Design Principles for the Optimization of Guest Binding in Aromatic-Paneled Fe^II₄L₆ Cages

Abstract

A series of aromatic-paneled FeL cages was synthesized through iron(II)-templated subcomponent self-assembly of 2-formylpyridine and C-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.

Trigonal Prismatic Cu6L3 Coordination Cage: Encapsulation of Aromatic Molecules and Tuned Photoluminescence

“…[1] Coordination cages are three-dimensional ordered structures with nano-cavity and they are constructed in solution by self-assembly of metal ions and organic ligands through dative bonds. Coordination cages have been well-developed in both efficient construction strategies and various functions by Raymond, [2] Stang, [3] Fujita, [4] Nitschke, [5] etc, which exhibit rich and interesting host-guest chemistry. The most common geometry for coordination cages are Platonic solid, such as tetrahedron, [6] octahedron, [7] and cube.…”

Dimensional Matching versus Induced‐Fit Distortions: Binding Affinities of Planar and Curved Polyaromatic Hydrocarbons with a Tetragold Metallorectangle

“…Our interest was motivated by the fact that these organic molecules are hazardous materials that have gathered significant environmental concern . It should be noted that the use of SCCs for the encapsulation of PAHs has provided a large number of examples for the last 15 years, but the search for more efficient metallocage‐based receptors continuous to be a challenge because the binding affinities found are still far from those shown by the most efficient pure organic‐based systems…”

“…The introduction of pyrene into the structure of a SCC also brings the advantage that it can magnify the affinity of the metal complex toward planar aromatic guests by favoring π–π stacking interactions. Not surprisingly, Nietschke and co‐workers recently demonstrated that the incorporation of large aromatic panels into the structure of metallocages brings about more effective hosts for the encapsulation of polyaromatic guests …”

Reversible reduction drives anion ejection and C60 binding within an FeII4L6 cage

“…S26 †) to accommodate C 60 , and analogous cages have been shown to bind fullerenes well. 14,18 C 60 thus appeared to be an ideal guest molecule to test the catch-and-release cycle shown in Fig. 5.…”