Aquo Ions Research Articles

Electrochemical oxidation of organic substrates mediated by the high-valent ruthenyl species

The ruthentium(V)—oxo species [Ru V(O)(H 2O) 3Cl 2] + was generated in situ by the coulometric oxidation of ruthenium(III) aquo ion, [Ru III(H 2O) 4Cl 2] + 1. The electrochemical oxidation of 1 proceeds by a stepwise electron transfer at + 0.386 V (Ru III/Ru IV) and + 0.870 V (Ru IV/Ru V) vs. SCE at pH 1.85. Coulometric oxidation of 1 to the stable oxo species 2 was performed at an electrode potential of + 0.950 V. Complex 2 was isolated and characterised by UV—Vis, IR and electrochemical studies. The oxo species 2 was then used as an electrocatalyst for the effective oxidation of olefinic substrates. The oxidation products were identified and analysed by GLC and HPLC techniques. A ‘shuttle’ mechanism was proposed for the conversion of Ru(III) 1 to Ru(V) 2 and back to 1 continuously at the electrode surface.

Chemical Composition and Metal Speciation in Porewaters from the Upper Qu’Appelle River Basin, Saskatchewan

Abstract Porewater samples from the upper Qu’Appelle River basin in Saskatchewan, Canada, were analyzed to obtain metal, inorganic ligand and amino add profiles. These data were used to compute the aqueous speciation of the metals in each porewater using the computer program GEOCHEM-PC. The porewaters were classified as slightly to moderately saline. Metal concentrations reflected both the geology of the drainage basin and the impact of anthropogenic activities. Whereas K and Na were present almost entirely as the free aquo ions, carbonate equilibria dominated the speciation of Ca. Mg and Mn (the predominant metal ligand species were of the type MCO3 (s). MCO30. and MHCO3+). Trace metal concentrations were generally within the ranges reported for non-polluted freshwater systems. Whereas the speciation of the trace metals Cr(III) and Co(II) was dominated by carbonate equilibria, Hg(II)-, Zn(II)- and Fe(II)-speciation was dominated by hydroxy-metal complexes of the type M(OH)+ and M(OH)2°. The speciation of Fe(III) was dominated by Fe(OH)3 (s). In porewaters with high chloride concentrations (> 2 mM), however, significant amounts of Hg(II) were bound as HgCl20 and HgClOH0. The aqueous speciation of Al was dominated by Al(OH)4− and Al2Si2O4(OH)6 (s). Total concentrations of dissolved free amino acids varied from 15.21 to 25.17 umole L−1. The most important metal scavenging amino acids were histidine (due to high stability constants for the metal-histidine complexes) and tryptophan (due to its relatively high concentration in the porewaters. i.e., 5.96 to 7.73 umole L−1). Secondary concentrations of various trace metal-amino add complexes were computed for all the porewaters, but metal-amino acid complexes dominated the speciation of Cu(II) in all the porewaters and Ni(II) in two of the porewaters.

Metal ion‐promoted hydrolysis of uridine 2′, 3′ ‐ cyclic monophosphate: Effect of metal chelates and uncomplexed aquo ions

AbstractThe effect of a wide variety of metal ious and metal ion complexes on the hydrolysis of uridine 2′,3′‐cyclic monophosphate was studied over the pH range 4.5–8.0. The greatest rate accelerations observed were 107–108‐fold. The kinetic data obtained are interpreted in terms of a mechanism involving arapid monodentate binding of the metal ion to the monoanionic phosphodiester and a subsequent intracomplex participation of the metal‐bound hydroxo ligand, either as a nucleophile attacking the tetracoordinated phosphorus or as a base facilitating an intermolecular attack of a water molecule. No effect on the product distribution between uridine 2′‐ and 3′‐monophosphates was observed.

Solvent water 1H NMRD study of oxovanadium(IV) aquo ion

The nuclear magnetic relaxation dispersions (NMRD) of the oxovanadium(IV) ion in water and water-glycerol solutions were studied at different temperatures and viscosities. A contact contribution to T 1M −1 in water was detected at low magnetic fields. At higher magnetic fields, the Solomon dipolar contribution holds. A quantitative analysis of the data allowed us to give an estimation of the electronic relaxation and the correlation time for the electron-lattice interaction. The electronic relaxation time at low field is found to be sizably shorter than that estimated from extrapolation of EPR values. By increasing viscosity, the evidence of the contact contribution disappears and deviation from the Solomon behavior occurs. Theoretical calculations indicated that both the hyperfine coupling between the unpaired electron and the nucleus and the field dependence of the correlation time are responsible for the deviation.

Charge dependence of the distribution of contrast agents in rat cerebral ventricles.

