Spin State Of Iron Research Articles

Influence of η1‐Acetylide Ligands on the Spin State and Catalytic Activity of Iron(II) PNP Pincer Complexes

Here, we report the preparation, structures and reactivity of iron(II) PNP pincer complexes bearing one, two or three η1‐acetylide ligands bound to the metal center. The described complexes are accessible through the reaction of either one, two or ≥ 3 equivalent of LiCCR (R = SiMe3, Ph) with the dibromide complex [Fe(PNP)Br2] (1). Through increasing the number of η1‐acetylides the overall ligand field strength can be modulated resulting in S = 2, S = 1 and S = 0 spin states for mono‐, bis‐ and tris‐acetylated complexes, respectively. These findings are supported by DFT calculations on the spin state spitting in these complexes and appear to affect their binding affinity towards additional co‐ligands. The Fe(II) mono‐, bis‐ and tris‐acetylides were tested as catalyst in the catalytic dimerization of terminal alkynes. These test reactions revealed that only the bis‐acetylated complexes are catalytically competent while mono‐acetylated species are inactive. The tris‐acetylated complexes were found to be off‐cycle intermediates in the presence of lithium acetylides.

Dehydration Achieving the Iron Spin State Regulation of Prussian Blue for Boosted Sodium-Ion Storage Performance.

Prussian blue analogs (PBAs) show promise as cathodes for sodium-ion batteries due to their notable cycle stability, cost-effectiveness, and eco-friendly nature, yet the presence of interstitial water limits the specific capacity and obstructs Na+ mobility within the material. Although considerable experimental efforts are focused on dehydrating water for capacity enhancement, there is still a deficiency of a comprehensive understanding of the low capacity of low-spin Fe resulting from interstitial water, which holds significance in Na+ storage. This study introduces a novel gas-assisted heat treatment method to efficiently remove interstitial water from Fe-based PBA (NaFeHCF) electrodes and combines experiments and theoretical calculations to reveal the iron spin state regulation that is related to the capacity enhancement mechanism. This dehydration strategy significantly enhances battery capacity, especially the portion at higher voltages (3.4-4.0V). The increase in capacity is attributed to the following factors: an enhanced proportion of Fe2+, reduced water content which facilitates faster charge transfer, and the activation of low spin Fe2+. The optimized NaFeHCF demonstrated impressive half-cell performance of retaining 87.3% capacity after 2000 cycles at a 5C rate and achieving 100mAhg-1 capacity over 200 cycles when being paired with hard carbon, exhibiting its practical potential.

S-Doping Regulated Iron Spin States in Fe-N-C Single-Atom Material for Enhanced Peroxidase-Mimicking Activity at Neutral pH.

Designing biomimetic nanomaterials with peroxidase (POD)-like activity at neutral pH remains a significant challenge. An S-doping strategy is developed to afford an iron single-atom nanomaterial (Fe1@CN-S) with high POD-like activity under neutral conditions. To the best of knowledge, there is the first example on the achievement of excellent POD-like activity under neutral conditions by regulating the active site structure. S-doping not only promotes the dissociation of the N─H bond in 3,3″,5,5″-tetramethylbenzidine (TMB), but also facilitates the desorption of OH* by the transformation of iron species' spin states from middle-spin (MS FeII) to low-spin (LS FeII). Meanwhile, LS FeII sites typically have more unfilled d orbitals, thereby exhibiting stronger interactions with H2O2 than MS FeII, which can enhance POD-like activity. Finally, a one-pot visual detection of glucose at pH 7 is performed, demonstrating the best selectivity and sensitivity than previous reports.

Naphthalimide-Tagged Iron(II) Spin Crossover Complex with Synergy of Ratiometric Fluorescence for Thermosensing.

Spin crossover (SCO) materials that possess switchable and cooperative fluorescence have long focused interest in photonic sensor devices to monitor the variations in the physicochemical parameters of the external environment. However, the lack of quantified cooperativity for the SCO transition operating in isolated molecules is detrimental to short-term technological applications. In this study, a pretwisted energy D-A system combining the deep-blue naphthalimide fluorophore (donor) and the FeN6 SCO chromophore (switchable acceptor) has been developed with the formula of Fe(naph-abpt)2(NCS)2·2DMF (1), where naph-abpt is N-[3,5-di(pyridin-2-yl)-4H-1,2,4-triazol-4-yl]-1,8-naphthalimide. Dual emission from the naphthalimide function based on its vibronic structure exhibits a different synergy effect with SCO, providing a new platform for ratiometric fluorescence thermosensing. Theoretical calculations and optical experimental results demonstrate an excellent correlation between luminescence intensity ratio signals and magnetic data of spin transition, promising a high sensitivity of the optical activity of the ligand to the spin state of the active iron(II) ions, with the maximum relative sensitivity as 0.7% K-1 around T1/2.

