A highly catalytically active Hf(IV) metal-organic framework for Knoevenagel condensation

We report a highly crystalline, Hf(IV) metal-organic framework (MOF) containing 2-hydrazinyl-1,4-benzene-dicarboxylic acid (H2BDC-N2H3) as the ligand. The resulting Hf-UiO-66-N2H3 MOF (1) exhibited efficient and recyclable catalytic activity in its activated form (1') for Knoevenagel condensation of benzaldehyde with malononitrile. Moreover, in traditional solvothermal synthesis, compound 1 was produced with a very high yield. Compound 1' showed very high N-2 adsorption capacity, excellent chemical and thermal stability. Material 1' could catalyze Knoevenagel condensation of benzaldehyde with malononitrile, giving quantitative yield under the optimum reaction conditions. Moreover, several control experiments indicated that Hf-based MOFs exhibit superior activity than Zr-based MOFs under identical conditions. Furthermore, the stability of 1' was surveyed by conducting reusability and leaching experiments. These results indicate that 1' is active up to five cycles and catalytic reaction is heterogeneous in nature. Furthermore, the scope of 1' was also studied with a series of aldehydes differing from their substituents and molecular sizes, and high yields were observed in many cases. Interestingly, the order of reactivity of the aldehydes with malononitrile in the presence of 1' under identical conditions was: benzaldehyde > 1-naphthaldehyde > 9-anthracenealdehyde. These observations suggest that the catalysis is size-selective.