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

Vanadium Alumanyl Complex: Synthesis, Characterization, Reactivity, and Application as a Catalyst for C-H Alumanylation of Alkenes

A complex containing a V-Al bond is described. This species can be prepared by either transmetalation of a previously disclosed alumanylpotassium with Cp2VCl or photolytic oxidative alumination of Cp2V using the corresponding dialumane. Reaction of the resulting V-Al complex with H2 gave a Cp2V-dihydridoaluminate complex. These complexes were studied with X-ray crystallography, vanadium K-edge XANES spectroscopy, and DFT calculations. Finally, the reactivity of these molecules was studied opening the way to a catalytic C-H alumanylation of alkenes.

A Combined Wave Function and Density Functional Approach for K-Edge X-ray Absorption Near-Edge Spectroscopy: A Case Study of Hydrated First-Row Transition Metal Ions

The prediction of X-ray absorption spectra (XAS) of transition metal complexes has important and broad application areas in chemistry and biology. In this letter, we have investigated the predictive ability of multiconfiguration pair-density functional theory (MC-PDFT) for X-ray absorption spectra by calculating the metal K pre-edge features of aquated 3d transition metal ions in common oxidation states. MC-PDFT results were compared with experimentally measured spectra as well as analyzed against results from restricted active-space second-order perturbation theory (RASPT2) and time-dependent density functional theory (TDDFT). As expected, TDDFT performs well for excited states that can be accurately represented by singly excited configurations but fails for excited states where higher order excitations become important. On the other hand, both RASPT2 and MC-PDFT provide quantitatively accurate results for all excited states irrespective of their character. While core-level spectroscopy with RASPT2 is accurate, it is computationally expensive. Our results show that MC-PDFT performs equally well with significantly lower computational cost and is an encouraging alternate approach for X-ray spectroscopies.

Beware of beam damage under reaction conditions: X-ray induced photochemical reduction of supported VOx catalysts during in situ XAS experiments

In situ X-ray absorption spectroscopy (XAS) is a powerful technique for the investigation of heterogeneous catalysts and electrocatalysts. The obtained XAS spectra are usually interpreted from the point of view of the investigated chemical processes, thereby sometimes omitting the fact that intense X-ray irradiation may induce additional transformations in metal speciation and, thus, in the corresponding XAS spectra. In this work, we report on X-ray induced photochemical reduction of vanadium in supported vanadia (VOx) catalysts under reaction conditions, detected at a synchrotron beamline. While this process was not observed in an inert atmosphere and in the presence of water vapor, it occurred at room temperature in the presence of a reducing agent (ethanol or hydrogen) alone or mixed with oxygen. Temperature programmed experiments have shown that X-ray induced reduction of VOx species appeared very clear at 30-100 °C but was not detected at higher temperatures, where the thermocatalytic ethanol oxidative hydrogenation (ODH) takes place. Similar to other studies on X-ray induced effects, we suggest approaches, which can help to mitigate vanadium photoreduction, including defocusing of the X-ray beam and attenuation of the X-ray beam intensity by filters. To recognize beam damage under in situ/operando conditions, we suggest performing X-ray beam switching (on and off) tests at different beam intensities under in situ conditions.

Minor Elements and Color Causing Role in Spinel: Multi-Analytical Approaches

Natural spinel (MgAl2O44) usually contains some minor and trace elements (e.g., Cr, Co, Fe, V) that may cause various hues. The ratios of these chromophores directly affect the color composition. The red color in spinel is attributed to the combination of significant Cr and V. Magenta and purple to blue and green colors in spinels are affected by the significant Fe concentration, whereas orange color in spinel shows the contribution of significant V content compared to Cr and Fe. After the heating experiment, advanced gemological investigation reveals some noteworthy characteristic features. X-ray absorption spectroscopy (XAS) indicates a greater change in oxidation state, as well as disordering of Fe and V. Broadening of the dominant peak at around 406 cm−1 with occurrences of additional small peaks at around 715–719 cm−1 in Raman spectra, as well as broadening of the 685 nm (R-line) and poorly defined structure of additional peaks (N-lines) in photoluminescence spectra should be significant indicators of spinel undergone heat treatment.