Effect of content and spin state of iron on electronic properties and floatability of iron-bearing sphalerite: A DFT+U study

Iron is an impurity widely occurred in sphalerite, and its effect on sphalerite flotation is complex. In this work, the effects of iron content and spin state on electronic properties and floatability of iron-bearing sphalerite are comprehensively studied using density functional theory Hubbard U (DFT+U) calculations combined with coordination chemistry flotation. The band gap of ideal sphalerite is 3.723 eV, and thus electron transition is difficult to occur, resulting in poor floatability. The results suggest the band gap of sphalerite decreases with increasing iron content. For low iron content, the decreased band gap facilitates electron transition; at this case, Fe2+ in a high-spin state possesses one π electron pair, which can form a weak π-backbonding with xanthate, causing increasing floatability. However, for medium and high iron-bearing sphalerite, with the further decrease of band gap, Fe2+ is oxidized to Fe3+ due to electrochemical interaction, and hence π-backbonding is eliminated, leading to lower floatability of iron-bearing sphalerite, which is consistent with the flotation experimental results. This work could give a deeper understanding of how sphalerite flotation behaviors are affected by iron content.

Phase transitions and spin state of iron in FeO under the conditions of Earth's deep interior

Phase transitions and spin state of iron in FeO under the conditions of Earth's deep interior

Characterization of the interaction between the tumour suppressor p53 and heme and its role in the protein conformational dynamics studied by various spectroscopic techniques and hydrogen/deuterium exchange coupled with mass spectrometry

The tumour suppressor p53 regulates the expression of a myriad of proteins that are important for numerous cellular processes, including apoptosis, cell cycle arrest, DNA repair, metabolism, and even autophagy and ferroptosis. Aside from DNA, p53 can interact with many types of partners including proteins and small organic molecules. The ability of p53 to interact with heme has been reported so far. In this study, we used various spectroscopic studies to conduct a thorough biophysical characterization of the interaction between p53 and heme concerning the oxidation, spin, coordination, and ligand state of heme iron. We found that the p53 oligomeric state and zinc biding ability are preserved upon the interaction with heme. Moreover, we described the effect of heme binding on the conformational dynamics of p53 by hydrogen/deuterium exchange coupled with mass spectrometry. Specifically, the conformational flexibility of p53 is significantly increased upon interaction with heme, while its affinity to a specific DNA sequence is reduced by heme. The inhibitory effect of DNA binding by heme is partially reversible. We discuss the potential heme binding sites in p53 with respect to the observed conformational dynamics changes and perturbed DNA-binding ability of p53 upon interaction with heme.

Modulation of iron spin states in highly distorted iron(III) porphyrins: H-bonding interactions and implications in hemoproteins.

