Publications by authors named "Serena DeBeer"

138 Publications

Impact of Single-Pulse, Low-Intensity Laser Post-Processing on Structure and Activity of Mesostructured Cobalt Oxide for the Oxygen Evolution Reaction.

ACS Appl Mater Interfaces 2021 Jul 29. Epub 2021 Jul 29.

Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, Mülheim an der Ruhr 45470, Germany.

Herein, we report nanosecond, single-pulse laser post-processing (PLPP) in a liquid flat jet with precise control of the applied laser intensity to tune structure, defect sites, and the oxygen evolution reaction (OER) activity of mesostructured CoO. High-resolution X-ray diffraction (XRD), Raman, and X-ray photoelectron spectroscopy (XPS) are consistent with the formation of cobalt vacancies at tetrahedral sites and an increase in the lattice parameter of CoO after the laser treatment. X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) further reveal increased disorder in the structure and a slight decrease in the average oxidation state of the cobalt oxide. Molecular dynamics simulation confirms the surface restructuring upon laser post-treatment on CoO. Importantly, the defect-induced PLPP was shown to lower the charge transfer resistance and boost the oxygen evolution activity of CoO. For the optimized sample, a 2-fold increment of current density at 1.7 V vs RHE is obtained and the overpotential at 10 mA/cm decreases remarkably from 405 to 357 mV compared to pristine CoO. Post-mortem characterization reveals that the material retains its activity, morphology, and phase structure after a prolonged stability test.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acsami.1c08034DOI Listing
July 2021

Surface-Promoted Evolution of Ru-bda Coordination Oligomers Boosts the Efficiency of Water Oxidation Molecular Anodes.

J Am Chem Soc 2021 Aug 22;143(30):11651-11661. Epub 2021 Jul 22.

Institute of Chemical Research of Catalonia (ICIQ). Barcelona Institute of Science and Technology (BIST), Avenida Països Catalans 16, 43007 Tarragona, Spain.

A new Ru oligomer of formula {[Ru(bda-κ-NO)(4,4'-bpy)](4,4'-bpy)}, (bda is [2,2'-bipyridine]-6,6'-dicarboxylate and 4,4'-bpy is 4,4'-bipyridine), was synthesized and thoroughly characterized with spectroscopic, X-ray, and electrochemical techniques. This oligomer exhibits strong affinity for graphitic materials through CH-π interactions and thus easily anchors on multiwalled carbon nanotubes (CNT), generating the molecular hybrid material . The latter acts as a water oxidation catalyst and converts to a new species, , during the electrochemical oxygen evolution process involving solvation and ligand reorganization facilitated by the interactions of molecular Ru catalyst and the surface. This heterogeneous system has been shown to be a powerful and robust molecular hybrid anode for electrocatalytic water oxidation into molecular oxygen, achieving current densities in the range of 200 mA/cm at pH 7 under an applied potential of 1.45 V vs NHE. The remarkable long-term stability of this hybrid material during turnover is rationalized based on the supramolecular interaction of the catalyst with the graphitic surface.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/jacs.1c04738DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8343522PMC
August 2021

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

Chem Sci 2021 Apr 29;12(22):7888-7901. Epub 2021 Apr 29.

Max Planck Institute for Chemical Energy Conversion Stiftstr. 34-36 D-45470 Mülheim an der Ruhr Germany

Phosphorus is ubiquitous in biochemistry, being found in the phosphate groups of nucleic acids and the energy-transferring system of adenine nucleotides ( 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.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/d1sc01266eDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8188515PMC
April 2021

Structure, reactivity, and spectroscopy of nitrogenase-related synthetic and biological clusters.

Chem Soc Rev 2021 Aug;50(15):8743-8761

Max Planck Institute for Chemical Energy Conversion, Stiftstr. 34-36, 45470 Mülheim an der Ruhr, Germany.

The reduction of dinitrogen (N2) is essential for its incorporation into nucleic acids and amino acids, which are vital to life on earth. Nitrogenases convert atmospheric dinitrogen to two ammonia molecules (NH3) under ambient conditions. The catalytic active sites of these enzymes (known as FeM-cofactor clusters, where M = Mo, V, Fe) are the sites of N2 binding and activation and have been a source of great interest for chemists for decades. In this review, recent studies on nitrogenase-related synthetic molecular complexes and biological clusters are discussed, with a focus on their reactivity and spectroscopic characterization. The molecular models that are discussed span from simple mononuclear iron complexes to multinuclear iron complexes and heterometallic iron complexes. In addition, recent work on the extracted biological cofactors is discussed. An emphasis is placed on how these studies have contributed towards our understanding of the electronic structure and mechanism of nitrogenases.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/d1cs00381jDOI Listing
August 2021

Towards a unified understanding of the copper sites in particulate methane monooxygenase: an X-ray absorption spectroscopic investigation.

