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 Fe(II)8L12 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 Fe(II)4L6 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 Pd(II)4L8 assembly, whereas an octahedral Pd(II)6L12 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 Pd(II)4L8 and Pd(II)6L12 cages, whereas the other did not.

The reaction of cis-blocked, square-planar M complexes with tetratopic N-donor ligands is known to give metallasupramolecular assemblies of the formula ML. These assemblies typically adopt barrel-like structures, with the ligands paneling the sides of the barrels. However, alternative structures are possible, as demonstrated by the recent discovery of a PtL cage with unusual gyrobifastigium-like geometry. To date, the factors that govern the assembly of ML complexes are not well understood. Herein, we provide a geometric analysis of ML complexes, and we discuss how size and geometry of the ligand is expected to influence the self-assembly process. The theoretical analysis is complemented by experimental studies using different cis-blocked Pt complexes and metalloligands with four divergent pyridyl groups. Mononuclear metalloligands gave mainly assemblies of type PtL, which adopt barrel- or gyrobifastigium-like structures. Larger assemblies can also form, as evidenced by the crystallographic characterization of a PtL complex and a PtL complex. The former adopts a pentagonal barrel structure, whereas the latter displays a barrel structure with a distorted square orthobicupola geometry. The PtL complex has a molecular weight of more than 23 kDa and a diameter of 4.5 nm, making it the largest, structurally characterized ML complex described to date. A dinuclear metalloligand was employed for the targeted synthesis of pentagonal PtL barrels, which are formed in nearly quantitative yields.

Only stable in the dark: when mixed with a metastable-state photoacid, metallasupramolecular structures become light sensitive. The photo-induced disassembly of the structures is reversed when the light is switched off.

Large (M > 10 kDa) heterometallic coordination cages with gyrobifastigium-like geometry are obtained by using metalloligands with sterically demanding Fe clathrochelate cores and four divergent pyridyl groups. Upon reaction with cis-blocked Pt and Pd complexes, ML cages are formed. The gyrobifastigium geometry of these cages is in contrast to the barrel-like structures which are typically observed for metallasupramolecular assemblies with ML stoichiometry.