Fabrication of porous covalent organic cages using cyclodextrin metal-organic frameworks as template

Abstract

Up to now, transformation from metal-organic frameworks (MOFs) to covalent-organic-framework cages (COF-Cages) has never been reported. In this report, we demonstrated an organic cage crystal by transformation from a cyclodextrin MOF, via boronate ester formation reaction of the hydroxy groups of γ-CD inside the MOF, followed by removing of the potassium ions. First, CD-MOF was prepared by reacting γ-CD with potassium hydroxide in aqueous solution, followed by vapor diffusion of methanol into the solution according to a previously reported method. The freshly prepared CD-MOF was first washed with ethanol three times to remove the unreacted reactants, and then added to an ethanol saturated solution of benzene-1,4-diboronic acid (BDBA) in a screw top vial, and kept it at 65℃ for three days. Finally, the covalent cross-linked CD-MOF (CL-CD-MOF) was obtained by forming boronic esters between the uncoordinated C(2) and C(3) hydroxy groups of contiguous γ-CD sides in the CD-MOF pores and two boronic acid groups of BDBA. Structure and physical properties of Z-Cages were fully characterized by thermogravimetric analysis (TGA), infrared spectroscopy (IR), powder X-ray diffraction (PXRD), solid-state 13C and 11B cross polarization/magic angle spining nuclear magnetic resonance (CP/MAS/NMR) spectroscopy and nitrogen adsorption. The obtained zeolite-type organic cage (Z-cage) displayed a targeted sodalite-type crystalline structure and permanent porosity with the surface area of 862 m2·g-1. A control experiment, the cross-linked polymers (CL-polymer) formed by coupling of γ-CD and BDBA was done by solvothermal method. The CL-polymer was synthesized by the heating of a 4:1 stoichiometric mixture of BDBA and γ-CD at 90℃ for three days in dimethylformamide (DMF). PXRD pattern shows the CL-polymer are crystalline, but totally different with Z-cage. This transformation from crystalline inorganic-organic hybrid framework of MOF to crystalline organic framework provides an opportunity for crystal-to-crystal in porous crystalline materials.