Near-Quantitative Predictions of the First-Shell Coordination Structure of Hydrated First-Row Transition Metal Ions Using K-Edge X-ray Absorption Near-Edge Spectroscopy

The solvation structure of transition metal ions is important for applications in geochemistry, biochemistry, energy storage, and environmental chemistry. We study the X-ray absorption pre-edge and near-edge spectra at the K-edge of a nearly complete series of hydrated first-row transition metal ions with d orbital occupancy from d² to d¹⁰. We optimize all of the structures at the density functional theory (DFT) level with explicit solvation and then compute the pre-edge X-ray absorption spectra with time-dependent DFT (TDDFT) and restricted active space second-order perturbation theory (RASPT2). TDDFT provides accurate results for spectra that are dominated by single excitations, while RASPT2 correctly distinguishes between singly and doubly excited states with quantitative accuracy compared with experiment. We analyze the pre-edge features for each metal ion to reveal the impact of the variations in d orbital occupancy on the first-shell coordination environment. We also report the lowest-energy ligand field d-d transitions using complete active space second-order perturbation theory.

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.

Redox Dynamics of Active VO x Sites Promoted by TiO x during Oxidative Dehydrogenation of Ethanol Detected by Operando Quick XAS

Titania-supported vanadia (VO _x /TiO₂) catalysts exhibit outstanding catalytic in a number of selective oxidation and reduction processes. In spite of numerous investigations, the nature of redox transformations of vanadium and titanium involved in various catalytic processes remains difficult to detect and correlate to the rate of products formation. In this work, we studied the redox dynamics of active sites in a bilayered 5% V₂O₅/15% TiO₂/SiO₂ catalyst (consisting of submonolayer VO _x species anchored onto a TiO _x monolayer, which in turn is supported on SiO₂) during the oxidative dehydrogenation of ethanol. The VO _x species in 5% V₂O₅/15% TiO₂/SiO₂ show high selectivity to acetaldehyde and an ca. 40 times higher acetaldehyde formation rate in comparison to VO _x species supported on SiO₂ with a similar density. Operando time-resolved V and Ti K-edge X-ray absorption near-edge spectroscopy, coupled with a transient experimental strategy, quantitatively showed that the formation of acetaldehyde over 5% V₂O₅/15% TiO₂/SiO₂ is kinetically coupled to the formation of a V⁴⁺ intermediate, while the formation of V³⁺ is delayed and 10-70 times slower. The low-coordinated nature of various redox states of VO _x species (V⁵⁺, V⁴⁺, and V³⁺) in the 5% V₂O₅/15% TiO₂/SiO₂ catalyst is confirmed using the extensive database of V K-edge XANES spectra of standards and specially synthesized molecular crystals. Much weaker redox activity of the Ti⁴⁺/Ti³⁺ couple was also detected; however, it was found to not be kinetically coupled to the rate-determining step of ethanol oxidation. Thus, the promoter effect of TiO _x is rather complex. TiO _x species might be involved in a fast electron transport between VO _x species and might affect the electronic structure of VO _x , thereby promoting their reducibility. This study demonstrates the high potential of element-specific operando X-ray absorption spectroscopy for uncovering complex catalytic mechanisms involving the redox kinetics of various metal oxides.

