Cation-Modulated Hydrophilicity-Directed Diverse Catalysis Performance: Polyoxovanadate-Based Ni^II/Co^II Complexes...

The Baeyer-Villiger (B-V) oxidation is a widely utilized reaction for the conversion of ketones to esters or lactones, making the development of efficient catalysts for this reaction highly significant. In this work, two isomorphous polyoxovanadate-based metal-organic complexes, [Ni(L)_0.5(H₂O)][V₂O₆] (V-Ni-MOC) and [Co(L)_0.5(H₂O)][V₂O₆] (V-Co-MOC) (L = N²-(pyridin-2-yl)-N⁵-(pyridin-3-yl)pyrazine-2,5-dicarboxamide), were synthesized and characterized. After removing the coordinated water molecules from Ni^II/Co^II ions by vacuum drying, two activated complexes (V-Ni-MOC-a and V-Co-MOC-a) can be used as heterogeneous catalysts with multiple sites (V^V and O atoms in {V₂O₆} and Ni^II/Co^II sites) for the B-V oxidation reaction. V-Ni-MOC-a exhibited efficient catalysis for the conversion of cyclohexanone to ε-caprolactone under a O₂ atmosphere, achieving 99.3% conversion and 100% selectivity at 50 °C within 4 h. Meanwhile, isostructural V-Co-MOC-a exhibited lower catalytic activity (69.2% yield), which can be attributed to its different hydrophilicity, depending on the different central-metal electronic structure as confirmed by density functional theory.