Molecular shuttle 154+ consists of a tetracationic cyclophane macrocycle, a linear thread containing two hydroquinol stations and a polyether spacer. The macrocycle binds the stations via n - n and charge-transfer interactions between the electron-poor cyclophane and the electron-rich hydroquinols. As explained above, because both stations are energetically degenerate (they are chemically identical) the macrocyclic unit has no preference for either of them and randomly shuttles between them, in this case at a rate of k = 2360 s 1 in (CDs CO at 34 °C, measured by JH NMR spectroscopy. It was already noted in Stoddart s seminal 1991 paper that including two stations of different binding affinity in the thread could allow a stimuli-induced change of position of the macrocycle in a molecular shuttle. 

In heterocircuit [2]catenane 174+, the tetracationic cyclophane initially encircles the more electron-rich tetrathiafulvalene station (TTF) as evidenced... 

The macrocyclization reaction described above has been used to generate a great number of catenane (12) and rotaxane (13) architectures (Scheme 10.4) using both crown ethers [preformed macrocyclic components 14 (strategy A)] and hydroqui-none-based dumbbell-shaped polyethers [preformed acyclic components 15 ( clipping )] as templates [14b, 15]. These templates are also relatively robust with regard to the substitution of different groups into both the tetracationic cyclophane and the neutral frameworks. 

Scheme 10.3 Template-directed synthesis of a tetracationic cyclophane by Stoddart and coworkers.

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