Phosphorus-Atom Transfer from Phosphaethynolate to an Alkylidyne

A low-spin and mononuclear vanadium complex, (Me nacnac)V(CO)(η2 -P≡Ct Bu) (2) (Me nacnac- =[ArNC(CH3 )]2 CH, Ar=2,6-i Pr2 C6 H3 ), was prepared upon treatment of the vanadium neopentylidyne complex (Me nacnac)V≡Ct Bu(OTf) (1) with Na(OCP)(diox)2.5 (diox=1,4-dioxane), while the isoelectronic ate-complex [Na(15-crown-5)]{([ArNC(CH2 )]CH[C(CH3 )NAr])V(CO)(η2 -P≡Ct Bu)} (4), was obtained via the reaction of Na(OCP)(diox)2.5 and ([ArNC(CH2 )]CH[C(CH3 )NAr])V≡Ct Bu(OEt2 ) (3) in the presence of crown-ether. Computational studies suggest that the P-atom transfer proceeds by [2+2]-cycloaddition of the P≡C bond across the V≡Ct Bu moiety, followed by a reductive decarbonylation to form the V-C≡O linkage. The nature of the electronic ground state in diamagnetic complexes, 2 and 4, was further investigated both theoretically and experimentally, using a combination of density functional theory (DFT) calculations, UV/Vis and NMR spectroscopies, cyclic voltammetry, X-ray absorption spectroscopy (XAS) measurements, and comparison of salient bond metrics derived from X-ray single-crystal structural characterization. In combination, these data are consistent with a low-valent vanadium ion in complexes 2 and 4. This study represents the first example of a metathesis reaction between the P-atom of [PCO]- and an alkylidyne ligand.

Phosphorus Kβ X-ray emission spectroscopy detects non-covalent interactions of phosphate biomolecules in situ

Phosphorus is ubiquitous in biochemistry, being found in the phosphate groups of nucleic acids and the energy-transferring system of adenine nucleotides (e.g. ATP). Kβ X-ray emission spectroscopy (XES) of phosphorus has been largely unexplored, with no previous applications to biomolecules. Here, the potential of P Kβ XES to study phosphate-containing biomolecules, including ATP and NADPH, is evaluated, as is the application of the technique to aqueous solution samples. P Kβ spectra offer a detailed picture of phosphate valence electronic structure, reporting on subtle non-covalent effects, such as hydrogen bonding and ionic interactions, that are key to enzymatic catalysis. Spectral features are interpreted using density functional theory (DFT) calculations, and potential applications to the study of biological energy conversion are highlighted.

Phosphorus X-ray emission spectroscopy probes non-covalent interactions and electronic structure of phosphate biomolecules in both solid and solution samples.

Characterizing Polyoxovanadate-Alkoxide Clusters Using Vanadium K-Edge X-Ray Absorption Spectroscopy

A number of technologies would benefit from developing inorganic compounds and materials with specific electronic and magnetic exchange properties. Unfortunately, designing compounds with these properties is difficult because metal⋅⋅⋅metal coupling schemes are hard to predict and control. Fully characterizing communication between metals in existing compounds that exhibit interesting properties could provide valuable insight and advance those predictive capabilities. One such class of molecules are the series of Lindqvist iron-functionalized and hexavanadium polyoxovanadate-alkoxide clusters, which we characterized here using V K-edge X-ray absorption spectroscopy. Substantial changes in the pre-edge peak intensities were observed that tracked with the V 3d-electron count. The data also suggested substantial delocalization between the vanadium cations. Meanwhile, the Fe^III cations were electronically isolated from the polyoxovanadate core.

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.

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.

Time-dependent DFT study of the K-edge spectra of vanadium and titanium complexes: effects of chloride ligands on pre-edge features

X-ray absorption near edge structures (XANES) of vanadium and titanium complexes were investigated with time-dependent density functional theory (TDDFT).

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.

