Circumrotation, catenanes

Figure 13.14 Dynamic processes associated with circumrotation of one ring in a catenane made of two different macrocycles, each incorporating two identical recognition sites. Asterisks are used to highlight the exchange of position of identical units.

Figure 13.22 The circumrotation of the tetracationic cyclophane component of catenane 254+ can be controlled reversibly by adding-protonating -hexylamine that forms a charge transfer adduct with the diazapyrenium unit of the catenane.

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. 

Figure 13.33 (a) The circumrotation of the tetracationic cyclophane component of catenane... 

Figure 5. Restricted circumrotation of [2]catenane 7 the steric demand of the cyclohexane groups only allows a 90° rocking motion and prevents inner (gray) and outer isophtha-loyl units from exchanging (legend Figure 3) [15].

Supposing this mechanism and considering the restricted circumrotation, the positioning of substituents on the reactants should result in stable isomeric [2]catenanes. Indeed the well-aimed variation of the substitution pattern resulted in three different [2]catenanes - 14-16 (Figures 6 and 7) [20, 21], When isophthaloyl dichloride (3) was reacted with the methoxy-substituted diamine 13 (pathway... 

Figure 15. Synthesis of the partially aliphatic [2]catenane 33 with free circumrotation because of the long aliphatic chains.

Figure 38. The free circumrotation of the partial aliphatic catenane 33 can be restricted by attachment of bulky substituents, e.g. 98.

For instance, the [2]catenane, 51 [112] has been synthesized incorporating an anthracene subunit—an important electrochemical and photochemical active site which is a major building block required for switches and machines at the molecular level. The bulky anthracene stops the normal dynamic circumrotation... 

Figure 17.11 Chemical formula of the bis-phenathroline catenane ligand 12 that was adsorbed on a Ag(l 11) surface.109 In solution, complexation of 12 by Cu+ ions is accompanied by conformational changes involving the circumrotation of the macrocyclic components through each other s cavity.110...

The current-voltage curve was interpreted on the basis of the mechanism illustrated in Figure 17.15a, which is derived from the behavior of the same catenane 134+ in solution.116,117 Conformation I is the switch open state and conformation IV the switch closed state of the device. When 134+ is oxidized (+2 V), the TTF unit is ionized in state II and experiences a Coulombic repulsion inside the tetra-cationic cyclophane component, resulting in circumrotation of the crown ether and formation of conformation III (note that in solution at +2 V TTF undergoes two-electron oxidation and the dioxynaphthalene unit is also oxidized).116 When the voltage is reduced to near-zero bias, a metastable conformation IV is obtained... 

Because these approaches can often be equally well applied to the shuttling motion of rotaxanes, the circumrotation of catenanes, and the threading/dethreading of pseudorotaxanes, we do not restrict this discussion to rotation only, but present a small selection of different examples. 

Derivative 178 containing a dibenzo-34-crown-10 ring interlocked with macrocycles incorporating two 4,4 -dipyr-idyl moieties tethered by different aryl spacers acts as bistable [2]catenane <20060L2119>. Variable-temperature (VT) NMR studies were used to determine the activation energy required for the conformational interconversions and to demonstrate that, by appropriate incorporation of bulky groups on one or both of the aryl linkers, it was possible to block one or both of the two circumrotation pathways. 

When the synthesis of a [2]catenane leads to the interlocking of two different macrocycles each containing two identical recognition sites, then circumrotation of a macrocycle through the cavity of the other leads to degenerate equilibrium states. An example of a degenerate [2]catenane is shown in Figure 28, wherein the dynamic processes in solution are illustrated [23]. 

Figure 30. Circumrotation of the macrocyclic polyether component of the [2]catenane can be controlled oxidizing/reducing reversibly the tetrathiafulvalene unit [59. ...

Figure 31. Circumrotation of the tetracationic cyclophane component of the [2]catenane 19 + can be controlled reversibly by reducing/oxidizing electrochemically its bipyridinium unit [36a].

