A Multiplet Analysis of Fe K-Edge 1s → 3d Pre-Edge Features of Iron Complexes

Electronic Structures of Iron in Oxide Glasses via 1s3p Resonant Inelastic X-ray Scattering

Electronic structures of iron in glasses are essential for unraveling the effect of transition metals on amorphous networks and controlling the electro-optical and transport properties of advanced glasses and amorphous energy-storing materials. The electronic configurations around iron in glasses, however, remain not well understood due to the structural disorders arising from multiple iron species with distinct valence, coordination, and spin states. Here, the first 1s3p resonant inelastic X-ray scattering (RIXS) for oxide glasses identifies hidden electronic configurations for Fe2+ and Fe3+ in amorphous networks. The results allow us to quantify the composition-induced evolution of oxygen ligand-field interactions of high-spin Fe 3d states with varying valence and coordination environments in complex glasses. The distinct electronic structures account for the electronic origins of iron-induced changes in the glass properties. The results offer prospects for a simultaneous probing of valence, coordination, and spin states of transition metals in diverse multicomponent oxide glasses and functional amorphous solids via 1s3p RIXS.

Synthesis and Reactivity of a Non-Heme μ-Oxodicobalt(IV) Complex

A mononuclear CoIII complex (1) of a bisamide-bisalkoxide donor ligand was synthesized and thoroughly characterized. The reaction of 1 with 0.5 equiv. of m-chloroperbenzoic acid (m-CPBA) in acetonitrile at -40 °C resulted in the formation of a μ-oxodicobalt(IV) complex (2), which was characterized by an array of spectroscopic techniques, including X-ray absorption spectroscopy which revealed a short Co-Ooxo distance of 1.67 Å. Reactivity studies of 2 towards oxidation/oxygenation of hydrocarbon C-H bond and triphenylphosphine or thioanisole derivatives have been examined. UV-vis spectroscopy studies showed the appearance of clear isosbestic points during the oxidation of substrates together with a neat transformation of 2 to 1. Detailed kinetic investigations established that 2 follows a Concerted Proton-Electron Transfer (CPET) mechanism for hydrocarbon oxidation and has a weak electrophilic character. Catalytic behavior of 1 was noted towards the oxygen atom transfer reactions. This study showcases the spectroscopic investigation and reactivity studies of a CoIV(μ-O)CoIV moiety. Although the FeIV analog of such a core has been described before, the study describes the first example with a CoIV center.

N2O Activation and NO Adsorption Control the Simultaneous Conversion of N2O and NO Using NH3 over Fe-ZSM-5

Fe-exchanged zeolites are heterogeneous catalysts that can potentially ensure simultaneous conversion of nitrous oxide (N2O) and nitric oxide (NO) using ammonia (NH3) as a selective reducing agent through their selective catalytic reduction reaction (N2O-NO-SCR). In this study, we rationalize the origin of the beneficial effect of N2O on the NO conversion by combining catalytic experiments with ex situ characterization and in situ/operando X-ray absorption spectroscopy (XAS) and infrared spectroscopy in diffuse reflectance mode (DRIFTS) on a series of Fe-ZSM-5 catalysts where we attempted to control Fe speciation at constant Fe content. The catalytic activity data revealed that N2O can promote NO conversion at different temperatures and to different extents. This behavior was found to be related to the activity of the catalysts in the NO-mediated N2O decomposition reaction, which ensures the oxidative transformation of NO and thus sustains the N2O-NO-SCR chemistry. The oxidation activity is in turn determined by processes of N2O activation and NO adsorption, which are a function of the Fe speciation and are likely catalyzed by a minority of isolated Fe2+ sites coordinated in different cationic environments. In agreement, the concentrations of the Fe species able to activate N2O (Cα) and of the Fe species able to coordinate NO (CFe-NO) decrease with an increasing degree of Fe agglomeration and govern especially the promotion of the NO conversion induced by N2O in this dual-site mechanism. Maximization of the concentration of both species is therefore essential to design Fe-exchanged zeolites with the highest activity toward the N2O-NO-SCR reaction.

Experimental electronic structures of the FeIV=O bond in S=1 heme vs. nonheme sites: Effect of the porphyrin ligand

High-valent FeIV=O species are common intermediates in biological and artificial catalysts. Heme and nonheme S=1 FeIV=O sites have been synthesized and studied for decades but little quantitative experimental comparison of their electronic structures has been available, due to the lack of direct methods focused on the iron. This study allows a rigorous determination of the electronic structure of a nonheme FeIV=O center and its comparison to an FeIV=O heme site using 1s2p resonant inelastic X-ray scattering (RIXS) and Fe L-edge X-ray absorption spectroscopy (XAS). Further, variable temperature magnetic circular dichroism (VT-MCD) of the ligand field transitions, combined with nuclear resonance vibrational spectroscopy of the two S=1 FeIV=O systems show that the equatorial ligand field decreases from a nonheme to a heme FeIV=O site. Alternatively, RIXS and Fe L-edge XAS combined with MCD show that the Fe dπ orbitals are unperturbed in the FeIV=O heme relative to the nonheme site because the strong axial Fe-O bond uncouples the Fe dπ orbitals from the porphyrin π-system. As a consequence, the thermodynamics and kinetics of the H-atom abstraction reactions are actually very similar for heme compound II and nonheme FeIV=O active sites.

Femto-microsecond electron transfer and intermediates in Al/Fe CO2 photoreduction systems through optical and X-ray spectroscopy

This study reveals the reaction pathways of 2 Al-Fe earth-abundant photocatalytic systems for CO2 reduction through femto-nanosecond optical transient absorption and microsecond X-ray spectroscopies. Time-resolved experimental findings with time-dependent density functional theory illustrate the formation of an elusive FeI octahedral species with bound aqua ligands and lifetimes of 23-29 μs.

The role of Tyr34 in proton coupled electron transfer and product inhibition of manganese superoxide dismutase

Human manganese superoxide dismutase (MnSOD) plays a crucial role in controlling levels of reactive oxygen species (ROS) by converting superoxide (O 2 ∙ - ) to molecular oxygen (O2) and hydrogen peroxide (H2O2) with proton-coupled electron transfers (PCETs). A key catalytic residue, Tyr34, determines the activity of human MnSOD and also becomes post-translationally inactivated by nitration in various diseases associated with mitochondrial dysfunction. Tyr34 has an unusual pKa due to its proximity to the Mn metal and undergoes cyclic deprotonation and protonation events to promote the electron transfers of MnSOD. Neutron diffraction, X-ray spectroscopy, and quantum chemistry calculations in oxidized, reduced and product inhibited enzymatic states shed light on the role of Tyr34 in MnSOD catalysis. The data identify the contributions of Tyr34 in MnSOD activity that support mitochondrial function and give a thorough characterization of how a single tyrosine modulates PCET catalysis. Product inhibition occurs by an associative displacement mechanism.

Operando Characterization of Fe in Doped Nix(Fe1-x)OyHz Catalysts for Electrochemical Oxygen Evolution

Iron-doped nickel oxyhydroxides, Nix(Fe1-x)OyHz, are among the most promising oxygen evolution reaction (OER) electrocatalysts in alkaline environments. Although iron (Fe) significantly enhances the catalytic activity, there is still no clear consensus on whether Fe directly participates in the reaction or merely acts as a promoter. To elucidate the Fe's role, we performed operando X-ray spectroscopy studies supported by DFT on Nix(Fe1-x)OyHz electrocatalysts. We probed the reversible changes in the structure and electronic character of Nix(Fe1-x)OyHz as the electrode potential is cycled between the resting (here at 1.10 VRHE) and operational states (1.66 VRHE). DFT calculations and XAS simulations on a library of Fe structures in various NiOyHz environments are in favor of a distorted local octahedral Fe(III)O3(OH)3 configuration at the resting state with the NiOyHz scaffold going from α-Ni(OH)2 to γ-NiOOH as the potential is increased. Under catalytic conditions, EXAFS and HERFD spectra reveal changes in p-d mixing (covalency) relative to the resting state between O/OH ligands and Fe leading to a shift from octahedral to square pyramidal coordination at the Fe site. XES measurements and theoretical simulations further support that the Fe equilibrium structure remains in a formal Fe(III) state under both resting and operational conditions. These spectral changes are attributed to potential dependent structural rearrangements around Fe. The results suggest that ligand dissociation leads to the C4v symmetry as the most stable intermediate of the Fe during OER. This implies that Fe has a weakly coordinated or easily dissociable ligand that could serve to coordinate the O-O bond formation and, tentatively, play an active role in the Nix(Fe1-x)OyHz electrocatalyst.

