A Cavity-Tailored Metal-Organic Cage Entraps Gases Selectively in Solution and the Amorphous Solid State

Abstract

Here we report the subcomponent self-assembly of a truxene-faced Zn₄ L₄ tetrahedron, which is capable of binding the smallest hydrocarbons in solution. By deliberately incorporating inward-facing ethyl groups on the truxene faces, the resulting partially-filled cage cavity was tailored to encapsulate methane, ethane, and ethene via van der Waals interactions at atmospheric pressure in acetonitrile, and also in the amorphous solid state. Interestingly, gas capture showed divergent selectivities in solution and the amorphous solid state. The selective binding may prove useful in designing new processes for the purification of methane and ethane as feedstocks for chemical synthesis.

Keywords: gas adsorption; gas encapsulation; host-guest chemistry; metal-organic cage; supramolecular chemistry.

Figure 1

a) Subcomponent self‐assembly of tetrahedron 1 and b) its ¹H NMR spectrum (CD₃CN, 600 MHz, 298 K). The peaks of the two ethyl groups on each truxene are labelled.

Figure 2

Crystal structure of tetrahedron 1 with its void space outlined in green mesh. The inset at right shows the cavity void surrounded by the ethyl substituents of the ligands. Ethyl carbon atoms are colored cyan. Disorder, counterions, and solvents of crystallization are omitted for clarity.

Figure 3

a) Encapsulation of the gases methane, ethane, and ethene within tetrahedron 1 in solution. Partial ¹H NMR spectra (CD₃CN, 600 MHz, 238 K) of b) free cage 1; 1 binding c) methane, d) ethane, and e) ethene. f) ¹H–¹H NOESY spectrum (CD₃CN, 600 MHz, 238 K) of 1 binding ethane. Cross‐peaks between encapsulated ethane and the ethyl groups of 1 are circled, and an exchange cross peak between the free and encapsulated ethane guest is highlighted by a square. Peaks from water and CHD₂CN are indicated by asterisks.

Figure 4

Gas adsorption isotherms of 1 for CH₄ (circles), C₂H₆ (squares), and C₂H₄ (triangles) at 295 K.