Showing papers on "Stability constants of complexes published in 2007"

Synthesis of bispyrenyl sugar-aza-crown ethers as new fluorescent molecular sensors for Cu(II).

Abstract: Two N-pyrenylacetamide-substituted sugar-aza-crown ethers have been synthesized as new fluorescent chemosensors. The designed ligands 1 and 2 exhibit fluorescence characteristics of a pyrene monomer and a dynamic excimer emission when compared to N-pyrenylacetamide as a model compound. Both ligands displayed a Cu2+-sensitive fluorescence quenching with a 1:1 stoichiometry and high stability constants (log K = 6.7 for 1 and 7.8 for 2). The quenching effect was rationalized on the basis of photoinduced electron transfer from the excited pyrene to the complexed Cu2+ cation, while the changes in excimer-to-monomer ratio were explained by a conformational analysis through DFT calculations. The predicted structure suggests that the Cu2+ cation is coordinated with the two carbonyl groups and the sugar-aza-crown ethers which rigidified the complex structure and placed the two pyrene moieties far apart.

Structural characterization and chemical response of a Ag-coordinated supramolecular gel.

Abstract: Supramolecular gels exhibit potential applications in the areas of sensors, nanodevices, drug and catalyst carriers, and so on. To develop a novel organogel with a multiresponse, we designed a component molecule bearing a pyridyl group for metal coordination and an amide group for the formation of intermolecular hydrogen bonding. A complex building block with a symmetrical structure was selectively constructed by the coordination of a silver cation to the organic component. The coordination existing in the complex and the hydrogen bonding existing between complexes were examined by IR, Raman, and 1H NMR spectroscopy. The gel formation and phase transition were examined by viscosity and differential scanning calorimetry measurements. The selection of metal ions for the formation of a gel has proved to be crucial as only the complex of a binary coordinated metal ion, Ag+, was found to form a gel structure. From the band shift of the L1 solution with different amounts of silver ion, the binding ratio of silv...

Uranium(VI) complexation by humic acid under neutral pH conditions studied by laser-induced fluorescence spectroscopy

Abstract: The complexation of U(VI) with Aldrich humic acid (HA) was studied at pH 7 under exclusion of CO 2 . Using two independent laser-induced spectroscopic methods, time-resolved laser-induced fluorescence spectroscopy (TRLFS) and TRLFS with ultrafast pulses (fs-TRLFS), the formation of the ternary U(VI) mono hydroxo humate complex UO 2 (OH)HA(I) by reaction of UO 2 OH + with HA was studied. Assuming that all proton exchanging functional groups of the HA are able to contribute to the complex formation, a mean stability constant of 6.58±0.24 was derived for UO 2 (OH)HA(I). Alternatively, the analytical data were evaluated based on the metal ion charge neutralization model resulting in a complexation constant of 6.95±0.10 and a loading capacity of 0.76±0.28. An overall complexation constant of log β 0.1M = 14.89±0.54 was calculated for the total reaction of U(VI) with HA starting from the non-hydrolyzed UO 2 2+ ion. This value agrees very well with literature data. Taking into account the UO 2 (OH)HA(I) complex, the speciation of U(VI) in presence of HA was recalculated. It was found, that the formation of UO 2 (OH)HA(I) can significantly influence the U(VI) distribution in the environmentally relevant pH region. As a consequence, the mobility of U(VI) in natural aquifer systems could be enhanced.

Complexation of palladium(II) with chloride and hydroxide

Abstract: Complex formation of Pd(II) with chloride and hydroxide has been investigated by spectrophotometry at 25°C and ionic strength 1.0 M. For the chloride complexes [PdCl n (H2O)4− n ]2− n (n = 0–4), the formation constants, β n , have been determined and the values are: log β1 = 4.47, log β2 = 7.80, log β3 = 10.18 and log β4 = 11.53. The formation of mixed chloro-hydroxo complexes, [PdCl p (OH) q ]2−, has been investigated at three chloride concentrations, 1.0, 0.5 and 0.2 M over the pHc range 4–12. The average values determined for the formation constants, β pq , where p and q refer to the number of chloride and hydroxide ions in the complex, are: log β31 = 16.48, log β22 = 20.63, log β13 = 24.02 and log β04 = 26.23. Molar absorption spectra for all the complexes in question have been obtained.

Stability constants of amino acids, peptides, proteins, and other biomolecules determined by CE and related methods: recapitulation of published data.

