Capped Polyoxometalate-Based Metal-Organic Complex with Mixed-Valence CuI/CuII for Synergistic Catalytic Synthesis of p-Benzoquinone.

Abstract

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 CuI/CuII, [Cu2ICu2II(pyca)4(OH)3(PMo12O40Cu2II)(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 CuI/CuII and POM sites but also contains a rare double Cu-capped POM structure. Due to the ternary synergistic effect of CuI, CuII, 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.