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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., The addition of a metastable-state photoacid to solutions containing metal–ligand assemblies renders the systems light responsive. Upon irradiation, proton transfer from the photoacid to the ligand is observed, resulting in disassembly of the metallasupramolecular structure. In the dark, the process is fully reversed. Light-induced switching was demonstrated for six different metal–ligand assemblies containing PdII, PtII or RuII complexes and bridging polypyridyl ligands. The methodology allows liberating guest molecules with light.

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.

The reaction of cis-blocked, square-planar M-II complexes with tetratopic N-donor ligands is known to give metallasupramolecular assemblies of the formula M2nLn. 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 Pt8L4 cage with unusual gyrobifastigium-like geometry. To date, the factors that govern the assembly of (M2nLn)-L-II complexes are not well understood. Herein, we provide a geometric analysis of M2nLn 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-II complexes and metalloligands with four divergent pyridyl groups. Mononuclear metalloligands gave mainly assemblies of type Pt8L4, which adopt barrel- or gyrobifastigium-like structures. Larger assemblies can also form, as evidenced by the crystallographic characterization of a Pt10L5 complex and a Pt16L8 complex. The former adopts a pentagonal barrel structure, whereas the latter displays a barrel structure with a distorted square orthobicupola geometry. The Pt16L8 complex has a molecular weight of more than 23 kDa and a diameter of 4.5 nm, making it the largest, structurally characterized M2nLn complex described to date. A dinuclear metalloligand was employed for the targeted synthesis of pentagonal Pt10L5 barrels, which are formed in nearly quantitative yields.

Large (Mw > 10 kDa) heterometallic coordination cages with gyrobifastigium-like geometry are obtained by using metalloligands with sterically demanding FeII clathrochelate cores and four divergent pyridyl groups. Upon reaction with cis-blocked PtII and PdII complexes, MII8L4 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 M8L4 stoichiometry.