Nives Galić

The Schiff bases of salicylaldehyde and aminopyridines as highly sensitive analytical reagents

The analytical applicability of heteroaromatic Schiff bases, 2-(3-pyridylmethyliminomethyl)phenol (compound 1), 2-(2-pyridyliminomethyl)phenol (compound 2), 2-(2-amino-3-pyridyliminomethyl)phenol (compound 3), N,N′-bis(salicylidene)-2,3-pyridinediamine (compound 4), N,N′-bis(salicylidene)-2,6-pyridinediamine (compound 5) and 2-(2-amino-4-methox-ymethyl-6-methyl-3-pyridylmethyliminomethyl)phenol (compound 6), was proved. The investigation included a study of the characteristics that are essential for solvent extraction and for the spectrophotometric and spectrofluorimetric determinations of copper. Despite structural similarities, the investigated Schiff bases exhibited different behaviour in solution. Compounds 2–5 adopted predominantly the enolimino form in most solvents, whereas compound 1 was partially present as ketoamine in the solutions of polar solvents. Compound 6 was found to be the tautomeric mixture of cyclic diamine and enolimine in most solvents. All the investigated Schiff bases were capable of quantitatively extracting transition metals into chloroform. In the case of Cu(II), compound 4 proved to be an effective extractant in the range 5 <pH < 12, compounds 1 and 6 were effective in the range 6 <pH < 12, whereas compounds 2, 3 and 5 were effective only in the alkaline medium above pH 9. The spectrophotometric determination of Cu(II) after extraction was very sensitive and selective with regard to Cd(II) and Pb(II). Highest sensitivity was achieved with compound 4 (ϵ430 = 2.1 × 104 mol−1 1 cm−1). This characteristic was favourable compared to other well-known copper reagents. Since the fluorescence of Schiff bases was quenched by Cu(II), Mn(II), Fe(III), Ni(II) and Cr(III), the sensitivity as well as the selectivity were further enhanced using the spectrofluorimetric procedure. Additional advantages of these compounds to other reagents for copper(II) are simple preparation and a detailed knowledge of their equilibria.

Structural and spectroscopic characteristics of aroylhydrazones derived from nicotinic acid hydrazide

Abstract Aroylhydrazones derived from salicylaldehyde, o-vanillin and nicotinic acid hydrazide have been synthesized and characterized on the basis of NMR, IR and UV/Vis spectral data. The crystal and molecular structure of N′-salicylidene-3-pyridinecarbohydrazide has been determined by X-ray diffraction.

Tautomeric and protonation equilibria of Schiff bases of salicylaldehyde with aminopyridines

Abstract The equilibria of the Schiff bases of salicylaldehyde with 2-aminopyridine, 2,3-diaminopyridine, 2,6-diaminopyridine and 3-aminomethylpyridine were studied by means of spectroscopic methods. In non-polar solvents enolimines were predominantly present. In polar solvents rapid tautomeric interconversion of enolimines to ketoamines as well as slow hydrolysis were noted. The tendency to tautomeric interconversion was significant in the case of 2-(3-pyridylmethylimi-nomethyl)phenol, whereas in the case of other Schiff bases it was very low. The corresponding tautomeric constants were estimated in a variety of solvents on the basis of UV spectral data. The protonation constants of all compounds were determined in methanol/water mixtures by the spectrophotometric method and discussed with respect to their structural characteristics and solvent properties.

Spectrometric study of tautomeric and protonation equilibria of o-vanillin Schiff base derivatives and their complexes with Cu(II)

Electronic absorption and emission properties of a series of Schiff bases derived from 2-hydroxy-3-methoxybenzaldehyde and 2-aminopyridine, 2,3-diaminopyridine, 2,6-diaminopyridine, or 3-aminomethylpyridine were studied in solvents of different polarities. The interconversion of the enolimine to the ketoamine tautomeric form was observed for compound 1, 6-methoxy-2-(3-pyridylmethyliminomethyl)phenol, and the corresponding equilibrium constant was estimated in several solvents. Protonation constants of all the investigated compounds were determined spectrophotometrically in the methanol/water 1/4 system. The effect of copper(II) ions on absorption and on the emission spectra of these ligands was examined in the buffered dioxane/water 1/1 system (pH 5.8). Strong complexation of Cu(II) and formation of a 1:1 complex were observed for the bis-Schiff base derived from 2,3-diaminopyridine. The complex of copper(II) with compound 1 was isolated and characterized by elemental analysis, magnetic susceptibility measurement, UV-vis and IR spectrometry.