Chem Sci 2021 Mar 30;12(17):6194-6209. Epub 2021 Mar 30.

Max Planck Institute for Chemical Energy Conversion Stiftstrasse 34-36 D-45470 Mülheim an der Ruhr Germany

The enzymatic conversion of the greenhouse gas, methane, to a liquid fuel, methanol, is performed by methane monooxygenases (MMOs) under mild conditions. The copper stoichiometry of particulate MMO (pMMO) has been long debated, with a dicopper site previously proposed on the basis of a 2.51 Å Cu-Cu feature in extended X-ray absorption fine structure (EXAFS) data. However, recent crystallographic data and advanced electron paramagnetic resonance (EPR) characterization support the presence of only mononuclear copper sites. To reconcile these data, we have collected high-energy resolution fluorescence detected (HERFD) and partial fluorescence yield (PFY) EXAFS spectra of (.) (Bath) pMMO. Both methods reveal only monocopper sites. These data were compared to previously published pMMO PFY-EXAFS data from (Bath) and 20Z, supporting dicopper and monocopper sites, respectively. The FT-EXAFS feature previously attributed to a dicopper site can be reproduced by the inclusion of a metallic copper background signal. The exact position of this feature is dependent on the nature of the sample and the percentage of background contamination, indicating that visual inspection is not sufficient for identifying background metallic contributions. Additionally, an undamaged X-ray absorption spectrum was obtained, consistent with the copper oxidation-state speciation determined by EPR quantification. X-ray photodamage studies suggest that the previously observed Cu(i) XAS features are in part attributable to photodamage. This study illustrates the complex array of factors involved in EXAFS measurement and modeling of pMMO and more generally, dilute metalloproteins with multiple metal centers.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/d1sc00676bDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8098663PMC
March 2021

Structure-Spectroscopy Correlations for Intermediate Q of Soluble Methane Monooxygenase: Insights from QM/MM Calculations.

J Am Chem Soc 2021 May 22;143(17):6560-6577. Epub 2021 Apr 22.

Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany.

The determination of the diiron core intermediate structures involved in the catalytic cycle of soluble methane monooxygenase (sMMO), the enzyme that selectively catalyzes the conversion of methane to methanol, has been a subject of intense interest within the bioinorganic scientific community. Particularly, the specific geometry and electronic structure of the intermediate that precedes methane binding, known as intermediate Q (or MMOH), has been debated for over 30 years. Some reported studies support a bis-μ-oxo-bridged Fe(IV)O closed-core conformation Fe(IV)O core, whereas others favor an open-core geometry, with a longer Fe-Fe distance. The lack of consensus calls for a thorough re-examination and reinterpretation of the spectroscopic data available on the MMOH intermediate. Herein, we report extensive simulations based on a hybrid quantum mechanics/molecular mechanics approach (QM/MM) approach that takes into account the complete enzyme to explore possible conformations for intermediates MMOH and MMOH of the sMMOH catalytic cycle. High-level quantum chemical approaches are used to correlate specific structural motifs with geometric parameters for comparison with crystallographic and EXAFS data, as well as with spectroscopic data from Mössbauer spectroscopy, Fe K-edge high-energy resolution X-ray absorption spectroscopy (HERFD XAS), and resonance Raman O-O difference spectroscopy. The results provide strong support for an open-core-type configuration in MMOH, with the most likely topology involving mono-oxo-bridged Fe ions and alternate terminal Fe-oxo and Fe-hydroxo groups that interact via intramolecular hydrogen bonding. The implications of an open-core intermediate Q on the reaction mechanism of sMMO are discussed.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/jacs.1c01180DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8154522PMC
May 2021

Comment on "Structural evidence for a dynamic metallocofactor during N reduction by Mo-nitrogenase".

Science 2021 02;371(6530)

Chemistry and Biochemistry, Utah State University, Logan, UT 84322, USA.

