Insights into the geometric and electronic structure of transition metal centers from valence-to-core X-ray emission spectroscopy

Valence-to-core X-ray emission spectroscopy of transition metal tetrahalides: mechanisms governing intensities

Valence-to-core (VtC) X-ray emission spectroscopy offers the opportunity to probe the valence electronic structure of a system filtered by selection rules. From this, the nature of its ligands can be inferred. While a preceding 1s ionization creates a core hole, in VtC XES this core hole is filled with electrons from mainly ligand based orbitals. In this work, we investigated the trends in the observed VtC intensities for a series of transition metal halides, which spans the first row transition metals from manganese to copper. Further, with the aid of computational studies, we corroborated these trends and identified the mechanisms and factors that dictate the observed intensity trends. Small amounts of metal p contribution to the ligand orbitals are known to give rise to intensity of a VtC transition. By employing an LCAO (linear combination of atomic orbitals) approach, we were able to assess the amount of metal p contribution to the ligand molecular orbitals, as well as the role of the transition dipole moment and correlate these factors to the experimentally observed intensities. Finally, by employing an ano (atomic natural orbital) basis set within the calculations, the nature of the metal p contribution (3p vs. 4p) was qualitatively assessed and their trends discussed within the same transition metal halide series.

X-ray absorption and emission spectroscopy of N2S2 Cu(II)/(III) complexes

This study investigates the influence of ligand charge on transition energies in a series of CuN2S2 complexes based on dithiocarbazate Schiff base ligands using Cu K-edge X-ray absorption spectroscopy (XAS) and Kβ valence-to-core (VtC) X-ray emission spectroscopy (XES). By comparing the formally Cu(II) complexes [CuII(HL1)] (HL12- = dimethyl pentane-2,4-diylidenebis[carbonodithiohydrazonate]) and [CuII(HL2)] (HL22- = dibenzyl pentane-2,4-diylidenebis[carbonodithiohydrazonate]) and the formally Cu(III) complex [CuIII(L2)], distinct changes in transition energies are observed, primarily attributed to the metal oxidation state. Density functional theory (DFT) calculations demonstrate how an increased negative charge on the deprotonated L23- ligand stabilizes the Cu(III) center through enhanced charge donation, modulating the core transition energies. Overall, significant shifts to higher energies are noted upon metal oxidation, emphasizing the importance of scrutinizing ligand structure in XAS/VtC XES analysis. The data further support the redox-innocent role of the Schiff base ligands and underscore the criticality of ligand protonation levels in future spectroscopic studies, particularly for catalytic intermediates. The combined XAS-VtC XES methodology validates the Cu(III) oxidation state assignment while offering insights into ligand protonation effects on core-level spectroscopic transitions.

PINK: a tender X-ray beamline for X-ray emission spectroscopy

A high-flux beamline optimized for non-resonant X-ray emission spectroscopy (XES) in the tender X-ray energy range has been constructed at the BESSY II synchrotron source. The beamline utilizes a cryogenically cooled undulator that provides X-rays over the energy range 2.1 keV to 9.5 keV. This energy range provides access to XES [and in the future X-ray absorption spectroscopy (XAS)] studies of transition metals ranging from Ti to Cu (Kα, Kβ lines) and Zr to Ag (Lα, Lβ), as well as light elements including P, S, Cl, K and Ca (Kα, Kβ). The beamline can be operated in two modes. In PINK mode, a multilayer monochromator (E/ΔE ≃ 30-80) provides a high photon flux (1014 photons s-1 at 6 keV and 300 mA ring current), allowing non-resonant XES measurements of dilute substances. This mode is currently available for general user operation. X-ray absorption near-edge structure and resonant XAS techniques will be available after the second stage of the PINK commissioning, when a high monochromatic mode (E/ΔE ≃ 10000-40000) will be facilitated by a double-crystal monochromator. At present, the beamline incorporates two von Hamos spectrometers, enabling time-resolved XES experiments with time scales down to 0.1 s and the possibility of two-color XES experiments. This paper describes the optical scheme of the PINK beamline and the endstation. The design of the two von Hamos dispersive spectrometers and sample environment are discussed here in detail. To illustrate, XES spectra of phosphorus complexes, KCl, TiO2 and Co3O4 measured using the PINK setup are presented.

Highly efficient anion exchange membrane water electrolyzers via chromium-doped amorphous electrocatalysts

In-depth comprehension and modulation of the electronic structure of the active metal sites is crucial to enhance their intrinsic activity of electrocatalytic oxygen evolution reaction (OER) toward anion exchange membrane water electrolyzers (AEMWEs). Here, we elaborate a series of amorphous metal oxide catalysts (FeCrOx, CoCrOx and NiCrOx) with high performance AEMWEs by high-valent chromium dopant. We discover that the positive effect of the transition from low to high valence of the Co site on the adsorption energy of the intermediate and the lower oxidation barrier is the key factor for its increased activity by synchrotron radiation in-situ techniques. Particularly, the CoCrOx anode catalyst achieves the high current density of 1.5 A cm-2 at 2.1 V and maintains for over 120 h with attenuation less than 4.9 mV h-1 in AEMWE testing. Such exceptional performance demonstrates a promising prospect for industrial application and providing general guidelines for the design of high-efficiency AEMWEs systems.

Stimulated X-ray emission spectroscopy

We describe an emerging hard X-ray spectroscopy technique, stimulated X-ray emission spectroscopy (S-XES). S-XES has the potential to characterize the electronic structure of 3d transition metal complexes with spectral information currently not reachable and might lead to the development of new ultrafast X-ray sources with properties beyond the state of the art. S-XES has become possible with the emergence of X-ray free-electron lasers (XFELs) that provide intense femtosecond X-ray pulses that can be employed to generate a population inversion of core-hole excited states resulting in stimulated X-ray emission. We describe the instrumentation, the various types of S-XES, the potential applications, the experimental challenges, and the feasibility of applying S-XES to characterize dilute systems, including the Mn4Ca cluster in the oxygen evolving complex of photosystem II.

