Zoltán Homonnay

Detailed Spectroscopic, Thermodynamic, and Kinetic Studies on the Protolytic Equilibria of FeIIIcydta and the Activation of Hydrogen Peroxide

The crystal structure of the as-yet-unknown salt K[Fe(III)(cydta)(H(2)O)].3H(2)O, where cydta = (+/-)-trans-1,2-cyclohexanediaminetetraacetate, has been resolved: orthorhombic space group Pbca with R1 = 0.0309, wR2 = 0.0700, and GOF = 0.99. There are two independent [Fe(III)(cydta)(H(2)O)](-) anions in the asymmetric unit, and the ligand is (R,R)-cydta in both cases. The coordination polyhedron is a seven-coordinate capped trigonal prism where the quadrilateral face formed by the four ligand donor oxygen atoms is capped by the coordinated water molecule. The speciation of [Fe(III)(cydta)(H(2)O)](-) in water was studied in detail by a combination of techniques: (i) Measurements of the pH dependence of the Fe(III/II)cydta redox potentials by cyclic voltammetry enabled the estimation of the stability constants (0.1 M KNO(3), 25 degrees C) of [Fe(III)(cydta)(H(2)O)](-) (log beta(III)(110) = 29.05 +/- 0.01) and [Fe(II)(cydta)(H(2)O)](2-) (log beta(II)(110) = 17.96 +/- 0.01) as well as pK(III)(a1OH) = 9.57 and pK(II)(a1H) = 2.69. The formation enthalpy of [Fe(III)(cydta)(H(2)O)](-) (DeltaH degrees = -23 +/- 1 kJ mol(-1)) was measured by direct calorimetry and is compared to the corresponding value for [Fe(III)(edta)(H(2)O)](-) (DeltaH degrees = -31 +/- 1 kJ mol(-1)). (ii) pH-dependent spectrophotometric titrations of Fe(III)cydta lead to pK(III)(a1OH) = 9.54 +/- 0.01 for deprotonation of the coordinated water and a dimerization constant of log K(d) = 1.07. These data are compared with those of Fe(III)pdta (pdta = 1,2-propanediaminetetraacetate; pK(III)(a1OH) = 7.70 +/- 0.01, log K(d) = 2.28) and Fe(III)edta (pK(III)(a1OH) = 7.52 +/- 0.01, log K(d) = 2.64). Temperature- and pressure-dependent (17)O NMR measurements lead to the following kinetic parameters for the water-exchange reaction at [Fe(III)(cydta)(H(2)O)](-) (at 298 K): k(ex) = (1.7 +/- 0.2) x 10(7) s(-1), DeltaH(++) = 40.2 +/- 1.3 kJ mol(-1), DeltaS(++) = +28.4 +/- 4.7 J mol(-1) K(-1), and DeltaV(++) = +2.3 +/- 0.1 cm(3) mol(-1). A detailed kinetic study of the effect of the buffer, temperature, and pressure on the reaction of hydrogen peroxide with [Fe(III)(cydta)(H(2)O)](-) was performed using stopped-flow techniques. The reaction was found to consist of two steps and resulted in the formation of a purple Fe(III) side-on-bound peroxo complex [Fe(III)(cydta)(eta(2)-O(2))](3-). The peroxo complex and its degradation products were characterized using Mossbauer spectroscopy. Formation of the purple peroxo complex is only observable above a pH of 9.5. Both reaction steps are affected by specific and general acid catalysis. Two different buffer systems were used to clarify the role of general acid catalysis in these reactions. Mechanistic descriptions and a comparison between the edta and cydta systems are presented. The first reaction step reveals an element of reversibility, which is evident over the whole studied pH range. The positive volume of activation for the forward reaction and the positive entropy of activation for the backward reaction suggest a dissociative interchange mechanism for the reversible end-on binding of hydrogen peroxide to [Fe(III)(cydta)(H(2)O)](-). Deprotonation of the end-on-bound hydroperoxo complex leads to the formation of a seven-coordinate side-on-bound peroxo complex [Fe(III)(cydta)(eta(2)-O(2))](3-), where one carboxylate arm is detached. [Fe(III)(cydta)(eta(2)-O(2))](3-) can be reached by two different pathways, of which one is catalyzed by a base and the other by deprotonated hydrogen peroxide. For both pathways, a small negative volume and entropy of activation was observed, suggesting an associative interchange mechanism for the ring-closure step to the side-on-bound peroxo complex. For the second reaction step, no element of reversibility was found.

