The Intricate Structural Chemistry of ML-Type Assemblies.

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.