Mechanosynthesis of new azine-functionalized Zn(II) metal–organic frameworks for improved catalytic performance

Knoevenagel condensation of aldehydes with active methylene compounds is a usefultransformation that has been widely employed for carbon–carbon bond formation in the synthesis of several fine chemicals. The condensation is usually catalyzed by solid bases like alkali or alkaline-earth metal oxides. Over the last few years, a wide range of catalysts have been investigated for this reaction such as Lewis acids, amine-functionalized solid supports, cation-exchanged zeolites, ionic liquids and organometallic catalysts. However, most of these methods have significant drawbacks, such as using hazardous and carcinogenic solvents, high catalyst loading or non-recoverable catalysts that sometimes contain toxic metals. Therefore, there is a great need for new catalytic methods that do not have these problems. MOFs have shown a high catalytic activity to promote condensation reactions. Three 3D, porous Zn(II)-based metal–organic frameworks, TMU-4, TMU-5 and TMU-6, containing azine-functionalized pores, were readily and quickly prepared via mechanosynthesis. Catalytic performance of these MOFs and the effect of N-donor ligands with diverse basicity in the Knoevenagel condensation reaction are investigated. Increasing the basicity of N-donor ligands leads to improvement in catalytic activity. Results show that among the three compounds, TMU-5 has the highest catalytic activity upon increasing its basicity of azine function in the N-donor ligand. These catalysts maintain their crystalline framework after the reaction and are easily recycled [1,2].