Showing papers on "Schiff base published in 1999"

Antimicrobial activity studies of the binuclear metal complexes derived from tridentate Schiff base ligands

Abstract: Three novel tridentate Schiff base ligands derived from␣the 3-hydroxysalicylaldehyde (H2L1), 4-hydroxysalicylaldehyde (H2L2) and 5-bromosalicylaldehyde (H2L3) with a new amine N-(pyridyl)-2-hydroxy-3-methoxy-5-aminobenzylamine (2) have been prepared. The ligands and their metal complexes have been characterized by elemental analyses, conductivity and magnetic susceptibility measurements, i.r., electronic absorption and 1H and 13C n.m.r. spectroscopy. All complexes are binuclear and, in some, the H2O molecules are coordinated to the metal ion. Antimicrobial activities of the ligands and their complexes have been tested against to the Bacillus subtilis IMG 22 (bacteria), Micrococcus luteus LA 2971 (bacteria) Saccharamyces cerevisiae WET 136 (yeast), and Candida albicans CCM 314 (yeast). Thermal properties of all complexes have been studied by t.g. and d.t.a techniques.

Singly anti-anti carboxylate-bridged zig-zag chain complexes from a carboxylate-containing tridentate schiff base ligand and m(hfac)2 m = mnii, niii, and cuii : synthesis, crystal structure, and magnetic properties

Abstract: The reaction of M(hfac)2 with the tridentate Schiff base H2L (where H2L stands for the 1:1 condensation product of 2-imidazolecarboxaldehyde with β-alanine) leads to the complexes [M(HL)(hfac)]n [M = MnII, NiII, and CuII; hfac = hexafluoroacetylacetonate anion] (1–3). The structures of the complexes 1 and 3 have been solved by X-ray crystallographic methods. The structures are very similar and consist of infinite zig-zag chains, running parallel to the b axis, in which the metal ions are bridged sequentially by anti-anti carboxylate groups with intrachain metal–metal distances of 6.134 A for 1 and 6.239 A for 3. Each monodeprotonated HL ligand acts as a tridentate one to a metal(II) ion and as a monodentate one to a neighbouring metal(II) centre. Metal atoms exhibit distorted octahedral coordination spheres comprised of two oxygen atoms from the hexafluoroacetylacetonate ligand, three donor atoms from the HL ligand and the oxygen atom belonging to the carboxylate group of an adjacent molecule. The complexes 1–3 have been confirmed to be isomorphous and isostructural on the basis of X-ray powder diffraction and IR spectra. The magnetic properties of the three compounds were studied by susceptibility measurements as a function of the temperature and successfully analyzed in terms of the isotropic spin Hamiltonian for one-dimensional infinite chain systems to give the coupling parameters J = –0.91 cm−1, g = 2.03 (1); J = –13.2 cm−1, g = 2.24 (2); and J = 0.40 cm−1, g = 2.11 (3). The magnetic behaviour for all three complexes can be satisfactorily explained in terms of the conformation of the bridge and the interaction between the d orbitals of the metal centre and the bridge.

Antimicrobial activity studies of mononuclear and binuclear mixed-ligand copper(II) complexes derived from Schiff base ligands and 1,10-phenanthroline

Abstract: Mixed-ligand complexes of copper(II) with 1,10-phenanthroline and various Schiff bases have been prepared and characterized by elemental analysis, electronic and i.r. spectra, magnetic moment and molar conductance data. The Schiff base behave as bidentate ligands, and the mixed-ligand copper(II) complexes of the ligands HL1′′, HL2′′ and HL4′′ are binuclear. The conductivity data for all the complexes are consistent with those expected for an electrolyte. Antimicrobial activities of some of the ligands and complexes have been tested against Bacillus megaterium and Candida tropicalis. 1H and 13C n.m.r. spectra have been recorded in order to solve the solution structure of the ligands. Thermal properties of all complexes have been studied by the d.t.a. and t.g.a. techniques.

