Ligand-Mediated Cu Valence States in Polyoxometalate-Based Complexes for Enhanced Baeyer-Villiger Oxidation

The Baeyer-Villiger (B-V) oxidation reaction was a key process in organic synthesis for the efficient and stereospecific conversion of ketones into esters or lactones. In this work, by adjusting the amounts of the Hpyca ligand, three novel polyoxometalate-based metal-organic complexes (POMOCs), [Cu₃^ICu_0.5^II(pz)₃(SiW₁₂O₄₀)] (Cu^II-Cu^I-SiW₁₂), [Cu₆^I(pyca)₂(pz)₂(SiW₁₂O₄₀)(H₂O)]·5H₂O (Cu^I-SiW₁₂), and [Cu₃^ICu_0.5^II(pz)₃(SiMo₁₂O₄₀)] (Cu^II-Cu^I-SiMo₁₂) (Hpyca = pyrazine-2-carboxylic acid; pz = pyrazine), with distinct valence states of Cu centers and compositions of organic ligands were synthesized under hydrothermal or solvent thermal conditions. Notably, Hpyca was found to decompose into pz, and the amount of Hpyca significantly influenced both the decomposition degree of Hpyca and the reduction of Cu(II). Three POMOCs can be used as heterogeneous catalysts in the B-V oxidation reaction with high catalytic activity. Notably, the varying valence states of the Cu center and different POMs in three POMOCs resulted in distinct catalytic effects. Among them, Cu^II-Cu^I-SiW₁₂ was able to catalyze the conversion of cyclohexanone to ε-caprolactone with a high conversion of 98.7% owing to the ternary synergistic catalysis between Cu(I), Cu(II), and [SiW₁₂O₄₀]^4-.