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.

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.

Unraveling Metal-Ligand Bonding in an HNO-Evolving {FeNO}6 Complex with a Combined X-ray Spectroscopic Approach

Photolytic delivery of nitric oxide and nitroxide has substantial biomedical and phototherapeutic applications. Here, we utilized hard X-ray spectroscopic methods to identify key geometric and electronic structural features of two photolabile {FeNO}6 complexes where the compounds differ in the presence of a pendant thiol in [Fe(NO)(TMSPS2)(TMSPS2H)] and thioether in [Fe(NO)(TMSPS2)(TMSPS2CH3)] with the former complex being the only transition metal system to photolytically generate HNO. Fe Kβ XES identifies the photoreactant systems as essentially Fe(II)-NO+, while valence-to-core XES extracts a NO oxidation state of +0.5. Finally, the pre-edge of the Fe high-energy-resolution fluorescence detected (HERFD) XAS spectra is shown to be acutely sensitive to perturbation of the Fe-NO covalency enhanced by the 3d-4p orbital mixing dipole intensity contribution. Collectively, this X-ray spectroscopic approach enables future time-resolved insights in these systems and extensions to other challenging redox noninnocent {FeNO}x systems.

Full Spectroscopic Characterization of the Molecular Oxygen-Based Methane to Methanol Conversion over Open Fe(II) Sites in a Metal-Organic Framework

Iron-based enzymes efficiently activate molecular oxygen to perform the oxidation of methane to methanol (MTM), a reaction central to the contemporary chemical industry. Conversely, a very limited number of artificial catalysts have been devised to mimic this process. Herein, we employ the MIL-100(Fe) metal-organic framework (MOF), a material that exhibits isolated Fe sites, to accomplish the MTM conversion using O2 as the oxidant under mild conditions. We apply a diverse set of advanced operando X-ray techniques to unveil how MIL-100(Fe) can act as a catalyst for direct MTM conversion. Single-phase crystallinity and stability of the MOF under reaction conditions (200 or 100 °C, CH4 + O2) are confirmed by X-ray diffraction measurements. X-ray absorption, emission, and resonant inelastic scattering measurements show that thermal treatment above 200 °C generates Fe(II) sites that interact with O2 and CH4 to produce methanol. Experimental evidence-driven density functional theory (DFT) calculations illustrate that the MTM reaction involves the oxidation of the Fe(II) sites to Fe(III) via a high-spin Fe(IV)═O intermediate. Catalyst deactivation is proposed to be caused by the escape of CH3• radicals from the relatively large MOF pore cages, ultimately resulting in the formation of hydroxylated triiron units, as proven by valence-to-core X-ray emission spectroscopy. The O2-based MTM catalytic activity of MIL-100(Fe) in the investigated conditions is demonstrated for two consecutive reaction cycles, proving the MOF potential toward active site regeneration. These findings will desirably lay the groundwork for the design of improved MOF catalysts for the MTM conversion.

Resonant Excitation Unlocks Chemical Selectivity of Platinum Lβ Valence-to-Core X-ray Emission Spectra

Valence-to-core X-ray emission spectroscopy (VtC XES) is an emerging technique that uses hard X-rays to probe the valence electronic structure of an absorbing atom. Despite finding varied applications for light elements and first row transition metals, little work has been done on heavier elements such as second and third row transition metals. This lack of application is at least partially due to the relatively low resolution of the data at the high energies required to measure these elements, which obscures the useful chemical information that can be extracted from the lower energy, higher resolution spectra of lighter elements. Herein, we collect data on a set of platinum-containing compounds and demonstrate that the VtC XES resolution can be dramatically enhanced by exciting the platinum atom in resonance with its L3-edge white line absorption. Whereas spectra excited using standard nonresonant absorption well above the Pt L3-edge display broad, unfeatured VtC regions, resonant XES (RXES) spectra have more than twofold improved resolution and are revealed to be rich in chemical information with the ability to distinguish between even closely related species. We further demonstrate that these RXES spectra may be used to selectively probe individual components of a mixture of Pt-containing compounds, establishing this technique as a viable probe for chemically complex samples. Lastly, it is shown that the spectra are interpretable using a molecular orbital framework and may be calculated using density functional theory, thus suggesting resonant excitation as a general strategy for extracting chemically useful information from heavy element VtC spectra.

Cu4S Cluster in "0-Hole" and "1-Hole" States: Geometric and Electronic Structure Variations for the Active CuZ* Site of N2O Reductase

