Showing papers on "Schiff base published in 1997"

Atom Transfer Radical Polymerization of Methyl Methacrylate Initiated by Alkyl Bromide and 2-Pyridinecarbaldehyde Imine Copper(I) Complexes

Abstract: It is shown the 2-pyridinecarbaldehyde imines may be used in place of bipyridines in atom transfer polymerization in conjunction with copper(I) bromides and alkyl bromides. The ligands are simple to prepare and allow a wide range of versatility with regards to electron donating and -withdrawing capability as well as producing soluble Cu(I) species and homogeneous reactions

Spectroscopy and Electrochemistry of Cobalt(III) Schiff Base Complexes

Abstract: The structural, spectroscopic, and electrochemical properties of cobalt(III) derivatives of acacen (H2acacen = bis(acetylacetone) ethylenediimine) and related ligands have been investigated. Electronic structure calculations indicate that the absorption between 340 and 378 nm in CoIII(acacen) spectra is attributable to the lowest π−π* intraligand charge-transfer transition. Equatorial ligand substitutions affect reduction potentials less than axial ligand changes, consistent with an electronic structural model in which dz2 is populated in forming cobalt(II). The crystal structure of [Co(3-Cl-acacen)(NH3)2]BPh4 has been determined: The compound crystallizes in the monoclinic space group P21/m (No. 11) with a = 9.720(2) A, b = 18.142(4) A, c = 10.046(2) A, β = 100.11(3)°, Dc = 1.339 g cm-3, and Z = 2; the complex cation, [Co(3-Cl-acacen)(NH3)2]+, exhibits a slightly distorted octahedral coordination geometry. The distances between the cobalt atom and the two axial nitrogen donor atoms differ only slightly ...

Synthesis, characteristic spectral studies and in vitro antimicrobial and antitumour activities of organotin(IV) complexes of Schiff bases derived from amino-acids

Abstract: Equimolar reactions of dibutyltin(IV) oxide with Schiff bases derived from amino-acids led to the formation of a new series of dibutyltin(IV) complexes of general formula, Bu2SnL [L=dianion of tridentate Schiff bases derived from the condensation of 2-hydroxy-1-naphthaldehyde or acetyl acetone with glycine (L-1), L-β-alanine (L-2), DL-valine (L-3), DL-4-aminobutyric acid (L-4), L-methionine (L-5), L-leucine (L-6) and phenylglycine (L-7)]. An attempt has been made to prove the structures of the resulting complexes on the basis of elemental analyses, conductance measurements and electronic, IR, multinuclear magnetic resonance (1H, 13C and 117Sn) and 119Sn Mossbauer spectral studies. The complexes have been tested against various bacteria [Streptococcus faecalis, Klebsiella pneumoniae, Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus penicillin resistance (2500 units)] and fungi (Candida albicans, Cryptococcus neoformans, Sporotrichum schenckii,Trichophyton mentagrophytes and Aspergillus fumigatus). All the complexes showed moderate activity. The cytotoxicity of a few compounds has been screened in vitro against seven human tumour cell lines, viz. MCF-7, EVSA-T, WiDr, IGROV, M19 MEL, A498 and H226. The activities found experimentally were better than those obtained for cisplatin and carboplatin © 1997 John Wiley & Sons, Ltd.

Catalytic Mechanism of the Quinoenzyme Amine Oxidase from Escherichia coli: Exploring the Reductive Half-Reaction†,‡

Abstract: The crystal structure of the complex between the copper amine oxidase from Escherichia coli (ECAO) and a covalently bound inhibitor, 2-hydrazinopyridine, has been determined to a resolution of 2.0 A. The inhibitor covalently binds at the 5 position of the quinone ring of the cofactor, 2,4,5-trihydroxyphenylalaninequinone (TPQ). The inhibitor complex is analogous to the substrate Schiff base formed during the reaction with natural monoamine substrate. A proton is abstracted from a methylene group adjacent to the amine group by a catalytic base during the reaction. The inhibitor, however, has a nitrogen at this position, preventing proton abstraction and trapping the enzyme in a covalent complex. The electron density shows this nitrogen is hydrogen bonded to the side chain of Asp383, a totally conserved residue, identifying it as the probable catalytic base. The positioning of Asp383 is such that the pro-S proton of a substrate would be abstracted, consistent with the stereospecificity of the enzyme determi...

