Credit: © 2008 Wiley

Metal–organic frameworks — porous materials prepared by connecting metal ions together with organic linkers — offer great flexibility in terms of pore characteristics. They could, ideally, be designed from appropriate building units to suit specific host–guest applications such as gas separation and storage, catalysis or drug delivery. Most approaches to tailor pores have so far focused on building complex structures or changing the linkers, and by comparison relatively little emphasis has been placed on modifying the materials themselves.

Now, Andrew Burrows and co-workers at the University of Bath have successfully altered a metal–organic framework by 'tagging' it1. Zinc oxide clusters and linear dicarboxylate linkers were assembled into a stable, porous framework. Aldehyde 'tags' were branched onto the ligand before the synthesis, then partially converted (50–60%) into hydrazone groups once the framework had been prepared. Although some distortion occurred to accommodate the larger hydrazone groups, the material retained its structure.

Partially replacing aldehyde tags by methoxy groups — which cannot be converted into larger hydrazones — also made it possible to control, to some extent, the porous volume of the framework obtained. This tagging approach enables groups to be incorporated in pores, where they could not be added by direct synthesis — either because they would not survive the preparation conditions or because they would compete with the ligand for coordination with the metal centres.