Supramolecular knots

Sauvage, J.-P. Dietrich-Buchecker, C. O. Chambron, J.-C. Transition metals as assembling and templating species synthesis of catenanes and molecular knots. In Comprehensive Supramolecular Chemistry, Sauvage, J.-P. Hosseini, M.W., Ed., Publisher Elsevier, Oxford, UK 1996 Vol. 2, p 43. 

Thus there is an essential difference between classical homogeneous reactions in organic chemistry and reactions such as those in which catenanes and knots are formed. In the latter, there are heterogeneities on the micro scale. Thus supramolecular chemistry lies also in the border area between classical organic chemistry and surface chemistry. 

This process involves the covalent locking in of structures formed by reversible self-assembly. The irreversible, post-assembly step switches off the equilibrium process involved in the self-assembly. As we will see in the following sections, self assembly with covalent postmodification is involved in a range of biochemistry (e.g. insulin synthesis) and elegant abiotic supramolecular synthesis as in the formation of catenanes and knots. 

Fritz Vogtle is Professor and Director at the Kekule-Institute for Organic Chemistry and Biochemistry at the University of Bonn, Germany. His research interests are supramolecular chemistry deformed helical molecules and their chiroptical properties and compounds with appealing architectures such as rotaxanes, catenanes, knots, and dendrimers [37-40],... 

There are also chiral topological supramolecular structures in molecular knots, rotaxanes, catenanes, and Borromean rings with metal complexes.73,74... 

Only lately have covalently linked species also become included in supramolecular structures since they are formed of particular building blocks, e.g. the Starburstjcascade dendrimers (see Chapter 6 of this volume). Moreover, the catenands and knots (as shown in Chapter 5) are not subject either to covalent or typical noncovalent bonds, although the latter are important in the formation step, but rest upon a mechanical interlock or coding. 

The Stoddart group has used the principles employed in their catenane studies to synthesise both a molecular trifoil knot and its isomeric large ring in very low yield. These isomers were separated by HPLC and characterised by liquid secondary ion mass spectrometry. The synthetic procedure, which is illustrated schematically in Figure 5.26, was based on the formation of a double-stranded supramolecular complex between an acyclic 7C-electron-rich, 1,5-dioxynaphthalene-based polyether and an acyclic 7i-electron-deficient bipyridinium-based tetracation. The extremely low efficiency of the synthesis in this case appears to reflect an unfavourable solution geometry of the intermediate host-guest complex - the latter... 

Numerous reviews as well as perspectives, feature articles, tutorials, and mini-reviews have appeared throughout 2005 that are of particular interest to the macroheterocyclic enthusiast and those delving into supramolecular chemistry at the macromolecular level, as well as those studying nanoconstructs the chemistry and chiral properties of molecular knots... 

Sauvage et al. have elaborated special molecules with the topologies of catenated rings and knots [29]. The syntheses of these depend on metal coordination sites which direct the assembly of the components, but in general these catenanes and knots have few bonded connections. They are special because non-bonded sections are topologically trapped to coexist, thereby creating new opportunities for investigation of supramolecular relationships. A... 

To make this review self-contained and to provide a foundatitMi for further discussion, we have included the experimental methods and theoretical models of mechanical degradation for linear chains in the second and third sections, respectively. From the fourth to seventh sections, the mechanochemistry of cyclic polymers, graft polymers, star-shaped polymers (star-shaped polymers), dendrimers, and hyperbranched polymers is summarized. In the eighth section, we survey the mechanochemistry of supramolecular aggregates and knotted polymers, where the topology constraints are temporal. We hope our overview can serve as a guideline for the future work in the field of polymer mechanochemistry. 

It should be noted that the two paradigmatic systems are of biological relevance. Proteins and double-strand DNAs have complex topologies with domains held by supramolecular interactions. Knots are widely found in chromosomes [269], viral capsid DNAs [270, 271], and proteins [272, 273], and probably take part in their physiological functions. 

With precise design criteria established, supramolecular interactions have been exploited to design and synthesise new and functional mechanically interlocked molecules in high yields [9]. Over a relatively short period of time, mastery of controlled synthesis of mechanically interlocked molecules has led to an extraordinary array of functional materials and molecular machines [10]. Examples of these include logic gates, sensors for explosives, anions and cations, in addition to exquisite molecules with complex topologies (such as molecular knots) that have yet to be thoroughly exploited. 

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