Electronic Modulation and Symmetry-Breaking Engineering of Single-Atom Catalysts Driving Long-Cycling Li-S Battery

Developing efficient and durable single-atom catalysts is vitally important for the sulfur redox reaction (SROR) in Li-S battery, while it remains enormous challenging. Herein, undercoordinated Ni-N3 moieties anchored on N,S-codoped porous carbon (Ni-NSC) is obtained to enhance the SROR. The experiments and theoretical calculations indicate that the symmetry-breaking charge transfer in Ni single-atom catalyst originates from tuning effect of sulfur atoms mediated Ni-N3 moieties, which can both facilitate the chemical adsorption by formation of N-Ni⋅⋅⋅Sn 2-, and achieve a rapid redox conversion of polysulfides because of the enhanced electron transfer. As results, the Ni-NSC based Li-S battery delivers a very high initial reversible capacity (1025 mAh g-1 at 1 C), as well as outstanding cycling-stability for 2400 cycles at 2 C and 3 C, respectively. Noteworthy, the areal capacity can reach 7.8 mAh cm-2 at 0.05 C and a retention capacity of 4.7 mAh cm-2 after 100 cycles at 0.2 C for Ni-NSC based Li-S battery with sulfur loading of 5.88 mg cm-2. This work provides profound insight for rational optimizing microscopic electronic density of active site to promoting SROR in metal-sulfur batteries.

The activation of the metal-containing regulatory protein NiaR from Thermotoga maritima by its effector, nicotinic acid

NiaR is a regulatory protein that represses the expression of proteins involved in the de novo biosynthesis and uptake of nicotinic acid (NA), with NA acting as a co-repressor. The previously published structure of NiaR from Thermotoga maritima (TmNiaR) identified it as a functional homodimer containing a transition metal ion in a suspected NA-binding pocket. Here, we present the crystal structure of NA bound to the iron-metalated form of TmNiaR. Supported by spectroscopic and solution studies, this structure shows that NA binds to a protein-bound ferrous ion via its ring nitrogen. In addition, the carboxylate group on NA interacts with Tyr108 from the dyad-related subunit, repositioning the likely DNA-binding domains of the dimer to promote high-affinity interactions with DNA operators. The specificity of TmNiaR for NA can be explained by the hydrogen bonding scheme within the NA-binding pocket.

Enhance the Proportion of Fe3+ in NiFe-Layered Double Hydroxides by utilizing Citric Acid to Improve the Efficiency and Durability of the Oxygen Evolution Reaction

NiFe-layered double hydroxides (NiFe-LDH) are a type of catalyst known for their exceptional catalytic performance during the oxygen evolution reaction (OER). In this study, citric acid was incorporated into the synthesis process of NiFe-LDH, resulting in the NiFe-LDH-CA catalyst with superior OER performance. The catalytic efficacy was evaluated using linear sweep voltammetry (LSV), which demonstrated a significant reduction in the overpotential for OER from 320 mV to 240 mV at a current density of 100 mA cm-2. X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) indicate that the distribution of nickel valence states showed no significant difference between two samples. However, the NiFe-LDH-CA exhibits a markedly higher proportion of Fe3+ ions in its iron content. In-situ Raman spectroscopes reveal that Fe3+ broadens the redox potential of nickel and Pourbaix diagrams indicate that higher Fe3+ levels could facilitate the interaction with oxygen active sites. Based on the analysis of test data, we propose a hypothesis that the high proportion of Fe3+ in catalysts may accelerate the oxygen evolution process by modulating the redox potential of nickel and engaging with reactive oxygen species. This provides valuable insights into how to improve the reaction rate of nickel-based catalysts.

No Bridge between Us: EXAFS and Computations Confirm Two Distant Iron Ions Comprise the Active Site of Alkane Monooxygenase (AlkB)

Alkane monooxygenase (AlkB) is the dominant enzyme that catalyzes the oxidation of liquid alkanes in the environment. Two recent structural models derived from cryo-electron microscopy (cryo-EM) reveal an unusual active site: a histidine-rich center that binds two iron ions without a bridging ligand. To ensure that potential photoreduction and radiation damage are not responsible for the absence of a bridging ligand in the cryo-EM structures, spectroscopic methods are needed. We present the results of extended X-ray absorption fine structure (EXAFS) experiments collected under conditions where photodamage was avoided. Careful data analysis reveals an active site structure consistent with the cryo-EM structures in which the two iron ions are ligated by nine histidines and separated by at least 5 Å. The EXAFS data were used to inform structural models for molecular dynamics (MD) simulations. The MD simulations corroborate EXAFS observations that neither of the two conserved carboxylate-containing residues (E281 and D190) near the active site are likely candidates for metal ion bridging. Mutagenesis experiments, spectroscopy, and additional MD simulations were used to further explore the role of these carboxylate residues. A variant in which a carboxylate containing residue (E281) was changed to a methyl residue (E281A) showed little change in pre-edge features, consistent with the observation that it is not essential for activity and hence unlikely to serve as a bridging ligand at any point in the catalytic cycle. D190 variants had substantially diminished activity, suggesting an important role in catalysis not yet fully understood.

Operando Spectroelectrochemistry Unravels the Mechanism of CO2 Electrocatalytic Reduction by an Fe Porphyrin

Iron porphyrins are molecular catalysts recognized for their ability to electrochemically and photochemically reduce carbon dioxide (CO2). The main reduction product is carbon monoxide (CO). CO holds significant industrial importance as it serves as a precursor for various valuable chemical products containing either a single carbon atom (C1), like methanol or methane, or multiple carbon atoms (Cn), such as ethanol or ethylene. Despite the long-established efficiency of these catalysts, optimizing their catalytic activity and stability and comprehending the intricate reaction mechanisms remain a significant challenge. This article presents a comprehensive investigation of the mechanistic aspects of the selective electroreduction of CO2 to CO using an iron porphyrin substituted with four trimethylammonium groups in the para position [(pTMA)FeIII-Cl]4+. By employing infrared and UV/Visible spectroelectrochemistry, changes in the electronic structure and coordination environment of the iron center can be observed in real-time as the electrochemical potential is adjusted, offering new insights into the reaction mechanisms. Catalytic species were identified, and evidence of a secondary reaction pathway was uncovered, potentially prompting a re-evaluation of the nature of the catalytically active species.

Triggering Reversible Intercalation-Conversion Combined Chemistry for High-Energy-Density Lithium Battery Cathodes

Combining intercalation and conversion reactions maximizes the utilization of redox-active elements in electrodes, providing a means for overcoming the current capacity ceiling. However, integrating both mechanisms within a single electrode material presents significant challenges owing to their contrasting structural requirements. Intercalation requires a well-defined host structure for efficient lithium-ion diffusion, whereas conversion reactions entail structural reorganization, which can undermine intercalation capabilities. Based on the previous study that successfully demonstrated reversible intercalation-conversion chemistry in amorphous LiFeSO4F, this study aims to provide an in-depth understanding on how this can be enabled. Experimental and theoretical investigations of a model system based on tavorite-structured LiFeSO4F revealed that amorphization governs the activation and reversibility of the combined reactions. Enhanced reversibility is achieved through the facile migration of transition metals within the amorphous matrix. Unexpectedly, it is found that amorphization also narrowed the voltage gap between the intercalation and conversion reactions. This voltage-gap reduction is explained by the thermodynamic metastability of the amorphous phase. The applicability of the approach to other intercalation hosts is further demonstrated, showing that amorphization enables reversible intercalation and conversion. These findings suggest a new strategy that leverages the full potential of intercalation and conversion reactions, introducing new avenues for cathode design.

