Polyoxomolybdates as efficient catalysts for Knoevenagel condensation reaction of benzaldehyde and ethyl cyanoacetate under mild condition

The development of new heterogeneous catalysts for Knoevenagel condensation reaction is critical for practical applications. Herein, we prepared three organophosphonate-functionalized polyoxomolybdates, Cs2Na9H9[{Co(H2O)4}(TeMo6O21)2{Co(OOCCH2NCH2PO3)2}3]·41H2O (1), Cs2Na8[H6{Co(H2O)4}3(SeMo6O21)2{Co(OOCCH2NCH2PO3)2}3]·31H2O (2) and Cs2Na9H3[H6{Ni(H2O)4}(SeMo6O21)2{Ni(OOCCH2NCH2PO3)2}3]·28H2O (3), which exhibit high catalytic activity for gram-scale Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate. Especially, under the optimal conditions, compound 1 can achieve a satisfactory yield (98%) and show high TON value (1960), which is superior to that of compounds 2 and 3. Notably, the TON value is much higher than that of previously reported catalysts. Such high catalytic performance could be attributed to the co-existence of Lewis acid and Lewis base sites in catalysts. In addition, based on substrates screening and recyclability tests, 1 has exhibited broad catalytic generality and good recyclability.