Mössbauer and electrical investigation of sulfur-doped sodium borate glasses containing iron

Three series of sulfur-doped sodium tetraborate glasses containingiron was prepared with the composition (Na₂ B₄ O₇ ) _1–x (Fe₂ O ₃ ) x , wherex equals 0.1, 0.2 and 0.3. One series of the glass was left free of sulfur,while sulfur was added to the other two series of 2.5 and 5 wt%, respectively,to investigate the role of sulfur ions on some physical properties of thistype of glass. Melting process was carried out in platinum crucibles at 1100°C for one hour. The glasses were quenched in air at room temperature.X-ray diffraction measurement was carried out to be sure that the pure glassystate is obtained. ⁵⁷Fe Mössbauer spectroscopy and dc conductivitymeasurements were carried out. It was found that iron is present in form ofFe²⁺ and Fe ³⁺ ions. Sulfur ions increased the valuesof dc conductivity. The isomer shift of the two states of iron is stable whilethe quadrupole splitting value changed with increasing Fe₂ O₃ content or as sulfur was added.     

Low‐Spin Pseudotetrahedral Iron(I) Sites in Fe2(μ‐S) Complexes

Fe^I centers in iron–sulfide complexes have little precedent in synthetic chemistry despite a growing interest in the possible role of unusually low valent iron in metalloenzymes that feature iron–sulfur clusters. A series of three diiron [(L₃Fe)₂(μ‐S)] complexes that were isolated and characterized in the low‐valent oxidation states Fe^II? S? Fe^II, Fe^II? S? Fe^I, and Fe^I? S? Fe^I is described. This family of iron sulfides constitutes a unique redox series comprising three nearly isostructural but electronically distinct Fe₂(μ‐S) species. Combined structural, magnetic, and spectroscopic studies provided strong evidence that the pseudotetrahedral iron centers undergo a transition to low‐spin S=1/2 states upon reduction from Fe^II to Fe^I. The possibility of accessing low‐spin, pseudotetrahedral Fe^I sites compatible with S^2? as a ligand was previously unknown.     

UV-absorption and radiation effects in different glasses doped with iron and tin in the ppm range

Intrinsic and extrinsic ultraviolet absorption and radiation-induced effects were investigated in different glass types, fluorides, phosphates and borosilicates. High-purity glass samples were prepared and their intrinsic absorption was measured in the vacuum ultraviolet region. The influence of doped iron and tin species in the ppm range on the ultraviolet absorption and radiation-induced effects were studied. The maximum of the dominating Fe³⁺ charge transfer band has the lowest energy (4.8 eV) and intensity in the fluoride glass and the highest energy (5.6 eV) and intensity in the borosilicate glass samples. The charge-transfer band for Fe²⁺ has much lower intensity and higher energy (∼5.7 eV) than those for Fe³⁺ in all glasses investigated. Photo-oxidation of Fe²⁺ to (Fe²⁺)⁺ hole centres and glass-matrix-related electron centres by UV irradiation increases the UV absorption drastically in all glasses. The kinetics was measured and simulated depending on the glass matrix. In fluoride and phosphate glasses, Fe³⁺ complexes are very stable against UV irradiation and do not participate in UV-radiation-induced processes. Only in silicate glasses, Fe³⁺ is able to form a (Fe³⁺)^– electron centre defect which decreases the charge transfer absorption of Fe³⁺ near 220 nm, but increase the absorption of hole centre defects, with a maximum at 280 nm. So, the defect generation in the ultraviolet region increases drastically with increasing Fe content in the range 10–200 ppm. Three or four electronic s → p transitions for Sn²⁺ were detected by optical absorption and luminescence spectroscopy shifted to longer wavelength in the range fluoride → phosphate → silicate glass samples. Sn⁴⁺ absorption bands were found at shorter wavelength in the vacuum ultraviolet region in all cases investigated. Sn²⁺ ions are photo-oxidised under UV radiation very fast, which leads to an decrease of absorption near 200 nm and to an increase near 250 nm. Both Sn²⁺ and Sn⁴⁺ are involved in the radiation-induced processes. In contrast to phosphate and silicate glasses, tin-doped fluoride glasses are very resistant against UV lamp but not against UV laser irradiation. The mechanisms are very complicated, with maximums and minimums in the defect formation curves.     