A family of five- and six-coordinated Fe-porphyrins which enable us to scrutinize the effects of non-covalent interactions on the out-of-plane displacement of iron and its spin-states and axial ligand orientation in a single distorted macrocyclic environment has been reported. Combined analysis using single-crystal X-ray structure determination and EPR spectral investigation revealed the stabilization of the high-spin state of iron in the five-coordinate complex FeIII(TPPBr8)(OCHMe2), while six-coordinate complexes [FeIII(TPPBr8)(MeOH)2]ClO4, [FeIII(TPPBr8)(H2O)2]ClO4 and [FeIII(TPPBr8)(1-MeIm)2]ClO4 stabilize admixed-high, admixed-intermediate and low-spin states, respectively. The H-bonding interactions between the weak axial H2O/MeOH and perchlorate anion resulted in an elongation of the Fe-O bond which eventually shortened the Fe-N(por) distances leading to the stabilization of the admixed spin state of iron which, otherwise, stabilizes the high-spin (S = 5/2) state only. In addition, the iron atom in [FeIII(TPPBr8)(H2O)2]ClO4 is displaced by 0.02 Å towards one of the water molecules engaged in the H-bonding interactions leading to two different Fe-O (H2O) distances of 2.098(8) and 2.122(9) Å. In contrast, iron in [FeIII(TPPBr8)(MeOH)2]ClO4 sits on the plane of the porphyrin since both the axial methanol units are engaged in similar H-bonding interactions with the ClO4- ion. Moreover, the X-ray structure of low-spin FeII(TPPBr8)(1-MeIm)2 revealed a dihedral angle of 63.0° between two imidazoles which deviates largely from the expected angle of 90° (perpendicular orientations) since the axial imidazole protons are engaged in strong intermolecular C-H⋯π interactions which thereby restrict the axial ligand movement. The complex also displays the shortest Fe-N(1-MeIm) bond along with smallest dihedral angles of 7.8° and 22.4° between the axial imidazole ring and the closest Fe-Np axis due to strong π-interactions between iron and the axial imidazole ligand. Our work highlights the influence of non-covalent interactions on the out-of-plane displacement and spin state of iron and axial ligand orientations which are indeed important steps in the functioning of various hemoproteins.

57Fe Mössbauer and DFT study of the electronic and spatial structures of the iron(II) (pseudo)clathrochelates: the effect of ligand field strength.

Combined experimental 57Fe Mössbauer and theoretical DFT study of a series of iron(II)-centered (pseudo)macrobicyclic analogs and homologs was performed. The field strength of the corresponding (pseudo)encapsulating ligand was found to affect both the spin state of a caged iron(II) ion and the electron density at its nucleus. In a row of the iron(II) tris-dioximates, passing from the non-macrocyclic complex to its monocapped pseudomacrobicyclic analog caused an increase both in the ligand field strength and in the electron density at the Fe2+ ion, and, therefore, a decrease in the isomer shift (IS) value (so-called "semiclathrochelate effect"). Its macrobicyclization, giving the quasiaromatic cage complex, caused a further increase in the two former parameters and a decrease in IS (so-called "macrobicyclic effect"). The trend of their IS values was successfully predicted using the performed quantum-chemical calculations and the corresponding linear correlation with the electron density at their 57Fe nuclei was plotted. A variety of different functionals can be successfully used for such excellent prediction. The slope of this correlation was found to be unaffected by the used functional. In contrast, the predictions of both the sign and the values of quadrupole splitting (QS) for them, based on the theoretical calculations of EFG tensors, were found to be a real great challenge, which could not be solved at the moment even in the case of these C3-pseudosymmetric iron(II) complexes with known XRD structures. The latter experimental data allowed us to deduce a sign of the QSs for them. The straightforwarded molecular design of a (pseudo)encapsulating ligand is proposed to control both the spin state and the redox characteristics of an encapsulated metal ion.

Manipulating fluorescence by photo-switched spin-state conversions in an iron(ii)-based SCO-MOF.

Manipulating fluorescence by photo-switched spin-state conversions is an attractive prospect for applications in smart magneto-optical materials and devices. The challenge is how to modulate the energy transfer paths of the singlet excited state by light-induced spin-state conversions. In this work, a spin crossover (SCO) FeII-based fluorophore was embedded into a metal-organic framework (MOF) to tune the energy transfer paths. Compound 1 {Fe(TPA-diPy)[Ag(CN)2]2}·2EtOH (1) has an interpenetrated Hofmann-type structure, wherein the FeII ion is coordinated by a bidentate fluorophore ligand (TPA-diPy) and four cyanide nitrogen atoms and acts as the fluorescent-SCO unit. Magnetic susceptibility measurements revealed that 1 underwent an incomplete and gradual spin crossover with T1/2 = 161 K. Photomagnetic studies confirmed photo-induced spin state conversions between the low-spin (LS) and high-spin (HS) states, where the irradiation of 532 and 808 nm laser lights converted the LS and HS states to the HS and LS states, respectively. Variable-temperature fluorescence spectra study revealed an anomalous decrease in emission intensity upon the HS → LS transition, confirming the synergetic coupling between the fluorophore and SCO units. Alternating irradiation of 532 and 808 nm laser lights resulted in reversible fluorescence intensity changes, confirming spin state-controlled fluorescence in the SCO-MOF. Photo-monitored structural analyses and UV-vis spectroscopic studies demonstrated that the photo-induced spin state conversions changed energy transfer paths from the TPA fluorophore to the metal-centered charge transfer bands, ultimately leading to the switching of fluorescence intensities. This work represents a new prototype compound showing bidirectional photo-switched fluorescence by manipulating the spin states of iron(ii).