Deciphering Water Oxidation Catalysts: The Dominant Role of Surface Chemistry over Reconstruction Degree in Activity Promotion

Water splitting hinges crucially on the availability of electrocatalysts for the oxygen evolution reaction. The surface reconstruction has been widely observed in perovskite catalysts, and the reconstruction degree has been often correlated with the activity enhancement. Here, a systematic study on the roles of Fe substitution in activation of perovskite LaNiO3 is reported. The substituting Fe content influences both current change tendency and surface reconstruction degree. LaNi0.9Fe0.1O3 is found exhibiting a volcano-peak intrinsic activity in both pristine and reconstructed among all substituted perovskites in the LaNi1-xFexO3 (x = 0.00, 0.10, 0.25, 0.50, 0.75, 1.00) series. The reconstructed LaNi0.9Fe0.1O3 shows a higher intrinsic activity than most reported NiFe-based catalysts. Besides, density functional theory calculations reveal that Fe substitution can lower the O 2p level, which thus stabilize lattice oxygen in LaNi0.9Fe0.1O3 and ensure its long-term stability. Furthermore, it is vital interesting that activity of the reconstructed catalysts relied more on the surface chemistry rather than the reconstruction degree. The effect of Fe on the degree of surface reconstruction of the perovskite is decoupled from that on its activity enhancement after surface reconstruction. This finding showcases the importance to customize the surface chemistry of reconstructed catalysts for water oxidation.

New insights into the dissolution mechanisms of iron oxides and combusted iron particles in oxalic acid

The influence of oxidation state and crystalline structure on the dissolution mechanisms of both pure iron oxides and combusted iron particles in aqueous oxalic acid (0.5 mol/l) at 60 °C was systematically investigated. Dissolution experiments were carried out in a temperature-controlled, continuous-flow capillary reactor, allowing for the removal of reaction products and thereby suppressing the autocatalytic reaction mechanism. The non-reductive dissolution of α-Fe2O3 was observed through in situ x-ray absorption measurements. In contrast, the dissolution of spinel-type oxides such as γ-Fe2O3 and Fe3O4 proceeded reductively, indicated by gradual changes in characteristic spectral features. Given that γ-Fe2O3 and Fe3O4 share a similar crystal structure but differ in the nominal oxidation state, this implies that the phase composition is decisive for the reductive dissolution. For mixed-phase particles consisting of spinel and rhombohedral phases (maghemite and hematite), the preferential dissolution of the spinel phase was observed. Despite the similar bulk composition of spinel and rhombohedral phases in the combusted iron particles (as confirmed by Mössbauer spectroscopy and x-ray diffraction analysis), dissolution predominantly follows a non-reductive pathway, with no preferential dissolution of the γ-phase. This unique dissolution behavior of combusted iron particles arises from their layered microstructure.

Characterization of copper speciation on waste biomass of phytofiltration systems using energy dispersive Inelastic X-ray scattering

BACKGROUND

Remediation of heavy metal-contaminated water using phytoremediation with accumulator aquatic plants is a promising low-cost emerging technology that adapts very well to the surrounding ecosystem. For the system to work efficiently, metal-saturated plants must be replaced, producing a potentially toxic amount of biomass that is usually stored dry to reduce its volume. The speciation of the high metal content in this biomass is crucial to define its final destination. This work explores the application of synchrotron-based EDIXS (Energy Dispersive Inelastic X-ray Scattering) to monitor the speciation of copper in regional aquatic plants from a laboratory-scale phytoremediation system.

RESULTS

The phytofiltration system utilized Lemna minor L. and Salvinia biloba Raddi species grown under controlled conditions of light and nutrient availability. Both species are known hyperaccumulators of copper and are prevalent in lakes and rivers across South America. The validation of EDIXS was previously carried out by comparing the results of copper standard samples with those obtained by XANES. The findings revealed that both plant species retained copper in chemical complexes exhibiting octahedral coordination with a Cu valence of 2. Notably, differences emerged between the leaves and roots of Lemna minor L., suggesting a more pronounced adsorption of copper in its leaves, a trend that intensified with exposure. In opposite, for Salvinia the differences between leaves and roots suggests the presence of specific protective mechanisms to cope the copper exposure. Surprisingly, no significant dependence on copper concentration of the aqueous media was observed for either species.

SIGNIFICANCE AND NOVELTY

These promising results endorse the viability of the proposed methodology in identifying the most effective fate of biomass generated in phytoremediation systems. EDIXS provides a valid tool for performing local copper speciation in aquatic plants with sufficient selectivity to identify subtle differences in various biological tissues. The simplicity of this methodology renders it a valuable tool for advancing our comprehension of metal speciation within waste biomass, thereby holding significant implications for the development of environmental remediation strategies.

Molecular catalyst coordinatively bonded to organic semiconductors for selective light-driven CO2 reduction in water

The selective photoreduction of CO2 in aqueous media based on earth-abundant elements only, is today a challenging topic. Here we present the anchoring of discrete molecular catalysts on organic polymeric semiconductors via covalent bonding, generating molecular hybrid materials with well-defined active sites for CO2 photoreduction, exclusively to CO in purely aqueous media. The molecular catalysts are based on aryl substituted Co phthalocyanines that can be coordinated by dangling pyridyl attached to a polymeric covalent triazine framework that acts as a light absorber. This generates a molecular hybrid material that efficiently and selectively achieves the photoreduction of CO2 to CO in KHCO3 aqueous buffer, giving high yields in the range of 22 mmol g-1 (458 μmol g-1 h-1) and turnover numbers above 550 in 48 h, with no deactivation and no detectable H2. The electron transfer mechanism for the activation of the catalyst is proposed based on the combined results from time-resolved fluorescence spectroscopy, in situ spectroscopies and quantum chemical calculations.

Experimentally Assessing the Electronic Structure and Spin-State Energetics in MnFe Dimers Using 1s3p Resonant Inelastic X-ray Scattering

The synergistic interaction between Mn and Fe centers is investigated via a comprehensive analysis of full 1s3p resonant inelastic X-ray scattering (RIXS) planes at both the Fe and Mn K-edges in a series of homo- and heterometallic molecular systems. Deconvolution of the experimental two-dimensional 1s3p RIXS maps provides insights into the modulation of metal-ligand covalency and variations in the metal multiplet structure induced by subtle electronic structural differences imposed by the presence of the second metal. These modulations in the electronic structure are key toward understanding the reactivity of biological systems with active sites that require heterometallic centers, including MnFe purple acid phosphatases and MnFe ribonucleotide reductases. Herein, we demonstrate the capabilities of 1s3p RIXS to provide information on the excited state energetics in both element- and spin-selective fashion. The contributing excited states are identified and isolated by their multiplicity and π- and σ-contributions, building a conceptual bridge between the electronic structures of metal centers and their reactivity. The ability of the presented 1s3p RIXS methodology to address fundamental questions in transition metal catalysis reactivity is highlighted.

Mechanochemical Polyoxometalate Super-Reduction with Lithium Metal

In this first systematic investigation of mechanochemical polyoxometalate (POM) reduction, (TBA)3[PMo12O40] was reacted with n equiv of lithium metal (n = 1-24) to generate PMo12/n products which were shown to be mixtures of electron-rich PMo12Lix species. FTIR analysis revealed the lengthening/weakening of terminal Mo═O bonds with increasing levels of reduction, while EXAFS spectra indicated the onset of Mo-Mo bond formation at n ∼ 8 and a significant structural change at n > 12. Successive MoVI reductions were monitored by XANES and XPS, and at n = 24, results were consistent with the formation of at least one MoIV-MoIV bonded {MoIV3} triad together with MoV. Upon dissolution, the PMo12Lix species present in the solid PMo12/n products undergo electron exchange and single-peak 31P NMR spectra were observed for n = 1-12. For n ≥ 16, changes in solid state and solution 31P NMR spectra coincided with the emergence of features in the UV-vis spectra associated with MoV-MoV and {MoIV3} bonding in an ε-Keggin structure. Bonding between {Li(NCMe)}+ and 2-electron-reduced PMo12 in (TBA)4[PMo12O40{Li(NCMe)}] suggests that super-reduction gives rise to more extensive Li-O bonding that ultimately causes lithium-oxide-promoted TBA cation decomposition and POM degradation, which might explain the appearance of XPS peaks for Mo2C at n ≥ 16. This work has revealed some of the complex, unexplored chemistry of super-reduced POMs and establishes a new, solvent-free approach in the search for a better fundamental understanding of the electronic properties and reactivity of electron-rich nanoscale metal oxides.

