Longhai Chen

Asymmetric epoxidation of olefins using novel chiral dinuclear Mn(III)‐Salen complexes with inherent phase‐transfer capability in ionic liquids

Two new chiral dinuclear Mn(III)-Salen complexes with inherent phase-transfer capability have been synthesized, which serve as catalysts in the asymmetric epoxidation of nonfunctionalized alkenes. Experimental results show these complexes are effective catalysts for the asymmetric epoxidation of some cyclic alkenes and the catalysts have certain inherent phase-transfer capability during the epoxidation because of their weak water solubility. In general, good enantioselectivity and acceptable yields were achieved when NaClO was used as oxidant under three different reaction systems. Among these alkenes, the catalyst 6a gave the highest ee (94%) for 6-chloro-2,2-dimethylchromene in the presence of ionic liquid 2. Additionally, the recovery and recycling of one dimeric Mn(III)-Salen complex were tested to investigate atom efficiency of the catalyst in different reaction systems on the alkenes epoxidations. The catalyst 6a could be recovered and recycled for six times without losing activity and selectivity.

Asymmetric Epoxidation of Chromenes Using Manganese(III) Complexes with Novel Chiral Salen-like Schiff Base Ligands

Three novel chiral salen-like schiff base ligands and their Mn(III) complexes containing different amino acid unit have been synthesized and characterized. Asymmetric epoxidation reactions show these complexes are effective catalysts for the chromenes with buffer NaOCl as terminal oxidant and pyridine N-oxide as co-catalyst in the presence of ionic liquid. Good-to -excellent enantioselectivity and acceptable yields can be obtained under optimum reaction conditions. Catalyst 4c gives the highest ee (95%) for 6-chloro-2,2-dimethylchromene among these catalytic performances. Furthermore, compared the enantioselectivity of catalyst 4c with the other two catalysts 4a and 4b, the positive experimental results suggest that the steric effect of the ligands plays an important role in the asymmetric catalysis.Graphical AbstractAsymmetric epoxidation of chromenes with Mn (III) complex containing amino acid moiety

Synthesis, photophysical, and electrochemical properties of a new family of trinuclear Ru(II) polypyridine complexes

A series of four tripodal ligands L1–4 were prepared by the reaction of 9-(2-hydroxy)phenylimino-4,5-diazafluorene with 1,3,5-tris(bromomethyl)benzene, 1,3,5-tris(bromomethyl)-2,4,6-trimethylbenzene, tris(2-chloroethyl)amine hydrochloride, and pentaerythrityl tetratosylate, respectively, in DMF solution under nitrogen. For each ligand, Ru(II) complexes were prepared by refluxing Ru(bipy)2Cl2 · 2H2O and ligand in 2-methoxyethanol. Photophysical behaviors of these Ru(II) complexes have been investigated by UV-Vis absorption and luminescence spectrometry. They display metal-to-ligand charge transfer absorption at 442 nm in MeCN solution at room temperature and emission at 574 nm in EtOH–MeOH (4 : 1, v/v) glassy matrix at 77 K. Electrochemical studies of the Ru(II) complexes show one Ru(II)-centered oxidation at 1.33 V and three ligand-centered reductions.

Preparation, Photophysical, Electrochemical, and Ion-Binding Properties of Ru(II) Polypyridyl Complexes Containing a Crown Ether Unit

Two polypyridyl ligands, 4`-(4,5-diazafluoren-9-ylimino)benzo-15-crown-5 (L1) and 4` -(4,5-diazafluoren- 9-ylimino)benzo-12-crown-4 (L2), and their Ru(II) complexes [(bpy)2RuL1](PF6)2 and [(bpy)2RuL2](PF6)2, respectively, have been synthesized and characterized. The two complexes display metal-to-ligand charge transfer absorptions at around 444 nm in CH3CN solution at r. t. and emission at around 573 nm in an EtOH-MeOH (4 : 1, v/v) glassy matrix at 77 K. Electrochemical studies of the complexes show one Ru(II)-centered oxidation at around 1.33 V and three ligandcentered reductions. The binding ability of the complexes with Na+ and Li+ has been investigated by UV/Vis absorption and emission spectroscopy and electrochemical titrations. Addition of Na+ and Li+ to solutions of the complexes results in a progressive quenching of the emission, a hyperchromic effect of the UV/Vis absorption, and a progressive cathodal shift of the Ru(II)-centered E1/2 potential. The stability constants for the stoichiometric 1 : 1 ratio of the complexes and the cations have been obtained by UV/Vis absorption titration. Graphical Abstract Preparation, Photophysical, Electrochemical, and Ion-Binding Properties of Ru(II) Polypyridyl Complexes Containing a Crown Ether Unit