Imidazolium-based ionic liquid functionalized chiral metal–organic framework as an efficient catalyst for asymmetric catalytic sulfoxidation

Abstract

Ionic liquid (IL) units in heterogeneous catalysts exhibit synergistic effects to enhance catalytic performance and stabilize catalytically active centers, while also preventing the degradation of catalysts during the reaction process. Ionic liquid units in IL-functionalized CMOF catalysts enhance their catalytic performance in a synergistic manner. However, not only are the yields of IL-functionalized CMOFs obtained with post-synthesis methods low, but they also lead to blocking of the MOF pores and leaching of the ionic liquid. Hence, we designed and synthesized IL-functionalized chiral Ti(salen)-derivatized dicarboxylic acid as a linker coordinated with zinc nitrate to form ionic liquid functionalized CMOF (IL-Ti(salen) CMOF-n) (n = 1 and 2) catalysts. The synthesised IL-Ti(salen) CMOF-n catalyst was characterized by FT-IR, PXRD, SEM, TEM, TGA and N₂ adsorption–desorption, and the catalytic performance of the IL-Ti(salen) CMOF-n was assessed using the asymmetric oxidation of methyl phenyl sulfide as a model reaction and H₂O₂ as an oxidant. (R)-Methyl phenyl sulfoxide with remarkable chemoselectivity (93%) and enantioselectivity (>99%) was obtained under the optimal reaction conditions. Furthermore, the catalysts demonstrated excellent recyclability after seven consecutive reuses without obvious loss of activity. This work highlights the significance of developing a new type of ionic liquid functionalized chiral heterogeneous catalyst with remarkable catalytic performance, and provides valuable guidance for the design and preparation of novel catalysts for asymmetric catalysis.