Metal-ligand delocalization of iron and cobalt porphyrin complexes in aqueous solutions probed by soft X-ray absorption spectroscopy

Metal-ligand delocalization of metal porphyrin complexes in aqueous solutions was investigated by analyzing the electronic structure of both the metal and ligand sides using soft X-ray absorption spectroscopy (XAS) at the metal L2,3-edges and nitrogen K-edge, respectively. In the N K-edge XAS spectra of the ligands, the energies of the CN π* peaks of cobalt protoporphyrin IX (CoPPIX) are higher than iron protoporphyrin IX (FePPIX). The energy difference between the two lowest peaks in the XAS spectrum of CoPPIX is also larger than that of FePPIX. Nitrogen K-edge inner-shell calculations of metalloporphyrins with different central metals indicate that the energy differences between these peaks reflect the electronic configurations and spin multiplicities of metalloporphyrins. We also investigated the hydration structure of CoPPIX in aqueous solution by analyzing the electronic structure of the ligand and revealed that CoPPIX maintains its five-coordination geometry in aqueous solution. The present study shows high performance of N K-edge XAS of ligands for studying the coordination structures of metalloporphyrins in solutions rather than the metal L2,3-edges of central metals.

Isolation and Crystallographic Characterization of an Octavalent Co2O2 Diamond Core

High-valent cobalt oxides play a pivotal role in alternative energy technology as catalysts for water splitting and as cathodes in lithium-ion batteries. Despite this importance, the properties governing the stability of high-valent cobalt oxides and specifically possible oxygen evolution pathways are not clear. One root of this limited understanding is the scarcity of high-valent Co(IV)-containing model complexes; there are no reports of stable, well-defined complexes with multiple Co(IV) centers. Here, an oxidatively robust fluorinated ligand scaffold enables the isolation and crystallographic characterization of a Co(IV)2-bis-μ-oxo complex. This complex is remarkably stable, in stark contrast with previously reported Co(IV)2 species that are highly reactive, which demonstrates that oxy-Co(IV)2 species are not necessarily unstable with respect to oxygen evolution. This example underscores a new design strategy for highly oxidizing transition-metal fragments and provides detailed data on a previously inaccessible chemical unit of relevance to O-O bond formation and oxygen evolution.

A Redox-Active Iron-Organic Framework Cathodes for Sustainable Magnesium Metal Batteries

Rechargeable magnesium metal batteries (RMBs) have shown promising prospects in sustainable energy storage due to the high crustal abundance, safety, and potentially large specific capacity of magnesium. However, their development is constrained by the lack of effective cathode materials that can achieve high capacity and stable magnesium storage at a practically reasonable rate. Herein, we construct a three-dimensional (3D) iron(III)-dihydroxy-benzoquinone (Fe2(DHBQ)3) metal-organic framework (MOF) material with dual redox centers of Fe3+ cations and DHBQ2- anions for reversible storage of Mg2+ in RMBs. Spectroscopic analysis and density functional theory (DFT) calculations reveal the redox chemistry of both Fe3+ ions and carbonyls from DHBQ ligands during electrochemical processes. Benefiting from the rational structure, the Fe2(DHBQ)3∥Mg cells exhibit a high reversible capacity of 395.3 mAh/g, large energy density of 463.5 Wh/kg, and high power density of 2456.0 W/kg. Moreover, the high electronic conductivity (8.35 × 10-5 S/cm) and favorable diffusion path of Mg2+ in Fe2(DHBQ)3 endow the cells with exceptional cycling stability and rate capability with a long life of 5000 cycles at 2000 mA/g. The dual redox-active MOF demonstrates a category of advanced cathode materials for high-performance RMBs.

Preparation and characterization of MnIIMnIII complexes with relevance to class Ib ribonucleotide reductases

The Mn2 complex [MnII2(TPDP)(O2CPh)2](BPh4) (1, TPDP = 1,3-bis(bis(pyridin-2-ylmethyl)amino)propan-2-ol, Ph =phenyl) was prepared and subsequently characterized via single-crystal X-ray diffraction, X-ray absorption, electronic absorption, and infrared spectroscopies, and mass spectrometry. 1 was prepared in order to explore its properties as a structural and functional mimic of class Ib ribonucleotide reductases (RNRs). 1 reacted with superoxide anion (O2•-) to generate a peroxido-MnIIMnIII complex, 2. The electronic absorption and electron paramagnetic resonance (EPR) spectra of 2 were similar to previously published peroxido-MnIIMnIII species. Furthermore, X-ray near edge absorption structure (XANES) studies indicated the conversion of a MnII2 core in 1 to a MnIIMnIII state in 2. Treatment of 2 with para-toluenesulfonic acid (p-TsOH) resulted in the conversion to a new MnIIMnIII species, 3, rather than causing O-O bond scission, as previously encountered. 3 was characterized using electronic absorption, EPR, and X-ray absorption spectroscopies. Unlike other reported peroxido-MnIIMnIII species, 3 was capable of oxidative O-H activation, mirroring the generation of tyrosyl radical in class Ib RNRs, however without accessing the MnIIIMnIV state.

A new parameterization of the DFT/CIS method with applications to core-level spectroscopy

Time-dependent density functional theory (TD-DFT) within a restricted excitation space is an efficient means to compute core-level excitation energies using only a small subset of the occupied orbitals. However, core-to-valence excitation energies are significantly underestimated when standard exchange-correlation functionals are used, which is partly traceable to systemic issues with TD-DFT's description of Rydberg and charge-transfer excited states. To mitigate this, we have implemented an empirically modified combination of configuration interaction with single substitutions (CIS) based on Kohn-Sham orbitals, which is known as "DFT/CIS." This semi-empirical approach is well-suited for simulating x-ray near-edge spectra, as it contains sufficient exact exchange to model charge-transfer excitations yet retains DFT's low-cost description of dynamical electron correlation. Empirical corrections to the matrix elements enable semi-quantitative simulation of near-edge x-ray spectra without the need for significant a posteriori shifts; this should be useful in complex molecules and materials with multiple overlapping x-ray edges. Parameter optimization for use with a specific range-separated hybrid functional makes this a black-box method intended for both core and valence spectroscopy. Results herein demonstrate that realistic K-edge absorption and emission spectra can be obtained for second- and third-row elements and 3d transition metals, with promising results for L-edge spectra as well. DFT/CIS calculations require absolute shifts that are considerably smaller than what is typical in TD-DFT.

Revealing the atomic and electronic mechanism of human manganese superoxide dismutase product inhibition

Human manganese superoxide dismutase (MnSOD) is a crucial oxidoreductase that maintains the vitality of mitochondria by converting superoxide (O2●-) to molecular oxygen (O2) and hydrogen peroxide (H2O2) with proton-coupled electron transfers (PCETs). Human MnSOD has evolved to be highly product inhibited to limit the formation of H2O2, a freely diffusible oxidant and signaling molecule. The product-inhibited complex is thought to be composed of a peroxide (O22-) or hydroperoxide (HO2-) species bound to Mn ion and formed from an unknown PCET mechanism. PCET mechanisms of proteins are typically not known due to difficulties in detecting the protonation states of specific residues that coincide with the electronic state of the redox center. To shed light on the mechanism, we combine neutron diffraction and X-ray absorption spectroscopy of the product-bound, trivalent, and divalent states of the enzyme to reveal the positions of all the atoms, including hydrogen, and the electronic configuration of the metal ion. The data identifies the product-inhibited complex, and a PCET mechanism of inhibition is constructed.

