pH-induced octamolybdate-based metal–organic complexes with different coordinated metal centers for catalytic epoxidation reaction of alkenes

The epoxidation of olefins is considered an essential reaction in chemistry, petrochemistry, and materials science. Developing highly efficient heterogeneous catalysts for the epoxidation reaction of olefins is of great significance. In this work, Co²⁺ was selected as the central metal to assemble with ammonium molybdate and a N,N’-bis(4-pyrimidinecarboxaido)-1,4-butane (bpyb) ligand under hydrothermal conditions, and two new polyoxometalate (POM)-based cobalt complexes, [Co(bpyb)(β-Mo₈O₂₆)_0.5(H₂O)]·H₂O (Co-POMOC-1) and [Co(bpyb)(β-Mo₈O₂₆)_0.5(H₂O)]·2H₂O (Co-POMOC-2) were synthesized by adjusting the pH of the solutions, which were characterized by single crystal X-ray diffraction, XPS, PXRD and IR. Two Co-POMOCs exhibited different coordination modes of Co(II) atoms, endowing them with different accessibilities of Co(II) sites and different catalytic activities. Co-POMOC-1 exhibits excellent heterogenous catalytic activity in the epoxidation reaction of olefins, achieving 99 % yield and 100 % selectivity of 1,2-epoxycyclooctane under O₂ atmosphere at 55°C within 22 min. Furthermore, a new [Zn(bpyb)(β-Mo₈O₂₆)_0.5(H₂O)]·H₂O (Zn-POMOC), which is isomorphous to Co-POMOC-1, was synthesized to explore the catalytic mechanism by comparison. The synergistic catalysis and catalysis mechanism of Co(II) and {β-Mo₈} sites in Co-POMOCs for the olefin epoxidation reaction were investigated systematically.