Polyoxometalate-based supramolecular porous frameworks with dual-active centers towards highly efficient synthesis of functionalized p-benzoquinones

Selective oxidation of substituted phenols is an ideal method for preparing functionalized p-benzoquinones (p-BQs), which serve as versatile raw materials for the synthesis of a variety of biologically active compounds. Herein, two new polyoxometalate-based supramolecular porous frameworks, K-3(H2O)(4)[Cu(tza)(2)(H2O)](2)[Cu(Htza)(2)(H2O)(2)][BW12O40]center dot 6H(2)O (1) and H3K3(H2O)(3)[Cu(Htza)(2)(H2O)](3)[SiW12O44]center dot 14H(2)O (2) (Htza = tetrazol-1-ylacetic acid), were synthesized and structurally characterized by elemental analysis, infrared spectroscopy, thermal analysis, UV-vis diffuse reflectance spectroscopy, and single-crystal X-ray and powder diffraction. The single-crystal X-ray diffraction analysis indicates that both compounds possess unique petal-like twelve-nucleated Cu-organic units composed of triangular and hexagonal metal-organic loops. In 1, the Cu-organic units are isolated and [BW12O40](5-) polyoxoanions are sandwiched between staggered adjacent triangular channels in the structure. However in 2, the Cu-organic units extend into a two-dimensional layered structure, and the [SiW12O44](12-) polyoxoanions occupy the larger hexagonal channels in the stacked structure. Both compounds as heterogeneous catalysts can catalyze the selective oxidation of substituted phenols to high value-added p-BQs under mild conditions (60 degrees C) with TBHP as the oxidant, particularly in the oxidation of 2,3,6-trimethylphenol to 2,3,5-trimethyl-p-benzoquinone (TMBQ, key intermediate in vitamin E production). Within 8-10 min, the yield of TMBQ is close to 100%, and oxidant utilization efficiency is up to 94.2% for 1 and 90.9% for 2. The turnover frequencies of 1 and 2 are as high as 5000 and 4000 h(-1), respectively. No obvious decrease in the yield of TMBQ was observed after five cycles, which indicates the excellent sustainability of both compounds. Our study of the catalytic mechanism suggests that there is a two-site synergetic effect: (i) the copper ion acts as the catalytic site of the homolytic radical pathway; and (ii) the polyoxoanion acts as the active center of the heterolytic oxygen atom transfer pathway.

Automated summary

Two new polyoxometalate-based supramolecular porous frameworks were synthesized and characterized, showing high efficiency as heterogeneous catalysts for the selective oxidation of substituted phenols to prepare value-added p-BQs. Both compounds exhibited excellent catalytic performance in the oxidation reaction under mild conditions, with high turnover frequencies.