A knotty problem

When the bow on your shoelace collapses into a small undoable knot, it is a source of frustration, whereas in chemistry, tight knots are viewed as a challenge. Tying molecules into knots may sound pointless, but the more we understand how to spatially manipulate molecules at this level, the closer we get to being able to perform chemical achievements currently the preserve of biology. As published last month, the smallest and tightest molecular knot has now been achieved by Richard Puddephatt,? working with colleagues at the Chinese Academy of Sciences in Dalia, and it was made by accident.

There is no absolute way of determining the tightness of molecular knots, but they can be ranked according to the number of atoms in the shortest path along the knotted strand divided by the number of crossings, known as the backbone crossing ratio (BCR). Up until now the smallest molecular knot was the 819 knot with a chain length of 192 atoms and a BCR of 24, but this paper presents a molecular trefoil knot (main image) with only 54 atoms in the backbone and a remarkable BCR of 18.

Even more remarkable is that this tiny knot was achieved with gold organometallic chemistry, an element not usually first choice in complex molecular wrangling, but it is the reversibility of these gold bonds that facilitates assembly. As shown in the structures below, the cyclic complex Au2 rearranges into the trefoil knot structure Au6 on crystallisation. This spontaneous rearrangement seems to be driven by the serendipitous arrangement of the bulky groups on the chain which naturally orient on the outside of the knot in combination with linearity of the gold acetylide bonds. The result is this wonderful molecule and unlike shoelaces, all you have to do to untie this knot is to pour dichloromethane on it.

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