Prof. Dr. Ulf-Peter Apfel

Inorganic Chemistry
Ruhr-Universität Bochum


  • A dinuclear porphyrin-macrocycle as efficient catalyst for the hydrogen evolution reaction
    Jökel, J. and Schwer, F. and Von Delius, M. and Apfel, U.-P.
    Chemical Communications 56 (2020)
    We report an unprecedented dinuclear catalyst for the electrochemical hydrogen evolution reaction (HER). A macrocyclic porphyrin complex comprising two nickel centres connected via redox mediating linker molecules gives rise to efficient catalysis, significantly outperforming a mononuclear reference catalyst. This journal is © The Royal Society of Chemistry.
    view abstract10.1039/d0cc05229a
  • Assessing the Influence of Supercritical Carbon Dioxide on the Electrochemical Reduction to Formic Acid Using Carbon-Supported Copper Catalysts
    Junge Puring, K. and Evers, O. and Prokein, M. and Siegmund, D. and Scholten, F. and Mölders, N. and Renner, M. and Roldan Cuenya, B. and Petermann, M. and Weidner, E. and Apfel, U.-P.
    ACS Catalysis 10 (2020)
    The electrocatalytic reduction of carbon dioxide (CO2) by means of renewable energies is widely recognized as a promising approach to establish a sustainable closed carbon cycle economy. However, widespread application is hampered by the inherent difficulty in suppressing the hydrogen evolution reaction and controlling the overall process selectivity. Further critical parameters are the limited solubility of CO2 in many electrolytes and its hindered mass transport to the electrodes. Herein we report on a series of nanoparticle Cu electrocatalysts on different carbon supports and their potential to perform the electrochemical CO2 reduction under supercritical conditions (scCO2). Herein, CO2 serves as the reaction medium and reactant alike. By a detailed comparison to ambient conditions we show that scCO2 conditions largely suppress the undesirable hydrogen evolution and favor the production of formic acid by the Cu electrodes. Furthermore, we show that scCO2 conditions significantly prevent Cu nanoparticle agglomeration during electrocatalysis. © 2020 American Chemical Society.
    view abstract10.1021/acscatal.0c02983
  • Catalytically Active Iron(IV)oxo Species Based on a Bis(pyridinyl)phenanthrolinylmethane
    Hüppe, H.M. and Keisers, K. and Fink, F. and Mürtz, S.D. and Hoffmann, A. and Iffland, L. and Apfel, U.-P. and Herres-Pawlis, S.
    Israel Journal of Chemistry 60 (2020)
    Starting from the mononuclear iron(II) complex [Fe(MeCPy2Phen)(MeCN)2]2+, a non-heme Fe(IV)oxo complex [FeIV(MeCPy2Phen)O]2+ was synthesized via oxidation with meta-chloroperoxybenzoic acid (mCPBA). The Fe(IV)oxo complex was characterized using UV/Vis spectroscopy, Mößbauer spectroscopy and CSI mass spectrometry. The ability of this species to oxidize C−H bonds was tested with cyclohexane and adamantane as model substrates. For cyclohexane, an alcohol-to-ketone ratio (A/K) of 6.1 and efficiencies up to 55 were obtained. In case of adamantane, the ratio of tertiary over secondary products (3°/2°) is 38. In combination, this indicates the iron(IV)oxo complex being the catalytically active species. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA
    view abstract10.1002/ijch.202000009
  • Dual-Heteroatom-Doped Reduced Graphene Oxide Sheets Conjoined CoNi-Based Carbide and Sulfide Nanoparticles for Efficient Oxygen Evolution Reaction
    Zakaria, M.B. and Zheng, D. and Apfel, U.-P. and Nagata, T. and Kenawy, E.-R.S. and Lin, J.
    ACS Applied Materials and Interfaces 12 (2020)
    Intensive research is being conducted into highly efficient and cheap nanoscale materials for the electrocatalytic oxidation of water. In this context, we built heterostructures of multilayered CoNi-cyanide bridged coordination (CoNi-CP) nanosheets and graphene oxide (GO) sheets (CoNi-CP/GO) as a source for heterostructured functional electrodes. The layered CoNi-CP/GO hybrid components heated in nitrogen gas (N2) at 450 °C yield CoNi-based carbide (CoNi-C) through thermal decomposition of CoNi-CP, while GO is converted into reduced GO (rGO) to finally form a CoNi-C/rGO-450 composite. The CoNi-C/rGO-450 composite shows a reasonable efficiency for oxygen evolution reaction (OER) through water oxidations in alkaline solution. Meanwhile, regulated annealing of CoNi-CP/GO in N2 with thiourea at 450 and 550 °C produces CoNi-based sulfide (CoNi-S) rather than CoNi-C between rGO sheets co-doped by nitrogen (N) and sulfur (S) heteroatoms (NS-rGO) to form CoNi-S/NS-rGO-450 and CoNi-S/NS-rGO-550 composites, respectively. The CoNi-S/NS-rGO-550 shows the best efficiency for electrocatalytic OER among all electrodes with an overpotential of 290 mV at 10 mA cm-2 and a Tafel slope of 79.5 mV dec-1. By applying the iR compensation to remove resistance of the solution (2.1 ω), the performance is further improved to achieve a current density of 10 mA cm-2 at an overpotential of 274 mV with a Tafel slope of 70.5 mV dec-1. This result is expected to be a promising electrocatalyst compared to the currently used electrocatalysts and a step for fuel cell applications in the future. © 2020 American Chemical Society.
    view abstract10.1021/acsami.0c06141
  • Electrocatalytic Reduction of CO2 to Acetic Acid by a Molecular Manganese Corrole Complex
    De, R. and Gonglach, S. and Paul, S. and Haas, M. and Sreejith, S.S. and Gerschel, P. and Apfel, U.-P. and Vuong, T.H. and Rabeah, J. and Roy, S. and Schöfberger, W.
    Angewandte Chemie - International Edition 59 (2020)
    The controlled electrochemical reduction of carbon dioxide to value added chemicals is an important strategy in terms of renewable energy technologies. Therefore, the development of efficient and stable catalysts in an aqueous environment is of great importance. In this context, we focused on synthesizing and studying a molecular MnIII-corrole complex, which is modified on the three meso-positions with polyethylene glycol moieties for direct and selective production of acetic acid from CO2. Electrochemical reduction of MnIII leads to an electroactive MnII species, which binds CO2 and stabilizes the reduced intermediates. This catalyst allows to electrochemically reduce CO2 to acetic acid in a moderate acidic aqueous medium (pH 6) with a selectivity of 63 % and a turn over frequency (TOF) of 8.25 h−1, when immobilized on a carbon paper (CP) electrode. In terms of high selectivity towards acetate, we propose the formation and reduction of an oxalate type intermediate, stabilized at the MnIII-corrole center. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.202000601
  • Electrochemical CO2 and Proton Reduction by a Co(dithiacyclam) Complex
    Iffland, L. and Siegmund, D. and Apfel, U.-P.
    Zeitschrift fur Anorganische und Allgemeine Chemie 646 (2020)
    While [Ni(cyclam)]2+ and [Ni(dithiacyclam)]2+ complexes were shown to be potent electrocatalysts for the CO2 conversion, their respective Co complexes hitherto received only little attention. Herein, we report on the CoII complexes of the cyclam and dithiacyclam platform, describe their synthesis and reveal their rich solvent dependent coordination chemistry. We show that sulfur implementation into the cyclam moiety leads to a switch from a low spin CoII complex in [Co(cyclam)]2+ to a high spin form in [Co(dithiacyclam)]2+. Notably, while both complexes are capable to perform the reduction of CO2 to CO, H2 formation is generally preferred. Along this line, the complexes were shown to enable proton reduction from acetic acid. However, in comparison to [Co(cyclam)]2+, the altered electronics make [Co(dithiacyclam)]2+ complexes prone to deposit on the glassy carbon working electrode over time leading to an overall low faradaic efficiency for the reduction of protons or CO2. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/zaac.201900356
  • Electrochemical CO2 Reduction-The Effect of Chalcogenide Exchange in Ni-Isocyclam Complexes
    Gerschel, P. and Battistella, B. and Siegmund, D. and Ray, K. and Apfel, U.-P.
    Organometallics 39 (2020)
    Among the numerous homogeneous electrochemical CO2 reduction catalysts, [Ni(cyclam)]2+ is known as one of the most potent catalysts. Likewise, [Ni(isocyclam)]2+ was reported to enable electrochemical CO2 conversion but has received significantly less attention. However, for both catalysts, a purposeful substitution of a single nitrogen donor group by chalcogen atoms was never reported. In this work, we report a series of isocyclam-based Ni complexes with {ON3}, {SN3}, {SeN3}, and {N4} moieties and investigated the influence of nitrogen/chalcogen substitution on electrochemical CO2 reduction. While [Ni(isocyclam)]2+ showed the highest selectivity toward CO2 reduction within this series with a Faradaic efficiency of 86% for the generation of CO at an overpotential of-1.20 V and acts as a homogeneous catalyst, the O-and S-containing Ni complexes revealed comparable catalytic activities at ca. 0.3 V milder overpotential but tend to form deposits on the electrode, acting as precursors for a heterogeneous catalysis. Moreover, the heterogeneous species generated from the O-and S-containing complexes enable a catalytic hydride transfer to acetonitrile, resulting in the generation of acetaldehyde. The incorporation of selenium, however, resulted in loss of CO2 reduction activity, mainly leading to hydrogen generation that is also catalyzed by a heterogeneous electrodeposit. Copyright © 2020 American Chemical Society.
    view abstract10.1021/acs.organomet.0c00129
  • Enantioselective epoxidation by flavoprotein monooxygenases supported by organic solvents
    Eggerichs, D. and Mügge, C. and Mayweg, J. and Apfel, U.-P. and Tischler, D.
    Catalysts 10 (2020)
    Styrene and indole monooxygenases (SMO and IMO) are two-component flavoprotein monooxygenases composed of a nicotinamide adenine dinucleotide (NADH)-dependent flavin adenine dinucleotide (FAD)-reductase (StyB or IndB) and a monooxygenase (StyA or IndA). The latter uses reduced FAD to activate oxygen and to oxygenate the substrate while releasing water. We circumvented the need for the reductase by direct FAD reduction in solution using the NAD(P)H-mimic 1-benzyl-1,4-dihydronicotinamide (BNAH) to fuel monooxygenases without NADH requirement. Herein, we report on the hitherto unknown solvent tolerance for the indole monooxygenase from Gemmobacter nectariphilus DSM15620 (GnIndA) and the styrene monooxygenase from Gordonia rubripertincta CWB2 (GrStyA). These enzymes were shown to convert bulky and rather hydrophobic styrene derivatives in the presence of organic cosolvents. Subsequently, BNAH-driven biotransformation was furthermore optimized with regard to the applied cosolvent and its concentration as well as FAD and BNAH concentration. We herein demonstrate that GnIndA and GrStyA enable selective epoxidations of allylic double bonds (up to 217 mU mg–1) in the presence of organic solvents such as tetrahydrofuran, acetonitrile, or several alcohols. Notably, GnIndA was found to resist methanol concentrations up to 25 vol.%. Furthermore, a diverse substrate preference was determined for both enzymes, making their distinct use very interesting. In general, our results seem representative for many IMOs as was corroborated by in silico mutagenetic studies. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/catal10050568
  • Enhancing the CO2 Electroreduction of Fe/Ni-Pentlandite Catalysts by S/Se Exchange
    Pellumbi, K. and Smialkowski, M. and Siegmund, D. and Apfel, U.-P.
