The 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 FeII8L12 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 FeII4L6 cages instead of cubic cages. In a related fashion, it was possible to control the geometry of PdII-based coordination cages. A metalloligand with a large aspect ratio gave an entropically favored tetrahedral PdII4L8 assembly, whereas an octahedral PdII6L12 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 PdII cages. Out of two metalloligands with only marginally different aspect ratios, one gave rise to a self-sorted collection of PdII4L8 and PdII6L12 cages, whereas the other did not.