Rotaxane electrochemical oxidation

In a similar manner donor-acceptor [2]rotaxanes such as the derivative 213 can be assembled. This example has been described as a molecular switch. The CBPQT4+ ring can occupy two positions as shown in the translational isomers 213 and 214 (Scheme 18). At equilibrium in acetonitrile solution the CBPQT4+ ring mainly occupies the benzidine site (84%) 213. Protonation (or electrochemical oxidation) eliminates the favorable CBPQT4+/benzidine donor-acceptor interaction and results in preferential occupation of the biphenol site 214 <2008T8231>. 

The controlled motion of the ring between the two coordinating sites of the string (schematically represented in Figure 2.33) in Cu(I)-complexed [2]-rotaxane 96 takes place as follows in the initial metallorotaxane the complexed ring stays at the phenanthroline site, because of the stereoelectronic requirements (tetrahedral coordination sphere) of Cu(I). Electrochemical oxidation of Cu(I) to Cu(II) resulted in the movement of the macrocycle to the terpyridine site, since Cu(II) requires higher coordination numbers than Cu(I). This translational motion occurs at a rate of 1.5 x 10-4 s-1 at room temperature... 

Figure 2.35. Diagram of the molecular motions in copper(I)-complexed [2]-rotaxane 101 controlled by the redox state dependence of the stereoelectronic requirements of Cu. Cu(I) and Cu(II) are represented by black and white circles, respectively. Chelating sites are represented by thick lines. Initially, the string coordinates Cu(I) together with the bidentate site of the macrocycle in a tetrahedral geometry, affording the state 101(4). Electrochemical oxidation of Cu(I) to Cu(II) produces the state 101+(4), which slowly converts into the state 101+(5) after rotation of the Cu(II)-complexed macrocycle. The cycle is completed by reduction of Cu(II), which produces the state 101(5), converting to the initial state by back motion of the Cu(I)-complexed macrocycle.

Fig. 31 Mechanical actuation of a gold-coated microcantilever by molecular muscles [227]. (a) Structural formula of a palindromic, bistable [3]rotaxane with gold-binding dithiolane groups attached to the cyclophanes. (b) Reversible bending up and down of a cantilever by actuation of a monolayer ( 8 billion molecules) of the rotaxanes on its surface. The gold surface bends when the rotaxanes contract under the influence of an electrochemical oxidation that causes the cyclophanes to shuttle inward from the periphery of the molecule, (c) Electrochemical cell (Ag/AgCl, Pt, and the cantilever are the reference, counter, and working electrodes, respectively) and combined AFM device used to measure the bending by detecting a laser beam reflected off of the cantilever s surface...

Fig. 3. A switchable rotaxane based on acceptor-donor complexes [24]. Before oxidation, the electron acceptor (ring) interacts preferentially with the benzidine nucleus (donor). After electrochemical oxidation of the latter, the ring is shifted towards the biphenol group. The process is reversible...

Electrochemical oxidation of the benzidine guest in 24+ creates a positive charge that forces the tetracationic bead to move over to the biphenol station (Scheme 5). This was clearly verified by the half-wave potential values obtained in cyclic voltammetric experiments with this rotaxane [7]. The reversible character of the one-electron oxidation of the benzidine unit in 24+ provides a useful electrochemical mechanism to control the sliding motion of this rotaxane s bead. Although we did not verify this point experimentally, it should be equally possible to oxidize the benzidine unit using homogeneous... 

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...

Figure 12 A rotaxane in motion (12) based on the redox state dependence of the stereoelectronic requirements of a transition metal (Cu) [19]. Cu(I) and Cu(ll) arc represented by black and white circles respectively. The linear component incorporates a bidentate and a separated terdentate chelating site. The macrocycle coordinates Cu(I) together with the bidentate site of the linear component in a tetrahedral fashion. Electrochemical oxidation of Cu(I) to Cuffl) triggers the transfer of the Cu(lI)-complcxed ring to the terdentate chelate of the thread. I hc timescale of this process is ca. l-2h. Interestingly, the back motion is faster it takes place in minutes.

When rotaxanes and catenanes contain redox-active units, electrochemical techniques are a very powerful means of characterization. They provide a fingerprint of these systems giving fundamental information on (i) the spatial organization of the redox sites within the molecular and the supramolecular structure, (ii) the entity of the interactions between such sites, and (iii) the kinetic and thermodynamic stabilities of the reduced/oxidized and charge-separated species. 

The electrochemical behavior of rotaxane 76+ can be straightforwardly explained on the basis of the above discussion. On reduction, a first monoelectronic process, assigned to the first reduction of the free bipyridinium unit, is followed by a bielectronic process, assigned to the first reduction of the two bipyridinium units encircled by the ring. Even the second reduction of the three bipyridinium units, which occurs at more negative potentials, occurs with the same 1 2 pattern. On oxidation, the behavior of rotaxane 76+ is again similar to that of rotaxane 66+, with a more intense process in correspondence of oxidation of the DMB units. 

In rotaxane 86+, the three bipyridinium units are expected to be electrochemically equivalent because each one is encircled by the ring. In agreement with this expectation, it has been found two trielectronic processes corresponding to the first and the second reduction of the bipyridinium units, a situation similar to that observed for compound 56+. In the case of the rotaxane, however, the processes occur at more negative potentials because of the CT interaction. As far as oxidation is concerned, the behavior of 86+ is in line with that of 66+ and 76+. 

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. 

Fig. 13 Principle of the electrochemically induced molecular motion in a rotaxane copper complex. The stable, four-coordinate monovalent complex is oxidized to an intermediate tetrahedral divalent species. This compound undergoes a rearrangement to afford the stable, five-coordinate copper(u) complex.

Cyclic voltammetry shows that the [2]rotaxane 22 1 h can also undergo electrochemical switching (Figure 16) by a monoelectronic oxidation process to give the radical pentacationic species 225+. On oxidation, the benzidine unit is converted to its monocationic radical state, generating an electrostatic repulsion, which causes the tetracationic cyclophane to move to the biphenol unit in [2]rotaxane 224+. This redox procedure is completely reversible. 

As already pointed out in the case of rotaxanes, mechanical movements can also be induced in catenanes by chemical, electrochemical, and photochemical stimulation. Catenanes 164+ and 174+ (Fig. 19) are examples of systems in which the conformational motion can be controlled electrochemically [82, 83], They are made of a symmetric electron acceptor, tetracationic cyclophane, and a desymmetrized ring comprising two different electron donor units, namely a tetrathiafulvalene (TTF) and a dimethoxybenzene (DOB) (I64 1) or a dimethoxynaphthalene (DON) (174+) unit. Because the TTF moiety is a better electron donor than the dioxyarene units, as witnessed by the potentials values for their oxidation, the thermodynamically stable conformation of these catenanes is that in which the symmetric cyclophane encircles the TTF unit of the desymmetrized macrocycle (Fig. 19a, state 0). 

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).

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