    Chemistry - A European Journal 26 (2020)
    The electrochemical reduction of CO2 is an attractive strategy towards the mitigation of environmental pollution and production of bulk chemicals as well as fuels by renewables. The bimetallic sulfide Fe4.5Ni4.5S8 (pentlandite) was recently reported as a cheap and robust catalyst for electrochemical water splitting, as well as for CO2 reduction with a solvent-dependent product selectivity. Inspired by numerous reports on monometallic sulfoselenides and selenides revealing higher catalytic activity for the CO2 reduction reaction (CO2RR) than their sulfide counterparts, the authors investigated the influence of stepwise S/Se exchange in seleno-pentlandites Fe4.5Ni4.5S8-YSeY (Y=1–5) and their ability to act as CO2 reducing catalysts. It is demonstrated that the incorporation of higher equivalents of selenium favors the CO2RR with Fe4.5Ni4.5S4Se4 revealing the highest activity for CO formation. Under galvanostatic conditions in acetonitrile, Fe4.5Ni4.5S4Se4 generates CO with a Faradaic Efficiency close to 100 % at applied current densities of −50 mA cm−2 and −100 mA cm−2. This work offers insight into the tunability of the pentlandite based electrocatalysts for the CO2 reduction reaction. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/chem.202001289
  • Homolytic versus Heterolytic Hydrogen Evolution Reaction Steered by a Steric Effect
    Guo, X. and Wang, N. and Li, X. and Zhang, Z. and Zhao, J. and Ren, W. and Ding, S. and Xu, G. and Li, J. and Apfel, U.-P. and Zhang, W. and Cao, R.
    Angewandte Chemie - International Edition 59 (2020)
    Several H−H bond forming pathways have been proposed for the hydrogen evolution reaction (HER). Revealing these HER mechanisms is of fundamental importance for the rational design of catalysts and is also extremely challenging. Now, an unparalleled example of switching between homolytic and heterolytic HER mechanisms is reported. Three nickel(II) porphyrins were designed and synthesized with distinct steric effects by introducing bulky amido moieties to ortho- or para-positions of the meso-phenyl groups. These porphyrins exhibited different catalytic HER behaviors. For these Ni porphyrins, although their 1e-reduced forms are active to reduce trifluoroacetic acid, the resulting Ni hydrides (depending on the steric effects of porphyrin rings) have different pathways to make H2. Understanding HER processes, especially controllable switching between homolytic and heterolytic H−H bond formation pathways through molecular engineering, is unprecedented in electrocatalysis. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.202002311
  • Interplay of Spin Crossover and Coordination-Induced Spin State Switch for Iron Bis(pyrazolyl)methanes in Solution
    Keisers, K. and Hüppe, H.M. and Iffland-Mühlhaus, L. and Hoffmann, A. and Göbel, C. and Apfel, U.-P. and Weber, B. and Herres-Pawlis, S.
    Inorganic Chemistry 59 (2020)
    Bis(pyrazolyl)bipyridinylmethane iron(II) complexes show a versatile spin state switching behavior in different solvents. In the solid, the magnetic properties of the compounds have been characterized by X-ray diffraction, Mößbauer spectroscopy, and SQUID magnetometry and point toward a high spin state. For nitrilic solvents, the solvation of the complexes leads to a change of the coordination environment from {N5O} to {N6} and results in a temperature-dependent SCO behavior. Thermodynamic properties of this transformation are obtained via UV/vis spectroscopy, SQUID measurements, and the Evans NMR method. Moreover, a coordination-induced spin state switch (CISSS) to low spin is observed by using methanol as solvent, triggered through a rearrangement of the coordination sphere. The same behavior can be observed by changing the stoichiometry of the ligand-to-metal ratio in MeCN, where the process is reversible. This transformation is monitored via UV/vis spectroscopy, and the resulting new bis-meridional coordination motif, first described for bis(pyrazolyl)methanes, is characterized in the solid state via X-ray diffraction, Mößbauer spectroscopy, and SQUID measurements. The sophisticated correlation of these switchable properties in dependence on different types of solvents reveals that the influence of the solvent on the coordination environment and magnetic properties should not be underestimated. Furthermore, careful investigation is necessary to differentiate between a thermally-induced spin crossover and a coordination-induced spin state switch. © 2020 American Chemical Society.
    view abstract10.1021/acs.inorgchem.0c02306
  • Metal-Rich Chalcogenides as Sustainable Electrocatalysts for Oxygen Evolution and Reduction: State of the Art and Future Perspectives
    Amin, H.M.A. and Apfel, U.-P.
    European Journal of Inorganic Chemistry 2020 (2020)
    The rational design of high-performance and cost-effective electrocatalysts is a key for the development of sustainable energy systems such as electrolyzers, fuel cells and metal-air batteries. Although water splitting and fuel cells are commercially mature technologies, they are still limited on large scale primarily due to the abundancy of the currently utilized expensive materials as well as the sluggish kinetics of the underlaying reactions, oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), and thus the required large observed overpotentials. Therefore, an efficient inexpensive catalyst is necessary. In the last decade, metal chalcogenides have been attractive materials in electrocatalysis of OER and ORR. Herein, we provide an overview on the recent advances on particularly metal-rich chalcogenides such as heazlewoodite- and pentlandite-types including their electrochemical activities and OER mechanisms. Likewise, examples of state-of-the-art metal chalcogenides revealing bifunctional activity for both OER and ORR are also presented. Diverse strategies to improve the catalytic performance are discussed and current challenges and future perspectives towards further development in this field are addressed. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/ejic.202000406
  • Metal-Rich Chalcogenides for Electrocatalytic Hydrogen Evolution: Activity of Electrodes and Bulk Materials
    Siegmund, D. and Blanc, N. and Smialkowski, M. and Tschulik, K. and Apfel, U.-P.
    ChemElectroChem 7 (2020)
    Metal-rich chalcogenides composed of highly abundant elements recently emerged as promising catalysts for the electrocatalytic hydrogen evolution reaction (HER). Many of these materials benefit from a high intrinsic conductivity as compared to their chalcogen-rich congeners, greatly reducing the necessity for conductive additives or sophisticated nanostructuring. Herein, we showcase the high potential of metal-rich transition-metal chalcogenides for the electrocatalytic hydrogen formation by summarizing the recent progress achieved with M9S8 (pentlandite type) and M3S2 (heazlewoodite type) based materials, which represent the most frequently applied compositions for this purpose. By a detailed electrochemical comparison of bulk as well as pellet electrodes of metal-rich Fe4.5Ni4.5S8, we also aim at raising awareness in the community for the inherent differences in catalytic properties of the materials themselves and those of the fabricated electrodes, a point that is often disregarded in reports on HER-catalyst systems. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/celc.201902125
  • New phosphorous-based [fefe]-hydrogenase models
    Wittkamp, F. and Boydas, E.B. and Roemelt, M. and Apfel, U.-P.
    Catalysts 10 (2020)
    [FeFe]-hydrogenases have attracted research for more than twenty years as paragons for the design of new catalysts for the hydrogen evolution reaction (HER). The bridging dithiolate comprising a secondary amine as bridgehead is the key element for the reactivity of native [FeFe]hydrogenases and was therefore the midpoint of hundreds of biomimetic hydrogenase models. However, within those mimics, phosphorous is barely seen as a central element in the azadithiolato bridge despite being the direct heavier homologue of nitrogen. We herein synthesized three new phosphorous based [FeFe]-hydrogenase models by reacting dithiols (HSCH2)2P(O)R (R = Me, OEt, OPh) with Fe3(CO)12. All synthesized mimics show catalytic reactivity regarding HER and change their mechanisms depending on the strength of the used acid. In all presented mimics, the oxide is the center of reactivity, independent of the nature of the bridgehead. However, the phosphorous atom might be reduced by the methods we present herein to alter the reactivity of the model compounds towards protons and oxygen. © 2020 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/catal10050522
  • Plasmachemical Trace-Oxygen Removal in a Coke Oven Gas with a Coaxial Packed-Bed-DBD Reactor
    Nitsche, T. and Budt, M. and Apfel, U.-P.
    Chemie-Ingenieur-Technik 92 (2020)
    The trace-O2 removal in coke oven gas, which enables better utilization of its contained H2, is investigated with combinations of atmospheric nonthermal plasma and a Pt/γ-Al2O3 catalyst. Herein it is shown that a coaxial packed-bed dielectric barrier discharge (DBD) reactor removes up to 80 % O2 in a model coke oven gas. Along this line, the H2 content and the usage of Al2O3 granules in the plasma zone have been identified as major factors for the plasmachemical trace-O2 conversion. In contrast to the Pt/γ-Al2O3 catalyst, nonthermal plasma converts trace O2 at coke oven gas temperatures below 100 °C. © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstract10.1002/cite.202000052
  • Powering Artificial Enzymatic Cascades with Electrical Energy
    Al-Shameri, A. and Petrich, M.-C. and junge Puring, K. and Apfel, U.-P. and Nestl, B.M. and Lauterbach, L.
    Angewandte Chemie - International Edition 59 (2020)
    We have developed a scalable platform that employs electrolysis for an in vitro synthetic enzymatic cascade in a continuous flow reactor. Both H2 and O2 were produced by electrolysis and transferred through a gas-permeable membrane into the flow system. The membrane enabled the separation of the electrolyte from the biocatalysts in the flow system, where H2 and O2 served as electron mediators for the biocatalysts. We demonstrate the production of methylated N-heterocycles from diamines with up to 99 % product formation as well as excellent regioselective labeling with stable isotopes. Our platform can be applied for a broad panel of oxidoreductases to exploit electrical energy for the synthesis of fine chemicals. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/anie.202001302
  • Shedding Light on Proton and Electron Dynamics in [FeFe] Hydrogenases
    Lorent, C. and Katz, S. and Duan, J. and Kulka, C.J. and Caserta, G. and Teutloff, C. and Yadav, S. and Apfel, U.-P. and Winkler, M. and Happe, T. and Horch, M. and Zebger, I.
    Journal of the American Chemical Society 142 (2020)
    [FeFe] hydrogenases are highly efficient catalysts for reversible dihydrogen evolution. H2 turnover involves different catalytic intermediates including a recently characterized hydride state of the active site (H-cluster). Applying cryogenic infrared and electron paramagnetic resonance spectroscopy to an [FeFe] model hydrogenase from Chlamydomonas reinhardtii (CrHydA1), we have discovered two new hydride intermediates and spectroscopic evidence for a bridging CO ligand in two reduced H-cluster states. Our study provides novel insights into these key intermediates, their relevance for the catalytic cycle of [FeFe] hydrogenase, and novel strategies for exploring these aspects in detail. Copyright © 2020 American Chemical Society.
    view abstract10.1021/jacs.9b13075
  • Sustainable and rapid preparation of nanosized Fe/Ni-pentlandite particles by mechanochemistry
    Tetzlaff, D. and Pellumbi, K. and Baier, D.M. and Hoof, L. and Shastry Barkur, H. and Smialkowski, M. and Amin, H.M.A. and Grätz, S. and Siegmund, D. and Borchardt, L. and Apfel, U.-P.
    Chemical Science 11 (2020)
    In recent years, metal-rich sulfides of the pentlandite type (M9S8) have attracted considerable attention for energy storage applications. However, common synthetic routes towards pentlandites either involve energy intensive high temperature procedures or solvothermal methods with specialized precursors and non-sustainable organic solvents. Herein, we demonstrate that ball milling is a simple and efficient method to synthesize nanosized bimetallic pentlandite particles (Fe4.5Ni4.5S8, Pn) with an average size of ca. 250 nm in a single synthetic step from elemental- or sulfidic mixtures. We herein highlight the effects of the milling ball quantity, precursor types and milling time on the product quality. Along this line, Raman spectroscopy as well as temperature/pressure monitoring during the milling processes provide valuable insights into mechanistic differences between the mechanochemical Pn-formation. By employing the obtained Pn-nanosized particles as cathodic electrocatalysts for water splitting in a zero-gap PEM electrolyzer we provide a comprehensive path for a potential sustainable future process involving non-noble metal catalysts. © 2020 The Royal Society of Chemistry.
    view abstract10.1039/d0sc04525j
  • Tailoring the Size, Inversion Parameter, and Absorption of Phase-Pure Magnetic MgFe2O4Nanoparticles for Photocatalytic Degradations
    Bloesser, A. and Kurz, H. and Timm, J. and Wittkamp, F. and Simon, C. and Hayama, S. and Weber, B. and Apfel, U.-P. and Marschall, R.
