Sensitivity of X-ray core spectroscopy to changes in metal ligation: a systematic study of low-coordinate, high-spin ferrous complexes

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.

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]).

Electronic Structure of Three-Coordinate FeII and CoII β-Diketiminate Complexes

The β-diketiminate supporting group, [ArNCRCHCRNAr]-, stabilizes low coordination number complexes. Four such complexes, where R = tert-butyl, Ar = 2,6-diisopropylphenyl, are studied: (nacnactBu)ML, where M = FeII, CoII and L = Cl, CH3. These are denoted FeCl, FeCH3, CoCl, and CoCH3 and have been previously reported and structurally characterized. The two FeII complexes (S = 2) have also been previously characterized by Mössbauer spectroscopy, but only indirect assessment of the ligand-field splitting and zero-field splitting (zfs) parameters was available. Here, EPR spectroscopy is used, both conventional field-domain for the CoII complexes (with S = 3/2) and frequency-domain, far-infrared magnetic resonance spectroscopy (FIRMS) for all four complexes. The CoII complexes were also studied by magnetometry. These studies allow accurate determination of the zfs parameters. The two FeII complexes are similar with nearly axial zfs and large magnitude zfs given by D = -37 ± 1 cm-1 for both. The two CoII complexes likewise exhibit large and nearly axial zfs, but surprisingly, CoCl has positive D = +55 cm-1 while CoCH3 has negative D = -49 cm-1. Theoretical methods were used to probe the electronic structures of the four complexes, which explain the experimental spectra and the zfs parameters.

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.

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.

Iron Insertion at the Assembly Site of the ISCU Scaffold Protein Is a Conserved Process Initiating Fe-S Cluster Biosynthesis

Iron-sulfur (Fe-S) clusters are prosthetic groups of proteins biosynthesized on scaffold proteins by highly conserved multi-protein machineries. Biosynthesis of Fe-S clusters into the ISCU scaffold protein is initiated by ferrous iron insertion, followed by sulfur acquisition, via a still elusive mechanism. Notably, whether iron initially binds to the ISCU cysteine-rich assembly site or to a cysteine-less auxiliary site via N/O ligands remains unclear. We show here by SEC, circular dichroism (CD), and Mössbauer spectroscopies that iron binds to the assembly site of the monomeric form of prokaryotic and eukaryotic ISCU proteins via either one or two cysteines, referred to the 1-Cys and 2-Cys forms, respectively. The latter predominated at pH 8.0 and correlated with the Fe-S cluster assembly activity, whereas the former increased at a more acidic pH, together with free iron, suggesting that it constitutes an intermediate of the iron insertion process. Iron not binding to the assembly site was non-specifically bound to the aggregated ISCU, ruling out the existence of a structurally defined auxiliary site in ISCU. Characterization of the 2-Cys form by site-directed mutagenesis, CD, NMR, X-ray absorption, Mössbauer, and electron paramagnetic resonance spectroscopies showed that the iron center is coordinated by four strictly conserved amino acids of the assembly site, Cys35, Asp37, Cys61, and His103, in a tetrahedral geometry. The sulfur receptor Cys104 was at a very close distance and apparently bound to the iron center when His103 was missing, which may enable iron-dependent sulfur acquisition. Altogether, these data provide the structural basis to elucidate the Fe-S cluster assembly process and establish that the initiation of Fe-S cluster biosynthesis by insertion of a ferrous iron in the assembly site of ISCU is a conserved mechanism.

Probing Physical Oxidation State by Resonant X-ray Emission Spectroscopy: Applications to Iron Model Complexes and Nitrogenase

The ability of resonant X-ray emission spectroscopy (XES) to recover physical oxidation state information, which may often be ambiguous in conventional X-ray spectroscopy, is demonstrated. By combining Kβ XES with resonant excitation in the XAS pre-edge region, resonant Kβ XES (or 1s3p RXES) data are obtained, which probe the 3dn+1 final-state configuration. Comparison of the non-resonant and resonant XES for a series of high-spin ferrous and ferric complexes shows that oxidation state assignments that were previously unclear are now easily made. The present study spans iron tetrachlorides, iron sulfur clusters, and the MoFe protein of nitrogenase. While 1s3p RXES studies have previously been reported, to our knowledge, 1s3p RXES has not been previously utilized to resolve questions of metal valency in highly covalent systems. As such, the approach presented herein provides chemists with means to more rigorously and quantitatively address challenging electronic-structure questions.

