Nanoscale Polyoxometalate-Based Cu-Aminotriazole Complexes with Cooperative Acid–Base Sites for the Baeyer–Villiger Oxidation Reaction

Abstract The development of efficient, stable, and recyclable heterogeneous catalysts is the key to enabling greener Baeyer–Villiger (B–V) oxidation processes. In this work, a new polyoxometalate-based metal–organic complex (POMOC) with both Lewis acidic sites and basic –NH2 groups, [Cu3(atrz)8(H2O)2(PW12O40)2] (denoted as 1-SC in its single-crystal form, atrz = 4-amino-1,2,4-triazole), was successfully synthesized. To address the limited accessibility of active sites in the nonporous POMOC material, micro- and nanosized catalysts with average particle sizes of 0.5 μm (1-0.5 μm) and 60 nm (1-60 nm) were obtained through dry ball milling and solvent-assisted ball milling of 1-SC, respectively. In the B–V oxidation reaction of cyclohexanone using O2 as the oxidant and benzaldehyde as the sacrificial agent, all prepared catalysts exhibited high catalytic activities. Notably, the nanoscale catalyst 1-60 nm demonstrated the best performance (99% conversion) due to the enhanced exposure of active sites. Furthermore, a synergistic catalytic mechanism was proposed, wherein the Cu(II) center, the [PW12O40]3– anion, and the –NH2 group cooperatively facilitated the B–V oxidation reaction via a proton-coupled electron transfer pathway. This work not only provides a facile approach for fabricating efficient and recoverable POMOC-based heterogeneous catalysts but also offers new insights into constructing green oxidation catalytic systems with acid–base multifunctional active sites.