Showing papers on "Schiff base published in 1996"

Highly stereoelective polymerization of rac-(D,L)-lactide with a chiral schiff's base/aluminium alkoxide initiator

Abstract: A chiral Schiff's base/aluminium alkoxide initiator bearing a ligand derived from R-(+)-1,1′-binaphthyl-2,2′-diamine was synthesized and used for the stereoelective polymerization of rac-(D,L)-lactide. Rather high stereoelectivity is observed: a polymer with 88% enantiomeric enrichment in D units is obtained at 19% conversion. At high conversions a stereocomplex between D- and L-enriched stereocopolymers is formed. The polymerization reaction shows living type features, and narrow molecular weight distributions (Mw/Mn = 1,05–1,30) are obtained up to very high conversions. This indicates that transesterification reactions do not occur significantly with this sterically hindered initiator.

Complexes derived from the reaction of manganese(III) Schiff base complexes and hexacyanoferrate(III): Syntheses, multidimensional network structures, and magnetic properties

Abstract: The reaction between the [Mn(BS)(H2O)]+ monomeric and [Mn2(μ-BS)2(H2O)2]2+ dimeric cations and [Fe(CN)6]3- gave rise to cation−anion interaction via the formation of [FeC⋮NMn] bridges. Depending on the nature of the Schiff base and regardless of the stoichiometry used, either the trimeric anion [{Mn(BS)}2{Fe(CN)6}]- (BS = 3-MeOsalen, 6; 5-Clsalen, 7; 5-Brsalen, 8; salcy, 10) or the pentameric cation [{Mn(BS)}4{Fe(CN)6}]+ (BS = saltmen, 9) is formed, which has been assembled by the K+ cation or the ClO4- anion, respectively. The X-ray analysis of 6 revealed a two-dimensional network layer structure. The magnetic measurements showed its metamagnetic behavior, where the ferromagnetic interaction operates within each layer and the antiferromagnetic interaction operates between the layers. The Neel temperature, TN, is 9.2 K, and the critical field at 2 K is 300 Oe. The temperature dependent magnetic susceptibilities of 7 and 8 are in agreement with a discrete, symmetrical, trinuclear structure Mn(III)Fe(III)Mn...

Synthesis and Characterization of New Chiral Schiff Base Complexes with Diiminobinaphthyl or Diiminocyclohexyl Moieties as Potential Enantioselective Epoxidation Catalysts.

Abstract: New chiral Schiff base complexes have been obtained by condensation of 2,2‘-diamino-1,1‘-binaphthalene or 1,2-diaminocyclohexane and various salicylaldehydes and by subsequent metalation with manganese, iron, cobalt, nickel, copper, or zinc. The complete 1H and 13C NMR characterization of the ligands is reported, as are the X-ray crystal structures of (1R,2R)-(−)-N,N‘-bis[3-(N,N-dimethylamino)salicylidene]-trans-1,2-cyclohexanediimine and [(1R,2R)-(−)-N,N‘-bis(salicylidene)-trans-1,2-cyclohexanediiminato]copper(II). The new chiral manganese complexes have been evaluated in the oxygenation of prochiral olefins and sulfides using sodium hypochlorite, hydrogen peroxide, or N-methylmorpholine N-oxide/m-chloroperbenzoic acid as oxidant.

Model for the Reduced Schiff Base Intermediate between Amino Acids and Pyridoxal: Copper(II) Complexes of N-(2-Hydroxybenzyl)amino Acids with Nonpolar Side Chains and the Crystal Structures of [Cu(N-(2-hydroxybenzyl)-D,L-alanine)(phen)].H(2)O and [Cu(N-(2-hydroxybenzyl)-D,L-alanine)(imidazole)].

