Triazole-directed fabrication of polyoxovanadate-based metal–organic frameworks as efficient multifunctional heterogeneous catalysts for the Knoevenagel condensation and oxidation of alcohols

Abstract

By introducing 4-amino-1,2,4-triazole (4-NH₂-trz), three new polyoxovanadate-based metal–organic frameworks (PMOFs) [Ni₃(4-NH₂-trz)₆][V₆O₁₈]·3H₂O (1), [Co₃(4-NH₂-trz)₆][V₆O₁₈]·3H₂O (2) and [Cu₃OH(4-NH₂-trz)₃H₂O][VO₃]₅·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]₅⁵⁻ 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]₅⁵⁻ anions were balanced by trinuclear complex cations [Ni₃(4-NH₂-trz)₆]⁶⁻ for 1, [Co₃(4-NH₂-trz)₆]⁶⁻ for 2 and [Cu₃OH(4-NH₂-trz)₃H₂O]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.