Angew. Chem. Int. Ed. 50, 8279–8282 (2011)

Credit: © 2011 WILEY

A myriad of discrete and extended molecular assemblies have been prepared by coordinating metal ions to organic ligands, yet only a few have relied on planar components with a five-fold symmetry. Pentagonal or star-shaped building blocks cannot tile a planar surface without tilting or curving, and the few metal–organic structures assembled with such ligands have thus been discrete spherical cages exhibiting fullerene-like morphology. Now, John Bacsa and colleagues from Cambridge University and the University of Liverpool have described a three-dimensional metal–organic framework based on the fullerene units.

An anionic five-membered aromatic ring decorated by five cyanide moieties {C5(CN)5−} was chosen as the ligand. On crystallizing this ligand with sodium ions by slow vapour diffusion of ether in a nitromethane solution, the researchers observed the assembly — directed by the solvent molecules — of a highly solvated, highly metastable, three-dimensional metal–organic framework. Characterization by X-ray diffraction revealed that each ligand binds five sodium centres through nearly linear C≡N–Na bonds. These in turn organize into a coordination network comprising sodium–ligand pentagonal and hexagonal faces, making up units that resemble fullerene moieties. The resulting MOF exhibits both discrete cavities and linear non-intersecting channels that run through the empty faces.

Removal of the solvent molecules under mild conditions does not cause the MOF to collapse but leads to a different morphology. The unsolvated framework adopts a more densely packed layered arrangement, favoured by π-stacking between the ligands, in which the C≡N–Na bonds are no longer linear.