A Bifunctional Keggin-Type POM-Based Cu(II) Complex for Oxidizing Phenolic Compounds and Trace Detecting Phenol

Abstract

Given the broad applications of p-benzoquinones in medicine and materials, developing efficient and environmentally benign catalysts for their synthesis from phenolic compounds is of great significance. Herein, a polyoxometalate (POM)-based Cu(II)-complex (POMOC), formulated as {[Cu2(cmbpy)4(H2O)2](PMo12O39)}·(OH)3·2H2O (1, HcmbpyCl = 1-(carboxymethyl)-(4,4′-bipyridine)chlorine)), was fabricated by integrating Keggin-type POMs, copper ions, and HcmbpyCl ligands. In complex 1, the cmbpy ligands direct Cu2+ ions into zigzag one-dimensional (1D) chains [Cu2(cmbpy)2(H2O)2]n, which are extended into 2D layers via hydrogen bonding. These layers stack to encapsulate independent 1D {PMo12O39}n chains within their pores, ultimately constructing a 3D supramolecular framework. Complex 1 efficiently catalyzes the oxidation of 2,3,6-TMP to TMBQ, delivering 98% conversion and 99% selectivity in merely 7 min at 60 °C. After 5 cycles, the catalyst maintains good structural stability and recyclability. Mechanistic explorations reveal a Cu(II)-POM synergistic effect that accelerates 2,3,6-TMP oxidation through a dual-pathway mechanism. Electron paramagnetic resonance (EPR) and free radical trapping experiments confirm that hydroxyl radicals (•OH) dominate the catalytic process. Additionally, complex 1 exhibits phenol detection capability with a detection limit of 1.02 μM.