Study of iron-chelates in solid state and aqueous solutions using Mössbauer spectroscopy - PDF

Scientific Supervisors: Prof. Zoltán Homonnay, professor and Prof. Azzedine Bousseksou, research director Laboratory of Nuclear Chemistry, Institute of Chemistry PhD School of Chemistry, Dr. György Inzelt

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Scientific Supervisors: Prof. Zoltán Homonnay, professor and Prof. Azzedine Bousseksou, research director Laboratory of Nuclear Chemistry, Institute of Chemistry PhD School of Chemistry, Dr. György Inzelt Theoretical and Physical Chemistry, Research of Material Structure PhD Programme, Dr. Péter Surján Eötvös Loránd University of Budapest, Hungary Switchable Molecular Materials Group Laboratory of Coordination Chemistry, CNRS, France PhD School Material Science, Dr. Roland Martinot Nanophysics PhD Programme, Dr. Jean-Claude Ousset Paul Sabatier University of Toulouse, France DOCTORATE THESIS of Petra Ágota Szilágyi Study of iron-chelates in solid state and aqueous solutions using Mössbauer spectroscopy Submitted for the degree of PhD 2007 Abstract Introduction The ferric-ethylenediaminetetraacetate complex (Fe III EDTA) features several interesting properties in both solution phase and solid state. This compound is widely utilised in industry and is being focussed on by many basic and applied scientific projects to discover more domains where the compound and its analogues could be used. A few examples for application of these compounds will be presented below, in which they are already utilised or their application is known to be possible but the actual reaction mechanisms are not completely understood yet. The most interesting property of the ferric-ethylenediaminetetraacetate complex is probably its reaction with hydrogen peroxide, which results in a peroxo-bounded complex. This latter complex is known to be, on the one hand, a good mimic of certain enzymes, therefore it has been widely studied in basic research, on the other hand, it was also studied in applied research, for it can be used as a very strong oxidising agent. Furthermore, the products of its reactions are not hazardous. This advantage, related to this complex gives rise to another major application field: purification of waste water by the photodegradation of the ferricethylenediaminetetraacetate. In fact, due to the wide utilisation of the ethylenediaminetetraacetic acid as a cheap chelating agent, its complexes formed with heavy metal ions are some of the most common water pollutants, therefore the decomposition of the chelating agent, which can be led through the photodegradation of its ferric complex, is one of the biggest challenges in waste water treatment. Nevertheless, the mechanism of these reactions is not yet completely described, which could be due to the fact that the structures of these ferric complexes are not very well known. Thus, I intended to contribute to our knowledge about the mechanism of these reactions and about their possible application, by studying the structure, physical proerties and the reactions of these ferric complexes. The problems I worked on and the methods I used are shortly described below: New Scientific Results 1 Study of the of the structure of Fe III EDTA/CDTA/EDDA Complexes in Aqueous Medium as a Function of ph Solutions of different ph values have been studied using Mössbauer Spectroscopy with and without applied external magnetic field at different temperatures, ESR Spectroscopy at different temperatures and Magnetic Measurements in order to better describe the speciation of the iron species and their dimerisation. In the Fe III EDTA system, a protolytic reaction was observed between ph=0.6 and 4.1. Beside the paramagnetic spin relaxation which is a common feature of high spin ferric species, especially in solutions (spin-spin relaxation in this case), the system displays another interesting magnetic phenomenon. 1.1) In the Mössbauer spectra recorded in an applied external magnetic field, two distinct ferric species have been observed in solutions at low ph ( ), which display a ferrimagnetic behaviour in frozen solutions 1.2) The two species (being present at each ph value, regardless of their protonation degree) differ probably in their coordination numbers: i.e. the spectrum component with lower isomer shift must refer to a species with sixfold coordination, whereas the other spectrum component refers to a species with sevenfold coordination and the ligand spheres of the ferric ions differ in the coordination of one water molecule. 