Rapid Copper(I)-Catalyzed Synthesis of p-Benzoquinone within Minutes Using a Keggin-Type POM Dimer-Based Metal-Organic Complex

Abstract

The advancement of highly efficient and environmentally friendly catalysts is crucial for the selective oxidation of phenol to benzoquinone. In this work, we successfully synthesized a new polyoxometalate (POM)-based metal-organic complex (POMOC):Single-crystal X-ray diffraction analysis reveals that complex 1 is a novel (6,6)-connected three-dimensional (3D) framework constructed from Keggin-type POM dimers {(PW 12 O 39.5 ) 2 } and linearly dicoordinated complexes{Cu Ⅰ (HBpyen) 2 } interconnected via hydrogen bonds. Complex 1 exhibited outstanding catalytic performance in the oxidation of 2,3,6-trimethylphenol (2,3,6-TMP) to 2,3,5-trimethyl-1,4-benzoquinone (TMBQ) using H 2 O 2 as the oxidant. Remarkably, the reaction achieved 99% conversion and 97% selectivity in only 4 min, corresponding to a high turnover frequency (TOF) of 4125 h -1 , highlighting the highly efficient catalytic role of the Cu(I) center. Kinetic studies indicate that the reaction follows pseudo-first-order kinetics with an apparent activation energy of 119.66 J•mol -1 . Furthermore, complex 1 showed broad applicability toward various phenol derivatives with consistently high performance. Notably, complex 1 maintained its structural integrity and catalytic efficiency after five consecutive cycles, demonstrating excellent recyclability and stability. The catalytic mechanism was investigated in detail to elucidate the critical role of Cu(I) in driving the rapid catalytic oxidation process. Radical trapping experiments confirmed that Cu(I) is essential for the reaction kinetics, a finding further supported by electron paramagnetic resonance (EPR) spectroscopy. The results indicate that oxygen-and carbon-centered radicals serve as the primary active species in the catalytic oxidation of phenols.