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

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 Co^II complexes [CoL¹]–[CoL⁴] in diethyleneglycol/dimethyl ether solution were determined at different temperatures. The oxygenation constant (Ko₂) and thermodynamic parameters (ΔH°, ΔS°) were calculated. The Mn^III 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.