The electrochemical reduction of oxygen to hydrogen peroxide at the dropping mercury electrode: Part II. Its kinetics at 0.4≤pH≤5.9

From experiments in acidic solutions it is shown that though the mechanism found in alkaline and neutral solutions (Part I) remains valid, the rate constant of the protonation step of the superoxide ion becomes pH-dependent. The experimental results can be quantitatively explained on the assumption of simultaneous protonation by water and free protons, the rate constants of electron transfer and protonation by water retaining their value.Therefore, the kinetics of the reduction of oxygen can be described by four parameters only, in the range 0.4≤pH≤12.5.The applicability of some other conceivable mechanisms is discussed.     

Azobenzene palladium(II) complexes with β-diketones

A procedure has been developed for the synthesis of cyclometalated azobenzene palladium(II) complexes [Pd(O^O)Azb], where Azb^– is deprotonated azobenzene and (O^O)^– is deprotonated β-diketone (acetylacetone, thenoyltrifluoroacetone, benzoylacetone, or dibenzoylmethane). Electronic absorption spectra and electrochemical behavior of the synthesized complexes have been studied. It has been found that the lowest unoccupied molecular orbital of the complexes is contributed most by the π* orbital of azobenzene, and the highest occupied molecular orbital, by the palladium d orbital.     

Packing of complexes and intermolecular interactions in the crystals of volatile palladium(II) β-diketonates. crystal structure of palladium(II) benzoyltrifluoroacetonate

This paper reports on the synthesis and crystal structure analysis of palladium(II) bis-(trans-benzoyltrifluoroacetonate). Crystal data for PdO₄C₂₀F₆H₁₂:a= 4.748, b = 22.237, c = 9.410 Å, P = 105.44° Z = 2, space group P2₁/c, d_calc. = 1.86 g/cm . CAD-4 diffmctometer(MoKa), heavy atom method, R = 0.041 for 986 reflections. The structure is molecular; the planar molecules are stacked. The Pd atom is surrounded by a square of four O atoms (Pd?O = 1.97 Å, chelate angle 94.4°); Pd is involved in additional intemctions with the C_γ atoms of two neighboring molecules (Pd...C_γ= 3.44 Å). The molecular structure of palladium β-diketonate complexes, molecular packing in crystal, and intermolecular intemctions are considered. The van der Waals energies of the crystal lattice are calculated. The results are compared with the experimental values. In the structures of palladium( 11) β-diketonates, three groups of crystals are isolated: one group with 2 nearest-neighbor interactions (stacks), the second with 4 nearest-neighbor interactions (two-dimensional nets), and the third with 1 nearest-neighbor interaction (dimers). The latter are characteristic of the cis-complexes. The intemctions with 10–12 nearest neighbor molecules determine the energetics of the whole crystal. The high volatility of Pd(hfa)₂ is explained by the weaker intermolecular contacts between stacks in crystal.     

The adsorption of mercury(II) on the surface of silica modified with β-cyclodextrin

Multistage chemical modification of the surface of silica with β-cyclodextrin was performed. IR spectroscopy and quantitative analysis of surface compounds were used to prove the structure of modified silica. The adsorption of Hg(II) from dilute solutions was studied. The adsorption affinity of silica for mercury ions increased because of the formation of supramolecular structures with chemically immobilized β-cyclodextrin.     

Thermal decomposition kinetics of polymeric complexes of copper(II) and mercury(II) with N,N′-bis(dithiocarboxy)piperazine

The thermal decomposition behaviour of polymeric complexes of Cu(II) and Hg(II) with N,N-bis(dithiocarboxy)piperazine is investigated in air by thermogravimetric (TG), derivative thermogravimetric (DTG) and differential thermal analysis (DTA) techniques. The kinetic parameters (non-isothermal method) for their decomposition have been evaluated by graphical as well as by least-squares methods. The equations of Coats-Redfern, Freeman-Carroll and Horowitz-Metzger have been applied. The results indicate that the values ofE,A and S obtained by these three different methods agree well. It was also found that the decomposition of these metal chelates follow first-order kinetics.