Theoretical Analysis of Fe K-Edge XANES on Iron Pentacarbonyl

Iron pentacarbonyl (Fe(CO)₅) is a versatile material that is utilized as an inhibitor of flame, shows soot suppressibility, and is used as a precursor for focused electron-beam-induced deposition (FEBID). X-ray absorption near-edge structure (XANES) of the K edge, which is a powerful technique for monitoring the oxidation states and coordination environment of metal sites, can be used to gain insight into Fe(CO)₅-related reaction mechanisms in in situ experiments. We use a finite difference method (FDM) and molecular-orbital-based time-dependent density functional theory (TDDFT) calculations to clarify the Fe K-edge XANES features of Fe(CO)₅. The two pre-edge peaks P₁ and P₂ are mainly the Fe(1s) → Fe-C(σ*) and Fe(1s) → Fe-C(π*) transitions, respectively. When the geometry transformed from D _3h to C _4v symmetry, a ∼30% decrease of the pre-edge P₂ intensity was observed in the simulated spectra. This implies that the π bonding of Fe and CO is sensitive to changes in geometry. The following rising edge and white line regions are assigned to the Fe(1s) → Fe(4p)(mixing C(2p)) transitions. Our results may provide useful information to interpret XANES spectra variations of in situ reactions of metal-CO or similar compounds with π acceptor ligandlike metal-CN complexes.

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.

From Ylides to Doubly Yldiide-Bridged Iron(II) High Spin Dimers via Self-Protolysis

A synthetic strategy for the preparation of novel doubly yldiide bridged iron(II) high spin dimers ([(μ2-C)FeL]2, L = N(SiMe3)2, Mesityl) has been developed. This includes the synthesis of ylide-iron(II) monomers [(Ylide)FeL2] via adduct formation. Subsequent self-protolysis at elevated temperatures by in situ deprotonation of the ylide ligands results in a dimerization reaction forming the desired bridging μ2-C yldiide ligands in [(μ2-C)FeL]2. The comprehensive structural and electronic analysis of dimers [(μ2-C)FeL]2, including NMR, Mössbauer, and X-ray spectroscopy, as well as X-ray crystallography, SQUID, and DFT calculations, confirm their high-spin FeII configurations. Interestingly, the Fe2C2 cores display very acute Fe-C-Fe angles (averaged: 78.6(2)°) resulting in short Fe···Fe distances (averaged: 2.588(2) Å). A remarkably strong antiferromagnetic coupling between the Fe centers has been identified. Strongly polarized Fe-C bonds are observed where the negative charge is mostly centered at the μ2-C yldiide ligands.

The Bonding Situation in the Dinuclear Tetra-Hydrido Complex [{5CpFe}2(μ-H)4] Revisited by Hard X-Ray Spectroscopy

High energy resolution fluorescence detected XANES (HERFD-XANES) and valence-to-core X-ray emission spectroscopy (VtC-XES) are introduced as powerful tools to investigate hydride-iron interaction, the possible iron-iron bond, and iron spin state of the dinuclear tetra-hydrido complex [{⁵CpFe}₂(μ-H)₄] (1H, ⁵Cp = η⁵-C₅ iPr₅) by thoroughly accessing the geometric and electronic structure of this complex in comparison to the nonhydride reference [⁵CpCpFe] (1, Cp = C₅H₅). The so far observed most intense hydride induced signals in the pre-edge feature of the HERFD-XANES and in the VtC-XES spectra at the iron K-edge allow a precise analysis of the LUMO and HOMO states, respectively, by application of time-dependent density function theory (TD-DFT) and density functional theory (DFT) calculations. The results of these calculations are further employed to understand the oxidation state, spin states, and potential Fe-Fe bonds in this complex.

