Ligand Aspect Ratio as a Decisive Factor for the Self-Assembly of Coordination Cages

It is possible to control the geometry and the composition of metallasupramolecular assemblies via the aspect ratio of their ligands. This point is demonstrated for a series of iron- and palladium-based coordination cages. Functionalized clathrochelate complexes with variable aspect ratios were used as rod-like metalloligands. A cubic (Fe8L12)-L-II cage was obtained from a metalloligand with an intermediate aspect ratio. By increasing the length or by decreasing the width of the ligand, the self assembly process resulted in the clean formation of tetrahedral (Fe4L6)-L-II cages instead of cubic cages. In a related fashion, it was possible to control the geometry of Pd-II-based coordination cages. A metalloligand with a large aspect ratio gave an entropically favored tetrahedral (Pd4L8)-L-II assembly, whereas an octahedral (Pd6L12)-L-II cage was formed with a ligand of the same length but with an increased width. The aspect ratio can also be used to control the composition of dynamic mixtures of Pd-II cages. Out of two metalloligands with only marginally different aspect ratios, one gave rise to a self-sorted collection of (Pd4L8)-L-II and (Pd6L12)-L-II cages, whereas the other did not.