Mercury(II) cysteine complexes in alkaline aqueous solution

Mercury(II) complexes with l-cysteine (H(2)Cys) in alkaline aqueous solutions have been structurally characterized by means of extended X-ray absorption fine structure (EXAFS) spectroscopy. The distribution of [Hg(Cys)(n)] (n = 2, 3, and 4) species in approximately 0.09 mol dm(-3) mercury(II) solutions with H(2)Cys/Hg(II) ratios varying from 2.2 to 10.1 has been evaluated by fitting linear combinations of simulated EXAFS functions for the separate complexes to the experimental EXAFS data, aided by (199)Hg NMR and Raman results. For the [Hg(Cys)(2)](2-) and [Hg(Cys)(3)](4-) complexes and the novel four-coordinated Hg(Cys)(4) species that dominates in solutions with excess of cysteine (H(2)Cys/Hg(II) > 5), the mean Hg-S bond distances were found to be 2.35(2), 2.44(2), and 2.52(2) Angstroms, respectively. The minor amount of the linear [Hg(Cys)(2)](2-) complex that can still be discerned in solutions with ratios up to H(2)Cys/Hg(II) = 5 was derived from the distinct S-Hg-S symmetric stretching Raman band at 334 cm(-1). From (199)Hg NMR spectra, the chemical shift of the Hg(Cys)(4) species was estimated to -340 ppm with an amount exceeding 85% in the highest excess of cysteine, consistent with the EXAFS data.     

Cadmium(II) N-acetylcysteine complex formation in aqueous solution

The complex formation between Cd(II) ions and N-acetylcysteine (H(2)NAC) in aqueous solution was investigated using Cd K- and L(3)-edge X-ray absorption and (113)Cd NMR spectroscopic techniques. Two series of 0.1 M Cd(II) solutions with the total N-acetylcysteine concentration c(H2NAC) varied between 0.2-2 M were studied at pH 7.5 and 11.0, respectively. At pH = 11 a novel mononuclear [Cd(NAC)(4)](6-) complex with the average Cd-S distance 2.53(2) ? and the chemical shift δ((113)Cd) = 677 ppm was found to dominate at a concentration of the free deprotonated ligand [NAC(2-)] > 0.1 M, consistent with our previous reports on cadmium tetrathiolate complex formation with cysteine and glutathione. At pH 7.5 much higher ligand excess ([HNAC(-)] > 0.6 M) is required to make this tetrathiolate complex the major species. The (113)Cd NMR spectrum of a solution containing c(Cd(II)) = 0.5 M and c(H2NAC) = 1.0 M measured at 288 K showed three broad signals at 421, 583 and 642 ppm, which can be attributed to CdS(3)O(3), CdS(3)O and CdS(4) coordination sites, respectively, in oligomeric Cd(II)-NAC species with single thiolate bridges between the cadmium ions.     

Thermodynamics of complex formation of Cu(II) ethylenediaminetetraacetate with ethylenediamine in aqueous solution

The formation of mixed-ligand complexes in the system Cu²⁺-Edta^4?-En was studied by the calorimetric and pH-potentiometric methods at 298.15 K and I = 0.5(KNO₃). The thermodynamic characteristic of the CuEdtaEn^2? complex formation were determined.     

Mixed complex formation of lead(II) and mercury(II) ethylenediaminetetraacetates with thiourea in an aqueous solution

The formation of mixed-ligand complexes HgEdtaThio²⁻, HgEdtaS₂O₃⁴⁻, PbEdtaThio²⁻, and Pb(Thio) _i ²⁺, i = 1, 2; Thio is thiourea) was studied by calorimetry, pH metry, and ¹H and ¹³C NMR spectroscopy. The thermodynamic parameters (logK, Δ _r G ⁰, Δ _r H, and Δ _r S) for the formation of the complexes at 298.15 K and the ionic strength I = 0.5(NaClO₄) were determined. The most probable coordination mode of the ligands in the mixed complex was considered.     

Thermodynamic characteristics of complex formation in the zinc(II)-glycylglycine system in aqueous solution

The heats of reaction of zinc(II) with glycylglycine at temperatures 288.15, 298.15, and 308.15 K and ionic strengths 0.25, 0.50, and 0.75 (potassium nitrate as a supporting electrolyte) were determined by calorimetry. The thermochemical results were processed with inclusion of stepwise equilibria. In addition to complexation reactions, “side” protolytic processes were considered. Standard heats of complexation in the system were found by extrapolation to the zero ionic strength by an equation with one individual parameter. The influence of the supporting electrolyte concentration and temperature on the thermodynamic characteristics of the complexation reactions in the glycylglycine-zinc(II) system was considered. The standard enthalpies of formation of ZnGlyGly⁺, Zn(GlyGly)₂, and Zn(GlyGly)₃⁻ species in aqueous solution were calculated.     

