Rotaxanes deprotonated

Figure 13.10 Cyclic voltammetric behavior on reduction of the protonated 9H3+ and deprotonated 92+ rotaxane shown in Fig. 13.9 and of its protonated and deprotonated dumbbell-shaped component (argon-purged MeCN/EtrNPFe 0.05 M, 298 K, glassy carbon electrode, scan rate 50mV/s). The current intensity has been corrected to account for the differences in diffusion coefficients.

It can be concluded, as shown in the square scheme reported in Fig. 13.27, that in the deprotonated rotaxane (i) the first reduction of the bipyridinium weakens the charge transfer interactions and promotes the displacement of the ring far from the monoreduced unit, and (ii) the reoxidation of such a unit, restoring its electron acceptor power, causes the back movement of the ring. 

From the electrochemical viewpoint, this system is particularly interesting because, by changing its protonation state, electrons can play different roles in protonated rotaxane 9H3+ (see Section 13.2.2), they are simply used to read the state of the system, whereas in deprotonated rotaxane 92+, they play the dual role of writing and reading the system. 

Figure 13.27 Dual-pathway square scheme mechanism that accounts for the rearrangements induced by the monoelectronic reduction of deprotonated rotaxane 92+. The species A and C represent the stable structure of the deprotonated rotaxane and its monoreduced form, respectively, whereas and D are metastable intermediates. Note that the exact position of the macrocycle along the axle in the reduced forms and C is not known. From a simple digital simulation of the cyclic voltammetric patterns, the following values have been obtained = - 0.59V, E°dc = - 0.34V, /cAD 0.15S- da<2.5s kBC > 100 s and kCB 1 s V...

A very surprising and fruitful result was obtained when a control experiment related to an amide templated synthesis was made. Dibromo compound 19 utilized in the reaction looked similar to the axle centerpiece used in the amide template synthesis but lacked the amide in the middle which was crucial for this purpose. However, when the reaction was complete, it was found that rotaxane 24 was formed with 80-95% yield [12] (Figure 9). It seemed reasonable to assume that this time not the axle but the stopper coordinated to the macrocycle [27], This suggestion was supported by the high binding constant of the deprotonated stopper-wheel complex 21 18 (> 105 M"1) derived from H NMR titrations. In the rotaxane synthesis, this complex reacts with the semiaxle 23 producing a rotaxane. 

One highly interesting feature of this synthetic approach is that rotaxanes are generated that contain a functional group at their axle centers. This permits to control the rotaxanes properties by external stimuli, e.g. by protonation and deprotonation. 

The [2]rotaxane 224+ can be switched (Figure 15) by controlling the pH. Upon addition of an excess of trifluoroacetic acid, the benzidine unit becomes protonated, generating the [2]rotaxane [22-2H]6+. The tetracationic cyclophane moves away from this newly generated dicationic unit because of electrostatic repulsion. In this case, the absorption spectrum lacks the 690 nm band, confirming the deprotonation of the benzidine unit and the relocation of the cyclophane to the biphenol unit. The [2]rotaxane [22-2H]6+ can be subsequently de-protonated by the addition of pyridine, regenerating the [2]rotaxane 224+. 

Cu(I)-complexed [2]-rotaxane 96 was synthesized as follows (Figure 2.32)16f 67 2-rnethy l-9-(p-anisy I)-1, 10-phenanthroline 91 was deprotonated (lithium diisopropylamide) and the resulting anion was alkylated with the... 

Figure 14. Shuttling of the macrocyclic component of [2]rotaxane 13 along its dumbbell-shaped component can be controlled electrochemically by oxidizing/reducing the benzidine unit [43]. Shuttling of the macrocycle component can also be controlled by protonating/deprotonating the benzidine unit (see text).

Figure 20. Ring motions in [2]rotaxane upon deprotonation/protonation [49].

Fig. 5.10. CID experiments conducted with (top to bottom) the mass-selected axle, rotaxane and the non-intertwined hydrogen-bonded axle-wheel complex. The inset on top shows the fragmentations of the deprotonated axle.

Fig. 16A-D. Mechanical switching in rotaxanes. A Rotaxanes may exist in isomeric states by the movement of the ring component between dissymmetric sites on the string component. B A redox- or pH-switchable [2]rotaxane. While the cyclophane complexes the native benzidine site (spectrum, curve a), the reduced or protonated benzidine repels the cyclophane, causing it to move to the dioxybiphenylene site (spectrum, curve b). C An azobenzene-based switchable [2]rotaxane. The cyclodextrin ring complexes the azobenzene site in the trans-state, but it is repelled from the ds-azobenzene. The state of the system is measurable by circular dichroism (plot). D A pH-switchable rotaxane. When the amine on the string component is protonated, it complexes the crown ether ring by hydrogen-bonding interactions (40a). When the amine is deprotonated, however, the ring component moves to the bipyridinium unit, where it is complexed by n donor-acceptor interactions (40b). The plots in B and C are adapted from [67] and [69], respectively, with permission...

Fig. 3 A chemically controllable molecular shuttle the macrocyelic ring can be switched between the two stations of the dumbbell-shaped component by base/acid inputs. Additionally, in the deprotonated rotaxane, the ring can be displaced from the bipyridinium station through reduction of such unit. (View this art in color at www.dekker.com.)...

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