A polyoxometalate-based dicopper complex as a heterogeneous catalyst for the selective oxidation of phenols and 2-chloroethyl ethyl sulfide under mild conditions
摘要
The development of efficient and eco-friendly heterogeneous catalysts for the selective oxidation of phenols to benzoquinones is of significafant importance in synthetic chemistry. In this work, the hydrothermal synthesis of a novel polyoxometalate (POM)-based metal-organic complex (POMOC), H2[Cu2(pbba)2 (Mo8O27)]·4H2O (Cu-pbba-Mo8, H2pbbaBr2 = 1,1′-(1,4-phenylene-bis(methylene))-bis(pyridine-3-carboxylic acid) dibromo) is reported. This structure was fully characterized by single-crystal X-ray diffraction, infrared radiation spectroscopy, powder X-ray diffraction (PXRD), thermogravimetric analysis, and X-ray photoelectron spectroscopy (XPS), revealing a unique architecture composed of [Mo8O27]6− clusters and one-dimensional (1D) chains [Cu2(pbba)2]n, in which dinuclear Cu(II) units are bridged by pbba ligands. Cu-pbba-Mo8 demonstrates outstanding catalytic performance in the oxidation of phenols and 2-chloroethyl ethyl sulfide under mild conditions. Notably, it achieves 98% conversion with 99% selectivity in the oxidation of 2,3,6-trimethylphenol (2,3,6-TMP) to 2,3,6-trimethylbenzoquinone within just 15 min at 60 °C, and it maintains high activity for other phenolic substrates. Remarkably, Cu-pbba-Mo8 also catalyzes the 99% conversion of 2-chloroethyl ethyl sulfide with 99% selectivity in only 3-min at room temperature. The superior catalytic performance of Cu-pbba-Mo8 significantly surpasses that of its individual precursors (NH4)6Mo7O24·4H2O, CuCl2·2H2O, H2pbbaBr2, as well as most reported POM-based catalysts. As a heterogeneous catalyst, Cu-pbba-Mo8 displays outstanding recyclability and stability, maintaining its high catalytic activity through five consecutive reaction cycles without significant structural degradation. Detailed mechanistic studies were conducted to elucidate the catalytic pathways.
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