Polyoxometalate-embedded copper-organic network as dual-site synergetic catalyst for the selective oxidation of benzylic C−H bond

Abstract

Selective activation and oxidization of inert C−H bonds into value-added chemicals affords attractively economic and ecological benefits, as well as central challenge in modern synthetic chemistry. Herein, by integrating the redox polyoxometalate (POM) centers and multinuclear copper sites in one system, four compounds were constructed with formulas of [Cu2(μ2-Cl)(H2O)2L3][PM12O40]·4H2O (M = Mo for 1, W for 2), and [Cu4(μ2-OH)4L2] [H3PM11CuO40]·12H2O (M = Mo for 3, W for 4, L = 1,4-di[4H-1,2,4-triazol-4-yl]-benzene). Both compounds 1–2 exhibited 3-D POM-embedded supramolecular networks constructed by discrete [PM12O40]3− clusters and binuclear {Cu2(μ2-Cl)(N–N)2} unit-based metal-organic sheets. While compounds 3–4 displayed 3-D POM-supported supramolecular networks composed of monosubstituted Keggin-type anions [H3PM11CuO40]4− and {Cu2(μ2-OH)(N–N)2} chain-based metal-organic layers. Four crystalline structures with distinct dual-active site distribution showed significant peroxidase-like activities owing to the collaborative catalysis of multinuclear copper units and polyanionic clusters, which can smoothly catalyze the C−H bond oxidation of diphenylmethane to benzophenone with ca. 97 % conversion and >98 % selectivity within 24 h. Notably, the monosubstituted [H3PM11CuO40]4−-based compounds 3−4 showed higher catalytic activities than [PM12O40]3−-based compounds 1−2 (88 % conversion and >97 % selectivity within 12 h). This kind of crystalline structures also showed good recoverability. The synergetic reaction mechanism was also explored. This work provides a new strategy to design robust heterogeneous POM-based synergetic catalysts for C−H bond functionalization.