Complex Formation in the Region of Metal Hydrolysis and M(OH)2 Precipitation. A Glass Electrode Potentiometric and Polarographic Study of Cd–(AMPSO)x–(OH)y and Zn–(AMPSO)x–(OH)y Systems

The interaction between cadmium or zinc and AMPSO was investigated by DCP and GEP, at fixed total ligand to total metal concentration ratios and various pH values, at 25.0 °C and 0.1 M KNO₃ ionic strength. For Cd–(AMPSO)_x–(OH)_y system, CdL and CdL(OH) species, were identified, with stability constants values set to (as log β): 2.1±0.1 and 6.2±0.2, respectively. For Zn–(AMPSO)_x–(OH)_y system, the proposed final model with stability constants set to (as log β) is: ZnL=2.5±0.1 and ZnL(OH)₂=12.9±0.2. For both systems, the fact that AMPSO deprotonation occurs in the metal hydrolysis and M(OH)₂ precipitation and the complexes formed are not too strong added a real challenge to data interpretation.     

Determination of the stability constants of Pb–(DIPSO)x–(OH)y and Pb–(AMPSO)x–(OH)y systems

In this work, complexation between lead ion and the ligands 3-[N,N-bis(2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid (DIPSO) and N-(1,1-dimethyl-2-hydroxyethyl)-3-amino-2-hydroxypropanesulfonic acid (AMPSO), which are commercial pH buffers, is presented. Both ligands form complexes with lead in their pH buffer range (between pH 6.5 and 8.5 for DIPSO and between pH 8.0 and 9.0 for AMPSO). The final models and the overall stability constants, which are reported here, were determined by direct current polarography and glass electrode potentiometry [only for the Pb–(DIPSO)_x–(OH)_y system] at 25.0 °C and 0.1 M KNO₃ ionic strength. For the Pb–(DIPSO)_x–(OH)_y system, the proposed final model contains PbL, PbL₂, PbL₂(OH), and PbL₂(OH)₂ with stability constants, as log β, of 3.4 ± 0.1, 6.35 ± 0.15, 12.8 ± 0.2, and 18.0 ± 0.3, respectively. For the Pb–(AMPSO)_x–(OH)_y system, the species observed are PbL, PbL(OH), and PbL(OH)₂ with stability constants, as log β, of 2.9 ± 0.5, 9.4 ± 0.1, and 14.5 ± 0.2, respectively. For AMPSO, the possible adsorption of the ligand at the mercury electrode surface was evaluated by alternating current polarography through calculation of the capacitance of the double layer.     

Challenges in modelling and optimisation of stability constants in the study of Cu-(TAPS)x-(OH)y system by polarography

This work describes the application of polarography, a technique scarcely used for modelling and optimisation of stability constants, in the study of copper complexes with [(2-hydroxy-1,1-bis(hydroxymethyl)ethyl)amino]-1-propanesulfonic acid (TAPS). Direct current polarography (DCP), using low total copper ion and large total ligand to total copper concentration, enabled the full characterization of Cu-(TAPS)_x-(OH)_y system, whose complexation occurs in the pH range of copper hydrolysis and Cu(OH)₂ precipitation. Cu-(TAPS)_x-(OH)_y system was studied by DCP and glass electrode potentiometry (GEP) in aqueous solution at fixed total ligand to total metal concentrations ratios and varied pH values (25.0 °C; I = 0.1 M, KNO₃). The predicted model, as well as the overall stability constants values, are (as log β): CuL⁺ = 4.2, CuL₂ = 7.8, CuL₂(OH)⁻ = 13.9 and CuL₂(OH)₂²⁻ = 18.94. GEP only allowed confirming the stability constants for CuL⁺ and CuL₂ and was used to determine the pKa of TAPS, 8.342.Finally, a briefly comparative analysis between TAPS and other structural related buffers was done. Evaluation based on log β_CuL versus pKa revealed that TES, TRIS, TAPS and AMPSO coordinated via amino and hydroxymethylgroups forming a five-membered chelate ring. For BIS-TRIS and TAPSO, and possibly DIPSO, one or more five-membered chelate rings involving additional hydroxyl groups are also likely formed.     