The active site of nitrous oxide reductase (N2OR), a key enzyme in denitrification, features a unique μ4-sulfido-bridged tetranuclear Cu cluster (the so-called CuZ or CuZ* site). Details of the catalytic mechanism have remained under debate and, to date, synthetic model complexes of the CuZ*/CuZ sites are extremely rare due to the difficulty in building the unique {Cu4(μ4-S)} core structure. Herein, we report the synthesis and characterization of [Cu4(μ4-S)]n+ (n = 2, 2; n = 3, 3) clusters, supported by a macrocyclic {py2NHC4} ligand (py = pyridine, NHC = N-heterocyclic carbene), in both their 0-hole (2) and 1-hole (3) states, thus mimicking the two active states of the CuZ* site during enzymatic N2O reduction. Structural and electronic properties of these {Cu4(μ4-S)} clusters are elucidated by employing multiple methods, including X-ray diffraction (XRD), nuclear magnetic resonance (NMR), UV/vis, electron paramagnetic resonance (EPR), Cu/S K-edge X-ray emission spectroscopy (XES), and Cu K-edge X-ray absorption spectroscopy (XAS) in combination with time-dependent density functional theory (TD-DFT) calculations. A significant geometry change of the {Cu4(μ4-S)} core occurs upon oxidation from 2 (τ4(S) = 0.46, seesaw) to 3 (τ4(S) = 0.03, square planar), which has not been observed so far for the biological CuZ(*) site and is unprecedented for known model complexes. The single electron of the 1-hole species 3 is predominantly delocalized over two opposite Cu ions via the central S atom, mediated by a π/π superexchange pathway. Cu K-edge XAS and Cu/S K-edge XES corroborate a mixed Cu/S-based oxidation event in which the lowest unoccupied molecular orbital (LUMO) has a significant S-character. Furthermore, preliminary reactivity studies evidence a nucleophilic character of the central μ4-S in the fully reduced 0-hole state.

Kβ X-ray Emission Spectroscopy of Cu(I)-Lytic Polysaccharide Monooxygenase: Direct Observation of the Frontier Molecular Orbital for H2O2 Activation

Lytic polysaccharide monooxygenases (LPMOs) catalyze the degradation of recalcitrant carbohydrate polysaccharide substrates. These enzymes are characterized by a mononuclear Cu(I) active site with a three-coordinate T-shaped "His-brace" configuration including the N-terminal histidine and its amine group as ligands. This study explicitly investigates the electronic structure of the d10 Cu(I) active site in a LPMO using Kβ X-ray emission spectroscopy (XES). The lack of inversion symmetry in the His-brace site enables the 3d/p mixing required for intensity in the Kβ valence-to-core (VtC) XES spectrum of Cu(I)-LPMO. These Kβ XES data are correlated to density functional theory (DFT) calculations to define the bonding, and in particular, the frontier molecular orbital (FMO) of the Cu(I) site. These experimentally validated DFT calculations are used to evaluate the reaction coordinate for homolytic cleavage of the H2O2 O-O bond and understand the contribution of this FMO to the low barrier of this reaction and how the geometric and electronic structure of the Cu(I)-LPMO site is activated for rapid reactivity with H2O2.

Ferricyanide photo-aquation pathway revealed by combined femtosecond Kβ main line and valence-to-core x-ray emission spectroscopy

Reliably identifying short-lived chemical reaction intermediates is crucial to elucidate reaction mechanisms but becomes particularly challenging when multiple transient species occur simultaneously. Here, we report a femtosecond x-ray emission spectroscopy and scattering study of the aqueous ferricyanide photochemistry, utilizing the combined Fe Kβ main and valence-to-core emission lines. Following UV-excitation, we observe a ligand-to-metal charge transfer excited state that decays within 0.5 ps. On this timescale, we also detect a hitherto unobserved short-lived species that we assign to a ferric penta-coordinate intermediate of the photo-aquation reaction. We provide evidence that bond photolysis occurs from reactive metal-centered excited states that are populated through relaxation of the charge transfer excited state. Beyond illuminating the elusive ferricyanide photochemistry, these results show how current limitations of Kβ main line analysis in assigning ultrafast reaction intermediates can be circumvented by simultaneously using the valence-to-core spectral range.

Oxygen Evolution/Reduction Reaction Catalysts: From In Situ Monitoring and Reaction Mechanisms to Rational Design

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are core steps of various energy conversion and storage systems. However, their sluggish reaction kinetics, i.e., the demanding multielectron transfer processes, still render OER/ORR catalysts less efficient for practical applications. Moreover, the complexity of the catalyst-electrolyte interface makes a comprehensive understanding of the intrinsic OER/ORR mechanisms challenging. Fortunately, recent advances of in situ/operando characterization techniques have facilitated the kinetic monitoring of catalysts under reaction conditions. Here we provide selected highlights of recent in situ/operando mechanistic studies of OER/ORR catalysts with the main emphasis placed on heterogeneous systems (primarily discussing first-row transition metals which operate under basic conditions), followed by a brief outlook on molecular catalysts. Key sections in this review are focused on determination of the true active species, identification of the active sites, and monitoring of the reactive intermediates. For in-depth insights into the above factors, a short overview of the metrics for accurate characterizations of OER/ORR catalysts is provided. A combination of the obtained time-resolved reaction information and reliable activity data will then guide the rational design of new catalysts. Strategies such as optimizing the restructuring process as well as overcoming the adsorption-energy scaling relations will be discussed. Finally, pending current challenges and prospects toward the understanding and development of efficient heterogeneous catalysts and selected homogeneous catalysts are presented.

A compact von Hámos spectrometer for parallel X-ray Raman scattering and X-ray emission spectroscopy at ID20 of the European Synchrotron Radiation Facility

A compact spectrometer for medium-resolution resonant and non-resonant X-ray emission spectroscopy in von Hámos geometry is described. The main motivation for the design and construction of the spectrometer is to allow for acquisition of non-resonant X-ray emission spectra while measuring non-resonant X-ray Raman scattering spectra at beamline ID20 of the European Synchrotron Radiation Facility. Technical details are provided and the performance and possible use of the spectrometer are demonstrated by presenting results of several X-ray spectroscopic methods on various compounds.

An Fe6 C Core in All Nitrogenase Cofactors

The biological process of dinitrogen reduction to ammonium occurs at the cofactors of nitrogenases, the only enzymes that catalyze this challenging chemical reaction. Three types of nitrogenases have been described, named according to the heterometal in their cofactor: molybdenum, vanadium or iron nitrogenases. Spectroscopic and structural characterization allowed the unambiguous identification of the cofactors of molybdenum and vanadium nitrogenases and revealed a central μ₆ -carbide in both of them. Although genetic studies suggested that the cofactor of the iron nitrogenase contains a similar Fe₆ C core, this has not been experimentally demonstrated. Here we report Valence-to-Core X-ray Emission Spectroscopy providing experimental evidence that this cofactor contains a carbide, thereby making the Fe₆ C core a feature of all nitrogenase cofactors.