Abstract: The stability (affinity, association, binding, complexation, formation) constant characterizes binding interaction between the analyte and the complexing agent. Knowledge of the stability constant makes possible the prediction and estimation of the binding behavior of constituents (amino acids, peptides, proteins, drugs, antibiotics, enzymes, enantiomers) to their partners, and the finding of a suitable partner for the given analyte to form a stable complex. The present paper summarizes the stability constant determination methods and the approaches used to evaluate the experimental data. Further, the paper recapitulates the published stability constant values determined, mainly, by capillary electrophoretic methods, taken from the Web of Science database covering the last decade. Details of the experimental conditions employed for the determination of the stability constants are also given. The review attempts to give a critical evaluation of the problems that accompany the determination of stability constant and discusses their solution.

Solvent Extraction Behavior of Plutonium (IV) Ions in the Presence of Simple Hydroxamic Acids

Abstract: Formo‐ and aceto‐hydroxamic acids are very effective reagents for stripping Pu(IV) ions from a tri‐butyl phosphate phase into nitric acid. Distribution data for Pu(IV) in the presence of these hydroxamate ions have been obtained and trends established. The affinity of aceto‐hydroxamic acid for Pu(IV) ions and its selectivity over U(VI) ions is demonstrated by the values of the stability constants in HClO4. These data support the applications of simple hydroxamic acids in advanced Purex‐type solvent extraction systems.

Deriving formation constants for aqueous metal complexes from XANES spectra: Zn2+ and Fe2+ chloride complexes in hypersaline solutions

Abstract: The development of numerical modeling of reactive transport relies on the availability of thermodynamic properties for the solid, surface, aqueous, and vapor species stable at the conditions of interest. The lack of experimental studies and comprehensive activity-composition models severely limits the predictive capabilities of these models in systems involving highly saline fluids or low-density volatile-rich fluids. X-ray absorption near-edge structure spectroscopy (XANES) is a powerful technique to study the speciation of transition metals in aqueous fluids: it is element specific; sensitive to the oxidation state of the metal, the ligand field and the coordination geometry of the complex; and it is suitable for measuring trace amounts of metals ( Formation constants for the aqueous complexes of transition metals can be determined from a series of XANES spectra obtained on solutions containing a constant amount of the metal of interest and variable concentrations of a ligand. The method relies on a non-linear, least-squares-fitting approach, with full distribution of species calculations based on a complete thermodynamic model for the experimental system under consideration. The technique is particularly suitable for following octahedral to tetrahedral transitions among weak chloride complexes of transition metals. The log K for the reaction Zn 2+ (octahedral) + 4Cl − = ZnCl 4 2− (tetrahedral) at 25 °C is retrieved to be 0.1(6), within error of the accepted literature value. The same method applied to Fe 2+ -chloride complexes shows that the log K for the reaction Fe 2+ (octahedral) + 4Cl − = FeCl 4 2− (tetrahedral) increases from −6.2(6) at 25 °C to −2.9(3) at 150 °C. This study confirms that tetrahedral chloride complexes play an important role in Fe transport in hypersaline brines especially at elevated temperatures, and shows that XANES is well suited to study systems that may be difficult to study with other techniques.

Synthesis of Novel Supramolecular Assemblies Based on Hyaluronic Acid Derivatives Bearing Bivalent β-Cyclodextrin and Adamantane Moieties

Abstract: New supramolecular assemblies based on hyaluronic acid stabilized by divalent β-cyclodextrin/adamantane (β-CD/AD) complexes were synthesized with the aim to compare their viscoelastic behavior in aqueous medium with systems resulting from monovalent complexes. For this purpose, dimers of AD and β-CD to be coupled to hyaluronic acid were prepared by multistep chemical synthetic pathways. Investigation of the complexation properties of these dimers by isothermal titration calorimetry (ITC) experiments revealed a pronounced increase of the stability constant. These results can be attributed to divalency as suggested by the value of the complexation enthalpy (ΔH0) which is twice that of the monovalent complex, in agreement with literature data. Nevertheless, a decrease of the complexation properties of these dimers when fixed on the polysaccharide backbone was observed as a result of an unfavorable entropy of binding. The viscoelastic properties of the supramolecular assemblies were examined by dynamic rheolo...

Imidazolate bridged Cu(II)-Cu(II) and Cu(II)-Zn(II) complexes of a terpyridinophane azamacrocycle: a solution and solid state study.