Previous studies of MRI contrast resulting from the intracerebral administration of several contrast agents have suggested that the distribution of these agents may be dependent on the net charge. In order to further evaluate the significance of this parameter, the distribution of both aquo and CDTA-chelated lanthanide and transition metal ions in the cerebral ventricles of rats has been evaluated based on their enhancement of MRI contrast. The agents were injected directly into the lateral ventricles of Sprague-Dawley rats. Each of five different positively charged paramagnetic metal ions tested selectively enhanced the inner cellular layers of ventricular luminal wall of the rat brain, while such enhancement was absent using the corresponding negatively charged metal-CDTA complexes. The abundance of negatively charged residues on the cell surface of the inner luminal layers is suggested to be the source of the observed charge affinity. Differences in the distribution of Mn2+ ions administered intracerebrally compared with intraperitoneal (ip) injection suggest that while the first treatment may result primarily in uncomplexed aquo ions which then interact with the luminal surface, manganese ions administered ip behave more like chelated ions and are most probably complexed to transferrin.

Self-diffusion coefficients and structure of the trivalent transplutonium ion curium and gadolinium in aqueous solution

Self-diffusion coefficients D of the trivalent aquo ion Cm3+ have been determined in aqueous Nd(ClO4)3−HClO4 solutions (pH 2.5) at 25°C, by the open-end capillary method (O. E. C. M.). The variation of D versus the square root of the concentration of inactive solution is an exponential form in the studied range of concentration. The limiting value D0 at zero ionic strength is 6.0·10−6 cm2·s−1. The curve\(D = f(\sqrt c )\) relating to Cm3+ can be compared to those of241Am3+ and153Gd3+ obtained under similar conditions. We find a similar ionic structure of Cm3+ with Am3+ and Gd3+. They have the same hydration as a tripositive of 5f and 4f ions in the absence of hydrolysis, complexing, or pairing at pH 2.5. The present study contributes to show the analogy of the solvation structure of trivalent actinide ions in aqueous solution at pH 2.5 with that of the trivalent lanthanide ions as a help for predicting the thermodynamic properties.

Fe 3+ and Al 3+ hydrolysis equilibria. Cooperativity in Al 3+ hydrolysis reactions

The hydrolysis equilibria of Fe 3+ and Al 3+ play important roles in geology and biology. While the four successive deprotonations from the aquo Fe 3+ are spaced at about two pH unit intervals, the four Al 3+ deprotonations occur cooperatively and squeeze into an interval of less than one pH unit. This dramatic contrast is accounted for by proposing that throughout the deprotonations, the hexacoordinate aquo ion remains six-coordinate for Fe 3+, but undergoes a regular reduction for Al 3+ from six through five to four-coordinate in the aluminate ion, Al(OH) 4 −. An acidity constant comparison predicts that the aluminum dimer analogous to Fe 2(OH) 2 4+is too weak to be detected in aqueous solutions, in agreement with recent results.

Effect of ionic strength on the polarographic half-wave potential

The shift in the polarographic half-wave potential caused by changes in the concentration of a non-complexing supporting electrolyte was investigated. The depolarizer systems studied were the cadmium(II) and zinc(II) amine complex ions, and the cadmium(II), zinc(II), indium(III) and thallium(I) aquo ions. These systems exhibited half-wave potential shifts that were dependent on five factors affected by the ionic strength of the solution: the activity and diffusion coefficients of the depolarizer ions, the activity of the complexing agent and/or water and the liquid junction potential. An equation was derived relating these factors to the half-wave potential shift and evaluated for the change in the activity coefficient of the depolarizer over the ionic strength range 4–10. As expected, the observed potential shift showed a strong correlation with the charge and primary hydration numbers of the aquo ions.

Mechanism of cation uptake by zirconia gels

A study has been made of the uptake of the cations Cr 3+, Co 2+, Ni 2+, Cu 2+ and Ca 2+ by zirconia gels prepared at pH values of 3.75, 7.45 and 9.00 using zirconyl chloride as starting material and ammonia solution as precipitating agent. The base exchange capacities of the gels as measured by the uptake of Na + and Cl − ions were found to depend on the preparative pH. The anionic capacity decreases and the cationic capacity increases when precipitation occurs in a basic medium. Before the isoelectric point (≈5.0), the uptake of Cu 2+, Co 2+, and Ni 2+ seems to involve complex formation by exchange of water molecules of the aquo ion with gel OH groups. The K d values appear to depend not only on the first hydrolysis constant but also may be affected by the textural characteristics of the gels brought about by differences in preparative pH values. The uptake of Cr 3+ is suggested to follow a different mechanism mainly involving electrostatic attraction between negatively charged anions on the surface and the aquo chromium ion as well as any other hydrolyzed species formed in solution. The uptake of Ca 2+ appears to involve electrostatic interaction. After the isoelectric point, the uptake involves a simple ion-exchange mechanism. The effect of equilibrium pH as well as the preparative pH on the distribution ratio K d has been discussed.