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Zusammenfassung Mittels TG, DTG und DTA wurde das thermische Zersetzungsverhalten von Polymerkomplexen aus Cu(II) bzw. Hg(II) mit N,N-Bis(dithiocarboxy)piperazin an Luft untersucht. Für ihre Zersetzung (nichtisotherme Methode) wurden die kinetischen Parameter sowohl graphisch als auch durch Methoden mit den kleinsten Fehlerquadraten ermittelt. Dabei wurden die Gleichungen von Coats-Redfern, Freeman-Carroll und von Horowitz-Metzger angewendet. Alle drei Verfahren zeigen übereinstimmende Resultate fürE, A undS. Es wurde weiterhin gefunden, daß diese Metallchelate einer Reaktion erster Ordnung unterliegen.

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The authors are thankful to Prof. C. G. R. Nair, Head of the Department of Chemistry, University of Kerala and Dr. M. P. Kannan, Department of Chemistry, University of Calicut for some helpful discussions.     

The stability constants of ML and ML2 complexes of Be(II) and Co(II)—isoleucine according to paper ionophoretic data

Complexation reactions of isoleucine with beryllium(II) and cobalt(II) have been studied in solution phase using paper ionophoretic technique. The stability constants of ML and ML₂ complexes of Be(II)—isoleucine and Co(II)—isoleucine systems have been found to be (7.49 ± 0.07, 5.89 ± 0.03) and (4.57 ± 0.11, 2.99 ± 0.09) (logarithm stability constant values), respectively at ionic strength of 0.1 mol L^?1 and a temperature of 35°C.     

Surface electrochemistry of the oxidation reactions of α- and β-alanine at a platinum electrode

The electrochemical oxidation reactions of α- and β-alanine at a Pt electrode were investigated in aqueous solutions at pH 1, 7, and 13 using steady-state current-potential measurements, cyclic voltammetry, and open circuit potential decay. The capacitance behaviour and the high Tafel slopes suggest the production of free radicals at the surface of the electrode accompanied by a second reaction involving loss of CO₂ which is the rate determining step. In the surface electro-oxidation of α-alanine, it appears that the adsorbed intermediate species is either hydrolyzed anodically to acetaldehyde and ammonia, or is oxidized to a carbonium ion which is subsequently hydrolyzed to acetaldehyde and ammonia in solution, analogous to the behaviour observed for glycine [D.G. Marangoni, R.S. Smith and S.G. Roscoe, Can. J. Chem., 67 (1989) 921]. The mechanisms for β-alanine would be similar except carbonium ion formation would probably be accompanied by a hydride transfer to form acetaldehyde. No dimerized products were detected by gas chromatography. These mechanisms differ from the dimerization process typical of the radical reactions associated with the Kolbe mechanism.     

The structure and EPR spectra of binuclear copper(II) complexes with acyldihydrazones of β-diketones

The synthesis and structure of binuclear copper(II) complexes with acyldihydrazones of some β-diketones are described. The molecular structure of the copper(II) complex [Cu₂L·4Py] with trifluoroacetylacetone succinyldihydrazone (H₄L) was determined by the single-crystal X-ray analysis. Central atoms are bridged by a chain of seven σ-bonds with a Cu...Cu distance of 8.750 Å. EPR spectra of copper(II) complexes with acyldihydrazones of trifluoroacetylacetone and lower dicarboxylic acids containing from one to four methylene groups in the aliphatic spacer show the seven-line HFS due to spin-spin coupling of unpaired electrons with the two equivalent copper nuclei (g = 2.110, a = (38.5–40.5)⊙10^?4 cm^?1). By extending polymethylene spacer, as well as by modifying starting β-diketone, an exchange interaction between paramagnetic centers is suppressed.     

