Tetra rotaxane

The electrochemical and chemical behavior of rotaxane 7 + was analyzed by CV and controlled potential electrolysis experiments.34,35 From the CV measurements at different scan rates (from 0.005 to 2 V/s) both on the copper(I) and on the copper(II) species, it could be inferred that the chemical steps (motions of the ring from the phenanthroline to the terpyridine and vice versa) are slow on the timescale of the experiments. As the two redox couples involved in these systems are separated by 0.7 V, the concentrations of the species in each environment (tetra- or pentacoor-dination) are directly deduced from the peak intensities of the redox signals. In Fig. 14.13 are displayed some voltammograms (curves a-e) obtained on different oxidation states of the rotaxane 7 and at different times. 

We have recently reported that compound 7a shows a smectic-smectic phase transition associated with the change of the photoluminescent color (Fig. 7) [33]. Compound 7a has a 2,6-diethynylanthracene group as a photoluminescent core. This molecule has fork-like mesogens which consist of tetra(ethylene oxide) and p-(4-/ra .v-pentylcyclohexyl)phenyl moieties. A similar molecular design was previously applied to induce smectic liquid crystallinity for rotaxanes and catenanes [39—41 ]. 

Tetra-urea Calix[4]arenes - From Dimeric Capsules to Novel Catenaries and Rotaxanes... 

Scheme 5.20 Two possibilities to synthesize tetra[2]rotaxanes (schematic) (a) by stoppering (FC represents a suitable functional group) or (b) by clipping (ring closure by metathesis).

The preorganization of functional groups attached to the urea residues could also be used for the synthesis of multiple rotaxanes. Schematically, this is shown for tetra[2] rotaxanes in Scheme 5.20. Bis[2]rotaxanes will be available analogously, using a bisloop compound 8 instead of the tetraloop compound 9. 

Multiple ring-closure reactions between adjacent urea functions of a suitably functionalized tetra-urea in a heterodimer with a second tetra-urea bearing bulky groups can also create the structural elements of a rotaxane. This strategy is usually called clipping (see Scheme 5.7). 

Tetra-ureas 20 form homodimers in solvents such as CDC13 or C6D6, but they are completely converted to heterodimers upon addition of an equimolar amount of a tetra-loop compound 9. When a 5-10% excess of the anthracene derivative 21 was applied, a quantitative conversion was obtained in refluxing toluene after 72 h (as judged by NMR), and the tetra[2]rotaxane 22 was isolated in 40-50% yield. 

The rotaxane structure of 22 can be demonstrated by ESI-MS or MALDI-TOF-MS. It remains intact in TH F-d8, a solvent in which the tetra-urea dimers usually dissociate. However, an upfield shift of the NH-signals in 1 H NMR indicates that the... 

The synthesis of fourfold [2]rotaxanes by clipping requires the efficient formation of heterodimers between a tetra-urea substituted by bulky stopper groups and an octaalk-enyl urea 6. We initially hoped that the steric crowding in homodimers of a tetra-tritylphenylurea calix[4]arene would be sufficient to shift the equilibrium toward the heterodimers in a mixture with 6. However, the distribution of the dimers was close to the statistical ratio and the desired rotaxane could be obtained in only 5% yield [59]. 

Figure 5.19 Molecular shape of rotaxane 23 (n = 6), a hetero-dimeric assembly of a tetraloop tetra-urea 9 (stick representation) and tetra-tosylurea 4 (space filling), based on MD simulations. Ether groups (OY = OC5Hnn,) areomitted in the formula.

The principal idea of this present essay was to show how the unique preorganization of functional groups in self-assembled dimers of tetra-urea calix[4]arenes can be used to prepare novel multi-rotaxanes and -catenanes or topologically even more complex molecules and supramolecular structures. We will conclude by summarizing some related studies in which calixarenes were used in a different way as building blocks for the construction of such structures or assemblies. 

Pseudo polyrotaxanes consist of low molecular weight rotaxanes which can assemble via non-covalent interactions. An illustrative example has recently been described by Stoddart, using a tetra-cationic bis(bipyridyl-phenyl)-cyclophane as macrocyclic ring component and phenylene-l,4-bis(oxy-oligoethyleneoxy)-co-carboxylic acid [385]. 

Fig. 3.13. Structure of Kaneda s cyclic di[2]rotaxane (a) and cyclic tetra[2]rotaxane (b) formed by permethylated a-CDs with an azobenzene derivative [31, 32].

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