Capped Polyoxometalate-Based Metal-Organic Complex with Mixed-Valence Cu^I/Cu^II for Synergistic Catalytic Synthesis of p-Benzoquinone

The design and synthesis of efficient and environmentally friendly heterogeneous catalysts for the oxidation of phenols to benzoquinones are of great significance. Considering the highly catalytic activity of mixed-valence metals and polyoxometalates (POMs), a three-dimensional polyoxometalate-based metal-organic complex with mixed-valence Cu^I/Cu^II, [Cu₂^ICu₂^II(pyca)₄(OH)₃(PMo₁₂O₄₀Cu₂^II)(H₂O)₄]·3.5H₂O (1, Hpyca = 2-pyrazinecarboxylic acid) has been successfully synthesized under solvothermal conditions. Surprisingly, complex 1 not only combines the desired mixed-valent Cu^I/Cu^II and POM sites but also contains a rare double Cu-capped POM structure. Due to the ternary synergistic effect of Cu^I, Cu^II, and POM, complex 1 exhibits excellent catalytic activity in the oxidation reaction of 2,3,6-trimethylphenol to the corresponding p-benzoquinone. The conversion and selectivity can reach 98 and 99% within 5 min, respectively, and the turnover frequency (TOF) value is as high as 8167 h^-1, which is superior to most catalysts.