Showing papers on "Schiff base published in 2002"
TL;DR: Rational optimization of catalyst structure based on the mechanistic insight led to an improved catalyst for the asymmetric Strecker reaction.
Abstract: A mechanistic investigation of the asymmetric Strecker reaction catalyzed by a metal-free Schiff base catalyst was conducted. The active site of the catalyst, the relevant stereoisomer of the imine substrate, and the solution structure of the imine−catalyst complex were elucidated using a series of kinetics, structure−activity, and NMR experiments. An unusual bridging interaction between the imine and the urea hydrogens of the catalyst was identified and supported by computation. Rational optimization of catalyst structure based on the mechanistic insight led to an improved catalyst for the asymmetric Strecker reaction.
492 citations
TL;DR: Stereoselective polymerization of racemic lactide (rac-LA) was examined using Al-achiral ligand complexes and a higher selectivity was obtained without any chiral auxiliaries in the catalyst via a chain-end control mechanism.
Abstract: Stereoselective polymerization of racemic lactide (rac-LA) was examined using Al-achiral ligand complexes. By introduction of substituents in aromatic rings of Schiff base ligands, a higher selectivity was obtained without any chiral auxiliaries in the catalyst via a chain-end control mechanism. The T(m) values (T(m) 170-192 degrees C) were comparable to or higher than that of homochiral polymer, poly(L-LA) (T(m) 162 degrees C), and a thermally more stable polylactide than poly(L-LA) was prepared from rac-LA.
437 citations
TL;DR: In this paper, three kinds of Schiff bases, N,N′-o-phenylen-bis(3-methoxysalicylidenimine) (V−o-PhýV), N, N′-p-phenylonen-, N−n−n −n−p−n, N−p-phýN−n and N−methylen-methylene]-histidine (V-His), were used for copper corrosion in aerated 0.5 mol dm−3 NaCl and Na
Abstract: The inhibitive action of three Schiff bases, N,N′-o-phenylen-bis(3-methoxysalicylidenimine) (V–o-Ph–V), N,N′-p-phenylen-bis (3-methoxysalicylidenimine) (V–p-Ph–V) and N-[(2-hydroxy-3-methoxyphenyl)methylene]-histidine (V-His), on copper corrosion in aerated 0.5 mol dm−3 NaCl and NaBr solutions was investigated using EIS and steady-state polarization techniques. The inhibitor effectiveness depended strongly on the geometric structure of the Schiff bases. Among the three kinds of Schiff base used, the inhibition efficiency of V–o-Ph–V on copper corrosion was the highest, V–p-Ph–V the next and V-His the lowest. The Schiff bases inhibited the cathodic current more significantly than the anodic current. The different influences of V–o-Ph–V or V–p-Ph–V on the anodic and cathodic reactions led to the appearance of a low frequency capacitive loop in the impedance spectra. The inhibition action of the Schiff bases was due to their adsorption on the copper surface followed by complexation with Cu(I) or Cu(II) ions, forming a blocking barrier to copper corrosion.
248 citations
TL;DR: There is no striking correlation between the nature of the ligand systems and the insulin-mimetic potency in these cell culture tests, encompassing 41 vanadium compounds, the results on 22 of which are reported in detail here.
