Wells-Dawson-Type Polyoxometalate-Based Copper Complexes with Dual Catalytic Active Sites for Highly Selective...

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), [Cu₃^II(L)₂(P₂Mo₁₈O₆₂)(H₂O)₆]·16H₂O (Cu-P₂Mo₁₈) and [Cu₃^II(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.