Synergistic crystalline catalysts assembled with Wells–Dawson-type polyoxometalates and heterovalent metal complexes 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 the Wells–Dawson-type [P₂W₁₈O₆₂]⁶⁻ cluster and heterovalent Cu complexes were synthesized for benzylic C–H oxidation, with the formulae [Cu^I₃Cu^II₃(μ₂-OH)₆(H₂O)_1.5(DTAB)₃][H₃P₂W₁₈O₆₂]·18H₂O (1) and [Cu^I₂(DTAB)₂]{[Cu^I₂(μ₃-Cl)][Cu^I₄(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 cationic layers composed of heterovalent [Cu^I₃Cu^II₃(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 {Cu^I₂}, trinuclear {Cu^I₃Cl}, and tetranuclear {Cu^I₄Cl} units), in which the [P₂W₁₈O₆₂]⁶⁻ clusters are encapsulated. The heterovalent copper species and [P₂W₁₈O₆₂]⁶⁻ clusters within the two compounds make them dual-site synergistic catalysts and the roles of each component in the selective oxidation of benzyl C–H bonds were explored. Using the oxidation of diphenylmethane (DPM) as a model reaction, the two synergistic catalysts displayed ca. 95% and 92% DPM conversion with >98% benzophenone (BP) selectivity within 8 h. Mechanism investigations revealed that both copper sites and polyanionic clusters as active centers corporately promoted the adsorption and activation of the 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 catalyst 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.