    ACS Applied Nano Materials 3 (2020)
    Phase-pure magnesium ferrite (MgFe2O4) spinel nanocrystals are synthesized by a fast microwave-assisted route. The elemental composition is optimized via the ratio of the precursor mixture and controlled by energy-dispersive X-ray spectroscopy. Fine-tuning of the magnetic properties without changing the overall elemental composition is demonstrated by superconducting quantum interference device (SQUID) magnetometry and Mössbauer spectroscopy. Together with X-ray absorption spectroscopy and X-ray emission spectroscopy, we confirm that the degree of cation inversion is altered by thermal annealing. We can correlate the magnetic properties with both the nanosize influence and the degree of inversion. The resulting nonlinear course of saturation magnetization (Ms) in correlation with the particle diameter allows to decouple crystallite size and saturation magnetization, by this providing a parameter for the production of very small nanoparticles with high Ms with great potential for magnetic applications like ferrofluids or targeted drug delivery. Our results also suggest that the optical band gap of MgFe2O4 is considerably larger than the fundamental electronic band gap because of the d5 electronic configuration of the iron centers. The presented different electronic transitions contributing to the absorption of visible light are the explanation for the large dissent among the band gaps and band potentials found in the literature. © 2020 American Chemical Society.
    view abstract10.1021/acsanm.0c02705
  • The effect of flue gas contaminants on the CO2electroreduction to formic acid
    Legrand, U. and Apfel, U.-P. and Boffito, D.C. and Tavares, J.R.
    Journal of CO2 Utilization 42 (2020)
    Contaminants in an electrochemical cell converting CO2 to formic acid can lead to the deactivation of cathode catalysts through several pathways, causing severe performance loss over time. Potential contaminants from flue gas emissions of principal fossil fuels include N2, O2, H2O, CO, NO2, SO2, particulate matter and hydrocarbons. Contaminant effects on the CO2 to formic acid electroreduction are scarcely covered in the literature. We describe in the present study these effects based on catalysts reported for the electroreduction of CO2 to formic acid, focusing principally on copper, tin and lead in the two most popular configurations, 2 and 3 compartment cells. Water solubility, metal affinity through chemisorption, known chemical reactions and altered electrochemical activities are the main focus of this review. We herein highlight that O2, SO2 and particulate matter have especially detrimental effects. While O2 can be efficiently removed from flue gas, additional treatment to remove SO2 and particulate matter is required. Our conclusions should raise interest in experimentally validating the effect of such contaminants. © 2020 Elsevier Ltd.
    view abstract10.1016/j.jcou.2020.101315
  • Water-Soluble Polymers with Appending Porphyrins as Bioinspired Catalysts for the Hydrogen Evolution Reaction
    Xie, L. and Tian, J. and Ouyang, Y. and Guo, X. and Zhang, W. and Apfel, U.-P. and Zhang, W. and Cao, R.
    Angewandte Chemie - International Edition 59 (2020)
    Molecular design to improve catalyst performance is significant but challenging. In enzymes, residue groups that are close to reaction centers play critical roles in regulating activities. Using this bioinspired strategy, three water-soluble polymers were designed with appending Co porphyrins and different side-chain groups to mimic enzyme reaction centers and activity-controlling residue groups, respectively. With these polymers, high hydrogen evolution efficiency was achieved in neutral aqueous media for electro- (turnover frequency >2.3×104 s−1) and photocatalysis (turnover number >2.7×104). Porphyrin units are surrounded and protected by polymer chains, and more importantly, the activity can be tuned with different side-chain groups. Therefore, instead of revising molecular structures that is difficult from both design and synthesis points of view, polymers can be used to improve molecular solubility and stability and simultaneously regulate activity by using side-chain groups. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.202003836
  • Bio-inspired design: Bulk iron-nickel sulfide allows for efficient solvent-dependent CO 2 reduction
    Piontek, S. and Junge Puring, K. and Siegmund, D. and Smialkowski, M. and Sinev, I. and Tetzlaff, D. and Roldan Cuenya, B. and Apfel, U.-P.
    Chemical Science 10 (2019)
    The electrocatalytic reduction of carbon dioxide (CO 2 RR) to valuable bulk chemicals is set to become a vital factor in the prevention of environmental pollution and the selective storage of sustainable energy. Inspired by structural analogues to the active site of the enzyme CODH Ni , we envisioned that bulk Fe/Ni sulfides would enable the efficient reduction of CO 2 . By careful adjustment of the process conditions, we demonstrate that pentlandite (Fe 4.5 Ni 4.5 S 8 ) electrodes, in addition to HER, also support the CO 2 RR reaching a peak faradaic efficiency of 87% and 13% for the formation of CO and methane, respectively at 3 mA cm -2 . The choice of solvent, the presence of water/protons and CO 2 solubility are identified as key-properties to adjust the balance between HER and CO 2 RR in favour of the latter. Such experiments can thus serve as model reactions to elucidate a potential catalyst within gas diffusion electrodes. © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c8sc03555e
  • Bioinspired reactivity and coordination chemistry
    Apfel, U.-P. and Suntharalingam, K.
    Dalton Transactions 48 (2019)
    view abstract10.1039/c9dt90082a
  • Differential Protonation at the Catalytic Six-Iron Cofactor of [FeFe]-Hydrogenases Revealed by 57 Fe Nuclear Resonance X-ray Scattering and Quantum Mechanics/Molecular Mechanics Analyses
    Mebs, S. and Duan, J. and Wittkamp, F. and Stripp, S.T. and Happe, T. and Apfel, U.-P. and Winkler, M. and Haumann, M.
    Inorganic Chemistry (2019)
    [FeFe]-hydrogenases are efficient biological hydrogen conversion catalysts and blueprints for technological fuel production. The relations between substrate interactions and electron/proton transfer events at their unique six-iron cofactor (H-cluster) need to be elucidated. The H-cluster comprises a four-iron cluster, [4Fe4S], linked to a diiron complex, [FeFe]. We combined 57 Fe-specific X-ray nuclear resonance scattering experiments (NFS, nuclear forward scattering; NRVS, nuclear resonance vibrational spectroscopy) with quantum-mechanics/molecular-mechanics computations to study the [FeFe]-hydrogenase HYDA1 from a green alga. Selective 57 Fe labeling at only [4Fe4S] or [FeFe], or at both subcomplexes was achieved by protein expression with a 57 Fe salt and in vitro maturation with a synthetic diiron site precursor containing 57 Fe. H-cluster states were populated under infrared spectroscopy control. NRVS spectral analyses facilitated assignment of the vibrational modes of the cofactor species. This approach revealed the H-cluster structure of the oxidized state (Hox) with a bridging carbon monoxide at [FeFe] and ligand rearrangement in the CO-inhibited state (Hox-CO). Protonation at a cysteine ligand of [4Fe4S] in the oxidized state occurring at low pH (HoxH) was indicated, in contrast to bridging hydride binding at [FeFe] in a one-electron reduced state (Hred). These findings are direct evidence for differential protonation either at the four-iron or diiron subcomplex of the H-cluster. NFS time-traces provided Mössbauer parameters such as the quadrupole splitting energy, which differ among cofactor states, thereby supporting selective protonation at either subcomplex. In combination with data for reduced states showing similar [4Fe4S] protonation as HoxH without (Hred′) or with (Hhyd) a terminal hydride at [FeFe], our results imply that coordination geometry dynamics at the diiron site and proton-coupled electron transfer to either the four-iron or the diiron subcomplex discriminate catalytic and regulatory functions of [FeFe]-hydrogenases. We support a reaction cycle avoiding diiron site geometry changes during rapid H 2 turnover. © 2019 American Chemical Society.
    view abstract10.1021/acs.inorgchem.9b00100
  • FexNi9-xS8 (x = 3-6) as potential photocatalysts for solar-driven hydrogen production?
    Tetzlaff, D. and Simon, C. and Achilleos, D.S. and Smialkowski, M. and Junge Puring, K. and Bloesser, A. and Piontek, S. and Kasap, H. and Siegmund, D. and Reisner, E. and Marschall, R. and Apfel, U.-P.
    Faraday Discussions 215 (2019)
    The efficient reduction of protons by non-noble metals under mild conditions is a challenge for our modern society. Nature utilises hydrogenases, enzymatic machineries that comprise iron- and nickel- containing active sites, to perform the conversion of protons to hydrogen. We herein report a straightforward synthetic pathway towards well-defined particles of the bio-inspired material FexNi9-xS8, a structural and functional analogue of hydrogenase metal sulfur clusters. Moreover, the potential of pentlandites to serve as photocatalysts for solar-driven H2-production is assessed for the first time. The FexNi9-xS8 materials are visible light responsive (band gaps between 2.02 and 2.49 eV, depending on the pentlandite's Fe:Ni content) and display a conduction band energy close to the thermodynamic potential for proton reduction. Despite the limited driving force, a modest activity for photocatalytic H2 has been observed. Our observations show the potential for the future development of pentlandites as photocatalysts. This work provides a basis to explore powerful synergies between biomimetic chemistry and material design to unlock novel applications in solar energy conversion. © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c8fd00173a
  • Geometry of the Catalytic Active Site in [FeFe]-Hydrogenase Is Determined by Hydrogen Bonding and Proton Transfer
    Duan, J. and Mebs, S. and Laun, K. and Wittkamp, F. and Heberle, J. and Happe, T. and Hofmann, E. and Apfel, U.-P. and Winkler, M. and Senger, M. and Haumann, M. and Stripp, S.T.
    ACS Catalysis 9 (2019)
    [FeFe]-hydrogenases are efficient metalloenzymes that catalyze the oxidation and evolution of molecular hydrogen, H2. They serve as a blueprint for the design of synthetic H2-forming catalysts. [FeFe]-hydrogenases harbor a six-iron cofactor that comprises a [4Fe-4S] cluster and a unique diiron site with cyanide, carbonyl, and hydride ligands. To address the ligand dynamics in catalytic turnover and upon carbon monoxide (CO) inhibition, we replaced the native aminodithiolate group of the diiron site by synthetic dithiolates, inserted into wild-type and amino acid variants of the [FeFe]-hydrogenase HYDA1 from Chlamydomonas reinhardtii. The reactivity with H2 and CO was characterized using in situ and transient infrared spectroscopy, protein crystallography, quantum chemical calculations, and kinetic simulations. All cofactor variants adopted characteristic populations of reduced species in the presence of H2 and showed significant changes in CO inhibition and reactivation kinetics. Differences were attributed to varying interactions between polar ligands and the dithiolate headgroup and/or the environment of the cofactor (i.e., amino acid residues and water molecules). The presented results show how catalytically relevant intermediates are stabilized by inner-sphere hydrogen bonding suggesting that the role of the aminodithiolate group must not be restricted to proton transfer. © 2019 American Chemical Society.
    view abstract10.1021/acscatal.9b02203
  • How [FeFe]-Hydrogenase Facilitates Bidirectional Proton Transfer
    Senger, M. and Eichmann, V. and Laun, K. and Duan, J. and Wittkamp, F. and Knör, G. and Apfel, U.-P. and Happe, T. and Winkler, M. and Heberle, J. and Stripp, S.T.
    Journal of the American Chemical Society 141 (2019)
    Hydrogenases are metalloenzymes that catalyze the conversion of protons and molecular hydrogen, H2. [FeFe]-hydrogenases show particularly high rates of hydrogen turnover and have inspired numerous compounds for biomimetic H2 production. Two decades of research on the active site cofactor of [FeFe]-hydrogenases have put forward multiple models of the catalytic proceedings. In comparison, our understanding of proton transfer is poor. Previously, residues were identified forming a hydrogen-bonding network between active site cofactor and bulk solvent; however, the exact mechanism of catalytic proton transfer remained inconclusive. Here, we employ in situ infrared difference spectroscopy on the [FeFe]-hydrogenase from Chlamydomonas reinhardtii evaluating dynamic changes in the hydrogen-bonding network upon photoreduction. While proton transfer appears to be impaired in the oxidized state (Hox), the presented data support continuous proton transfer in the reduced state (Hred). Our analysis allows for a direct, molecular unique assignment to individual amino acid residues. We found that transient protonation changes of glutamic acid residue E141 and, most notably, arginine R148 facilitate bidirectional proton transfer in [FeFe]-hydrogenases. © 2019 American Chemical Society.
    view abstract10.1021/jacs.9b09225
  • Insights from 125Te and 57Fe nuclear resonance vibrational spectroscopy: A [4Fe-4Te] cluster from two points of view
    Wittkamp, F. and Mishra, N. and Wang, H. and Wille, H.-C. and Steinbrügge, R. and Kaupp, M. and Cramer, S.P. and Apfel, U.-P. and Pelmenschikov, V.