Detection and Characterization of Hydride Ligands in Iron Complexes by High-Resolution Hard X-ray Spectroscopy and Implications for Catalytic Processes

Two hydride catalysts [Fe(CO)(dppp)H(NO)] (dppp = 1,3-bis(diphenylphosphino)propane) and [Fe(CO)H(NO)(PPh₃)₂] in comparison with nonhydride analogues [Fe(dppe)(NO)₂] (dppe = 1,3-bis(diphenylphosphino)ethane) and [Fe(NO)₂(PPh₃)₂] are investigated with a combination of valence-to-core X-ray emission spectroscopy (VtC-XES) and high-energy resolution fluorescence detected X-ray absorption near-edge structure (HERFD-XANES). To fully understand the experiments and to obtain precise information about molecular levels being involved in the spectral signals, time-dependent density functional theory (TD-DFT) calculations and ground state density functional theory (DFT) calculations are necessary. An excellent agreement between experiment and theory allows the identification of particular spectral signals of the Fe-H group. Antibonding Fe-H interactions clearly contribute to pre-edge signals in HERFD-XANES spectra, while bonding Fe-H interactions cause characteristic signatures in the VtC-XES spectra. The sensitivity of both methods with respect to the Fe-H distance is demonstrated by a scanning simulation approach. The results open the way to study metal hydride complexes in situ, their formation, and their fate during catalytic reactions, using high-resolution XANES and valence-to-core X-ray emission spectroscopy.

Cobalt Kβ valence-to-core X-ray emission spectroscopy: a study of low-spin octahedral cobalt(iii) complexes

Kβ valence-to-core (V2C) X-emission spectroscopy (XES) has gained prominence as a tool for molecular inorganic chemists to probe the occupied valence orbitals of coordination complexes, as illustrated by recent evaluation of Kβ V2C XES ranging from titanium to iron. However, cobalt Kβ V2C XES has not been studied in detail, limiting the application of this technique to probe cobalt coordination in molecular catalysts and bioinorganic systems. In addition, the community still lacks a complete understanding of all factors that dictate the V2C peak area. In this manuscript, we report experimental cobalt Kβ V2C XES spectra of low-spin octahedral Co(iii) complexes with different ligand donors, in conjunction with DFT calculations. Cobalt Kβ V2C XES was demonstrated to be sensitive to cobalt-ligand coordination environments. Notably, we recognize here for the first time that there is a linear correlation between the V2C area and the spectrochemical series for low-spin octahedral cobalt(iii) complexes, with strong field π acceptor ligands giving rise to the largest V2C area. This unprecedented correlation is explained by invoking different levels of π-interaction between cobalt p orbitals and ligand orbitals that modulate the percentage of cobalt p orbital character in donor MOs, in combination with changes in the average cobalt-ligand distance.

Experimental and theoretical correlations between vanadium K-edge X-ray absorption and Kβ emission spectra

A series of vanadium compounds was studied by K-edge X-ray absorption (XAS) and K[Formula: see text] X-ray emission spectroscopies (XES). Qualitative trends within the datasets, as well as comparisons between the XAS and XES data, illustrate the information content of both methods. The complementary nature of the chemical insight highlights the success of this dual-technique approach in characterizing both the structural and electronic properties of vanadium sites. In particular, and in contrast to XAS or extended X-ray absorption fine structure (EXAFS), we demonstrate that valence-to-core XES is capable of differentiating between ligating atoms with the same identity but different bonding character. Finally, density functional theory (DFT) and time-dependent DFT calculations enable a more detailed, quantitative interpretation of the data. We also establish correction factors for the computational protocols through calibration to experiment. These hard X-ray methods can probe vanadium ions in any oxidation or spin state, and can readily be applied to sample environments ranging from solid-phase catalysts to biological samples in frozen solution. Thus, the combined XAS and XES approach, coupled with DFT calculations, provides a robust tool for the study of vanadium atoms in bioinorganic chemistry.