N,N-phthaloylamino acids as chiral auxiliaries in asymmetric mannich-type reactions

Abstract: High stereoselectivity of the Mannich reaction of N,N-phthaloylamino acids as chiral auxiliaries is achieved by choice of the aniline part of the Schiff base 1. Treatment of ortho-substituted N-arylimines of type 1 and N,N-phthaloyl-tert-leucine chloride (2) with a silylketene acetal provides the corresponding Mannich adducts 3 with diastereomer ratios in excess of 99:1.

Synthesis and magnetic properties of a series of bi- and tri-nuclear complexes of copper(II) with the unsymmetrical tetradentate Schiff-base ligand 3-[N-2-(pyridylethyl)formimidoyl]salicylic acid, H2fsaaep, and crystal structures of [{Cu(Hfsaaep)Cl}2] and [{Cu(fsaaep)(H2O)}2]

Abstract: Three oligonuclear complexes of copper(II), [{Cu(Hfsaaep)Cl}2] 1, [{Cu(fsaaep)(H2O)}2] 2 and [Cu3(fsaaep)2][ClO4]2 3 have been synthesized, using the unsymmetrical tetradentate Schiff-base ligand 3-[N-2-(pyridylethyl)formimidoyl]salicylic acid, H2fsaaep. The crystal structures of complexes 1 and 2 have been solved. That of 1 consists of discrete binuclear entities with copper atoms bridged by two chloride anions. The copper atoms are related by an inversion center and exhibit a slightly distorted square pyramidal stereochemistry. The Cu· · ·Cu separation within the binuclear unit is 3.825(2) A. The structure of 2 consists of neutral centrosymmetric binuclear entities. The copper(II) ions are bridged by phenolic oxygen atoms, assuming a square-pyramidal geometry. The distance between the copper atoms is 3.0279(4) A. Variable-temperature magnetic susceptibility measurements indicated a very weak ferromagnetic coupling of the copper(II) ions in compound 1 (J = 0.15 cm–1) and an antiferromagnetic coupling in compounds 2 and 3 (J = –617 and –228 cm–1, respectively).

Amino acid Schiff base complex catalyst for effective oxidation of olefins with molecular oxygen

Abstract: The amino acid Schiff base manganese complex (Sal–Phe–Mn) was prepared with l -phenylalanine, salicylaldehyde and Mn(OAc) 2 ·4H 2 O. The ligand and the complex were characterized by the infrared spectra, small area X-ray photoelectron spectroscopy, and ICP-AES. In the presence of the manganese complex, cyclohexene was effectively oxidized by molecular oxygen without reductant. The major products of the reaction were 2-cyclohexen-1-ol (–OH), 2-cyclohexen-1-one (CO) and 2-cyclohexen-1-hydroperoxide (–OOH), which was different with typical oxidation of cyclohexene. The influence of reaction temperature and additive for oxidation had been studied. The selectivity of 2-cyclohexen-1-hydroperoxide varied with reaction time and different additives. The mechanism of cyclohexene oxidation had also been discussed.

Spectrophotometric study of the equilibria between nickel(ii) schiff-base complexes and alkaline earth or nickel(ii) cations in acetonitrile solution

Abstract: Adduct formation between M(ClO4)2 and nickel(II) complexes of tetradentate Schiff-base ligands ([NiL]) have been investigated in acetonitrile by means of UV−vis spectrophotometric analysis (M2+= Mg2+, Ca2+, Sr2+, Ba2+, Ni2+; [NiL] = [Ni(salen)] = [N,N‘-ethylenebis(salicylideneiminato)]nickel(II), [Ni(salphen)] = [N,N‘-o-phenylenebis(salicylideneiminato)]nickel(II), [Ni(salpren)] = [N,N‘-propylenebis(salicylideneiminato)]nickel(II)). [NiL] complexes act as bidentate oxygen-donor ligands forming adducts of the type [(NiL)M]2+ and [(NiL)2M]2+ whose general order of increasing stability was correlated to the dimensions of the M2+ cations. Complexes of general composition 2[Ni(salen)]·M(ClO4)2 were isolated as powders in the solid state (M = Sr, Ca, Mg). X-ray structures of the crystalline complexes [{Ni(salen)}2Ba(ClO4)2(thf)] (1) (thf = tetrahydrofuran) and [{Ni(salen)}2Ni(H2O)2](ClO4)2 (2) are reported. 1 crystallizes in the space group P21/c (a = 10.995 A; b = 20.830 A; c = 19.567 A, β = 116.94°) and 2 in ...