Figure 29. Photoconlrollable molecular-level brake. The thermally activated circumrotation of the macrocyclic polyether component of 2 catenane 31 can be modulated reversibly by cisitrans photoisomerization of the azobenzene unit incorporated into the tetracationic macrocycle [119, 120].

Fig. 24. Three different dynamic processes occur in the [2]catenane 174+. The crown ether circumrotates through the cyclophane (a c and b d) 27 times/s at 298 K, the cyclophane circumrotates through the cavity of the crown ether(a b and c d) 8000 times/s, and the two components rock with respect to each other (e f) nearly 2 million times/s at 298 K...

The rationale behind this design was justified upon electrochemical investigation of the [2]catenane 184+. This catenane - synthesized in 43% yield (Fig. 26) from crown ether BPP34C10, the bipyridinium dibromide derivative 192+ and ( )-l,2-bis(4,4 -bipyridyl)ethylene - was demonstrated to consist, in solution, of mainly co-conformer A, with the more powerful n-electron-accepting bipyridinium unit located inside the cavity of the crown ether. Upon electrochemical reduction of this bipyridinium unit, the cyclophane undergoes a circumrotational movement with respect to the crown ether such that the profoundly more electron-deficient 7t-extended bipyridinium unit resides inside the cavity of the crown ether, affording co-conformer B. When the bipyridinium radical cation is oxidized back down to its dicationic state, the opposite circumrotational process occurs and the system reverts back to co-conformer A, its ground state [49]. 

Fig. 26. When the dibromide salt 19-2PF6 is reacted with bis (pyridine) ethylene in the presence of BPP34C10 in acetonitrile, the resultant [2]catenane 18-4PF6 is formed in 43% yield. The crown ether resides preferentially around the bipyridinium site in a 92 8 ratio with respect to occupancy around the bis(pyridinium) ethylene site - co-conformer A. a Upon electrochemical reduction, the best electron donor - the bipyridinium site - is reduced first. This reduction leads b to the unfavorable situation in which the bipyridinium radical cation is located within the cavity of the crown ether, and so the cyclophane circumrotates c to locate the bis(pyridinium)ethylene site within the crown ether cavity - co-conformer B. This process is reversible, in that d reoxidation of the bipyridinium radical cation leads to e circumrotation of the cyclophane to yield the [2]catenane in its original state - co-conformer A...

Fig. 29. The UV-Vis spectrum of [2]catenane 21-4PF6 shows - a by virtue of a strong charge transfer band at 830 nm - exclusive occupancy of the cyclophane cavity by the TTF unit, b Addition of one equivalent of Fe(Cl04)3 oxidizes the TTF unit to its radical cation, which results in the circumrotation of the crown ether, such that the 1,5-dioxynaphthalene unit resides within the cyclophane s cavity, as indicated by the appearance of a charge-transfer band at 515 nm. c Addition of a further equivalent of iron perchlorate yields the TTF dication which remains outside the cavity. Chemical reduction of the TTF dication back to its neutral form yields the [2]catenane in which the TTF resides exclusively within the cyclophane s cavity...

It was noted that, although the circumrotational motion followed mostly the general mechanism pertaining to the catenane in the solution phase, upon... 

Fig. 35. The device functions in a manner not entirely unlike the solution-state behavior of the catenane, with some subtle differences. The ground state of the device - [A0] -represents the switch open state, i.e., the switch is off . Oxidation of the TTF unit yields [A+], which spontaneously circumrotates, resulting in the co-conformer [B+]. Subsequent reduction of the TTF radical cation yields the stable co-conformer [B°], that represents the closed state of the switch, i.e., the switch is on . The switch is reset only by (partial) reduction of the bipyridinium units - a reaction which allows the departure of the iz-electron-rich unit from the cavity of the cyclophane. Reoxidation of the bipyridinium units completes the closing of the switch, i.e., it is once again off , on its return to [A0]...

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