Two Organic-Inorganic Hybrid Polyoxovanadates as Reusable Catalysts for Knoevenagel Condensation

Two novel organic-inorganic hybrid polyoxovanadates, i.e., [Ni(1-mIM)(4)(H2O)(2)][Ni(H2O)(5)](2)V(10)O(28)5.5H(2)O (compound 1) and [V(O)(1-vIM)(4)](2)V4O12H2O (compound 2) (1-mIM = 1-methylimidazole; 1-vIM = 1-vinylimidazole) were synthesized under routine conditions. The two polyoxovanadate hybrids were further unequivocally characterized by the usual physicochemical techniques such as single crystal X-ray diffraction (SCXRD), Fourier transform infrared spectroscopy (FT-IR), powder X-ray diffraction (PXRD), thermogravimetric (TG) analyses and elemental analyses. The SCXRD analyses demonstrated that the three Ni2+ atoms of compound 1 presented distorted octahedral geometry; one of the three Ni2+ atoms coordinated with four 1-mIM molecules and two coordinated water molecules, thereby exhibiting a [NiN4O2] binding set. Furthermore, the other two Ni2+ atoms were coordinated with five water molecules and one decavanadate polyanion, presenting an [NiO6] coordination environment. The structure of 2 exhibited a central [V4O12](4-) unit attached to the opposite sides of two [V(O)(1-vIM)(4)](2) groups, in which the V center displayed distorted octahedral configuration. More importantly, the two organic-inorganic hybrid polyoxovanadates were further studied using the Knoevenagel condensation reaction, and they were found to display efficient heterogeneous catalytic activities (yield up to 99%). The catalytic performance of compound 1 did not decrease even after three runs of catalytic cycling.