Abstract: Copper(II) complexes with reduced Schiff base ligands of amino acids possessing nonpolar side chains with salicylaldehyde have been synthesized. Ternary complexes with imidazole, 1,10-phenanthroline, and pyridine have been prepared and characterized for N-(2-hydroxybenzyl)-D,L-alanine. The crystal structures of [(N-(2-hydroxybenzyl)-D,L-alanine)(1,10-phenanthroline)Cu(II)] monohydrate ([Cu(SAla)phen].H(2)O) and [(N-(2-hydroxybenzyl)-D,L-alanine)(imidazole)Cu(II)] ([Cu(SAla)Him]), have been determined. [Cu(SAla)phen].H(2)O crystallized in space group Po, with a = 8.718(2) A, b = 10.886(3) A, c = 11.693(2) A, alpha = 71.32(2) degrees, beta = 85.27(2) degrees, gamma = 70.21(2) degrees, and Z = 2. The copper atom is five coordinate, with SAla acting as a tridentate ONO chelator through the carboxylato and phenolato oxygens and the amine nitrogen. The remaining donors are provided by the phen nitrogens. [Cu(SAla)Him] crystallized in space group P2(1)/n, with a = 10.353(1) A, b = 6.714(1) A, c = 18.769(2) A, beta = 91.71(1) degrees, and Z = 4. The copper atom is four coordinate, with SAla acting as a tridentate ONO chelator with the neutral imidazole moiety coordinated through nitrogen. In both complexes the ligand has two chiral centers due to the coordination of the N. Molecular mechanics calculations show that unfavorable steric interactions would occur in the nonobserved R,R and S,S diastereomers. Compounds prepared have been characterized by a range of physicochemical techniques. The complexes may serve as stable models for the intermediates in enzymatic amino acid transformations.

Binucleating Ligands: Synthesis of Acyclic Achiral and Chiral Schiff Base−Pyridine and Schiff Base−Phosphine Ligands

Abstract: 5-tert-Butyl-3-(2‘-pyridyl)salicyaldehyde and 5-tert-butyl-3-(diphenylphosphino)salicyaldehyde were synthesized from 4-tert-butylphenol in good overall yields. Condensation of the salicyaldehydes with 2,3-diamino-2,3-dimethylbutane afforded the desired dinucleating Schiff base−pyridine and Schiff base−phosphine ligands, respectively. 5-tert-Butyl-3-(2‘-pyridyl)salicyaldehyde reacted with optically active 1,2-diaminocyclohexanes to give chiral dinucleating Schiff base−pyridine ligands.

Electronic and steric effects in manganese Schiff-base complexes as models for the water oxidation complex in photosystem II. The isolation of manganese-(II) and -(III) complexes of 3- and 3,5-substituted N,N′-bis(salicylidene)ethane-1,2-diamine (H2salen) ligands

Abstract: Manganese-(II) and -(III) complexes of substituted N,N′-bis(salicylidene)ethane-1,2-diamine (H2salen) ligands H2L (substituents are in the 3, 5 or 3,5 positions of the phenyl rings of the salen moiety) have been prepared and thoroughly characterised. The reaction of Mn(ClO4)2·6H2O with H2L in ethanol in air normally leads to manganese(III) complexes ligated by both the N2O2 ligand and water molecule(s). However, by employing electron-withdrawing substituents on the ligand, e.g. 3-Br,5-NO2, a manganese(II) complex can be obtained. A ‘borderline’ ligand is represented by the 5-NO2 derivative (nsalen), which produces a manganese(II) complex contaminated with a small amount of a manganese(III) species. Using a more rigorous oxidising agent in the synthesis, [Fe(η-C5H5)2][FeCl4], drives the reaction totally to a manganese(III) complex [Mn(nsalen)Cl(H2O)]. In addition to magnetic susceptibility studies, cyclic voltammetry has been employed. All the complexes exhibit an oxidation and reduction peak, the reversible character being confirmed by pulse voltammetry. Pulse voltammetry also confirmed the nature of the manganese(II) species [Mn(bnsalen)(H2O)2·2H2O [H2bnsalen =N,N′-bis(3-bromo-5-nitrosalicylidene)ethane-1,2-diamine] and that a slight amount of a manganese(III) species is present in [Mn(nsalen)(H2O)2]·2H2O. Six complexes have been crystallographically characterised. Despite the retention of an octahedral manganese environment in all of them, the supramolecular structures exhibit a wide diversity. The 3,5-dichloro and 5-bromo salen complexes containing co-ordinated water display combined π and hydrogen bonding, as well as dimerisation. The complex [{Mn(µ-dbsalen)(µ-O)}2](dbsalen = 3,5-dibromo derivative) offers an alternative bridging arrangement, and [Mn(bsalen)(MeOH)(OClO3)]·H2O (bsalen = 5-bromo derivative) highlights the versatility of the manganese centre in these systems where, unexpectedly, perchlorate is co-ordinated in place of a lattice water. A more subtle rearrangement of supramolecular structure is obtained in [Mn(nsalen)Cl(H2O)] where the usual combination of π- or hydrogen-bonding interaction is modified by the corresponding ability of the 5-NO2 substituent.

