14 | Structural Motifs

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In some cases it is possible to communicate a complex structure by analogy to a simpler form, e.g. the common elemental structures. In such cases larger molecules or subunits of a structure may have the same arrangement in space or the same connectivity of units as a well know structure type.

One such crystal structure is the of the molecular material fullerene (\(C_{60}\)), Fig. 27. Here the individual \(C_{60}\) polyhedra approximate the shape of a sphere and we can imaging the packing of these molecules to follow a simple packing of sphere just like we say for other elemental structures.

Fig. 27 Crystal structure of \(C_{60}\) which adopts a cubic close packed motif akin to metals like Ni and Au.

Another structure that also follows this ccp motif that that of the metal-organic framework \(Zr_6O_4(OH)_4(\text{1,4-benzenedicarboxylate})_6\), Fig. 28. Owing to the less spherical shape of the metal clusters this relationship may be less obvious but note their centering on the vertices and faces of the unit cell and that each cluster is connected to 12 nearest neighbor clusters. This structure also nicely illustrates that within inside a close packing arrangement we can find void spaces that are shape either like tetrahedra or octahedra. We can use these voids to describe a large number of simple ionic structures.

Fig. 28 Crystal structure of \(Zr_6O_4(OH)_4(\text{1,4-benzenedicarboxylate})_6\) also known as UIO-66. The Zr clusters (yellow) are face centered and each as 12 nearest neighbor clusters. Not the presence of both tetrahedral and octahedral voids in the crystal structure.