Interlocked rotaxanes

Squaraines 17b and 17c have terminal acetylene residues, which allowed to convert the squaraine dyes and tetralactam macrocycles into permanently interlocked rotaxane structures using copper-catalyzed and copper-free cycloaddition reactions with bulky stopper groups [58]. 

Because of their extraordinary dynamic properties, interlocked architectures [36-38] are particularly well suited for the development of molecular machinery [9]. Catenanes (from the Latin catena = chain) are molecules in which two or more macrocycles are interlocked. Rotaxanes (from the Latin rota = wheel and axis = axle) are molecular species in which one or more macrocycles are threaded onto a linear component and de-threading is prevented by bulky stoppers (Fig. 4). 

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. 

The nse of templates for the formation of macrocyclic compounds has been reviewed comprehensivelyand the most important concepts and a personal selection of representative examples are given here. The use of templates in the preparation of macrocycles to form molecules with an aspect of interlocking—rotaxanes, catenanes, knots, and the like—is reviewed in other contributions to this section, and only representative examples are presented. 

Association of these two components is mediated by the chloride anion via N-H -Cl" and C-H -Cl hydrogen bonding and further stabilized by donor-acceptor interactions between the electron-rich hydroquinone groups of 9 and electron-deficient pyridinium axle 8. Conversion of this orthogonal association complex into a permanently interlocked rotaxane was achieved by RCM clipping to afford 10 in 47% yield. 

PRF-23 and PRF-24 demonstrate a novel way to create PRFs by using permanently interlocked rotaxanes which are connected by a mechanical linkage and not merely held together by relatively weak non-covalent interactions prior to PRF synthesis. This synthetic approach allows for the use of harsher reaction conditions since the linker cannot simply unthread when exposed to competitive solvation during PRF formation. 

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. 

A useful summary of the various and numerous types of rotaxanes, catenanes, and knots can be found in a review of template routes to interlocked molecular structures 468). Inorganic chemistry is centrally involved in the templating involved in self-assembly and in controlled synthesis of such species. 

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. 

The dynamic nature of the system offers the crown ether access to the ammonium centre allowing self-assembly of the corresponding dynamic [2]ro-taxanes 31 to occur. Fixing of the interlocked assembly can be achieved by reducing the imine groups in 31 to the corresponding amine so that a kinetically inert [2]rotaxane forms. 

The anion-templated synthesis of rotaxanes first introduced by Vogtle has now been employed by others to produce other interlocked assemblies. Schalley for example [68], has reported an improved yield for the synthesis of rotaxanes by... 

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... 

Further studies by the same authors have led to the formation of [2]rotaxanes, [3]rotaxanes and pseudo-polyrotaxanes [85-87]. In all these interlocked species, in spite of the presence of aromatic rings in the axle and wheel, tt-ti interactions do not seem to play a role in the templating process. This highlights once again the importance of C-H---0 hydrogen bonding in the assembly of interlocked species. 

Leigh DA, Venturini A, Wilson AJ, Wong JKY, Zerbetto F (2004) The mechanism of formation of amide-based interlocked compounds prediction of a new rotaxane-forming motif. Chem Eur J 10 4960 -969... 

Ceo-TTF molecules, interlocked molecules (catenanes, rotaxanes), donor-acceptor macrocycles, cage molecules, etc. (Jeppesen et al, 2004). It is beyond the scope of this book to review such developments and I appeal to the curiosity of researchers really not familiar with such macromolecules to see how big the molecules can become. 

Catenanes and rotaxanes represent examples of interlocked architectures. A catenane consists of two interlocked rings (it is called in this case a [2]catenane),... 

Besides their topology, rotaxanes and catenanes are also appealing systems for the construction of molecular machines because (i) the mechanical bond allows a large variety of mutual arrangements of the molecular components, while conferring stability to the system, (ii) the interlocked architecture limits the amplitude of the intercomponent motion in the three directions, (iii) the stability of a specific... 

Interestingly, the dumbbell component of a molecular shuttle exerts on the ring motion the same type of directional restriction as imposed by the protein track for linear biomolecular motors (an actin filament for myosin and a microtubule for kinesin and dynein).4 It should also be noted that interlocked molecular architectures are largely present in natural systems—for instance, DNA catenanes and rotaxanes... 

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).

Several examples of catenanes and rotaxanes have been constructed and investigated on solid surfaces.1 la,d f 12 13 26 If the interlocked molecular components contain electroactive units and the surface is that of an electrode, electrochemical techniques represent a powerful tool to study the behavior of the surface-immobilized ensemble. Catenanes and rotaxanes are usually deposited on solid surfaces by employing the Langmuir-Blodgett technique27 or the self-assembled monolayer (SAM) approach.28 The molecular components can either be already interlocked prior to attachment to the surface or become so in consequence of surface immobilization in the latter setting, the solid surface plays the dual role of a stopper and an interface (electrode). In most instances, the investigated compounds are deposited on macroscopic surfaces, such as those of metal or semiconductor electrodes 26 less common is the case of systems anchored on nanocrystals.29... 

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