Determination of stability constants of chelate complexes: Part II. Applications

The proposed simplified method for calculating the stability constants of chelate complexes from pH and pM measurements (Part I) is applied to two systems. On the basis of data reported by Österberg, the stability constants of copper o-phosphorylserylglutamic acid are calculated and good agreement is achieved. In addition, the stability constants of the mononuclear, binuclear and trinuclear silver complexes of TTHA (triethylenetetraminehexaacetic acid) were calculated from potentiometric pAg data. These calculations yielded the following values of the cumulative constants: log β_AgL=8.7, log β_AgHL= 17.6, log β_AgH2L= 23.8, log β_Ag2L= 14.0, log β_Ag2HL = 20.5, log β_Ag2H2L = 25.6, log β_Ag2L =17.0.     

Iron(III)‐Complexes Engaged in the Biochemical Processes in Seawater. II. Voltammetry of Fe(III)‐Malate Complexes in Model Aqueous Solution

Characteristics of iron(III) complexes with malic acid in 0.55 mol L^?1 NaCl were investigated by voltammetric techniques. Three iron(III)‐malate redox processes were detected in the pH range from 4.5 to 11: first one at ?0.11 V, second at ?0.35 V and third at ?0.60 V. First process was reversible, so stability constants of iron(III) and iron(II) complexes were calculated: log K₁(Fe^III(mal))=12.66±0.33, log β₂(Fe^III(mal)₂)=15.21±0.25, log K₁(Fe^II(mal))=2.25±0.36, and log β₂(Fe^II(mal)₂)=3.18±0.32. In the case of second and third reduction process, conditional cumulative stability constants of the involved complexes were determined using the competition method: log β(Fe(mal)₂(OH)_x)=15.28±0.10 and log β(Fe(mal)₂(OH)_y)=27.20±0.09.     

Complexation of iron(III) by pimelyldihydroxamic acid

Pimelyldihydroxamic acid forms strong complexes with iron(III) in aqueous solution at pH 2–9. Plots of n? and proton liberation against pH show plateaux regions at values of 1.5 and 3.0, respectively, over the pH range 4.0–8.0 supporting a formulation of Fe₂L₃ (logβ = 41.06). The orage-red complex exhibits maximum absorbance at 420 nm, and a well-defined peak at 0.6 V vs. SCE in differential pulse polarography.     

Potentiometric investigations of the equilibria between caffeic acid and copper(II), zinc(II), iron(II) and hydrogen ions in aqueous solution

Interactions in aqueous solution of caffeate with copper(II), zinc(II), iron(II) and iron(III) have been investigated. Virtually instantaneous and complete reduction of iron(III) was observed. Glass electrode potentiometry was used to determine the speciation and corresponding formation constants of caffeate with each of the other three metal ions named. Conditions were: temperature, 25°C; ionic strength, 0.100 mol dm⁻³ with respect to chloride. Values obtained for the logarithms of the stepwise protonation constants of the singly protonated dianion of caffeate (L²⁻) are 8.72 and 4.41. The titration data carried out in the presence of the three metal(II) ions can be explained by postulating the major complexes: LCu, logβ₁₁₀=6.02; LCuH₋⁻₁, logβ_11-1=0.25; L₃Cu₂H₋₃⁻⁵, logβ_32-3=0.97; LZnH₋₁⁻, logβ_11-1 = −3.03; L₃ZnH₋₂⁶⁻, logβ_31-2 = −5.51; LFe, logβ₁₁₀ = 3.86; LFeH₋₁⁻, logβ_11-1 = −3.83; L₃FeH₋₂⁶⁻, logβ_31-2 = −6.14, together with a variety of minor species. Complexation in the major species involves, predominantly, chelation by the catecholic site of caffeate whereas coordination to the carboxylate group together with catechol-type chelation featured amongst the minor species. The tendency of copper(II) to form oligonuclear complexes is evident. A single dinuclear iron(II) complex was also found amongst the minor species.     

Complex formation of iron (III) with eriochrome cyanine r

The complex formation of iron(III) with the trisodium salt of 5-[α-(3-carboxy-5-methyl-4-oxo-2,5-cyclohexadien-1-ylidene)-2-sulfobenzyl]-3-methylsalicylic acid (eriochrome cyanine R) was studied by spectrophotometry. The pure tetrabasic acid of the ligand (H₄ER) was prepared and used in the investigation. In the pH range 2.7–4.2 three complexes were detected: a ring-formed dimer Fe₂(ER)₂, Fe₂(ER) and Fe(ER) ; the absolute stability constants (at an ionic strength of 0.1 M and at room temperature (20 ± 3°)) were log k = 37.9, log k = 22.5 and log k = 17.9, respectively.     