Informed Chemical Classification of Organophosphorus Compounds via Unsupervised Machine Learning of X-ray Absorption Spectroscopy and X-ray Emission Spectroscopy

We analyze an ensemble of organophosphorus compounds to form an unbiased characterization of the information encoded in their X-ray absorption near-edge structure (XANES) and valence-to-core X-ray emission spectra (VtC-XES). Data-driven emergence of chemical classes via unsupervised machine learning, specifically cluster analysis in the Uniform Manifold Approximation and Projection (UMAP) embedding, finds spectral sensitivity to coordination, oxidation, aromaticity, intramolecular hydrogen bonding, and ligand identity. Subsequently, we implement supervised machine learning via Gaussian process classifiers to identify confidence in predictions that match our initial qualitative assessments of clustering. The results further support the benefit of utilizing unsupervised machine learning as a precursor to supervised machine learning, which we term Unsupervised Validation of Classes (UVC), a result that goes beyond the present case of X-ray spectroscopies.

The Inner Shell Spectroscopy beamline at NSLS-II: a facility for in situ and operando X-ray absorption spectroscopy for materials research

The Inner Shell Spectroscopy (ISS) beamline on the 8-ID station at the National Synchrotron Light Source II (NSLS-II), Upton, NY, USA, is a high-throughput X-ray absorption spectroscopy beamline designed for in situ, operando, and time-resolved material characterization using high monochromatic flux and scanning speed. This contribution discusses the technical specifications of the beamline in terms of optics, heat load management, monochromator motion control, and data acquisition and processing. Results of the beamline tests demonstrating the quality of the data obtainable on the instrument, possible energy scanning speeds, as well as long-term beamline stability are shown. The ability to directly control the monochromator trajectory to define the acquisition time for each spectral region is highlighted. Examples of studies performed on the beamline are presented. The paper is concluded with a brief outlook for future developments.

Relativistic Orbital-Optimized Density Functional Theory for Accurate Core-Level Spectroscopy

Core-level spectra of 1s electrons of elements heavier than Ne show significant relativistic effects. We combine advances in orbital-optimized density functional theory (OO-DFT) with the spin-free exact two-component (X2C) model for scalar relativistic effects to study K-edge spectra of third period elements. OO-DFT/X2C is found to be quite accurate at predicting energies, yielding a ∼0.5 eV root-mean-square error versus experiment with the modern SCAN (and related) functionals. This marks a significant improvement over the >50 eV deviations that are typical for the popular time-dependent DFT (TDDFT) approach. Consequently, experimental spectra are quite well reproduced by OO-DFT/X2C, sans empirical shifts for alignment. OO-DFT/X2C combines high accuracy with ground state DFT cost and is thus a promising route for computing core-level spectra of third period elements. We also explored K and L edges of 3d transition metals to identify limitations of the OO-DFT/X2C approach in modeling the spectra of heavier atoms.

Combining Valence-to-Core X-ray Emission and Cu K-edge X-ray Absorption Spectroscopies to Experimentally Assess Oxidation State in Organometallic Cu(I)/(II)/(III) Complexes

A series of organometallic copper complexes in formal oxidation states ranging from +1 to +3 have been characterized by a combination of Cu K-edge X-ray absorption (XAS) and Cu Kβ valence-to-core X-ray emission spectroscopies (VtC XES). Each formal oxidation state exhibits distinctly different XAS and VtC XES transition energies due to the differences in the Cu Z_eff, concomitant with changes in physical oxidation state from +1 to +2 to +3. Herein, we demonstrate the sensitivity of XAS and VtC XES to the physical oxidation states of a series of N-heterocyclic carbene (NHC) ligated organocopper complexes. We then extend these methods to the study of the [Cu(CF₃)₄]⁻ ion. Complemented by computational methods, the observed spectral transitions are correlated with the electronic structure of the complexes and the Cu Z_eff. These calculations demonstrate that a contraction of the Cu 1s orbitals to deeper binding energy upon oxidation of the Cu center manifests spectroscopically as a stepped increase in the energy of both XAS and Kβ_2,5 emission features with increasing formal oxidation state within the [Cun+(NHC₂)]ⁿ⁺ series. The newly synthesized Cu(III) cation [Cu^III(NHC₄)]³⁺ exhibits spectroscopic features and an electronic structure remarkably similar to [Cu(CF₃)₄]⁻, supporting a physical oxidation state assignment of low-spin d⁸ Cu(III) for [Cu(CF₃)₄]⁻. Combining XAS and VtC XES further demonstrates the necessity of combining multiple spectroscopies when investigating the electronic structures of highly covalent copper complexes, providing a template for future investigations into both synthetic and biological metal centers.

