Positional Effect of a Neutral Linker on the Three-Component MOFs: Room Temperature Synthesis, Structural Characterization, and Bifunctional Sustainable Catalysis

This work reports the room temperature synthesis of two 2D metal-organic frameworks (MOFs), {[Zn₂(3-tpom)₂(oxdz)₂]·4H₂O}_<i>n</i> (<b>1</b>) and {[Cd₂(3-tpom)₂(oxdz)₂(H₂O)₂]·6H₂O}_<i>n</i> (<b>2</b>), in good yields utilizing a mixed-ligand approach, where 3-tpom = tetrakis(3-pyridyloxymethylene)methane and oxdz^2- = 4,4'-(1,3,4-oxadiazole-2,5-diyl)-dibenzoate). Both <b>1</b> and <b>2</b> exhibit thermal stability up to 305 °C and chemical resistance toward polar solvents (water and methanol). The single-crystal X-ray structure of <b>2</b> reveals that each Cd center is heptacoordinated in a pentagonal bipyramidal geometry with two pyridyl nitrogen atoms from two different 3-tpom ligands, four oxygen atoms from two different chelated oxdz^2- ligands, and one water molecule. Based on the comparison of their FTIR spectroscopic data and powder diffraction patterns, <b>1</b> and <b>2</b> are found to be isostructural. Additional characterization of <b>1</b> and <b>2</b> includes elemental, microscopic (FESEM and TEM), and X-ray photoelectron spectroscopy analysis. Their rectangular pores are lined with uncoordinated dangling pyridyl groups. The synergistic effect of Lewis acidic metal sites and uncoordinated N/O Bronsted basic sites of <b>1</b> and <b>2</b> is judiciously explored for the heterogeneous catalysis of the synthesis of coumarin-3-carboxylic acid (C-3-CA) and its derivatives following the Knoevenagel intramolecular cyclization tandem reaction between salicylaldehyde and Meldrum's acid under sustainable reaction conditions (solvent: methanol; temperature: 25 °C; and catalyst: 1.5 mol %).