Electron Correlation in the Iron(II) Porphyrin by Natural Orbital Functional Approximations.

The relative stability of the singlet, triplet, and quintet spin states of iron(II) porphyrin (FeP) represents a challenging problem for electronic structure methods. While it is currently accepted that the ground state is a triplet, multiconfigurational wave function-based methods predict a quintet, and density functional approximations vary between triplet and quintet states, leading to a prediction that highly depends on the features of the method employed. The recently proposed Global Natural Orbital Functional (GNOF) aims to provide a balanced treatment between static and dynamic correlation, and together with the previous Piris Natural Orbital Functionals (PNOFs), allowed us to explore the importance of each type of correlation in the stability order of the states of FeP with a method that conserves the spin of the system. It is noteworthy that GNOF correlates all electrons in all available orbitals for a given basis set; in the case of the FeP with a double-ζ basis set as used in this work, this means that GNOF can properly correlate 186 electrons in 465 orbitals, significantly increasing the sizes of systems amenable to multiconfigurational treatment. Results show that PNOF5, PNOF7s, and PNOF7 predict the quintet to have a lower energy than the triplet state; however, the addition of dynamic correlation via second-order Møller-Plesset corrections (NOF-MP2) turns the triplet state to be lower than the quintet state, a prediction also reproduced by GNOF that incorporates much more dynamic correlation than its predecessors.

Electronic Coupling and Electrocatalysis in Redox Active Fused Iron Corroles.

Conjugated arrays composed of corrole macrocycles are increasingly more common, but their chemistry still lags behind that of their porphyrin counterparts. Here, we report on the insertion of iron(III) into a β,β-fused corrole dimer and on the electronic effects that this redox active metal center has on the already rich coordination chemistry of [H3tpfc] COT, where COT = cyclo-octatetraene and tpfc = tris(pentafluorophenyl)corrole. Synthetic manipulations were performed for the isolation and full characterization of both the 5-coordinate [FeIIItpfc(py)]2COT and 6-coordinate [FeIIItpfc(py)2]2COT, with one and two axial pyridine ligands per metal, respectively. X-Ray crystallography reveals a dome-shaped structure for [FeIIItpfc(py)]2COT and a perfectly planar geometry which (surprisingly at first) is also characterized by shorter Fe-N (corrole) and Fe-N (pyridine) distances. Computational investigations clarify that the structural phenomena are due to a change in the iron(III) spin state from intermediate (S = 3/2) to low (S = 1/2), and that both the 5- and 6-coordinated complexes are enthalpically favored. Yet, in contrast to iron(III) porphyrins, the formation enthalpy for the coordination of the first pyridine to Fe(III) corrole is more negative than that of the second pyridine coordination. Possible interactions between the two corrole subunits and the chelated iron ions were examined by UV-Vis spectroscopy, electrochemical techniques, and density functional theory (DFT). The large differences in the electronic spectra of the dimer relative to the monomer are concluded to be due to a reduced electronic gap, owing to the extensive electron delocalization through the fusing bridge. A cathodic sweep for the dimer discloses two redox processes, separated by 230 mV. The DFT self-consistent charge density for the neutral and cationic states (1- and 2-electron oxidized) reveals that the holes are localized on the macrocycle. A different picture emerges from the reduction process, where both the electrochemistry and the calculated charge density point toward two consecutive electron transfers with similar energetics, indicative of very weak electron communication between the two redox active iron(III) sites. The binuclear complex was determined to be a much better catalyst for the electrochemical hydrogen evolution reaction (HER) than the analogous mononuclear corrole.