Abstract: The dinuclear Cu2+ and Zn2+ as well as the mixed Cu2+–Zn2+ complexes of a 5,5″-pentaazaterpyridinophane ligand (L) are able to incorporate imidazolate (Im−) as a bridging ligand. The crystal structure of [Cu2L(Im)(Br)(H2O)](CF3SO3)2·3H2O (1) shows one copper coordinated by the three pyridine nitrogens of the terpyridine unit, one nitrogen of the imidazolate bridge (Im−) and one bromide anion occupying the axial position of a distorted square pyramid. The second copper atom is coordinated by the remaining imidazolate nitrogen, the three secondary nitrogens at the centre of the polyamine bridge and one water molecule that occupies the axial position. Magnetic measurements have been performed in the 2.0–300.0 K temperature range. Experimental data could be satisfactorily reproduced by using an isotropic exchange model H = –JS1S2 with J = −52.3 cm−1 and g = 2.09. Potentiometric studies have provided details of the speciation and stability constants for the mixed Cu2+–L–HIm, Zn2+–L–HIm (HIm = imidazole) and Cu2+–Zn2+–L–HIm systems. The apparent stability constant obtained at pH = 9 for the addition of imidazole to the dinuclear Cu2+ complexes is one of the highest so far reported (log K = 7.5). UV-Vis spectroscopy and paramagnetic NMR data show that imidazole coordinates to the Cu2+ ions as a bridging imidazolate ligand from pH 5 to 10. Electrochemical reduction of the Cu2+–Zn2+–L complex occurs in two successive one-electron per copper ion quasi-reversible steps. The formal potential of the Cu2+–Zn2+–L/Cu+–Zn2+–L couple is close to that of SOD. The IC50 values measured at pH 7.8 by means of the nitro blue tetrazolium method show significant SOD activity for the dinuclear Cu2+ complexes (IC50 = 2.5 µM) and moderate activity for the Cu2+–Zn2+ mixed systems (IC50 = 30 µM).

Complex formation between poly(vinyl alcohol) and metallic ions in aqueous solution

Abstract: As the basic research for the study of complex reactions in polymers, the complex formation of poly(vinyl alcohol) (PVA) with copper and other metals was tried. Spectroscopy, pH titration, and elementary analysis were employed in the reaction of PVA and Cu(II) ions in aqueous solution. The complexing protonation was found in the higher pH region (above 7.0), and the maximum absorptions at 640 and 260 nm were observed. These ϵmax were proportional to the concentrations of Cu(II) ions bound to the hydroxyl groups of PVA. Formation constants of the PVA–Cu(II) complexes were measured by applying a modified Bjerrum's method. The effects of the ratio Tcu2+/THL, the anions in copper salts, and the degree of polymerization on the formation constants were studied. Four hydroxyl groups combined with one Cu(II) ion in the pH range above 7.0. The stability constants for the complexes of PVA with bivalent transition metallic ions were in agreement with the Irving-William's series. The changes in viscosity, solubility, tacticity, mechanochemical phenomenon, and the other properties based on the complex formation are discussed.

Study of Complex Formation between 4-(2-Pyridylazo) Resorcinol and Al3+, Fe3+, Zn2+, and Cd2+ Ions in an Aqueous Solution at 0.1 M Ionic Strength

Abstract: The complexation reactions between 4-(2-pyridylazo)resorcinol (PAR) and Al3+, Fe3+, Zn2+, and Cd2+ ions at 25 °C and 0.1 M ionic strength are performed spectrophotometrically. A critical comparison of the various PCA methods on the absorbance matrix data concerning the number of light-absorbing species has been made. Knowing the number of light-absorbing species is a critical step for subsequent quantitative and qualitative solution equilibria studies. Therefore, the nine selected index functions for the prediction of the number of light-absorbing components which contribute to a set of spectra are critically tested using the algorithms implemented in INDICES software. Behind the number of various complexes formed the stability constants of species ML and MLH, type log β110 and log β111, for the system of PAR (ligand L) with Zn2+ and Cd2+ ions and the stability constants of species MLH and ML2H, type log β111 and log β121, for the system of PAR (ligand L) with Al3+ and Fe3+ ions at 25 °C and 0.1 M ionic s...

Potentiometric and spectrophotometric studies of copper(II) complexes of some ligands in aqueous and nonaqueous solution

Abstract: Stoichiometry and equilibrium study of copper-ligands including mercaptobenzoxazole (MBO), 4-propyl 2-thiouracyl (PTU), methyl-2-pyridylketone oxime (MPKO), phenyl-2-pyridylketone oxime (PPKO), 4,6-dihydroxy-2-mercaptopyrimidine (DHMP), N,N'-phenylene bis(salicylaldimine) (PBS) and l,2-bis(2-hydroxyphenyl)naphtaldiimine (BHNPDI) were conducted in aqueous and nonaqueous solution by potentiometry and spectrophotometry. Stability constants of the complexes are determined at 25 ± 1 °C and 0.1 or 0.05 M ionic strength in water or acetonitrile solvents. Oximes ligand protonation constants and copper-ligands complexes' stability and hydrolysis constants were calculated using the BEST program in aqueous solution. The stability constants of copper-ligands complexes were calculated using the KINFIT program in acetonitrile solution. The results of these two methods are made self-consistent, then rationalized assuming an equilibrium model including the species, ML, MLH, MLOH and ML 2 (where the charges of the species have been ignored for the sake of simplicity) (L = MBO, PTU, MPKO, PPKO, DHMP, BHNPDI and PBS).