The structure and tautomeric properties of 2-(3-pyridylmethyliminomethyl)phenol☆

Abstract The structure and properties of 2-(3-pyridylmethyliminomethyl)phenol have been determined by X-ray crystallography, NMR, UV and IR spectroscopy and compared with the characteristics of the Schiff bases derived from salicylaldehyde and aminopyridines. Crystal data: C13H12N2O, Mr = 212.250, monoclinic, space group P21/n, a = 10.49(1) A, b = 9.002(4) A, c = 12.42(1) A, β = 109.10(4)°, V = 1109(2) A3, Z = 4, Dx = 1.271 g cm−3, Mokα (λ = 0.71073 A), μ = 0.8 cm−1, F(000) = 448, T = 297 K, R = 0.049, RW = 0.053 for 886 reflections with I 2σ(I). Bond distances and bond angles are similar to those of 2-(pyridyliminomethyl)phenols. In the title molecule the dihedral angle between the pyridine and benzene least-squares planes is 89.9(2)°. There is an intramolecular O1H … N1 hydrogen bond of 2.572(5) A. In solutions of polar solvents tautomeric interconversion of the Schiff base into the ketoamine is observed. Corresponding equilibrium constants are estimated in a variety of solvents on the basis of UV spectral data.

STRUCTURAL CHARACTERISTICS OF N,N'-BIS(SALICYLIDENE)-2,6-PYRIDINEDIAMINE

Abstract The Schiff base N , N ′-bis(salicylidene)-2,6-pyridinediamine has been synthesized and characterized in the solid state and in solution using X-ray analysis, IR, UV/Vis and NMR spectroscopy. Crystal data: C 19 H 15 N 3 O 2 , M r =317.34, monoclinic, space group P2 1 /c, a =19.313(5), b =5.854(2), c =14.957(6) A, β =110.45(2)°, V =1584.4(9) A 3 , Z =4, R =0.049, R w =0.095 for 3323 independent reflections with I >2 σ ( I ). There are two intramolecular hydrogen bonds O–H⋯N between the hydroxyl and imino groups of 2.564(3) and 2.633(3) A. The enolimino form is found in the solid state and is also the predominant tautomeric form in solution. A tendency of interconversion to ketoamine has been observed only in rather polar solvents, such as methanol and methanol/water mixtures and has been found to be very low: tautomeric constants K t =[ketoamine]/[enolimine] amount to 0.02 and 0.03 in methanol and methanol/water 4/1, respectively.© 1997 Elsevier Science B.V.

Vanadium-induced formation of thiadiazole and thiazoline compounds. Mononuclear and dinuclear oxovanadium(V) complexes with open-chain and cyclized thiosemicarbazone ligands

Reactions of the salicylaldehyde 4-phenylthiosemicarbazone (H(2)L) with selected vanadium(iv) and vanadium(v) precursors ([VO(acac)(2)], [VO(OAc)(2)], VOSO(4), [V(2)O(4)(acac)(2)]) were investigated under aerobic conditions in different alcohols (methanol, ethanol, propanol). In all examined cases mononuclear alkoxo vanadium(v) complexes [VOL(OR)] (1) (OR = OMe, OEt, OPr) were isolated as major products. On prolonged standing, mother liquids afforded dinuclear vanadium(v) complexes [V(2)O(3)(L(cycl))(2)(OR)(2)] (3) (OR = OMe, OEt, OPr), where L(cycl)(-) represents 1,3,4-thiadiazole ligand, formed by vanadium-induced oxidative cyclization of H(2)L. When [VO(acac)(2)] or [V(2)O(4)(acac)(2)] were used as precursors, in addition to products 1 and 3, a thiazoline derivative HL(acac)(cycl) (2) was isolated. This compound, formed by a reaction between acetylacetone and H(2)L, represented the second type of cyclic product. The products were characterized by IR and NMR spectroscopies, TG analysis, and in some cases by single-crystal X-ray diffraction. To the best of our knowledge, compounds [V(2)O(3)(L(cycl))(2)(OR)(2)] represent the first structurally characterized dinuclear vanadium(v) complexes with a thiadiazole moiety acting as a bridging ligand. Complexes 1 and 3, when dissolved in an appropriate alcohol, underwent substitution of the alkoxo ligand as confirmed by XRPD. The kinetics of reactions in methanolic solutions was qualitatively studied by UV-Vis and ESMS spectrometries. Under the experimental conditions applied, a relatively slow formation of the mononuclear complex [VOL(OMe)] and an even slower formation of the cyclic species 2 were observed, whereas the presence of dinuclear compound [V(2)O(3)(L(cycl))(2)(OMe)(2)] in the reaction mixture could not be detected.