Abstract: A representative set of vanadium(IV and V) compounds in varying coordination environments has been tested in the concentration range 1 to 10–6 mM, using transformed mice fibroblasts (cell line SV 3T3), with respect to their short-term cell toxicity (up to 36 hours) and their ability to stimulate glucose uptake by cells. These insulin-mimetic tests have also been carried out with non-transformed human fibroblasts (cell line F26). The compounds under investigation comprise established insulin-mimetic species such as vanadate ([H2VO4]–), [VO(acetylacetonate)2], [VO2(dipicolinate)]– and [VO(maltolate)2], and new systems and coordination compounds containing OO, ON, OS, NS and ONS donor atom sets. A vitality test assay, measuring the reduction equivalents released in the mitochondrial respiratory chain by intracellular glucose degradation, is introduced and the results are counter-checked with 3H-labelled glucose. Most compounds are toxic at the 1 mM concentration level, and most compounds are essentially non-toxic and about as effective as or more potent than insulin at concentrations of 0.01 mM and below. VV compounds tend to be less toxic than VIV compounds, and complexes containing thio functional ligands are somewhat more toxic than others. Generally, ON ligation is superior in insulin-mimetic efficacy to OO or O/NS coordination, irrespective of the vanadium oxidation state. There is, however, no striking correlation between the nature of the ligand systems and the insulin-mimetic potency in these cell culture tests, encompassing 41 vanadium compounds, the results on 22 of which are reported in detail here. The syntheses and characteristics of various new compounds are provided together with selected speciation results. The crystal and molecular structures of {[VO(naph-tris)]2} [where naph-tris is the Schiff base formed between o-hydroxynaphthaldehyde and tris(hydroxymethyl)amine] are reported. Electronic supplementary material to this paper can be obtained by using the Springer Link server located at http://dx.doi.org/10.1007/s00775-001-0311-5
226 citations
TL;DR: Screening of the compounds for their cytotoxicities against T-lymphoblastic leukemia cancer cells has shown that the acetone Schiff base of S-methyldithiocarbazate (Hasme) exhibits a very weak activity, whereas the S-benzyl derivative (Hasbz) is inactive.
204 citations
TL;DR: In this article, a self-complementary copper-II complexes with multidentate Schiff-base ligands containing imidazole groups have potentially donor and acceptor character in the formation of coordination bond and a hydrogen bond.
166 citations
TL;DR: In this article, six Mn(III) quadridentate Schiff base compounds with N,N′-(1,1,2,2-tetramethylethylene)bis(salicylideneiminato) dianion (saltmen2−) and N, N′-( 1, 1, 2, 2, 2, 3, 4 and 6) have been prepared and structurally characterized.
Abstract: Six Mn(III) quadridentate Schiff base compounds with N,N′-(1,1,2,2-tetramethylethylene)bis(salicylideneiminato) dianion (saltmen2−) and N,N′-(1,1,2,2-tetramethylethylene)bis(naphthylideneiminato) dianion (naphtmen2−) have been prepared and structurally characterized: [Mn(saltmen)(H2O)]ClO4 (1), [Mn(naphtmen)(H2O)]ClO4 (2), [Mn(saltmen)(NCS)] (3), [Mn(naphtmen)(NCS)] (4), [Mn(saltmen)(Cl)] (5) and [Mn(naphtmen)(Cl)] (6). Among them, 1 and 2 form phenolate-bridged out-of-plane dimers with Mn–Ophenolate
bond distances of 2.434(2) and 2.662(3) A, respectively. X-Ray diffraction analysis shows that compounds 3, 4 and 6 can also be considered as out-of-plane dimers in spite of long Mn–Ophenolate interacting distances (3.441(2) A for 3, 3.758(3) A for 4 and 3.505(5) A for 6). In contrast with the above compounds, 5 is a discrete Mn(III) mononuclear complex with a square-pyramidal geometry. In the dimer series (compounds 1–4 and 6), the out-of-plane intermolecular distance varies dramatically according to equatorial ligands, saltmen2−
or naphtmen2−, and axial ligands, H2O, NCS− and Cl−. The relation between substitution of the ligands and structural parameters of the dimeric molecules are discussed. Magnetic susceptibility studies reveal interesting intra-dimer ferromagnetic interactions between Mn(III) ions. Our work reports on these new S
= 4 building blocks that open new possibilities in the design of magnetic molecule-based materials.
154 citations
TL;DR: In this paper, the Schiff base derived from the condensation of o-aminophenol with benzaldehydes was induced to undergo oxidative cyclization in the presence of DDQ, and the resulting 2-arylbenzoxazoles were separated from the reduced DDQ byproduct by treatment of reaction mixture with a strongly basic ion exchange resin.
154 citations
TL;DR: Rate, recycling, and other data suggest that the palladacycles serve as sources of palladium nanoparticles, which are the dominant active catalysts.