Syntheses and X-ray Structures of Mixed-Ligand Salicylaldehyde Complexes of Mn(III), Fe(III), and Cu(II) Ions: Reactivity of the Mn(III) Complex toward Primary Monoamines and Catalytic Epoxidation of Olefins by the Cu(II) Complex

Abstract: The Schiff base (L), synthesized from 2-(dimethylamino)ethylamine and salicylaldehyde acts as a tridentate ligand. This ligand, when stirred with 1 equiv of KOH in methanol, undergoes partial hydrolysis of the imine bond. This solution readily takes up Mn(II)/Mn(III) acetate or Fe(III) chloride/perchlorate to form mixed-ligand Mn(III) or Fe(III) complexes, respectively. The neutral dark brown complex, [Mn(L-H)(NCS){o-(CHO)C6H4O-}] (1), crystallizes in the presence of thiocyanate in the orthorhombic space group Pbca with a = 15.271(8), b = 19.522(7), c = 13.213(7) A; Z = 8; R = 0.060; and Rw = 0.062. The coordination geometry around Mn(III) ion is distorted octahedral with donation from one L-H, one salicylaldehyde and, one thiocyanate ligand. With Fe(III), the dark red complex isolated in the solid state is found to be a neutral μ-oxo Fe(III) dimer with the formula [{o-(CHO)C6H4O-}(L-H)Fe]2O (3). The structure of 3 has been solved and successfully refined in the monoclinic space group C2/c with a = 18.558...

Ethylenebis(salicylideneiminato)zirconium dichloride : Crystal structure and use as a heterogeneous catalyst in the polymerization of ethylene

Abstract: In the solid-state structure of the Schiff base salen [salen = N,N‘-ethylenebis(salicylideneiminato)] zirconium dichloride complex Zr(salen)Cl2(THF), C20H22Cl2N2O3Zr, a quadridentate ligand, occupies four sites in the pseudopentagonal bipyramidal coordination. The coordination sphere is completed by one THF molecule, and two chloride ions are forced into the trans-position. Removal of THF, while Zr(salen)Cl2(THF) is heated in toluene, leads to a six-coordinated complex with the two chloride ligands assuming the cis-position. This property was successfully utilized in the preparation of a homogeneous Zr(salen)Cl2 catalyst precursor. Heating of Zr(salen)Cl2(THF) in toluene in the presence of SiO2 causes deposition of Zr(salen)Cl2 on to the silica support, producing a heterogeneous catalyst, which shows an acceptable activity in ethylene polymerization at 80 °C and at low ethylene pressures in the presence of the cocatalyst methylaluminoxane.

A local electrostatic change is the cause of the large-scale protein conformation shift in bacteriorhodopsin

Abstract: During light-driven proton transport bacteriorhodopsin shuttles between two protein conformations. A large-scale structural change similar to that in the photochemical cycle is produced in the D85N mutant upon raising the pH, even without illumination. We report here that (i) the pKa values for the change in crystallographic parameters and for deprotonation of the retinal Schiff base are the same, (ii) the retinal isomeric configuration is nearly unaffected by the protein conformation, and (iii) preventing rotation of the C13—C14 double bond by replacing the retinal with an all-trans locked analogue makes little difference to the Schiff base pKa. We conclude that the direct cause of the conformational shift is destabilization of the structure upon loss of interaction of the positively charged Schiff base with anionic residues that form its counter-ion.

Dicopper Complexes with a Dissymmetric Dicompartmental Schiff Base−Oxime Ligand: Synthesis, Structure, and Magnetic Interactions

Abstract: A dicopper(II) complex with an unsymmetrical dicompartmental ligand containing bridging phenolic oxygen atoms was synthesized by stepwise condensation, in the presence of copper(II) ions, of 2,6-diformyl-4-methylphenol and diaminopropane, followed by hydroxylamine. The complex was isolated and characterized as Cl-, [Cu2Cl4]2-, and ClO4- derivatives. A byproduct of the reaction, a copper(II) complex with a substituted salycilaldoxime, was also structurally characterized. The structure of the binuclear cation of major interest is nearly planar, with the copper(II) ions slightly displaced to opposite sides of the plane of the donor atoms (<0.20 A). [Cu2Cl4]2- is an unusual anion, having been characterized previously in only five other compounds. It is flat, with both copper(I) ions in trigonal planar environments and two chlorides bridging the two copper(I) centers. X-ray structure determinations of the [Cu2Cl4]2- and ClO4- derivatives of the dicopper complex reveal a strong tendency for the formation of coo...

