Metal-organic frameworks (MOFs) are porous, cage-like structures formed from metal ions connected by organic linkers; these constituents are used to prepare many different architectures. These porous networks have several potential applications such as the storage and separation of gases, catalysis and molecular recognition. They have an advantage over inorganic porous materials in that they can be rationally designed, such that the architecture can be controlled and the pores functionalized. As this field develops, more increasingly complex structures are being prepared from equally complicated molecular building blocks.

Fig. 1: Build-up of the MOF. a, The structure of the “paddlewheel” (left) and a schematic representation of it (right). b, The paddlewheels assemble into a 3D network with polyhedral voids.Reproduced with permission. © 2008 JACS.

In a departure from current trends, Hyungphil Chun, from Hanyang University at Ansan, Korea describes the preparation of a highly porous, highly symmetrical structure using a simple combination of easily available building blocks1. Chun approach employs the self-assembly of Zn2+ and two simple unmodified organic ligands, H2mip and dabco—reaction between these moieties in dimethyl formamide produced crystals of a non-interpenetrating three-dimensional framework based on units of Zn2(O2C)4 “paddlewheels” (Fig. 1a). The paddlewheels had a square pyramid-type structure, with the five tips of the ‘paddles’ providing connecting nodes. When assembled, they formed a 3D framework with triangular, square and hexagonal shapes forming the faces of polyhedral voids (Fig. 1b); the overall framework had a face-centred cubic symmetry.

The voids are described as cuboctahedra (CO), truncated tetrahedra (TT), and truncated octahedra (TO), and provide 67% of the crystal volume. X-ray powder diffraction and gas sorption measurements showed the structure to remain permanently porous; retaining its 3D architecture even after evacuation. The structure was adapted to accommodate different types of molecules, as the centre of the CO space contains coordinated water molecules, which could be replaced by other solvents or removed altogether. Also, as the CO cages are connected to TT and TO cages only by dabco ligands, it is possible that other structures containing only CO cages could be prepared using similar ligands to dabco but of different lengths. Chun says that the most anticipated use for these MOFs is in the storage of industrially important gases such as hydrogen, methane and carbon dioxide.