Potentiometric study of the protonation and binary complexation equilibria between the hydrogen ion,copper(II) ion and zinc(II) ion and the picolinate ion in dioxane-water mixtures

Protonation constants of picolinic acid and stability constants of Cu(II) and Zn(II) picolinate complexes were determined potentiometrically in 50% (v/v) dioxane-water solution at 25°C and 0.2 M KNO₃. The values obtained for the constants were: protonation constants for picolinate ion: logβ₁ = 5.36±0.01 and logβ₂ = 6.80±0.04; stability constants for copper(II) complexes: logβ_1.1 = 7.766±0.001 and logβ_1.2 = 16.826±0.007; stability constants for the Zn(II) complexes: logβ_1.1 = 6.10±0.05, logβ_1.2 = 11.47±0.03 and logβ_1.3 = 15.77±0.08. No protonated nor hydroxo-complex was detected in the metal ion-picolinate systems.     

Evaluation of Vanadium(IV) as a Non-radioactive Surrogate for Technetium(IV) by Comparison of Stability Constants for Polyamino Polycarboxylate Ligand Complexation

The stability constants for the Tc(IV) and V(IV) complexation with the polyamino polycarboxylate ligands IDA, NTA, HEDTA and DTPA were determined using liquid–liquid extraction techniques. These stability constants were then used to evaluate the validity of using V(IV) as a chemical analogue for Tc(IV). Results suggest that Tc(IV), as TcOOH⁺, will form β _1?11 complexes with the selected ligands, while V(IV), as VO²⁺, will form β ₁₀₁ complexes. The values for these determined stability constants are (in log₁₀ unit) 10.9 ± 0.1, 11.4 ± 0.1, 14.9 ± 0.1, and 20.1 ± 0.1 for Tc(IV) in 0.5 mol·L^?1 NaCl at 25 °C, for IDA, NTA, HEDTA and DTPA, respectively, they are 9.3 ± 0.1, 11.6 ± 0.2, 15.8 ± 0.1, and 20.8 ± 0.1 for V(IV) in 0.5 mol·L^?1 NaCl at 25 °C, for the same suite of ligands. The incorporation of a hydroxide into the metal ligand complexes formed by Tc(IV) is proposed as the largest factor differentiating the apparent stability constants of Tc(IV) and V(IV). This work shows that V(IV) is a poor analog for Tc(IV); however, despite the differences in complexation mechanism between V(IV) and Tc(IV), V(IV) still appears to have some use for predicting Tc(IV) complexation behavior.     

Hydrolytic Behavior of La<Superscript>3+</Superscript> and Sm<Superscript>3+</Superscript> at Various Temperatures

The hydrolytic species of lanthanide ions, La³⁺ and Sm³⁺, in water at I = 0.1 mol·dm^?3 KCl ionic strength and temperatures of 298.15, 310.15 and 318.15 K were investigated by potentiometry. The hydrolytic species were modeled by the HySS simulation program. From the results, the hydrolytic species of each metal ion at different temperatures were calculated using the program HYPERQUAD2013. The hydrolysis constants (log₁₀ β) of [La(OH)]²⁺ and La(OH)₃ were calculated as ?8.52 ± 0.46, ?26.84 ± 0.48, and log₁₀ β values of [Sm(OH)]²⁺, [Sm(OH)₂]⁺, Sm(OH)₃ were calculated as ?7.11 ± 0.21, ?15.84 ± 0.25 and ?23.44 ± 0.52 in aqueous media at 298.15 K, respectively. The dependence of the hydrolysis constants on the temperature allowed us to calculate the enthalpy, entropy, and Gibbs energy of hydrolysis values of each species.     

