Mechanically interlocked molecules
Catenanes are the other main type of MIM, and are in fact slightly simpler, as they are simply two or more interlocked rings/macrocycles. Similarly to the rotaxane, the macrocycles can rotate around each other and, if one ring is much bigger than the other, the smaller ring can shuttle around, forming what is known as a ‘molecular train.’ If the same ‘docking sites’ are introduced onto the larger macrocycle, you can control the train’s movement. 3
Crystal structure of a Catenane
The final type of MIM is the Molecular Knot, which is simply defined as a structure analogous to a macroscopic knot, 4 meaning it is the only type which can be formed with only one molecule. In addition, it was the only type to be discovered in nature before being made artificially, as it is seen in DNA and various other proteins (rotaxane sub-structures have been found in peptides since its synthesis).
Synthesis
As mentioned in the introduction, the synthesis of mechanically interlocked molecules has evolved massively from the inelegant methods of the 60s, which relied on the ‘statistical approach’, and now is done almost exclusively using a ‘template’, which allows for both higher -yielding and more precise syntheses. This progress has also of course enabled the production of more and more complex molecules, which allows for more and more potential uses. To be specific, the production of rotaxanes has seen the most development since the dark ages of MIMs. When they were first synthesized by Ian Harrison and Shuyen Harrison in 1967, they simply reacted two halves of the axle molecule in proximity with the macrocycle, in the hopes that one reaction would take place through the ring, which gave a measly yield of ~6%. Nowadays though, the components of the rotaxane are designed to recognize each other and preorganize themselves through a variety of
intermolecu lar forces (it could be hydrogen bonding, π - π interactions, coulombic interactions, etc.), before the components are fully interlocked by the final reaction. As displayed in the diagram there are a variety of ways of achieving this, namely clipping (a), capping (b), and slipping (c). Clipping involves a partial macrocycle being fitted around the axle through intermolecular forces, before being closed by a ring closing reaction. An example of this would be how Stoddart used the hydrogen bonds between dialkylammonium ions (the axles) and an acyclic crown ether derivative (what became the macrocycle) to bring the components together before adding a dialdehyde
to close the macrocycle. 5 Capping involves forming a pseudorotaxane (i.e. a rotaxane without any stoppers) through intermolecular means (e.g. cyclodextrin macrocycles use their hydrophobic effects
3 Ball 1994: 175-178. 4 https://en.wikipedia.org/wiki/Molecular_knot#cite_note-:1-1 5 Atwood 2017: volume 5, chapter 17.
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