Functionalization of Magnetic UiO-66-NH₂ with Schiff Base Copper(II) Complex for the One-Pot Synthesis of Pyran-Annulated Heterocyclic Scaffolds

This study explores the design and synthesis of the magnetic UiO-66-NH₂(Zr) functionalized with a Schiff base copper(II) complex via the postsynthesis method, forming a core-shell structure that acts as a magnetically recyclable nanocatalyst. The nanocatalyst was synthesized using an affordable and straightforward method and abbreviated as magnetic@UiO-66-NH-SB-Cu(II). For this purpose, magnetic@UiO-66-NH₂ was synthesized by modifying Fe₃O₄ with tetraethyl orthosilicate, ZrCl₄, and 2-aminoterephthalic acid, stabilizing the core, and promoting Zr-MOF growth. The surface was functionalized with pyrrole-2-carboxaldehyde and CuCl₂, followed by characterization through various techniques. Using particle size distributions from transmission electron microscopy images, the average size was determined to be roughly 10-22 nm. The presence of cupric ions (Cu²⁺) within the catalyst was confirmed through X-ray photoelectron spectroscopy analysis, which distinctly revealed two peaks at 940.2 and 960.1 eV corresponding to Cu 2p_3/2 and Cu 2p_1/2, respectively. The synthesized nanocatalyst exhibits improved performance in the formation of chromene-annulated heterocycles from various aromatic aldehydes and various phenols with malononitrile in ethanol under reflux conditions. The catalyst can be reused by a magnet field up to seven times with only a minimal drop in the yield of the product. Cu leaching is confirmed by inductively coupled plasma analysis to occur at a rate of 0.7 wt %.