The thermodynamic characteristics of complex formation in the zinc(II) ion-threonine system in aqueous solution

The heat effects of complex formation between D,L-threonine and zinc(II) ions at 288.15, 298.15, and 308.15 K and ionic strengths of 0.25, 0.50, and 0.75 were determined calorimetrically against the background of potassium nitrate. The thermochemical results were processed taking step equilibria into account. Along with complex formation, side protolytic processes were considered. The standard heat effects of complex formation in the system studied were determined by extrapolation to zero ionic strength with the use of an equation with one individual parameter. The heats of solution of D,L-threonine in water and aqueous alkali were used to calculate the standard enthalpies of formation of the complexes.     

Thermodynamics of mixed-ligand complex formation of copper (II) ethylenediaminetetraacetate with hexamethylenediamine in an aqueous solution

The mixed-ligand complex formation in the system Cu²⁺−Edta⁴⁻−(CH₂)₆(NH₂)₂ (L), where L is hexamethylenediamine has been calorimetrically, pH-potentiometrically and spectrophotometrically studied in aqueous solution at 298.15 K and the ionic strength of I = 0.5 (KNO₃). The thermodynamic parameters of formation of the CuEdtaL²⁻, CuEdtaHL⁻ (CuEdta)₂L⁴⁻ and (CuEdta)₂En⁴⁻ complexes have been determined. The most probable coordination mode for the complexone and the ancillary ligand in the mixed-ligand complexes was discussed.     

Thermodynamic characteristics of complex formation in the nickel(II) ion-β-alanine systems in aqueous solution

Heats of reactions between a nickel(II) ion and β-alanine were measured calorimetrically at 288.15, 298.15, and 308.15 K and ionic strengths of 0.5, 1.0, and 1.5 (KNO₃). Thermochemical data were processed with account for stepwise equilibria; attendant protolytic processes were taken into account together with complexing reactions. Extrapolation to the zeroth ionic strength with the use of a one-parameter equation gave standard thermodynamic characteristics of complex formation in the system. The influence of the supporting electrolyte and temperature on the heats of complex formation reactions was considered. Standard enthalpies of formation were calculated for NiAla⁺, NiAla₂, and NiAla₃⁻ species. Original Russian Text ? L.A. Kochergina, O.V. Platonycheva, O.M. Drobilova, V.V. Chernikov, 2009, published in Zhurnal Neorganicheskoi Khimii, 2009, Vol. 54, No. 2, pp. 377–384.     

Temperature effect on the heats of copper(II) ion complex formation with L-phenylalanine in aqueous solution

The heats of complex formation in the L-phenylalanine-copper(II) ion system in aqueous solution were determined calorimetrically at 288.15, 298.15, and 308.15 K for an ionic strength of 0.5 (KNO₃ supporting electrolyte). The thermodynamic parameters of formation of phenylalaninatocopper(II) complexes were calculated for various temperatures.     

Thermochemical study of the complex formation of mercury(II) with nitrilotriacetic acid in an aqueous solution

The formation constant of the Hg(Nta) ₂ ^4? complex, where Nta^3? is the nitrilotriacetate ion, is determined by pH-metric titration at 298.15 K and ionic strength I = 0.5 (KNO₃) (logβ = 21.49 ± 0.10). The thermal effects for the formation of the Hg(NTa) _i2?3i complexes (i = 1, 2) are determined by a direct calorimetric method (?56.69 ± 1.04 and ?85.88 ± 1.32 kJ/mol for i = 1 and 2, respectively).     