Design, Synthesis, and X-ray Structural Characterization of New Dinucleating Macrocyclic Ligands and a Novel Phenolate-Bridged Dilanthanum(III) Complex

Abstract: The [2+2] template condensation of 6,6‘-bis(aminomethyl)-2,2‘-bipyridyl with 2,6-diformyl-p-cresol in the presence of lanthanum acetate yields a homodinuclear complex of a Schiff base macrocycle (babp)2(dfc)2, 1. The crystal of [La21(OAc)4]·4CH3CN·3H2O is triclinic, space group P1, with cell constants a = 10.986(6) A, b = 12.231(8) A, c = 23.818(12) A, α = 86.63(5)°, β = 85.99(5)°, γ = 79.45(4)°, V = 3135(3) A3, and Z = 2. The decacoordination geometry around each identical La3+ ion is a bicapped dodecahedron made up of two bidentate acetate anions, two phenolate oxygens, and four imine nitrogens. The 1H, 13C, and 2D-COSY NMR and UV−visible spectra of the complex are consistent with the solid state structural information obtained by IR and X-ray diffraction studies. Treatment of a solution of La21(OAc)4 in methanol with sodium cyanotrihydroborate yields the hydrogenated octaaza macrocycle R(babp)2(dfc)2, 2. The [2+2] metal-free condensation of 2,6-bis(aminomethyl)-p-cresol with 2,6-diformylpyridine yield...

Conformational effects on the proton affinity of the Schiff base in bacteriorhodopsin: A density functional study

Abstract: Density functional theory (DFT) calculations have been performed on a number of Schiff base structures related to the retinal Schiff base in bacteriorhodopsin (BR) The proton affinity (PA) of the Schiff base group was calculated in species with different lengths of the conjugated double-bond system and at different cis/trans isomerization states The results show that the length of the conjugated electronic structure has a positive effect on the PA of the system which can be related to the more delocalized electronic structures in the longer chains Although there is no significant difference of PA between different cis/trans isomers at single or double bonds in the main chain of the polyene structure, very pronounced PA changes are predicted during the rotation of these bonds The calculations show that the rotation of the single bond adjacent to the Schiff base group significantly decreases the PA of the Schiff base The results of closed-shell DFT calculations indicate that the rotation of the second

Langmuir-Blodgett layers from Schiff base copper(II) complexes

Abstract: Metal complexes of salicylideneamines exhibit very interesting properties and have been well characterized since the late 1920s. Amphiphilic salicylideneamines have been synthesized in order to int...

Crystal structures, spectroscopic and nonlinear optical properties of metal complexes of Schiff-base ligands containing nitrogen and sulfur donors

Abstract: ML2 type CuII chelates with the Schiff base ligands R-C6H4CHNNHC(S)SCH2Ph (R=NMe2 or -OMe) have been prepared and characterized. Spectroscopic data suggest that the Schiff bases act as singly charged anionic bidentate ligands, forming stable neutral metal complexes. Magnetic and e.s.r. data support a square-planar coordination geometry for both complexes. Single crystal X-ray diffraction analysis of the complexes has established that the Schiff base ligand is deprotonated to give the thiol tautomer, coordinated via the thiolato sulfur and β-nitrogen. The geometry around the metal is square-planar with two equivalent Cu-N and Cu-S bonds; the two phenyl rings and the donor atoms are in one plane forming an extensive electronic delocalization system. Third-order optical nonlinearity measurements show that the complexes exhibit fairly large second-order hyperpolarizabilities.

Copper(II) Template Synthesis of a 20-Membered [1+1] Schiff Base Macrocycle and Nickel(II) Template Synthesis of a 40-Membered [2+2] Schiff Base Macrocycle from 2,6-Pyridinedicarboxaldehyde and 1,13-Diamino-4,7,10-trioxatridecane

Abstract: Copper(II) and nickel(II) are effective template ions for the synthesis of the large, polyether Schiff base macrocyclic ligands in the complexes CuL4Cl(H2O)(PF6) and NiL5(PF6)2. These complexes show that large metal ion templates are not always required to produce large macrocycles.

Kinetics and Equilibria of Dioxygen Binding to a Vacant Site in Cobalt(II) Complexes With Pentadentate Ligands

Abstract: Here we show that dioxygen binding to vacant metal ion sites proceeds at rapid rates, with very low enthalpic barriers but negative activation entropies; the rates are relatively insensitive to the electronic structure of the other ligands bound to the metal. While these properties are shared by vacant sites exposed to noncoordinating solvents and by those having sites protected from solvent, the latter react substantially more rapidly. The oxygenation of cobalt(II) complexes with pentadentate Schiff base ligands dissolved in the noncoordinating solvent acetone has been studied. For nonbridged complexes, reversible formation of 1:1 adducts with O2 was observed only at low temperatures (from −75 to −40 °C for acetylacetone derivatives and from −75 to −20 °C for salicylaldehyde derivatives), whereas the oxygenation of the corresponding lacunar species is reversible at room temperature. The dioxygen affinity of the salicylaldehyde derivative (CoSalMeDPT) (0.024 Torr-1 at −39 °C) is significantly smaller than...