Angew. Chem. Int. Ed.http://doi.org/f2cd3h (2013)

Credit: © 2013 WILEY

The challenge of making molecules with non-trivial topologies has proved to be a popular one over the past 50 years or so. Two archetypal examples of such systems are catenanes — compounds made up of two or more macrocyles that are held together simply by being mechanically interlocked with one another — and knots (molecular trefoils being the most common). Weaving and tying molecular strands into complex architectures not only produces eye-catching structures, but has also been used to make nanoscale machines and motors.

Now, a team based in Cambridge and Berlin, headed up by Christoph Schalley, Jonathan Nitschke and Jeremy Sanders, have studied a system in which six crown ethers can thread on to the edges of a metal–organic tetrahedron to produce an unusual [7]catenane — one member of a new class of mechanically interlocked molecules. The central tetrahedral cage is pieced together from four Fe(ii) ions (these are the corners) and six identical rigid-rod organic ligands (the edges) that have an electron-poor aromatic group in the middle. The glue that holds these pieces together comes from 12 formylpyridine molecules — the aldehydes react with amines at the ends of the edges to form imines and the pyridine rings coordinate to the metal ions. The structure is dynamic because the metal–pyridine bonds are reversible, and when one of the corners opens up, an electron-rich macrocycle (a dinaphtho crown ether) can slip on to an edge to bind the electron-deficient region.

It is shown that each edge can accommodate a threaded macrocycle, resulting in the formation of a [7]catenane if a large enough excess of the crown ether is present in solution. After a mixture containing a 10:1 ratio of crown ether (C) to tetrahedron (T) was allowed to reach equilibrium, six catenated compounds (TC1–6 — [2]catenane through to [7]catenane and all those in between) were observed by mass spectrometry. The next step is to explore whether catenation of the edges of the tetrahedron can be used to control the capture and release of guest molecules from the inside of the metal–organic cage.