The determination of the stability constants of the lanthanide α-hydroxyisobutyrate and lactate complexes by potentiometric titration

The stability constants of the lanthanide and yttrium, complexes with α-hydroxyisobutyrate and lactate, have been determined by potentiometric titration. The average numbor of ligands bound per metal ion has been calculated by the method of calvin-wilson and indicates the formation of uninegative tetra-ligand complexes of the form ML₄-. The ₄ formation constants have been derived by ₄ procedures; bjerrum's half-n¯;-method. fronaeus' and poulsen's extrapolation methods, and by least squares calculation, using an IBM 1620 digital computer.     

Stability constants of the complexes of Zn(II) and Mn(II) with N,N′-bis(2-hydroxyethyl) aminomethyl

The acid dissociation constants of N,N′-bis(2-hydroxyethyl) aminomethyl phosphonic acid (BHAMP) and the stability constants with Zn⁺⁺ and Mn⁺⁺ were determined at 25°C and at an ionic strength of 0.10 by means of potentiometric methods. The existence of a stable chelate species YM is illustrated. The equilibrium constants reported were calculated by the method developped by Schwartzenbach.     

Complexes of vitamin B6—IV: Polarographic determination of the stability constants of the mixed-ligand complexes of cadmium(II) with amino acids and pyridoxamine

Ternary complexes of cadmium(II)-pyridoxamine (PM-amino acid (L) systems were studied polarographically in aqueous solution at 0.50M KNO₃ of ionic strength. The amino acids considered were glycine, dl-alanine,dl-valine and β-phenylalanine. Three complexes of the forms Cd PM L, Cd PM L₂ and Cd(PM)₂L coexist in the pH range 8.0–10.5. The tendency toward mixed 1:1:1 complex formation was expressed in terms of the two equilibria Cd PM₂+Cd L₂⇋2 Cd PM L and Cd PM⁺ + Cd L⁺⇋Cd PM L + Cd², i.e. log X and Δ log K respectively.     

β-diketone complexes of manganese(II), cobalt(II), nickel(II) and palladium(II)

Summary The reactions of manganese(II), cobalt(II) and nickel(II) acetates (1 mole) with antipyrine-4-azo--ethylcyanoacetate (HL¹) and antipyrine-4-azo--acetylacetone (HL²) (1 mole) produce complexes of the M(L)₂ type. K₂PdCl₄ (1 mole) reacts with HL¹ and HL² (1 mole) to yield complexes of the general formula PdLCl, the ligands behaving as monobasic tridentates. The electronic spectral and magnetic data show the complexes to be high-spin octahedral, whereas the palladium(II) complexes are diamagnetic square planar. The complexes were characterized by elemental analyses, conductance measurements and i.r. and electronic spectra as well as magnetic susceptibility measurements and thermal (t.g.a. and d.t.a.) analysis.Nuclear Material Authority.     

The thermodynamic parameters of complex formation between the copper(II) Ion and β-alanine in aqueous solution

The heats of formation of β-alanine complexes with the doubly charged copper(II) ion were determined calorimetrically. The heat effects of interaction of a solution of the amino acid with a solution of copper(II) were measured at 288.15, 298.15, and 308.15 K and ionic strengths of 0.50, 0.75, and 1.00 against the background of KNO₃. The heats of dilution of a solution of copper nitrate with solutions of the background electrolyte were also determined for the introduction of the corresponding corrections. The standard thermodynamic characteristics of complex formation were calculated. The influence of temperature on the thermal effects of complex formation in the β-alanine-copper(II) ions system were considered. The standard enthalpies of formation of CuAla⁺ and CuAla₂ complex particles in aqueous solution were calculated.     