Kinetics and mechanism of copper(II) complex formation with tripodal aminopolythiaether and aminopolypyridyl ligands in aqueous solution

The complex formation kinetics of aquated copper(II) ion reacting with 12 related tripodal ligands have been studied in aqueous solution at 25 degrees C, mu = 0.10 M (NaClO4). For most of the ligands studied, specific formation rate constants have been resolved for both the unprotonated and monoprotonated ligand species. All of the tripodal ligands included in this study contain a bridgehead amine nitrogen with the three legs consisting of 2-methylthioethyl or 2-ethylthioethyl and/or 2-pyridylethyl or 2-pyridylmethyl. Since the bridgehead nitrogen is too sterically hindered to participate in initial coordinate bond formation, the first bond must involve a thiaether sulfur or a pyridine nitrogen on one of the pendant legs followed by coordination to the bridgehead nitrogen to complete the first chelate ring. All kinetic data are interpreted in terms of this presumed sequence in the bond formation steps. For the two ligands in which all three pendant legs contain thiaether sulfur donor atoms, the rate-determining step appears to be at the point of second bond formation (chelate ring closure), although the distinction is not well defined. For all other unprotonated ligands, the kinetic behavior is consistent with the first-bond formation being rate-determining. Upon protonation, the rate-determining step appears to shift to the point of proton loss associated with second-bond formation in several cases. A particularly interesting observation is that the tripodal ligand tris(ethylthioethyl)amine (TEMEA) exhibits specific Cu(II) complex formation rate constants that are virtually identical to those for a closely related macrocyclic ligand, 1,4,8-trithia-11-azacyclotetradecane ([14]aneNS3), but the calculated CuIIL dissociation rate constants differ by a factor of 1000. A further comparison of the calculated dissociation rate constants for Cu(II)-tripodal ligand complexes indicates that a Cu(II)-N(pyridine) bond is approximately 10(4) times stronger than a Cu(II)-SR2 bond. This leads to the conclusion that a 1:1 Cu(II)-SR2 complex would have a predicted stability constant of about 0.04 M-1 in aqueous solution--the first estimate obtained for the strength of a single Cu(II)-S(thiaether) bond.     

Interactions of silicate ions with zinc(II) and aluminum(III) in alkaline aqueous solution

We present (29)Si, (27)Al, and (67)Zn NMR evidence to show that silicate ions in alkaline solution form complexes with zinc(II) (present as zincate, Zn(OH)(3)(-) or Zn(OH)(4)(2-)) and, concomitantly, with aluminate (Al(OH)(4)(-)). Zincate reacts with monomeric silicate at pH 14-15 to form [(HO)O(2)Si-O-Zn(OH)(3)](4-) and with dimeric silicate to produce [HO-SiO(2)-O-SiO(2)-O-Zn(OH)(3)](6-). The exchange of Si between these free and Zn-bound sites is immeasurably fast on the (29)Si NMR time scale. The cyclic silicate trimer reacts relatively slowly and incompletely with zincate to form [(HO)(3)Zn{(SiO(3))(3)}](7-). The concentration of the cyclic trimer becomes further depleted because zincate scavenges the silicate monomer and dimer, with which the cyclic trimer is in equilibrium on the time scale of sample preparation. Identification of these zincate-silicate complexes is supported by quantum chemical theoretical calculations. Aluminate and zincate, when present together, compete roughly equally for a deficiency of silicate to form [(HO)(3)ZnOSiO(2)OH](4-) and [(HO)(3)AlOSiO(2)OH](3-) which exchange (29)Si at a fast but measurable rate.     

A study of polyamine complex formation with H+, Cu(II), Zn(II), Pb(II), and Mg(II) in aqueous solution

Summary The composition and stability of the following biogenic amine complexes have been investigated: 1,4-diaminobutane(Put), 4-azaoctane-1,8-diamine(Spd), 4,9-diazadodecan-1, 12-diamine(Spm) as well as homologues such as 1,3-diaminopropane(Put3), 4-azaheptane-1, 7-diamine(Spd3,3) and 4,8-diazaundecan-1,11-diamine(Spm3,3,3) with H⁺, Cu(II), Zn(II), Pb(II) and Mg(II). A potentiometric method was used. The VIS technique enabled the determination of coordination mode in copper/amine systems. It was found that Mg(II) does not form coordination compounds with any of the studied polyamines in solution. An increase in the concentration of ligand and metal was found to result in a stronger tendency towards the formation of protonated compounds accompanied by a decrease in the concentration of hydroxocomplexes. At physiologicalpH (7.4) an increase in the concentration of protonated compounds by approximately 15% was observed within the ligand concentration range from 0.001 mol dm^–3 to 0.0001 mol dm^–3 at a Cu(II) concentration of 0.000177 mol dm^–3.

