Showing papers on "Stability constants of complexes published in 2014"

Optical Chemosensors Based on Transmetalation of Salen-Based Schiff Base Complexes

Abstract: We report our systematic studies of novel, simple, selective, and sensitive optical (both colorimetric and fluorescent) chemosensors for detecting Al3+ based on transmetalation reactions (metal displacement or exchange reactions) of a series of K(I), Ca(II), Zn(II), Cu(II), and Pt(II) complexes containing different ligands of salen-based Schiff bases. Both the chemical structure of the salen ligand and the identity of the central metal ion have a tremendous impact on the sensing performance, which is mainly determined by the stability constant of the complex. Moreover, the selectivities of the salen-complex-based chemosensors are much better than those of the corresponding free salen ligands because of the shielding function of the filled-in metal ion in the complex. Therefore, the present work potentially provides a new and simple way to design optical probes via complex-based transmetalation reactions.

Universal reaction mechanism of boronic acids with diols in aqueous solution: kinetics and the basic concept of a conditional formation constant.

Abstract: To establish a detailed reaction mechanism for the condensation between a boronic acid, RB(OH)2, and a diol, H2L, in aqueous solution, the acid dissociation constants (Ka(BL)) of boronic acid diol esters (HBLs) were determined based on the well-established concept of conditional formation constants of metal complexes. The pKa values of HBLs were 2.30, 2.77, and 2.00 for the reaction systems, 2,4-difluorophenylboronic acid and chromotropic acid, 3-nitrophenylboronic acid and alizarin red S, and phenylboronic acid and alizarin red S, respectively. A general and precise reaction mechanism of RB(OH)2 with H2L in aqueous solution, which can serve as a universal reaction mechanism for RB(OH)2 and H2L, was proposed on the basis of (a) the relative kinetic reactivities of the RB(OH)2 and its conjugate base, that is, the boronate ion, toward H2L, and (b) the determined pKa values of HBLs. The use of the conditional formation constant, K', based on the main reaction: RB(OH)2 + H2L (K1)⇌ RB(L)(OH)(-) + H3O(+) instead of the binding constant has been proposed for the general reaction of uncomplexed boronic acid species (B') with uncomplexed diol species (L') to form boronic acid diol complex species (esters, BL') in aqueous solution at pH 5-11: B' + L' (K')⇌ BL'. The proposed reaction mechanism explains perfectly the formation of boronic acid diol ester in aqueous solution.

Selective fluorescence assay of aluminum and cyanide ions using chemosensor containing naphthol

Abstract: The selective assay of aluminum and cyanide ions is reported using fluorescence enhancement and quenching of a phenol–naphthol based chemosensor (PNI) in aqueous and nonaqueous solvents, respectively. PNI gave no significant fluorescence in water. The binding properties of PNI with metal ions were investigated by UV-vis, fluorescence, and electrospray ionization mass spectrometry in a Bis–Tris buffer solution. The addition of aluminum ions switches on the fluorescence of the sensor PNI in water, comparable to relatively very low fluorescence changes in the presence of various other metal ions. The complex stability constant (Ka) for the stoichiometric 1 : 1 complexation of PNI with aluminium ions was obtained by fluorimetric titrations and NMR experiments. However, upon treatment with cyanide ions, the fluorescence of PNI was selectively turned off and the yellow solution of PNI turned to red in methanol. Other comparable anions, such as F−, Cl−, Br−, I−, CH3COO−, and H2PO4−, afforded no apparent fluorescence quenching. The interaction of PNI with cyanide ions was studied by NMR experiments.

Thermodynamic data for Pb2+ and Zn2+ sequestration by biologically important S-donor ligands, at different temperatures and ionic strengths

Abstract: Thermodynamic parameters for the interactions of cysteine (cys) and penicillamine (psh) with Pb(II) and glutathione (gsh) with Pb(II) and Zn(II) were determined in NaNO3 or NaCl aqueous solution by potentiometry, at different ionic strengths (0 < I ≤ 1 mol L−1) and temperatures (15 ≤ t ≤ 45 °C). For Pb2+ systems, the formation of PbL, PbLH, PbLH2 and PbL2 species was found, together with PbLOH for cys and psh, and PbL2H and PbL2H2 for gsh. The speciation models for Pb2+–psh and –gsh systems were confirmed by UV spectrophotometric measurements. For the Zn2+–gsh system, a more complex speciation model was obtained with the formation of ZnL, ZnLH, ZnLH2, ZnL2, ZnL2H, ZnL2H2, ZnLOH and ZnL2OH species. From the dependence of formation constants on temperature, rough ΔH values were evaluated: the main contribution to the complexation free energy is entropic in nature, with small enthalpic values. Moreover, from the dependence on ionic strength, formation constants extrapolated at I = 0 mol L−1 were obtained. The sequestering ability of the ligands towards Pb2+ and Zn2+ was evaluated by determining the pL0.5, i.e. the −log of the concentration of the ligand able to complex half of the metal ion fraction. All these ligands show good sequestering ability. For Pb2+–cys, –psh and –gsh systems, pL0.5 reaches the value of 8.2, 9.0 and 5.9, respectively, at pH = 7, I = 0.1 mol L−1 and t = 25 °C. Under the same experimental conditions, the sequestering ability of gsh towards Zn2+ is lower, with pL0.5 = 4.1.