Potentiometric and Theoretical Studies of (2Z, 3Z)-2H-benzo[b][1,4]thiazine-2,3(4H)-dionedioxime with Some Divalent Transition Metal Ions

The protonation equilibria of (2Z, 3Z)-2H-benzo[b][1,4]thiazine-2,3(4H)-dionedioxime (BTDH₂) together with the equilibria of its bis- binary complexes of Co(II), Ni(II), Cu(II) and Zn(II) were investigated potentiometrically. The investigation was carried out at 25 ± 0.1 °C, in aqueous solution, with a constant ionic strength of 0.100 mol·dm^?3 NaCl. The protonation constants of the ligand together with the stability constants of a variety of complexes were determined potentiometrically in 10 % ethanol–water mixed solution using the SUPERQUAD computer program. Theoretical calculations were set up to assist in understanding the protonation sequence in the ligand molecule via the semi-empirical molecule orbital method of parameterized model number 3. Results are discussed in connection to the basicity of the donor atoms and structural arrangement of the ligand. Although BTDH₂ has two dissociable protons, four protonation constants can be measured under the experimental conditions presented. These four protonation constants (as log₁₀ βs) are 10.245, 19.397, 22.414 and 25.176.     

Effect of Solvent Composition on Protonation Constants of Some Glycine Dipeptides

Stoichiometric protonation constants (log₁₀ K ₁ and log₁₀ K ₂) of some aliphatic dipeptides (Gly–Tyr, Gly–Phe, Gly–Val, Gly–Leu, Gly–Thr, Gly–Met and Gly–Pro) were determined potentiometrically in 20, 40, and 60 % (v/v) 1,4-dioxane–water and dimethyl sulfoxide–water mixtures at 25.0 (±0.1) °C with an ionic strength of 0.10 mol·L^?1 sodium chloride. The protonation constants were calculated with the computer program PKAS and selection of the best fit chemical models is based on the statistical parameters. The effects of solvent composition on these protonation constants are discussed to determine the factors which control these processes. It has been observed that, while the correlation between log₁₀ K ₁ and log₁₀ K ₂ with the percentages of dimethyl sulfoxide in the dimethyl sulfoxide–water mixtures are not linear, these values linearly increase as the concentration of 1,4-dioxane increases in the solvent mixtures.     

Complexation of M–(buffer) x –(OH) y Systems Involving Divalent Ions (Cobalt or Nickel) and Zwitterionic Biological Buffers (AMPSO, DIPSO, TAPS and TAPSO) in Aqueous Solution

The complexation behavior of eight M–(buffer) _x –(OH) _y systems involving two divalent ions (cobalt and nickel) and four zwitterionic biological buffers (AMPSO, DIPSO, TAPS and TAPSO) were characterized. Complex formation was detected for all eight M–(buffer) _x –(OH) _y systems studied, but fully defined final models were obtained for only four of these systems. For systems involving cobalt or nickel with AMPSO or TAPS, a complete characterization of the systems was not possible in the studied buffer pH-range. Metal complexation was studied by glass-electrode potentiometry (GEP) and UV-Vis spectroscopy at 25.0 °C and I=0.1 mol⋅dm⁻³ KNO₃ ionic strength. For the Ni–(L) _x –(OH) _y and Co–(L) _x –(OH) _y systems, with L = TAPSO or DIPSO, the proposed final models and overall stability constants were obtained by combining results from both techniques. For the Ni–(L) _x –(OH) _y systems, the measured values of the stability constants are log ₁₀ β _NiL=3.0±0.1 and log ₁₀ β _NiL2=4.8±0.1 for L = TAPSO, and log ₁₀ β _NiL=2.7±0.1 and log ₁₀ β _NiL2=4.6±0.1 for L = DIPSO. For the Co–(L) _x –(OH) _y systems, the overall stability constants are log ₁₀ β _CoL=2.2±0.1, log ₁₀ β _CoL2=3.6±0.2 and log ₁₀ β _CoL(OH)=7.6±0.1 for L = TAPSO, and log ₁₀ β _CoL=2.0±0.1 and log ₁₀ β _CoL(OH)=7.8±0.1 for L = DIPSO. For both buffers, the CoL(OH) species is characterized by a major structural rearrangement.     