149 citations
TL;DR: The reaction of the zinc(II) Schiff base complex ZnL [H2L=N,N′-bis(3-methoxysalicylidene)ethylene-1,2-diamine] with one equivalent of Ln(NO3)3·xH2O (Ln=Nd, Ho, Er or Yb) gives the neutral 3d-4f bi-metallic Schiff base complexes [ZnL].
Abstract: The reaction of the zinc(II) Schiff base complex ZnL [H2L=N,N′-bis(3-methoxysalicylidene)ethylene-1,2-diamine] with one equivalent of Ln(NO3)3·xH2O (Ln=Nd, Ho, Er or Yb) gives the neutral 3d-4f bi-metallic Schiff base complexes [Zn(NO3)(μ-L)Ln(NO3)2(H2O)], which in solution exhibit emission corresponding to the Ln(III) ions (Ln=Nd, Er and Yb) in the near-infrared region.
148 citations
TL;DR: In this article, a tetradentate Schiff-base aluminum complexes (designated as SalenAlX) were prepared and used as catalysts for the synthesis of ethylene carbonate from supercritical carbon dioxide/ethylene oxide mixture.
Abstract: Various tetradentate Schiff-base aluminum complexes (designated as SalenAlX) were prepared and used as catalysts for the synthesis of ethylene carbonate from supercritical carbon dioxide/ethylene oxide mixture. Their catalytic activities can be markedly enhanced in the presence of a Lewis base or quaternary salt. With SalenAlCl/ n -Bu 4 NBr as catalyst, the formation rate of ethylene carbonate is up to 2220 turnovers/h and about two times that under 4.0 MPa CO 2 constant pressure at 110 °C. The high rate of reaction may be attributed to rapid diffusion and high miscibility of ethylene oxide in supercritical carbon dioxide under employed conditions. Axial X– group and substitution on the aromatic rings of SalenAlX also affect catalytic properties of these aluminum complexes. Compared to SalenAlX, some other metal–Salen complexes, alone or combined with a Lewis base as co-catalyst, show very little catalytic activity towards the reaction under employed conditions. However, with a quaternary salt as co-catalyst, these metal–Salen complexes all exhibit catalytic activities, which are in the following order: SalenCrCl>SalenCo>SalenNi>SalenMg, SalenCu, SalenZn.
TL;DR: UV spectrophotometric investigations indicate that conversion of Cu(I) to Cu(II) is not essential for catalysis but that Cu( II) is probably also a competent system.
Abstract: Racemic 2,2‘-diamino-6,6‘-dimethylbiphenyl is resolved using simulated moving bed chromatography, and the absolute configuration of the enantiomers is confirmed via the X-ray crystal structure of a derivative. The diamine is condensed with a range of aldehydes to give bidentate aldimine proligands L. Molecular structures of the complexes formed between L and Cu(I) fall into two classes; bimetallic double helices ([Cu2L2]2+) and monometallic ([CuL]+). The latter are strikingly more efficient in the aziridination of alkenes than are the former in terms of rate, turnover, and enantioselection. In particular, the imine ligand formed from the diamine and 2,6-dichlorobenzaldehyde gives, in combination with Cu(I) or Cu(II), up to 99% ee in the aziridination of 6-acyl-2,2-dimethylchromene and 88−98% ee for a range of cinnamate esters. Styrenic and other alkenes are converted with lower selectivities (5−54%). The catalytic system shows a linear response in product ee to catalyst ee, and the product ee does not var...
TL;DR: The substituent effect, UV-vis absorption, and EPR spectral studies indicate oxygen atom transfer from the oxidant to the substrate in the rate-determining step during the oxidation of para-substituted phenyl methyl sulfides in acetonitrile.
Abstract: The oxidation of a series of para-substituted phenyl methyl sulfides was carried out with several oxo(salen)iron (salen = N,N‘-bis(salicylidine)ethylenediaminato) complexes in acetonitrile. The oxo complex [OFeIV(salen)]•+, generated from an iron(III)−salen complex and iodosylbenzene, effectively oxidizes the organic sulfides to the corresponding sulfoxides. The formation of [OFeIV(salen)]•+ as the active oxidant is supported by resonance Raman studies. The kinetic data indicate that the reaction is first-order in the oxidant and fractional-order with respect to sulfide. The observed saturation kinetics of the reaction and spectral data indicate that the substrate binds to the oxidant before the rate-controlling step. The rate constant (k) values for the product formation step determined using Michaelis−Menten kinetics correlate well with Hammett σ constants, giving reaction constant (ρ) values in the range of −0.65 to −1.54 for different oxo(salen)iron complexes. The log k values observed in the oxidatio...