Revealing Structural Evolution of Nickel Phosphide-Iron Oxide Core-Shell Nanocatalysts in Alkaline Medium for the Oxygen Evolution Reaction

Metal phosphide-containing materials have emerged as a potential candidate of nonprecious metal-based catalysts for alkaline oxygen evolution reaction (OER). While it is known that metal phosphide undergoes structural evolution, considerable debate persists regarding the effects of dynamics on the surface activation and morphological stability of the catalysts. In this study, we synthesize NiP x -FeO x core-shell nanocatalysts with an amorphous NiP x core designed for enhanced OER activity. Using ex situ X-ray absorption spectroscopy, we elucidate the local structural changes as a function of the cyclic voltammetry cycles. Our studies suggest that the presence of corner-sharing octahedra in the FeO x shell improves structural rigidity through interlayer cross-linking, thereby inhibiting the diffusion of OH-/H2O. Thus, the FeO x shell preserves the amorphous NiP x core from rapid oxidation to Ni3(PO4)2 and Ni(OH)2. On the other hand, the incorporation of Ni from the core into the FeO x shell facilitates absorption of hydroxide ions for OER. As a result, Ni/Fe(OH) x at the surface oxidizes to the active γ-(oxy)hydroxide phase under the applied potentials, promoting OER. This intriguing synergistic behavior holds significance as such a synthetic route involving the FeO x shell can be extended to other systems, enabling manipulation of surface adsorption and diffusion of hydroxide ions. These findings also demonstrate that nanomaterials with core-shell morphologies can be tuned to leverage the strength of each metallic component for improved electrochemical activities.

The role of Tyr34 in proton-coupled electron transfer of human manganese superoxide dismutase

Human manganese superoxide dismutase (MnSOD) plays a crucial role in controlling levels of reactive oxygen species (ROS) by converting superoxide (O2 ●-) to molecular oxygen (O2) and hydrogen peroxide (H2O2) with proton-coupled electron transfers (PCETs). The reactivity of human MnSOD is determined by the state of a key catalytic residue, Tyr34, that becomes post-translationally inactivated by nitration in various diseases associated with mitochondrial dysfunction. We previously reported that Tyr34 has an unusual pKa due to its proximity to the Mn metal and undergoes cyclic deprotonation and protonation events to promote the electron transfers of MnSOD. To shed light on the role of Tyr34 MnSOD catalysis, we performed neutron diffraction, X-ray spectroscopy, and quantum chemistry calculations of Tyr34Phe MnSOD in various enzymatic states. The data identifies the contributions of Tyr34 in MnSOD activity that support mitochondrial function and presents a thorough characterization of how a single tyrosine modulates PCET catalysis.

Tailoring Oxygen Reduction Reaction Kinetics of Fe-N-C Catalyst via Spin Manipulation for Efficient Zinc-Air Batteries

The interaction between oxygen species and metal sites of various orbitals exhibits intimate correlation with the oxygen reduction reaction (ORR) kinetics. Herein, a new approach for boosting the inherent ORR activity of atomically dispersed Fe-N-C matrix is represented by implanting Fe atomic clusters nearby. The as-prepared catalyst delivers excellent ORR activity with half-wave potentials of 0.78 and 0.90 V in acidic and alkaline solutions, respectively. The decent ORR activity can also be validated from the high-performance rechargeable Zn-air battery. The experiments and density functional theory calculations reveal that the electron spin-state of monodispersed Fe active sites is transferred from the low spin (LS, t2g 6 eg 0) to the medium spin (MS, t2g 5 eg 1) due to the involvement of Fe atomic clusters, leading to the spin electron filling in σ∗ orbit, by which it favors OH- desorption and in turn boosts the reaction kinetics of the rate-determining step. This work paves a solid way for rational design of high-performance Fe-based single atom catalysts through spin manipulation.

Enhanced Fe(OH)2-driven reductive Dechlorination via shortened Fe-O bonds and colloidal medium

Fe2+ is usually adsorbed to the surface of iron-bearing clay, and iron (hydr)oxide in groundwater. However, the reductive activity of Fe(OH)2, a prevalent intermediate during the transformation of Fe2+, remains unclear. In this study, high-purity Fe(OH)2 was synthesized and tested for its activity in the degradation of carbon tetrachloride (CT). XRD data confirm that the synthesized material is a pure Fe(OH)2 crystal, exhibiting sharp peaks of (001) and (100) facets. Zeta potential analysis confirms that the off-white Fe(OH)2 is a colloidal suspension with a positive charge of ∼+35-50 mV. FTIR spectra reveal the formation of a coordination compound Fe2+ with OH-/OD-, derived from NaOH/OD. SEM and HRTEM results demonstrate that the Fe(OH)2 crystal has a regular octahedral structure with a size of ∼30-70 nm and average lattice spacings of 2.58 Å. Mössbauer spectrum verifies that the Fe2+ in Fe(OH)2/Fe(OD)2 is hexacoordinated with six Fe-O bonds. XAFS data demonstrate that the Fe-O bonds become shorter as the OH-:Fe(II) ratios increase. DFT results indicate that the (100) crystal face of Fe(OH)2 more readily transfers electrons to CT. In addition to being adsorbed to iron compounds, structural Fe2+ compounds such as Fe(OH)2 could also accelerate the electron transfer from Fe2+ to CT through shortened Fe-O bonds. The rate constant of CT reduction by Fe(OH)2 is as high as 0.794 min-1 when the OH-:Fe(II) ratio is 2.5 in water. This study aims to enhance our understanding of the structure-reactivity relationship of Fe2+ compounds in groundwater, particularly in relation to electron transfer mechanisms.

Directed Electron Delivery from a Pb-Free Halide Perovskite to a Co(II) Molecular Catalyst Boosts CO2 Photoreduction Coupled with Water Oxidation

The development of high-performance photocatalytic systems for CO2 reduction is appealing to address energy and environmental issues, while it is challenging to avoid using toxic metals and organic sacrificial reagents. We here immobilize a family of cobalt phthalocyanine catalysts on Pb-free halide perovskite Cs2AgBiBr6 nanosheets with delicate control on the anchors of the cobalt catalysts. Among them, the molecular hybrid photocatalyst assembled by carboxyl anchors achieves the optimal performance with an electron consumption rate of 300±13 μmol g-1 h-1 for visible-light-driven CO2-to-CO conversion coupled with water oxidation to O2, over 8 times of the unmodified Cs2AgBiBr6 (36±8 μmol g-1 h-1), also far surpassing the documented systems (<150 μmol g-1 h-1). Besides the improved intrinsic activity, electrochemical, computational, ex-/in situ X-ray photoelectron and X-ray absorption spectroscopic results indicate that the electrons photogenerated at the Bi atoms of Cs2AgBiBr6 can be directionally transferred to the cobalt catalyst via the carboxyl anchors which strongly bind to the Bi atoms, substantially facilitating the interfacial electron transfer kinetics and thereby the photocatalysis.

Can the Rate of a Catalytic Turnover Be Altered by Ligands in the Absence of Direct Binding Interactions?

Second sphere coordination effects ubiquitous in enzymatic catalysis occur through direct interactions, either covalent or non-covalent, with reaction intermediates and transition states. We present herein evidence of indirect second sphere coordination effects in which ligation of water/alkanols far removed from the primary coordination sphere of the active site nevertheless alter energetic landscapes within catalytic redox cycles in the absence of direct physicochemical interactions with surface species mediating catalytic turnovers. Density functional theory, in situ X-ray absorption and infrared spectroscopy, and a wide array of steady-state and transient CO oxidation rate data suggest that the presence of peripheral water renders oxidation half-cycles within two-electron redox cycles over μ3-oxo-bridged trimers in MIL-100(M) more kinetically demanding. Communication between ligated water and the active site appears to occur through the Fe-O-Fe backbone, as inferred from spin density variations on the central μ3-oxygen 'junction'. Evidence is provided for the generality of these second sphere effects in that they influence different types of redox half-cycles or metals, and can be amplified or attenuated through choice of coordinating ligand. Specifically in the case of MIL-100(M) materials, the Cr isostructure can be made to kinetically mimic the Fe variant by disproportionately hindering oxidation half-cycles relative to the reduction half-cycles. Kinetic and spectroscopic inferences presented here significantly expand both the conceptual definition of second sphere effects as well as the palette of synthetic levers available for tuning catalytic redox performance through chemical ligation.

