Credit: © 2008 RSC

The topology of a molecule has a crucial role in determining its properties, and so the synthesis of entangled species such as knots and links has attracted considerable interest. Molecular knots can be made by using a number of different covalent or non-covalent templating strategies, but such approaches have traditionally involved multi-step syntheses.

Now, Mir Wais Hosseini, Jean-Marc Planeix and co-workers1 at the Université Louis Pasteur in France have observed the self-assembly of silver ions and organic ligands directly into molecular knots. The ligands consist of a flexible oligo-ether chain with a rigid quinoline group at each end. The two different types of coordinating sites — oxygen atoms in the linker and nitrogen atoms in the aromatic end groups — combine to wrap around a small number of silver ions to produce knotted structures when the components slowly diffuse together in a mixture of solvents. Different knotted complexes were formed depending on the length of the oligo-ether chain.

The ligand with a longer ether chain (ten carbon atoms) formed a trinuclear knot — in which three ligands combine with three silver ions — whereas the ligand with the eight-carbon-atom chain resulted in a tetranuclear knot comprising four ligands and four metal ions. In both cases, the metal centres act as connecting nodes and cannot be removed without disrupting the entire structure.