Metal complexes of a chiral quadridentate Schiff base

Abstract: Chiral metal Schiff-base complexes [AlL(Et)], [TiLCl2], [VO(L)], [(FeL)2O], [CoL], [NiL], [CuL], [ZrLCl2], [RuL(CO)2] and trans-[RuL(NO)Cl]([H2L =(R,R)-(–)-N,N′-bis(3,5-di-tert-butylsalicylidene)cyclohexane-1,2-diamine] were synthesized and characterized. The structures of [CoL] and trans-[RuL(NO)Cl] have been established by X-ray crystallography. The former has a pseudo-square-planar geometry with Co–N and Co–O distances of 1.88 and 1.84 A, respectively. The geometry around Ru in trans-[RuL(NO)Cl] is octahedral with Ru–N (nitrosyl) and Ru–Cl distances of 1.72(2) and 2.354(4)A, respectively, and Ru–N–O 175(2)°. The cyclic voltammograms for the metal Schiff-base complexes show reversible MIII–MII and ligand-centred oxidation couples. Treatment of NiL with AgBF4 afforded air-stable [NiL]BF4, which is formulated as a nickel(II) complex of the Schiff-base cation radical.

Coordination Compounds of Schiff-Base Ligands Derived from Diaminomaleonitrile (DMN): Mononuclear, Dinuclear, and Macrocyclic Derivatives

Abstract: Copper(II) and VIVO complexes of an open chain (1:2) Schiff-base ligand (H2L1), derived by the template condensation of diaminomaleonitrile (DMN) and salicylaldehyde, and dicopper(II) complexes of (2:2) macrocyclic Schiff-base ligands derived by template condensation of diformylphenols and diaminomaleonitrile, have been synthesized and studied. Structures have been established for the first time for mononuclear Cu(II) and VIVO derivatives of the open chain ligand H2L1 (1:2), a dinuclear macrocyclic Cu(II) complex derived from a 2:2 macrocyclic ligand (H2M1), and the half-condensed 1:1 salicylaldehyde ligand (H2L2). [Cu(L1)] (1) (L1 = C18H10N4O2) crystallized in the monoclinic system, space group P21/n (No. 14), with a = 11.753(6) A, b = 7.708(5) A, c = 16.820(1) A, and Z = 4. [VO(L1)(DMSO] (2) crystallized in the orthorhombic system, space group Pbca (No. 61), with a = 22.534(9) A, b = 23.31(1) A, c = 7.694(5) A, and Z = 8. H2L2 (C18H8N4O) (3) crystallized in the monoclinic system, space group P21/c (No. ...

Structure, characterization and photoreactivity of monomeric dioxovanadium(v) schiff-base complexes of trigonal-bipyramidal geometry

Abstract: Five mononuclear cis-dioxovanadium(V) complexes of tridentate Schiff bases derived from salicylaldehyde and its derivatives and 8-aminoquinoline have been synthesized and characterized. Single-crystal X-ray analyses were performed with [VO2L1]1 and [VO2L2]2(L1 and L2 denote the [1 + 1] Schiff-base anions derived from salicylaldehyde or naphthaldehyde and 8-aminoquinoline). While the structure of 2 was refined to a final R= 0.074 (R′= 0.055), that of 1 was refined only to R= 0.134 (R′= 0.139) due to its poor diffraction quality. The complexes contain cis-oxo groups in the equatorial plane and a trigonal-bipyramidal geometry around the vanadium at which the Schiff base binds meridionally. Photoirradiation of these complexes in CH2Cl2 yielded chloride-bound VO3+ species, as studied using absorption and 51V NMR spectroscopy. These species are convertible back to the dioxovanadium(V) complexes upon addition of water to the photoirradiated solution.

