Feixiang Cheng

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.

Two dinuclear Ru(II) polypyridyl complexes with different photophysical and cation recognition properties.

Two dinuclear Ru(II) polypyridyl complexes functionalized with vacant coordination sites have been designed and synthesized. Their photophysical properties and interactions with various metal ions have been investigated at room temperature. The two complexes exhibit different UV/Vis absorption and emission intensities. When titrated with various metal ions, complex [{Ru(bpy)2}2(μ2-L(1))](4+) exhibits a notable fluorescence quenching in the presence of Cu(2+) in H2O-CH3CN media (1:1, v/v); its analogous complex [{Ru(bpy)2}2(μ2-L(2))](4+) exhibits no cation selectivity, the fluorescence intensity of complex [{Ru(bpy)2}2(μ2-L(2))](4+) has been enhanced by several transition metal ions due to prevention of the photo-induced electron transfer process. The fluorescence titration spectra and Benesi-Hildebrand expression reveal the formation of a 1:1 bonding mode between [{Ru(bpy)2}2(μ2-L(1))](4+) and Cu(2+) ion with the association constant of 5.50×10(4) M(-1).

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.

Three trinuclear Ru(II) complexes containing 4,5-diazafluorene and 2,2′-bipyridine: synthesis, absorption spectrum, luminescence, and redox behavior

Three heterotopic ligands L1, L2, and L3 have been prepared by the reaction of 4,4′-bis(bromomethyl)-2,2′-bipyridine with 4,5-diazafluoren-9-oxime, 9-(2-hydroxy)phenylimino-4,5-diazafluorene, and 9-(4-hydroxy)phenylimino-4,5-diazafluorene, respectively, in DMF. The three ligands consist of two 4,5-diazafluorene units and one 2,2′-bipyridine unit. Ru(II) complexes [{Ru(bpy)2}3(μ3-L1−3)](PF6)6 (bpy = 2,2′-bipyridine) were prepared by refluxing Ru(bpy)2Cl2·2H2O and the ligands in 2-methoxyethanol. The three Ru(II) complexes display metal-to-ligand charge-transfer absorption at 445–450 nm and one Ru(II)-centered oxidation at 1.32 V in CH3CN solution at room temperature. Upon excitation into the metal-to-ligand charge-transfer band, the emission intensities of [{Ru(bpy)2}3(μ3-L2)]6+ and [{Ru(bpy)2}3(μ3-L3)]6+ are almost equal to that of [{Ru(bpy)2}3(μ3-L1)]6+ in CH3CN solution at room temperature, but weaker than that of [{Ru(bpy)2}3(μ3-L1)]6+ in EtOH–MeOH (4 : 1, v/v) glassy matrix at 77 K.

Synthesis, photophysical, and electrochemical properties of six Ru(II) complexes: asymmetric ligands composed of 6-phenyl substituted 2,2′-bipyridine and 4,5-diazafluorene derivatives

Abstract Six asymmetric ligands, 6-[(4,5-diazafluoren-9-ylimino)methyoxyphenyl]-2,2′-bipyridine (L1), 4-[(4,5-diazafluoren-9-ylimino)methyoxyphenyl]-6-phenyl-2,2′-bipyridine (L2), 6-[4-(4,5-diazafluoren-9-ylimino)phenoxybenzyl]-2,2′-bipyridine (L3), 4-[4-(4,5-diazafluoren-9-ylimino)phenoxybenzyl]-6-phenyl-2,2′-bipyridine (L4), 6-[2-(4,5-diazafluoren-9-ylimino)phenoxybenzyl]-2,2′-bipyridine (L5) and 4-[2-(4,5-diazafluoren-9-ylimino)phenoxybenzyl]-6-phenyl-2,2′-bipyridine (L6), and corresponding Ru(II) complexes [(bpy)2Ru(L1–6)](PF6)2 (bpy = 2,2′-bipyridine) were synthesized. All six ligands have two kinds of nonequivalent chelating sites: one involving the 6-phenyl substituted 2,2′-bipyridine moiety, and the other involving the 4,5-diazafluorene moiety. All six Ru(II) complexes have metal-to-ligand charge transfer absorptions around 447 nm and one Ru(II)-centered oxidation around 1.35 V versus SCE in CH3CN solution at room temperature. These complexes are non-emissive in CH3CN solution at room temperature. The emission intensities of [(bpy)2Ru(L1)]2+ and [(bpy)2Ru(L2)]2+ are stronger than that of [(bpy)2Ru(L3–6)]2+ in EtOH-MeOH (4 : 1, v/v) glassy matrix at 77 K.

Preparation, Spectroscopic, and Electrochemical Properties of Two 4,5-Diazafluorene-Containing Ruthenium(II) Complexes: Bridging Ligands Composed of Two Kinds of Nonequivalent Coordinating Sites

Two tetrapodal ligands L1 and L2 containing 4,5-diazafluorene fragments have been prepared and characterized. Both ligands are composed of two kinds of nonequivalent coordinating sites. Ligand L1 consists of the 4-(4,5-diazafluoren-9-ylimino)phenoxy and 4,5-diazafluoren-9-yliminoxy moieties, ligand L2 involves the 2-(4,5-diazafluoren-9-ylimino)phenoxy and 4,5-diazafluoren-9-yliminoxy moieties. The Ru(II) complexes [(bpy)8Ru4(L1)](PF6)8 and [(bpy)8Ru4(L2)](PF6)8 (bpy = 2,2′-bipyridine) have been synthesized by refluxing Ru(bpy)2Cl2·2H2O and each ligand in 2-methoxyethanol. Both complexes exhibit metal-to-ligand charge transfer absorptions at around 443 nm, and emission at around 577 nm. Electrochemical studies of both complexes display one Ru(II)-centered oxidation at around 1.32 V and four ligand-centered reductions.

Preparation, Characterization, and Proton-induced Fluorescence Switching of Two Ru(II) Polypyridyl Complexes containing Different N-Heterocyclic Groups

Two ligands H2L1 and HL2 containing imidazole rings and piperazine or morpholine units have been prepared by the reaction of 1,10-phenanthroline-5,6-dione with 4,4´- (1,4-piperazinediyl)bisbenzaldehyde and 4-morpholinobenzaldehyde, respectively. The Ru(II) polypyridyl complexes [(bpy)2Ru(H2L1)Ru(bpy)2]4+ and [(bpy)2Ru(HL2)]2+ have been synthesized by the reaction of Ru(bpy)2Cl2·2H2O with ligands H2L1 and HL2, respectively. The pH effects on the UV=Vis absorption and fluorescence spectra of both complexes have been studied. The ground-state and excited-state ionization constants of the acid-base equilibria have been calculated according to the absorbance and emission data. The photophysical properties of both complexes are strongly dependent on the solution pH. They act as proton-induced “off-on-off” fluorescence pH sensors through protonation and deprotonation of the imidazole, piperazine or morpholine groups, with a maximum on-off ratio of 6 in buffer solution at room temperature. Graphical Abstract Preparation, Characterization, and Proton-induced Fluorescence Switching of Two Ru(II) Polypyridyl Complexes containing Different N-Heterocyclic Groups

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