Hydrogen-bond-mediated template synthesis of rotaxanes, catenanes, and knotanes

The synthesis of rotaxanes, catenanes, and trefoil knotanes can only be efficiently accomplished, when a suitable template effect is used that provides a means either to thread a string-like molecule through a macrocycle or to wrap an open macrocycle around a string. This overview is devoted to the template effects for the synthesis of mechanically bound species which are mediated by hydrogen bonding. Three major classes can be distinguished by the charges involved: (i) catien-mediated template effects utilizing the formation of crown ether complexes with secondary ammonium ions, (ii) recognition of neutral amides by oligolactam macrocycles, and (iii) threading through hydrogen bonding to anions. The basic idea which we put forward in this chapter is that none of the template effects found so far and discussed here is "traceless" in the sense that they would not leave functional groups behind in the final rotaxane, catenane, or knotane structure. Consequently, the variability of the structures available through these template syntheses is limited in this respect. Also, the properties of the mechanically bound species are intimately interconnected with the template effect used for their synthesis. However, what appears here as a drawback may also be an advantage, when the functional groups used for templating can be utilized to control the properties of the final product which hopefully can in future be exploited for the development of new materials. We aim at providing sufficient examples in order to strengthen this point in throughout the article. Among the properties and functions discussed are the realization of molecular machines, topological chirality; and the stability of rotaxanes as determined from deslipping experiments.