Triazole-Directed Fabrication Of Polyoxovanadate-Based Metal-Organic Frameworks As Efficient Multifunctional Heterogeneous Catalysts For The Knoevenagel Condensation And Oxidation Of Alcohols

By introducing 4-amino-1,2,4-triazole (4-NH2-trz), three new polyoxovanadate-based metal-organic frameworks (PMOFs) [Ni-3(4-NH2-trz)(6)][V6O18]center dot 3H(2)O (1), [Co-3(4-NH2-trz)(6)][V6O18]center dot 3H(2)O (2) and [Cu3OH(4-NH2-trz)(3)H2O][VO3](5)center dot H2O (3) have been synthesized and thoroughly characterized by single-crystal X-ray diffraction (SXRD), powder X-ray diffraction (PXRD), infrared spectroscopy (FT-IR), thermogravimetric (TG) analysis and elemental analysis (EA). Among them, PMOFs 1 and 2 had similar structures containing [V6O18](6-) clusters; however, PMOF 3 was isolated as a structure containing a [VO3](5)(5-) cluster when the amount of the 4-NH2-trz ligand was reduced to half with the other synthesis conditions being the same as those of PMOFs 1 and 2 except for the transition-metal chlorides. Furthermore, the negative charges of polyoxovanadate [V6O18](6-) and [VO3](5)(5-) anions were balanced by trinuclear complex cations [Ni-3(4-NH2-trz)(6)](6-) for 1, [Co-3(4-NH2-trz)(6)](6-) for 2 and [Cu3OH(4-NH2-trz)(3)H2O](5-) for 3, respectively. PMOFs 1-3 were further used as heterogeneous catalysts in the Knoevenagel condensation under solvent-free conditions and showed high catalytic activity. PMOF 1 showed moderate catalytic activities in the oxidation of various aromatic alcohols using H2O2 as an oxidant. Moreover, PMOF 1 could be reused at least three times without losing its activity.