Spin State of Iron(II) Clathrochelate in the Cocrystallization Products with 2-Aminopyridine and 2-Hydroxypyridine

Spin State of Iron(II) Clathrochelate in the Cocrystallization Products with 2-Aminopyridine and 2-Hydroxypyridine

Non-resonant and resonant X-ray emission at high pressure using a von Hámos setup: the case of FeO

We present a setup exploiting a von Hámos spectrometer in order to study (resonant) X-ray emission of matter exposed to high pressure. The capabilities of this setup are demonstrated for the case of FeO at pressures between 13 GPa and 75 GPa. The setup provides high-quality Kβ 1,3 X-ray emission spectra at high pressures for iron spin state analysis within minutes and iron valence-to-core spectra in less than one hour. Resonant X-ray emission maps can be obtained on a timescale of one hour with 1.0 eV and in approximately 3 hours with 0.2 eV incident energy resolution. Both Kα and Kβ emission can be utilized to gain L-edge and M-edge-like information, respectively, with the option of measuring both simultaneously. The spin state results on FeO between 13 GPa and 75 GPa are in accordance with recent literature. The structural distortion is reflected in both, valence-to-core spectra and resonant X-ray emission maps, which showcase the great potential of the presented setup. The achieved data acquisition times are promising to couple pressure with temperature by laser heating.

Rational Design of Iron Spin-Crossover Complexes Using Heteroscorpionate Chelates.

The optical, structural, and magnetic properties of iron(II,III) sandwich complexes, Fe(Tp')2n+ (Tp' = bis(3,5-dimethylpyrazolyl)benzotriazolylborate), are described. The intensely colored FeII(Tp')2 (orange) and FeIII(Tp')2+ (purple) show strong MLCT bands. Geometric isomerism for M(Tp')2 is established crystallographically in the racemate of chiral cis-Fe(Tp')2. For the first time, paramagnetic 11B NMR describes solution-phase low-spin (LS, S = 0) to high-spin (HS, S = 2) crossover behavior in Fe(Tp')2. Thermochemical parameters for solution-phase SCO of Fe(Tp')2 demonstrate the endothermic LS to HS conversion and entropic preference of the HS state. Entropy changes for both Fe(Tp')2 isomers are significantly larger than for the majority of iron scorpionate SCO systems. Solid-state magnetic and thermochemical measurements show cis-Fe(Tp')2 to be thermally stable up to 520 K, allowing experimental investigation of a solid-state SCO magnetic hysteresis of over 45 K. A large solution vs solid-state SCO difference was observed: cis-Fe(Tp')2 shows Tc ≈ 270 K (solution) and Tc ≈ 385 K (solid), with the remarkably wide ΔTc ≈ 115 K; trans-Fe(Tp')2 shows Tc ≈ 278 K (solution) and Tc ≈ 372 K (solid). Solid-state Tc values are among the highest seen for iron(II) molecular systems. The large solution/solid ΔTc difference is explained by "anchoring" intermolecular interactions in the solid state that prevent thermal expansion of the LS iron(II) coordination sphere in its transition to the HS state. DFT calculations, validated against LS cis-Fe(Tp')2 crystallography and LS to HS SCO thermochemical parameters, demonstrate the role the benzotriazole rings play in its structural and optical properties. The Lewis basicity of M(Tp')2 is shown with the structural characterization of the air-stable tin(II) adduct [cis-Fe(Tp')2-SnCl2]; tin(II) coordination does not alter the iron(II) spin state. The Tp' chelate adds functionality (asymmetry, chirality, chemical reactivity) to the array of iron SCO materials for potential incorporation into nanoscale magnetic switches and spintronic devices.

Oxygen Reduction Reaction on Fe-N-C Catalysts: A Computational Spectroscopy Study