Controlled Incorporation of Nitrides into W‐Fe‐S Clusters

Incorporation of monatomic 2p ligands into the core of iron–sulfur clusters has been researched since the discovery of interstitial carbide in the FeMo cofactor of Mo‐dependent nitrogenase, but has proven to be a synthetic challenge. Herein, two distinct synthetic pathways are rationalized to install nitride ligands into targeted positions of W‐Fe‐S clusters, generating unprecedented nitride‐ligated iron–sulfur clusters, namely [(Tp*)₂W₂Fe₆(μ₄‐N)₂S₆L₄]^2? (Tp*=tris(3,5‐dimethyl‐1‐pyrazolyl)hydroborate(1?), L=Cl^? or Br^?). ⁵⁷Fe Mössbauer study discloses metal oxidation states of W^IV₂Fe^II₄Fe^III₂ with localized electron distribution, which is analogous to the mid‐valent iron centres of FeMo cofactor at resting state. Good agreement of Mössbauer data with the empirical linear relationship for Fe–S clusters indicates similar ligand behaviour of nitride and sulfide in such clusters, providing useful reference for reduced nitrogen in a nitrogenase‐like environment.     

Electronic structure and magnetism at the active site in ferredoxin: Ab initio approach to (Fe2S2)2+ complex with the 1st peptide shell

We have studied the iron–sulfur cluster systems which model an active site of ferredoxin proteins by using the first-principles electronic structure calculation. The modeled molecule is a complex between the (Fe₂S₂)²⁺ core and the amino acid residues which surround the core. The electronic structure of oxidized state for the molecules is presented. The antiferromagentic arrangement for Fe atomic magnetizations was obtained as the ground state. The spin polarized state of the half-filled Fe 3d orbitals is consistent with the formal valence of Fe³⁺. The induced spin density on the cysteine S atoms was found to be parallel to the direction of magnetization on the nearest Fe atom. The hybridized states consisting of N 2p and C 2p orbitals at the side chain of Arg residue appeared just above the highest occupied molecular orbital level for the free standing peptide.     

Application of Synchrotron Radiation in Neuromicrobiology: Role of Iron in Parkinson's Disease

In order to understand the role of iron (Fe) in the oxidative stress underlying the pathogenesis of Parkinson's disease (PD) and parkinsonism–dementia complex (PDC), we investigate distributions and chemical states of Fe within a single neuron of the two disease cases, using synchrotron radiation (SR) micro beam. In the X-ray fluorescence (XRF) spectroscopic study, an excessive accumulation of Fe can be seen in the melanized neurons and free-neuromelanin (MN) aggregates in the substantia nigra tissue of both PD and PDC midbrains. X-ray absorption near-edge structure (XANES) analyses of PD revealed that the chemical state of Fe in the melanized neurons and free-MN aggregates shifted toward Fe³⁺ with a pre-edge peak at Fe K-edge due to a 1s 3d transition, indicating a breaking of the inversion symmetry around the Fe site. In PDC, however, the melanized neurons and free-MN aggregates showed mixed states of Fe²⁺ and Fe³⁺ without any pre-edge peak in the spectra. This tendency was also observed in the control case. These results suggest that the changes in distributions and chemical states of Fe may endogenously play a crucial role in the oxidative damage of the melanized neurons in PD, but through a different mechanism other than PDC.     

An integrated experimental and analytical approach to the chemical state imaging of iron in brain gliomas using X-ray absorption near edge structure spectroscopy

X-ray absorption near-edge structure spectroscopy is used for human neoplastic tissues in order to investigate distributions and chemical states of iron. The specimens used in this study were obtained intraoperatively from brain gliomas of different types and various grades of malignancy and from a control subject. An integrated experimental and analytical approach toward topographic and quantitative analysis in thin freeze-dried cryo-sections is presented. The full XANES spectra at the Fe absorption K edge show the presence of both chemical forms of Fe in the analyzed points of the tissues. The main goal of the work is the chemical state imaging of Fe in tissue areas. Topographic analysis of Fe speciation in the tissues investigated with the use of the XANES technique indicates the presence of microstructures where Fe²⁺ is dominant as well as those with a high abundance of the oxidized form of Fe. The quantitative analysis shows that for all cases the content of the oxidized form of Fe is significantly higher in comparison with Fe²⁺. The highest level of Fe³⁺ is found in the control sample, and the lowest one for the glioma of the highest grade of malignancy. The content of either Fe²⁺ or Fe³⁺ is increased in low grade gliomas in comparison to high-grade malignant tumors.     