Structural Elucidation, Aggregation, and Dynamic Behaviour of N,N,N,N ‐Copper(I) Schiff Base Complexes in Solid and in Solution: A Combined NMR, X‐ray Spectroscopic and Crystallographic Investigation

A series of Cu(I) complexes of bidentate or tetradentate Schiff base ligands bearing either 1‐H‐imidazole or pyridine moieties were synthesized. The complexes were studied by a combination of NMR and X‐ray spectroscopic techniques. The differences between the imidazole‐ and pyridine‐based ligands were examined by ¹H, ¹³C and ¹⁵N NMR spectroscopy. The magnitude of the ¹⁵N_imine coordination shifts was found to be strongly affected by the nature of the heterocycle in the complexes. These trends showed good correlation with the obtained Cu−N_imine bond lengths from single‐crystal X‐ray diffraction measurements. Variable‐temperature NMR experiments, in combination with diffusion ordered spectroscopy (DOSY) revealed that one of the complexes underwent a temperature‐dependent interconversion between a monomer, a dimer and a higher aggregate. The complexes bearing tetradentate imidazole ligands were further studied using Cu K‐edge XAS and VtC XES, where DFT‐assisted assignment of spectral features suggested that these complexes may form polynuclear oligomers in solid state. Additionally, the Cu(II) analogue of one of the complexes was incorporated into a metal‐organic framework (MOF) as a way to obtain discrete, mononuclear complexes in the solid state.

Unsupervised machine learning for unbiased chemical classification in X-ray absorption spectroscopy and X-ray emission spectroscopy

We report a comprehensive computational study of unsupervised machine learning for extraction of chemically relevant information in X-ray absorption near edge structure (XANES) and in valence-to-core X-ray emission spectra (VtC-XES) for classification of a broad ensemble of sulphorganic molecules. By progressively decreasing the constraining assumptions of the unsupervised machine learning algorithm, moving from principal component analysis (PCA) to a variational autoencoder (VAE) to t-distributed stochastic neighbour embedding (t-SNE), we find improved sensitivity to steadily more refined chemical information. Surprisingly, when embedding the ensemble of spectra in merely two dimensions, t-SNE distinguishes not just oxidation state and general sulphur bonding environment but also the aromaticity of the bonding radical group with 87% accuracy as well as identifying even finer details in electronic structure within aromatic or aliphatic sub-classes. We find that the chemical information in XANES and VtC-XES is very similar in character and content, although they unexpectedly have different sensitivity within a given molecular class. We also discuss likely benefits from further effort with unsupervised machine learning and from the interplay between supervised and unsupervised machine learning for X-ray spectroscopies. Our overall results, i.e., the ability to reliably classify without user bias and to discover unexpected chemical signatures for XANES and VtC-XES, likely generalize to other systems as well as to other one-dimensional chemical spectroscopies.

Photon-in/photon-out spectroscopy at the I20-scanning beamline at diamond light source

A scanning multi-crystal x-ray emission spectrometer to perform photon-in/photon-out spectroscopy at the I20-Scanning beamline at Diamond Light Source is described. The instrument, equipped with three analyzer crystals, is based on a 1 m Rowland circle spectrometer operating in the vertical plane. The energy resolution of the spectrometer is of the order of 1 eV, having sufficient resolving power to overcome the core-hole lifetime broadening of most of the transition metalsK-edges. Examples showing the capability of the beamline for performing high energy resolution fluorescence detection x-ray absorption spectroscopy (HERFD-XAS), non-resonant x-ray emission spectroscopy (XES) and resonant x-ray emission spectroscopy are presented. The comparison of the Zn and MnK-edge HERFD-XANES of ZnO and MnO withab initiocalculations shows that the technique provides enhanced validation of the models by making subtle spectral features more visible.

Capturing photochemical and photophysical transformations in iron complexes with ultrafast X-ray spectroscopy and scattering

Light-driven chemical transformations provide a compelling approach to understanding chemical reactivity with the potential to use this understanding to advance solar energy and catalysis applications. Capturing the non-equilibrium trajectories of electronic excited states with precision, particularly for transition metal complexes, would provide a foundation for advancing both of these objectives. Of particular importance for 3d metal compounds is characterizing the population dynamics of charge-transfer (CT) and metal-centered (MC) electronic excited states and understanding how the inner coordination sphere structural dynamics mediate the interaction between these states. Recent advances in ultrafast X-ray laser science has enabled the electronic excited state dynamics in 3d metal complexes to be followed with unprecedented detail. This review will focus on simultaneous X-ray emission spectroscopy (XES) and X-ray solution scattering (XSS) studies of iron coordination and organometallic complexes. These simultaneous XES-XSS studies have provided detailed insight into the mechanism of light-induced spin crossover in iron coordination compounds, the interaction of CT and MC excited states in iron carbene photosensitizers, and the mechanism of Fe-S bond dissociation in cytochrome c.

Using X-ray free-electron lasers for spectroscopy of molecular catalysts and metalloenzymes

The metal centres in metalloenzymes and molecular catalysts are responsible for the rearrangement of atoms and electrons during complex chemical reactions, and they enable selective pathways of charge and spin transfer, bond breaking/making and the formation of new molecules. Mapping the electronic structural changes at the metal sites during the reactions gives a unique mechanistic insight that has been difficult to obtain to date. The development of X-ray free-electron lasers (XFELs) enables powerful new probes of electronic structure dynamics to advance our understanding of metalloenzymes. The ultrashort, intense and tunable XFEL pulses enable X-ray spectroscopic studies of metalloenzymes, molecular catalysts and chemical reactions, under functional conditions and in real time. In this Technical Review, we describe the current state of the art of X-ray spectroscopy studies at XFELs and highlight some new techniques currently under development. With more XFEL facilities starting operation and more in the planning or construction phase, new capabilities are expected, including high repetition rate, better XFEL pulse control and advanced instrumentation. For the first time, it will be possible to make real-time molecular movies of metalloenzymes and catalysts in solution, while chemical reactions are taking place.

