Catenanes bipyridinium based

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. 

The Stoddart group has used the principles employed in their catenane studies to synthesise both a molecular trifoil knot and its isomeric large ring in very low yield. These isomers were separated by HPLC and characterised by liquid secondary ion mass spectrometry. The synthetic procedure, which is illustrated schematically in Figure 5.26, was based on the formation of a double-stranded supramolecular complex between an acyclic 7C-electron-rich, 1,5-dioxynaphthalene-based polyether and an acyclic 7i-electron-deficient bipyridinium-based tetracation. The extremely low efficiency of the synthesis in this case appears to reflect an unfavourable solution geometry of the intermediate host-guest complex - the latter... 

Stoddart s extensive work on catenanes started with the realization that diben-zocrown ethers contained two environments, the polyether electron-donor region and the electron-rich region with 7r-stacking potential. The aromatic regions were used to hold a 4,4/-bipyridinium-based molecular clip in place while cyclization with 1,4-di(bromomethyl)benzene was effected. The interlocked complex was obtained in 70 percent yield as the tetracationic salt with hexafluorophosphate counterions [4]. 

Figure B. Pictorial representation of the self-assembly of pseudorotaxa-nes based on (a) charge-transfer and C-H—O hydrogen-bonding interactions between 1,1 -diben-zyl-4,4 -bipyridinium dication and 1,5-dinaphtho[38] crown-10 (1/5DN38C10), and (b) hydrogen-bonding interactions between dibenzyl ammonium ion and dibenzo[24]crown-8 (DB24C8). A possible route towards the synthesis of rotaxanes and catenanes is also schematized.

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],... 

Stoddart and co-workers developed the efficient template-directed one-pot synthesis of the [2]catenane from a bipyridinium salt, bis(bromomethyl)benzene, and a paracyclophane crown ether [63], The dominant, noncovalent interactions involved are based on electrostatic and dispersive forces brought about by the reci-... 

When conferred with a hydrophilic head (in this case a substituted trityl unit), and a hydrophobic (benzylic alcohol) tail, rotaxanes - branched [55] or otherwise - can be formed into Langmuir films in a manner similar to catenanes. Rotaxane 224+ - synthesized from its corresponding thread via the slipping approach - when incorporated into a device in a manner similar to the catenane 214+ also exhibited interesting electron-transport properties [56], Unlike the catenane-based device, there is no switching element built into the molecule. However, like the switchable catenane, the rotaxane 224+ has electroactive bipyridinium sites, whose presence can mediate the tunneling of... 

Rationale and efficient synthetic approaches for the preparation of complicated supramolecular systems like pseudorotaxanes, rotaxanes and catenanes have been devised only recently [14]. The strategies chosen by Stoddart and coworkers are based on (i) charge-transfer (CT) interactions between electron-acceptor (e.g., bipyridinium-type) and electron-donor (e.g., dimethoxybenzene-type) units [15], or... 

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