From Pure Inorganic to Hybrid Structure: Rational Design and Knoevenagel Reaction Performance of Polyoxovanadate Catalysts

The type of transition-metal center and bound organic ligands serve as critical regulators for the catalytic properties of polyoxovanadate (POV) clusters in organic transformations. Herein, a series of pure inorganic POVs with distinct transition-metal ions, namely [M(H2O)4][β-VIV14Sb8O40(OH)2(H2O)]·6H2O (M = Co (1), Ni (2), Zn (3)) were hydrothermally synthesized. By further introducing the 5,5′-mbpy (5,5′-dimethyl-2,2′-dipyridyl) organic ligand into the identical metal centers, corresponding inorganic-organic hybrid POVs [M(5,5′-mbpy)2(H2O)]2[β-VIV14Sb8O42(H2O)]·3H2O (M = Co (4), Ni (5), Zn (6)) were successfully synthesized. These compounds were characterized by infrared spectroscopy (IR), single crystal X-ray diffraction (SCXRD), powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TG). Their catalytic activities were systematically evaluated for the Knoevenagel condensation of benzaldehyde and malononitrile. Compound 4 demonstrated superior catalytic performance, affording nearly quantitative benzaldehyde conversion (>99%) with >99% selectivity within 90 min at room temperature. Five successive recycling experiments confirmed its excellent reusability and cycling stability. This work demonstrates that both transition-metal identity and organic-ligand incorporation can effectively tune the catalytic efficiency and structural robustness of POV clusters in Knoevenagel condensation.