Urinary metabolites as biomarkers of human exposure to atrazine: atrazine mercapturate in agricultural workers.

Human exposure to atrazine and other triazine herbicides results in urinary excretion of traces of parent compounds and of their metabolites formed by N-dealkylation or conjugation with mercapturic acid. In contrast to N-dealkylated metabolites, which are not compound-specific, the measurement of atrazine mercapturate and unchanged atrazine provides an unambiguous confirmation of exposure to this herbicide. The aim of this study was to investigate the levels of these two compounds in a group of agricultural workers who may be considered representative for typical behaviour and procedures during the atrazine application in Croatia. The spot urine samples were collected at the beginning (samples A) and at the end (samples B) of a working day and 12h after exposure has ended (samples C). Atrazine and atrazine mercapturate were extracted from acidified urine samples (pH 2) with ethyl acetate and the extracts were analysed using high performance liquid chromatography-tandem mass spectrometry with a turbo ion spray (electrospray) ionization interface. The detection limits based on treatment of 2ml urine samples were 0.2ngml(-1) for both analytes. Atrazine was not detected in any of 27 analysed urine samples but traces of atrazine mercapturate were measured in about a third of pre-exposure and in all post-exposure urine samples in mass concentrations ranging from 0.3 to 10.4ngml(-1) (0.3 to 8.0μgg(-1) of creatinine). The metabolite concentrations in B and C group of post-exposure samples were not significantly different. The urinary atrazine mercapturate post-exposure concentrations were comparable to those reported for U.S. farmers engaged in a single field application of atrazine.

Fluorescent phenanthridine-based calix[4]arene derivatives: synthesis and thermodynamic and computational studies of their complexation with alkali-metal cations

New fluorescent calix[4]arene derivatives 1, 2, and 3 were synthesized by introducing phenanthridine moieties at a lower calixarene rim. It was shown that due to the prominent fluorescence of compounds 1 and 3, they could be considered as potential sensitive fluorimetric cation sensors. Complexation of the prepared compounds with alkali-metal cations was studied at 25 °C in acetonitrile–dichloromethane and methanol–dichloromethane solvent mixtures (φ = 0.5) by means of fluorimetric, spectrophotometric, potentiometric, and microcalorimetric titrations as well as NMR spectroscopy. The stability constants of the corresponding complexes were determined, as were the enthalpies and entropies of the complexation reactions. In addition, equilibrium constants of ion-pairing reactions between alkali-metal cations and several anions in the solvents used were measured conductometrically. It was found that the cation-binding affinity of ligand 1 with four phenanthridine subunits was much higher than that of 2 and 3, with the complex stabilities in all cases being significantly lower in methanol–dichloromethane mixture compared to that in acetonitrile–dichloromethane. These findings were thoroughly discussed by taking into account the determined thermodynamic complexation data, structural properties of the ligand and free and complexed cations, as well as the solvation abilities of the solvents examined. The conclusions made in that way were corroborated by the results of the molecular dynamics simulations of the systems studied. An attempt to get an insight into the possible structures of the alkali-metal cation complexes with ligand 1 was made by carrying out the corresponding density functional theory calculations.

Structural investigation of aroylhydrazones in dimethylsulphoxide/water mixtures.

Molecular structures of aroylhydrazones derived from salicylaldehyde, o-vanilin and nicotinic acid hydrazide in DMSO and DMSO/H(2)O mixtures have been studied by NMR, UV-Vis, ATR and Raman spectroscopy. The addition of water to the system did not induce the tautomeric conversion of the existing form constituted of the ketoamino hydrazide part and the enolimino aldehyde part, but it was involved in the formation of hydrated molecules. Vibrational spectra (ATR and Raman) clearly indicated hydrogen bonding of the studied hydrazones through the carbonyl, amino and hydroxyl groups with water molecules. Increasing the water content conversion from E to Z isomer was not observed.