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

Abstract

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 %.