Electron transfer reactions in metal ammine complexes—IV: Reduction of some amino acid,aminobenzoic acid and their Schiff base substituted pentaammine Cr(III) complexes at the dropping mercury electrode

Aminoacid, aminobenzoic acid and their Schiff base substituted pentaammine Cr(III) perchlorate complexes undergo an irreversible, one electron reduction process at the dropping mercury electrode. The rates of the electrode process k_app, calculated from the values of diffusion current at the end of the life of the mercury drop, show small enhancements in presence of iodide ions. On comparing these rate enhancement values with the corresponding value for the reduction of [Cr(NH₃)₆]³⁺, which undergoes a completely outer-sphere path, it is concluded that these complexes follow a mixed inner/outer-sphere path. The contribution of the inner-sphere path is maximum in the case of aminoacid complexes and minimum in the case of Schiff bases of aminobenzoic acids. These results are further supported by the values of α_app, obtained from Tafel plots and other electrical double layer parameters like (δΦ_r/δE)_μ, (δδ_r/δq^m)μ and α_corr.     

Synthesis of Co–Pd alloys by co-electroreduction of aquachloro-cobalt(II) and palladium(II) complexes

The work presents results of the studies on the synthesis of Co–Pd alloys from acid electrolytes containing chloride ions. The main aim of the tests was to identify reactions responsible for alloy formation and to determine an influence of the electrolysis parameters, i.e. working electrode potential, electrolyte composition and temperature on the composition of the resulted alloy coatings. Electrochemical investigations were performed by applying cyclic voltammetry (CV) combined with electrochemical quartz crystal microbalance (EQCM). The electrolyte composition was selected based on a thermodynamic analysis and spectrophotometric tests which were described in our previous papers [1, 2]. They allowed determination of equilibrium distribution of the metals complex forms and a stability analysis of the electrolyte. The alloys were synthesized within the potential range from ?0.7 to ?1.1 V. The tests indicate a possibility of alloys synthesis already at the potential range 相似文献   

Spectrophotometric study of stability constants of cimetidine–Ni(II) complex at different temperatures

Cimetidine, a histamine H₂-receptor, has a structure which enables it to act as a chelating agent. The formation of nickel(II) complex with cimetidine has been studied spectrophotometrically at an absorption maximum of 622 nm at different temperatures. The data show that nickel(II) and cimetidine combine in the molar ratio of 1:2. The stability constants of the complex were calculated to be 1.40–2.4 × 10⁸ by continuous variation method and 1.24–2.4 × 10⁸ by mole ratio method at 25 and 40 °C, respectively. The immediately formed complex shows stability with respect to time and temperature.     

Quantum chemical calculations of stability constants: study of ligand effects on the relative stability of Pd(II)�Cpeptide complexes

Replacement of [Pd(H₂O)₄]²⁺ by cis-[Pd(en)(H₂O)₂]²⁺, [PdCl₄]^2?, and [Pd(NH₃)₄]²⁺ on the hydrolytic cleavage of the Ace-Ala-Lys-Tyr-Gly?CGly-Met-Ala-Ala-Arg-Ala peptide is theoretically investigated by using different quantum chemical methods both in the gas phase an in water solution. First, we carry out a series of validation calculations on small Pd(II) complexes by computing high-level ab initio [MP2 and CCSD(T)] and Density Functional Theory (B3LYP) electronic energies while solvent effects are taken into account by means of a Poisson-Boltzmann continuum model coupled with the B3LYP method. After having assessed the actual performance of the DFT calculations in predicting the stability constants for selected Pd(II)-complexes, we compute the relative free energies in solution of several Pd(II)?Cpeptide model complexes. By assuming that the reaction of the peptide with cis-[Pd(en)(H₂O)₂]²⁺, [Pd(Cl)₄]^2?, and [Pd(NH₃)₄]²⁺ would lead to the initial formation of the respective peptide-bound complexes, which in turn would evolve to afford a hydrolytically active complex [Pd(peptide)(H₂O)₂]²⁺ through the displacement of the en, Cl^?, and NH₃ ligands by water, our calculations of the relative stability of these complexes allow us to rationalize why [Pd(H₂O)₄]²⁺ and [Pd(NH₃)₄]²⁺ are more reactive than cis-[Pd(en)(H₂O)₂]²⁺ and [PdCl₄]^2? as experimentally found.     