Photocatalytic CO2 Reduction Using Mixed Catalytic Systems Comprising an Iron Cation with Bulky Phenanthroline Ligands

This study reports on efficient photocatalytic CO2 reduction reactions using mixed catalytic systems of an Fe ion source and various 1,10-phenanthroline derivatives (R1R2p) as ligands in the presence of triethanolamine (TEOA). As the relatively bulky substituents at positions 2 and 9 of R1R2p weakened the ability to coordinate to the Fe ion, the Fe ion formed TEOA complexes. The free R1R2p accepted an electron from the reduced photosensitizer through proton-coupled electron transfer (PCET) using protons of TEOA dissolved in a CH3CN solution in a CO2 atmosphere as the initial step of the catalytic cycle. Although the mixed system of the nonsubstituted 1,10-phenanthroline generates a stable tris(phenanthroline)-Fe(II) complex in solution, this complex could not function as a CO2 reduction catalyst. The mechanism in which R1R2p interacts with the Fe ion after PCET was proposed for this efficient photocatalytic CO2 reduction. The proposed photocatalytic system using the 2,9-di-sec-butyl-phenanthroline ligand could produce CO with high efficiency (quantum yield of 8.2%) combined with a dinuclear Cu(I) complex as a photosensitizer.

Correlating Structure with Spectroscopy in Ascorbate Peroxidase Compound II

Structural and spectroscopic investigations of compound II in ascorbate peroxidase (APX) have yielded conflicting conclusions regarding the protonation state of the crucial Fe(IV) intermediate. Neutron diffraction and crystallographic data support an iron(IV)-hydroxo formulation, whereas Mössbauer, X-ray absorption (XAS), and nuclear resonance vibrational spectroscopy (NRVS) studies appear consistent with an iron(IV)-oxo species. Here we examine APX with spectroscopy-oriented QM/MM calculations and extensive exploration of the conformational space for both possible formulations of compound II. We establish that irrespective of variations in the orientation of a vicinal arginine residue and potential reorganization of proximal water molecules and hydrogen bonding, the Fe-O distances for the oxo and hydroxo forms consistently fall within distinct, narrow, and nonoverlapping ranges. The accuracy of geometric parameters is validated by coupled-cluster calculations with the domain-based local pair natural orbital approach, DLPNO-CCSD(T). QM/MM calculations of spectroscopic properties are conducted for all structural variants, encompassing Mössbauer, optical, X-ray absorption, and X-ray emission spectroscopies and NRVS. All spectroscopic observations can be assigned uniquely to an Fe(IV)═O form. A terminal hydroxy group cannot be reconciled with the spectroscopic data. Under no conditions can the Fe(IV)═O distance be sufficiently elongated to approach the crystallographically reported Fe-O distance. The latter is consistent only with a hydroxo species, either Fe(IV) or Fe(III). Our findings strongly support the Fe(IV)═O formulation of APX-II and highlight unresolved discrepancies in the nature of samples used across different experimental studies.

Chemical and Redox Noninnocence of Pentane-2,4-dione Bis(S-methylisothiosemicarbazone) in Cobalt Complexes and Their Application in Wacker-Type Oxidation

Cobalt complexes with multiproton- and multielectron-responsive ligands are of interest for challenging catalytic transformations. The chemical and redox noninnocence of pentane-2,4-dione bis(S-methylisothiosemicarbazone) (PBIT) in a series of cobalt complexes has been studied by a range of methods, including spectroscopy [UV-vis, NMR, electron paramagnetic resonance (EPR), X-ray absorption spectroscopy (XAS)], cyclic voltammetry, X-ray diffraction, and density functional theory (DFT) calculations. Two complexes [CoIII(H2LSMe)I]I and [CoIII(LSMe)I2] were found to act as precatalysts in a Wacker-type oxidation of olefins using phenylsilane, the role of which was elucidated through isotopic labeling. Insights into the mechanism of the catalytic transformation as well as the substrate scope of this selective reaction are described, and the essential role of phenylsilane and the noninnocence of PBIT are disclosed. Among the several relevant species characterized was an unprecedented Co(III) complex with a dianionic diradical PBIT ligand ([CoIII(LSMe••)I]).

Functional Model of Compound II of Cytochrome P450: Spectroscopic Characterization and Reactivity Studies of a FeIV-OH Complex

Herein, we show that the reaction of a mononuclear FeIII(OH) complex (1) with N-tosyliminobenzyliodinane (PhINTs) resulted in the formation of a FeIV(OH) species (3). The obtained complex 3 was characterized by an array of spectroscopic techniques and represented a rare example of a synthetic FeIV(OH) complex. The reaction of 1 with the one-electron oxidizing agent was reported to form a ligand-oxidized FeIII(OH) complex (2). 3 revealed a one-electron reduction potential of -0.22 V vs Fc+/Fc at -15 °C, which was 150 mV anodically shifted than 2 (Ered = -0.37 V vs Fc+/Fc at -15 °C), inferring 3 to be more oxidizing than 2. 3 reacted spontaneously with (4-OMe-C6H4)3C• to form (4-OMe-C6H4)3C(OH) through rebound of the OH group and displayed significantly faster reactivity than 2. Further, activation of the hydrocarbon C-H and the phenolic O-H bond by 2 and 3 was compared and showed that 3 is a stronger oxidant than 2. A detailed kinetic study established the occurrence of a concerted proton-electron transfer/hydrogen atom transfer reaction of 3. Studying one-electron reduction of 2 and 3 using decamethylferrocene (Fc*) revealed a higher ket of 3 than 2. The study established that the primary coordination sphere around Fe and the redox state of the metal center is very crucial in controlling the reactivity of high-valent Fe-OH complexes. Further, a FeIII(OMe) complex (4) was synthesized and thoroughly characterized, including X-ray structure determination. The reaction of 4 with PhINTs resulted in the formation of a FeIV(OMe) species (5), revealing the presence of two FeIV species with isomer shifts of -0.11 mm/s and = 0.17 mm/s in the Mössbauer spectrum and showed FeIV/FeIII potential at -0.36 V vs Fc+/Fc couple in acetonitrile at -15 °C. The reactivity studies of 5 were investigated and compared with the FeIV(OH) complex (3).

Asymmetric Fe-O2-Ti structures accelerate reduced-layer-FeII "electron" conversion: Facilitating photocatalytic nitrogen fixation

As a green and sustainable method for ammonia production, solar photocatalytic nitrogen fixation (PNRR) provides a new approach to slowing down the consumption of non-renewable energy resources. Given the extremely huge energy required to activate inert nitrogen, a rational design of efficient nitrogen fixation catalytic materials is essential. This study constructs defective Ti3+-Ti3C2Ox to regulate the NH2-MIL-101(Fe) reduced layer-FeII 'electron' transition; meanwhile, the heterojunction interface electronic structure formed by coupling promotes catalytic charges' transfer/separation, while the interface-asymmetric Fe-O2-Ti structure accelerates the response with nitrogen. It is shown that the heterojunction NM-101(FeII/FeIII)-1.5 exhibits a 75.1 % FeII enrichment (FeII:FeIII), which successfully impedes the fouling relationship between the two (FeII/FeIII). Mössbauer spectroscopy analysis demonstrates that the presence of D1-high spin state FeIII and D2-low/medium spin state FeII structures in the heterojunction boosts the PNRR activity. Furthermore, it is found that the defective state Ti3+-Ti3C2Ox modulation enhances the reduced nitrogen fixation capacity of the heterojunction (CB = -0.84 eV) and decreases the interfacial charge transfer resistance, yielding 450 umol·g-1·h-1 ammonia. Furthermore, this study modulates the charge ration of the catalyst reduction layer by constructing a charge-asymmetric structure with Ti3+-deficient carriers; this method provides a potential opportunity for enhancing photocatalytic nitrogen fixation in the future.