Kang (Reports, 19 June 2020, p. 1381) report a structure of the nitrogenase MoFe protein that is interpreted to indicate binding of N or an N-derived species to the active-site FeMo cofactor. Independent refinement of the structure and consideration of biochemical evidence do not support this claim.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1126/science.abe5481DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7931246PMC
February 2021

The Asp1 pyrophosphatase from S. pombe hosts a [2Fe-2S] cluster in vivo.

J Biol Inorg Chem 2021 02 5;26(1):93-108. Epub 2021 Feb 5.

Institut Für Physikalische Biologie, Heinrich-Heine-Universität Düsseldorf, Universitätsstr. 1, 40225, Düsseldorf, Germany.

The Schizosaccharomyces pombe Asp1 protein is a bifunctional kinase/pyrophosphatase that belongs to the highly conserved eukaryotic diphosphoinositol pentakisphosphate kinase PPIP5K/Vip1 family. The N-terminal Asp1 kinase domain generates specific high-energy inositol pyrophosphate (IPP) molecules, which are hydrolyzed by the C-terminal Asp1 pyrophosphatase domain (Asp1). Thus, Asp1 activities regulate the intracellular level of a specific class of IPP molecules, which control a wide number of biological processes ranging from cell morphogenesis to chromosome transmission. Recently, it was shown that chemical reconstitution of Asp1 leads to the formation of a [2Fe-2S] cluster; however, the biological relevance of the cofactor remained under debate. In this study, we provide evidence for the presence of the Fe-S cluster in Asp1 inside the cell. However, we show that the Fe-S cluster does not influence Asp1 pyrophosphatase activity in vitro or in vivo. Characterization of the as-isolated protein by electronic absorption spectroscopy, mass spectrometry, and X-ray absorption spectroscopy is consistent with the presence of a [2Fe-2S] cluster in the enzyme. Furthermore, we have identified the cysteine ligands of the cluster. Overall, our work reveals that Asp1 contains an Fe-S cluster in vivo that is not involved in its pyrophosphatase activity.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1007/s00775-020-01840-wDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8038993PMC
February 2021

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

Angew Chem Int Ed Engl 2021 04 23;60(18):10112-10121. Epub 2021 Mar 23.

Department of Inorganic Spectroscopy, Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, 45470, Mülheim an der Ruhr, Germany.

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 3d 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.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/anie.202015669DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8252016PMC
April 2021

Preparation and spectroscopic characterization of lyophilized Mo nitrogenase.

J Biol Inorg Chem 2021 02 30;26(1):81-91. Epub 2020 Dec 30.

Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, 45470, Mülheim an der Ruhr, Germany.

Mo nitrogenase is the primary source of biologically fixed nitrogen, making this system highly interesting for developing new, energy efficient ways of ammonia production. Although heavily investigated, studies of the active site of this enzyme have generally been limited to spectroscopic methods that are compatible with the presence of water and relatively low protein concentrations. One method of overcoming this limitation is through lyophilization, which allows for measurements to be performed on solvent free, high concentration samples. This method also has the potential for allowing efficient protein storage and solvent exchange. To investigate the viability of this preparatory method with Mo nitrogenase, we employ a combination of electron paramagnetic resonance, Mo and Fe K-edge X-ray absorption spectroscopy, and acetylene reduction assays. Our results show that while some small distortions in the metallocofactors occur, oxidation and spin states are maintained through the lyophilization process and that reconstitution of either lyophilized protein component into buffer restores acetylene reducing activity.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1007/s00775-020-01838-4DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8038959PMC
February 2021

Organometallic Synthesis of Bimetallic Cobalt-Rhodium Nanoparticles in Supported Ionic Liquid Phases (Co Rh @SILP) as Catalysts for the Selective Hydrogenation of Multifunctional Aromatic Substrates.

Small 2021 Feb 21;17(5):e2006683. Epub 2020 Dec 21.

Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, Mülheim an der Ruhr, 45470, Germany.