Tetradentate Schiff base oxovanadium(IV) complexes

Abstract: Eleven oxovanadium(IV) complexes of tetradentate Schiff bases, obtained by condensating two moles of an o-hydroxycarbonyl compound with a diamine, have been prepared and characterized by elemental analysis, m.p., and i.r. and electronic spectra. The i.r. and electronic spectra of the free ligand and the complexes are compared and discussed. The Gaussian analysis of the vis. spectra of the complexes, normally C1 or Cs, in MeCN yielded four peaks at ca. 12000, 15000, 17700 and 20000–23000cm−1, assigned to the four d-d transitions.

Cobalt Schiff Base Complex-Catalyzed Oxidation of Anilines with tert-Butyl Hydroperoxide

Abstract: Cobalt Schiff base complexes [Co(SB)] catalyze the oxidation of anilines (1) with tert-butyl hydroperoxide to give nitrobenzenes 2 and 4-(tert-butylperoxy)-2,5-cyclohexadien-1-imine derivatives 3 in yield distributions depending on the substitution mode of the substrate. 4-Alkyl- and 4-aryl-2,6-di-tert-butylanilines gave mixtures of 2 and 3, where the higher the bulkiness of the 4-substituent, the higher the yield of 2. With 2,4,6-trimethylaniline, the ratio of oxidations of the nitrogen and C-4 atoms was almost the same; but a hydrolyzed product 5 of the imine was obtained. 2,4,6-Triphenylaniline gave only 2. Nitrobenzene derivatives were also obtained from 2,6-dialkylanilines and 4-substituted anilines. The catalytic activity of Co(SB) depended on the nature of the SB ligand: the formal potential E° and steric factors seem to affect the reaction rate. Kinetic studies showed that the key step may involve hydrogen abstraction from the aniline, presumably by t-BuO• generated from homolytic decomposition o...

Synthesis and Characterisation of Binuclear Cu(II), Ni(II) and Co(II) Chelates with Tetradentate Schiff Base Ligands Derived from 1,5-Diaminonaphthalene

Abstract: A new series of copper(II), nickel(II) and cobalt(II) chelates containing the binucleating tetradentate Schiff base ligands N,N′-bis(salicylidene)-1,5-diaminonaphthalene (H2L), N,N′-bis(3-methoxysalicylidene)-1,5-diaminonaphthalene (H2L′) and N,N′-bis(4-hydroxysalicylidene)-1,5-diaminonaphthalene (H2L) have been synthesized. These compounds have been characterized by elemental analyses, conductivity measurements, magnetic susceptibility, thermoanalyses (TG-DTA), infrared and electronic spectral data. The results suggest that the Schiff bases function as tetradentate ligands coordinating through the ONNO donor system and indicate a binuclear structure for the Cu(II) complexes with pseudo-tetrahedral geometry and binuclear structures for the Co(II) and Ni(II) complexes with pseudo-octahedral geometry. All complexes are non-electrolytes.

Synthesis, structure and reactions of the first tellurium-containing macrocyclic Schiff base

Abstract: The condensation of bis(2-formylphenyl) telluride 1 with ethane-1,2-diamine yielded the novel macrocyclic tellurium ligand 2via metal-free dimerisation. Crystals of 2 are triclinic, space group P with a= 7.956(3), b= 9.885(2), c= 10.068(2)A, Z= 1. Hydrogenation of macrocycle 2 provided the corresponding saturated tetraazamacrocycle 3, protonation of which with HBr afforded 4. The co-ordination chemistry of 2 has been studied with ‘soft’ metal ions such as palladium(II) and mercury(II). N,N′-Bis[(2-chlorotelluranyl)benzylidene]ethane-1,2-diamine has also been characterised by an X-ray diffraction study, with triclinic space group P, a= 7.71(1), b= 7.90(1), c= 8.52(1)A and Z= 1.

