Luminescent amino-functionalized Pb-MOFs based on benzoic acid ligands: design, physicochemical characterization, and application as heterogeneous catalysts for Knoevenagel–Doebner reactions†

Abstract

Metal–organic frameworks (MOFs), known for their unique properties and adaptability, have garnered considerable interest in the development of advanced materials and technologies in chemistry and engineering. Due to the versatility of the metal centers and ligands present in their structure, MOFs have emerged as excellent catalytic platforms and tend to exhibit superior properties when functionalized. This study explores the synthesis, characterization, and catalytic application of lead-based MOFs (Pb-MOFs) containing terephthalic acid (BDC) and 2-aminoterephthalic acid (BDC-NH₂) ligands in the Knoevenagel–Doebner reaction, a key transformation in organic synthesis. While Knoevenagel reactions have been extensively studied, there are currently no reports on the use of MOFs in the Knoevenagel–Doebner reaction. The Pb(BDC)_0.20−x(BDC-NH₂)_x (x = 0.01, 0.02 or 0.04) compounds were synthesized using the hydrothermal method and characterized using X-ray diffraction (XRD), which confirmed their orthorhombic crystal structure and high purity, even with the introduction of amino groups. Scanning electron microscopy (SEM) revealed morphological changes correlated with different BDC-NH₂ concentrations, transitioning from rod-like crystals to small particle conglomerates. Fourier transform infrared (FT-IR) spectroscopy verified the successful incorporation of amine groups, while UV-Vis diffuse reflectance spectroscopy (DRS) and emission spectra highlighted changes in photophysical properties due to the mixed ligands. The catalytic performance of Pb(BDC)_0.16(BDC-NH₂)_0.04 with amine groups was tested in the Knoevenagel–Doebner reaction with benzaldehyde, 4-(trifluoromethyl)benzaldehyde 4-chlorobenzaldehyde and malonic acid. The catalytic performance of Pb(BDC)_0.16(BDC-NH₂)_0.04, featuring amine groups, was evaluated in the Knoevenagel–Doebner reaction, yielding cinnamic acids with a range of 80–91%. These results demonstrate the effectiveness of Pb-MOFs as heterogeneous catalysts, showing that structural and functional modifications can significantly enhance both catalytic activity and selectivity. This study underscores the potential of Pb-MOFs to improve catalytic processes for the synthesis of industrially relevant compounds.