Acid Dissociation Constants and Rare Earth Stability Constants for DTPA

Abstract: Diethylenetriamine-N,N,N′,N″,N″-pentaacetic acid (DTPA) is an octadentate aminopolycarboxylate complexing agent whose f-element complexes find important practical applications in nuclear medicine and in advanced nuclear fuel reprocessing. This investigation focuses primarily on the latter application, specifically on characterization of lanthanide–DTPA complexes of relevance to the Trivalent Actinide–Lanthanide Separations by Phosphorus reagent Extraction and Aqueous Komplexants (TALSPEAK) process. To function acceptably, the TALSPEAK process requires the presence of moderate concentrations (0.5–2.0 mol·L−1) of a (Na+/H+) lactate (or citrate) buffer. Competition between DTPA, lactate, and the extractant bis(2-ethylhexyl)phosphoric acid (HDEHP) for the lanthanides and trivalent actinides governs the course of the extraction process. To facilitate modeling and to support process improvements, the acid dissociation constants and stability constants for rare earth complexes with DTPA have been determined in 2.0 mol·L−1 ionic strength (NaClO4) media. The acid dissociation constants for DTPA and the stability constant for [Eu(DTPA)]2− also were determined in sodium trifluoromethanesulfonate at 2.0 mol·L−1 ionic strength to evaluate the potential impact of changing the nature of the electrolyte. The thermodynamic data are compared with earlier reports of similar data at lower ionic strength and used to complete calculations exploring the relative stability of lanthanide–DTPA and lactate complexes under TALSPEAK extraction conditions. Lanthanide–DTPA stability trends are discussed in comparison with literature data on a variety of other metal ions.

Electrode Reaction and Mass-Transport Mechanisms Associated with the Iodide/Triiodide Couple in the Ionic Liquid 1-Ethyl-3-methylimidazolium Bis(trifluoromethanesulfonyl)imide

Abstract: The oxidation and reduction processes associated with the iodide/triiodide (I–/I3–) couple have been investigated in the room temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide using cyclic voltammetry, convolution voltammetry, and chronoamperometry. Analogous to the case in acetonitrile, two processes with a relative electron stoichiometry of 2:1 are observed at a platinum electrode and are assigned to the I–/I3– and I3–/I2 processes at lower and higher potentials, respectively. The electro-oxidation of I– has been simulated using a termolecular charge transfer mechanism: I2 + 2e– ⇌ I– + I– (E°, ks, α), I2 + I– ⇌ I3– (Keq, kf), where E°, ks, α, Keq, and kf are the standard potential, standard heterogeneous electron-transfer rate constant, charge transfer coefficient, equilibrium (stability) constant, and bimolecular (forward) rate constant, respectively. The stability constant of I3– in this ionic liquid (Keq = 2.3 × 106, estimated by simulation) is comparable to that...

Effect of chemical parameters on the interaction between cationic dyes and poly(acrylic acid)

Abstract: Being polymers and electrolytes, polyelectrolytes are widely used in many applications. In perspective of dye removal from textile effluents, they have been used in coagulation–flocculation and to a lesser extent in polyelectrolyte enhanced ultrafiltration. To optimize the abovementioned process we need to study in detail the interaction of dye with polyelectrolyte in conditions near the industry reality. In this context we investigate herein the effect of chemicals additives, NaCl, surfactant, pH on the behavior of two model cationic dyes, methylene blue (MB) and toluidine blue (TB), with polyacrylic acid polymer (PAA). Using spectrophotometric method it was found that the poly(acrylic acid) induced metachromasy in the dyes resulting by the blue shift of the absorbance confirming the formation of dye polymer complexes. The stoichiometry of MB–PAA and TB–PAA complexes determined by the ratio method was found to be 2:1. The stability constant of the complexes at 298 K was found to be 5332 for MB–PAA and 4358 dm3 mol−1 for TB–PAA and the amount of additives inducing total destabilization of the complex were determined.

