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.