The structure of Fe(III) ions in strongly alkaline aqueous solutions from EXAFS and Mössbauer spectroscopy

To establish the structure of ferric ions in strongly alkaline (pH > 13) environments, aqueous NaOH solutions supersaturated with respect to Fe(III) and the solid ferric-hydroxo complex salts precipitating from them have been characterized with a variety of experimental techniques. From UV measurements, in solutions of pH > 13, only one kind of Fe(III)-hydroxo complex species was found to be present. The micro crystals obtained from such solutions were proven to be a new, so far unidentified solid phase. Mössbauer spectra of the quick-frozen solution and that of the complex salt indicated a highly symmetrical ferric environment in both systems From the EXAFS and XANES spectra, the environment of the ferric ion in these solutions (both native and quick-frozen) and in the complex salt was found to be different. In the complex salt, the bond lengths are consistent with an octahedral coordination around the ferric centres. In solution, the coordination geometry of Fe(III) is most probably tetrahedral. Our results demonstrate that in strongly alkaline aqueous solutions, ferric ions behave very similarly to other structurally related tervalent ions, like Al(III) or Ga(III).

In situ spectroscopic studies related to the mechanism of the Friedel–Crafts acetylation of benzene in ionic liquids using AlCl3 and FeCl3

Several aspects of the mechanism of the Friedel–Crafts acetylation of benzene were studied by in situ spectroscopic methods in ionic liquids, prepared from MCl3 (M = Al or Fe) and 1-butyl-3-methylimidazolium chloride ([bmim]Cl). Mossbauer measurements have revealed that the addition of FeCl3 to [bmim]Cl leads to an equilibrium mixture that contains solid FeCl3, [bmim][Fe2Cl7], and Fe2Cl6 and/or [bmim][FeCl4], depending on the molar ratio of FeCl3 and [bmim]Cl. The formation of [(CH3CO)2CHCO]+[MCl4]−, a potential side product in the Friedel–Crafts acetylation of benzene, was shown to require the presence of both the acetylium ion [CH3CO]+[MCl4]− and free acetyl chloride. We have confirmed that [(CH3CO)2CHCO]+[MCl4]− does not involve in the Friedel–Crafts acetylation of benzene. Experimental data and theoretical calculations indicate that the acetylium ion [CH3CO]+[MCl4]− is the key intermediate in the Friedel–Crafts acetylation of benzene and the reaction proceeds through an ionic mechanism.

Decomposition of Potassium Ferrate(VI) (K2FeO4) and Potassium Ferrate(III) (KFeO2): In‐situ Mössbauer Spectroscopy Approach

Mossbauer spectroscopy was shown to be very useful technique studying the mechanism of thermal decomposition or aging processes of the most known ferrate(VI), K2FeO4. In‐situ Mossbauer spectroscopy approach was used to monitor the phase composition during the studied processes. The experimental set‐up was designed to perform in‐situ measurements at high temperatures and at different air humid conditions at room temperature. The potassium ferrate(III), KFeO2 was demonstrated to be the primary product of thermal decomposition of K2FeO4. The KFeO2 was unstable in a humid air at room temperature and reacted with components of air, H2O and CO2 to give Fe2O3 nanoparticles and KHCO3. The aging kinetics of K2FeO4 and KFeO2 under humid air were significantly dependent on the relative air humidity.

Mössbauer studies of the interaction of oxygen with solid β-FeII-phthalocyanine

Oxygen has been diffused into the interplanar spacings of solid β-FeIIPc at relatively low temperatures in the dry state and in aqueous suspensions. A variety of oxygen adducts are stabilized in the solid matrix, which do not exist in solution. Tentative assignment of species have been made with the help of Mossbauer and IR spectroscopy.