TL;DR: A mechanism is proposed wherein water dissociation is effected by means of a chemical desiccant (BaO or silyl enol ether, thereby revealing the site for aldehyde complexation) and a bridged, dimeric structure bearing aquo-bound six-coordinate Cr(III) centers is revealed.
Abstract: Tridentate Schiff base chromium(III) complex 1 catalyzes the asymmetric hetero-ene reaction between aryl aldehydes and either 2-methoxypropene or 2-trimethylsilyloxypropene to provide a series of beta-hydroxyenol ether products in high yields and enantioselectivities. X-ray crystallographic analysis of a closely related chromium complex reveals a bridged, dimeric structure bearing aquo-bound six-coordinate Cr(III) centers. A mechanism is proposed wherein water dissociation is effected by means of a chemical desiccant (BaO or silyl enol ether), thereby revealing the site for aldehyde complexation.
TL;DR: In this paper, two series of diimino tetradentate Schiff bases derived from phenylenediamine and 1,3-naphthalene diamine and their vanadyl complexes were synthesized by template and non-template methods and characterized by 1H, 13C NMR, IR, UV-Vis and elemental analysis.
TL;DR: In this article, bidentate isomeric NS and NS′ Schiff bases were derived from the condensation of S-benzyldithiocarbazate (SBDTC) with 5-methyl-2-furyldehyde and 2furyl-methylketone.
TL;DR: In this article, the electrochemical reduction of copper(II) complexes with salen Schiff-base ligands derived from ethylenediamine or (R,R) or (S,S)-1,2-diphenylethylenedienine and 5-methoxy, 5-bromo and 5nitrosalicylaldehyde have been studied by cyclic voltammetry in the potential range +1 to −2.3 V in dimethyl sulfoxide (DMSO) as a solvent.
TL;DR: A tetranuclear double-open dicubane complex has been characterised structurally and magnetostructurally in this paper, where it has been shown to have a strong magnetic property.
TL;DR: In this paper, bimetallic and binuclear carbonylruthenium(II) complexes with N2O2 Schiff base ligands based on 3,5-di-tert-butylsalicylaldehyde and three different ortho-diamines have been prepared.
Abstract: Mononuclear and binuclear carbonylruthenium(II) complexes with N2O2 Schiff base ligands based on 3,5-di-tert-butylsalicylaldehyde and three different ortho-diamines have been prepared. The mononuclear Ru(BSP)(CO) [BSP = N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-phenylenediamine] complex 4 acts as a versatile supramolecular synthon, as illustrated by the fact that it spontaneously forms linear and three-dimensional assemblies through axial coordination with pyridyl Lewis bases. Using this motif, neutral and charged assemblies 6, 9, and 12 were prepared. The versatility of the salophen ligand was highlighted by the preparation of bimetallic carbonylruthenium(II) compounds 14−17 from 1,2,4,5-tetraaminobenzene and 2,3,7,8-tetraaminodibenzo[1,4]dioxin. The bimetallic complexes were isolated as a mixture of cis and trans diastereomers with respect to the spatial relationship between the two axially bound carbon monoxide ligands. The electronic spectral and electrochemical properties of the pyridyl adducts 5, 15, and 17 were compared. The properties of 17 closely resembled 5 due to the insulating effect of the extended central tetraamino fragment, while 15 behaved as a single, novel chromophore. The electrochemical studies revealed that the central tetraamino linker regulates the communication between the two metal centers of 15 and 17. It was found that the two metal atoms of 15 sense each other to a larger extent than those of 17.
TL;DR: Chiral ruthenium(II)−salen complexes [RuII(salen)(PPh3)2] catalyse asymmetric aziridination of alkenes with up to 83% ees, asymmetric amidation of silyl enol ethers and cholesteryl acetates with good regioselectivity as discussed by the authors.
