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]2V10O28·5.5H2O (compound 1) and [V(O)(1-vIM)4]2V4O12·H2O (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.