Modular syntheses of H4octapa and H2dedpa, and yttrium coordination chemistry relevant to 86Y/90Y radiopharmaceuticals

Abstract: The ligands H2dedpa, H4octapa, p-SCN-Bn-H2dedpa, and p-SCN-Bn-H4octapa were synthesized using a new protection chemistry approach, with labile tert-butyl esters replacing the previously used methyl esters as protecting groups for picolinic acid moieties. Additionally, the ligands H2dedpa and p-SCN-Bn-H2dedpa were synthesized using nosyl protection chemistry for the first time. The use of tert-butyl esters allows for deprotection at room temperature in trifluoroacetic acid (TFA), which compares favorably to the harsh conditions of refluxing HCl (6 M) or LiOH that were previously required for methyl ester cleavage. H4octapa has recently been shown to be a very promising 111In and 177Lu ligand for radiopharmaceutical applications; therefore, coordination chemistry studies with Y3+ are described to assess its potential for use with 86Y/90Y. The solution chemistry of H4octapa with Y3+ is shown to be suitable via solution NMR studies of the [Y(octapa)]− complex and density functional theory (DFT) calculations of the predicted structure, suggesting properties similar to those of the analogous In3+ and Lu3+ complexes. The molecular electrostatic potential (MEP) was mapped onto the molecular surface of the DFT-calculated coordination structures, suggesting very similar and even charge distributions between both the Lu3+ and Y3+ complexes of octapa4−, and coordinate structures between 8 (ligand only) and 9 (ligand and one H2O). Potentiometric titrations determined H4octapa to have a formation constant (log KML) with Y3+ of 18.3 ± 0.1, revealing high thermodynamic stability. This preliminary work suggests that H4octapa may be a competent ligand for future 86Y/90Y radiopharmaceutical applications.

Ion-pair formation in aqueous strontium chloride and strontium hydroxide solutions under hydrothermal conditions by AC conductivity measurements

Abstract: Frequency-dependent electrical conductivities of solutions of aqueous strontium hydroxide and strontium chloride have been measured from T = 295 K to T = 625 K at p = 20 MPa, over a very wide range of ionic strength (3 × 10−5 to 0.2 mol kg−1), using a high-precision flow AC conductivity instrument. Experimental values for the concentration-dependent equivalent conductivity, Λ, of the two electrolytes were fitted with the Turq–Blum–Bernard–Kunz (“TBBK”) ionic conductivity model, to determine ionic association constants, KA,m. The TBBK fits yielded statistically significant formation constants for the species SrOH+ and SrCl+ at all temperatures, and for Sr(OH)02 and SrCl02 at temperatures above 446 K. The first and second stepwise association constants for the ion pairs followed the order KA1(SrOH+) > KA1(SrCl+) > KA2[Sr(OH)02] > KA2[SrCl02], consistent with long-range solvent polarization effects associated with the lower static dielectric constant and high compressibility of water at elevated temperatures. The stepwise association constants to form SrCl+ agree with previously reported values for CaCl+ to within the combined experimental error at high temperatures and, at temperatures below ∼375 K, the values of log10KA1 for strontium are lower than those for calcium by up to ∼0.3–0.4 units. The association constants for the species SrOH+ and Sr(OH)02 are the first accurate values to be reported for hydroxide ion pairs with any divalent cation under these conditions.

Lower denticity leading to higher stability: structural and solution studies of Ln(III)-OBETA complexes.