Olefin epoxidation catalysed by Schiff-base complexes of Mn and Ni in heterogenised-homogeneous systems

Abstract: M(salen) complexes (M=Mn(III), Ni(II); salen=bis-(salicyldene)ethylenediamine) have been encapsulated in zeolite Y and characterised. Mn(salen)+ complex was also anchored in montmorillonite clay and characterised. Epoxidation of olefins, viz. cyclohexene, cyclooctene and 1-hexene with terminal oxidants (NaOCl, KHSO5) was carried out with the anchored catalyst complexes and found that the epoxidation of linear olefin (1-hexene) is selectively facile than cyclic olefins. Experimental results are compared with those reported for M(salen) complexes catalysed olefin epoxidation in homogeneous and heterogenised-homogeneous catalytic conditions.

Titanium and ruthenium binaphthyl Schiff base complexes as catalysts for asymmetric trimethylsilylcyanation of aldehydes

Abstract: Investigations on the catalytic behaviour of titanium complexes formed in situ from Ti(OPri)4 and a variety of Schiff bases, mainly the binaphthyl derivatives 2,2′-bis(3-R1-5-R2-2-hydroxybenzylideneamino)-1,1′-binaphthyl, toward the asymmetric trimethylsilylcyanation of some aromatic and aliphatic aldehydes demonstrated that the titanium complex of the binaphthyl Schiff base with R1 = R2 = But is one of the best catalysts for such a process, with an enantiomeric excess (e.e.) as high as 96% obtained for m-tolualdehyde. Crystal structure determination of a nitrosylruthenium complex, [RuII(L)(NO)Cl] (L is the dianion of the binaphthyl Schiff base with R1 = R2 = Cl), revealed that the complex assumes a cis-β configuration with the binaphthyl moiety having a dihedral angle of 70.2°. After treatment with AgPF6, the ruthenium complex also exhibited a good catalytic property for the trimethylsilylcyanation of benzaldehyde albeit with a lower e.e. (24%).

Magic angle spinning NMR of the protonated retinylidene Schiff base nitrogen in rhodopsin: Expression of 15N-lysine- and 13C-glycine-labeled opsin in a stable cell line

Abstract: The apoprotein corresponding to the mammalian photoreceptor rhodopsin has been expressed by using suspension cultures of HEK293S cells in defined media that contained 6-15N-lysine and 2-13C-glycine. Typical yields were 1.5–1.8 mg/liter. Incorporation of 6-15N-lysine was quantitative, whereas that of 2-13C-glycine was about 60%. The rhodopsin pigment formed by binding of 11-cis retinal was spectrally indistinguishable from native bovine rhodopsin. Magic angle spinning (MAS) NMR spectra of labeled rhodopsin were obtained after its incorporation into liposomes. The 15N resonance corresponding to the protonated retinylidene Schiff base nitrogen was observed at 156.8 ppm in the MAS spectrum of 6-15N-lysine-labeled rhodopsin. This chemical shift corresponds to an effective Schiff base-counterion distance of greater than 4 A, consistent with structural water in the binding site hydrogen bonded with the Schiff base nitrogen and the Glu-113 counterion. The present study demonstrates that structural studies of rhodopsin and other G protein-coupled receptors by using MAS NMR are feasible.

Copper(II)-selective membrane electrode based on a recently synthesized naphthol-derivative Schiff’s base

Abstract: A PVC membrane electrode for copper(II) ions based on a recently synthesized naphthol-derivative Schiff’s base as membrane carrier was prepared. The sensor exhibits a Nernstian response for Cu2+ ions over a wide concentration range (5.0 × 10–6–5.0 × 10–2 mol/L) with a detection limit of 3.1 × 10–6 mol/L (0.2 μg/mL). It has a very short response time of about 5 s and can be used for ¶3 months without any divergence in potential. The proposed electrode revealed good selectivities over a wide variety of other cations including alkali, alkaline earth, transition and heavy metal ions and could be used in a pH range of 4.0–7.0. It was successfully applied to the direct determination and potentiometric titration of copper ion.

