Highly Efficient Supramolecular Catalysts Assembled by Dawson-Type POMs and Metal-Organic Complexes for the SynergisticCatalytic Synthesis ofp-Benzoquinones

Selective oxidation of alkyl-substituted phenolsoffers an ideal pathway to synthesizep-benzoquinones (p-BQs),but it is known to be inefficient because of the parallel competingreactions. Aiming at the design and synthesis of more powerfulheterogeneous catalysts for highly efficient synthesizing function-alizedp-BQs, herein, we report two crystalline catalysts withsupramolecular 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]2middotinamiddot21H2O(1)and[Cu4.5(ptz)4(OH)3(H2O)4][P2W18O62]middot10H2O(2), where ptz = 5-(4-pyridyl)-1H-tetrazole and ina =isonicotinic acid. Structural analysis shows that1is formed by thesupramolecular contact between one-dimensional POM inorganicchains and two kinds of trinuclear Cu-organic clusters, while2isformed by supramolecular stacking of ladder-like one-dimensional chains that are composed of ribbonlike nonanuclear Cu-organicclusters and POM anions. Both supramolecular catalysts exhibit high catalytic activity in H2O2-based oxidation of phenols/aromaticstop-BQs. Strikingly, in the synthesis of trimethyl-p-benzoquinone (TMBQ, an intermediate of vitaminE) by oxidizing 2,3,6-trimethylphenol, the conversion, selectivity, and oxidant efficiency can reach >99, 97.9, and 85.0% for1, as well as 95.4, >99, and79.8% for2within 5 min, respectively. Also, the catalytic activities of1and2presented by the turnover frequency are 12 000 (thehighest thus far) and 7632 h-1, respectively. Detailed structure-activity analysis revealed that the higher performance of1isattributed to the more active-site accessibility due to the supramolecular interactions between POM and Cu organic units in theframework. 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 reactionprocesses (radical and oxygen transfer) were illuminated.

Automated summary

This study reports two crystalline catalysts based on Dawson-type polyoxometalates (POMs) and metal-organic subunits, which exhibit high catalytic activity in the H2O2-based oxidation of phenols/aromatics to p-BQs. The catalysts show high conversion, selectivity, and oxidant efficiency, with catalyst 1 displaying the highest performance. The structure-activity analysis reveals that the active-site accessibility due to the supramolecular interactions between POM and Cu organic units contributes to the superior performance of catalyst 1.