Four μ4-oxo-bridged copper(ii) complexes: magnetic properties and catalytic applications in liquid phase partial oxidation reactions

Abstract

Four copper(II) complexes, [Cu₄(O)(Lⁿ)₂(CH₃COO)₄] with N₂O-donor Schiff-base ligands, where HL¹ = 4-methyl-2,6-bis(cyclohexylmethyliminomethyl)phenol for complex 1, HL² = 4-methyl-2,6-bis(phenylmethyliminomethyl)phenol for complex 2·CH₃CN, HL³ = 4-methyl-2,6-bis(((3-tri-fluoromethyl)phenyl)methyliminomethyl)phenol for complex 3, HL⁴ = 4-methyl-2,6-bis(((4-tri-fluoromethyl)phenyl)methyliminomethyl)phenol for complex 4, were synthesized and characterized by elemental analysis, FT-IR, UV-vis spectroscopy and finally by single crystal X-ray diffraction study. X-Ray analysis reveals that all of these are μ₄-oxo-bridged tetrameric copper(II) complexes. Four copper atoms arrange themselves around an oxygen atom tetrahedrally. Magnetic susceptibility measurements show the existence of very strong antiferromagnetic coupling among these ions (J = −210.1 to −271.3 cm⁻¹), mediated by the oxygen atoms. Catalysis of the epoxidation of cyclohexene, styrene, α-methylstyrene and trans-stilbene by these complexes has been carried out homogeneously as well as heterogeneously by immobilizing the metal complexes over 2D-hexagonal mesoporous silica. The results obtained in both the catalytic conditions show that the olefins are converted to the respective epoxides in good yield together with high selectivity.