Soft X-ray spectromicroscopy studies of pitting corrosion of reinforcing steel bar

Pitting corrosion of reinforcing steel bar (rebar) imbedded in concrete by chloride ions can cause concrete degradation. It is thus necessary to develop methods to mitigate concrete corrosion which could include using a protective polymer rebar coating. Corrosion studies of polyvinyl butyral-carbon black polymer-coated rebar using soft X-ray fluorescence (XRF) microprobe and micro-X-ray absorption near edge spectroscopy (μ-XANES) are reported in this study. After removal of the polyvinyl butyral-carbon black polymer coating, Fe Lα₁, Mn Lα₁, and O Kα₁ XRF maps were collected as well as Fe and Mn L₃-edge μ-XANES spectra from different regions across the whole rebar surface by collection of total electron yield (TEY) and partial XRF yield (PFY) spectra. The distribution of metallic Fe and Fe corrosion products was determined by analysis of the Fe XRF map. The μ-XANES spectra indicated a higher fraction of Fe (III) phases on the corroded rebar surface, while Fe (II/III) phases were the major corrosion products beneath the surface region. In addition, Mn (II) and Mn (III) were determined as the valence states of the manganese corrosion products.     

Synthesis and X-ray and Spectral Study of the Compounds [Q4N]2[Fe2(S2O3)2(NO)4] (Q = Me,Et, n-Pr,n-Bu)

Iron nitrosyl complexes with general formula [Q₄N]₂[Fe₂(S₂O₃)₂(NO)₄] (Q = Me, Et, n-Pr, n-Bu) were synthesized by the exchange reaction of K₂[Fe₂(S₂O₃)₂(NO)₄] with tetraalkylammonium bromides. The molecular and crystal structure of [(CH₃)₄N]₂[Fe₂(S₂O₃)₂(NO)₄] were studied by X-ray diffraction analysis. The iron atom in the four-membered cycle of the [2Fe–2S] anion is bound to another Fe atom and to two sulfur atoms and is coordinated by two nonequivalent NO groups, each bridging sulfur atom being bound to the SO₃group. The structurally equivalent iron atoms are in the state Fe^1–(S= 1/2). The Mössbauer spectroscopy method shows that the complexes are diamagnetic due to the strong Fe–Fe bond. It is found that the SO₃group provides higher stability of the thiosulfate anion than the anion in Roussin's red salt [Fe₂S₂(NO)₄]^2–.     

A spectromicroscopy study of the corrosion of fusion-bonded epoxy–coated rebar

Reported herein is an analysis of accelerated corrosion of a commercially coated fusion-bonded epoxy rebar sample using an X-ray spectromicroscopy methodology that involves the combination of X-ray fluorescence mapping (XRF) and micro X-ray absorption near-edge spectroscopy (μ-XANES). XRF maps collected using an excitation energy of 7115 eV reveal a sharp contrast between corroded and noncorroded sample areas after correction for topography effects that can be confirmed by Fe K-edge μ-XANES. This X-ray spectromicroscopy methodology allows for the study of corrosion through commercially prepared polymer-coated rebar coatings without any modification to the polymer film prior to analysis and is suitable for long-term testing of the corrosion of these materials under real-world conditions.     

Forms of iron occurrence in a series of binary tellurides Fe<Subscript>x</Subscript>Ti<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Te<Subscript>1.45</Subscript>

Nonstoichiometric tellurides Fe_xTi_1–x Te_1.45 synthesized at 850°C were studied by X-ray phase and X-ray fluorescence analysis and by ⁵⁷Fe Mössbauer spectroscopy. The mutual iron-titanium substitution is limited in this series. The system contains four individual phases in which iron is in three different states: Fe²⁺ in an asymmetric environment, Fe²⁺ in a symmetric environment, and Fe⁰. The distribution of various iron states in the system depends not only on the Fe : Ti ratio, but also on the structure of phases.Translated from Zhurnal Obshchei Khimii, Vol. 74, No. 11, 2004, pp. 1761–1764.Original Russian Text Copyright © 2004 by Pankratova, Zabolotnaya, Panchuk, Semenov, Zvinchuk, Suvorov.This revised version was published online in April 2005 with a corrected cover date.     

Synthesis of an All‐Ferric Cuboidal Iron–Sulfur Cluster [FeIII4S4(SAr)4]

An unprecedented, super oxidized all‐ferric iron–sulfur cubanoid cluster with all terminal thiolates, Fe₄S₄(STbt)₄ ( 3 ) [Tbt=2,4,6‐tris{bis(trimethylsilyl)methyl}phenyl], has been isolated from the reaction of the bis‐thiolate complex Fe(STbt)₂ ( 2 ) with elemental sulfur. This cluster 3 has been characterized by X‐ray crystallography, zero‐field ⁵⁷Fe Mössbauer spectroscopy, cyclic voltammetry, and other relevant physico‐chemical methods. Based on all the data, the electronic ground state of the cluster has been assigned to be S_tot=0.     