TL;DR: The vanadyl Schiff base complexes with the general formula VOL x (x = 7-18) were synthesized and characterized by physicochemical methods as mentioned in this paper, which indicated that oxovanadium(IV) exhibits a coordination number of five.
Abstract: Novel vanadyl Schiff base complexes with the general formula VOL x (x = 7–18) were synthesized and characterized by physicochemical methods. Only an orange polymeric form was obtained for VOL 13 , both brown polymeric and green monomeric forms were obtained for VOL x , x = 16, 17. These observations suggest that in these complexes oxovanadium(IV) exhibit a coordination number of five. Systematic substitution on the ancillary ligand have allowed V(V)/V(IV) reduction potentials to be tuned over a range of approximately 130 mV. The complexes are catalysts for the aerobic oxidation of cyclohexene. Catalytic activity increases with increasing V(V)/V(IV) reduction potential and the catalytic selectivity varied by changes in the ligands. The catalytic system described here is an efficient and inexpensive method for oxidation of olefins, with advantages of high activity, selectivity, re-usability and short reaction times. © 2002 Elsevier Science B.V. All rights reserved.
TL;DR: In this paper, the products obtained by reacting ruthenium (II) complexes [RuHCl(CO)(PPh3)2(B)] [B = PPh3, pyridine (py) or piperidine (pip)] with tridentate Schiff base ligands derived by condensing salicylaldehyde or o-vanillin with o-aminophenol and o-iamothiophenol, have been characterised by analytical, i.m., electronic, 1H-n.m.
Abstract: The products obtained by reacting ruthenium (II) complexes [RuHCl(CO)(PPh3)2(B)] [B = PPh3, pyridine (py) or piperidine (pip)] with tridentate Schiff base ligands derived by condensing salicylaldehyde or o-vanillin with o-aminophenol and o-aminothiophenol, have been characterised by analytical, i.r., electronic, 1H-n.m.r. and 31P-n.m.r. spectral studies and formulated as [Ru(L)(CO)(PPh3)(B)] (L = bifunctional tridentate Schiff base anion, B = PPh3, py or pip). An octahedral structure has been tentatively proposed for the new complexes. Some have been tested for the in vitro growth inhibitory activity against bacteria Escherichia coli, Bacillus sp. and Pseudomonas sp.
TL;DR: Eight selective nitrogen-sulfur donor ligands have been synthesized from the condensation of S-methyldithiocarbazate with aldehydes and ketones with a view to evaluating their antimicrobial and cytotoxic activities, and also to correlate the biological properties with the structure of the ligands.
Abstract: Eight selective nitrogen-sulfur donor ligands have been synthesized from the condensation of S-methyldithiocarbazate (SMDTC) with aldehydes and ketones with a view to evaluating their antimicrobial and cytotoxic activities, and also to correlate the biological properties with the structure of the ligands. The compounds were all characterized by elemental analyses and other physicochemical techniques. SMDTC and the Schiff bases were screened for antimicrobial and cytotoxic activities. SMDTC showed very large inhibition zones (24-44 mm) against bacteria and fungi with a minimum inhibitory concentration (MIC) of 390-25,000 and 1562-6250 microg ml(-1), against different bacteria and fungi, respectively. Streptomycin and nystatin were used as the internal standards against bacteria and fungi, respectively. SMDTC along with its Schiff bases with pyridine-2-carboxaldehyde, acetylacetone and 2,3-butanedione were strongly antifungal and the MIC values were comparable to nystatin. Most of the Schiff bases were strongly cytotoxic. In particular, those with pyridine-2-carboxaldehyde and 2,3-butanedione have CD(50) values of 5.5, 1.9-2.0 microg ml(-1), respectively, against leukemic cells, while against colon cancer cells, the values were 3.7 and 2.0 microg ml(-1), respectively. The glyoxal Schiff base was strongly active only against leukemic cell with CD(50) value of 4.0 microg ml(-1). The present findings have been compared with standard drugs.
TL;DR: The Schiff base ligands [3,5-But2-2-(OH)C6H2CHNL] were accessed in good yields (>85%) via standard imine condensation reactions.
