A 4,4-connected 2D POM-based complex with synergistic mixed-valence CuI/CuII for mild and efficient catalytic oxidation of phenols

Abstract

The development of efficient and sustainable catalysts for oxidizing phenols to benzoquinone is critical, as it facilitates the green production of this essential intermediate, which is widely used in synthesizing hydroquinone dyes, pharmaceuticals and advanced materials. In this work, a novel polyoxometalate (POM)-based metal–organic complex (POMOC), {[Cu₂^I(cmbpy)₂(H₂O)₄][Cu₂^II(cmbpy)₂(H₂O)₆](PW₁₂O₄₀)}(PW₁₂O₄₀)·30H₂O (1, HcmbpyCl = 1-(carboxymethyl)-(4,4′-bipyridine)chlorine), was successfully synthesized by hydrothermal reactions of [PW₁₂O₄₀]³⁻ with HcmbpyCl in the presence of a Cu²⁺ ligand. Complex 1 features two distinct cyclic metal–organic units, [Cu₂^I(cmbpy)₂(H₂O)₄] and [Cu₂^II(cmbpy)₂(H₂O)₆]. In complex 1, [Cu₂^I(cmbpy)₂(H₂O)₄] units are bridged by [PW₁₂O₄₀]³⁻ polyoxoanions, generating a one-dimensional (1D) chain structure. These 1D chains are cross-linked by [Cu₂^II(cmbpy)₂(H₂O)₆] units to form a 4,4-connected 2D layer. Furthermore, the resulting 2D layers extend into a 3D supramolecular architecture via H-bonds with free [PW₁₂O₄₀]³⁻ clusters. Complex 1 exhibited outstanding catalytic activity in the oxidation of 2,3,6-trimethylphenol (2,3,6-TMP) to 2,3,5-trimethy1-1,4-benzoquinone (TMBQ) using tert-butyl hydroperoxide (TBHP) as an oxidant achieving 98% conversion with >99% selectivity within 13 minutes at 50 °C. The reaction followed quasi-first-order kinetics with a rate constant of 0.24 min⁻¹ and a half-life of 2.89 min. The catalyst also displayed broad substrate applicability toward various phenol derivatives while retaining structural integrity and high performance over four consecutive cycles, demonstrating excellent stability and recyclability. Mechanistic studies revealed that Cu^I/Cu^II redox pairs serve as the primary active sites, with the POM playing an auxiliary role in facilitating the oxidation process. Radical trapping experiments and electron paramagnetic resonance (EPR) spectroscopy confirmed the involvement of oxygen- and carbon-centered radicals as dominant reactive species. The synergistic interaction between mixed-valence copper centers significantly enhances oxidant activation and electron transfer, leading to superior catalytic efficiency.