Hexakis(urea-O)iron Complex Salts as a Versatile Material Family: Overview of Their Properties and Applications

Due to their Fe- and N-containing reactive urea ligand content, the hexakis(urea-O)iron(II) and hexakis(urea-O)iron(III) complexes were found to be versatile materials in various application fields of industry and environmental protection. In our present work, we have comprehensively reviewed the synthesis, structural and spectroscopic details, and thermal properties of hexakis(urea-O)iron(II) and hexakis(urea-O)iron(III) salts with different anions (NO3-, Cl-, Br- I-, I3-, ClO4-, MnO4-, SO42-, Cr2O72-, and S2O82-). We compared and evaluated the structural, spectroscopic (IR, Raman, UV-vis, Mössbauer, EPR, and X-ray), and thermogravimetric data. Based on the thermal behavior of these complexes, we evaluated the solid-phase quasi-intramolecular redox reactions of anions and urea ligands in these complexes and summarized the available information on the properties of the resulting simple and mixed iron-containing oxides. Furthermore, we give a complete overview of the application of these complexes as catalysts, reagents, absorbers, or agricultural raw materials.

A Double-ligand Chelating Strategy to Iron Complex Anolytes with Ultrahigh Cyclability for Aqueous Iron Flow Batteries

Aqueous all-iron flow batteries (AIFBs) are attractive for large-scale and long-term energy storage due to their extremely low cost and safety features. To accelerate commercial application, a long cyclable and reversible iron anolyte is expected to address the critical barriers, namely iron dendrite growth and hydrogen evolution reaction (HER). Herein, we report a robust iron complex with triethanolamine (TEA) and 2-methylimidazole (MM) double ligands. By introducing two ligands into one iron center, the binding energy of the complex increases, making it more stable in the charge-discharge reactions. The Fe(TEA)MM complex achieves reversible and stable redox between Fe3+ and Fe2+ , without metallic iron growth and HER. AIFBs based on this anolyte perform a high energy efficiency of 80.5 % at 80 mA cm-2 and exhibit a record durability among reported AIFBs. The efficiency and capacity retain nearly 100 % after 1,400 cycles. The capital cost of this AIFB is $ 33.2 kWh-1 (e.g., 20 h duration), cheaper than Li-ion battery and vanadium flow battery. This double-ligand chelating strategy not only solves the current problems faced by AIFBs, but also provides an insight for further improving the cycling stability of other flow batteries.

Iron Kβ X-ray Emission Spectroscopy: The Origin of Spectral Features from Atomic to Molecular Systems Using Multi-configurational Calculations

Kβ X-ray emission spectroscopy (XES) is widely used to fingerprint the local spin of transition-metal ions, including in pump-probe experiments, to identify excited states or in chemical and biological reactions to characterize short-lived intermediates. In this study, the spectra of ferrous and ferric complexes for various spin states were measured experimentally and described theoretically through restricted active space (RAS) calculations including dynamic correlations. Through the RAS calculations from simple atomic models to complex molecular systems, spectral effects such as the exchange interactions, crystal-field strength, and covalent orbital mixing were evaluated and discussed. The calculations find that only the spectral features of low-spin cases show a dependence on the crystal-field strength, particularly for ferrous low spin. The effect of the covalent orbital mixing strength on the first moment of the Kβ1,3 main line and the Kβ1,3-Kβ' energy splitting is quantitatively described. Clear relationships are found within a given nominal spin but less between different spin states, which calls for careful selection of reference spectra in future experiments. This study further advances our understanding of the correlation between changes in experimental spectral features and their corresponding electronic structure information.

Revealing the atomic and electronic mechanism of human manganese superoxide dismutase product inhibition

Human manganese superoxide dismutase (MnSOD) is a crucial oxidoreductase that maintains the vitality of mitochondria by converting O 2 ∙ - to O 2 and H 2 O 2 with proton-coupled electron transfers (PCETs). Since changes in mitochondrial H 2 O 2 concentrations are capable of stimulating apoptotic signaling pathways, human MnSOD has evolutionarily gained the ability to be highly inhibited by its own product, H 2 O 2 . A separate set of PCETs is thought to regulate product inhibition, though mechanisms of PCETs are typically unknown due to difficulties in detecting the protonation states of specific residues that coincide with the electronic state of the redox center. To shed light on the underlying mechanism, we combined neutron diffraction and X-ray absorption spectroscopy of the product-bound, trivalent, and divalent states to reveal the all-atom structures and electronic configuration of the metal. The data identifies the product-inhibited complex for the first time and a PCET mechanism of inhibition is constructed.

Highly loaded bimetallic iron-cobalt catalysts for hydrogen release from ammonia

Ammonia is a storage molecule for hydrogen, which can be released by catalytic decomposition. Inexpensive iron catalysts suffer from a low activity due to a too strong iron-nitrogen binding energy compared to more active metals such as ruthenium. Here, we show that this limitation can be overcome by combining iron with cobalt resulting in a Fe-Co bimetallic catalyst. Theoretical calculations confirm a lower metal-nitrogen binding energy for the bimetallic catalyst resulting in higher activity. Operando spectroscopy reveals that the role of cobalt in the bimetallic catalyst is to suppress the bulk-nitridation of iron and to stabilize this active state. Such catalysts are obtained from Mg(Fe,Co)2O4 spinel pre-catalysts with variable Fe:Co ratios by facile co-precipitation, calcination and reduction. The resulting Fe-Co/MgO catalysts, characterized by an extraordinary high metal loading reaching 74 wt.%, combine the advantages of a ruthenium-like electronic structure with a bulk catalyst-like microstructure typical for base metal catalysts.

A natural biogenic nanozyme for scavenging superoxide radicals

Biominerals, the inorganic minerals of organisms, are known mainly for their physical property-related functions in modern living organisms. Our recent discovery of the enzyme-like activities of nanomaterials, coined as nanozyme, inspires the hypothesis that nano-biominerals might function as enzyme-like catalyzers in cells. Here we report that the iron cores of biogenic ferritins act as natural nanozymes to scavenge superoxide radicals. Through analyzing eighteen representative ferritins from three living kingdoms, we find that the iron core of prokaryote ferritin possesses higher superoxide-diminishing activity than that of eukaryotes. Further investigation reveals that the differences in catalytic capability result from the iron/phosphate ratio changes in the iron core, which is mainly determined by the structures of ferritins. The phosphate in the iron core switches the iron core from single crystalline to amorphous iron phosphate-like structure, resulting in decreased affinity to the hydrogen proton of the ferrihydrite-like core that facilitates its reaction with superoxide in a manner different from that of ferric ions. Furthermore, overexpression of ferritins with high superoxide-diminishing activities in E. coli increases the resistance to superoxide, whereas bacterioferritin knockout or human ferritin knock-in diminishes free radical tolerance, highlighting the physiological antioxidant role of this type of nanozymes.

Heterologous expression of a fully active Azotobacter vinelandii nitrogenase Fe protein in Escherichia coli

IMPORTANCE

The heterologous expression of a fully active Azotobacter vinelandii Fe protein (AvNifH) has never been accomplished. Given the functional importance of this protein in nitrogenase catalysis and assembly, the successful expression of AvNifH in Escherichia coli as reported herein supplies a key element for the further development of heterologous expression systems that explore the catalytic versatility of the Fe protein, either on its own or as a key component of nitrogenase, for nitrogenase-based biotechnological applications in the future. Moreover, the "clean" genetic background of the heterologous expression host allows for an unambiguous assessment of the effect of certain nif-encoded protein factors, such as AvNifM described in this work, in the maturation of AvNifH, highlighting the utility of this heterologous expression system in further advancing our understanding of the complex biosynthetic mechanism of nitrogenase.