The oxygen reduction reaction (ORR) plays an important role in proton exchange fuel cells (PEFCs). In PEFCs, ORR is the cathodic half-cell reaction complementary to the oxidation of the fuel, but since ORR has slow kinetics, it requires high amounts of catalyst. State-of-the-art ORR catalysts are based on the expensive metal platinum. Even though the amount of platinum needed for ORR in PEFCs has been reduced significantly over the last decade, it is still the major contributor to the cost of PEFCs, thus hindering the commercialization and accessibility of this technology.[1]Iron and nitrogen doped carbon (Fe-N-C) catalysts have gained a lot of research attention due to their high ORR activity, which makes them potential substitutes for platinum-based catalysts. In Fe-N-C catalysts, iron is thought to be atomically dispersed as pseudo-molecular active centres with four- or fivefold nitrogen coordination spheres which are embedded in graphene layers. Since Fe-N-C catalysts are typically prepared via pyrolysis, they have a highly amorphous structure and can contain multiple iron phases, which makes them difficult to characterize structurally and spectroscopically. Consequently, there is still a scientific debate on the exact nature of the active site, in terms of iron spin and oxidation states and its precise coordination environment.[2-4]Fe-57 Mössbauer spectroscopy can provide direct insights on iron spin and oxidation states and is used successfully to characterise the amorphous Fe-N-C catalysts. Until recently, the interpretation of Mössbauer spectra was limited to comparisons with small reference complexes which lack the extended π-systems of Fe-N-C catalysts.[2] Since synthesis of such extended π-systems as references is difficult, we have developed a library of computational models that encompasses different structural motifs and electronic structures. With increasing use of in situ and operando experiments on Fe-N-C catalysts, the interest in computational models for the interpretation of experimental Mössbauer spectra has grown.[4-6] In this contribution, we present our density functional theory results for different molecular Fe-N-C models with extended π-systems and discuss their electronic structures and spectroscopic properties.[1] L. Osmieri, et al. Current Opinion in Electrochemistry 2021, 25, 100627.[2] U. I. Kramm, et al. Advanced Materials 2019, 31, 1805623.[3] S. Wagner, et al. Angewandte Chemie International Edition 2019, 58, 10486.[4] L. Ni, C. Gallenkamp, et al. Advanced Energy & Sustainability Research 2021, 2, 2000064.[5] J. Li, et al. Nature Catalysis, 2021, 4, 10.[6] X. Liu, et al. Chem 2020, 6, 3440.

Precise Regulation of Iron Spin States in Single FeN4 Sites for Efficient Peroxidase-Mimicking Catalysis.

The catalytic activity and selectivity of single-atomsites catalysts is strongly dependent on the supports structure and central metal coordination environment. However, the further optimization of electronic configuration to improve the catalytic performance is usually hampered by the strong coordination effect between the support and metal atoms. Herein, it is discovered that enzyme-mimicking catalytic performance can be enhanced at the fixed coordination single-atom Fe sites by regulating the Fe spin states. The X-ray absorption fine structure, 57 Fe Mössbauer spectrum, and temperature-dependent magnetization measurements reveal that the spin states of Fe in single FeN4 sites can be well manipulated via changing the pyrolysis temperature. The intermediate-spin Fe sites catalyst (t2g 4eg 1) demonstrates a much higher peroxidase-mimicking activity in comparison with high-spin structure (t2g 3eg 2). More importantly, the based enzymes system realizes sensitive detection of H2 O2 and glucose by colorimetric sensors with high catalytic activity and selectivity. Furthermore, theoretical calculations unveil that the intermediate-spin FeN4 promotes the OH* desorption process, thus greatly reducing the reaction energy barrier. These findings provide a route to design highly active enzyme-mimicking catalysts and an engineering approach for regulating spin states of metal sites to enhance their catalytic performance.

Fe3+ -hosting carbon phases in the deep Earth

Iron-bearing carbonates play an important role in Earth's carbon cycle. Owing to their stability at mantle conditions, recently discovered iron carbonates with tetrahedrally coordinated carbon atoms are candidates for carbon storage in the deep Earth. The carbonates' iron oxidation and spin state at extreme pressure and temperature conditions contribute to the redox conditions and element partitioning in the deep mantle. By laser heating ${\mathrm{FeCO}}_{3}$ at pressures of about 83 GPa, ${\mathrm{Fe}}_{4}^{3+}{\mathrm{C}}_{3}{\mathrm{O}}_{12}$ and ${\mathrm{Fe}}_{2}^{2+}{\mathrm{Fe}}_{2}^{3+}{\mathrm{C}}_{4}{\mathrm{O}}_{13}$ were synthesized and then investigated by x-ray emission spectroscopy to elucidate their spin state, both in situ and temperature quenched. Our experimental results show both phases in a high-spin state at all pressures and over the entire temperature range investigated, i.e., up to 3000 K. The spin state is conserved after temperature quenching. A formation path is favored where ${\mathrm{Fe}}_{4}^{3+}{\mathrm{C}}_{3}{\mathrm{O}}_{12}$ forms first and then reacts to ${\mathrm{Fe}}_{2}^{2+}{\mathrm{Fe}}_{2}^{3+}{\mathrm{C}}_{4}{\mathrm{O}}_{13}$, most likely accompanied by the formation of oxides. Density functional theory calculations of ${\mathrm{Fe}}_{2}^{2+}{\mathrm{Fe}}_{2}^{3+}{\mathrm{C}}_{4}{\mathrm{O}}_{13}$ at 80 GPa confirm the experimental findings with both ferric and ferrous iron in high-spin state with antiferromagnetic order at 80 GPa. As the intercrystalline cation partitioning between the Fe-bearing carbonates and the surrounding perovskite and ferropericlase depends on the spin state of the iron, an understanding of the redox conditions prevalent in subducted slab regions in the lower mantle has to take the latter into account. Especially, ${\mathrm{Fe}}_{2}^{2+}{\mathrm{Fe}}_{2}^{3+}{\mathrm{C}}_{4}{\mathrm{O}}_{13}$ may play a key role in subducted material in the lower mantle, potentially with a similar role as silicate perovskite.