Revealing the bonding of solvated Ru complexes with valence-to-core resonant inelastic X-ray scattering

Ru-complexes are widely studied because of their use in biological applications and photoconversion technologies. We reveal novel insights into the chemical bonding of a series of Ru(ii)- and Ru(iii)-complexes by leveraging recent advances in high-energy-resolution tender X-ray spectroscopy and theoretical calculations. We perform Ru 2p4d resonant inelastic X-ray scattering (RIXS) to probe the valence excitations in dilute solvated Ru-complexes. Combining these experiments with a newly developed theoretical approach based on time-dependent density functional theory, we assign the spectral features and quantify the metal-ligand bonding interactions. The valence-to-core RIXS features uniquely identify the metal-centered and charge transfer states and allow extracting the ligand-field splitting for all the complexes. The combined experimental and theoretical approach described here is shown to reliably characterize the ground and excited valence states of Ru complexes, and serve as a basis for future investigations of ruthenium, or other 4d metals active sites, in biological and chemical applications.

Quantification of Ni-N-O Bond Angles and NO Activation by X-ray Emission Spectroscopy

A series of β-diketiminate Ni-NO complexes with a range of NO binding modes and oxidation states were studied by X-ray emission spectroscopy (XES). The results demonstrate that XES can directly probe and distinguish end-on vs side-on NO coordination modes as well as one-electron NO reduction. Density functional theory (DFT) calculations show that the transition from the NO 2s2s σ* orbital has higher intensity for end-on NO coordination than for side-on NO coordination, whereas the 2s2s σ orbital has lower intensity. XES calculations in which the Ni-N-O bond angle was fixed over the range from 80° to 176° suggest that differences in NO coordination angles of ∼10° could be experimentally distinguished. Calculations of Cu nitrite reductase (NiR) demonstrate the utility of XES for characterizing NO intermediates in metalloenzymes. This work shows the capability of XES to distinguish NO coordination modes and oxidation states at Ni and highlights applications in quantifying small molecule activation in enzymes.

Kβ X-ray Emission Spectroscopy as a Probe of Cu(I) Sites: Application to the Cu(I) Site in Preprocessed Galactose Oxidase

Cu(I) active sites in metalloproteins are involved in O₂ activation, but their O₂ reactivity is difficult to study due to the Cu(I) d¹⁰ closed shell which precludes the use of conventional spectroscopic methods. Kβ X-ray emission spectroscopy (XES) is a promising technique for investigating Cu(I) sites as it detects photons emitted by electronic transitions from occupied orbitals. Here, we demonstrate the utility of Kβ XES in probing Cu(I) sites in model complexes and a metalloprotein. Using Cu(I)Cl, emission features from double-ionization (DI) states are identified using varying incident X-ray photon energies, and a reasonable method to correct the data to remove DI contributions is presented. Kβ XES spectra of Cu(I) model complexes, having biologically relevant N/S ligands and different coordination numbers, are compared and analyzed, with the aid of density functional theory (DFT) calculations, to evaluate the sensitivity of the spectral features to the ligand environment. While the low-energy Kβ_2,5 emission feature reflects the ionization energy of ligand np valence orbitals, the high-energy Kβ_2,5 emission feature corresponds to transitions from molecular orbitals (MOs) having mainly Cu 3d character with the intensities determined by ligand-mediated d-p mixing. A Kβ XES spectrum of the Cu(I) site in preprocessed galactose oxidase (GO_pre) supports the 1Tyr/2His structural model that was determined by our previous X-ray absorption spectroscopy and DFT study. The high-energy Kβ_2,5 emission feature in the Cu(I)-GO_pre data has information about the MO containing mostly Cu 3d_x²-y² character that is the frontier molecular orbital (FMO) for O₂ activation, which shows the potential of Kβ XES in probing the Cu(I) FMO associated with small-molecule activation in metalloproteins.

Probing a Silent Metal: A Combined X-ray Absorption and Emission Spectroscopic Study of Biologically Relevant Zinc Complexes

As the second most common transition metal in the human body, zinc is of great interest to research but has few viable routes for its direct structural study in biological systems. Herein, Zn valence-to-core X-ray emission spectroscopy (VtC XES) and Zn K-edge X-ray absorption spectroscopy (XAS) are presented as a means to understand the local structure of zinc in biological systems through the application of these methods to a series of biologically relevant molecular model complexes. Taken together, the Zn K-edge XAS and VtC XES provide a means to establish the ligand identity, local geometry, and metal-ligand bond lengths. Experimental results are supported by correlation with density-functional-theory-based calculations. Combining these theoretical and experimental approaches will enable future applications to protein systems in a predictive manner.

Chemical Sensitivity of Kβ and Kα X-ray Emission from a Systematic Investigation of Iron Compounds

K-fluorescence X-ray emission spectroscopy (XES) is receiving growing interest in all fields of natural sciences to investigate the local spin. The spin sensitivity in Kβ (Kα) XES stems from the exchange interaction between the unpaired 3p (2p) and the 3d electrons, which is greater for Kβ than for Kα. We present a thorough investigation of a large number of iron-bearing compounds. The experimental spectra were analyzed in terms of commonly used quantitative parameters (Kβ_1,3-first moment, Kα₁-full width at half-maximum, and integrated absolute difference -IAD-), and we carefully examined the difference spectra. Multiplet calculations were also performed to elucidate the underlying mechanisms that lead to the chemical sensitivity. Our results confirm a strong influence of covalency on both Kβ and Kα lines. We establish a reliable spin sensitivity of Kβ XES as it is dominated by the exchange interaction, whose variations can be quantified by either Kβ_1,3-first moment or Kβ-IAD and result in a systematic difference signal line shape. We find an exception in the Kβ XES of Fe³⁺ and Fe²⁺ in water solution, where a new difference spectrum is identified that cannot be reproduced by scaling the exchange integrals. We explain this by strong differences in orbital mixing between the valence orbitals. This result calls for caution in the interpretation of Kβ XES spectral changes as due to spin variations without a careful analysis of the line shape. For Kα XES, the smaller exchange interaction and the influence of other electron-electron interactions make it difficult to extract a quantity that directly relates to the spin.

