The simplest example of a catenane is one in which two macrocycles are mechanically interlocked with just a single link. Increasingly complicated structures result when the two rings are linked together more than once; for example, a doubly linked catenane has the same interwoven topology as the ancient motif known as Solomon's knot. When it comes to interlocked molecules made up of three macrocycles, the situation is even more complex. The most common arrangement is a catenane in which the three rings are (singly) linked together linearly to form a chain, but other topologies are possible.
Now, Thorfinnur Gunnlaugsson and co-workers at Trinity College Dublin have prepared a catenane in which each ring in the structure is interlocked with the other two. Using an Eu(iii) ion as a template, three copies of the same pyridyl diamide ligand can be brought together in a coordination complex. This acyclic ligand has an alkene group at each end and can, in isolation, undergo a ring-closing metathesis (RCM) reaction in the presence of a ruthenium catalyst to form a macrocycle. When three of these ligands are wrapped around a metal centre, however, the same RCM procedure can, in principle, yield a range of different products with interwoven topologies. If each ligand in the complex reacts with itself to form a macrocycle around the metal template — and the ligands are organized in the appropriate fashion — the result is a catenane in which each ring is threaded through both of the others (pictured).
NMR spectroscopy, in addition to mass spectrometry, indicates that both - and catenanes are formed during the reaction, but other knotted structures could also be produced by inter-ligand metathesis reactions. Although lanthanide ions have been used previously to make another class of interlocked compounds called rotaxanes, Gunnlaugsson and colleagues suggest that this is the first time that they have been used to template the formation of catenanes. Moreover, this particular arrangement of three interlocking rings is somewhat unusual and differs from the topologies observed in related systems, including linear catenanes and molecular Borromean rings. SC
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Cantrill, S. Triple-clipped links. Nature Chem 6, 270 (2014). https://doi.org/10.1038/nchem.1914