Distinctive features and challenges in catenane chemistry

Abstract

From being an aesthetic molecular object to a building block for the construction of molecular machines, catenanes and related mechanically interlocked molecules (MIMs) continue to attract immense interest in many research areas. Catenane chemistry is closely tied to that of rotaxanes and knots, and involves concepts like mechanical bonds, chemical topology and co-conformation that are unique to these molecules. Yet, because of their different topological structures and mechanical bond properties, there are some fundamental differences between the chemistry of catenanes and that of rotaxanes and knots although the boundary is sometimes blurred. Clearly distinguishing these differences, in aspects of bonding, structure, synthesis and properties, between catenanes and other MIMs is therefore of fundamental importance to understand their chemistry and explore the new opportunities from mechanical bonds.

Fig. 1. Comparison of catenanes, rotaxanes and knots.

Fig. 2. The rotaxanes reported by Pöthig and co-workers referred to as [3]- and [2]rotaxane when the interlocked macrocycles are separated and connected by Ag⁺ ions respectively.

Fig. 3. Possible structures of (a) [6]catenanes and (b) [4]rotaxanes.

Fig. 4. Possible types of catenane isomerism.

Fig. 5. General templated synthesis of a catenane.

Fig. 6. Example of catenane synthesis using (a) an active copper template and (b) CB[6]-mediated azide–alkyne cycloaddition.

Fig. 7. A hydrocarbon [2]catenane obtained through directed synthesis.

Fig. 8. Templated synthesis of (a) a Hopf link using a tetrahedral template can lead to a molecular figure-8 and (b) Solomon link from either a helicate or 2 × 2 grid template.

Fig. 9. Synthesis of (a) a radial [n]catenane with one ring-closing reaction and (b) a linear [n]catenane with sequential interlocking of the macrocycles.

Fig. 10. Synthesis of (a) linear [5]catenane (olympiadane) using an aromatic donor–acceptor template; and (b) [n]catenanes from a metallosupramolecular polymer.

Fig. 11. Breaking a covalent bond in (a) a Hopf link could result in either a [2]rotaxane that contains a mechanical bond or separated derivatives from the dumbbell and macrocycle; (b) a Hopf link and a Solomon link could both result in the breaking of the mechanical bond.

Fig. 12. Effect of tightness (top) and stickiness (bottom) of interlocked rings on the kinetics and thermodynamics of catenane co-conformational change.

Fig. 13. Co-conformational switching of an acid–base responsive DPAC-containing [2]catenane with different fluorescence properties.

Fig. 14. Co-conformational switching in (a) linear and (b) cyclic daisy chains results in length and aperture control.

Fig. 15. Co-conformational switching in a chemically fuelled molecular motor results in a unidirectional motion.

Fig. 16. Examples of polymer materials that incorporate MIMs with new properties: (a) slide-ring gel of unique elasticity and softness; (b) a catenane polymer that shows selective mechanophore activation; and (c) a catenane-crosslinked polymer with switchable rigidity and hardness.

Fig. 17. Examples of guest-binding catenanes with a well-defined cavity: (a) an anion-binding Star-of-David catenane; and (b) a Solomon link that binds to four water molecules.

Fig. 18. Examples of reactive species confined and stabilised within an MIM: (a) a cation radical pair; (b) a charge-separated radical pair generated upon photoexcitation; and (c) a metal carbene species.

Fig. 19. Recent examples of catenane catalysts for (a) photoredox, (b) transition metal and (c) asymmetric catalysis.

Ho Yu Au-Yeung

Yulin Deng