The synthesis, characterization, and catalytic properties of bimetallic cobalt-rhodium nanoparticles of defined Co:Rh ratios immobilized in an imidazolium-based supported ionic liquid phase (Co Rh @SILP) are described. Following an organometallic approach, precise control of the Co:Rh ratios is accomplished. Electron microscopy and X-ray absorption spectroscopy confirm the formation of small, well-dispersed, and homogeneously alloyed zero-valent bimetallic nanoparticles in all investigated materials. Benzylideneacetone and various bicyclic heteroaromatics are used as chemical probes to investigate the hydrogenation performances of the Co Rh @SILP materials. The Co:Rh ratio of the nanoparticles is found to have a critical influence on observed activity and selectivity, with clear synergistic effects arising from the combination of the noble metal and its 3d congener. In particular, the ability of Co Rh @SILP catalysts to hydrogenate 6-membered aromatic rings is found to experience a remarkable sharp switch in a narrow composition range between Co Rh (full ring hydrogenation) and Co Rh (no ring hydrogenation).
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/smll.202006683DOI Listing
February 2021

Ligand Protonation Triggers H Release from a Dinickel Dihydride Complex to Give a Doubly "T"-Shaped Dinickel(I) Metallodiradical.

Angew Chem Int Ed Engl 2021 Jan 24;60(4):1891-1896. Epub 2020 Nov 24.

Universität Göttingen, Institut für Anorganische Chemie, Tammannstrasse 4, 37077, Göttingen, Germany.

The dinickel(II) dihydride complex (1 ) of a pyrazolate-based compartmental ligand with β-diketiminato (nacnac) chelate arms (L ), providing two pincer-type {N } binding pockets, has been reported to readily eliminate H and to serve as a masked dinickel(I) species. Discrete dinickel(I) complexes (2 , 2 ) of L are now synthesized via a direct reduction route. They feature two adjacent T-shaped metalloradicals that are antiferromagnetically coupled, giving an S=0 ground state. The two singly occupied local type magnetic orbitals are oriented into the bimetallic cleft, enabling metal-metal cooperative 2 e substrate reductions as shown by the rapid reaction with H or O . X-ray crystallography reveals distinctly different positions of the K in 1 and 2 , suggesting a stabilizing interaction of K with the dihydride unit in 1 . H release from 1 is triggered by peripheral γ-C protonation at the nacnac subunits, which DFT calculations show lowers the barrier for reductive H elimination from the bimetallic cleft.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/anie.202011494DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7894169PMC
January 2021

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

Inorg Chem 2020 Sep 6;59(18):13551-13560. Epub 2020 Sep 6.

Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, D-45470 Mülheim an der Ruhr, Germany.

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.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.inorgchem.0c01931DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7509839PMC
September 2020

Spectroscopy and scattering for chemistry: new possibilities and challenges with large scale facilities.

Nanoscale 2020 Sep;12(35):17968-17970

Center for Hybrid Nanostructures (CHyN), Institute of Nanostructure and Solid State Physics, University of Hamburg, 22761 Hamburg, Germany.

View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/d0nr90182bDOI Listing
September 2020

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

Nanoscale 2020 Jul 13;12(28):15185-15192. Epub 2020 Jul 13.

Max Planck Institute for Chemical Energy Conversion, Stiftstr. 34-36, D-45470 Mülheim an der Ruhr, Germany.

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-AlO. Following the reductive activation, it was found that the NiCoO/γ-AlO catalyst required a relatively higher temperature compared to the monometallic NiO/γ-AlO 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.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/d0nr01960gDOI Listing
July 2020

In Situ X-ray Microscopy Reveals Particle Dynamics in a NiCo Dry Methane Reforming Catalyst under Operating Conditions.

ACS Catal 2020 Jun 1;10(11):6223-6230. Epub 2020 May 1.

Max Planck Institute for Chemical Energy Conversion, Stiftstraße 34-36, Mülheim an der Ruhr D-45470, Germany.

Herein, we report the synthesis of a γ-AlO-supported NiCo catalyst for dry methane reforming (DMR) and study the catalyst using in situ scanning transmission X-ray microscopy (STXM) during the reduction (activation step) and under reaction conditions. During the reduction process, the NiCo alloy particles undergo elemental segregation with Co migrating toward the center of the catalyst particles and Ni migrating to the outer surfaces. Under DMR conditions, the segregated structure is maintained, thus hinting at the importance of this structure to optimal catalytic functions. Finally, the formation of Ni-rich branches on the surface of the particles is observed during DMR, suggesting that the loss of Ni from the outer shell may play a role in the reduced stability and hence catalyst deactivation. These findings provide insights into the morphological and electronic structural changes that occur in a NiCo-based catalyst during DMR. Further, this study emphasizes the need to study catalysts under operating conditions in order to elucidate material dynamics during the reaction.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acscatal.9b05517DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7295368PMC
June 2020

Dual Role of Silver Moieties Coupled with Ordered Mesoporous Cobalt Oxide towards Electrocatalytic Oxygen Evolution Reaction.