Thermodynamics and kinetics of complexation of iron(III) ion by picolinic and dipicolinic acids

The complexation reactions of iron(III) with 2-pyridine carboxylic acia (picolinic acid) and 2,6-pyridine dicarboxylic acid (dipicolinic acid) in aqueous solutions have been studied by spectrophotometric and stopped flow techniques. Equilibrium constants were determined for the 1 : 1 complexes at temperatures between 25 and 80°C. The values obtained are: Picolinic Acid (HL): Fe³⁺+ H₂L⁺? FeHL³⁺+H+(K₁ = 2.8,ΔH = 2 kcal mole^?1 at 25°C, μ = 2.67 M) Dipicolinic Acid (H₂D): Fe³⁺+H₂D? FeD⁺+2H⁺(K₁K_1A= 227 M, ΔH = 3.4 kcal mole^?1 at 25°C,μ = 1.0 M). The rate constants for the formation of these complexes are also given. The results are used to evaluate the effects of these two acids upon the rate of dissolution of iron(III) from its oxides.     

Mononuclear cyano- and hydroxo-complexes of iron(III)

A detailed investigation of the iron(III)-cyanide and iron(III)-hydroxide systems has been made in NaClO(4) media at 25 degrees C, using combined UV-vis spectrophotometric and pH-potentiometric titrations. For the Fe(III)/OH- system, use of low total Fe(III) concentrations (< or =10 microM) and a wide pH range (0 < or = pH < or = 12.7) enabled detection of six mononuclear complexes, corresponding to the following equilibria: Fe3+(aq)+rH2O<=>Fe(OH)r(3-r)+(aq) + rH(+)(aq), where r = 1-6 with stability constants (log *beta 1r) of -2.66, -7.0, -12.5, -20.7, -30.8, and -43.4, respectively, at I = 1 M (NaClO(4)). It was also found to be possible to measure, for the first time, stability constants for most of the following equilibria: Fe3+(aq)+qCN-(aq)<=>Fe(CN)q(3-q)+(aq), despite a plethora of complicating factors. Values of log beta(1q) = 8.5, 15.8, 23.1, and 38.8 were obtained at I = 1.0 M (NaClO(4)) for q = 1-3 and 6, respectively. No reliable evidence could be obtained for the intermediate (q = 4 or 5) complexes. Similar results were obtained for both systems at I = 0.5 M(NaClO(4)). Spectra for the individual mononuclear complexes detected for Fe(III) with OH- and CN- are reported. Attempted measurements on the Fe(II)/CN- system were unsuccessful, but values of log beta(16)(Fe(CN)(6)(4-)) = 31.8 and log beta(15)(Fe(CN)(5)(3-) approximately 24 were estimated from well established electrode potential and other data.     

Trace metal speciation in sea water — a paper electrophoretic approach

Stability constants of individual trace metal complexes form the basis for calculations predicting the distribution of trace metal species in complexing media, such as sea water. In this study, the electrophoretic mobility of radiotracer ²¹⁰Pb is measured as a function of ligand concentration in chloride and sulfate solutions of constant ionic strength and temperature. A theoretically-derived expression, relating mobility to ligand concentration and complex stability constants, is fitted by the method of least squares to the experimental data to obtain estimates of the conditional stability constants of lead(II) chloro and sulfato complexes at 23°C and ionic strength 0.7 i.e., under conditions resembling those of ocean water. The values obtained are: log β₁ = 0.999 ± 0.014, log β₂ = 1.037± 0.032, log β₃ = 1.250 ± 0.015 for lead(II) chloro complexes, and log β₁ = 1.048 ± 0.015 and log β₂ = 1.183 ± 0.025 for lead(II) sulfato complexes. Experiments with eight other metal ions [Au(III), Bi(III), Cd(II), Co(II), Cu(II), Hg(II), Ni(II), and Po(IV)] and with sea water as electrolyte indicate the general applicability of the method.     