    Chemical Science 10 (2019)
    Iron-sulfur clusters are common building blocks for electron transport and active sites of metalloproteins. Their comprehensive investigation is crucial for understanding these enzymes, which play important roles in modern biomimetic catalysis and biotechnology applications. We address this issue by utilizing (Et4N)3[Fe4Te4(SPh)4], a tellurium modified version of a conventional reduced [4Fe-4S]+ cluster, and performed both 57Fe- and 125Te-NRVS to reveal its characteristic vibrational features. Our analysis exposed major differences in the resulting 57Fe spectrum profile as compared to that of the respective [4Fe-4S] cluster, and between the 57Fe and 125Te profiles. DFT calculations are applied to rationalize structural, electronic, vibrational, and redox-dependent properties of the [4Fe-4Te]+ core. We herein highlight the potential of sulfur/tellurium exchange as a method to isolate the iron-only motion in enzymatic systems. © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c9sc02025j
  • Loss of Specific Active-Site Iron Atoms in Oxygen-Exposed [FeFe]-Hydrogenase Determined by Detailed X-ray Structure Analyses
    Esselborn, J. and Kertess, L. and Apfel, U.-P. and Hofmann, E. and Happe, T.
    Journal of the American Chemical Society 141 (2019)
    The [FeFe]-hydrogenases catalyze the uptake and evolution of hydrogen with unmatched speed at low overpotential. However, oxygen induces the degradation of the unique [6Fe-6S] cofactor within the active site, termed the H-cluster. We used X-ray structural analyses to determine possible modes of irreversible oxygen-driven inactivation. To this end, we exposed crystals of the [FeFe]-hydrogenase CpI from Clostridium pasteurianum to oxygen and quantitatively investigated the effects on the H-cluster structure over several time points using multiple data sets, while correlating it to decreases in enzyme activity. Our results reveal the loss of specific Fe atoms from both the diiron (2FeH) and the [4Fe-4S] subcluster (4FeH) of the H-cluster. Within the 2FeH, the Fe atom more distal to the 4FeH is strikingly more affected than the more proximal Fe atom. The 4FeH interconverts to a [2Fe-2S] cluster in parts of the population of active CpIADT, but not in crystals of the inactive apoCpI initially lacking the 2FeH. We thus propose two parallel processes: dissociation of the distal Fe atom and 4FeH interconversion. Both pathways appear to play major roles in the oxidative damage of [FeFe]-hydrogenases under electron-donor deprived conditions probed by our experimental setup. Copyright © 2019 American Chemical Society.
    view abstract10.1021/jacs.9b07808
  • Molecular cobalt corrole complex for the heterogeneous electrocatalytic reduction of carbon dioxide
    Gonglach, S. and Paul, S. and Haas, M. and Pillwein, F. and Sreejith, S.S. and Barman, S. and De, R. and Müllegger, S. and Gerschel, P. and Apfel, U.-P. and Coskun, H. and Aljabour, A. and Stadler, P. and Schöfberger, W. and Roy, S.
    Nature Communications 10 (2019)
    Electrochemical conversion of CO2 to alcohols is one of the most challenging methods of conversion and storage of electrical energy in the form of high-energy fuels. The challenge lies in the catalyst design to enable its real-life implementation. Herein, we demonstrate the synthesis and characterization of a cobalt(III) triphenylphosphine corrole complex, which contains three polyethylene glycol residues attached at the meso-phenyl groups. Electron-donation and therefore reduction of the cobalt from cobalt(III) to cobalt(I) is accompanied by removal of the axial ligand, thus resulting in a square-planar cobalt(I) complex. The cobalt(I) as an electron-rich supernucleophilic d8-configurated metal centre, where two electrons occupy and fill up the antibonding dz 2 orbital. This orbital possesses high affinity towards electrophiles, allowing for such electronically configurated metals reactions with carbon dioxide. Herein, we report the potential dependent heterogeneous electroreduction of CO2 to ethanol or methanol of an immobilized cobalt A3-corrole catalyst system. In moderately acidic aqueous medium (pH = 6.0), the cobalt corrole modified carbon paper electrode exhibits a Faradaic Efficiency (FE%) of 48 % towards ethanol production. © 2019, The Author(s).
    view abstract10.1038/s41467-019-11868-5
  • Ni-Metalloid (B, Si, P, As, and Te) Alloys as Water Oxidation Electrocatalysts
    Masa, J. and Piontek, S. and Wilde, P. and Antoni, H. and Eckhard, T. and Chen, Y.-T. and Muhler, M. and Apfel, U.-P. and Schuhmann, W.
    Advanced Energy Materials 9 (2019)
    Breakthroughs toward effective water-splitting electrocatalysts for mass hydrogen production will necessitate material design strategies based on unexplored material chemistries. Herein, Ni-metalloid (B, Si, P, As, Te) alloys are reported as an emergent class of highly promising electrocatalysts for the oxygen evolution reaction (OER) and insight is offered into the origin of activity enhancement on the premise of the surface electronic structure, the OER activation energy, influence of the guest metalloid elements on the lattice structure of the host metal (Ni), and surface-oxidized metalloid oxoanions. The metalloids modify the lattice structure of Ni, causing changes in the nearest Ni–Ni interatomic distance (dNi–Ni). The activation energy Ea scales with dNi–Ni indicating an apparent dependence of the OER activity on lattice properties. During the OER, surface Ni atoms are oxidized to nickel oxyhydroxide, which is the active state of the catalyst, meanwhile, the surface metalloids are oxidized to the corresponding oxoanions that affect the interfacial electrode/electrolyte properties and hence the adsorption/desorption interaction energies of the reacting species. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/aenm.201900796
  • Seleno-analogues of pentlandites (Fe4.5Ni4.5S8-: YSey, y = 1-6): Tuning bulk Fe/Ni sulphoselenides for hydrogen evolution
    Smialkowski, M. and Siegmund, D. and Pellumbi, K. and Hensgen, L. and Antoni, H. and Muhler, M. and Apfel, U.-P.
    Chemical Communications 55 (2019)
    We herein present a series of hitherto unprecedented seleno-pentlandites (Fe4.5Ni4.5S8-YSeY). By analysing the influence of S/Se exchange on the catalyst structure and activity in the electrochemical hydrogen evolution reaction we herein showcase the potential and limitations of homologous S/Se exchanges within pentlandite HER catalysts. © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c9cc01842e
  • Solvent-Controlled CO 2 Reduction by a Triphos-Iron Hydride Complex
    Iffland, L. and Khedkar, A. and Petuker, A. and Lieb, M. and Wittkamp, F. and Van Gastel, M. and Roemelt, M. and Apfel, U.-P.
    Organometallics 38 (2019)
    The selective reduction of CO 2 is of high interest toward future applications as a C1-building block. Therefore, metal complexes that allow for the formation of specific CO 2 reduction products under distinct reaction conditions are necessary. A detailed understanding of the CO 2 reduction pathways on a molecular level is, however, required to help in designing catalytic platforms for efficient CO 2 conversion with specific product formation. Reported herein is a unique example of a solvent-controlled reduction of CO 2 using a Triphos-based iron hydride complex. In THF, CO 2 reduction selectively leads to CO formation, whereas experiments in acetonitrile exclusively afford formate, HCOO - . In order to explain the experimental findings, theoretical calculations of the reaction pathways were performed and further demonstrate the importance of the applied solvent for a selective reduction of CO 2 . © Copyright © 2019 American Chemical Society.
    view abstract10.1021/acs.organomet.8b00711
  • Sulfur substitution in a Ni(cyclam) derivative results in lower overpotential for CO2 reduction and enhanced proton reduction
    Gerschel, P. and Warm, K. and Farquhar, E.R. and Englert, U. and Reback, M.L. and Siegmund, D. and Ray, K. and Apfel, U.-P.
    Dalton Transactions 48 (2019)
    The replacement of the opposing nitrogen atoms in 1,4,8,11-tetraazacyclotetradecane (cyclam) with two sulfur atoms in 1,8-dithia-4,11-diazacyclotetradecane (dithiacyclam) enables the electrochemical reduction of protons and CO2via the corresponding nickel(ii) complex at more positive potentials. In addition, a 10-fold enhancement in the proton reduction rate of [Ni(dithiacyclam)]2+ relative to [Ni(cylcam)]2+ was observed. The study provides vital insight into Nature's choice of employing predominantly sulfur based ligand platforms in achieving biological proton and CO2 reductions. © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c8dt04740e
  • Synthetic approaches to artificial photosynthesis: General discussion
    Aitchison, C.M. and Andrei, V. and Antón-García, D. and Apfel, U.-P. and Badiani, V. and Beller, M. and Bocarsly, A.B. and Bonnet, S. and Brueggeller, P. and Caputo, C.A. and Cassiola, F. and Clausing, S.T. and Cooper, A.I. and Creissen, C.E. and De La Peña O'Shea, V.A. and Domcke, W. and Durrant, J.R. and Grätzel, M. and Hammarström, L. and Hankin, A. and Hatzell, M.C. and Karadas, F. and König, B. and Kuehnel, M.F. and Lamaison, S. and Lin, C.-Y. and Maneiro, M. and Minteer, S.D. and Paris, A.R. and Pastor, E. and Pornrungroj, C. and Reek, J.N.H. and Reisner, E. and Roy, S. and Sahm, C. and Shankar, R. and Shaw, W.J. and Shylin, S.I. and Smith, W.A. and Sokol, K. and Soo, H.S. and Sprick, R.S. and Viertl, W. and Vogel, A. and Wagner, A. and Wakerley, D. and Wang, Q. and Wielend, D. and Zwijnenburg, M.A.
    Faraday Discussions 215 (2019)
    view abstract10.1039/C9FD90024A
  • 27. Bioorganik-Nachwuchssymposium in Bochum
    Merten, C. and Apfel, U.-P.
    Nachrichten aus der Chemie 66 (2018)
    view abstract10.1002/nadc.20184082053
  • Cobalt-metalloid alloys for electrochemical oxidation of 5-hydroxymethylfurfural as an alternative anode reaction in lieu of oxygen evolution during water splitting
    Weidner, J. and Barwe, S. and Sliozberg, K. and Piontek, S. and Masa, J. and Apfel, U.-P. and Schuhmann, W.
    Beilstein Journal of Organic Chemistry 14 (2018)
    The electrochemical water splitting commonly involves the cathodic hydrogen and anodic oxygen evolution reactions (OER). The oxygen evolution reaction is more energetically demanding and kinetically sluggish and represents the bottleneck for a commercial competitiveness of electrochemical hydrogen production from water. Moreover, oxygen is essentially a waste product of low commercial value since the primary interest is to convert electrical energy into hydrogen as a storable energy carrier. We report on the anodic oxidation of 5-hydroxymethylfurfural (HMF) to afford the more valuable product 2,5-furandicarboxylic acid (FDCA) as a suitable alternative to the oxygen evolution reaction. Notably, HMF oxidation is thermodynamically more favorable than water oxidation and hence leads to an overall improved energy efficiency for H2 production. In addition, contrary to the “waste product O2”, FDCA can be further utilized, e.g., for production of polyethylene 2,5-furandicarboxylate (PEF), a sustainable polymer analog to polyethylene terephthalate (PET) and thus represents a valuable product for the chemical industry with potential large scale use. Various cobalt-metalloid alloys (CoX; X = B, Si, P, Te, As) were investigated as potential catalysts for HMF oxidation. In this series, CoB required 180 mV less overpotential to reach a current density of 55 mA cm−2 relative to OER with the same electrode. Electrolysis of HMF using a CoB modified nickel foam electrode at 1.45 V vs RHE achieved close to 100% selective conversion of HMF to FDCA at 100% faradaic efficiency. © 2018 Weidner et al.
    view abstract10.3762/bjoc.14.121
  • Crystallographic and spectroscopic assignment of the proton transfer pathway in [FeFe]-hydrogenases
    Duan, J. and Senger, M. and Esselborn, J. and Engelbrecht, V. and Wittkamp, F. and Apfel, U.-P. and Hofmann, E. and Stripp, S.T. and Happe, T. and Winkler, M.