New reflections on hard X-ray photon-in/photon-out spectroscopy

Analysis of the electronic structure and local coordination of an element is an important aspect in the study of the chemical and physical properties of materials. This is particularly relevant at the nanoscale where new phases of matter may emerge below a critical size. X-ray emission spectroscopy (XES) at synchrotron radiation sources and free electron lasers has enriched the field of X-ray spectroscopy. The spectroscopic techniques derived from the combination of X-ray absorption and emission spectroscopy (XAS-XES), such as resonant inelastic X-ray scattering (RIXS) and high energy resolution fluorescence detected (HERFD) XAS, are an ideal tool for the study of nanomaterials. New installations and beamline upgrades now often include wavelength dispersive instruments for the analysis of the emitted X-rays. With the growing use of XAS-XES, scientists are learning about the possibilities and pitfalls. We discuss some experimental aspects, assess the feasibility of measuring weak fluorescence lines in dilute, radiation sensitive samples, and present new experimental approaches for studying magnetic properties of colloidal nanoparticles directly in the liquid phase.

In situ X-ray emission and high-resolution X-ray absorption spectroscopy applied to Ni-based bimetallic dry methane reforming catalysts

The promoting effect of cobalt on the catalytic activity of a NiCoO Dry Methane Reforming (DMR) catalyst was studied by a combination of in situ Kβ X-ray Emission Spectroscopy (XES) and Kβ-detected High Energy Resolution Fluorescence Detected X-ray absorption spectroscopy (HERFD XAS). Following the calcination process, Ni XES and Kβ-detected HERFD XAS data revealed that the NiO coordination in the NiCoO catalyst has a higher degree of symmetry and is different than that of pure NiO/γ-Al₂O₃. Following the reductive activation, it was found that the NiCoO/γ-Al₂O₃ catalyst required a relatively higher temperature compared to the monometallic NiO/γ-Al₂O₃ catalyst. This finding suggests that Co is hampering the reduction of Ni in the NiCoO catalyst by modulation of its electronic structure. It has also been previously shown that the addition of Co enhances the DMR activity. Further, the Kβ XES spectrum of the partly reduced catalysts at 450 °C reveals that the Ni sites in the NiCoO catalyst are electronically different from the NiO catalyst. The in situ X-ray spectroscopic study demonstrates that reduced metallic Co and Ni are the primary species present after reduction and are preserved under DMR conditions. However, the NiCo catalyst appears to always be somewhat more oxidized than the Ni-only species, suggesting that the presence of cobalt modulates the Ni electronic structure. The electronic structural modulations resulting from the presence of Co may be the key to the increased activity of the NiCo catalyst relative to the Ni-only catalyst. This study emphasizes the potential of in situ X-ray spectroscopy experiments for probing the electronic structure of catalytic materials during activation and under operating conditions.

Kβ X-Ray Emission Spectroscopic Study of a Second-Row Transition Metal (Mo) and Its Application to Nitrogenase-Related Model Complexes

In recent years, X-ray emission spectroscopy (XES) in the Kβ (3p-1s) and valence-to-core (valence-1s) regions has been increasingly used to study metal active sites in (bio)inorganic chemistry and catalysis, providing information about the metal spin state, oxidation state and the identity of coordinated ligands. However, to date this technique has been limited almost exclusively to first-row transition metals. In this work, we present an extension of Kβ XES (in both the 4p-1s and valence-to-1s [or VtC] regions) to the second transition row by performing a detailed experimental and theoretical analysis of the molybdenum emission lines. It is demonstrated in this work that Kβ2 lines are dominated by spin state effects, while VtC XES of a 4d transition metal provides access to metal oxidation state and ligand identity. An extension of Mo Kβ XES to nitrogenase-relevant model complexes shows that the method is sufficiently sensitive to act as a spectator probe for redox events that are localized at the Fe atoms. Mo VtC XES thus has promise for future applications to nitrogenase, as well as a range of other Mo-containing biological cofactors. Further, the clear assignment of the origins of Mo VtC XES features opens up the possibility of applying this method to a wide range of second-row transition metals, thus providing chemists with a site-specific tool for the elucidation of 4d transition metal electronic structure.

The Spectroscopy of Nitrogenases

Nitrogenases are responsible for biological nitrogen fixation, a crucial step in the biogeochemical nitrogen cycle. These enzymes utilize a two-component protein system and a series of iron-sulfur clusters to perform this reaction, culminating at the FeMco active site (M = Mo, V, Fe), which is capable of binding and reducing N2 to 2NH3. In this review, we summarize how different spectroscopic approaches have shed light on various aspects of these enzymes, including their structure, mechanism, alternative reactivity, and maturation. Synthetic model chemistry and theory have also played significant roles in developing our present understanding of these systems and are discussed in the context of their contributions to interpreting the nature of nitrogenases. Despite years of significant progress, there is still much to be learned from these enzymes through spectroscopic means, and we highlight where further spectroscopic investigations are needed.