Room-Temperature Energy-Sampling Kβ X-ray Emission Spectroscopy of the Mn4Ca Complex of Photosynthesis Reveals Three Manganese-Centered Oxidation Steps and Suggests a Coordination Change Prior to O2 Formation

In oxygenic photosynthesis, water is oxidized and dioxygen is produced at a Mn4Ca complex bound to the proteins of photosystem II (PSII). Valence and coordination changes in its catalytic S-state cycle are of great interest. In room-temperature (in situ) experiments, time-resolved energy-sampling X-ray emission spectroscopy of the Mn Kβ1,3 line after laser-flash excitation of PSII membrane particles was applied to characterize the redox transitions in the S-state cycle. The Kβ1,3 line energies suggest a high-valence configuration of the Mn4Ca complex with Mn(III)3Mn(IV) in S0, Mn(III)2Mn(IV)2 in S1, Mn(III)Mn(IV)3 in S2, and Mn(IV)4 in S3 and, thus, manganese oxidation in each of the three accessible oxidizing transitions of the water-oxidizing complex. There are no indications of formation of a ligand radical, thus rendering partial water oxidation before reaching the S4 state unlikely. The difference spectra of both manganese Kβ1,3 emission and K-edge X-ray absorption display different shapes for Mn(III) oxidation in the S2 → S3 transition when compared to Mn(III) oxidation in the S1 → S2 transition. Comparison to spectra of manganese compounds with known structures and oxidation states and varying metal coordination environments suggests a change in the manganese ligand environment in the S2 → S3 transition, which could be oxidation of five-coordinated Mn(III) to six-coordinated Mn(IV). Conceivable options for the rearrangement of (substrate) water species and metal-ligand bonding patterns at the Mn4Ca complex in the S2 → S3 transition are discussed.

Axial Ligation and Redox Changes at the Cobalt Ion in Cobalamin Bound to Corrinoid Iron-Sulfur Protein (CoFeSP) or in Solution Characterized by XAS and DFT

A cobalamin (Cbl) cofactor in corrinoid iron-sulfur protein (CoFeSP) is the primary methyl group donor and acceptor in biological carbon oxide conversion along the reductive acetyl-CoA pathway. Changes of the axial coordination of the cobalt ion within the corrin macrocycle upon redox transitions in aqua-, methyl-, and cyano-Cbl bound to CoFeSP or in solution were studied using X-ray absorption spectroscopy (XAS) at the Co K-edge in combination with density functional theory (DFT) calculations, supported by metal content and cobalt redox level quantification with further spectroscopic methods. Calculation of the highly variable pre-edge X-ray absorption features due to core-to-valence (ctv) electronic transitions, XANES shape analysis, and cobalt-ligand bond lengths determination from EXAFS has yielded models for the molecular and electronic structures of the cobalt sites. This suggested the absence of a ligand at cobalt in CoFeSP in α-position where the dimethylbenzimidazole (dmb) base of the cofactor is bound in Cbl in solution. As main species, (dmb)Co^III(OH₂), (dmb)Co^II(OH₂), and (dmb)Co^III(CH₃) sites for solution Cbl and Co^III(OH₂), Co^II(OH₂), and Co^III(CH₃) sites in CoFeSP-Cbl were identified. Our data support binding of a serine residue from the reductive-activator protein (RACo) of CoFeSP to the cobalt ion in the CoFeSP-RACo protein complex that stabilizes Co(II). The absence of an α-ligand at cobalt not only tunes the redox potential of the cobalamin cofactor into the physiological range, but is also important for CoFeSP reactivation.

X-ray Absorption and Emission Spectroscopic Studies of [L2Fe2S2](n) Model Complexes: Implications for the Experimental Evaluation of Redox States in Iron-Sulfur Clusters

Herein, a systematic study of [L2Fe2S2](n) model complexes (where L = bis(benzimidazolato) and n = 2-, 3-, 4-) has been carried out using iron and sulfur K-edge X-ray absorption (XAS) and iron Kβ and valence-to-core X-ray emission spectroscopies (XES). These data are used as a test set to evaluate the relative strengths and weaknesses of X-ray core level spectroscopies in assessing redox changes in iron-sulfur clusters. The results are correlated to density functional theory (DFT) calculations of the spectra in order to further support the quantitative information that can be extracted from the experimental data. It is demonstrated that due to canceling effects of covalency and spin state, the information that can be extracted from Fe Kβ XES mainlines is limited. However, a careful analysis of the Fe K-edge XAS data shows that localized valence vs delocalized valence species may be differentiated on the basis of the pre-edge and K-edge energies. These findings are then applied to existing literature Fe K-edge XAS data on the iron protein, P-cluster, and FeMoco sites of nitrogenase. The ability to assess the extent of delocalization in the iron protein vs the P-cluster is highlighted. In addition, possible charge states for FeMoco on the basis of Fe K-edge XAS data are discussed. This study provides an important reference for future X-ray spectroscopic studies of iron-sulfur clusters.