Generation, Spectroscopic Characterization, and Computational Analysis of a Six-Coordinate Cobalt(III)-Imidyl Complex with an Unusual S = 3/2 Ground State that Promotes N-Group and Hydrogen Atom-Transfer Reactions with Exogenous Substrates

We report the synthesis and characterization of a mononuclear nonheme cobalt(III)-imidyl complex, [Co(NTs)(TQA)(OTf)]+ (1), with an S = 3/2 spin state that is capable of facilitating exogenous substrate modifications. Complex 1 was generated from the reaction of CoII(TQA)(OTf)2 with PhINTs at -20 °C. A flow setup with ESI-MS detection was used to explore the kinetics of the formation, stability, and degradation pathway of 1 in solution by treating the Co(II) precursor with PhINTs. Co K-edge XAS data revealed a distinct shift in the Co K-edge compared to the Co(II) precursor, in agreement with the formation of a Co(III) intermediate. The unusual S = 3/2 spin state was proposed based on EPR, DFT, and CASSCF calculations and Co Kβ XES results. Co K-edge XAS and IR photodissociation (IRPD) spectroscopies demonstrate that 1 is a six-coordinate species, and IRPD and resonance Raman spectroscopies are consistent with 1 being exclusively the isomer with the NT ligand occupying the vacant site trans to the TQA aliphatic amine nitrogen atom. Electronic structure calculations (broken symmetry DFT and CASSCF/NEVPT2) demonstrate an S = 3/2 oxidation state resulting from the strong antiferromagnetic coupling of an •NTs spin to the high-spin S = 2 Co(III) center. Reactivity studies of 1 with PPh3 derivatives revealed its electrophilic characteristic in the nitrene-transfer reaction. While the activation of C-H bonds by 1 was proved to be kinetically challenging, 1 could oxidize weak O-H and N-H bonds. Complex 1 is, therefore, a rare example of a Co(III)-imidyl complex capable of exogenous substrate transformations.

Oxidative Grafting for Catalyst Synthesis in Surface Organometallic Chemistry

The development of new methods of catalyst synthesis with the potential to generate active site structures orthogonal to those accessible by traditional protocols is of great importance for discovering new materials for addressing challenges in the evolving energy and chemical economy. In this work, the generality of oxidative grafting of organometallic and well-defined molecular metal precursors onto redox-active surfaces such as manganese dioxide (MnO2) and lithium manganese oxide (LiMn2O4) is investigated. Nine molecular metal precursors are explored, spanning groups 4-11 and each of the three periods of the transition metal series. The byproducts of the oxidative grafting reaction, a mixture of protodemetalation and ligand homocoupling for several organometallic precursors, was found to provide insights into the mechanism of the grafting reaction, suggesting oxidation of both the metal d-orbitals, as well as the metal-carbon σ-bonds, resulting in ejection of the ligand radical fragment. Analysis of the supported structures and oxidation state by X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) suggests that several of the chemisorbed metal ions are intercalated into interstitial vacancies of the surface structure while other complexes form intact molecular fragments on the surface. Proof of concept for the use of this metalation protocol to generate diverse, metal-dependent catalytic performance is demonstrated by the application of these materials in the conversion of cyclohexane to K/A oil (cyclohexanol and cyclohexanone) with tert-butyl hydroperoxide, as well as in the low-temperature (T ≤ 50 °C) oxidation of carbon monoxide to carbon dioxide.

Mössbauer and Nuclear Resonance Vibrational Spectroscopy Studies of Iron Species Involved in N-N Bond Cleavage

Diketiminate-supported iron complexes are capable of cleaving the strong triple bond of N2 to give a tetra-iron complex with two nitrides (Rodriguez et al., Science, 2011, 334, 780-783). The mechanism of this reaction has been difficult to determine, but a transient green species was observed during the reaction that corresponds to a potential intermediate. Here, we describe studies aiming to identify the characteristics of this intermediate, using a range of spectroscopic techniques, including Mössbauer spectroscopy, electronic absorption spectroscopy, Raman spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and nuclear resonance vibrational spectroscopy (NRVS) complemented by density functional theory (DFT) calculations. We successfully elucidated the nature of the starting iron(II) species and the bis(nitride) species in THF solution, and in each case, THF breaks up the multiiron species. Various observations on the green intermediate species indicate that it has one N2 per two Fe atoms, has THF associated with it, and has NRVS features indicative of bridging N2. Computational models with a formally diiron(0)-N2 core are most consistent with the accumulated data, and on this basis, a mechanism for N2 splitting is suggested. This work shows the power of combining NRVS, Mössbauer, NMR, and vibrational spectroscopies with computations for revealing the nature of transient iron species during N2 cleavage.

Mapping the Ultrafast Mechanistic Pathways of Co Photocatalysts in Pure Water through Time-Resolved X-ray Spectroscopy

Nanosecond time-resolved X-ray (tr-XAS) and optical transient absorption spectroscopy (OTA) are applied to study 3 multimolecular photocatalytic systems with [Ru(bpy)3 ]2+ photoabsorber, ascorbic acid electron donor and Co catalysts with methylene (1), hydroxomethylene (2) and methyl (3) amine substituents in pure water. OTA and tr-XAS of 1 and 2 show that the favored catalytic pathway involves reductive quenching of the excited photosensitizer and electron transfer to the catalyst to form a CoII square pyramidal intermediate with a bonded aqua molecule followed by a CoI square planar derivative that decays within ≈8 μs. By contrast, a CoI square pyramidal intermediate with a longer decay lifetime of ≈35 μs is formed from an analogous CoII geometry for 3 in H2 O. These results highlight the protonation of CoI to form the elusive hydride species to be the rate limiting step and show that the catalytic rate can be enhanced through hydrogen containing pendant amines that act as H-H bond formation proton relays.

OSDA-Free Seeded Cu-Containing ZSM-5 Applied for NH3-SCR-DeNOx

A series of ZSM-5 zeolite materials were synthesized from organic structure-directing agent (OSDA)-free seeded systems, including nanosized silicalite-1 (12 wt % water suspension or in powder form) or nanosized ZSM-5 (powder form of ZSM-5 prepared at 100 or 170 °C). The physicochemical characterization revealed aggregated species in the samples based on silicalite-1. Contrarily, the catalysts based on ZSM-5 seeds revealed isolated copper species, and thus, higher NO conversion during the selective catalytic reduction of NOx with NH3 (NH3-SCR-DeNOx) was observed. Furthermore, a comparison of the Cu-containing ZSM-5 catalysts, conventionally prepared in the presence of OSDAs and prepared with an environmentally more benign approach (without OSDAs), revealed their comparable activity in NH3-SCR-DeNOx.

Transformation of 2-Line Ferrihydrite to Goethite at Alkaline pH

The transformation of 2-line ferrihydrite to goethite from supersaturated solutions at alkaline pH ≥ 13.0 was studied using a combination of benchtop and advanced synchrotron techniques such as X-ray diffraction, thermogravimetric analysis, and X-ray absorption spectroscopy. In comparison to the transformation rates at acidic to mildly alkaline environments, the half-life, t1/2, of 2-line ferrihydrite reduces from several months at pH = 2.0, and approximately 15 days at pH = 10.0, to just under 5 h at pH = 14.0. The calculated-first order rate constants of transformation, k, increase exponentially with respect to the pH and follow the progression log10 k = log10 k0 + a·pH3. Simultaneous monitoring of the aqueous Fe(III) concentration via inductively coupled plasma optical emission spectroscopy demonstrates that (i) goethite likely precipitates from solution and (ii) its formation is rate-limited by the comparatively slow redissolution of 2-line ferrihydrite. The analysis presented can be used to estimate the transformation rate of naturally occurring 2-line ferrihydrite in aqueous electrolytes characteristic to mine and radioactive waste tailings as well as the formation of corrosion products in cementitious pore solutions.

In situ X-ray spectroscopies beyond conventional X-ray absorption spectroscopy on deciphering dynamic configuration of electrocatalysts

Realizing viable electrocatalytic processes for energy conversion/storage strongly relies on an atomic-level understanding of dynamic configurations on catalyst-electrolyte interface. X-ray absorption spectroscopy (XAS) has become an indispensable tool to in situ investigate dynamic natures of electrocatalysts but still suffers from limited energy resolution, leading to significant electronic transitions poorly resolved. Herein, we highlight advanced X-ray spectroscopies beyond conventional XAS, with emphasis on their unprecedented capabilities of deciphering key configurations of electrocatalysts. The profound complementarities of X-ray spectroscopies from various aspects are established in a probing energy-dependent "in situ spectroscopy map" for comprehensively understanding the solid-liquid interface. This perspective establishes an indispensable in situ research model for future studies and offers exciting research prospects for scientists and spectroscopists.