The development of chiral metal–organic frameworks for enantioseparation of racemates

Abstract

MIL-101(Cr), an achiral metal–organic framework, made up of a terephthalic acid ligand and a metal chromium ion was selected as a template. Its structural features are unsaturated Lewis acid sites that can be easily activated and it has an extremely high specific surface area, big pore size, and good thermal/chemical/water stability. This achiral framework was modified to introduce chirality within the structure to develop chiral metal–organic frameworks (CMOFs). Here, natural chiral ligands, amino acids (L-proline, L-thioproline and L-tyrosine), were selected for post synthetic modification (PSM) of MIL-101(Cr). This is a very simple, clean and facile methodology with respect to the reactants and reaction conditions. CMOFs 1–3 abbreviated as MIL-101-L-proline (CMOF-1), MIL-101-L-thioproline (CMOF-2) and MIL-101-L-tyrosine (CMOF-3) were prepared by introducing L-proline, L-thioproline and L-tyrosine as chiral moieties within the framework of (Cr). These CMOFs were characterized by FTIR, PXRD, SEM, and thermo gravimetric analysis. Chirality within these CMOFs 1–3 was established by circular dichroism (CD) and polarimetric methods. These three CMOFs 1–3 showed enantioselectivity towards RS-ibuprofen, RS-mandelic acid and RS-1-phenylethanol to varying extents. Their enantioselectivity towards racemates was studied by chiral HPLC and polarimetry.