Probing Sulfur Chemical and Electronic Structure with Experimental Observation and Quantitative Theoretical Prediction of Kα and Valence-to-Core Kβ X-ray Emission Spectroscopy

An extensive experimental and theoretical study of the Kα and Kβ high-resolution X-ray emission spectroscopy (XES) of sulfur-bearing systems is presented. This study encompasses a wide range of organic and inorganic compounds, including numerous experimental spectra from both prior published work and new measurements. Employing a linear-response time-dependent density functional theory (LR-TDDFT) approach, strong quantitative agreement is found in the calculation of energy shifts of the core-to-core Kα as well as the full range of spectral features in the valence-to-core Kβ spectrum. The ability to accurately calculate the sulfur Kα energy shift supports the use of sulfur Kα XES as a bulk-sensitive tool for assessing sulfur speciation. The fine structure of the sulfur Kβ spectrum, in conjunction with the theoretical results, is shown to be sensitive to the local electronic structure including effects of symmetry, ligand type and number, and, in the case of organosulfur compounds, to the nature of the bonded organic moiety. This agreement between theory and experiment, augmented by the potential for high-access XES measurements with the latest generation of laboratory-based spectrometers, demonstrates the possibility of broad analytical use of XES for sulfur and nearby third-row elements. The effective solution of the forward problem, i.e., successful prediction of detailed spectra from known molecular structure, also suggests future use of supervised machine learning approaches to experimental inference, as has seen recent interest for interpretation of X-ray absorption near-edge structure (XANES).

Ruthenium 4d-to-2p X-ray Emission Spectroscopy: A Simultaneous Probe of the Metal and the Bound Ligands

Ruthenium 4d-to-2p X-ray emission spectroscopy (XES) was systematically explored for a series of Ru²⁺ and Ru³⁺ species. Complementary density functional theory calculations were utilized to allow for a detailed assignment of the experimental spectra. The studied complexes have a range of different coordination spheres, which allows the influence of the ligand donor/acceptor properties on the spectra to be assessed. Similarly, the contributions of the site symmetry and the oxidation state of the metal were analyzed. Because the 4d-to-2p emission lines are dipole-allowed, the spectral features are intense. Furthermore, in contrast with K- or L-edge X-ray absorption of 4d transition metals, which probe the unoccupied levels, the observed 4p-to-2p XES arises from electrons in filled-ligand- and filled-metal-based orbitals, thus providing simultaneous access to the ligand and metal contributions to bonding. As such, 4d-to-2p XES should be a promising tool for the study of a wide range of 4d transition-metal compounds.

Ruthenium 4d-to-2p XES was applied to a series of molecular Ru complexes with varied coordination environment, oxidation state and site symmetry. Through correlations to calculations, it is demonstrated the Ru 4d-to-2p XES provides a unique probe of both the filled ligand np and filled metal 4d orbitals, providing a promising new tool for the study of a wide range of 4d transition metals.

Electronic Structure of the Hieber Anion [Fe(CO)3(NO)]- Revisited by X-ray Emission and Absorption Spectroscopy

While the Hieber anion [Fe(CO)₃(NO)]^- has been reincarnated in the last years as an active catalyst in organic synthesis, there is still a debate about the oxidation state of the central Fe atom and the resulting charge of the NO ligand. To shed new light on this question and to understand the Fe-NO interaction in the Hieber anion, it is investigated in comparison to the formal 3d⁸ reference Fe(CO)₅ and the formal 3d¹⁰ reference [Fe(CO)₄]^2- by the combination of valence-to-core X-ray emission spectroscopy (VtC-XES), X-ray absorption near-edge structure spectroscopy (XANES), and high-energy-resolution fluorescence-detected XANES. In order to extract information about the electronic structure, time-dependent density functional theory and ground-state density functional theory calculations are applied. This combination of experimental and computational methods reveals that the electron density at the Fe center of the Hieber resembles that of the isoelectronic [Fe(CO)₄]^2-. These observations challenge recent descriptions of the Hieber anion and reopen the debate about the experimentally and computationally determined Fe oxidation state and charge on the NO ligand.

Excited state charge distribution and bond expansion of ferrous complexes observed with femtosecond valence-to-core x-ray emission spectroscopy

Valence-to-core x-ray emission spectroscopy (VtC XES) combines the sample flexibility and element specificity of hard x-rays with the chemical environment sensitivity of valence spectroscopy. We extend this technique to study geometric and electronic structural changes induced by photoexcitation in the femtosecond time domain via laser-pump, x-ray probe experiments using an x-ray free electron laser. The results of time-resolved VtC XES on a series of ferrous complexes [Fe(CN)_2n(2, 2'-bipyridine)_3-n]^-2n+2, n = 1, 2, 3, are presented. Comparisons of spectra obtained from ground state density functional theory calculations reveal signatures of excited state bond length and oxidation state changes. An oxidation state change associated with a metal-to-ligand charge transfer state with a lifetime of less than 100 fs is observed, as well as bond length changes associated with metal-centered excited states with lifetimes of 13 ps and 250 ps.

Active sites and mechanisms in the direct conversion of methane to methanol using Cu in zeolitic hosts: a critical examination

In this critical review we examine the current state of our knowledge in respect of the nature of the active sites in copper containing zeolites for the selective conversion of methane to methanol.

Calcium Valence-to-Core X-ray Emission Spectroscopy: A Sensitive Probe of Oxo Protonation in Structural Models of the Oxygen-Evolving Complex

Calcium is an abundant, nontoxic metal that finds many roles in synthetic and biological systems including the oxygen-evolving complex (OEC) of photosystem II. Characterization methods for calcium centers, however, are underdeveloped compared to those available for transition metals. Valence-to-core X-ray emission spectroscopy (VtC XES) selectively probes the electronic structure of an element's chemical environment, providing insight that complements the geometric information available from other techniques. Here, the utility of calcium VtC XES is established using an in-house dispersive spectrometer in combination with density functional theory. Spectral trends are rationalized within a molecular orbital framework, and Kβ2,5 transitions, derived from molecular orbitals with primarily ligand p character, are found to be a promising probe of the calcium coordination environment. In particular, it is shown that calcium VtC XES is sensitive to the electronic structure changes that accompany oxo protonation in Mn3CaO4-based molecular mimics of the OEC. Through correlation to calculations, the potential of calcium VtC XES to address unresolved questions regarding the mechanism of biological water oxidation is highlighted.