Source term identification of environmental radioactive Pu/U particles by their characterization with non-destructive spectrochemical analytical techniques

Six radioactive particles stemming from Thule area (NW-Greenland) were investigated by gamma-ray and L X-ray spectrometry based on radioactive disintegration, scanning electron microscopy coupled with energy-dispersive and wavelength-dispersive X-ray spectrometer, synchrotron radiation based techniques as microscopic X-ray fluorescence, microscopic X-ray absorption near-edge structure (μ-XANES) as well as combined X-ray absorption and fluorescence microtomography. Additionally, one particle from Mururoa atoll was examined by microtomography. From the results obtained, it was found out that the U and Pu were mixed in the particles. The U/Pu intensity ratios in the Thule particles varied between 0.05 and 0.36. The results from the microtomography showed that U/Pu ratio was not homogeneously distributed. The ²⁴¹Am/^{238 + 239 + 240}Pu activity ratios varied between 0.13 and 0.17, indicating that the particles originate from different source terms. The oxidation states of U and Pu as determined by μ-XANES showed that U(IV) is the preponderant species and for Pu, two types of particles could be evidenced. One set had about 90% Pu(IV) while in the other the ratio Pu(IV)/Pu(VI) was about one third.     

Synthesis, crystal structures, Mössbauer spectra, and redox properties of binuclear and tetranuclear iron-sulfur nitrosyl clusters

The iron-sulfur nitrosyl complexes A[Fe₄S₃(NO)₇], where A=Na⁺, NH₄ ⁺, or N(Bu ⁿ )₄ ⁺, and B₂[Fe₂S₂(NO)₄], where B=Na⁺, Cs⁺, or N(Buⁿ)₄ ⁺, were synthesized. Their structures and properties were studied by X-ray diffraction analysis, Mössbauer spectroscopy, and cyclic voltammetry. The effect of the crystal packing on the geometry of the tetranuclear NH₄[Fe₄S₃(NO)₇]·H₂O and binuclear Cs₂[Fe₂S₂(NO)₄]·2H₂O complexes was analyzed. The changes in the Fe⁵⁷ Mössbauer spectral parameters of the anion in the B₂[Fe₂S₂(NO)₄] series depend on the size of the B cation and agree with variations in the structural parameters of the Fe[S₂(NO)₂] chromophores as well as in the stretching vibrations of the NO groups caused by changes in intermolecular contacts. The presence of electronic states delocalized through the Fe?Fe bonds explains the fact that the electronic states of the Fe_a(S₃NO) and Fe_b(S₂(NO)₂) chromophores in the [Fe₄S₃(NO)₇]^? anion are nearly identical. The binuclear clusters are unstable upon storage in the solid phase and decompose in solutions to form the tetranuclear [Fe₄S₃(NO)₇]^? complexes, sulfur, and nitrogen oxides. The redox properties of the [Fe₄S₃(NO)₇]^? and [Fe₂S₂(NO₄)]^2? anions in CH₃CN and THF solutions were studied. The mechanism of reduction of the anion in the tetranuclear cluster is proposed.     

Spectroscopic Characterization of an Eight‐Iron Nitrogenase Cofactor Precursor that Lacks the “9th Sulfur”

Nitrogenases catalyze the reduction of N₂ to NH₄⁺ at its cofactor site. Designated the M‐cluster, this [MoFe₇S₉C(R‐homocitrate)] cofactor is synthesized via the transformation of a [Fe₄S₄] cluster pair into an [Fe₈S₉C] precursor (designated the L‐cluster) prior to insertion of Mo and homocitrate. We report the characterization of an eight‐iron cofactor precursor (designated the L*‐cluster), which is proposed to have the composition [Fe₈S₈C] and lack the “9^th sulfur” in the belt region of the L‐cluster. Our X‐ray absorption and electron spin echo envelope modulation (ESEEM) analyses strongly suggest that the L*‐cluster represents a structural homologue to the l ‐cluster except for the missing belt sulfur. The absence of a belt sulfur from the L*‐cluster may prove beneficial for labeling the catalytically important belt region, which could in turn facilitate investigations into the reaction mechanism of nitrogenases.     

