Credit: © 2008 Wiley

In principle, porous materials can be made by simply mixing together small molecular building blocks and metal ions that recognize one another and assemble into extended networks. Engineering the intermolecular interactions of the individual components — rather than relying on non-directional van der Waals forces — may enable a greater degree of control in the fabrication of specific three-dimensional structures containing pores with particular sizes and shapes.

Now, Jonathan Steed and co-workers1 from Durham University have used this approach to make coordination polymers from ligands that have a metal-binding pyridyl ring at each end, and two hydrogen-bonding urea groups in between. When mixed with silver nitrate, three pyridyl rings (each from a different ligand molecule) bond to the positively charged silver ions to form a network of large hexanuclear macrocycles. In addition to this coordination bonding, however, six urea groups bond to each negatively charged nitrate ion. This combination of intermolecular forces, as well as interactions between pairs of silver ions, gives rise to a two-dimensional sheet structure in which three independent networks weave together to form a so-called Borromean ring topology.

Two different types of cavities are found within the extended structure — smaller circular cavities within each interwoven sheet are occupied by nitrate anions, which form infinite stacks throughout the material. Larger triangular cavities in each sheet contain discrete clusters of seven water molecules. By changing the size of the ligand, Steed and co-workers hope to tune the size of the cavities in such materials, and include larger guest molecules within them.