Dioxygen affinities and catalytic oxidation performance of unsymmetrical bis-Schiff base transition-metal complexes with aza-crown pendant groups

摘要

Co~(II) and Mn~(III) 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 [CoL~1]–[CoL~4] in diethyleneglycol/dimethyl ether solution were determined at different temperatures. The oxygenation constant (Ko_2) and thermodynamic parameters (ΔH°, ΔS°) were calculated. The Mn~(III) complexes ([MnL~1Cl]–[MnL~4Cl]) were employed to catalyse styrene oxidation using molecular oxygen at ambient temperature and pressure. The modulation of O_2-binding capabilities and catalytic oxidation performance by the aza-crown ether pendant groups in [ML~3] and [ML~4] were investigated as compared with the parent complexes [ML~1] and morpholino-substituted analogue [ML~2]. The results indicate that the dioxygen affinities and catalytic oxidation activities of [CoL~3] and [CoL~4] have been much more enhanced by aza-crown pendants. Moreover, the O_2-binding capabilities of [CoL~3] and [CoL~4] 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, [MnL~3Cl] and [MnL~4Cl] exhibit the best catalytic activities:the conversion of styrene to benzaldehyde are up to 41.2% and 45.8% with more than 99% selectivity.