Light-induced spin crossover in Fe(II)-based complexes: The full photocycle unraveled by ultrafast optical and X-ray spectroscopies

The light-induced spin and structure changes upon excitation of the singlet metal-to-ligand charge transfer (¹MLCT) state of Fe(II)-polypyridine complexes are investigated in detail in the case of aqueous iron(II)-tris-bipyridine ([Fe^II(bpy)₃]²⁺) by a combination of ultrafast optical and X-ray spectroscopies. Polychromatic femtosecond fluorescence up-conversion, transient absorption studies in the 290–600 nm region and femtosecond X-ray absorption spectroscopy allow us to retrieve the entire photocycle upon excitation of the ¹MLCT state from the singlet low-spin ground state (¹GS) as the following sequence: ^1,3MLCT → ⁵T → ¹GS, which does not involve intermediate singlet and triplet ligand-field states. The population time of the HS state is found to be ～150 fs, leaving it in a vibrationally hot state that relaxes in 2–3 ps, before decaying to the ground state in 650 ps. We also determine the structure of the high-spin quintet excited state by picosecond X-ray absorption spectroscopy at the K-edge of Fe. We argue that given the many common electronic (ordering of electronic states) and structural (Fe–N bond elongation in the high-spin state, Fe–N mode frequencies, etc.) similarities between all Fe(II)-polypyridine complexes, the results on the electronic relaxation processes reported in the case of [Fe^II(bpy)₃]²⁺ are of general validity to the entire family of Fe(II)-polypyridine complexes.     

Mössbauer investigations on the redox equilibrium of iron in weldor safety glasses

In machine drawn weldor safety glasses with a different protection grade and a total iron oxide content of 6.8–9.5% the lattice coordination of iron and the Fe²⁺/Fe_tot ratio were determined by Mössbauer spectroscopy. In combination with electron beam microanalysis and specific sample preparation, homogeneity disturbances were observed inside the defect glass as well as on its surface.     

Site‐Specific Oxidation State Assignments of the Iron Atoms in the [4Fe:4S]2+/1+/0 States of the Nitrogenase Fe‐Protein

The nitrogenase iron protein (Fe‐protein) contains an unusual [4Fe:4S] iron‐sulphur cluster that is stable in three oxidation states: 2+, 1+, and 0. Here, we use spatially resolved anomalous dispersion (SpReAD) refinement to determine oxidation assignments for the individual irons for each state. Additionally, we report the 1.13‐Å resolution structure for the ADP bound Fe‐protein, the highest resolution Fe‐protein structure presently determined. In the dithionite‐reduced [4Fe:4S]¹⁺ state, our analysis identifies a solvent exposed, delocalized Fe^2.5+ pair and a buried Fe²⁺ pair. We propose that ATP binding by the Fe‐protein promotes an internal redox rearrangement such that the solvent‐exposed Fe pair becomes reduced, thereby facilitating electron transfer to the nitrogenase molybdenum iron‐protein. In the [4Fe:4S]⁰ and [4Fe:4S]²⁺ states, the SpReAD analysis supports oxidation states assignments for all irons in these clusters of Fe²⁺ and valence delocalized Fe^2.5+, respectively.     

Influence of Local Atomic and Electronic Structures of Magnetite on Subtle Effects in HERFD-XANES Spectra

Large iron oxide nanoparticles are studied by high-energy resolution fluorescence-detected X-ray absorption spectroscopy (HERFD XANES). A theoretical procedure within the finite-difference method framework is tested to calculate Fe K-edge X-ray absorption spectra for the magnetite structure. The influence of structural and non-structural calculation parameters on the subtle effects in the X-ray absorption spectra are considered. Tetrahedral positions of Fe³⁺ ions are shown to provide the main contribution to the spectral feature at 20–21 eV whose intensity grows together with the content of these ions.     

Spin-State Variations of Iron(III) Complexes with Tetracarbene Macrocycles

Starting from their six-coordinate iron(II) precursor complexes [L^8RFe(MeCN)]²⁺, a series of iron(III) complexes of the known macrocyclic tetracarbene ligand L^8H and its new octamethylated derivative L^8Me, both providing four imidazol-2-yliden donors, were synthesized. Several five- and six-coordinate iron(III) complexes with different axial ligands (Cl⁻, OTf⁻, MeCN) were structurally characterized by X-ray diffraction and analyzed in detail with respect to their spin state variations, using a bouquet of spectroscopic methods (NMR, UV/Vis, EPR, and ⁵⁷Fe Mößbauer). Depending on the axial ligands, either low-spin (S=1/2) or intermediate-spin (S=3/2) states were observed, whereas high-spin (S=5/2) states were inaccessible because of the extremely strong in-plane σ-donor character of the macrocyclic tetracarbene ligands. These findings are reminiscent of the spin state patterns of topologically related ferric porphyrin complexes. The ring conformations and dynamics of the macrocyclic tetracarbene ligands in their iron(II), iron(III) and μ-oxo diiron(III) complexes were also studied.