A correlation between spin state and electrochemical electron-transfer rate for tris(N,N-disubtituted dithiocarbamato) iron(III) complexes

The electrochemical electron-transfer rate constants for the redox systems Fe(IV)L₃⁺/Fe(III)L₃ (L=N,N-disubstituted dithicarbamate ion) and Fe(III)L₃/Fe(II)L₃^? with a variety of substituents were measured at a platinum electrode in acetonitrile with the galvanostatic double-pulse method. It is known that each of the Fe(III) complexes exists both in a highspin state ⁶A₁ and a low-spin state ²T₂ in equilibirium of which position is widely changed by a subtle change in substituent. The standard rate constants for Fe(IV)L₃⁺/Fe(III)L₃ were larger or smaller than those for Fe(III)L₃/Fe(II)L₃^? according as the Fe(III)L₃ complexes are predominantly low- or high-spin complexes. Since the Fe(IV) and Fe(II) complexes are low-and high-spin complexes respectively, these findings suggest that electrochemical electron-transfer reactions accompanied by a spin-state change are slower than those without it. Such spin-state effect on electrode reactions has rarely been discussed so far.     

Spectropolarimetric determination of copper(II), nickel(II) and iron(III) ions with n-carboxymethylpyrrolidine-2-carboxylic acid

N-Carboxymelhyl pyrrolidine-2-carboxylic acid (CMPCA) is suggested as an optically active titrant. The values of the acidity constants of the ligand were determined and the order of magnitude of the stability constants of the complexes formed by CMPCA with some metal ions was evaluated. In order to determine the best conditions for the spectropolarimetric titrations, the dependence of the molar rotation of CMPCA and its complexes on wavelength and pH was examined. The spectropolarimetric titrations ofcopper(II), nickel(II) and iron(III) ions were carried out successfully.     

Complexation between iron(III) and nicotinamide in aqueous dimethyl sulfoxide

The stability constants of iron(III) complexes with nicotinamide in water-DMSO mixtures (X _DMSO = 0–0.75) were determined by potentiometric titration at 25.0 ± 0.1°C and an ionic strength of 0.25 (NaClO₄). The contributions from the solvation of the reagents to the Gibbs energy of complexation transfer were analyzed. The stabilities of iron(III), copper(II), and silver(I) complexes with nicotinamide were compared. The observed decrease in the stability constants was attributed to the stabilization of iron(III) solvate complexes as the DMSO content increases.     

Synthesis and characterization of two binuclear iron(III) complexes of aminoethanol derivatives of aminophenol as models for non-heme iron enzymes active sites

Two aminoethanol derivatives of aminophenol ligands were synthesized and characterized by IR and ¹H NMR spectroscopies. The binuclear iron(III) complexes of these ligands have been prepared and characterized by IR, ¹H NMR and UV-Vis spectroscopic techniques, cyclic voltammetry, single crystal X-ray diffraction and magnetic susceptibility studies. X-ray analysis revealed binuclear complexes, Fe₂(L₂), in which Fe(III) centers are surrounded by two phenolate and hydroxyl oxygen atoms, and amine nitrogens of the ligands. The metal active sites of both complexes are held together by the two above mentioned hydroxyl bridges. Variable temperature magnetic susceptibility indicates antiferromagnetic coupling between the iron centers of both complexes. This exchange coupling is stronger for Fe₂(L^ae)₂, such that it shows a room temperature strong coupling between the two iron centers. The investigated complexes undergo irreversible electrochemical oxidation and reduction.     

Copper(II), iron(III), and chromium(III) complexes with 5,10-dioxo-4,5,9,10-tetrahydro-4,9-diazapyrene derivatives

Six copper(II), iron(III), and chromium(III) complexes with 5,10-dioxo-4,5,9,10-tetrahydro-4,9-diazapyrene derivatives (H₂L¹-H₂L³) have been synthesized and studied by physical methods (IR and electronic absorption spectroscopy, quantum-chemical calculations). The composition of the complexes has been determined and their stability constants in aqueous dimethylformamide solutions have been calculated. The energy characteristics, electronic structure and geometry of isolated diazapyrenes and their tautomeric forms have been calculated by the PM6 method, and their complexes have been modeled.     