Abstract: The heptadentate ligand OBETA (2,2'-oxybis(ethylamine)-N,N,N',N'-tetraacetic acid) was reported to form complexes with Ln(3+) ions more stable than those formed by the octadentate and more popular congener EGTA (ethylene glycol O,O'-bis(ethylamine)-N,N,N',N'-tetraacetic acid). The structural features leading to this puzzling coordination paradox were investigated by X-ray diffraction, solution state NMR, molecular modeling, and relaxometric studies. The stability constant of Gd(OBETA) (log KGdL = 19.37, 0.1 M KCl) is 2 orders of magnitude higher than that of the higher denticity analogue Gd(EGTA) (log KGdL = 17.66, 0.1 M KCl). The half-lives (t1/2) for the dissociation reactions of Gd(OBETA) and Gd(EGTA) ([Cu(2+)]tot = 0.2 mM, [Cit(3-)]tot = 0.5 mM, [PO4(3-)]tot = 1.0 mM, and [CO3(2-)]tot = 25 mM at pH = 7.4 and 25 °C in 0.1 M KCl solution) are 6.8 and 0.63 h, respectively, reflecting the much higher inertness of Gd(OBETA) near physiological conditions. NMR studies and DFT calculations using the B3LYP functional and a large-core ECP indicate that the [Gd(OBETA)(H2O)2](-) complex most likely exists in solution as the Δ(λλ)(δδδδ)A/Λ(δδ)(λλλλ)A enantiomeric pair, with an activation free energy for the enantiomerization process of ∼40 kJ·mol(-1). The metal ion is nine-coordinate by seven donor atoms of the ligand and two inner-sphere water molecules. The X-ray crystal structure of [C(NH2)3]3[Lu(OBETA)(CO3)]·2H2O is in agreement with the predictions of DFT calculations, the two coordinated water molecules being replaced by a bidentate carbonate anion. The (1)H NMRD and (17)O NMR study revealed that the two inner-sphere water molecules in Gd(OBETA) are endowed with a relatively fast water exchange rate (kex(298) = 13 × 10(6) s(-1)). The higher thermodynamic stability and inertness of Ln(OBETA) complexes, peaking in the center of the 4f series, combined with the presence of two coordinated water molecules suggests that Gd(OBETA) is a promising paramagnetic probe for MRI applications.

A Macrocyclic Chelator with Unprecedented Th4+ Affinity

Abstract: A novel macrocyclic octadentate ligand incorporating terephthalamide binding units has been synthesized and evaluated for the chelation of Th4+. The thorium complex was structurally characterized by X-ray diffraction and in solution with kinetic studies and spectrophotometric titrations. Dye displacement kinetic studies show that the ligand is a much more rapid chelator of Th4+ than prevailing ligands (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid and diethylenetriaminepentaacetic acid). Furthermore, the resulting complex was found to have a remarkably high thermodynamic stability, with a formation constant of 1054. These data support potential radiotherapeutic applications.

Role of basic sites of substituted ferrocenes in interaction with the trinuclear 3,5-bis(trifluoromethyl)pyrazolates: thermodynamics and structure of complexes

Abstract: Formation of complexes of the macrocycles (ML)3, where L = 3,5-(CF3)2Pz = 3,5-bis(trifluoromethyl)pyrazolate, M = Cu and Ag, and the acylferrocenes FcC(O)CH2R (Fc = (C5H5)Fe(C5H4); R = H (1), Ph (2)) was studied by means of variable temperature IR, UV-vis, NMR spectroscopy. The sole site of coordination in solution is the oxygen atom of the CO group. The complex composition (1 : 1) and thermodynamic parameters in hexane solution were determined, the formation constants and the enthalpies decreasing from 1 to 2 and from Ag to the Cu macrocycle. The same coordination site featuring triple coordination of oxygen to all metal atoms of a macrocycle was found in the solid state by single crystal X-ray diffraction. There are no shortened contacts of the metal in the macrocycles with π-electron system of the ferrocene's cyclopentadienyl ligands in all complexes. The complexes of (ML)3 with 1 have 1 : 2 composition and bipyramidal structure whereas 2 forms the 1 : 1 complex with (AgL)3. The latter is packed in the infinite stacks involving additional contacts with Ph groups.

Trace Amounts of Aqueous Copper(II) Chloride Complexes in Hypersaline Solutions: Spectrophotometric and Thermodynamic Studies

Abstract: Knowledge of the thermodynamic properties of aqueous copper(II) chloride complexes is important for understanding and quantitatively modeling trace copper behavior in hydrometallurgical extraction processing. In this paper, UV–Vis spectra data of Cu(II) chloride solutions with various salinities (NaCl, 0–5.57 mol·kg−1) are collected at 25 °C. The concentration distribution of Cu–Cl species is in good agreement with those calculated by a reaction model (RM). The simple hydrated ion, Cu2+, is dominant at low concentration, whereas [CuCl]+, [CuCl2]0 and [CuCl3]− become increasingly important as the chloride concentration rises. Moreover, the RM calculation suggests the present of a small amount of [CuCl4]2−. The de-convoluted molar spectrum of each species is in excellent agreement with our previous theoretical results predicted by time-dependent density functional theory treatment of aqueous Cu-containing systems. The formation constants for these copper chloride complexes have been reported and are to be preferred, except log10 K 2 ([CuCl2]0).