High affinity divalent cation exchange on actin. Association rate measurements support the simple competitive model.

Each actin molecule has one high affinity site which binds a divalent cation. It has been proposed that an isomerization of the actin molecule is involved in divalent cation exchange at this site ("isomerization model," Frieden, C. (1982) J. Biol. Chem. 257, 2882-2886); we have maintained that exchange is by a simple competitive mechanism (Estes, J. E., Selden, L. A., and Gershman, L. C. (1987) J. Biol. Chem. 262, 4952-4957). Here, using fluorescent-labeled actin, we measure the apparent rate constant for exchange (kapp) as a function of the ratio of free Ca2+ and Mg2+ concentrations, ([Ca]/[Mg]), and show that both models are consistent with the data. The major parameter controlling this relationship in the simple competitive exchange model, the ratio of the association rate constants for Ca2+ and Mg2+ to actin (kCa/kMg), is found to have a value of about 90. We have verified this parameter by direct measurements of kCa and kMg, finding that kCa = 1.9 x 10(7) M-1 s-1 and kMg = 2.3 x 10(5) M-1 s-1, consistent with the characteristics of the Ca2+ and Mg2+ aquo ions. The corresponding parameter derived from the isomerization model is not verifiable. We conclude that high affinity divalent cation exchange on actin proceeds by a simple competitive mechanism.

Epoxidation of cyclohexene, methylcyclohexene and cis-cyclooctene by molecular oxygen using ruthenium(III) aquo ion as catalyst: a kinetic study

Kinetics of the epoxidation of cyclohexene, methylcyclohexene and cis-cyclooctene by molecular oxygen catalysed by Ru(III) ion, [RuCl 2H( 2O) 4] + at pH 2.0 in a mixed water-dioxane solvent medium are reported. The reactions were studied manometrically as a function of catalyst, substrate and dioxygen concentrations. For all the substrates studied, the rate of oxidation is first order with respect to catalyst and substrate concentrations and one-half order with respect to dioxygen concentration. The oxidations yielded epoxide as the only product in all cases; these were identified and analysed by GLC technique. Based on the kinetics, a mechanism involving homolytic cleavage of the O-O (peroxo) bond with concerted transfer of the oxygen atom to the substrate was proposed. The presence of methyl group on the cyclohexene ring (reaction 2) and an increase in the number of carbon atoms in the ring for cyclooctene (reaction 3) cause a decrease in the rates of epoxidation. The rate of epoxide formation and the corresponding yields for reactions ( 1), ( 2) and ( 3) decreases in the order: cyclohexene > methylcyclohexene > cyclooctene.

Reduction mechanism of cobalt(II)-ammonia complexes on a dropping mercury electrode

The mechanism of the polarographic reduction of cobalt(II) complexes with ammonia at a dropping mercury electrode over a wide ligand concentration range was investigated. It was shown that the Co(II) aquo ion and the Co(NH3)2+ and Co(NH32+2 complexes participate in the electrode process. Transfer coefficients, α, for these species and the electrode reaction rates were evaluated. Stability constants of Co(II) complexes with ammonia in 0.5 M ammonium perchlorate were determined on the basis of the polarographic wave equation of totally irreversible reduction of complex specie.

Solution and solid state characterization of europium and gadolinium Schiff base complexes and assessment of their potential as contrast agents in magnetic resonance imaging

Two lanthanide Schiff base macrocyclic complexes, LnAM(OAc){sub 2}Cl{center dot}4H{sub 2}O (Ln = Eu, Gd; HAM - HexaAzaMacrocycle = C{sub 22}H{sub 26}N{sub 6}), have been characterized in view of the potential of the Gd complex as a magnetic resonance imaging (MRI) contrast agent. The relaxivity of GdHAM(OAc){sub 2}Cl was measured at 300 and 20 MHz and is as high as that for the gadolinium aquo ion. The number of coordinated waters, q, was measured by comparison of the luminescent lifetimes of EuHAM(OAc){sub 2}Cl in H{sub 2}O and D{sub 2}O and found to be between three and four. The complex GdHAM(OAc){sub 2}Cl{center dot}4H{sub 2}O was characterized by single-crystal X-ray diffraction. The complex crystallizes in space group P{bar 1} with Z = 2, a = 10.032 (2) {angstrom}, b = 12.765 (2) {angstrom}, c = 13.668 (3) {angstrom}, {alpha} = 69.190{degree} (9){degree}, {beta} = 72.405{degree} (9){degree}, and {gamma} = 74.07{degree} (1){degree}.