    Nature Communications 9 (2018)
    The unmatched catalytic turnover rates of [FeFe]-hydrogenases require an exceptionally efficient proton-transfer (PT) pathway to shuttle protons as substrates or products between bulk water and catalytic center. For clostridial [FeFe]-hydrogenase CpI such a pathway has been proposed and analyzed, but mainly on a theoretical basis. Here, eleven enzyme variants of two different [FeFe]-hydrogenases (CpI and HydA1) with substitutions in the presumptive PT-pathway are examined kinetically, spectroscopically, and crystallographically to provide solid experimental proof for its role in hydrogen-turnover. Targeting key residues of the PT-pathway by site directed mutagenesis significantly alters the pH-activity profile of these variants and in presence of H2 their cofactor is trapped in an intermediate state indicative of precluded proton-transfer. Furthermore, crystal structures coherently explain the individual levels of residual activity, demonstrating e.g. how trapped H2O molecules rescue the interrupted PT-pathway. These features provide conclusive evidence that the targeted positions are indeed vital for catalytic proton-transfer. © 2018, The Author(s).
    view abstract10.1038/s41467-018-07140-x
  • From Enzymes to Functional Materials—Towards Activation of Small Molecules
    Möller, F. and Piontek, S. and Miller, R.G. and Apfel, U.-P.
    Chemistry - A European Journal 24 (2018)
    The design of non-noble metal-containing heterogeneous catalysts for the activation of small molecules is of utmost importance for our society. While nature possesses very sophisticated machineries to perform such conversions, rationally designed catalytic materials are rare. Herein, we aim to raise the awareness of the overall common design and working principles of catalysts incorporating aspects of biology, chemistry, and material sciences. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/chem.201703451
  • Hydrogen and oxygen trapping at the H-cluster of [FeFe]-hydrogenase revealed by site-selective spectroscopy and QM/MM calculations
    Mebs, S. and Kositzki, R. and Duan, J. and Kertess, L. and Senger, M. and Wittkamp, F. and Apfel, U.-P. and Happe, T. and Stripp, S.T. and Winkler, M. and Haumann, M.
    Biochimica et Biophysica Acta - Bioenergetics 1859 (2018)
    [FeFe]-hydrogenases are superior hydrogen conversion catalysts. They bind a cofactor (H-cluster) comprising a four-iron and a diiron unit with three carbon monoxide (CO) and two cyanide (CN−) ligands. Hydrogen (H2) and oxygen (O2) binding at the H-cluster was studied in the C169A variant of [FeFe]-hydrogenase HYDA1, in comparison to the active oxidized (Hox) and CO-inhibited (Hox-CO) species in wildtype enzyme. 57Fe labeling of the diiron site was achieved by in vitro maturation with a synthetic cofactor analogue. Site-selective X-ray absorption, emission, and nuclear inelastic/forward scattering methods and infrared spectroscopy were combined with quantum chemical calculations to determine the molecular and electronic structure and vibrational dynamics of detected cofactor species. Hox reveals an apical vacancy at Fed in a [4Fe4S-2Fe]3 − complex with the net spin on Fed whereas Hox-CO shows an apical CN− at Fed in a [4Fe4S-2Fe(CO)]3 − complex with net spin sharing among Fep and Fed (proximal or distal iron ions in [2Fe]). At ambient O2 pressure, a novel H-cluster species (Hox-O2) accumulated in C169A, assigned to a [4Fe4S-2Fe(O2)]3 − complex with an apical superoxide (O2 −) carrying the net spin bound at Fed. H2 exposure populated the two-electron reduced Hhyd species in C169A, assigned as a [(H)4Fe4S-2Fe(H)]3 − complex with the net spin on the reduced cubane, an apical hydride at Fed, and a proton at a cysteine ligand. Hox-O2 and Hhyd are stabilized by impaired O2 – protonation or proton release after H2 cleavage due to interruption of the proton path towards and out of the active site. © 2017
    view abstract10.1016/j.bbabio.2017.09.003
  • Hydrogenases: Recent developments and future perspectives
    Wittkamp, F. and Senger, M. and Stripp, S.T. and Apfel, U.-P.
    Chemical Communications 54 (2018)
    [FeFe]-Hydrogenases are the most efficient enzymes for catalytic hydrogen turnover. Their H2 production efficiency is hitherto unrivalled. However, functional details of the catalytic machinery and possible modes of application are discussed controversially. The incorporation of synthetically modified cofactors and utilization of semi-artificial enzymes only recently allowed us to shed light on key steps of the catalytic cycle. Herein, we summarize the essential findings regarding the redox chemistry of [FeFe]-hydrogenases and discuss their catalytic hydrogen turnover. We furthermore will give an outlook on potential research activities and exploit the utilization of synthetic cofactor mimics. © 2018 The Royal Society of Chemistry.
    view abstract10.1039/c8cc01275j
  • Influence of the Fe:Ni Ratio and Reaction Temperature on the Efficiency of (FexNi1-x)9S8 Electrocatalysts Applied in the Hydrogen Evolution Reaction
    Piontek, S. and Andronescu, C. and Zaichenko, A. and Konkena, B. and Junge Puring, K. and Marler, B. and Antoni, H. and Sinev, I. and Muhler, M. and Mollenhauer, D. and Roldan Cuenya, B. and Schuhmann, W. and Apfel, U.-P.
    ACS Catalysis 8 (2018)
    Inspired by our recent finding that Fe4.5Ni4.5S8 rock is a highly active electrocatalyst for HER, we set out to explore the influence of the Fe:Ni ratio on the performance of the catalyst. We herein describe the synthesis of (FexNi1-x)9S8 (x = 0-1) along with a detailed elemental composition analysis. Furthermore, using linear sweep voltammetry, we show that the increase in the iron or nickel content, respectively, lowers the activity of the electrocatalyst toward HER. Electrochemical surface area analysis (ECSA) clearly indicates the highest amount of active sites for a Fe:Ni ratio of 1:1 on the electrode surface pointing at an altered surface composition of iron and nickel for the other materials. Specific metal-metal interactions seem to be of key importance for the high electrocatalytic HER activity, which is supported by DFT calculations of several surface structures using the surface energy as a descriptor of catalytic activity. In addition, we show that a temperature increase leads to a significant decrease of the overpotential and gain in HER activity. Thus, we showcase the necessity to investigate the material structure, composition and reaction conditions when evaluating electrocatalysts. © 2017 American Chemical Society.
    view abstract10.1021/acscatal.7b02617
  • Local Surface Structure and Composition Control the Hydrogen Evolution Reaction on Iron Nickel Sulfides
    Bentley, C.L. and Andronescu, C. and Smialkowski, M. and Kang, M. and Tarnev, T. and Marler, B. and Unwin, P.R. and Apfel, U.-P. and Schuhmann, W.
    Angewandte Chemie - International Edition 57 (2018)
    In order to design more powerful electrocatalysts, developing our understanding of the role of the surface structure and composition of widely abundant bulk materials is crucial. This is particularly true in the search for alternative hydrogen evolution reaction (HER) catalysts to replace platinum. We report scanning electrochemical cell microscopy (SECCM) measurements of the (111)-crystal planes of Fe4.5Ni4.5S8, a highly active HER catalyst. In combination with structural characterization methods, we show that this technique can reveal differences in activity arising from even the slightest compositional changes. By probing electrochemical properties at the nanoscale, in conjunction with complementary structural information, novel design principles are revealed for application to rational material synthesis. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.201712679
  • Protonation/reduction dynamics at the [4Fe-4S] cluster of the hydrogen-forming cofactor in [FeFe]-hydrogenases
    Senger, M. and Mebs, S. and Duan, J. and Shulenina, O. and Laun, K. and Kertess, L. and Wittkamp, F. and Apfel, U.-P. and Happe, T. and Winkler, M. and Haumann, M. and Stripp, S.T.
    Physical Chemistry Chemical Physics 20 (2018)
    The [FeFe]-hydrogenases of bacteria and algae are the most efficient hydrogen conversion catalysts in nature. Their active-site cofactor (H-cluster) comprises a [4Fe-4S] cluster linked to a unique diiron site that binds three carbon monoxide (CO) and two cyanide (CN-) ligands. Understanding microbial hydrogen conversion requires elucidation of the interplay of proton and electron transfer events at the H-cluster. We performed real-time spectroscopy on [FeFe]-hydrogenase protein films under controlled variation of atmospheric gas composition, sample pH, and reductant concentration. Attenuated total reflection Fourier-transform infrared spectroscopy was used to monitor shifts of the CO/CN- vibrational bands in response to redox and protonation changes. Three different [FeFe]-hydrogenases and several protein and cofactor variants were compared, including element and isotopic exchange studies. A protonated equivalent (HoxH) of the oxidized state (Hox) was found, which preferentially accumulated at acidic pH and under reducing conditions. We show that the one-electron reduced state Hred′ represents an intrinsically protonated species. Interestingly, the formation of HoxH and Hred′ was independent of the established proton pathway to the diiron site. Quantum chemical calculations of the respective CO/CN- infrared band patterns favored a cysteine ligand of the [4Fe-4S] cluster as the protonation site in HoxH and Hred′. We propose that proton-coupled electron transfer facilitates reduction of the [4Fe-4S] cluster and prevents premature formation of a hydride at the catalytic diiron site. Our findings imply that protonation events both at the [4Fe-4S] cluster and at the diiron site of the H-cluster are important in the hydrogen conversion reaction of [FeFe]-hydrogenases. © 2018 the Owner Societies.
    view abstract10.1039/c7cp04757f
  • Spectroscopical investigations on the redox chemistry of [FeFe]-hydrogenases in the presence of carbon monoxide
    Laun, K. and Mebs, S. and Duan, J. and Wittkamp, F. and Apfel, U.-P. and Happe, T. and Winkler, M. and Haumann, M. and Stripp, S.T.
    Molecules 23 (2018)
    [FeFe]-hydrogenases efficiently catalyzes hydrogen conversion at a unique [4Fe–4S]-[FeFe] cofactor, the so-called H-cluster. The catalytic reaction occurs at the diiron site, while the [4Fe–4S] cluster functions as a redox shuttle. In the oxidized resting state (Hox), the iron ions of the diiron site bind one cyanide (CN−) and carbon monoxide (CO) ligand each and a third carbonyl can be found in the Fe–Fe bridging position (µCO). In the presence of exogenous CO, A fourth CO ligand binds at the diiron site to form the oxidized, CO-inhibited H-cluster (Hox-CO). We investigated the reduced, CO-inhibited H-cluster (Hred´-CO) in this work. The stretching vibrations of the diatomic ligands were monitored by attenuated total reflection Fourier-transform infrared spectroscopy (ATR FTIR). Density functional theory (DFT) at the TPSSh/TZVP level was employed to analyze the cofactor geometry, as well as the redox and protonation state of the H-cluster. Selective 13CO isotope editing, spectro-electrochemistry, and correlation analysis of IR data identified a one-electron reduced, protonated [4Fe–4S] cluster and an apical CN− ligand at the diiron site in Hred´-CO. The reduced, CO-inhibited H-cluster forms independently of the sequence of CO binding and cofactor reduction, which implies that the ligand rearrangement at the diiron site upon CO inhibition is independent of the redox and protonation state of the [4Fe–4S] cluster. The relation of coordination dynamics to cofactor redox and protonation changes in hydrogen conversion catalysis and inhibition is discussed. © 2018 by the authors.
    view abstract10.3390/molecules23071669
  • Trendbericht Anorganische Chemie 2017: Koordinationschemie und Bioanorganik
    Apfel, U.-P. and Demir, S. and Berkefeld, A.
    Nachrichten aus der Chemie 66 (2018)
    view abstract10.1002/nadc.20184071717
  • Accumulating the hydride state in the catalytic cycle of [FeFe]-hydrogenases
    Winkler, M. and Senger, M. and Duan, J. and Esselborn, J. and Wittkamp, F. and Hofmann, E. and Apfel, U.-P. and Stripp, S.T. and Happe, T.