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Untersuchungen zur Komplexbildung von Polyaminen mit H⁺, Cu(II), Zn(II), Pb(II) und Mg(II) in wäßriger Lösung

Zusammenfassung Anhand einer Analyse von potentiometrischen Daten wurden Zusammensetzung und Beständigkeit folgender biogener Aminkomplexe untersucht: 1,4-Diaminobutan(Put), 4-Azaoktan-1,8-diamin(Spd), 4,9-Diazadodekan-1,12-diamin(Spm), sowie auch deren Homologen 1,3-Diaminopropan(Put3), 4-Azaheptan-1,7-diamin(Spd3,3) und 4,8-Diazaundekan-1,11-diamin(Spm3,3,3) mit H⁺, Cu(II), Zn(II), Pb(II) und Mg(II). Mit Hilfe der VIS-Technik wurde die Koordinationsweise in Kupfer/Amin-Systemen bestimmt. Es wurde festgestellt, daß Mg(II) keine Koordinationsverbindungen mit den untersuchten Polyaminen bildet. Eine höhere Konzentration von Ligand und Metall führte zu stärkerer Tendenz der Bildung protonierter Verbindungen, wobei die Konzentration von Hydroxokomplexen kleiner wurde. Bei physiologischempH (7.4) wurde im Bereich der Ligand-Konzentration von 0.001 mol dm^–3 bis 0.0001 mol dm^–3 bei einer Cu(II)-Konzentration von 0.000177 mol dm^–3 ein Anstieg der Konzentration protonierter Verbindungen um etwa 15% beobachtet.

     

Complex formation equilibria of some beta-amino-alcohols with lead(II) and cadmium(II) in aqueous solution

A study of complex formation equilibria of some beta-amino-alcohols with lead(II) and cadmium(II) ions at 25 degrees C and in 0.5 M KNO(3) is reported. The amino-alcohols considered are 2-amino-1-propanol, 2-amino-1-butanol, 2-amino-1-pentanol and 2-amino-1,3-propanediol. sec-Buthylamine and 2-amino-1-methoxy-propane have been also considered for comparison. The results are discussed in terms of ligand structure, paying attention to the number of hydroxyl groups and to the length of the alkyl residual. A weak contribution of the alcoholic oxygen in the coordination of cadmium(II) and the presence of a mixed hydroxyl species in lead(II) containing systems are hypothesized.     

Complex formation equilibria of L-Amino-Acid amides with copper(II) in aqueous solution

Protonation and Cu(II) complexation equilibria of L -phenyhilaninamide, N²-methyl-L-phenylalaninamide, N², N²-dimethyl-L-phenylalaninamide, L -valinamide, and L -prolinamide have been studied by potentiometry in aqueous solution. The formation constants of the species observed, CuL²⁺, CuL, CuLH, CuL₂H and CuL₂H_?2, are discussed in relation to the structures of the ligands. Possible structures of bisamidato complexes are proposed on the ground of VIS and CD spectra. Since Cu(II) complexes of the present ligands (pH range 6–8) perform chiral resolution of dansyl- and unmodified amino acids in HPLC (reversed phase), it is relevant for the investigation of the resolution mechanism to know which are the species potentially involved in the recognition process.     

High-turnover supramolecular catalysis by a protected ruthenium(II) complex in aqueous solution

The design of a supramolecular catalyst capable of high-turnover catalysis is reported. A ruthenium(II) catalyst is incorporated into a water-soluble supramolecular assembly, imparting the ability to catalyze allyl alcohol isomerization. The catalyst is protected from decomposition by sequestration inside the host but retains its catalytic activity with scope governed by confinement within the host. This host-guest complex is a uniquely active supramolecular catalyst, capable of >1000 turnovers.     

Photoinduced electron ejection from hydroxo complexes of thallium(I) and tin(II) in alkaline aqueous solution

In alkaline solution (1 M NaOH) irradiation (λ_ir=266 nm) of both TlOH and Sn(OH)⁻₃ leads to the formation of hydrated electron and oxidized complex in the primary photochemical step. In both cases nascent hydrated electrons react with the ground-state hydroxometalates to form the corresponding reduced compounds. The main reaction of these latter species is recombination (synproportionation) with the oxidized complexes, significantly diminishing the efficiency of the overall light-induced oxidation of TlOH and Sn(OH)⁻₃.     

The mechanism of the cobalt(II)-catalyzed electro-generated chemiluminescence of luminol in aqueous alkaline solution

A rotating ring—disc electrode system is used where the disc electrode (carbon) is maintained at a negative potential to reduce oxygen to hydrogen peroxide, and a symmetric double-step potential is applied to the ring electrode (platinum). Cobalt(II) catalyzes the electrogenerated chemiluminescence of luminol at the ring electrode during the negative pulse of the double-step potential. A possible reaction scheme for this cobalt(II)-catalyzed emission process is outlined.