Cation-Modulated Hydrophilicity-Directed Diverse Catalysis Performance: Polyoxovanadate-Based Ni II /Co II Complexes with Multiple Catalytic Sites in Baeyer–Villiger Oxidation Reaction

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 (H 2 O)][V 2 O 6 ] ( V–Ni-MOC ) and [Co(L) 0.5 (H 2 O)][V 2 O 6 ] ( V–Co-MOC ) (L = N 2 -(pyridin-2-yl)- N 5 -(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 {V 2 O 6 } 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 O 2 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.