    Nature Communications 8 (2017)
    H2 turnover at the [FeFe]-hydrogenase cofactor (H-cluster) is assumed to follow a reversible heterolytic mechanism, first yielding a proton and a hydrido-species which again is double-oxidized to release another proton. Three of the four presumed catalytic intermediates (Hox, Hred/Hred and Hsred) were characterized, using various spectroscopic techniques. However, in catalytically active enzyme, the state containing the hydrido-species, which is eponymous for the proposed heterolytic mechanism, has yet only been speculated about. We use different strategies to trap and spectroscopically characterize this transient hydride state (Hhyd) for three wild-type [FeFe]-hydrogenases. Applying a novel set-up for real-time attenuated total-reflection Fourier-transform infrared spectroscopy, we monitor compositional changes in the state-specific infrared signatures of [FeFe]-hydrogenases, varying buffer pH and gas composition. We selectively enrich the equilibrium concentration of H hyd, applying Le Chatelier's principle by simultaneously increasing substrate and product concentrations (H2/H+). Site-directed manipulation, targeting either the proton-transfer pathway or the adt ligand, significantly enhances Hhyd accumulation independent of pH. © The Author(s) 2017.
    view abstract10.1038/ncomms16115
  • Anorganische Chemie 2016: Koordinationschemie und Bioanorganik
    Apfel, U.-P. and Berkefeld, A. and Demir, S.
    Nachrichten aus der Chemie 65 (2017)
    view abstract10.1002/nadc.20174059829
  • Bridging Hydride at Reduced H-Cluster Species in [FeFe]-Hydrogenases Revealed by Infrared Spectroscopy, Isotope Editing, and Quantum Chemistry
    Mebs, S. and Senger, M. and Duan, J. and Wittkamp, F. and Apfel, U.-P. and Happe, T. and Winkler, M. and Stripp, S.T. and Haumann, M.
    Journal of the American Chemical Society 139 (2017)
    [FeFe]-Hydrogenases contain a H2-converting cofactor (H-cluster) in which a canonical [4Fe-4S] cluster is linked to a unique diiron site with three carbon monoxide (CO) and two cyanide (CN-) ligands (e.g., in the oxidized state, Hox). There has been much debate whether reduction and hydrogen binding may result in alternative rotamer structures of the diiron site in a single (Hred) or double (Hsred) reduced H-cluster species. We employed infrared spectro-electrochemistry and site-selective isotope editing to monitor the CO/CN- stretching vibrations in [FeFe]-hydrogenase HYDA1 from Chlamydomonas reinhardtii. Density functional theory calculations yielded vibrational modes of the diatomic ligands for conceivable H-cluster structures. Correlation analysis of experimental and computational IR spectra has facilitated an assignment of Hred and Hsred to structures with a bridging hydride at the diiron site. Pronounced ligand rotation during μH binding seems to exclude Hred and Hsred as catalytic intermediates. Only states with a conservative H-cluster geometry featuring a μCO ligand are likely involved in rapid H2 turnover. © 2017 American Chemical Society.
    view abstract10.1021/jacs.7b07548
  • Carbon/Silicon Exchange at the Apex of Diphos- and Triphos-Derived Ligands – More Than Just a Substitute?
    Petuker, A. and Reback, M.L. and Apfel, U.-P.
    European Journal of Inorganic Chemistry 2017 (2017)
    Diphos [R′2C(CH2PR′′2)2] and Triphos [R′C(CH2PR′′2)3] are convenient ligand platforms in catalysis. Likewise, their Si counterparts, [R′2Si(CH2PR′′2)2] and [R′Si(CH2PR′′2)3], are commonly employed as they have easier synthetic protocols towards the desired ligands. Alterations caused by C/Si exchange are often neglected and considered to have no significant influence on the properties of the complex; therefore, the differences are commonly not investigated. We show herein that C/Si backbone exchange does indeed have a significant influence on the stability and reactivity of metal complexes and should not be ignored. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/ejic.201700388
  • Chalcogenide substitution in the [2Fe] cluster of [FeFe]-hydrogenases conserves high enzymatic activity
    Kertess, L. and Wittkamp, F. and Sommer, C. and Esselborn, J. and Rüdiger, O. and Reijerse, E.J. and Hofmann, E. and Lubitz, W. and Winkler, M. and Happe, T. and Apfel, U.-P.
    Dalton Transactions 46 (2017)
    [FeFe]-Hydrogenases efficiently catalyze the uptake and evolution of H2 due to the presence of an inorganic [6Fe-6S]-cofactor (H-cluster). This cofactor is comprised of a [4Fe-4S] cluster coupled to a unique [2Fe] cluster where the catalytic turnover of H2/H+ takes place. We herein report on the synthesis of a selenium substituted [2Fe] cluster [Fe2{μ(SeCH2)2NH}(CO)4(CN)2]2- (ADSe) and its successful in vitro integration into the native protein scaffold of [FeFe]-hydrogenases HydA1 from Chlamydomonas reinhardtii and CpI from Clostridium pasteurianum yielding fully active enzymes (HydA1-ADSe and CpI-ADSe). FT-IR spectroscopy and X-ray structure analysis confirmed the presence of structurally intact ADSe at the active site. Electrochemical assays reveal that the selenium containing enzymes are more biased towards hydrogen production than their native counterparts. In contrast to previous chalcogenide exchange studies, the S to Se exchange herein is not based on a simple reconstitution approach using ionic cluster constituents but on the in vitro maturation with a pre-synthesized selenium-containing [2Fe] mimic. The combination of biological and chemical methods allowed for the creation of a novel [FeFe]-hydrogenase with a [2Fe2Se]-active site which confers individual catalytic features. © 2017 The Royal Society of Chemistry.
    view abstract10.1039/c7dt03785f
  • Electronic and molecular structure relations in diiron compounds mimicking the [FeFe]-hydrogenase active site studied by X-ray spectroscopy and quantum chemistry
    Kositzki, R. and Mebs, S. and Schüth, N. and Leidel, N. and Schwartz, L. and Karnahl, M. and Wittkamp, F. and Daunke, D. and Grohmann, A. and Apfel, U.-P. and Gloaguen, F. and Ott, S. and Haumann, M.
    Dalton Transactions 46 (2017)
    Synthetic diiron compounds of the general formula Fe2(μ-S2R)(CO)n(L)6-n (R = alkyl or aromatic groups; L = CN- or phosphines) are versatile models for the active-site cofactor of hydrogen turnover in [FeFe]-hydrogenases. A series of 18 diiron compounds, containing mostly a dithiolate bridge and terminal ligands of increasing complexity, was characterized by X-ray absorption and emission spectroscopy in combination with density functional theory. Fe K-edge absorption and Kβ main-line emission spectra revealed the varying geometry and the low-spin state of the Fe(i) centers. Good agreement between experimental and calculated core-to-valence-excitation absorption and radiative valence-to-core-decay emission spectra revealed correlations between spectroscopic and structural features and provided access to the electronic configuration. Four main effects on the diiron core were identified, which were preferentially related to variation either of the dithiolate or of the terminal ligands. Alteration of the dithiolate bridge affected mainly the Fe-Fe bond strength, while more potent donor substitution and ligand field asymmetrization changed the metal charge and valence level localization. In contrast, cyanide ligation altered all relevant properties and, in particular, the frontier molecular orbital energies of the diiron core. Mutual benchmarking of experimental and theoretical parameters provides guidelines to verify the electronic properties of related diiron compounds. © 2017 The Royal Society of Chemistry.
    view abstract10.1039/c7dt02720f
  • Interplay between CN- Ligands and the Secondary Coordination Sphere of the H-Cluster in [FeFe]-Hydrogenases
    Lampret, O. and Adamska-Venkatesh, A. and Konegger, H. and Wittkamp, F. and Apfel, U.-P. and Reijerse, E.J. and Lubitz, W. and Rüdiger, O. and Happe, T. and Winkler, M.
    Journal of the American Chemical Society 139 (2017)
    The catalytic cofactor of [FeFe]-hydrogenses (H-cluster) is composed of a generic cubane [4Fe-4S]-cluster (4FeH) linked to a binuclear iron-sulfur cluster (2FeH) that has an open coordination site at which the reversible conversion of protons to molecular hydrogen occurs. The (2FeH) subsite features a diatomic coordination sphere composed of three CO and two CN- ligands affecting its redox properties and providing excellent probes for FTIR spectroscopy. The CO stretch vibrations are very sensitive to the redox changes within the H-cluster occurring during the catalytic cycle, whereas the CN- signals seem to be relatively inert to these effects. This could be due to the more structural role of the CN- ligands tightly anchoring the (2FeH) unit to the protein environment through hydrogen bonding. In this work we explore the effects of structural changes within the secondary ligand sphere affecting the CN- ligands on FTIR spectroscopy and catalysis. By comparing the FTIR spectra of wild-type enzyme and two mutagenesis variants, we are able to assign the IR signals of the individual CN- ligands of the (2FeH) site for different redox states of the H-cluster. Moreover, protein film electrochemistry reveals that targeted manipulation of the secondary coordination sphere of the proximal CN- ligand (i.e., closest to the (4FeH) site) can affect the catalytic bias. These findings highlight the importance of the protein environment for re-adjusting the catalytic features of the H-cluster in individual enzymes and provide valuable information for the design of artificial hydrogenase mimics. © 2017 American Chemical Society.
    view abstract10.1021/jacs.7b08735
  • Mobile zinc increases rapidly in the retina after optic nerve injury and regulates ganglion cell survival and optic nerve regeneration
    Li, Y. and Andereggen, L. and Yuki, K. and Omura, K. and Yin, Y. and Gilbert, H.-Y. and Erdogan, B. and Asdourian, M.S. and Shrock, C. and De Lima, S. and Apfel, U.-P. and Zhuo, Y. and Hershfinkel, M. and Lippard, S.J. and Rosenberg, P.A. and Benowitz, L.
    Proceedings of the National Academy of Sciences of the United States of America 114 (2017)
    Retinal ganglion cells (RGCs), the projection neurons of the eye, cannot regenerate their axons once the optic nerve has been injured and soon begin to die.Whereas RGC death and regenerative failure are widely viewed as being cell-autonomous or influenced by various types of glia, we report here that the dysregulation of mobile zinc (Zn2+) in retinal interneurons is a primary factor. Within an hour after the optic nerve is injured, Zn2+ increases several-fold in retinal amacrine cell processes and continues to rise over the first day, then transfers slowly to RGCs via vesicular release. Zn2+ accumulation in amacrine cell processes involves the Zn2+ transporter protein ZnT-3, and deletion of slc30a3, the gene encoding ZnT-3, promotes RGC survival and axon regeneration. Intravitreal injection of Zn2+ chelators enables many RGCs to survive for months after nerve injury and regenerate axons, and enhances the prosurvival and regenerative effects of deleting the gene for phosphatase and tensin homolog (pten). Importantly, the therapeutic window for Zn2+ chelation extends for several days after nerve injury. These results show that retinal Zn2+ dysregulation is a major factor limiting the survival and regenerative capacity of injured RGCs, and point to Zn2+ chelation as a strategy to promote long-term RGC protection and enhance axon regeneration.
    view abstract10.1073/pnas.1616811114
  • Modulation of the CO2 fixation in dinickel azacryptands
    Möller, F. and Castañeda-Losada, L. and Junqueira, J.R.C. and Miller, R.G. and Reback, M.L. and Mallick, B. and Van Gastel, M. and Apfel, U.-P.
    Dalton Transactions 46 (2017)
    While bimetallic azacryptands are known to selectively coordinate CO2, there is little knowledge on how different substitution patterns of the azacryptand cage structure influence CO2 coordination. Stopped-flow UV-vis spectroscopy, electrochemical analysis and DFT calculations were performed on a series of dinickel azacryptands and showed different rates of CO2 coordination to the complexes. We herein present data showing that the different flexibility of the azacryptands is directly responsible for the difference in the CO2 uptake capability of dinickel azacryptand complexes. © 2017 The Royal Society of Chemistry.
    view abstract10.1039/c6dt04527h
  • Monodispersed Mesoporous Silica Spheres Supported Co3O4 as Robust Catalyst for Oxygen Evolution Reaction
    Deng, X. and Rin, R. and Tseng, J.-C. and Weidenthaler, C. and Apfel, U.-P. and Tüysüz, H.