Zinc 1s Valence-to-Core X-ray Emission Spectroscopy of Halozincate Complexes

The Zn 1s valence-to-core (VtC) X-ray emission spectra of seven ionic liquids have been measured experimentally and simulated on the basis of time-dependent density-functional theory (TDDFT) calculations. Six of the ionic liquids were made by mixing [C₈C₁Im]X and Zn(II)X₂ at three different ZnX₂ mole fractions (0.33, 0.50, or 0.67) for X = Cl or Br, and a further ionic liquid was made by mixing [P_6,6,6,14]Cl and a mole fraction of ZnCl₂ of 0.33. Calculations were performed for the [ZnX₄]^2-, [Zn₂X₆]^2-, and [Zn₄X₁₀]^2- ions to capture the expected metal complex speciation. The VtC emission spectra showed three bands arising from single-electron processes that can be assigned to emission from ligand p-type orbitals, zinc d-orbitals, and ligand s-type orbitals. For all seven ionic liquids, the highest occupied molecular orbital arises from the ligand p orbitals, and the spectra for the different size metal complexes for the same X were found to be very similar, in terms of both relative peak intensities and peak energies. For both experiments and TDDFT calculations, there was an energy difference of 0.5 eV between the Cl-based and Br-based metal complexes for the ligand s and p orbitals, while the Zn 3d orbital energies were relatively unaffected by the identity of the ligand. The TDDFT calculations find that for the ions with symmetrically equivalent zinc atoms ([Zn₂X₆]^2- and [Zn₄X₁₀]^2-), the most appropriate core-ionized reference state has a core-hole that is localized on a single zinc atom. In this framework, the spectra for the larger ions can be viewed as a sum of spectra for the tetrahedral complex with a single zinc atom with small variations in the structure of the coordinating ligands. Because the spectra are relatively insensitive to small changes in the geometry of the ligands, this is consistent with the small variation in the spectra measured in the experiment.

The influence of ZnII coordination sphere and chemical structure over the reactivity of metallo-β-lactamase model compounds

The first coordination sphere influences the reactivity of metallo-β-lactamase monozinc model complexes.

Temperature-Induced Formation of Lubricous Oxides in Vanadium Containing Iron-Based Arc Sprayed Coatings

In the field of surface engineering, the use of self-lubricous coatings with the incorporation of vanadium represent a promising approach to reduce friction, thus contributing to the wear behavior. For vanadium containing hard coatings produced by means of thin film technology, the reduction in friction at elevated temperatures was repeatedly attributed to temperature-induced and tribo-oxidatively formed oxides which act as solid lubricant. Only very few studies focused on the tribological characteristics of vanadium containing arc sprayed coatings. In this study, the tribological characteristics of a vanadium containing iron-based arc sprayed deposit were investigated in dry sliding experiments under ambient conditions and different temperatures. Types of wear at the worn surfaces and counterparts were examined by means of electron microscopy and energy dispersive X-ray (EDX) spectroscopy. The speciation of vanadium in the superficial layer was determined using X-ray absorption near edge structure (XANES) spectroscopy. It was found that the vanadium-containing coating exhibited a distinctly reduction of the coefficient of friction above 450 °C which further decreased with increasing temperature. XANES spectroscopy indicated an increased oxidation state for the V component on the coating surface, suggesting the prevalence of specific vanadium oxides which promote a self-lubricating ability of the coating.

A Restricted Open Configuration Interaction with Singles Method To Calculate Valence-to-Core Resonant X-ray Emission Spectra: A Case Study

In this work, a new protocol for the calculation of valence-to-core resonant X-ray emission (VtC RXES) spectra is introduced. The approach is based on the previously developed restricted open configuration interaction with singles (ROCIS) method and its parametrized version, based on a ground-state Kohn-Sham determinant (DFT/ROCIS) method. The ROCIS approach has the following features: (1) In the first step approximation, many-particle eigenstates are calculated in which the total spin is retained as a good quantum number. (2) The ground state with total spin S and excited states with spin S' = S, S ± 1, are obtained. (3) These states have a qualitatively correct multiplet structure. (4) Quasi-degenerate perturbation theory is used to treat the spin-orbit coupling operator variationally at the many-particle level. (5) Transition moments are obtained between the relativistic many-particle states. The method has shown great potential in the field of X-ray spectroscopy, in particular in the field of transition-metal L-edge, which cannot be described correctly with particle-hole theories. In this work, the method is extended to the calculation of resonant VtC RXES [alternatively referred to as 1s-VtC resonant inelastic X-ray scattering (RIXS)] spectra. The complete Kramers-Dirac-Heisenerg equation is taken into account. Thus, state interference effects are treated naturally within this protocol. As a first application of this protocol, a computational study on the previously reported VtC RXES plane on a molecular managanese(V) complex is performed. Starting from conventional X-ray absorption spectra (XAS), we present a systematic study that involves calculations and electronic structure analysis of both the XAS and non-resonant and resonant VtC XES spectra. The very good agreement between theory and experiment, observed in all cases, allows us to unravel the complicated intensity mechanism of these spectroscopic techniques as a synergic function of state polarization and interference effects. In general, intense features in the RIXS spectra originate from absorption and emission processes that involve nonorthogonal transition moments. We also present a graphical method to determine the sign of the interference contributions.