Optimization of the electron spin resonance spectrometric determination of transition-metal ions by variation of temperature

The effect of variation in temperature on the first-derivative electron spin resonance spectra of transition metal ions in aqueous solution is reported. For the aquo ions of Mn(II), Cr(III), Fe(III) and vanadyl, and for hexafluoroferrate(III), an increase in temperature results in a decrease in line width with a concomitant increase in signal amplitude. In contrast, [Cu(H 2O) 6] 2+ and [Ti (H 2O) 4F 2] + show an increase in line width and a marked decrease in derivative amplitude as the temperature is raised. Consequently, the choice of the optimum temperature for each system results in greatly improved sensitivity and lower limits of detection. Recording spectra at more than one temperature is also useful for detecting a given metal ion in a multicomponent system that has overlapping resonance lines.

A Comparison of Some Aspects of the Aqueous Coordination Chemistry of Aluminum(III) and Iron(III)

Abstract The aqueous coordination chemistry of Al3+ and Fe3+ is compared. The differences in electronegativities (Xm ), ionic radii (ri ) and electron configuration (dn ) for the two ions are shown to influence trends in aquo ion structure and reactivity, hydrolysis, and metal complex stability. Similarities in coordination chemistry result in similarities in distribution and modes of transport for Al3+ and Fe3+ in a biological system.

INFLUENCE OF SYNTHETIC CHELATING AGENTS ON THE SPECIATION OF COPPER, ZINC AND MANGANESE IN PLANT NUTRIENT SOLUTIONS

Abstract A chemical equilibrium model of a typical plant nutrient solution was constructed and applied, in conjunction with an appropriate data base of equilibrium constants, to the program MINEQL. The output yielded the speciation of the metals in the nutrient solution. Additions of DTPA or EDTA were found to suppress the concentration of copper(II), zinc(II) and manganese(II) aquo ions, the relative decrease in concentration being more marked at pH 7.5 than at pH 4.3 in the case of DTPA but more marked at pH 4.3 than at pH 7.5 in the case of EDTA. These results were found to correspond to the trends observed in experimental observations on hydroponically cultivated bush beans. The results are interpreted in terms of the aquo ions of the three metals considered being the bioavailable species. The possible importance of copper(II)-hydroxy and other dissolved copper(II) species as bioavailable forms was considered but no clear conclusions could be reached.

ChemInform Abstract: Sulfur‐Capped Triangular Tungsten(IV) Aqua Ion, W3O3S4+ and X‐Ray Structure of K2(W3O3S(N(CH2CO2H)(CH2CO2)2)3)·9 H2O.

AbstractA new triangular W(IV) aquo ion, W3O3S4+, in which besides S and O atoms the only other ligands are water, is prepared by the reaction of (NH4)2WS4 with K3W2Cl9 in HCl, followed by column chromatography.

Photodecomposition of europium(III) acetate and formate in aqueous solutions

The products obtained in the photodecomposition of europium(III) acetate and formate were determined quantitatively under various pH conditions. The two photoprocesses and the subsequent radical reactions have been found to proceed stoichiometrically with a cyclic redox reaction of the Eu/sup 3 +//Eu/sup 2 +/ couple being the key process. In the acetate system, (1) redox reactions of methyl radical with Eu/sup 2 +/ and Eu/sup 3 +/ aquo ions are supported to occur with formation of methane and methanol, respectively; (2) carboxymethyl radical produced through hydrogen abstraction from acetic acid by the methyl radical probably regenerates acetic acid via the intermediate formed through the radical capture reaction by Eu/sup 2 +/ ion, followed by acidolysis reaction. In the formate system, (1) it was confirmed that the photoprocess is followed by a chain reaction in which carboxyl radicals generated in the primary process and hydrogen atoms produced through reduction by illuminated Eu/sup 2 +/ ion behave as chain carriers; (2) evolution of carbon dioxide gas is mostly ascribed to the disproportionation of carboxyl radical and the reduction of Eu/sup 3 +/ ion by carboxyl radical. In both systems, minor products are formed by redox and disproportionation radical reactions.

ChemInform Abstract: A Temperature‐Jump Relaxation Study of the Zn2+‐Chlorophenol Red System

Abstracthas been performed in order to locate the octahedral/tetrahedral conversion involving the aquo ion.

Study of the prussian blue/prussian white redox reaction by cyclic voltammothermometry

In the reaction of the prussian blue (PB)/prussian white (PW) redox system, a pair of large peak currents was observed when potassium ion was used as the supporting cation. But when the other alkali halides were used, such peaks could not be observed. This phenomenon was studied by means of cyclic voltammothermometry (CVT). The contribution of the charging current was separated from that of the faradaic current. The charging current had a large contribution to the total current in the PW/PB redox system. The explanation given by the model for the appearance of the peak current is that the potassium ion could not enter the PB lattice hole but could enter the PW lattice hole. The heat generation of two typical Fe II/III redox systems, the aquo ion system and the ferrocyanide/ferricyanide system, was also measured for reference and the effect of the solvation states of the ions on the electrode reaction heat was examined.