    ChemCatChem 9 (2017)
    Monodispersed mesoporous silica spheres (MSS) with fibrous nanostructure and highly open porosity were fabricated by a facile one-pot synthetic route and loaded with Co3O4 nanoclusters for catalyzing the oxygen evolution reaction with Ru(bpy)3 2+–S2O8 2− photosensitizer and sacrificial reagent system. The effect of the loading amount on the morphology and microstructure of Co3O4 was investigated and it was found that lower Co3O4 content in the composite materials results in smaller crystallite size, which in turn leads to significantly enhanced oxygen evolution activity. Furthermore, owing to the monodispersity of the spheres and good accessibility of active species offered by the fibrous pore structure, the material shows a clear advantage over nonsupported Co3O4 nanoparticles and the commonly used ordered mesoporous silica supports such as KIT-6 and SBA-15. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cctc.201701001
  • Operando Phonon Studies of the Protonation Mechanism in Highly Active Hydrogen Evolution Reaction Pentlandite Catalysts
    Zegkinoglou, I. and Zendegani, A. and Sinev, I. and Kunze, S. and Mistry, H. and Jeon, H.S. and Zhao, J. and Hu, M.Y. and Alp, E.E. and Piontek, S. and Smialkowski, M. and Apfel, U.-P. and Körmann, F. and Neugebauer, J. and Hickel, T. and Roldan Cuenya, B.
    Journal of the American Chemical Society 139 (2017)
    Synthetic pentlandite (Fe4.5Ni4.5S8) is a promising electrocatalyst for hydrogen evolution, demonstrating high current densities, low overpotential, and remarkable stability in bulk form. The depletion of sulfur from the surface of this catalyst during the electrochemical reaction has been proposed to be beneficial for its catalytic performance, but the role of sulfur vacancies and the mechanism determining the reaction kinetics are still unknown. We have performed electrochemical operando studies of the vibrational dynamics of pentlandite under hydrogen evolution reaction conditions using 57Fe nuclear resonant inelastic X-ray scattering. Comparing the measured Fe partial vibrational density of states with density functional theory calculations, we have demonstrated that hydrogen atoms preferentially occupy substitutional positions replacing pre-existing sulfur vacancies. Once all vacancies are filled, the protonation proceeds interstitially, which slows down the reaction. Our results highlight the beneficial role of sulfur vacancies in the electrocatalytic performance of pentlandite and give insights into the hydrogen adsorption mechanism during the reaction. © 2017 American Chemical Society.
    view abstract10.1021/jacs.7b07902
  • Organometallic Fe–Fe Interactions: Beyond Common Metal–Metal Bonds and Inverse Mixed-Valent Charge Transfer
    Ringenberg, M.R. and Schwilk, M. and Wittkamp, F. and Apfel, U.-P. and Kaim, W.
    Chemistry - A European Journal 23 (2017)
    The compounds [Fe(CO)3(dRpf)]n+, n=0, 1, 2 and dRpf=1,1′-bis(dicyclohexylphosphino)ferrocene ([1]n+) or 1,1′-bis(diisopropylphosphino)ferrocene ([2]n+), were obtained as two-step reversible redox systems by photolytic and redox reactions. The iron–iron distance decreases from about 4 Å to about 3 Å on oxidation, which takes place primarily at the tricarbonyliron moiety. Whereas ferrocene oxidation is calculated to occur only in excited states, the near infrared absorptions of the mixed-valent monocations are due to an unprecedented “inverse” inter-valence charge transfer from the electron-rich iron(II) in the ferrocene backbone to the electron-deficient tricarbonyliron(I). Protonation of complex 1 results in the formation of the structurally characterized hydride [1H]BF4, which reacts with acetone to form the dication, 12+, and isopropanol. While the hydride [2H]BF4was found to be unstable, protonation of 2 in acetone resulted in the clean formation of 22+,formally a hydrogen transfer. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/chem.201605527
  • Proton-Coupled Reduction of the Catalytic [4Fe-4S] Cluster in [FeFe]-Hydrogenases
    Senger, M. and Laun, K. and Wittkamp, F. and Duan, J. and Haumann, M. and Happe, T. and Winkler, M. and Apfel, U.-P. and Stripp, S.T.
    Angewandte Chemie - International Edition 56 (2017)
    In nature, [FeFe]-hydrogenases catalyze the uptake and release of molecular hydrogen (H2) at a unique iron-sulfur cofactor. The absence of an electrochemical overpotential in the H2 release reaction makes [FeFe]-hydrogenases a prime example of efficient biocatalysis. However, the molecular details of hydrogen turnover are not yet fully understood. Herein, we characterize the initial one-electron reduction of [FeFe]-hydrogenases by infrared spectroscopy and electrochemistry and present evidence for proton-coupled electron transport during the formation of the reduced state Hred′. Charge compensation stabilizes the excess electron at the [4Fe-4S] cluster and maintains a conservative configuration of the diiron site. The role of Hred′ in hydrogen turnover and possible implications on the catalytic mechanism are discussed. We propose that regulation of the electronic properties in the periphery of metal cofactors is key to orchestrating multielectron processes. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.201709910
  • Redox Induced Configurational Isomerization of Bisphosphine-Tricarbonyliron(I) Complexes and the Difference a Ferrocene Makes
    Ringenberg, M.R. and Wittkamp, F. and Apfel, U.-P. and Kaim, W.
    Inorganic Chemistry 56 (2017)
    The tricarbonyliron (TCFe) complexes Fe(CO)3(dppf) and Fe(CO)3(dppp), where dppf = 1,1′-bis(diphenylphosphino)ferrocene and dppp = 1,3-bis(diphenylphosphino)propane, exhibit redox activity that induces configurational isomerization. The presence of the ferrocenyl (Fc) group stabilizes higher oxidized forms of TCFe. Using spectroelectrochemistry (IR, UV-vis, Mössbauer, and EPR) and computational analysis, we can show that the Fc in the backbone of the dppf ligand tends to form a weak dative bond to the electrophilic TCFeI and TCFeII species. The open shell TCFeI intermediate was stabilized by the distribution of spin between the two Fe centers (Fc and TCFe), whereas lacking the Fc moiety resulted in highly reactive TCFeI species. The [Fe(CO)3(dppf)]+ cation adopts two possible configurations, square-pyramidal (without an Fe-Fe interaction) and trigonal-bipyramidal (containing an Fe-Fe interaction). The two configurations are in equilibrium with the trigonal-bipyramidal configuration being enthalpically favored (ΔH = -7 kJ mol-1). There is an entropic penalty (ΔS = -20 J mol-1) due to tilting of the Cp (cyclopentadienide) rings of the dppf moieties by ∼8°. Additionally, the terminal iron hydride [FeH(CO)3(dppf)]BF4 was formed by protonation with a strong acid (HBF4·Et2O). © 2017 American Chemical Society.
    view abstract10.1021/acs.inorgchem.7b00957
  • Simple methods for the preparation of non-noble metal bulk-electrodes for electrocatalytic applications
    Puring, K.J. and Piontek, S. and Smialkowski, M. and Burfeind, J. and Kaluza, S. and Doetsch, C. and Apfel, U.-P.
    Journal of Visualized Experiments 2017 (2017)
    The rock material pentlandite with the composition Fe4.5Ni4.5S8 was synthesized via high temperature synthesis from the elements. The structure and composition of the material was characterized via powder X-ray diffraction (PXRD), Mössbauer spectroscopy (MB), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and energy dispersive X-ray spectroscopy (EDX). Two preparation methods of pentlandite bulk electrodes are presented. In the first approach a piece of synthetic pentlandite rock is directly contacted via a wire ferrule. The second approach utilizes pentlandite pellets, pressed from finely ground powder, which is immobilized in a Teflon casing. Both electrodes, whilst being prepared by an additive-free method, reveal high durability during electrocatalytic conversions in comparison to common drop-coating methods. We herein showcase the striking performance of such electrodes to accomplish the hydrogen evolution reaction (HER) and present a standardized method to evaluate the electrocatalytic performance by electrochemical and gas chromatographic methods. Furthermore, we report stability tests via potentiostatic methods at an overpotential of 0.6 V to explore the material limitations of the electrodes during electrolysis under industrial relevant conditions. © 2017 Journal of Visualized Experiments.
    view abstract10.3791/56087
  • Spectroscopic and reactivity differences in metal complexes derived from sulfur containing Triphos homologs
    Petuker, A. and Gerschel, P. and Piontek, S. and Ritterskamp, N. and Wittkamp, F. and Iffland, L. and Miller, R. and Van Gastel, M. and Apfel, U.-P.
    Dalton Transactions 46 (2017)
    Herein, we report a simplified method for the synthesis of Triphos homologs H3CC(CH2X)n(CH2Y)3-n (X = SPh, Y = PPh2, n = 0-3). The multidentate compounds were tested for their potential to coordinate metals such as Ni, Fe, and Mo under the same experimental conditions. Cyclic voltammetry, spectroelectrochemical IR investigations as well as DFT calculations were used to examine the electronic alterations in a series of [{H3CC(CH2X)n(CH2Y)3-n}Mo(CO)3] complexes and to evaluate their potential to open coordination sites or to release CO upon oxidation or in the presence of different solvents. In addition, we demonstrate that the catalytic hydrosilylation of N,N-dimethylbenzamide to N,N-dimethylbenzylamine is influenced by the applied tripodal ligand. Our investigations show the high potential of such manipulations to selectively alter the dynamics of the binding properties of Triphos-metal complexes and their reactivity. © 2017 The Royal Society of Chemistry.
    view abstract10.1039/c7dt01459g
  • Spontaneous Si-C bond cleavage in (TriphosSi)-nickel complexes
    Petuker, A. and Mebs, S. and Schüth, N. and Gerschel, P. and Reback, M.L. and Mallick, B. and Van Gastel, M. and Haumann, M. and Apfel, U.-P.
    Dalton Transactions 46 (2017)
    Herein, we report on the versatile reactions of CH3C(CH2PPh2)3 as well as CH3Si(CH2PPh2)3 derived Ni-complexes. While Ni[CH3C(CH2PPh2)3] complexes reveal high stability, the Ni[CH3Si(CH2PPh2)3] analogs show rapid decomposition at room temperature and afford the unprecedented pseudo-tetrahedral phosphino methanide complex 5. We provide a detailed electronic structure of 5 from X-ray absorption and emission spectroscopy data analysis in combination with DFT calculations, as well as from comparison with structurally related complexes. A mechanistic study for the formation of complex 5 by reaction with BF4 − is presented, based on a comparison of experimental data with quantum chemical calculations. We also show a simple route towards isolable Ni(i)-complexes on the gram scale. © The Royal Society of Chemistry.
    view abstract10.1039/c6dt04048a
  • Sunlight-Dependent Hydrogen Production by Photosensitizer/Hydrogenase Systems
    Adam, D. and Bösche, L. and Castañeda-Losada, L. and Winkler, M. and Apfel, U.-P. and Happe, T.
    ChemSusChem 10 (2017)
    We report a sustainable in vitro system for enzyme-based photohydrogen production. The [FeFe]-hydrogenase HydA1 from Chlamydomonas reinhardtii was tested for photohydrogen production as a proton-reducing catalyst in combination with eight different photosensitizers. Using the organic dye 5-carboxyeosin as a photosensitizer and plant-type ferredoxin PetF as an electron mediator, HydA1 achieves the highest light-driven turnover number (TONHydA1) yet reported for an enzyme-based in vitro system (2.9×106 mol(H2) mol(cat)−1) and a maximum turnover frequency (TOFHydA1) of 550 mol(H2) mol(HydA1)−1 s−1. The system is fueled very effectively by ambient daylight and can be further simplified by using 5-carboxyeosin and HydA1 as a two-component photosensitizer/biocatalyst system without an additional redox mediator. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cssc.201601523
  • A structural view of synthetic cofactor integration into [FeFe]-hydrogenases
    Esselborn, J. and Muraki, N. and Klein, K. and Engelbrecht, V. and Metzler-Nolte, N. and Apfel, U.-P. and Hofmann, E. and Kurisu, G. and Happe, T.
