Design and Room-Temperature Synthesis of a Multipurpose and Sustainable Heterogeneous Catalyst Decorated with Lewis Acidic Metal Sites and Dangling Pyridyl Groups

Abstract

In this work, we report a 2D metal-organic framework with dangling pyridine groups, {[Cd₂(4-tpom)₂(oxdz)₂(H₂O)₂]·4H₂O}_n (1), has been synthesized using a flexible N-donor linker, 4-tpom (tetrakis(4-pyridyloxymethylene)methane), and a bent dicarboxylate, oxdz²⁻(4,4′-(1,3,4-oxadiazole-2,5-diyl)-dibenzoate), to demonstrate its multipurpose as a bifunctional heterogeneous catalyst. Its thermal (up to 332 °C) and chemical stability in water and methanol and at different pH levels (5, 7.4, 9) provides sustainability for any application. Its structural features and properties are well established by elemental analysis, powder X-ray diffraction (PXRD), various spectroscopic/microscopic techniques, and geometry optimization. With unsaturated metal centers as the Lewis acidic sites and uncoordinated nitrogen/oxygen atoms as the Lewis/Bronsted basic sites, a very low catalyst loading (1.5 mol%) of activated 1 is found to be sufficient to catalyze two important C─C bond-forming reactions at room temperature: a) the cyanosilylation reaction converting benzaldehyde to trimethyl silyl ether (100% in 5 h) under solvent-free conditions, and b) the Knoevenagel-intramolecular cyclization reaction for converting salicylaldehyde to coumarin-3-carboxylic acid in methanol (99% in 1 h). Notably, this is the first time the same catalyst has been utilized for these two reactions, demonstrating its multipurpose nature. Furthermore, the spent catalyst is recyclable for multiple cycles, maintaining its structural integrity established via FTIR spectroscopy and PXRD.