The Intricate Structural Chemistry of M^II_2nL_n-Type Assemblies

Abstract

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.

Cages vs. Prisms: Controlling the Formation of Metallosupramolecular Architectures with Ligand Side‐Chains

“…Previously, we had used structurally related metalloligands such as 2 (Scheme ) for the construction of metallosupramolecular assemblies . A unique feature of 1 is the presence of three bulky and rigid phenanthrene groups, which point away from the ligand center.…”

“…We had previously investigated the reaction between ligand 2 and [(dppe)Pt(OTf) 2 ] and [(dppp)Pt(OTf) 2 ], and we had observed that the product in both cases was a cage structure with a gyrobifastigium‐like geometry . These cages ( 5 and 6 ) show a characteristic feature in the 31 P NMR spectra, namely the presence of two singlets of equal intensity.…”

“…The different outcome of the reactions must be related to the side chains of the central iron complex ( n ‐butyl vs. phenanthrene). We had reasoned previously that the lateral size of a ligand can be a decisive element for the assembly of M 8 L 4 ‐type structures, and geometrical considerations suggest that wider ligands should favor cage‐like structures over prisms , . However, it is difficult to argue that the lateral size of ligand 2 is larger than that of ligand 1 .…”

“…In 2016, we have shown that M 8 L 4 ‐type complexes can not only form tetragonal prisms, but also cages with a gyrobifastigium‐like geometry (Figure , bottom) . Subsequent studies by the group of Mukherjee led to the structural characterization of a Pd 12 L 6 cage with a triangular orthobicupola‐like structure, and we have recently reported the structural characterization of a Pt 16 L 8 cage with a square orthobicupola‐like structure …”

“…For example, the self‐assembly process can be influenced by the nature of the cis ‐blocked M II complex . Furthermore, we have reasoned that the lateral size of a tetratopic N‐donor ligand is a significant factor for M 8 L 4 ‐type assemblies , , . M 8 L 4 complexes can either form tetragonal prisms or gyrobifastigium‐like cages (Figure ).…”