Coordination and inclusion compounds formed by addition of quinoline (Q) or isoquinoline (Iq) to a metal(II) dibenzoylmethanate (Co, Ni, Zn, Cd)

Three isomorphous series of new compounds are reported: complexes [M(DBM)₂Q₂] and [M(DBM)₂Iq₂] (M = M(II) = Co, Ni, Zn, Cd; DBM is C₆H₅COCHCOC₆H₅ ^?) and inclusion compounds [M(DBM)₂Q₂]*Q (M = Co, Zn, Cd). All the compounds comprise a trans configured octahedral complex molecule. Inclusion compounds of modified Zn and Cd DBM complexes are reported for the first time and their inclusion ability is attributed to the trans isomeric state induced by the bulky Q or Iq ligand. The TG measurements indicate the following order of thermal stability of the complexes defined by the strength of the metal–ligand bonds: Ni > Co > Cd > Zn. The inclusion compounds do not follow this trend.     

Modelling of Pb-(TAPS)x-(OH)y system and refinement of stability constants in the region of lead hydrolysis and lead hydroxide precipitation

The influence of [(2-hydroxy-1,1-bis(hydroxymethyl)ethyl)amino]-1-propanesulfonic acid (TAPS) on solutions containing lead(II) was studied by direct current polarography (DCP) and glass electrode potentiometry (GEP). The readings were taken at fixed total TAPS to total lead(II) concentration ratios and various pH values, at 25.0 ± 0.1 °C and ionic strength 0.1 M KNO₃.Due to the basic pK_a of the ligand, which occurs in the pH range where large amount of lead polynuclear species are formed, and the occurrence of ligand adsorption, that disabled the use of high concentrations of TAPS on DCP experiments, GEP and DCP experimental conditions were put to the limit in order to provide the correct Pb-TAPS-OH model and reliable stability constants.The proposed final model is: PbL, PbL₂, PbL₂(OH) and PbL₂(OH)₂ with overall stability constants values, as log β, 3.27 ± 0.06, 6.5 ± 0.1, 12.7 ± 0.1 and 17.27 ± 0.06, respectively.A comparative analysis of the strength of complexation of TAPS and a structural related buffer, 2-hydroxy-3-[tris(hydroxymethyl)methylamino]-1-propanesulfonic acid (TAPSO), with lead is also discussed.     

Hydrolysis of platinum(II) phosphine complexes in aqueous 1.0 M NaNO3 medium

Hydrolysis of [Pt(PR₃)₂]²⁺ (R=Me, Et) have been investigated at 25.0°C in 1.000 M NaNO₃ by the combined emf–NMR method. Quantitative analysis of the emf and ³¹P NMR data revealed the formation of [{Pt(PR₃)₂(μ-OH)}₂]²⁺ as the only hydrolysis product in the range 1.5<−log h<5.7. Least-squares calculation gave log β_Me=−4.19±0.04 and log β_Et=−3.58±0.04, where β_Me and β_Et stand for the formation constants of the dimeric cations of methyl and ethyl derivatives, respectively. The ¹⁹⁵Pt NMR data were also consistent with this model.     

New cyclen derivative ligand for thorium(IV) separation by solvent extraction

A new N-containing ligand, 1,4,7,10-tetra-(4-nitrobenzyl)-1,4,7,10-tetraazacyclo-dodecane (L), was synthesized, and its structure was determined by ¹H NMR, high resolution mass spectrometry and X-ray diffraction. L crystallized in the monoclinic system (P2₁/n space group; a = 7.7895(2) Å, b = 22.9592(5) Å, c = 9.9204(2) Å; α = 90.00°, β = 105.481(3)°, γ = 90.00°; Z = 2). Slope analysis and the continuous variation method demonstrated that 1:2 complexes between Th(IV) and L are formed; furthermore, the XPS analysis suggested that two oxygen atoms might be provided by two water molecules and that eight nitrogen atoms might be provided by two L molecules to form a ten-coordinate compound with Th(IV). The extraction equilibrium constant for the complex formation between Th(IV) and L was logK _ex = 6.95 ± 0.15 (25 °C), and the Gibbs free energy, ΔG ^o (25 °C), of the 1:2 Th–L complex in dichloromethane was ?39.56 kJ/mol. The L ligand in dichloromethane only slightly extracted Th(IV) from HNO₃ solution at pH = 1–3; however, an extraction efficiency of E = 94.9 ± 0.3 % was observed at pH = 4.63. The selectivity of L for the Th(IV) cation over other cations (i.e., Cs(I), Sr(II), Y(III), La(III), Sm(III), Eu(III), U(VI), and ²⁴¹Am(III)) was evaluated. Furthermore, the stripping experiments showed that the stripping agent (0.5 mol/L Na₂CO₃ + 0.1 mol/L EDTA) could provide an optimal condition for stripping thorium, and thorium recovery was up to 91.6 ± 0.1 %.     