Mössbauer study of some novel iron-bis-glyoxime and iron-tris-glyoxime complexes

Dioximes as ligands are used as analytical reagents and serve as models for biological systems as well as catalysts in chemical processes. A number of novel mixed complexes of the type [Fe(DioxH)2(amine)2] have been prepared and characterised by FTIR, 57Fe Mössbauer and mass spectroscopy by us. We have found strong Fe–N donor acceptor interactions and iron occurred in low-spin FeII state in all complexes. Later, we have also found that the incorporation of branching alkyl chains (isopropyl) in the complexes alters the Fe–N bond length and results in high-spin iron(II) state [1, 2]. The question arises: can the spin state of iron be manipulated generally by replacing the short alkyl chains with high volume demand ones in Fe-azomethine-amine complexes? To answer the question we have synthetized novel iron-bis-glioxime and iron-tris-gloxime complexes when long chain alkyl or aromatic ligands replaced the short alkyl ones and studied by 57Fe Mössbauer spectroscopy, MS, FTIR, UV-VIS, TG-DTA-DTG and XRD methods. Novel iron-bis-glyoxime and iron-tris-glyoxime type complexes, [Fe(Diethyl-Diox)3(BOH)2], [Fe(Diethyl-Diox)3(BOEt)2] and [Fe(phenyl-Me-Diox)3(BOEt)2], were synthesized similarly as described in [2]. The FTIR, UV-VIS, TG-DTA-DTG and MS measurements indicated that the expected novel complexes could be successfully synthesized.

Fe-N-C Catalysts for Oxygen Reduction Reaction: A Computational Analysis of Electronic Structure and Spectroscopic Properties

In proton exchange fuel cells (PEFCs), the electrochemical reduction of oxygen (ORR) plays a key role as the complementary half-cell reaction to the oxidation of hydrogen. It requires the transport of four electrons per oxygen molecule associated with high activation barriers and slow kinetics and thus requires high amounts of catalyst. [1] State-of-the art catalysts for both reactions are based on platinum, an expensive metal.Iron and nitrogen doped carbon (Fe-N-C) catalysts show high ORR activity and are thus a very promising alternative to Pt catalysts. It is believed that in Fe-N-C catalysts, iron is atomically dispersed as pseudo-molecular active centres with four- or fivefold nitrogen coordination spheres which are embedded in a graphene matrix. However, the exact nature of the active site in terms of Fe spin and oxidation states, precise coordination environment and spatial extent of the graphene sheets are still debated, as is the influence of these factors on the ORR mechanism in Fe-N-C catalysts. [2-4]Unlike other spectroscopies, Mössbauer spectroscopy, which provides direct insights on iron spin and oxidation states, is used successfully to characterise the amorphous FeNC catalysts. Until recently, the interpretation of Mössbauer spectra was limited to comparisons with small reference complexes [2] which lack the extended π-systems of Fe-N-C catalysts. Recently, with increasing use of in situ and operando experiments on these catalysts, the interest in computational models for the interpretation of experimental Mössbauer spectra has grown. [4-6]. Using density functional theory, we show for different FeN4 environments how the electronic structure is influenced by the size and symmetry of the π-system. The influence is quantified in terms of structural parameters, relative spin state energies, spin distributions and spectroscopic properties.