Highly Efficient Supramolecular Catalysts Assembled by Dawson-Type POMs and Metal–Organic Complexes for the Synergistic Catalytic Synthesis of p-Benzoquinones

Selective oxidation of alkyl-substituted phenols offers an ideal pathway to synthesize p-benzoquinones (p-BQs), but it is known to be inefficient because of the parallel competing reactions. Aiming at the design and synthesis of more powerful heterogeneous catalysts for highly efficient synthesizing functionalized p-BQs, herein, we report two crystalline catalysts with supramolecular networks based on Dawson-type polyoxometalates (POMs) and metal–organic subunits, H2K(H2O)3[Cu3(H2O)8(ptz)4][Cu1.5(H2O)3(ptz)2(ina)][P2W18O62]2·ina·21H2O (1) and [Cu4.5(ptz)4(OH)3(H2O)4][P2W18O62]·10H2O (2), where ptz = 5-(4-pyridyl)-1H-tetrazole and ina = isonicotinic acid. Structural analysis shows that 1 is formed by the supramolecular contact between one-dimensional POM inorganic chains and two kinds of trinuclear Cu-organic clusters, while 2 is formed by supramolecular stacking of ladder-like one-dimensional chains that are composed of ribbonlike nonanuclear Cu-organic clusters and POM anions. Both supramolecular catalysts exhibit high catalytic activity in H2O2-based oxidation of phenols/aromatics to p-BQs. Strikingly, in the synthesis of trimethyl-p-benzoquinone (TMBQ, an intermediate of vitamin E) by oxidizing 2,3,6-trimethylphenol, the conversion, selectivity, and oxidant efficiency can reach >99, 97.9, and 85.0% for 1, as well as 95.4, >99, and 79.8% for 2 within 5 min, respectively. Also, the catalytic activities of 1 and 2 presented by the turnover frequency are 12 000 (the highest thus far) and 7632 h–1, respectively. Detailed structure–activity analysis revealed that the higher performance of 1 is attributed to the more active-site accessibility due to the supramolecular interactions between POM and Cu organic units in the framework. Moreover, the observed yield of TMBQ has not decreased and the skeleton of catalysts has not changed in >10 cycles. The reaction mechanism was also systematically studied, and the parallel dual center, namely, Cu-site and POM, -driven reaction processes (radical and oxygen transfer) were illuminated.