Thermodynamic Study on the Protonation and Na<Superscript>+</Superscript>, Ca<Superscript>2+</Superscript>, Mg<Superscript>2+</Superscript>-Complexation of a Biodegradable Chelant (HEIDA) at Different Ionic Strengths and Temperatures

A potentiometric method has been used for the determination of the protonation constants of N-(2-hydroxyethyl)iminodiacetic acid (HEIDA or L) at various temperatures 283.15?≤?T/K?≤?383.15 and different ionic strengths of NaCl_(aq), 0.12?≤?I/mol·kg^?1?≤?4.84. Ionic strength dependence parameters were calculated using a Debye–Hückel type equation, Specific Ion Interaction Theory and Pitzer equations. Protonation constants at infinite dilution calculated by the SIT model are \( \log_{10} \left( {{}^{T}K_{1}^{\text{H}} } \right) = 8.998 \pm 0.008 \) (amino group), \( \log_{10} \left( {{}^{T}K_{2}^{\text{H}} } \right) = 2.515 \pm 0.009 \) and \( \log_{10} \left( {{}^{T}K_{3}^{\text{H}} } \right) = 1.06 \pm 0.002 \) (carboxylic groups). The formation constants of HEIDA complexes with sodium, calcium and magnesium were determined. In the first case, the formation of a weak complex species, NaL, was found and the stability constant value at infinite dilution is log₁₀K_NaL?=?0.78?±?0.23. For Ca²⁺ and Mg²⁺, the CaL, CaHL, CaL₂ and MgL species were found, respectively. The calculated stability constants for the calcium complexes at T?=?298.15 K and I?=?0.150 mol·dm^?3 are: log₁₀β_CaL?=?4.92?±?0.01, log₁₀β_CaHL?=?11.11?±?0.02 and \( \log_{10} \beta_{\text{Ca{L}}_{2}} \)?=?7.84?±?0.03, while for the magnesium complex (at I?=?0.176 mol·dm^?3): log₁₀β_MgL?=?2.928?±?0.006. Protonation thermodynamic functions have also been calculated and interpreted.     

Fluorescence signaling of aromatic oxoanion inclusion within metal-ion activated molecular receptor complexes formed from 2-(9-anthracenylmethylamino)ethyl-appended cyclen

Our observations that 1-[2-[(9-anthracenylmethylamino)ethyl)-4,7,10-tris[(2S)-2-hydroxy-3-phenoxypropyl]-1,4,7,10-tetraazacyclododecane, L¹, complexes Cd(II) to form fluorescent [CdL¹]²⁺ which undergoes a fluorescence change when it acts as an aromatic anion receptor complex has caused us to explore further the potential development of an interesting sequestration/sensor system. Accordingly, three new, octadentate, fluorescent, macrocyclic ligands, 1-[2-[(9-anthracenylmethyl)((2S)-2-hydroxy-3-phenoxypropyl)amino]ethyl]-4,7,10-tris[(2S)-2-hydroxy-3-phenoxypropyl]-1,4,7,10-tetraazacyclododecane, (L²), 1-[2-[(9-anthracenyl-methyl)((2S)-2-hydroxy-3-(4??-methyl)phenoxypropyl)amino]ethyl]-4,7,10-tris[(2S)-2-hydroxy-3-(4??-methyl)phenoxypropyl]-1,4,7,10-tetraazacyclododecane, (L³), and 1-[2-[(9-anthracenylmethyl)((2S)-2-hydroxy-3-(4??-t-butyl)phenoxypropyl)amino]ethyl]-4,7,10-tris[(2S)-2-hydroxy-3-(4??-t-butyl)phenoxypropyl]-1,4,7,10-tetraazacyclododecane, (L⁴), have been prepared with a view to using their metal complexes to study aromatic anion sequestration. The eight-coordinate Cd(II) complexes of L² and L³, [CdL²](ClO₄)₂·5H₂O and [CdL³](ClO₄)₂·2H₂O·2Et₂O are both capable of acting as receptors for a range of aromatic oxoanions. This is demonstrated by perturbation of the anthracene derived fluorescence emission intensity as the guest aromatic oxoanion and the receptor complex combine. In 20% aqueous 1,4-dioxane the receptor complex/aromatic oxoanion association constants are in the range of 10^3.2 M^?1 (guest = p-hydroxybenzoate) to 10^7.3 M^?1 (guest = 3,4,5-trihydroxybenzoate).     

