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Ligands L1 (ligand + l1)
Selected AbstractsTriplet MLCT Photosensitization of the Ring-Closing Reaction of Diarylethenes by Design and Synthesis of a Photochromic Rhenium(I) Complex of a Diarylethene-Containing 1,10-Phenanthroline LigandCHEMISTRY - A EUROPEAN JOURNAL, Issue 22 2006Chi-Chiu Ko Dr. Abstract Synthesis of the diarylethene-containing ligand L1 based on Suzuki cross-coupling reaction between thienyl boronic acid and the dibromophenanthroline ligand is reported. On coordination to the rhenium(I) tricarbonyl complex system, the photochromism of L1 could be photosensitized and consequently extended from intraligand excitation at ,,340 nm in the free ligand to metal-to-ligand charge-transfer (MLCT) excitation at ,,480 nm in the complex. The photochromic reactions were studied by 1H NMR, UV/Vis, and steady-state emission spectroscopy. Photosensitization was further probed by ultrafast transient absorption and time-resolved emission spectroscopy. The results provided direct evidence that the formation of the closed form by the MLCT-sensitized photochromic process was derived from the 3MLCT excited state. This supports the photosensitization mechanism, which involves an intramolecular energy-transfer process from the 3MLCT to the 3IL(L1) state that initiated the ring-closure reaction. The photophysical and electrochemical properties of the complex were also investigated. [source] Monomeric Cu(II) Complex Containing Chiral Phase-transfer Catalyst as Ligand and Its Asymmetrically Catalytic ReactionCHINESE JOURNAL OF CHEMISTRY, Issue 2 2008Zhi-Rong QU Abstract The thermal treatment of CuCl2 with N -(4,-vinylbenzyl)cinchonidinium chloride (L1) afforded a monomeric discrete homochiral copper(II) complex N -4,-(vinylbenzyl)cinchonidinium trichlorocoprate(II) (1). Their applications to the enantioselectively catalytic alkylation reaction of N -(diphenylmethylidene)glycine tert -butyl ester (3) show that the higher ee value observed in catalyst 1 than that in the corresponding free ligand L1 is probably due to the rigidity enhancement after the coordination of N atom of quinoline ring to the copper ion. [source] Toward an Allosteric Metallated ContainerEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 2 2009Helga Szelke Abstract Polytopic ligands L1 and L2 in which three 2,2,-bipyridine units are linked to a central tris(pyrid-2-yl)amine (L1) or tris(pyrid-2-yl)methanol (L2) moiety by alkyl spacers were prepared by multistep organic syntheses. The parent tris(pyrid-2-yl)-type ligands were shown to be modest-to-good chelators for Zn2+ and Cu2+ ions in solution, and bi- and tridentate N-coordination was confirmed by crystal structures of CuII and RuII complexes, respectively. FeII and RuII smoothly form stable, cage-like 1:1 complexes with L1 and L2, in which the metal ion is coordinated to the tris(bpy) site of the ligands. The vacant tris(pyrid-2-yl) site of these complexes is, however, a poor donor site for Zn2+ and Cu2+ ions. In addition, FeII modulates the coordination behaviour of the tris(pyrid-2-yl) site toward Zn2+: Whereas tris(5-methylpyrid-2-yl)amine forms a 2:1 complex with Zn2+ in CH2Cl2, [Fe(L1)]2+ forms a 1:1 Zn complex. Spectrophotometric titrations suggest that [Fe(L2)]2+ forms a polynuclear Zn2+ complex in CH2Cl2, possibly involving bridging coordination of the alcohol OH group, which contrasts the smooth formation of a 2:1 complex of the parent tris(pyrid-2-yl)-type ligand with Zn. FeII might therefore be considered as an allosteric effector, which modulates the metal binding properties of the second tris(pyrid-2-yl) site of L1 and L2. Contrary to expectation, Zn2+ and Cu2+ appear to associate weakly with donor atoms directed toward the exterior of the cage-like complexes [Fe(Ln)]2+ and [Ru(L1)]2+, rather than locating in the interior of the container by tripodal coordination to the tris(pyrid-2-yl) site.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Sensitized Lanthanide-Ion Luminescence with Aryl-Substituted N -(2-Nitrophenyl)acetamide-Derived ChromophoresHELVETICA CHIMICA ACTA, Issue 11 2009Michael Andrews Abstract The syntheses of the two tetraazamacrocyclic ligands L1 and L2 bearing a [(methoxy-2-nitrophenyl)amino]carbonyl chromophore, i.e., an N -(methoxy-2-nitrophenyl)acetamide moiety, together with their corresponding lanthanide-ion complexes are described. A combined spectroscopic (UV/VIS, 1H-NMR), structural (X-ray), and theoretical (DFT) investigation revealed that the absorption properties of the chromophores were dictated by the extent of electronic delocalisation, which in turn was determined by the position of the MeO substituent at the aromatic ring. X-Ray crystallographic studies showed that when attached to the macrocycle, both isomeric forms of the N -(methoxy-2-nitrophenyl)acetamide