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

A long-standing goal of inorganic chemists is the ability to decipher the geometric and electronic structures of chemical species. This is particularly true for the study of small molecule and biological catalysts, where this knowledge is critical for understanding how these molecules effect chemical transformations. Numerous techniques are available for this task, and collectively they have enabled detailed understanding of many complex chemical systems. Despite this battery of probes, however, challenges still remain, particularly when the structural question involves subtle perturbations of the ligands bound to a metal center, as is often the case during chemical reactions. It is here that, as an emerging probe of chemical structure, valence-to-core (VtC) X-ray emission spectroscopy (XES) holds promise. VtC XES begins with ionization of a 1s electron from a metal ion by high energy X-ray photons. Electrons residing in ligand-localized valence orbitals decay to fill the 1s hole, emitting fluorescent photons in the process; in this manner, VtC XES primarily probes the filled, ligand-based orbitals of a metal complex. This is in contrast to other X-ray based techniques, such as K-edge X-ray absorption and EXAFS, which probe the unoccupied d-manifold orbitals and atomic scatterers surrounding the metal, respectively. As a hard X-ray technique, VtC XES experiments can be performed on a variety of sample states and environments, enabling application to demanding systems, such as high pressure cells and dilute biological samples. VtC XES thus can offer unique insights into the geometric and electronic structures of inorganic complexes. In recent years, we have sought to use VtC XES in the study of inorganic and bioinorganic complexes; doing so, however, first required a thorough and detailed understanding of the information content of these spectra. Extensive experimental surveys of model compounds coupled to the insights provided by DFT calculated spectra of real and hypothetical compounds allowed the development of a framework whereby VtC XES spectra may be understood in terms of a molecular orbital picture. Specifically, VtC spectra may be interpreted as a probe of electronic structure for the ligands bound to a metal center, enabling access to chemical information that can be difficult to obtain with other methods. Examples of this include the ability to (1) assess the identity and number of atomic/small molecule ligands bound to a metal center, (2) quantify the degree of bond activation of a small molecule substrate, and (3) establish the protonation state of donor atoms. With this foundation established, VtC has been meaningfully applied to long-standing questions in bioinorganic chemistry, with the potential for numerous future applications in all areas of metal-mediated catalysis.

Using solution- and solid-state S K-edge X-ray absorption spectroscopy with density functional theory to evaluate M-S bonding for MS4(2-) (M = Cr, Mo, W) dianions

Herein, we have evaluated relative changes in M-S electronic structure and orbital mixing in Group 6 MS4(2-) dianions using solid- and solution-phase S K-edge X-ray absorption spectroscopy (XAS; M = Mo, W), as well as density functional theory (DFT; M = Cr, Mo, W) and time-dependent density functional theory (TDDFT) calculations. To facilitate comparison with solution measurements (conducted in acetonitrile), theoretical models included gas-phase calculations as well as those that incorporated an acetonitrile dielectric, the latter of which provided better agreement with experiment. Two pre-edge features arising from S 1s → e* and t electron excitations were observed in the S K-edge XAS spectra and were reasonably assigned as (1)A1 → (1)T2 transitions. For MoS4(2-), both solution-phase pre-edge peak intensities were consistent with results from the solid-state spectra. For WS4(2-), solution- and solid-state pre-edge peak intensities for transitions involving e* were equivalent, while transitions involving the t orbitals were less intense in solution. Experimental and computational results have been presented in comparison to recent analyses of MO4(2-) dianions, which allowed M-S and M-O orbital mixing to be evaluated as the principle quantum number (n) for the metal valence d orbitals increased (3d, 4d, 5d). Overall, the M-E (E = O, S) analyses revealed distinct trends in orbital mixing. For example, as the Group 6 triad was descended, e* (π*) orbital mixing remained constant in the M-S bonds, but increased appreciably for M-O interactions. For the t orbitals (σ* + π*), mixing decreased slightly for M-S bonding and increased only slightly for the M-O interactions. These results suggested that the metal and ligand valence orbital energies and radial extensions delicately influenced the orbital compositions for isoelectronic ME4(2-) (E = O, S) dianions.

