Hybridization of NH2-MIL-88B/Deep eutectic solvent: A novel multifunctional heterogeneous catalyst for the synthesis of spirooxindoles

Abstract

Deep eutectic solvents (DESs) with outstanding environmental suitability have emerged as green alternatives to ionic liquids (ILs) and many other organic solvents. Nevertheless, their separation from reaction media and reuse is still problematic. Hence, the heterogenization of DESs on proper support can overcome their drawbacks. Herein, a DES-supported catalyst was prepared for the synthesis of organic compounds. Initially, the surface of Fe-based MOF, NH2–MIL-88B, was modified via a polymerization reaction between MOF amino groups, cyanuric chloride, and urea. Then, choline chloride (ChCl) was stabilized on the surface of modified MOF, NH2–MIL-88B-CU, through hydrogen bonding. NH2–MIL-88B-CU@ChCl as a multifunctional catalyst (Brønsted/Lewis acids and Brønsted base), with alluring attributes like environmental sustainability, chemical and thermal stability, retrievability, and reusability, was used for the one-pot three-component synthesis of spirooxindoles. Its high catalytic performance was related to a synergistic effect between active sites of NH2–MIL-88B-CU and ChCl. NH2–MIL-88B not only provided a platform for immobilizing of DESs but also showed catalytic activity through Fe-oxo nodes. Quantum chemical calculations were performed to shed light on the reaction mechanism. In addition, green metrics parameters have shown that the present method is more efficient than some reported homogeneous and heterogeneous catalytic systems while providing less waste, which is vital from the viewpoint of green chemistry. The hot filtration test and the retrievability/reusability of the catalyst proved the structural stability and heterogeneous nature of the hybrid system.