Complexation of the cesium cation with nonactin: extraction and DFT study

From extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium Cs⁺(aq) + A^?(aq) + 1(nb) ? 1·Cs⁺(nb) + A^?(nb) taking place in the two-phase water–nitrobenzene system (A^? = picrate, 1 = nonactin; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log K _ex (1·Cs⁺,A^?) = 2.8 ± 0.1. Further, the stability constant of the 1·Cs⁺ complex in nitrobenzene saturated with water was calculated for a temperature of 25 °C: log β _nb (1·Cs⁺) = 4.7 ± 0.1. Finally, by using quantum–mechanical DFT calculations, the most probable structure of the resulting cationic complex species 1·Cs⁺ was derived.     

Challenges in Modelling and Optimization of Stability Constants in the Study of Metal Complexes with Monoprotonated Ligands. Part II

The influence of 2‐hydroxy‐3‐[(2‐hydroxy‐1,1‐dimethylethyl)amino]propane‐1‐sulfonic acid (AMPSO=HL) on systems containing copper(II) was studied by glass‐electrode potentiometry (GEP) and direct‐current polarography (DCP), at fixed total‐ligand‐to‐total‐metal‐concentration ratios and various pH values (25°, 0.1M KNO₃ medium). The predicted model ([CuL]⁺, [CuL(OH)], [CuL₂], [CuL₂(OH)]^?, [CuL₂(OH)₂]^2?, and [CuL₃]^?) and the overall stability constants for species found were obtained by combining results from both electrochemical techniques. The last five complexes are reported for the first time. For the species [CuL]⁺, [CuL₂], [CuL₃]^?, and [CuL₂(OH)₂]^2?, it was possible to determine stability constants with reasonable certainty and their values, as log β, were found to be 4.62±0.04, 9.5±0.1, 13.4±0.1, and 21.2±0.1, respectively. For the species [CuL(OH)] and [CuL₂(OH)]^?, stability constants 11.7±0.2 and 15.6±0.2, respectively, are presented as indicative values. It was demonstrated that AMPSO buffer may decrease the Cu²⁺ concentration by ten orders of magnitude by forming complexes with Cu²⁺. For the first time, the correction in DCP waves for the adsorption of the ligand and quasi‐reversibility of the metal allowed to determine stability‐constant values that are in good agreement with the values obtained by GEP. The importance of graphic analysis of data and significance of employing two analytical techniques was demonstrated; neither GEP nor DCP would be able to provide the correct M/L/OH^? model and reliable stability constants when used independently.     

Composition and stability constants of copper(II) complexes with succinic acid determined by capillary electrophoresis

The advantage of capillary electrophoresis was demonstrated for studying a complicated system owing to the dependence of direction and velocity of the electrophoretic movement on the charge of complex species. The stability constants of copper(II) complexes with ions of succinic acid were determined by capillary electrophoresis, including the 1?:?2 metal to ligand complexes which are rarely mentioned. The measurements were carried out at 25 °C and ionic strength of 0.1, obtained by mixing the solutions of succinic acid and lithium hydroxide up to pH 4.2–6.2. It was shown that while pH was more than 4.5 the zone of copper(II) complexes with succinate moves as an anion. It is impossible to treat this fact using only the complexes with a metal-ligand ratio of 1?:?1 (CuL⁰, CuHL⁺). The following values of stability constants were obtained: log β(CuL) = 2.89 ± 0.02, log β(CuHL⁺) = 5.4 ± 0.5, log β(CuL₂^2?) = 3.88 ± 0.05, log β(CuHL₂^?) = 7.2 ± 0.3.