Binary Pd-polyoxometalates and isolation of a ternary Pd-V-polyoxomolybdate active species for selective aerobic oxidation of alcohols

Binary Pd-polyoxometalates [Pd(dpa)₂]₃[PW ₁₂O₄₀]₂×12 DMSO (2), [Pd(dpa) ₂]₃[PMo₁₂O₄₀]₂×12 DMSO×2 H₂O (3), and [Pd(dpa)(DMSO)₂] ₂[HPMo₁₀V₂O₄₀]×4 DMSO (4) were synthesized by reaction of [Pd(dpa)(OAc)₂]×2 H₂O (1; dpa=2,2′-dipyridylamine) with three Keggin-type polyoxometalates and fully characterized by single-crystal and powder XRD analyses, IR spectroscopy, and elemental analyses. The synthesis is facile and straightforward, and the complicated ligand-modification procedure often used in the traditional charge-transfer method can be omitted. In 2-4, Pd complexes and polyoxometalate anions are coupled through electrostatic interaction. Compound 4 is more active than the other three compounds in the selective aerobic oxidation of alcohols at ambient pressure. Interestingly, during catalytic recycling of compound 4, unprecedented ternary Pd-V-polyoxometalate [Pd(dpa)₂{VO(DMSO) ₅}₂][PMo₁₂O₄₀]₂×4 DMSO (5), which was captured and characterized by single-crystal XRD, proved to be the true active species and showed high catalytic activity for the selective aerobic oxidation of aromatic alcohols (98.1-99.8 % conversion, 91.5-99.1 % selectivity). Moreover, on the basis of control experiments and EPR and UV/Vis spectra, a plausible reaction mechanism for the oxidation of alcohols catalyzed by 5 was proposed. Binary noble metal-POM compounds: The unprecedented ternary Pd-V-POM compound (POM=polyoxometalate) [Pd(dpa)₂{VO(DMSO) ₅}₂][PMo₁₂O₄₀]₂×4 DMSO, which was isolated during the catalytic recycling of [Pd(dpa)(DMSO) ₂]₂[HPMo₁₀V₂O₄₀]×4 DMSO (dpa=2,2′-dipyridylamine) and characterized by single-crystal X-ray diffraction, proved to be the true active species and showed high catalytic activity for the aerobic oxidation of aromatic alcohols (see figure).