1.3) I studied the Fe III CDTA system within a ph range of 8.1 to I have evidenced a protolytic exchange reaction between these ph values, which had already been suggested according to protolytic titration data. 1.4) I also studied the dimerisation of the Fe III CDTA complex and I observed that the monomeric form of the Fe III CDTA complex is more stable than its dimeric form relative to the Fe III EDTA system, because at the same concentration and ph range the dimerisation degree was always higher for the EDTA than for the CDTA complex. I recorded Mössbauer spectra of these species and I have reinforced the hypotheses concerning the structures and coordination numbers of these CDTA-containing ferric species by the correlation of hyperfine parameters. 1.5) I observed that the differences in the ligands (CDTA, EDTA) coordinated to the ferric ions affect considerably the frequency of the paramagnetic spin relaxation. 1.6) The results of my experiments carried out in order to study the dimerisation of the Fe III EDDA complex showed that a different type of dimer forms than in the EDTA and CDTA cases. Because the quadrupole splitting of the doublet representing the Fe III EDDA dimer was remarkably lower than those observed for the EDTA and CDTA containing dimers, I assumed that the Fe-O-Fe bite angle is lower than 180. This assumption is reinforced by the observation that the dimerisation was not followed by the red colouration of the originally yellow solutions, which can be explained by the fact that the EDDA is only tetradentate, whereas the CDTA and EDTA chelating agents are hexadentate ligands and thus the structure of the dimer is less rigid. 2 Tuning the oxidation state: Study of the photodegradation of the Fe III EDTA Complex; Study of the Autoxidation of the Fe 2+ /EDTA System in Solid State and in Solution Phase Photodegradation and autoxidation of the Fe III EDTA complex have been studied with Mössbauer Spectroscopy. My results concerning the photodegradation, which, amongst others, consists of the reduction of the central ferric ion to ferrous species, of the Fe III EDTA complex led to three major conclusions: 2.1) I have evidenced spectroscopically, that, contrary to previous hypotheses, the photodegradation occurs at high ph values (ph 10), i.e. when the Fe III EDTA complex is in dimeric form. 2.2) No photodegradation occurs when no ferric-(oxy)hydroxide precipitate was present in the solutions (such precipitation often occurs when raising the ph of the aqueous solution), i.e. the smallest amount of precipitate (photodegradation occurred after filtering off the precipitate as well) can be enough, but precipitation is necessary to trigger the photoreduction (surface catalysis). 2.3) Contrary to the hypotheses published so far, I have found that the Fe II EDTA complex was not present in the solution phase, but the only ferrous species of an appreciable lifetime is the ferrous hexaaqua complex. air. I studied the autoxidation, i.e. the spontaneous oxidation of the Fe II /EDTA system in 2.4) No formation of Fe II EDTA complex was experienced in solution phase with this method. This is an important finding since the existence of this complex has always been supposed in previous literature, even if no spectroscopic evidence has been given so far for its formation.[1] 2.5) It was found that these reactions took place without any intermediate species of an appreciable lifetime, either in solution phase or solid state. The final product of the autoxidation reactions was found to be the dimeric Fe III EDTA complex, in both solid, and solution cases.[1] 3 Study of the reaction of Fe III EDTA/CDTA/EDDA with H 2 O 2 I studied the reaction of all these three complexes using Mössbauer spectroscopy. 3.1) The presence of two intermediate species in the course of the reaction between the Fe III EDTA complex with H 2 O 2 has been shown and possible structures have been proposed as well.[3] 3.2) In agreement with previous literature data about the kinetics of the reaction with hydrogen peroxide, I gave a possible reaction pathway including the intermediate species I identified.[3] 3.3) I also studied the effect of the photodegradation on the reaction between Fe III EDTA and hydrogen peroxide and it was found to be rather significant. The presence of ferrous ions, which are formed in the course of the photodegradation of the Fe III EDTA complex, seems to inhibit the formation of the dihapto-peroxo complex. 3.4) Despite the Fe III CDTA system being less studied and its reaction with hydrogen peroxide being less known than in the case of EDTA, I was able to determine the formation of distinct species in the course of this reaction. 3.5) The reaction mechanism for the ferric EDTA and CDTA complexes was found to be similar. In both cases the system relaxes back to its original state after all reactions have occurred (the chelating agent was added in excess to the systems in every case). 3.6) When Fe III EDDA was reacted with hydrogen peroxide, the spectra recorded of the system were completely different from those recorded when using EDTA or CDTA complexes, therefore another reaction mechanism can be assumed for the EDDA complex. Furthermore, the original state did not return when using Fe III EDDA complex as a reactant despite the considerable excess of the chelating agent. 