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Abstract
For metal‐mediated host compounds, the development of strategies to reduce symmetry and introduce multiple functionalities in a non‐statistical way is a challenging task. We show that the introduction of steric stress around the coordination environment of square‐planar PdII cations and bis‐monodentate nitrogen donor ligands allows to control the size and shape of the assembled product, from [Pd2L4] cages over [Pd2L3] bowl‐shaped structures to [Pd2L2] rings. Therefore, banana‐shaped ligand backbones were equipped with pyridines, two different quinoline isomers and acridine, the latter three introducing steric congestion through hydrogen substituents on annelated benzene rings. Differing behavior of the four resulting hosts towards the binding of C60 and C70 fullerenes was studied and related to structural differences by NMR spectroscopy, mass spectrometry and single crystal X‐ray diffraction. The three cages based on pyridine, 6‐quinoline or 3‐quinoline donors were found to either bind C60, C70 or no fullerene at all.
Highlights
The metal-mediated self-assembly of supramolecular host systems with nano-sized cavities has been extensively explored in the last decades
It was considered to exhibit a similar behavior in selfassembly as L2, forming a bowl-shaped structure in which the coordination of three quinolines to each PdII cation is supplemented by a solvent molecule as the fourth ligand
We describe the expansion of a family of bis-monodentate ligands based on a curved backbone that allows corresponding metallo-supramolecular hosts to bind fullerenes in their interior
Summary
When considering other sterically more demanding donor functionalities, we recently started to substitute pyridines with quinolines, which carry an hydrogen atom on the annelated benzene ring that causes steric congestion around the metal binding site.[14] We tested this method on bis-monodentate banana-shaped ligands with a dibenzo-2.2.2-bicyclo-octane backbone and showed that the pyridine derivative L1 leads to the formation of a typical [Pd2L14]4+ cage while 6-quinolinyl derivative L2 cleanly forms bowl [Pd2L23(MeCN)2]4+ when palladium precursor [Pd(MeCN)4](BF4) was mixed with L2 in a 2:3 ratio in acetonitrile.[15] As explanation for the latter finding, we identified steric congestion around the coordination site, as observed in the single crystal X-ray structure of the bowl-shaped compound While this situation disfavors (but not absolutely prevents) the binding of a fourth bis-monodentate quinoline ligand, the square-planar coordination sphere of the PdII cations is either completed by an acetonitrile molecule or a chloride ligand. The tendency of the isomeric quinoline ligands to form bowls or cages as well as the fullerene affinities of all examined cage and bowl systems in comparison show more variety than initially expected and we suggest a number of structural reasons to explain these observations
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