Catenane ring rotation

Figure 13.2 Schematic representation of the intercomponent motions that can be obtained with simple interlocked molecular architectures ring shuttling in rotaxanes (a), and ring rotation in rotaxanes (b) and catenanes (c).

Figure 13.35 Redox controlled ring rotation in solution for catenane 404+, which contains the symmetric electron acceptor cyclophane 124+ and a nonsymmetric electron donor ring.

Fig. 19 Redox controlled ring rotation in catenanes 164+ and I74+, which contain a symmetric electron-acceptor cyclophane and a desymmetrized electron-donor ring [82, 83]. Steps a-d are explained in the text...

Fig. 23 (a) Structural formula of catenane 20. (b) Processes that enable unidirectional ring rotation [92]. A, B recognition sites X], X2 blocking groups... 

The time scales and number of reactions involved for unidirectional ring rotation in catenane 20 and in other similar catenanes [93] make their operation as rotary motors somewhat unpractical. Nevertheless, analysis of the thermodynamic and kinetic aspects of the operation mechanisms provides a fundamental insight on how energy inputs can be used to harness thermal fluctuations and drive unidirectional motion. 

Supramolecular interactions are an important factor in catenane formation. Such interactions can be disrupted after the catenane has been built, making the catenane structure more flexible. This flexible nature can be an advantage because the catenane structure is then free to respond when external stimuli are applied. The catenane shown in Fig. 3.26 is one example where this structural flexibility is utilized. One of the rings of this catenane contains two kinds of ligands, and the nature of the coordination to the copper ion depends on the oxidation number of the copper. When the copper ion is in the Cu(I) state, fourway coordination is stabilized. However, five-way coordination becomes more favorable upon oxidation to Cu(II), and to accommodate this, the ring rotates... 

Figure 30. An example of externally induced ring rotation in a [2]catenane [92]. The motion could be triggered either electrochemically or chemically.

Figure 20. Pictorial representation of machine-like movements that can be obtained with pseudorotaxanes, rotaxanes, and catenanes (a) dethreading/rethreading of the molecular components in a [2]pseu-dorotaxane, (b) shuttling of the macrocyclic component along the axle in a [2]rotaxane, and (c) ring rotation in a [2]catenane.

Figure 30. Photoinduced rotation, based on the use of an oxidant scavenger (/>-nitrobenzylbromide), of the terpyridine-containing macrocycle in the copper-containing [2] catenane 32+ [121]. The system is brought back to the initial structure through another ring rotation, induced chemically by reduction with ascorbic acid.

A schematic representation of a catenane complex with donors from each of two interlinked rings binding to a central metal ion, and (at right) the process by which a system with differing ring components may operate as an electrochemically-driven molecular switch, involving ring rotation. 

Leigh. D.A. Troisi. A. Zesbetto. F. A quantum-mechanical description of macrocyclic ring rotation in benzylic amide [2]catenanes. Chem. Eur. J. 2002. 7. 1450-1454. 

Controlled rotation of the molecular rings has also been achieved in catenanes composed of three interlocked macrocycles. For example, catenane 42H26+ (Fig. 13.37) is made up of two identical macrocycles 2 interlocked with a cyclophane containing two bipyridinium and two ammonium units.44 Because of the type of the macrocycles used, the stable coconformation of 42H26+ is that in which the two rings surround the bipyridinium units (Fig. 13.37a, state 0). Upon addition of one electron in each of the bipyridinium units, the two macrocycles move on the ammonium stations (Fig. 13.37b, state 1) and move back to the original position when the bipyridinium units are reoxidized. 

A catenane is a molecule composed of two or more interlocked macrocyclic components. From a macroscopic mechanical viewpoint the movement of one ring relative to the other in a catenane is reminiscent of a ball and socket joint (Fig. 18, top) [81]. Similarly, twisting of one ring around the main axis of the catenane forces the other ring to rotate in the same direction in a manner reminiscent of an universal joint (Fig. 18, bottom) [81]. 

Controlled rotation of the molecular rings has also been achieved in catenanes composed of three interlocked macrocycles. For example, catenane 19H26+ (Fig. 21) is made up of two identical dioxybenzene-based macrocycles interlocked with a cyclophane containing two bipyridinium and two ammonium units [87],... 

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