Angew Chem Int Ed Engl 2020 Sep 21;59(38):16544-16552. Epub 2020 Jul 21.

Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470, Mülheim an der Ruhr, Germany.

Herein, we show that the performance of mesostructured cobalt oxide electrocatalyst for oxygen evolution reaction (OER) can be significantly enhanced by coupling of silver species. Various analysis techniques including pair distribution function and Rietveld refinement, X-ray absorption spectroscopy at synchrotron as well as advanced electron microscopy revealed that silver exists as metallic Ag particles and well-dispersed Ag O nanoclusters within the mesostructure. The benefits of this synergy are twofold for OER: highly conductive metallic Ag improves the charge transfer ability of the electrocatalysts while ultra-small Ag O clusters provide the centers that can uptake Fe impurities from KOH electrolyte and boost the catalytic efficiency of Co-Ag oxides. The current density of mesostructured Co O at 1.7 V is increased from 102 to 211 mA cm with incorporation of silver spices. This work presents the dual role of silver moieties and demonstrates a simple method to increase the OER activity of Co O .
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/anie.202003801DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7540465PMC
September 2020

Caught in the H : Crystal Structure and Spectroscopy Reveal a Sulfur Bound to the Active Site of an O -stable State of [FeFe] Hydrogenase.

Angew Chem Int Ed Engl 2020 09 23;59(38):16786-16794. Epub 2020 Jul 23.

Department of Inorganic Spectroscopy, Max Planck Institute for Chemical Energy Conversion, Stiftstraße 34-36, 45470, Mülheim an der Ruhr, Germany.

[FeFe] hydrogenases are the most active H converting catalysts in nature, but their extreme oxygen sensitivity limits their use in technological applications. The [FeFe] hydrogenases from sulfate reducing bacteria can be purified in an O -stable state called H . To date, the structure and mechanism of formation of H remain unknown. Our 1.65 Å crystal structure of this state reveals a sulfur ligand bound to the open coordination site. Furthermore, in-depth spectroscopic characterization by X-ray absorption spectroscopy (XAS), nuclear resonance vibrational spectroscopy (NRVS), resonance Raman (RR) spectroscopy and infrared (IR) spectroscopy, together with hybrid quantum mechanical and molecular mechanical (QM/MM) calculations, provide detailed chemical insight into the H state and its mechanism of formation. This may facilitate the design of O -stable hydrogenases and molecular catalysts.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/anie.202005208DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7540559PMC
September 2020

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

Inorg Chem 2020 Jun 11;59(12):8272-8283. Epub 2020 May 11.

Max Planck Institute for Chemical Energy Conversion, Stiftstraße 34-36, 45470 Mülheim an der Ruhr, Germany.

Ruthenium 4d-to-2p X-ray emission spectroscopy (XES) was systematically explored for a series of Ru and Ru 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.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.inorgchem.0c00663DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7298721PMC
June 2020

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

Angew Chem Int Ed Engl 2020 07 29;59(31):12965-12975. Epub 2020 May 29.

Department of Inorganic Spectroscopy, Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, 45470, Mülheim an der Ruhr, Germany.

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 Kβ 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.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/anie.202003621DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7496169PMC
July 2020

The Spectroscopy of Nitrogenases.

Chem Rev 2020 06 2;120(12):5005-5081. Epub 2020 Apr 2.

Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, 45470 Mülheim an der Ruhr, Germany.

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 N to 2NH. 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.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.chemrev.9b00650DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7318057PMC
June 2020

Mechanism of L-edge x-ray magnetic circular dichroism intensity from quantum chemical calculations and experiment-A case study on V/V complexes.

J Chem Phys 2020 Mar;152(11):114107

Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany.