Abstract: The Schiff base ligands [3,5-But2-2-(OH)C6H2CHNL] [L = CH2CH2NMe2 (1a), 2-(PhO)C6H4 (1b), 2-CH2C5H3N (1c), 8-C9H6N (quinoline) (1d) and 2-(PPh2)C6H4 (1e)] are accessed in good yields (>85%) via standard imine condensation reactions. Reaction of 1a–e with Me3Al at room temperature affords the corresponding complexes [(3,5-But2-2-(O)C6H2CHNL)AlMe2] (2a–e);
in the case of L = 8-quinoline, the same reaction conducted in refluxing toluene affords binuclear {[3,5-But2-2-(O)C6H2CHMeN-8-C9H6N]AlMe}2 (3) by methyl migration from metal to ligand. Further reaction of the dimethyl compounds with B(C6F5)3 in CD2Cl2 or C6D6 affords the cationic systems [(3,5-But2-2-(O)C6H2CHNL)AlMe]+ (4a–e). The crystal structures of 2a, 2c, 2e and 3 have been determined. In 2a and 2c
the respective ligands bind to the metal centre via all three heteroatoms, the aluminium having a trigonal bipyramidal geometry, whereas in 2e coordination is via nitrogen and oxygen only, and the aluminium is tetrahedral. Complex 3 has a dimeric structure with the ligand adopting both tridentate and binucleating roles; the aluminium centres are trigonal bipyramidal.
TL;DR: In this paper, a chiral chromium salen complex has been anchored on aminopropyl-functionalized SiO 2, ITQ-2 or MCM-41.
Abstract: Two series of solid catalysts in which a chiral chromium salen complex has been anchored on aminopropyl-functionalized SiO 2 , ITQ-2 or MCM-41 have been prepared. In those catalysts in which anchoring of the complex was accomplished through coordination with the metal high enantiomeric excesses were obtained (up to 70% enantiomeric excess (e.e.)), but the complex leaches to the solution on a large extent. In contrast, for those other solids in which the complex is attached to the surface by covalent linkage to the ligand no leaching was observed, but the e.e. were modest (below 20%) compared to those obtained in homogeneous catalysis under comparable conditions (above 50%).
TL;DR: In this paper, a new class of homobimetallic and monometallic Schiff base-substituted ruthenium olefin metathesis catalysts has been prepared, characterised and tested in ring-closing (RCM) and ring-opening (ROMP) reactions.
Abstract: A new class of homobimetallic and monometallic Schiff base-substituted ruthenium olefin metathesis catalysts has been prepared, characterised and tested in ring-closing metathesis (RCM) and ring-opening metathesis polymerisation (ROMP) reactions. The results obtained point out that the synergy of Schiff base ligands with coordinatively labile ligands leads to bimetallic catalytic systems that combine very high activity with excellent stability. Furthermore, the catalytic activity of these catalysts is very dependent on the steric and electronic environment of the Schiff base. To conclude, a mechanism that explains the obtained data is postulated.
TL;DR: The title compounds and their Zn(II) derivatives were evaluated for antibacterial activity against several bacterial strains which easily develop resistance to classical antibiotics, such as Escherichia coli, Staphylococcus aureus and Pseudomonas aeruginosa and some of them showed promising biological activity in inhibiting the growth of such organisms.
Abstract: Schiff bases derived from oxaldiamide/oxalylhydrazine and pyrrol-2-carbaldehyde, or salicylaldehyde respectively, as well as their Zn(II) complexes have been prepared and tested as antibacterial agents. These Schiff bases function as tetradentate ligands, forming octahedral Zn(II) complexes. The ketonic form for the diamide derived Schiff base and the enolic form of the hydrazide derived Schiff base were the preferred tautomers for coordination of the metal ions. The title compounds and their Zn(II) derivatives were evaluated for antibacterial activity against several bacterial strains which easily develop resistance to classical antibiotics, such as Escherichia coli, Staphylococcus aureus and Pseudomonas aeruginosa. Some of them showed promising biological activity in inhibiting the growth of such organisms.