Dioxygen Affinities and Catalytic Oxidation Performance of Unsymmetrical Bis-Schiff Base Transition-Metal Complexes with Aza-Crown Pendant Groups

Abstract

CoII and MnIII complexes with aza-crown substituted, unsymmetrical bis-Schiff base ligands have been synthesised starting from monoaza-15-crown-5 or benzo-10-aza-15-crown-5. The saturated oxygen uptakes of the CoII complexes [CoL1]–[CoL4] in diethyleneglycol/dimethyl ether solution were determined at different temperatures. The oxygenation constant ( Ko2) and thermodynamic parameters (Δ H°, Δ S°) were calculated. The MnIII complexes ([MnL1Cl]–[MnL4Cl]) were employed to catalyse styrene oxidation using molecular oxygen at ambient temperature and pressure. The modulation of O2-binding capabilities and catalytic oxidation performance by the aza-crown ether pendant groups in [ML3] and [ML4] were investigated as compared with the parent complexes [ML1] and morpholino-substituted analogue [ML2]. The results indicate that the dioxygen affinities and catalytic oxidation activities of [CoL3] and [CoL4] have been much more enhanced by aza-crown pendants. Moreover, the O2-binding capabilities of [CoL3] and [CoL4] were also improved by adding alkali metal cations (Li+, Na+ and K+) to the system, and adding Na+ shows the most significant enhancement of dioxygen affinities. Likewise, [MnL3Cl] and [MnL4Cl] exhibit the best catalytic activities: the conversion of styrene to benzaldehyde are up to 41.2% and 45.8% with more than 99% selectivity.