X-ray photoelectron study of electron density distribution in palladium(II) β-diketonate complexes

This work is devoted to experimental (X-ray photoelectron) and theoretical investigations of electron density distribution in Pd(II) β-diketonate complexes. Data about the electronic structure (effective charges, core level energies) of the compounds are compared with their thermodynamic parameters (thermal stability, vaporization enthalpy). In molecular crystals of Pd(II) β-diketonates, the volatility of the complexes and vaporization enthalpy ΔH _T ⁰ depend not only on van der Waals interactions, but also on electrostatic interactions of molecules in crystal.     

Structural effects of the lone pair on lead(II), and parallels with the coordination geometry of mercury(II). Does the lone pair on lead(II) form H-bonds? Structures of the lead(II) and mercury(II) complexes of the pendant-donor macrocycle DOTAM (1,4,7,10-tetrakis(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane)

The synthesis and structures of [Pb(DOTAM)](ClO4)2.4.5H2O (1) and [Hg(DOTAM)](ClO4)2.0.5CH3OH.1.5H2O (2) are reported, where DOTAM is 1,4,7,10-tetrakis(carbamoylmethyl)-1,4,7,10-tetraazacyclododecane. Compound 1 is triclinic, space group P, a = 12.767(3) A, b = 13.528(2) A, c = 18.385(3) A, alpha = 101.45(2) degrees, beta = 93.32(2) degrees, gamma = 90.53(2) degrees, Z = 4, R = 0.0500. Compound 2 is monoclinic, space group Cc, a = 12.767(3) A, b = 13.528(2) A, c = 18.385(3) A, beta = 101.91(2) degrees, Z = 4, R = 0.0381. The Pb(II) ion in 1 has an average Pb-N = 2.63 A to four N-donors from the macrocyclic ring, and four O-donors (average Pb-O = 2.77 A) from the amide pendant donors of the macrocycle, with a water molecule placed with Pb-O = 3.52 A above the proposed site of the lone pair (Lp) on Pb. The Hg(II) in 2 appears to be only six-coordinate, with four Hg-N bond lengths averaging 2.44 A, and two Hg-O from pendant amide donors at 2.41 A. The other two amide donors appear to be noncoordinating, with Hg-O distances of 2.74 and 2.82 A. A water situated 3.52 A above the proposed site of the lone pair on Pb(II) in 1 is oriented in such a way that it might be thought to be forming a Pb-Lp.H-O-H hydrogen bond. It is concluded that that this is not an H-bond, but that the presence of the lone pair allows a closer approach of the hydrogens to Pb than would be true otherwise. The structural analogy in the VSEPR sense between Pb(II), which has the 5d(10)6s(2) outer electron structure, and the Hg(II) ion, which has the 5d10 structure, is examined. The tendency of Hg(II) toward linear coordination, with two short Hg-L bonds (L = ligand) at 180 degrees to each other, and other donor groups at roughly 90 degrees to this and at much longer bond distances, is paralleled by Pb(II). One of the short Hg-L bonds is replaced in the Pb(II) structures by the lone pair (Lp), which is opposite the short Pb-L bond, or in some cases 2-4 shorter Pb-L bonds.     

Potentiometric investigation of ternary complexes of nickel(II), zinc(II) and cadmium(II) ions with β-alaninehydroxamic acid and ethylenediamine

Summary The formation constants of mixed ligand complexes of M^II (M = Ni, Zn or Cd) with -alaninehydroxamic acid (-Alaha) as primary ligand (A) and ethylenediamine (en) as secondary ligand (B) have been determined potentiometrically [298 K, = 0.2 (KCl)]. The formation constants for binary systems of -alaninehydroxamic acid and ethylenediamine were determined under identical experimental conditions. The pH-titration data were analysed using the SUPERQUAD computer program. The relative stability of each ternary complex was compared to that of its corresponding binary complex in terms of log X values.