X-ray Absorption and Emission Study of Dioxygen Activation by a Small-Molecule Manganese Complex

Manganese K-edge X-ray absorption (XAS) and Kβ emission (XES) spectroscopies were used to investigate the factors contributing to O-O bond activation in a small-molecule system. The recent structural characterization of a metastable peroxo-bridged dimeric Mn(III)2 complex derived from dioxygen has provided the first opportunity to obtain X-ray spectroscopic data on this type of species. Ground state and time-dependent density functional theory calculations have provided further insight into the nature of the transitions in XAS pre-edge and valence-to-core (VtC) XES spectral regions. An experimentally validated electronic structure description has also enabled the determination of structural and electronic factors that govern peroxo bond activation, and have allowed us to propose both a rationale for the metastability of this unique compound, as well as potential future ligand designs which may further promote or inhibit O-O bond scission. Finally, we have explored the potential of VtC XES as an element-selective probe of both the coordination mode and degree of activation of peroxomanganese adducts. The comparison of these results to a recent VtC XES study of iron-mediated dintrogen activation helps to illustrate the factors that may determine the success of this spectroscopic method for future studies of small-molecule activation at transition metal sites.

Feasibility of Valence-to-Core X-ray Emission Spectroscopy for Tracking Transient Species

X-ray spectroscopies, when combined in laser-pump, X-ray-probe measurement schemes, can be powerful tools for tracking the electronic and geometric structural changes that occur during the course of a photoinitiated chemical reaction. X-ray absorption spectroscopy (XAS) is considered an established technique for such measurements, and X-ray emission spectroscopy (XES) of the strongest core-to-core emission lines (Kα and Kβ) is now being utilized. Flux demanding valence-to-core XES promises to be an important addition to the time-resolved spectroscopic toolkit. In this paper we present measurements and density functional theory calculations on laser-excited, solution-phase ferrocyanide that demonstrate the feasibility of valence-to-core XES for time-resolved experiments. We discuss technical improvements that will make valence-to-core XES a practical pump-probe technique.

Valence to core X-ray emission spectroscopy

This Progress Report discusses the chemical sensitivity of Kβ valence to core X-ray emission spectroscopy (vtc-XES) and its applications for investigating 3d-transition-metal based materials. Vtc-XES can be used for ligand identification and for the characterization of the valence electronic levels. The technique provides information that is similar to valence band photoemission spectroscopy but the sample environment can be chosen freely and thus allows measurements in presence of gases and liquids and it can be applied for measurements under in situ/operando or extreme conditions. The theoretical basis of the technique is presented using a one-electron approach and the vtc-XES spectral features are interpreted using ground state density functional theory calculations. Some recent results obtained by vtc-XES in various scientific fields are discussed to demonstrate the potential and future applications of this technique. Resonant X-ray emission spectroscopy is briefly introduced with some applications for the study of 3d and 5d-transition-metal based systems.

The role of X-ray spectroscopy in understanding the geometric and electronic structure of nitrogenase

X-ray absorption (XAS) and X-ray emission spectroscopy (XES) provide element specific probes of the geometric and electronic structures of metalloprotein active sites. As such, these methods have played an integral role in nitrogenase research beginning with the first EXAFS studies on nitrogenase in the late 1970s. Herein, we briefly explain the information that can be extracted from XAS and XES. We then highlight the recent applications of these methods in nitrogenase research. The influence of X-ray spectroscopy on our current understanding of the atomic structure and electronic structure of iron molybdenum cofactor (FeMoco) is emphasized. Contributions of X-ray spectroscopy to understanding substrate interactions and cluster biosynthesis are also discussed. This article is part of a Special Issue entitled: Fe/S proteins: Analysis, structure, function, biogenesis and diseases.