Transition metal complexes of bidentate Schiff base ligands

Abstract: New Schiff base ligands derived from vanillin (HL1), 4-dimethylaminobenzaldehyde (HL2) and 3,5-di-t-butyl-4-hydroxybenzaldehyde (HL3) with N-(pyridyl)-3-methoxy-4-hydroxy-5-aminobenzylamine (2) and their copper(II), cobalt(II), nickel(II), oxovanadium(IV) and zinc(II) transition metal complexes have been synthesized and characterized by elemental analyses, electronic and i.r. spectra, molar conductance data and by 1H and 13C n.m.r. spectra. The results indicate that the ligands coordinate through azomethine nitrogen and phenolic oxygen to the metal ions. In like manner, it was found that the pyridine and amine nitrogen atoms are not coordinated to the metal ions. The 1H and 13C n.m.r. spectral data confirmed the suggested structure for the Schiff base ligands, and the mass spectra results confirmed the proposed structure of the ligands. The antimicrobial activity properties of the ligands and their metal complexes have been studied.

Vanadium catalyzed enantioselective oxidation of sulfides: easy transformation of bis(arylthio)alkanes into C2 symmetric chiral sulfoxides

Abstract: A facile and selective method for the title transformation is described. The two-phase oxidation of bis-sulfides with hydrogen peroxide catalyzed by vanadium complex of chiral Schiff base leads to the corresponding chiral mono- and bis-sulfoxides. In the case of 1,2-bis(arylthio)ethanes the respective optically active bis-sulfoxides are formed in up to over 95% e.e.

Structure of a Michaelis complex analogue: propionate binds in the substrate carboxylate site of alanine racemase.

Abstract: The structure of alanine racemase from Bacillus stearothermophilus with the inhibitor propionate bound in the active site was determined by X-ray crystallography to a resolution of 1.9 A. The enzyme is a homodimer in solution and crystallizes with a dimer in the asymmetric unit. Both active sites contain a pyridoxal 5'-phosphate (PLP) molecule in aldimine linkage to Lys39 as a protonated Schiff base, and the pH-independence of UV-visible absorption spectra suggests that the protonated PLP-Lys39 Schiff base is the reactive form of the enzyme. The carboxylate group of propionate bound in the active site makes numerous interactions with active-site residues, defining the substrate binding site of the enzyme. The propionate-bound structure therefore approximates features of the Michaelis complex formed between alanine racemase and its amino acid substrate. The structure also provides evidence for the existence of a carbamate formed on the side-chain amino group of Lys129, stabilized by interactions with one of the residues interacting with the carboxylate group of propionate, Arg136. We propose that this novel interaction influences both substrate binding and catalysis by precisely positioning Arg136 and modulating its charge.

Solid state 15N NMR evidence for a complex Schiff base counterion in the visual G-protein-coupled receptor rhodopsin.

Abstract: Using the baculovirus/Sf9 cell expression system, we have incorporated 99% 15N-enriched [alpha,epsilon-15N2]-L-lysine into the rod visual pigment rhodopsin. We have subsequently investigated the protonated Schiff base (pSB) linkage in the [alpha, epsilon-15N2]Lys-rhodopsin with cross-polarization magic angle spinning (CP/MAS) 15N NMR. The Schiff base (SB) 15N in [alpha, epsilon-15N2]Lys-rhodopsin resonates with an isotropic shift sigmaI of 155.9 ppm, relative to 5.6 M 15NH4Cl. This suggests that the SB in rhodopsin is protonated and stabilized by a complex counterion. The 15N shifts of retinal SBs correlate with the energy difference between the ground and excited states and the frequency of maximum visible absorbance, numax, associated with the pi-pi transition of the polyene chromophore. Experimental modeling of the relation between the numax and the size of the counterion with a set of pSBs provides strong evidence that the charged chromophore in rhodopsin is stabilized by a counterion with an estimated effective center-center distance (deff) between the counterion and the pSB of 0.43 +/- 0.01 nm. While selected prokaryotic proteins and complexes have been labeled before, this is the first time to our knowledge that a 15N-labeled eukaryotic membrane protein has been generated in sufficient amount for such NMR investigations.