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

Selective oxidation of substituted phenols is an ideal method for preparing functionalized p-benzoquinones (p-BQs), which are served as versatile raw materials for the synthesis of a variety of biologically active compounds. Herein, two new polyoxometalate-based supramolecular porous frameworks K3(H2O)4[Cu(tza)2(H2O)]2[Cu(Htza)2(H2O)2][BW12O40]·6H2O (1) and H3K3(H2O)3[Cu(Htza)2(H2O)]3[SiW12O44]·14H2O (2) (Htza = tetrazol-1-ylacetic acid) are synthesized and structurally characterized by elemental analysis, infrared spectroscopy, thermal analysis, UV−vis diffuse reflectance spectrum, and X-ray single crystal and powder diffraction. Single-crystal X-ray diffraction analysis indicates that both compounds possess unique petal-like twelve-nuclearity Cu-organic units composed of triangle 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. While 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 °C) with TBHP as oxidant, especially in the oxidation of 2,3,6-trimethylphenol to 2,3,5-trimethyl-p-benzoquinone (TMBQ, vitamin E key intermediate). 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 is observed after five cycles, which indicates the excellent sustainability of both compounds. Research on 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; (ii) the polyoxoanion acts as the active center of the heterolytic oxygen atom transfer pathway.