Wells-Dawson-Type Polyoxometalate-Based Copper Complexes with Dual Catalytic Active Sites for Highly Selective Oxidation of Alkylbenzene C–H Bonds

The selective oxidation of benzylic C–H bonds to C═O bonds is an economical strategy to produce aromatic ketones and aldehydes directly. However, the activation of C–H bonds is challenging due to their high bond energy. In this work, two isostructural three-dimensional (3D) Wells-Dawson-type polyoxometalate (POM)-based metal–organic complexes (POMOCs), [Cu3II(L)2(P2Mo18O62)(H2O)6]·16H2O (Cu–P2Mo18) and [Cu3II(L)2(P2W18O62)(H2O)6]·14H2O (Cu–P2W18) (L = N,N′-(ethane-1,2-diyl)bis(pyrazine-2-carboxamide)) were synthesized through the self-assembly of Wells-Dawson-type POMs with Cu2+ and a flexible dipyrazine-dicarboxamide ligand L under hydrothermal conditions, which were structurally characterized by infrared spectroscopy (IR) and powder X-ray diffraction (PXRD). As-synthesized POMOCs can be used as heterogeneous catalysts for the selective oxidation reaction of alkylbenzene C–H bonds. Cu–P2Mo18 achieved 90% conversion and 99% selectivity for diphenylmethane oxidation within 3 h, owing to the synergistic catalysis between POM anions and Cu(II) sites. Notably, its isostructural analogue Cu–P2W18 showed slightly lower catalytic activity under identical reaction conditions (88% conversion), likely due to the lower catalytic activity of [P2W18O62]6– compared to that of [P2Mo18O62]6–. The influence of the ratio of mixed solvents on the catalytic effect was investigated. Moreover, Cu–P2Mo18 exhibited excellent structural stability, which can be used for at least five cycles without significant inactivation. This work explores the catalytic potential of Wells-Dawson-type POMOCs for selective C–H bond oxidation in alkylbenzenes, expanding their application in oxidation catalysis.