unit can participate in coordination, via the CO, to an encapsulated potassium cation. Luminescence measurements confirmed that such a binding mode also exists in solution for the corresponding lanthanide complexes (q ca. ,1), with the para -MeO derivative allowing longer wavelength sensitization (,ex 330,nm). [source] A Density Functional Study of Ethylene Insertion into the M-methyl (M = Ti, Zr) Bond for Different Catalysts, with a QM/MM Model for the Counterion, B(C6F5)3CH3,ISRAEL JOURNAL OF CHEMISTRY, Issue 4 2002Kumar Vanka Single site homogeneous catalysts have been studied extensively in recent years as alternatives to traditional heterogeneous catalysts. The current theoretical study uses density functional theory to study the insertion process of the ethylene monomer into the titanium-carbon chain for contact ion-pair systems of the type [L1L2TiCH3 -,-CH3 -B(C6F5)3], where L1, L2, are Cp, NPH3, and other ligands. Different modes of approach cis and trans to the ,-CH3 bridge were considered. The counterion, B(C6F5)3CH3,, was modeled by QM/MM methods. The value of ,Htot,the total barrier to insertion,was found to be positive (in the range of 4,15 kcal/mol). The ability of the ancillary ligands, L1 and L2, to stabilize the ion-pair was found to be an important factor in determining the value of ,Htot. On replacing the titanium metal center with zirconium, the ,Htot values were found to be lowered (in the range of 2,9 kcal/mol), indicating that they would be better catalysts than their titanium analogues. The size of the ligands L1 and L2 was increased by replacing hydrogens in the ligands with tertiary butyl groups. The value of ,Htot was found to increase (in the range of 10,28 kcal/mol) in contrast to the simple systems, for both the cis and trans cases of approach, with the cis mode of approach giving lower values of ,Htot. Solvent effects were incorporated with cyclohexane (, = 2.023) as the solvent, and were found to have a minor influence, ±(0.5,1.5) kcal/mol) on the insertion barrier for all the cases studied. [source] Unexpected Aggregation of Neutral, Xylene-Cored Dinuclear GdIII Chelates in Aqueous SolutionCHEMISTRY - A EUROPEAN JOURNAL, Issue 26 2006Jérôme Costa Dr. Abstract We have synthesized ditopic ligands L1, L2, and L3 that contain two DO3A3, metal-chelating units with a xylene core as a noncoordinating linker (DO3A3, = 1,4,7,10-tetraazacyclododecane-1,4,7-triacetate; L1 = 1,4-bis{[4,7,10-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane-1-yl]methyl}benzene; L2 = 1,3-bis{[4,7,10-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane-1-yl]methyl}benzene; L3 = 3,5-bis{[4,7,10-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane-1-yl]methyl}benzoic acid). Aqueous solutions of the dinuclear GdIII complexes formed with the three ligands have been investigated in a variable-temperature, multiple-field 17O NMR and 1H relaxivity study. The 17O longitudinal relaxation rates measured for the [Gd2L1,3(H2O)2] complexes show strong field dependence (2.35,9.4 T), which unambiguously proves the presence of slowly tumbling entities in solution. The proton relaxivities of the complexes, which are unexpectedly high for their molecular weight, and in particular the relaxivity peaks observed at 40,50 MHz also constitute experimental evidences of slow rotational motion. This was explained in terms of self-aggregation related to hydrophobic interactions, , stacking between the aromatic linkers, or possible hydrogen bonding between the chelates. The longitudinal 17O relaxation rates of the [Gd2L1,3(H2O)2] complexes have been analysed with the Lipari,Szabo approach, leading to local rotational correlation times of 150,250 ps and global rotational correlation times of 1.6,3.4 ns (cGd: 20,50 mM), where is attributed to local motions of the Gd segments, while describes the overall motion of the aggregates. The aggregates can be partially disrupted by phosphate addition; however, at high concentrations phosphate interferes in the first coordination sphere by replacing the coordinated water. In contrast to the parent [Gd(DO3A)(H2O)1.9], which presents a hydration equilibrium between mono- and dihydrated species, a hydration number of q = 1 was established for the [Ln2L1,3(H2O)2] chelates by 17O chemical shift measurements on Ln = Gd and UV/Vis spectrophotometry for Ln = Eu. The exchange rate of the coordinated water is higher for [Gd2L1,3(H2O)2] complexes ( = 7.5,12.0×106 s,1) than for [Gd(DOTA)(H2O)],. The proton relaxivity of the [Gd2L1,3(H2O)2] complexes strongly decreases with increasing pH. This is related to the deprotonation of the inner-sphere water, which has also been characterized by pH potentiometry. The protonation constants determined for this process are logKOH = 9.50 and 10.37 for [Gd2L1(H2O)2] and [Gd2L3(H2O)2], respectively. [source] | ||||||||||