Preparation of nanosize FeS particles on SiO2 and clay mineral supports: SAXS and Mössbauer spectroscopic measurements

FeS particles were synthesized in the solid/liquid interfacial adsorption layer on the surface of highly disperse SiO2 particles, and hydrophobic montmorillonites of lamellar structure. It was established that the presence of nanoparticles can be verified by X-ray diffraction (XRD) and small angle X-ray scattering (SAXS) measurements and their diameter can be determined from the so-called Guinier plot. On the FeS-containing samples, the Mossbauer spectra proved that two components were formed in the studied samples: FeS and FeSO4 · H2O. The Mossbauer parameters showed that Fe2+ has a tetrahedral, rather than an octahedral, environment in FeS.

Speciation and structure of tin(II) in hyper-alkaline aqueous solution

The identity of the predominating tin(ii)-hydroxide complex formed in hyper-alkaline aqueous solutions (0.2 ≤CNaOH≤ 12 mol dm(-3)) is determined by potentiometric titrations, Raman, Mössbauer and XANES spectroscopy, supplemented by quantum chemical calculations. Thermodynamic studies using a H2/Pt electrode up to free hydroxide concentrations of 1 mol dm(-3) showed the presence of a single monomeric complex with a tin(II) : hydroxide ratio of 1 : 3. This observation together with Raman and Mössbauer spectroscopic measurements supplemented by quantum mechanical calculations proved that the predominating complex is [Sn(OH)3](-), and that the presence of the other possible complex, [SnO(OH)](-), could not be proven with either experiments or simulations. The structure of the trihydroxidostannate(II) complex, [Sn(OH)3](-), was determined by EXAFS and was found to be independent of the applied hydroxide and tin(II) concentrations. The mean Sn-O bond distance is short, 2.078 Å, and in very good agreement with the only structure reported in the solid state. It is also shown that at pH values above 13 the speciation of the predominant trihydroxidostannate(II) complex is not affected by the presence of high concentrations of chloride ions.

CO2 absorption of perovskites as seen by positron lifetime spectroscopy

Abstract The CO 2 absorption of several ABO 3 type perovskites was studied by positron lifetime spectroscopy. The longer positron lifetime was associated with positrons trapped by A site vacancies. The evaluated positron lifetime data indicated the relative stability of the crystal structure of Sr(Co 0.5 Fe 0.5 )O 3− δ against Ca doping at low Ca concentrations. Oxygen desorption and CO 2 absorption/desorption could also be followed by positron lifetime spectroscopy. It was shown that the concentration of oxygen vacancies has a large effect on positron lifetime data through the electron density of A site vacancies.

On the synthesis and steric distortion of the tris(2,2′-bipyridine)iron(II)complex ion in zeolite-Y

Mossbauer spectra and X-ray diffraction patterns of [Fe(bpy)3]2+synthesized in supercages of zeolite Y reveal three interesting features of this system: first; the spectra show no evidence of steric distortion of the complex, secondly, special pretreatment of zeolite (ion exchange with La3+ and a subsequent heat treatment in order to block the small cavities) can stimulate the complex formation in the zeolite, and thirdly, despite the fact that the large complex cannot be introduced directly into the Y zeolite, it can be removed (at least partially) by cation exchange, by taking advantage of the dynamic feature of complex equilibrium in the presence of a ligand-eliminating process.

Emission Mössbauer study of oxygen deficient PrBa2Cu3(57Co)O6+x

Abstract The variation of Mossbauer parameters as a function of oxygen content in PrBa 2 Cu 3 ( 57 Co ) O 6+x has been studied and compared to the results of similar studies on YBa 2 Cu 3 ( 57 Co)O 6+x . It was found that the Mossbauer isomer shift of 57 Co( 57 Fe) dopant at Cu(1) site shows the same behaviour as for YBa 2 Cu 3 ( 57 Co ) O 6+x except that all isomer shifts are significantly higher than those for YBa 2 Cu 3 ( 57 Co)O 6+x and several other superconducting 1-2-3 compounds. These findings are explained by the tendency of Pr III to form Pr IV , which can be responsible for the absence of superconductivity, and charge transfer between the Cu(2)-O sheets and Cu(1)-O chains. It is also concluded that at high oxygen stoichiometries the Cu(1)-O chains have metallic character for both compounds.