4 Solid State Study of the Fe III EDTA Complex: Study of the Magnetic Relaxation Properties of the Fe III EDTA; Study of the Thermal Stability of the Fe III EDTA Complex in its Monomeric Form The relaxation properties of the solid NaFeEDTA 3H 2 O have been studied using Mössbauer spectroscopy with and without external magnetic field at different temperatures, magnetic susceptibility measurements and single crystal XRD analysis. 4.1) The material is paramagnetic, obeying the Curie law. However, Mössbauer experiments revealed that at low temperature (12 K) and in an external magnetic field (7 T) there are two species displaying ferrimagnetic interaction. 4.2) The zero-field Mössbauer spectra revealed a paramagnetic spin relaxation in a wide temperature range. The fact that the spectral lineshape did not change with temperature, together with the fact that the spectrum recorded in a concentrated Fe III EDTA solution was identical to those recorded of the solid material (with and without crystal water molecules), shows that the nature of the relaxation is not spin-lattice but spin-spin. The decomposition of the Fe III EDTA complex was studied by carrying out DTG/TGA measurements to identify the temperature values of the different decomposition steps, then heat treatments were done in a furnace. 4.3) The degradation products were characterised using Mössbauer spectroscopy. Furthermore, I also studied the further oxidation of the resultant ferrous degradation products in order to obtain more structural information about the original degradation products. I showed that thermal degradation of the Fe III EDTA complex results in two ferrous degradation products, which are probably configurational isomers and have probably a µ-oxo dimer structure. Perspectives My results on the photodegradation of the Fe III EDTA and on the autoxidation of the Fe 2+ /EDTA system may be applied in wastewater treatment. These results may contribute to the global understanding of analogue systems as well. My results on the reaction of Fe III EDTA with hydrogen peroxide pointed out that Mössbauer spectroscopy can be also used to investigate relatively fast reactions qualitatively and one can also obtain information about short-lived intermediate species. Hopefully as enzyme-mimics, the identification of the intermediate species can enlighten many biological processes, such as the enzymatic activity of the superoxidise and dismutase. My publications List of publications in the research field of iron-chelates: 1. P. Á. Szilágyi, Z. Homonnay, R. Szalay, V. K. Sharma, A. Vértes, Mössbauer study of the autoxidation of ethylenediaminetetraacetato-ferrate(ii), Structural Chemistry, accepted, Z. Homonnay, N. Smith, V. K. Sharma, P. Á. Szilágyi, E. Kuzmann, Transformation of iron(vi) into iron(iii) in the presence of chelating agents: a frozen solution Mössbauer study, In: ACS Symposium Series: Ferrates: Synthesis, Properties, and Applications in Water and Wastewater Treatment, in press, V. K. Sharma, P. Á. Szilágyi, Z. Homonnay, E. Kuzmann, A. Vértes, Mössbauer Investigation of Peroxo Species in the Iron(III)-EDTA-H 2 O 2 System, European Journal of Inorganic Chemistry, 2005, K. Kovács, A. A. Kamnev, E. Kuzmann, Z. Homonnay, P. Á. Szilágyi, V. K. Sharma, A. Vértes, Mössbauer studies of iron(iii)-(indole-3-alkanoic acids) systems in frozen aqueous solutions, Journal of Radioanalytical and Nuclear Chemistry, 2005, List of publications in other research fields: 5. P. Tabero, A. Blonska-Tabero, P. Á. Szilágyi, Z. Homonnay, The investigations of phases with general formula M 2 FeV 3 O 11, where M=Mg, Co, Ni, Zn by IR and Mössbauer spectroscopy, Journal of Physics and Chemistry of Solids, in press, Z. Homonnay, P. Á. Szilágyi, E. Kuzmann, K. Varga, Z. Németh, A. Szabó, K. Radó, J. Schunk, P.Tilky, G. Patek, Corrosion study of heat exchanger tubes in pressurized water cooled nuclear reactors by conversion electron Mössbauer spectroscopy, Journal of Radioanalytical and Nuclear Chemistry, 2007, O. Graziani, P. Hamon, J-Y. Thépot, L. Toupet, P. Á. Szilágyi, G. Molnár, A. Bousseksou, M. Tilset, J.R. Hamon, Novel tert-butyl-tris(3-hydrocarbylpyrazol-1-yl)borate ligands: Synthesis, Spectroscopic studies and Coordination chemistry, Inorganic Chemistry, 2006, 8. C. Desroches, G. Pilet, P. Á. Szilágyi, G. Molnár, S. A. Borshch, A. Bousseksou, S. Parola, D. Luneau, Tetra- and Decanuclear Iron(II) Complexes of Thiacalixarene Macrocycles: Synthesis, Structure, Mössbauer Spectroscopy and Magnetic Properties, European Journal of Inorganic Chemistry, 2006, P. Á. Szilágyi, S. Dorbes, G. Molnár, J. A. Real, C. Faulmann, A. Bousseksou Temperature and Pressure Effects on the Spin State of Fe III Ions in the [Fe(sal 2 - trien)][ni(dmit) 2 ] Complex, J. Phys. Chem. Solids, submitted, 2007.
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