Interlocked pseudorotaxane

In particular, rotaxane dendrimers capable of reversible binding of ring and rod components, such as Type II, pseudorotaxane-terminated dendrimers, can be reversibly controlled by external stimuli, such as the solvent composition, temperature, and pH, to change their structure and properties. This has profound implications in diverse applications, for instance in the controlled drug release. A trapped guest molecule within a closed dendrimeric host system can be unleashed in a controlled manner by manipulating these external factors. In the type III-B rotaxane dendrimers, external stimuli can result in perturbations of the interlocked mechanical bonds. This behavior can be gainfully exploited to construct controlled molecular machines. 

The strong hydrogen bonding interactions observed between the oxygen atoms of crown ethers and the N-H groups of ammonium groups can be successfully employed to prepare pseudorotaxanes and rotaxanes by templated processes. This approach has been extensively utilised by Stoddart, Busch and others to obtain a wide range of interlocked species. 

Loeb has reported a series of pseudorotaxanes [84,85] and rotaxanes [86,87] where C-H- 0 hydrogen bonding interactions (together with N+- -O attractive forces) play an important contribution in templating the formation of the interlocked species. In particular, the formation of a pseudorotaxane was observed when equimolar amounts of [pyCH2CH2py]2+ and the crown ether 20 were mixed. The structural characterization of the resulting host-guest complex... 

Combination of 10.41 with crown ether derivative 10.48 and Ag0) gives a trimetallic sheathed rack in which a rack of Ag(I) ions is threaded through the cavities of three heterocrowns (Figure 10.42). This complex is an example of a pseudorotaxane (stricdy a [4] pseudorotaxane because there are four components - three loops and an axel) and we will return to these kinds of assemblies, which are precursors for the synthesis of complex interlocked molecules by postmodification techniques, in Section 10.7. 

Scheme 6.1 Schematic representation of interlocked molecules (a) pseudorotaxane. (b) rotaxane and (c) catenane.

Molecules incorporating interlocked [refs. 6, 7] components (Fig. 10.1) and their supramolecular analogs are suitable candidates for the generation of bistable chemical systems. A [2]pseudorotaxane is a supramolecular complex composed of a macrocyclic host encircling a linear guest. The two components are held together solely by noncovalent bonding interactions and they can... 

This topic was partially covered in CHEC-II(1996) <1996CHEC-II(9)809> under the subentry Catenanes and Rotaxanes . In this section, emphasis is given to the design and construction (and to some extent, the properties) of supramolecular architectures derived from or incorporating crown ethers rather than to the synthesis of the crown ether component present in them. The crown ether rings described herein are either covalently linked (dendrimers), mechanically interlocked (rotaxanes, catenanes), or just bound by noncovalent interactions (pseudorotaxanes) to the rest of the supermolecule to which they belong. 

Compound 129 - the first [2]rotaxane of the dialkylammonium ion/crown ether family ever to be reported - was obtained by Busch and co-workers via acylation of a pseudorotaxane precursor bearing an anthracenyl-substituted ethylenediammonium axle, in a biphasic CHC13/H20 system <1995CC1289>. Amide-bond formation has also been used for the attachment of the second stopper to [2]rotaxane 130 <20010L2485>. In this case, however, the final interlocking step was carried out between a preformed dicyclocarbodiimide-activated [2]rotaxane precursor and an N-substituted ethylenediamino stopper. 

Rotaxanes [a name derived from the Latin words rota (wheel) and axis (axle)] [1] and catenanes [from the Latin word catena (chain)] [1] are supramolecular (multi-component) species [1-10] strictly related (Figure 1) to pseudorotaxanes, which were described in Volume III, Part 2, Chapter 6. Whereas pseudorotaxanes can undergo dissociation into their wire-like and macrocyclic components, rotaxanes and catenanes are interlocked species, whose dissociation requires breaking of a covalent bond. It should be pointed out, however, that, as discussed in detail in the previous chapter 6, the boundary between rotaxanes and pseudorotaxanes is somewhat fuzzy because, for example, when the stoppers are not extremely bulky compared to the hole of the macrocyclic component, a rotaxane at low temperature might well be a pseudorotaxane at elevated temperature [11]. 

Gibson et al. reported that the copolymerization of poly(THF) and a diol-pseudorotaxane consisting of 4,4/-bipyridinium salt and bis-p-phenylene crown ether with diisocyanate afforded the corresponding polyurethane with the interlocked structure [48] (Scheme 6). Although this polyurethane has a pseudopolyrotaxane structure, the interlocked structure is stable because the interaction between 4,4/-bipyridinium salt and bis-p-phenylene crown ether is strong enough to keep the inclusion complex. In this elastic polyurethane, the rotaxane unit acted as a hard segment. 

The attractive interaction of crown ether with certain secondary ammonium salts is strong enough to prepare various interlocked compounds. The combination of 24-membered crown ether such as dibenzo-24-crown8 (DB24C8) and bis (primary alkyl) or dibenzyl ammonium salt has been widely used for the complex formation. While there are two types of complexes, side-on and inclusion complexes, inclusion complex, which has a pseudorotaxane struc-... 

Among the systems proposed as models for the photosynthetic reaction center, supramolecular assemblies in which Ru(II)-polypyridine complexes and 4,4 -bipyridinium units are held together noncovalently in threaded and interlocked structures have been extensively studied [43, 82-88]. In such assemblies, connections between the molecular components rely on charge transfer interactions between the electron acceptor bipyridinium units and aromatic electron donor groups (Fig. 3). For instance, in the various pseudorotaxanes formed in acetonitrile solution at 298 K by the threading of cyclophane 4 + by the dioxybenzene-containing tethers of 192+ (Fig. 17) [84], an efficient photoinduced electron... 

In this review, I describe our efforts to construct interlocked structures such as rotaxanes, polyrotaxanes and molecular necklaces incorporating cucurbituril as a molecular bead by utilizing the principles of self-assembly and coordination chemistry. A key to the success of this synthesis is the high affinity of cucurbituril toward alkyl diammonium ions, which allows formation of a stable pseudorotaxane... 

Before going on to discuss molecular electronic machines, it will be useful to describe their structural foundation at a molecular level, namely those based on interlocked molecules. Interlocked molecules can take on a variety of forms, the most common being catenanes, rotaxanes, knots [16], and carceplexes [17]. For the purpose of this review, only catenanes, rotaxanes and their geometrically related complexes - pseudorotaxanes [18] - will be discussed. When conferred with the ability to undergo some mechanical motion as a result of an applied stimulus - be it chemical, electrochemical, or photochemical - these interlocked molecular and interpenetrated supramo-lecular systems often take on the characteristics of molecular machines [19]. 

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