Synergistic Crystalline Catalysts Assembled with Wells-Dawson-type Polyoxometalate and Heterovalent Metal-Complex for Efficient Benzylic C-H Bond Oxidation

Abstract

Integrating polyoxometalate and catalytically active metal centers with multiple chemical states provides a promising approach for the design of efficient catalysts for benzyl C-H bond oxidation. Herein, two synergistic crystalline catalysts assembled with Wells-Dawson-type [P2W18O62]6- cluster and heterovalent Cu-complex were synthesized for benzylic C−H oxidation, with the formula of [Cu3ICu3II(μ2-OH)6(H2O)1.5(DTAB)3][H3P2W18O62]·18H2O (1), and [CuI2(DTAB)2]{[CuI2(μ3-Cl)][CuI4(μ3-Cl)](DTAB)4]}[P2W18O62]·9H2O (2) (DTAB = 1,4-di[4H-1,2,4-triazol-4-yl]-benzene). Compound 1 possesses 2-D copper-ligand cation layers composed of heterovalent [CuI3CuII3(μ2-OH)6]3+ chains, in which the protonated [H3P2W18O62]3- polyanions intercalate between them. Compound 2 has a sandwich-like 2-D lamellar structure consisting of three copper-organic units (binuclear {CuI2}, trinuclear {CuI3Cl}, and tetranuclear {CuI4Cl} units), in which the [P2W18O62]6- clusters are encapsulated. The heterovalent copper species and [P2W18O62]6- clusters within two compounds render them dual-site synergistic catalysts for exploring the roles of each component in the selective oxidation of benzyl C-H bond. Using the oxidation of diphenylmethane (DPM) as a model reaction, two synergistic catalysts displayed ca. 95% and 92% DPM conversion with >98% benzophenone (BP) selectivity within 8h. Mechanism investigations revealed that both copper sites and polyanionic clusters as active centers corporately promoted the absorption and activation of t-BuOOH oxidant in the radical-mediated DPM oxidation reaction via coordination and hydrogen bonding interactions to generate active t-BuO• and •OH radicals, accelerating the oxidation of DPM and the formation of BP. Additionally, this kind of synergistic catalysts showcased excellent substrate compatibility and recoverability, as well as robust structural stability. This work affords a promising approach for designing efficient POM-based crystalline catalysts for benzyl C-H bond oxidation.