Catenanes: The Art of Molecular Entanglement
There is a group of organic compounds called catenanes. In a catenane, two ring structures are interlocked with each other like a chain or a wire disentanglement puzzle (Figure 1). Catena means “chain” in Latin, so the naming is very fitting. It is a fascinating molecular architecture indeed, but how can you make such objects in the nanoscale world?
Fig. 1 schematic diagram of catenane
The First Catenane
The first synthesis of a catenane goes way back to 1960 and it was done by Professor Harry Wasserman’s research group. Their strategy was very simple: mix a large cyclic molecule and a long linear molecule in a solution and tie up the ends of the linear molecule. Most of the linear molecules become ordinary rings, but there should be at least some that cyclize while they are threading through a ring, making an interlocked double ring system (Figure 2). The chemical yield of this reaction was marginal, however, since it relied on pure statistical possibility (it was only about 0.0001%). A story has been told that these reactions were done in a bath tub because flasks weren’t big enough (to compensate the low yield and high dilution conditions)!
Fig. 2 Wasserman's trial
The synthetic efficiency of catenanes was improved dramatically by the group of Professor Jean-Pierre Sauvage in France. Their strategy was to use the metal-chelating property of nitrogen atoms. For example, two molecules of phenanthroline chelate (or bind to) a copper atom by orienting themselves orthogonal (or vertical) to each other (Figure 3). Sauvage’s idea was to use this kind of stable copper-nitrogen complex as a template.
Fig. 3 phenanthroline complex
They used a phenanthroline-type compound with two additional phenol substructures. When copper is added to this compound, the two molecules chelate a copper ion in the aforementioned orthogonal orientation, providing a perfect template. An interlocking ring system can then be made by connecting the phenolic oxygen atoms with appropriately long linear molecules (Figure 4). The synthesis is completed by removing the copper atom. This is really a creative strategy reminding me of the Egg of Columbus. Since the first reports, this “template-directed synthesis” has proved extremely useful and become a standard methodology in the field of supramolecular chemistry.
Fig. 4 Sauvage's approach
In the United States, Professor Fraser Stoddart has done successful syntheses of catenanes using pi-stacking force (an attractive interaction between aromatic ring systems). In 1994, his group synthesized a catenane composed of five interlocking rings. This molecule was named “olympiadane” after the symbol of the Olympic Games (Figure 5)
Fig. 5 olympiadane
Catenane Synthesis by Self Assembly
Professor Makoto Fujita of Tokyo University is a very active scientist in this field. He has been studying the reaction of various nitrogen containing organic molecules with metal ions to develop functional materials. An example is shown in Figure 6. The experimental plan here was to synthesize a macrocyclic molecule.
Fig. 6 Fujita's macrocyclic complex
However, the product of the experiment was not what was expected. Surprisingly, it was a catenane (Figure 7)! As said by Aristotle, “nature abhors a vacuum.” A large cyclic structure has a tendency to enclose something instead of leaving the inside an empty space. According to this natural tendency, the first ring is thought to capture and hold onto a linear component which forms the second ring. This reaction boasts an astonishing 90% yield. The kind of reaction which was once 0.00001% yield is now possible with such high efficiency by mixing the simple reactants. Talk about a great progress! Professor Fujita is a major contributor in this field, making many other complex and interesting catenanes.
Fig. 7 Fujita's catenane
Like this reaction, a phenomenon in which simple reactants gather spontaneously
to form a complex system is called "self assembly". Self assembly
has a potential power to construct very complex molecular systems with
little effort, so it is one of very hot fields right now.
The same concept has been applied to the synthesis of even more complex molecular systems. One of the really impressive examples is the molecular Borromean ring made by Professor Stoddart in 2004 (Figure 8). The name of the Borromean ring comes from the symbol of the Borromeo family, an aristocrat in the medieval Italy, and the same design is known in a Japanese family crest as “mitsuwachigai no mon.” It has an interesting property that the three rings are linked as a whole, but breaking any one of the three rings results in two unlinked rings.
Fig. 8 Borromean ring
Professor Stoddart synthesized a molecular Borromean ring from twelve reactants using six metal ions as controlling templates. Considering the level of topological complexity, this is truly a great art in the nanoscale world (Figure X.X). More recently, his group also reported the synthesis of the King Solomon’s knot from the same reactants by changing the type of the metal ion!
Fig. 9 Stoddart's Borromean catenane
Other than the ways mentioned so far, there is another synthetic possibility which utilizes the Möbius band. The Möbius band is a famous “non-orientable” shape made by twisting a piece of ribbon once (180 degrees) and connecting the ends. Do you know what happens to it if you cut it along the ribbon’s center line to halve the width of the ribbon? An ordinary ring would become two separate rings, but the Möbius band becomes one large ring.
Then, what happens if you do the same cut to a Möbius band made by a ribbon that has been given two twists (360 degrees). It turns out, that the result is two interlocking rings (Figure 10). So, in principle, this should be possible on molecular level to make catenanes.
Fig. 10 From Möbius band to catenane
There are actually researchers who are trying to realize this synthetic hypothesis. At this point though, only making a large ring from a single-twist Möbius band has been successful and I have yet to hear good news about the catenane synthesis. This kind of chemistry is interesting no doubt, but it’s really mind-twisting at the same time.
The New World of Catenanes
Catenanes are interesting molecules but they are not an aimless game played by chemists. For example, a polymer made of catenane units is expected to be a very strong and flexible fiber (although the longest catenane chain reported to date is seven to the best of my knowledge).
There are other applications being developed as well, making the use of catenane’s unique property of being “not rigidly bonded but not apart.” An example in this sense is the use of interlocking rings as molecular motors (Figure 11).
Fig. 11 molecular motor
This so-called molecular motor contains a copper atom within a catenane system. Copper ion can exist in two oxidation states and they differ in that Cu(I) makes a stable complex with five nitrogen ligands whereas Cu(II) tends to complex with four. Like the picture shows, when you reduce the catenane-Cu(II) complex (upper picture) into catenane-Cu(I) (lower picture) by applying electricity through the solution, the ring on the right rotates to form a five coordination complex. So the essence of what is happening here is that in this first molecular motor system, the rotational motion of the ring can be controlled by an externally applied electrical signal.
In recent years, the chemistry has gone one step further ahead and a catenane motor that rotates in only one direction has been reported. The development of these impressive “molecular machines” is a fast progressing field which I want to bring up in the other sections.
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