In this work, we present a combined experimental and theoretical study on the V L-edge x-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD) spectra of VO(acac) and V(acac) prototype complexes. The recorded V L-edge XAS and XMCD spectra are richly featured in both V L and L spectral regions. In an effort to predict and interpret the nature of the experimentally observed spectral features, a first-principles approach for the simultaneous prediction of XAS and XMCD spectra in the framework of wavefunction based ab initio methods is presented. The theory used here has previously been formulated for predicting optical absorption and MCD spectra. In the present context, it is applied to the prediction of the V L-edge XAS and XMCD spectra of the VO(acac) and V(acac) complexes. In this approach, the spin-free Hamiltonian is computed on the basis of the complete active space configuration interaction (CASCI) in conjunction with second order N-electron valence state perturbation theory (NEVPT2) as well as the density functional theory (DFT)/restricted open configuration interaction with singles configuration state functions based on a ground state Kohn-Sham determinant (ROCIS/DFT). Quasi-degenerate perturbation theory is then used to treat the spin-orbit coupling (SOC) operator variationally at the many particle level. The XAS and XMCD transitions are computed between the relativistic many particle states, considering their respective Boltzmann populations. These states are obtained from the diagonalization of the SOC operator along with the spin and orbital Zeeman operators. Upon averaging over all possible magnetic field orientations, the XAS and XMCD spectra of randomly oriented samples are obtained. This approach does not rely on the validity of low-order perturbation theory and provides simultaneous access to the calculation of XMCD A, B, and C terms. The ability of the method to predict the XMCD C-term signs and provide access to the XMCD intensity mechanism is demonstrated on the basis of a generalized state coupling mechanism based on the type of the excitations dominating the relativistically corrected states. In the second step, the performance of CASCI, CASCI/NEVPT2, and ROCIS/DFT is evaluated. The very good agreement between theory and experiment has allowed us to unravel the complicated XMCD C-term mechanism on the basis of the SOC interaction between the various multiplets with spin S' = S, S ± 1. In the last step, it is shown that the commonly used spin and orbital sum rules are inadequate in interpreting the intensity mechanism of the XAS and XMCD spectra of the VO(acac) and V(acac) complexes as they breakdown when they are employed to predict their magneto-optical properties. This conclusion is expected to hold more generally.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1063/1.5129029DOI Listing
March 2020

Resolving the structure of the E state of Mo nitrogenase through Mo and Fe K-edge EXAFS and QM/MM calculations.

Chem Sci 2019 Nov 4;10(42):9807-9821. Epub 2019 Sep 4.

Max-Planck Institute for Chemical Energy Conversion , Stiftstrasse 34-36 , 45470 Mülheim an der Ruhr , NRW , Germany . Email: ; Email:

Biological nitrogen fixation is predominately accomplished through Mo nitrogenase, which utilizes a complex MoFeSC catalytic cluster to reduce N to NH. This cluster requires the accumulation of three to four reducing equivalents prior to binding N; however, despite decades of research, the intermediate states formed prior to N binding are still poorly understood. Herein, we use Mo and Fe K-edge X-ray absorption spectroscopy and QM/MM calculations to investigate the nature of the E state, which is formed following the addition of the first reducing equivalent to Mo nitrogenase. By analyzing the extended X-ray absorption fine structure (EXAFS) region, we provide structural insight into the changes that occur in the metal clusters of the protein when forming the E state, and use these metrics to assess a variety of possible models of the E state. The combination of our experimental and theoretical results supports that formation of E involves an Fe-centered reduction combined with the protonation of a belt-sulfide of the cluster. Hence, these results provide critical experiment and computational insight into the mechanism of this important enzyme.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1039/c9sc02187fDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6984330PMC
November 2019

Spectroscopic and Computational Evidence that [FeFe] Hydrogenases Operate Exclusively with CO-Bridged Intermediates.

J Am Chem Soc 2020 01 30;142(1):222-232. Epub 2019 Dec 30.

Max Planck Institute for Chemical Energy Conversion , Stiftstrasse 34-36 , 45470 Mülheim an der Ruhr , Germany.

[FeFe] hydrogenases are extremely active H-converting enzymes. Their mechanism remains highly controversial, in particular, the nature of the one-electron and two-electron reduced intermediates called HH and HH. In one model, the HH and HH states contain a semibridging CO, while in the other model, the bridging CO is replaced by a bridging hydride. Using low-temperature IR spectroscopy and nuclear resonance vibrational spectroscopy, together with density functional theory calculations, we show that the bridging CO is retained in the HH and HH states in the [FeFe] hydrogenases from and , respectively. Furthermore, there is no evidence for a bridging hydride in either state. These results agree with a model of the catalytic cycle in which the HH and HH states are integral, catalytically competent components. We conclude that proton-coupled electron transfer between the two subclusters is crucial to catalysis and allows these enzymes to operate in a highly efficient and reversible manner.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/jacs.9b09745DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6956316PMC
January 2020

An Adaptable N-Heterocyclic Carbene Macrocycle Hosting Copper in Three Oxidation States.

Angew Chem Int Ed Engl 2020 Mar 24;59(14):5696-5705. Epub 2020 Jan 24.

