Synthesis, Oxygenation and Catalytic Epoxidation Performance of Salen and Salophen Transition-Metal Complexes with Aza-Crown or Morpholino Pendants

Abstract

CoII and MnIII complexes with aza-crown or morpholino substituted Salen and Salophen ligands were synthesized starting from benzo-10-aza-15-crown-5 or morpholine. The saturated oxygen uptake of the CoII complexes CoL1–CoL4 in MeOCH2CH2OMe solution was determined at different temperatures. The equilibrium constant (KO2) and thermodynamic parameters (ΔH0, ΔS0) for oxygenation were calculated. Meanwhile, the corresponding MnIII complexes, MnL1Cl–MnL4Cl, were employed as models of mimic mono-oxygenase to catalyze PhCH=CH2 epoxidation at ambient temperature and pressures. The modulation of O2-binding capabilities and catalytic oxidation performance by these pendant substituents in the complexes were investigated and compared with the parent complexes ML5(Msalen) and ML6(Msalophen). The results indicate that the dioxygen affinities and catalytic oxidation activities of these complexes have been much more enhanced by aza-crown pendants than by morpholino pendants. Moreover, the O2-binding capabilities of bis(aza-crown ether) CoII complexes, CoL1 and CoL2, would also be improved by adding alkali metal (Li+, Na+ and K+) cations to the system. Adding K+ shows the most significant enhancement of dioxygen affinity through its forming sandwich-type complexes with two aza-crown ethers of CoL1 and CoL2. Likewise, the bis(aza-crown ether) MnIII complexes, MnL1 and MnL2, exhibit the best catalytic activity: the conversions of PhCH=CH2 attain 76.6, 79.5% respectively.