Synthesis and catalytic activities of a Zn(ii) based metallomacrocycle and a metal–organic framework towards one-pot deacetalization-Knoevenagel tandem reactions under different strategies: a comparative study

Abstract

Solvothermal reactions between a pyridine based amide functionalized dicarboxylic acid, 4,4′-{(pyridine-2,6-dicarbonyl)bis(azanediyl)}dibenzoic acid (H₂L), and zinc(II) nitrate in the absence and presence of a base produced the binuclear metallomacrocyclic compound [Zn₂(L)₂(H₂O)₄]·2(H₂O)·6(DMF) (1) and the metallomacrocyclic based two dimensional MOF [Zn₅(L)₄(OH)₂(H₂O)₄]_n·8n(DMF)·4n(H₂O) (2), respectively. Compound 1 bears two tetrahedral Zn(II) centres, whereas the 2D framework 2 includes a penta-nuclear Zn(II) cluster as a secondary building block unit, with two of the metal cations assuming a tetrahedral type geometry and the remaining three an octahedral type geometry. The topological analyses reveal that compound 1 has a 2-connected uninodal net and framework 2 has a 2, 8-connected binodal net. These compounds heterogeneously catalyse the tandem deacetalization-Knoevenagel condensation reactions carried out under conventional heating, microwave irradiation or ultrasonic irradiation. Comparative studies show that ultrasonic irradiation (final product yield of 99% after 2 h of reaction time) provides the most favourable method (e.g., microwave irradiation leads to a final product yield of 91% after 3 h of reaction time). Moreover, the catalysts can be reused at least for five consecutive cycles without losing activity significantly.