Institute of Inorganic Chemistry, University of Göttingen, Tammannstrasse 4, 37077, Göttingen, Germany.

A neutral hybrid macrocycle with two trans-positioned N-heterocyclic carbenes (NHCs) and two pyridine donors hosts copper in three oxidation states (+I-+III) in a series of structurally characterized complexes (1-3). Redox interconversion of [LCu] is electrochemically (quasi)reversible and occurs at moderate potentials (E =-0.45 V and +0.82 V (vs. Fc/Fc )). A linear C -Cu-C arrangement and hemilability of the two pyridine donors allows the ligand to adapt to the different stereoelectronic and coordination requirements of Cu versus Cu /Cu . Analytical methods such as NMR, UV/Vis, IR, electron paramagnetic resonance, and Cu Kβ high-energy-resolution fluorescence detection X-ray absorption spectroscopies, as well as DFT calculations, give insight into the geometric and electronic structures of the complexes. The XAS signatures of 1-3 are textbook examples for Cu , Cu , and Cu species. Facile 2-electron interconversion combined with the exposure of two basic pyridine N sites in the reduced Cu form suggest that [LCu] may operate in catalysis via coupled 2 e /2 H transfer.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1002/anie.201912745DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7154638PMC
March 2020

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

Inorg Chem 2019 Dec 19;58(23):16292-16301. Epub 2019 Nov 19.

Max Planck Institute for Chemical Energy Conversion , Stiftstrasse 34-36 , D-45470 Mülheim an der Ruhr , Germany.

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 Kβ 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 MnCaO-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.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.inorgchem.9b02866DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6891804PMC
December 2019

High-resolution iron X-ray absorption spectroscopic and computational studies of non-heme diiron peroxo intermediates.

J Inorg Biochem 2020 02 22;203:110877. Epub 2019 Oct 22.

Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, D-45470 Mülheim an der Ruhr, Germany. Electronic address:

Ferritin-like carboxylate-bridged non-heme diiron enzymes activate O for a variety of difficult reactions throughout nature. These reactions often begin by abstraction of hydrogen from strong CH bonds. The enzymes activate O at their diferrous cofactors to form canonical diferric peroxo intermediates, with a range of possible coordination modes. Herein, we explore the ability of high-energy resolution fluorescence detected X-ray absorption spectroscopy (HERFD XAS) to provide insight into the nature of peroxo level intermediates in non-heme diiron proteins. Freeze quenched (FQ) peroxo intermediates from p-aminobenzoate N-oxygenase (AurF), aldehyde-deformylating oxygenase (ADO), and the β subunit of class Ia ribonucleotide reductase from Escherichia coli (Ecβ) are investigated. All three intermediates are proposed to adopt different peroxo binding modes, and each exhibit different Fe Kα HERFD XAS pre-edge features and intensities. As these FQ-trapped samples consist of multiple species, deconvolution of HERFD XAS spectra based on speciation, as determined by Mössbauer spectroscopy, is also necessitated - yielding 'pure' diferric peroxo HERFD XAS spectra from dilute protein samples. Finally, the impact of a given peroxo coordination mode on the HERFD XAS pre-edge energy and intensity is evaluated through time-dependent density functional theory (TDDFT) calculations of the XAS spectra on a series of hypothetical model complexes, which span a full range of possible peroxo coordination modes to a diferric core. The utility of HERFD XAS for future studies of enzymatic intermediates is discussed.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1016/j.jinorgbio.2019.110877DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7012765PMC
February 2020

Planar three-coordinate iron sulfide in a synthetic [4Fe-3S] cluster with biomimetic reactivity.

Nat Chem 2019 11 14;11(11):1019-1025. Epub 2019 Oct 14.

Department of Chemistry, Yale University, New Haven, CT, USA.

