Dual-Site Polyoxometalate-Based Cobalt Complexes for Catalytic Epoxidation Reaction of Olefins

The epoxidation reaction of olefins represents a critically important oxidative process in industrial and synthetic chemistry. The development of novel heterogeneous catalysts for efficiently catalyzing olefin epoxidation is of great practical significance. In this study, three new isostructural polyoxometalate-based metal-organic complexes (POMOCs), H[Co(bpyb)₂(PMo₁₂O₄₀)(H₂O)₂]·H₂O (Co-PMo₁₂), H[Co(bpyb)₂(PW₁₂O₄₀)(H₂O)₂]·H₂O (Co-PW₁₂), and H[Zn(bpyb)₂(PMo₁₂O₄₀)(H₂O)₂] (Zn-PMo₁₂), were synthesized by reacting Co^II or Zn^II as the metal centers with H₃PMo₁₂O₄₀ or H₃PW₁₂O₄₀ and the ligand N,N'-bis(4-pyrimidinecarboxaido)-1,4-butane (bpyb). The three POMOCs were characterized by single-crystal X-ray diffraction, Fourier transform infrared spectroscopy, powder X-ray diffraction, and X-ray photoelectron spectroscopy. In the epoxidation reaction of cis-cyclooctene, both Co-POMOCs exhibited high catalytic efficiency. Under an O₂ atmosphere at 55 °C, they achieved 98% conversion of cis-cyclooctene within 10 min (Co-PMo₁₂) and 14 min (Co-PW₁₂), respectively, with 100% selectivity for 1,2-epoxycyclooctane in both cases. A synergistic catalysis between the Co^II and POM sites in Co-PMo₁₂ for the epoxidation of olefins was confirmed through the catalytic activity comparison with those of its components and Zn-PMo₁₂.