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}n (1) and {[Cd2(3-tpom)2(oxdz)2(H2O)2]·6H2O}n (2), in good yields utilizing a mixed-ligand approach, where 3-tpom = tetrakis(3-pyridyloxymethylene)methane and oxdz2- = 4,4'-(1,3,4-oxadiazole-2,5-diyl)-dibenzoate). Both 1 and 2 exhibit thermal stability up to 305 °C and chemical resistance toward polar solvents (water and methanol). The single-crystal X-ray structure of 2 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 oxdz2- ligands, and one water molecule. Based on the comparison of their FTIR spectroscopic data and powder diffraction patterns, 1 and 2 are found to be isostructural. Additional characterization of 1 and 2 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 1 and 2 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 %).