Revealing the roles of the dual active-sites on a polyoxometalate-based metal–organic framework in catalyzing Knoevenagel condensations

Abstract

The efficient catalysis of Knoevenagel condensations under mild conditions is a significant way for obtaining olefin compounds, which can be used to produce a wide range of biologically active heterocyclic chemicals and fine chemical raw materials. Herein, a novel polyoxometalate-based metal–organic framework (POMOF), [Co(bix)(H2O)]{V2O6} (VMOF-1, bix = 1,4-bis(imidazol-1-ylmethyl)benzene), is synthesized and structurally characterized. After removal of the coordinated water molecules of CoII ions by vacuum drying, VMOF-1a with both Lewis acid and Lewis base catalytic active-centers is achieved. VMOF-1a as the heterogeneous catalyst for the conversion of benzaldehyde and malononitrile to high value-added olefinic compounds under mild conditions, exhibits excellent conversion and selectivity, good recyclability and broad substrate applicability. Experimental and theoretical studies have revealed that CoII ion and {V2O6}2− unit in VMOF-1a synergistically catalyze the Knoevenagel condensation reaction: (1) {V2O6}2− is used as a Lewis base to pull out the proton of the active methylene group on malononitrile; (2) CoII ion acts as a Lewis acid to activate the carbonyl oxygen on benzaldehyde. Furthermore, it is demonstrated for the first time that the Lewis acid sites in POMOF can play the role of stabilize the alcohol hydroxyl anion intermediates generated during the reaction process, which further elucidates the reason for the high catalytic activity of VMOF-1a in the Knoevenagel condensation. This study provides theoretical guidance for the design of efficient and sustainable catalysts for Knoevenagel condensation.