Determination of the transiently lowered pKa of the retinal Schiff base during the photocycle of bacteriorhodopsin

Abstract: Reprotonation of the transiently deprotonated retinal Schiff base in the bacteriorhodopsin photocycle is greatly slowed when the proton donor Asp-96 is removed with site-specific mutagenesis, but its rate is restored upon adding azide or other weak acids such as formate and cyanate. As expected, between pH 3 and 7 the rate of Schiff base protonation in the photocycle of the D96N mutant correlates with the concentrations of the acid forms of these agents. Dissection of the rates in the biexponential reprotonation kinetics of the Schiff base between pH 7 and 9 yielded calculated rate constants for the protonation equilibrium. Their dependencies on pH and azide or cyanate concentrations are consistent with both earlier suggested mechanisms: (i) azide and other weak acids may function as proton carriers in the protonation equilibrium of the Schiff base, or (ii) the binding of their anionic forms may catalyze proton conduction to and from the Schiff base. The measured rate constants allow the calculation of the pKa of the Schiff base during its reprotonation in the photocycle of D96N. It is 8.2-8.3, a value much below the pKa determined earlier in unphotolyzed bacteriorhodopsin.

A change in the internal aldimine lysine (K42) in O-acetylserine sulfhydrylase to alanine indicates its importance in transimination and as a general base catalyst

Abstract: O-Acetylserine sulfhydrylase (OASS) is a pyridoxal 5'-phosphate dependent enzyme that catalyzes a beta-replacement reaction forming L-cysteine and acetate from O-acetyl-L-serine (OAS) and sulfide. The pyridoxal 5'-phosphate (PLP) is bound at the active site in Schiff base linkage with a lysine. In the present study, the Schiff base lysine was identified as lysine 42, and its role in the OASS reaction was determined by changing it to alanine using site-directed mutagenesis. K42A-OASS is isolated as an external aldimine with methionine or leucine and shows no reaction with the natural substrates. Apo-K42A-OASS can be reconstituted with PLP, suggesting that K42 is not necessary for cofactor binding and formation of the external Schiff base. The apo-K42A-OASS, reconstituted with PLP, shows slow formation of the external aldimine but does not form the alpha-aminoacrylate intermediate on addition of OAS, suggesting that K42 is involved in the abstraction of the alpha-proton in the beta-elimination reaction. The external aldimines formed upon addition of L-Ala or L-Ser are stable and represent a tautomer that absorbs maximally at 420 nm, while L-Cys gives a tautomeric form of the external aldimine that absorbs at 330 nm, and is also seen in the overall reaction after addition of primary amines to the assay system. The use of a small primary amine such as ethylamine or bromoethylamine in the assay system leads to the initial formation of an internal (gamma-thialysine) or external (ethylamine) aldimine followed by the slow formation of the alpha-aminoacrylate intermediate on addition of OAS. Activity could not be fully recovered, and only a single turnover is observed. Data suggest a significant rate enhancement resulting from the presence of K42 for transimination and general base catalysis.

Electric-field-induced Schiff-base deprotonation in D85N mutant bacteriorhodopsin.

Abstract: The application of an external electric field to dry films of Asp-85-->Asn mutant bacteriorhodopsin causes deprotonation of the Schiff base, resulting in a shift of the optical absorption maximum from 600 nm to 400 nm. This is in marked contrast to the case of wild-type bacteriorhodopsin films, in which electric fields produce a red-shifted product whose optical properties are similar to those of the acid-blue form of the protein. This difference is due to the much weaker binding of the Schiff-base proton in the mutant protein, as indicated by its low pK of approximately 9, as compared with the value pK approximately 13 in the wild type. Other bacteriorhodopsins with lowered Schiff-base pK values should also exhibit a field-induced shift in the protonation equilibrium of the Schiff base. We propose mechanisms to account for these observations.