A CuI6L4 Cage Dynamically Reconfigures to Form Suit[4]anes and Selectively Bind Fluorinated Steroids

Abstract

Simple organic ligands can self-assemble with metal ions to generate metal-organic cages, whose cavities bind guests selectively. This binding may enable new methods of chemical separation or sensing, among other useful functions. Here we report the preparation of a Cu^I₆L₄ pseudo-octahedral metal-organic cage, the ligands of which self-assemble from simple organic building blocks. Temperature, solvent, and the presence of different guests governed which structure predominated from a dynamic mixture of cage diastereomers with different arrangements of right- or left-handed metal vertices. Dissolution in dimethyl sulfoxide or the binding of tetrahedral guests led to a chiral tetrahedral T-symmetric framework, whereas low temperatures favored the achiral S₄-symmetric diastereomer. Tetrahedral guests with long arms were encapsulated to form mechanically bonded suit[4]anes, with guest arms protruding out through host windows. The cage was also observed to bind fluorinated steroids, an important class of drug molecules, but not non-fluorinated steroids, providing the basis for new separation processes.

Figure 1

Preparation and characterization of Cu^I₆L₄ cage 1. (a) The self-assembly of subcomponents A and B with Cu^I in MeCN yielded 1. The blue spheres in 1 indicate Cu^I vertices adopting either Δ or Λ stereochemistry. (b) ¹H NMR spectrum of S₄-1 measured at 253 K in CD₃CN. The signal for each unique ligand arm environment is labeled using a different color; each set could not be conclusively assigned to a specific arm in the structure. (c) Single-crystal X-ray structure of S₄-1. (d) Space-filling representation of S₄-1.

Figure 2

Formation of three-dimensional suit[4]ane G3⊂T-1. (a) Transformation of 1 to mechanically interlocked G3⊂T-1 following the addition of G3. The blue spheres in 1 indicate Cu^I vertices adopting either Δ or Λ stereochemistry. (b) The three tetrahedral guests that induced the S₄-1 to T-1 transformation upon guest encapsulation. (c) Space-filling representation of the X-ray crystal structure of G3⊂T-1.

Figure 3

Cage 1 bound fluorinated corticosteroids selectively. (a) Cartoon illustrating the putative exo binding mode of fluorinated G11 by 1 over its non-fluorinated analogue G4. (b) The corticosteroids screened, showing those observed to bind and those that did not. The fluorine atoms in G10 and G11 are highlighted.