Dioxygen affinities and catalytic epoxidation performance of mono-Schiff base transition-metal complexes with aza-crown or morpholino pendants

Abstract

Mono-Schiff base Co^II complexes, CoL ¹₂ –CoL ⁶₂ with aza-crown or morpholino pendants were synthesized. The O₂ uptake of these complexes in MeOCH₂CH₂OMe solution was determined at different temperatures, and the equilibrium constants (KO₂) and thermodynamic parameters (ΔH ⁰, ΔS ⁰) for oxygenation were calculated. The corresponding Mn^III complexes, MnL ¹₂ Cl–MnL ⁶₂ Cl, were employed to catalyze epoxidation of styrene at ambient temperature and pressures. Crown ether effects on the modulation of O₂-binding capability and the catalytic oxidation of styrene are discussed. The results indicate that the dioxygen affinities of the Co^II complexes are much more enhanced by aza-crown pendants than that by morpholino pendants, and the O₂-binding capabilities of CoL ¹₂ –CoL ³₂ with aza-crown pendants can also be enhanced by adding alkali metal (Li⁺, Na⁺ and K⁺) cations; similarly, the catalytic activities of the Mn^III complexes with aza-crown pendants, MnL ¹₂ Cl–MnL ³₂ Cl, are higher than those of the Mn^III complexes with morpholino pendants, MnL ⁴₂ Cl–MnL ⁶₂ Cl.