Electrochemically Controlled Shuttling Processes

Sauvage has demonstrated both electrochemical and photochemical control over ring motions in a catenate, 18 [57,58]. The observed behavior of the catenate is essentially similar to the analogous rotaxane, the only difference being that the 4-coordinate to 5-coordinate (dpp -> terpy) shuttling process is slower in the catenate and the reverse step is faster. Again, the issue of directionality is not addressed in this system. 

No example has so far been reported of a shuttling process controlled by electron transfer chemical reactions. There are, however, very interesting examples of shuttling processes controlled by acid/base reactions. One case is that of the previously discussed compound 13 " (see Figure 14), in which the shuttling of the macrocycle component can be controlled not only electrochemically, but also by protonation/ deprotonation of the benzidine unit [43]. 

Fig. 15. A chemically and electrochemically controllable molecular shuttle 104+. When the dumbbell is in its unperturbed state, the cyclophane exists in an 84 16 ratio (CD3CN, -44 °C) encircling preferentially the comparatively more 7t-electron-rich benzidine site. The cyclophane can be enticed to translate to the biphenol site exclusively either chemically -through protonation of the benzidine nitrogen atoms - or electrochemically - through oxidation of the benzidine unit to its radical cation. Both of these processes are completely reversible, through addition of base, or electrochemical reduction, respectively...

Fig. 16. A photo- and electrochemically controllable molecular shuttle. The unperturbed rotaxane 116+ exists preferentially in the translational isomer in which the BPP34C10 crown ether resides around the bipyridinium unit, a Photochemical excitation of the Ru(bipy)3 unit results in PET to the bipyridinium site, and consequent translation of the crown ether to the 3,3dimethylbipyridinium unit, which is a less efficient recognition site for the cyclophane CBPQT4+ than a bipyridinium system. This process occurs only in the presence of a sacrificial reductant which reduces the Ru(III) center back to its Ru(II) state in order to prevent charge recombination, b Conversely, upon electrochemical reduction of the bipyridinium unit, the crown ether takes up residency around the 3,3 -dimethylbipyridi-nium site. This process is reversed through electrochemical oxidation of the bipyridinium radical cation back to the dication...

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