Mechanically interlocked molecules
to trap its guest), before adding on the stoppers. Slipping involves forcing the macrocycle over the bulky group using microwave irradiation or high temperatures. There is one method not on the diagram which is known as threading, where you react both halves of the axle in the cavity of the rotaxane, which has recently been developed by Leigh et al. They use an active-metal template (essentially, a metal ion acts as a way to both preorganize the various components of the rotaxane, and also to catalyse the formation of the final covalent bond between the two halves of the axle). They have recently used this method to form a [4]rotaxane (one with three rings) which demonstrates its effectiveness in developing even complex structures. 6 Catenane synthesis has also seen an improvement thanks to the use of template-based approaches. In 1960, Wasserman used a similarly simple process to Harrison, in that he simply performed many ring closing reactions with the idea being that some would close around each other to form the desired product. As this produced low yields and required a large excess of closed rings in relation to open ones, this method is rarely used. Instead, just like in rotaxane synthesis, different intermolecular forces are used to bring the components together before the ring closure reaction is carried out. In general, there are three different ways to form catenanes: single macrocyclization, where one acyclic molecule intertwined with a macrocycle undergoes cyclization; double macrocyclization, where both rings are acyclic and both are closed; and magic ring synthesis, where there are two macrocycles, and one opens and closes up again to form a catenane. Along with a high-pressure environment, these methods can give yields of up to 94%, which greatly improves on Wasserman’s original yields of <1%. For one example of a synthesis, Leigh and colleagues produced a [2]catenane with palladium(II) as its metal cation template (which favours square planar geometry), using a ‘tridentate -monodentate ligand combination to generate a crossover point prior to final ring closure reaction’, ensuring the two components were suitably interlocked before using RCM (Ring closing olefin metathesis) to close the acyclic component around the macrocycle. They obtained a 78% yield of the final product after separating it. 7 As molecular knots haven’t been around as long (they were first synthesized by Sauvage in 1989, when he formed a trefoil knot) they have always made use of template-based methodologies. Nevertheless, their production has been steadily developing, producing more and more complex knots, and with greater efficiency. Recently, in 2017, a molecular knot with 8 crossings was synthesized.
Potential Applications
As a result of these continual advances in the synthesis of MIMs, there has been a significant increase in the research into potential applications. Rotaxanes have seen by far the most development. In particular, their usage as molecular machines has sparked major interest, as they could be used in molecular electronics, mainly through their usage as ‘molecular switches’ as referred to earlier, which would allow for them to employed in molecular logic gates. It is unclear how the different states of a rotaxane would be differentiated to allow for this to function, but that has not stopped optimism and further research into this field. Additionally, rotaxanes could see usage in molecular wires. For the efficient transfer of electrons, the axle of the rotaxane must be rigid, and so some different concepts
6 Sluysmans and Stoddart 2019: 3. 7 Atwood 2017: volume 6, chapter 9.
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