Vanadyl as a Stable Structural Mimic of Reactive Ferryl Intermediates in Mononuclear Nonheme-Iron Enzymes

The iron(II)- and 2-(oxo)glutarate-dependent (Fe/2OG) oxygenases catalyze an array of challenging transformations via a common iron(IV)-oxo (ferryl) intermediate, which in most cases abstracts hydrogen (H•) from an aliphatic carbon of the substrate. Although it has been shown that the relative disposition of the Fe-O and C-H bonds can control the rate of H• abstraction and fate of the resultant substrate radical, there remains a paucity of structural information on the actual ferryl states, owing to their high reactivity. We demonstrate here that the stable vanadyl ion [(V^IV-oxo)²⁺] binds along with 2OG or its decarboxylation product, succinate, in the active site of two different Fe/2OG enzymes to faithfully mimic their transient ferryl states. Both ferryl and vanadyl complexes of the Fe/2OG halogenase, SyrB2, remain stably bound to its carrier protein substrate (l-aminoacyl-S-SyrB1), whereas the corresponding complexes harboring transition metals (Fe, Mn) in lower oxidation states dissociate. In the well-studied taurine:2OG dioxygenase (TauD), the disposition of the substrate C-H bond relative to the vanadyl ion defined by pulse electron paramagnetic resonance methods is consistent with the crystal structure of the reactant complex and computational models of the ferryl state. Vanadyl substitution may thus afford access to structural details of the key ferryl intermediates in this important enzyme class.

Protonation State of MnFe and FeFe Cofactors in a Ligand-Binding Oxidase Revealed by X-ray Absorption, Emission, and Vibrational Spectroscopy and QM/MM Calculations

Enzymes with a dimetal-carboxylate cofactor catalyze reactions among the top challenges in chemistry such as methane and dioxygen (O₂) activation. Recently described proteins bind a manganese-iron cofactor (MnFe) instead of the classical diiron cofactor (FeFe). Determination of atomic-level differences of homo- versus hetero-bimetallic cofactors is crucial to understand their diverse redox reactions. We studied a ligand-binding oxidase from the bacterium Geobacillus kaustophilus (R2lox) loaded with a FeFe or MnFe cofactor, which catalyzes O₂ reduction and an unusual tyrosine-valine ether cross-link formation, as revealed by X-ray crystallography. Advanced X-ray absorption, emission, and vibrational spectroscopy methods and quantum chemical and molecular mechanics calculations provided relative Mn/Fe contents, X-ray photoreduction kinetics, metal-ligand bond lengths, metal-metal distances, metal oxidation states, spin configurations, valence-level degeneracy, molecular orbital composition, nuclear quadrupole splitting energies, and vibrational normal modes for both cofactors. A protonation state with an axial water (H₂O) ligand at Mn or Fe in binding site 1 and a metal-bridging hydroxo group (μOH) in a hydrogen-bonded network is assigned. Our comprehensive picture of the molecular, electronic, and dynamic properties of the cofactors highlights reorientation of the unique axis along the Mn-OH₂ bond for the Mn1(III) Jahn-Teller ion but along the Fe-μOH bond for the octahedral Fe1(III). This likely corresponds to a more positive redox potential of the Mn(III)Fe(III) cofactor and higher proton affinity of its μOH group. Refined model structures for the Mn(III)Fe(III) and Fe(III)Fe(III) cofactors are presented. Implications of our findings for the site-specific metalation of R2lox and performance of the O₂ reduction and cross-link formation reactions are discussed.