Polarographic study of Eu(III)-succinate-ethylenediamine and Eu(III)-malonate-ethylenediamine mixed systems

The mixed complexes of Eu(III) with succinate (succ^2?) and malonate (mal^2?) and ethylenediamine (en) have been studied polarographically at 25°C and at constant ionic strength, μ = 0.1 (NaNO₃) and pH 6. The reduction of the complexes in each case is quasi-reversible and diffusion-controlled. In each system three mixed complexes are formed, viz. [Eu(succ)(en)]⁺, [Eu(succ)(en)2]⁺ and [Eu(succ)₂(en)]^? with stability constants log β₁₁ = 9.2, log β₁₂ = 17.5 and log β₂₁ = 11.7; and [Eu(mal)(en)]⁺, [Eu(mal)₂(en)₂]^? and [Eu(mal)₃(en)]^3? with stability constants log β₁₁ = 11.4, log β₂₂ = 19.08 and log β₃₁ = 13.5 respectively.     

The effect of 2,2′-bipyridyl on the stability of Cu(II) complexes with carboxylic acids

The stability constants β₂, and β₂ of simple Cu(II) complexes with methoxyacetic, phenylacetic, and cyclohexylacetic acid were determined spectrophotometrically and compared with the stability of composite complexes containing 2,2′-bipyridyl as the first ligand and the above mentioned acid as the second ligand. In each case the stability of the composite complex Cu(bip)L⁺ was found to exceed that of the simple complex CuL⁺ and the differences in the values of log β are comprised within 0.15–0.17.     

A spectrophotometric determination of the stability constants of the complexes of titanium(III) with acetylacetone

The stability constants of the consecutive complexes of titanium(III) with acetylacetone were determined by means of spectrophotometric methods. The values are log /gb₁ = 10.4, log β₂ = 18.8, and log β₃ = 24.9. Because of the fact that K₁K_a = 10^1.5 a high concentration of acetylacetone results in a considerable formation of TiL even at pH 0.     

Complex formation of copper(II) with chrome azurol s

The complex formation of copper(II) with chrome azurol S (CAS) was studied by spectrophotometric and potentiometric methods. In the pH range 5–7, two complexes with the composition Cu(H₂O)₂HCAS- and (Cu(H₂O)₂)₂CAS were detected; the stability constants were calculated to be log K = 4.02 ±0.05 and log K = 13.7±0.1, respectively (at 25° and ionic strength 0.1 (KCl)). A comparison is made between the copper(II)-CAS and iron(III)-CAS systems.     

A POTENTIOMETRIC STUDY OF METAL CHELATES WITH TETRAETHYLENEPENTAAMINEHEPTA-ACETIC ACID

Abstract

The protonation constants of tetraethylenepentaamineheptaacetic acid, TPHA, were determined by potentiometric titration in aqueous solution at an ionic strength of 0.10 M KNO₃ and at 25°C. The formation constants of various metal-TPHA complexes were also obtained by titrating mixtures of metal to ligand in molar ratios of 1 :1 and 2:1. Calculations were performed with the computer program BEST. Individual stability constants are reported for Co(II). Ni(II), Cu(II), Zn(II), Cd(II), Hg(II) and Pb(II) with TPHA as well as their related pro-tonated species. The stabilities of the 1:1 complexes parallel to those of similar complexes with DTPA and TTHA. However the 2: 1 complexes have significantly larger log K _ML's than their TTHA counterparts. The extra stability of the 2:1 metal-TPHA complexes is explained in terms of ligand denticity and steric effects. Mercury(II)-TPHA complexes exhibited the highest formation constants and the copper-TPHA complexes had slightly higher log K _ML's than those for Co(II), Ni(II), Zn(II), Cd(II) and Pb(II).     

Coordination of N-methylpyrrolidone to iron(II)

The use of NMP (N-methylpyrrolidone) as a cosolvent has been shown to improve the yield of iron-catalyzed cross-coupling reactions, but surprisingly there are no iron complexes of NMP in the literature. This paper reports two novel NMP complexes of iron(II): Fe₃Cl₆(NMP)₈ and L^tBuFe(NMP)Cl (L^tBu = bulky β-diketiminate ligand). The X-ray crystal structure of Fe₃Cl₆(NMP)₈ shows an octahedral cation and two tetrahedral FeCl₃(NMP)⁻ anions. ¹H NMR spectra show that the NMP ligands are labile, exchanging rapidly on the NMR time scale. The β-diketiminate complex has a trigonal pyramidal geometry with the NMP in an axial position. The use of NMP improves the yield of the catalytic cross-coupling of methyl 4-chlorobenzoate and 1-hexylmagnesium bromide using these and other iron complexes as precatalysts.