Efficient and Reusable Pb(II) Metal–Organic Framework for Knoevenagel Condensation

Abstract

A microporous lead–organic framework {[Pb4(µ8-MTB)2(H2O)4]·5DMF·H2O}n (MTB = methanetetrabenzoate, DMF = N,N′-dimethylformamide) was synthesized and studied as a catalyst in Knoevenagel condensation reactions. The framework is built from tetranuclear [Pb4(µ3-COO)(µ2-COO)6(COO)(H2O)4] clusters and exhibits a 3D structure, with repeated 1D jar-like cavities with sizes about 14.98 × 7.88 and 14.98 × 13.17 A2 and BET specific surface area of 980 m2 g−1. To obtain open framework with unsaturated Pb(II) sites needed for catalysis, the thermal activation of the solvent exchanged sample was performed (DMF was exchanged by EtOH). The activated compound was tested in Knoevenagel condensation of bulky aldehydes and active methylene compounds at different temperatures. Excellent catalytic conversion and selectivity in condensation of small-sized aldehydes with malononitrile was observed, which indicates that the opened Pb(II) sites play a significant role in the heterogeneous catalytic process. Leaching test confirmed the stability of the catalyst in catalytic reactions. Moreover, the compound displayed good recyclability after several reuses without significant decrease in the original catalytic activity. Novel Pb(II) metal–organic framework was tested in Knoevenagel condensation. The catalyst showed excellent catalytic conversion, selectivity and recyclability. Aldehydes with lower kinetic diameter demonstrated high conversions and yields. Catalyst is less efficient for condensation of larger aromatic aldehydes.