Circumrotation

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... 

In contrast, the bis (bipyridinium) macrocycle component (6.81) undergoes only two discrete two-electron reductions. The reason that four distinct reductions are not observed for 10.76 is that following the initial two reductions, the ring circumrotation process (process II, Figure 10.59) becomes fast on the cyclic voltammetric time scale and so the distinction between the inner and outer bipyridinium units is lost. The cyclic voltammetric waves are shown in Figure 10.60. 

In the [2]catenate, the circumrotation of the terpyridine-containing macrocyclic component can be reversibly controlled 21,22 (Figure 5), by altering the redox state of the metal. The absorption spectrum of a red-brown solution of [5 Cu] BF4 in MeCN shows a band, centered on 437 nm, characteristic of a Cu+ ion tetracoordinated to two phenanthroline ligands. Upon oxidative electrolysis, or upon addition of Br2, Cu+ is oxidized (step 1 in Figure 5) to Cu2+ and the solution turns deep green. The absorption spectrum shows a band, centered on 670 nm, typical of a Cu2+ ion tetracoordinated to two phenanthroline ligands. However, this absorption band shifts to... 

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. 

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