Iron-sulfur clusters are emerging as reactive sites for the reduction of small-molecule substrates. However, the four-coordinate iron sites of typical iron-sulfur clusters rarely react with substrates, implicating three-coordinate iron. This idea is untested because fully sulfide-coordinated three-coordinate iron is unprecedented. Here we report a new type of [4Fe-3S] cluster that features an iron centre with three bonds to sulfides, and characterize examples of the cluster in three oxidation levels using crystallography, spectroscopy, and ab initio calculations. Although a high-spin electronic configuration is characteristic of other iron-sulfur clusters, the three-coordinate iron centre in these clusters has a surprising low-spin electronic configuration due to the planar geometry and short Fe-S bonds. In a demonstration of biomimetic reactivity, the [4Fe-3S] cluster reduces hydrazine, a natural substrate of nitrogenase. The product is the first example of NH bound to an iron-sulfur cluster. Our results demonstrate that three-coordinate iron supported by sulfide donors is a plausible precursor to reactivity in iron-sulfur clusters like the FeMoco of nitrogenase.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1038/s41557-019-0341-7DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6858550PMC
November 2019

Spectroscopic X-ray and Mössbauer Characterization of M and M Iron(Molybdenum)-Carbonyl Carbide Clusters: High Carbide-Iron Covalency Enhances Local Iron Site Electron Density Despite Cluster Oxidation.

Inorg Chem 2019 Oct 25;58(19):12918-12932. Epub 2019 Sep 25.

Department of Inorganic Spectroscopy , Max Planck Institute for Chemical Energy Conversion , Stiftstrasse 34-36 , D-45470 , Mülheim an der Ruhr , Germany.

The present study employs a suite of spectroscopic techniques to evaluate the electronic and bonding characteristics of the interstitial carbide in a set of iron-carbonyl-carbide clusters, one of which is substituted with a molybdenum atom. The MC and MC clusters are the dianions (EtN)[(μ-C)(μ-CO)(CO)] (), [K(benzo-18-crown-6)][(μ-C)(μ-CO)(CO)] (), and [K(benzo-18-crown-6)][(μ-C)(μ-CO)(CO)] (). Because and have the same overall cluster charge (2-) but different numbers of iron sites (: 6 sites → : 5 sites), the metal atoms of are formally oxidized compared to those in . Despite this, Mössbauer studies indicate that the iron sites in possess (lower spectroscopic oxidation state) compared with those in . Iron K-edge X-ray absorption and valence-to-core X-ray emission spectroscopy measurements, paired with density functional theory spectral calculations, revealed the presence of significant metal-to-metal and carbide 2p-based character in the filled valence and low-lying unfilled electronic manifolds. In all of the above experiments, the presence of the molybdenum atom in (FeMo) results in somewhat unremarkable spectroscopic properties that are essentially a "hybrid" of (Fe) and (Fe). The overall electronic portrait that emerges illustrates that the central inorganic carbide ligand is essential for distributing charge and maximizing electronic communication throughout the cluster. It is evident that the carbide coordination environment is quite flexible and adaptive: it can drastically modify the covalency of individual Fe-C bonds based on local structural changes and redox manipulation of the clusters. In light of these findings, our data and calculations suggest a potential role for the central carbon atom in FeMoco, which likely performs a similar function in order to maintain cluster integrity through multiple redox and ligand binding events.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acs.inorgchem.9b01870DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6784818PMC
October 2019

Elucidation of Structure-Activity Correlations in a Nickel Manganese Oxide Oxygen Evolution Reaction Catalyst by Operando Ni L-Edge X-ray Absorption Spectroscopy and 2p3d Resonant Inelastic X-ray Scattering.

ACS Appl Mater Interfaces 2019 Oct 8;11(42):38595-38605. Epub 2019 Oct 8.

Max Planck Institute for Chemical Energy Conversion , Stiftstraße 34-36 , Mülheim an der Ruhr 45470 , Germany.

Herein, we report the synthesis and electrochemical oxygen evolution experiments for a graphene-supported NiMnO catalyst. The changes that occur at the Ni active sites during the electrocatalyic oxygen evolution reaction (OER) were elucidated by a combination of operando Ni L-edge X-ray absorption spectroscopy (XAS) and Ni 2p3d resonant inelastic X-ray scattering (RIXS). These data are compared to reference measurements on NiO, β-Ni(OH), β-NiOOH, and γ-NiOOH. Through this comparative analysis, we are able to show that under alkaline conditions (0.1 M KOH), the oxides of the NiMnO catalyst are converted to hydroxides. At the onset of catalysis (1.47 V), the β-Ni(OH)-like phase is oxidized and converted to a dominantly γ-NiOOH phase. The present study thus challenges the notion that the β-NiOOH phase is the active phase in OER and provides further evidence that the γ-NiOOH phase is catalytically active. The ability to use Ni L-edge XAS and 2p3d RIXS to provide a rational basis for structure-activity correlations is highlighted.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/acsami.9b06752DOI Listing
October 2019
-->