Schiff Base Complex of Copper Immobilized on Core–Shell Magnetic Nanoparticles Catalyzed One-Pot Syntheses of Polyhydroquinoline Derivatives under Mild Conditions Supported by a DFT Study

We synthesized a stable and reusable Schiff base complex of copper immobilized on core-shell magnetic nanoparticles [Cu(II)-SB/GPTMS@SiO₂@Fe₃O₄] with simple, efficient, and available materials. A variety of characterization analyses including Fourier transform infrared (FT-IR), X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET), thermogravimetric analysis (TGA), X-ray diffraction (XRD), vibrating-sample magnetometry (VSM), energy-dispersive X-ray spectrometry (EDX), and inductively coupled plasma (ICP) confirm that our synthesized nanocatalyst was obtained. The particle size distribution from the TEM image was obtained in the range of 42-55 nm. The existence of cupric species (Cu²⁺) in the catalyst was determined with XPS analysis and clearly indicated two peaks at 933.7 and 953.7 eV for Cu 2p_3/2 and Cu 2p_1/2, respectively. BET results showed that our catalyst synthesized with a mesoporous structure and with a specific area of 48.82 m² g^-1. After detailed characterization, the resulting nanocatalyst exhibited excellent catalytic performance for the explored catalytic reactions in the one-pot synthesis of polyhydroquinoline derivatives by the Hantzsch reaction of dimedone, ethyl acetoacetate, ammonium acetate, and various aldehydes under sustainable and mild conditions. The corresponding products 5a-l are achieved in yields of 88-97%. Additionally, density functional theory (DFT) calculations were carried out to investigate the electrostatic potential root (ESP), natural bond orbital (NBO), and molecular orbitals (MOs), drawing the reaction mechanism using the total energy of the reactant and product and the study of structural parameters.