Abstract
A series of aromatic-paneled FeII4L6 cages was synthesized through iron(II)-templated subcomponent self-assembly of 2-formylpyridine and C2-symmetric diamine building blocks having differing geometries, including many with a large degree of lateral offset between metal-binding sites. The new cages were characterized using X-ray crystallography, NMR spectroscopy, and mass spectrometry. Investigations of the guest binding properties of the cages provided insights into the structural factors important for the observation of guest binding. Both the size and arrangement of the aromatic panels were shown to be crucial for achieving effective encapsulation of large hydrophobic guests, including fullerenes, polycyclic aromatic hydrocarbons, and steroids, with subtle differences in the structure of subcomponents resulting in incommensurate effects on the binding abilities of the resulting hosts. Cages with large, offset aromatic panels were observed to be the most effective hosts as a result of a preference for a ligand conformation where the aromatic panels lie tangent to the edges of the tetrahedron, thus maximizing cavity enclosure.
Highlights
The study of self-assembled metal−organic cages[1] within the wider context of container molecules[2] has been a topic of great interest in recent years
Cages 5, 8, and 9 were reported previously, with full crystallographic characterization having been carried out on 5 and 9.22,24 ESI-MS confirmed the FeII4L6 composition of the assemblies, where L is the bis(pyridylimine) ligand derived from imine condensation of the respective diamine with 2-formylpyridine
We have focused on the size, shape, and degree of offset of the aromatic panels as the key parameters determining guest binding propensity
Summary
The study of self-assembled metal−organic cages[1] within the wider context of container molecules[2] has been a topic of great interest in recent years. The selective encapsulation of large aromatic molecules[11] such as fullerenes[6] and polycyclic aromatic hydrocarbons (PAHs)[12] is an attractive goal since the physicochemical properties of members of these classes of molecules are similar, which can render their separation difficult. Strong host−guest interactions can be further facilitated by cage walls with extended aromatic surfaces to provide favorable stacking interactions with aromatic guests.[16] One approach to achieving these criteria is to employ ligands with extended aromatic panels, which act to enclose the cavity and undergo aromatic stacking interactions with prospective guests.2d,17
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