    Chemical Science 7 (2016)
    [FeFe]-hydrogenases are nature's fastest catalysts for the evolution or oxidation of hydrogen. Numerous synthetic model complexes for the [2Fe] subcluster (2FeH) of their active site are known, but so far none of these could compete with the enzymes. The complex Fe2[μ-(SCH2)2X](CN)2(CO)4 2- with X = NH was shown to integrate into the apo-form of [FeFe]-hydrogenases to yield a fully active enzyme. Here we report the first crystal structures of the apo-form of the bacterial [FeFe]-hydrogenase CpI from Clostridium pasteurianum at 1.60 Å and the active semisynthetic enzyme, CpIADT, at 1.63 Å. The structures illustrate the significant changes in ligand coordination upon integration and activation of the [2Fe] complex. These changes are induced by a rigid 2FeH cavity as revealed by the structure of apoCpI, which is remarkably similar to CpIADT. Additionally we present the high resolution crystal structures of the semisynthetic bacterial [FeFe]-hydrogenases CpIPDT (X = CH2), CpIODT (X = O) and CpISDT (X = S) with changes in the headgroup of the dithiolate bridge in the 2FeH cofactor. The structures of these inactive enzymes demonstrate that the 2FeH-subcluster and its protein environment remain largely unchanged when compared to the active enzyme CpIADT. As the active site shows an open coordination site in all structures, the absence of catalytic activity is probably not caused by steric obstruction. This demonstrates that the chemical properties of the dithiolate bridge are essential for enzyme activity. © 2016 The Royal Society of Chemistry.
    view abstract10.1039/c5sc03397g
  • Bimetallic nickel complexes for selective CO2 carbon capture and sequestration
    Möller, F. and Merz, K. and Herrmann, C. and Apfel, U.-P.
    Dalton Transactions 45 (2016)
    Herein we report a dinickel azacryptand complex that enables fast, selective, and tight CO2 binding from air. Exploiting the affinity of the cavitand towards azides, CO2 release was observed. Despite the stability of the azido complex, UV irradiation under atmospheric conditions proved to be a suitable pathway for N3- replacement by CO2. © The Royal Society of Chemistry 2016.
    view abstract10.1039/c5dt04267d
  • Controlled Flexible Coordination in Tripodal Iron(II) Phosphane Complexes: Effects on Reactivity
    Petuker, A. and Merz, K. and Merten, C. and Apfel, U.-P.
    Inorganic Chemistry 55 (2016)
    The possibility to alter properties of metal complexes without significant steric changes is a useful tool to tailor the reactivity of the complexes. Herein we present the synthesis of iron complexes with the tripodal phosphane ligands Triphos and TriphosSi and report on their different coordination properties. Whereas reaction of TriphosSi and FeX2 (X = Cl, Br) exclusively afforded (TriphosSi)FeX2 with a κ2-coordinated ligand, the homologous C-derived Fe complexes show rapid conversion in solution to afford [(Triphos)Fe(CH3CN)3][Fe2Cl6] or [(Triphos)Fe(CH3CN)3][FeBr4], respectively. The structural conversion was found to be temperature- and solvent-dependent and was accompanied by a linear change of the overall magnetization. The different ligand influence was shown to have a significant effect on the ability of (TriphosSi)FeCl2 and (Triphos)FeCl2 to perform the Sonogashira cross-coupling reaction of 4-iodotoluene and phenyl acetylene as well as the hydrosilylation of acetophenone. The results presented herein show the different coordination properties of two structurally homologous tripodal ligands and demonstrate the importance of geometrically controlled ligand field splitting on the stability and reactivity of metal complexes. The C/Si exchange therefore provides a simple and straightforward tool to manipulate properties and reactivity of metal complexes. © 2016 American Chemical Society.
    view abstract10.1021/acs.inorgchem.5b02361
  • Electrochemical Investigations of the Mechanism of Assembly of the Active-Site H-Cluster of [FeFe]-Hydrogenases
    Megarity, C.F. and Esselborn, J. and Hexter, S.V. and Wittkamp, F. and Apfel, U.-P. and Happe, T. and Armstrong, F.A.
    Journal of the American Chemical Society 138 (2016)
    Protein film electrochemistry (PFE) has been used to study the assembly of the complex 6Fe active site of [FeFe]-hydrogenases (known as the H-cluster) from its precursors - the [4Fe-4S] domain that is already coordinated within the host, and the 2Fe domain that is presented as a synthetic water-soluble complex stabilized by an additional CO. Not only does PFE allow control of redox states via the electrode potential but also the immobilized state of the enzyme facilitates control of extremely low concentrations of the 2Fe complex. Results for two enzymes, CrHydA1 from Chlamydomonas reinhardtii and CpI from Clostridium pasteurianum, are very similar, despite large differences in size and structure. Assembly begins with very tight binding of the 34-valence electron 2Fe complex to the apo-[4Fe-4S] enzyme, well before the rate-determining step. The precursor is trapped under highly reducing conditions (<-0.5 V vs SHE) that prevent fusion of the [4Fe-4S] and 2Fe domains (via cysteine-S) since the immediate product would be too electron-rich. Relaxing this condition allows conversion to the active H-cluster. The intramolecular steps are relevant to the final stage of biological H-cluster maturation. © 2016 American Chemical Society.
    view abstract10.1021/jacs.6b09366
  • Pentlandite rocks as sustainable and stable efficient electrocatalysts for hydrogen generation
    Konkena, B. and Puring, K.J. and Sinev, I. and Piontek, S. and Khavryuchenko, O. and Dürholt, J.P. and Schmid, R. and Tüysüz, H. and Muhler, M. and Schuhmann, W. and Apfel, U.-P.
    Nature Communications 7 (2016)
    The need for sustainable catalysts for an efficient hydrogen evolution reaction is of significant interest for modern society. Inspired by comparable structural properties of [FeNi]-hydrogenase, here we present the natural ore pentlandite (Fe 4.5 Ni 4.5 S 8) as a direct rock' electrode material for hydrogen evolution under acidic conditions with an overpotential of 280 mV at 10 mA cm -2. Furthermore, it reaches a value as low as 190 mV after 96 h of electrolysis due to surface sulfur depletion, which may change the electronic structure of the catalytically active nickel-iron centres. The rock' material shows an unexpected catalytic activity with comparable overpotential and Tafel slope to some well-developed metallic or nanostructured catalysts. Notably, the rock' material offers high current densities (≤650 mA cm -2) without any loss in activity for approximately 170 h. The superior hydrogen evolution performance of pentlandites as rock' electrode labels this ore as a promising electrocatalyst for future hydrogen-based economy.
    view abstract10.1038/ncomms12269
  • Phosphine-ligated dinitrosyl iron complexes for redox-controlled NO release
    Wittkamp, F. and Nagel, C. and Lauterjung, P. and Mallick, B. and Schatzschneider, U. and Apfel, U.-P.
    Dalton Transactions 45 (2016)
    Here we present the syntheses and structural, spectroscopic, as well as electrochemical properties of four dinitrosyl iron complexes (DNICs) based on silicon- and carbon-derived di- and tripodal phosphines. Whereas CH3C(CH2PPh2)3 and Ph2Si(CH2PPh2)2 coordinate iron in a η2-binding mode, CH3Si(CH2PPh2)3 undergoes cleavage of one Si-C bond to afford [Fe(NO)2(P(CH3)Ph2)2] at elevated temperatures. The complexes were characterized by IR spectroelectrochemistry as well as UV-vis measurements. The oxidized {Fe(NO)2}9 compounds were obtained by oxidation with (NH4)2[Ce(NO3)6] and their properties evaluated with Mössbauer and IR spectroscopy. Stability experiments on the complexes suggest that they are capable of releasing their NO-ligands in the oxidized {Fe(NO)2}9 but not in the reduced {Fe(NO)2}10 form. A detailed DFT analysis is provided in order to understand the electronic configurations and the complexes' ability to release NO. © The Royal Society of Chemistry 2016.
    view abstract10.1039/c6dt01209d
  • Stepwise isotope editing of [FeFe]-hydrogenases exposes cofactor dynamics
    Senger, M. and Mebs, S. and Duan, J. and Wittkamp, F. and Apfel, U.-P. and Heberle, J. and Haumann, M. and Stripp, S.T.
    Proceedings of the National Academy of Sciences of the United States of America 113 (2016)
    The six-iron cofactor of [FeFe]-hydrogenases (H-cluster) is the most efficient H2-forming catalyst in nature. It comprises a diiron active site with three carbonmonoxide (CO) and two cyanide (CN-) ligands in the active oxidized state (Hox) and one additional CO ligand in the inhibited state (Hox-CO). The diatomic ligands are sensitive reporter groups for structural changes of the cofactor. Their vibrational dynamics were monitored by real- Time attenuated total reflection Fouriertransform infrared spectroscopy. Combination of 13CO gas exposure, blue or red light irradiation, and controlled hydration of three different [FeFe]-hydrogenase proteins produced 8 Hox and 16 Hox-CO species with all possible isotopic exchange patterns. Extensive density functional theory calculations revealed the vibrational mode couplings of the carbonyl ligands and uniquely assigned each infrared spectrum to a specific labeling pattern. For Hox-CO, agreement between experimental and calculated infrared frequencies improved by up to one order of magnitude for an apical CN- At the distal iron ion of the cofactor as opposed to an apical CO. For Hox, two equally probable isomers with partially rotated ligands were suggested. Interconversion between these structures implies dynamic ligand reorientation at the H-cluster. Our experimental protocol for site-selective 13CO isotope editing combined with computational species assignment opens new perspectives for characterization of functional intermediates in the catalytic cycle.
    view abstract10.1073/pnas.1606178113
  • [FeFe]-Hydrogenase with Chalcogenide Substitutions at the H-Cluster Maintains Full H2Evolution Activity
    Noth, J. and Esselborn, J. and Güldenhaupt, J. and Brünje, A. and Sawyer, A. and Apfel, U.-P. and Gerwert, K. and Hofmann, E. and Winkler, M. and Happe, T.
    Angewandte Chemie - International Edition 55 (2016)
    The [FeFe]-hydrogenase HYDA1 from Chlamydomonas reinhardtii is particularly amenable to biochemical and biophysical characterization because the H-cluster in the active site is the only inorganic cofactor present. Herein, we present the complete chemical incorporation of the H-cluster into the HYDA1-apoprotein scaffold and, furthermore, the successful replacement of sulfur in the native [4FeH] cluster with selenium. The crystal structure of the reconstituted pre-mature HYDA1[4Fe4Se]Hprotein was determined, and a catalytically intact artificial H-cluster variant was generated upon in vitro maturation. Full hydrogen evolution activity as well as native-like composition and behavior of the redesigned enzyme were verified through kinetic assays, FTIR spectroscopy, and X-ray structure analysis. These findings reveal that even a bioinorganic active site with exceptional complexity can exhibit a surprising level of compositional plasticity. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/anie.201511896
  • A sterically stabilized FeI-FeI semi-rotated conformation of [FeFe] hydrogenase subsite model
    Goy, R. and Bertini, L. and Elleouet, C. and Görls, H. and Zampella, G. and Talarmin, J. and De Gioia, L. and Schollhammer, P. and Apfel, U.-P. and Weigand, W.
    Dalton Transactions 44 (2015)
    The [FeFe] hydrogenase is a highly sophisticated enzyme for the synthesis of hydrogen via a biological route. The rotated state of the H-cluster in the [FeIFeI] form was found to be an indispensable criteria for an effective catalysis. Mimicking the specific rotated geometry of the [FeFe] hydrogenase active site is highly challenging as no protein stabilization is present in model compounds. In order to simulate the sterically demanding environment of the nature's active site, the sterically crowded meso-bis(benzylthio)diphenylsilane (2) was utilized as dithiolate linker in an [2Fe2S] model complex. The reaction of the obtained hexacarbonyl complex 3 with 1,2-bis(dimethylphosphino)ethane (dmpe) results three different products depending on the amount of dmpe used in this reaction: [{Fe2(CO)5{μ-(SCHPh)2SiPh2}}2(μ-dmpe)] (4), [Fe2(CO)5(κ2-dmpe){μ-(SCHPh)2SiPh2}] (5) and [Fe2(CO)5(μ-dmpe){μ-(SCHPh)2SiPh2}] (6). Interestingly, the molecular structure of compound 5 shows a [FeFe] subsite comprising a semi-rotated conformation, which was fully characterized as well as the other isomers 4 and 6 by elemental analysis, IR and NMR spectroscopy, X-ray diffraction analysis (XRD) and DFT calculations. The herein reported model complex is the first example so far reported for [FeIFeI] hydrogenase model complex showing a semi-rotated geometry without the need of stabilization via agostic interactions (Fe⋯H-C). This journal is © The Royal Society of Chemistry.
    view abstract10.1039/c4dt03223c
  • Modulating sonogashira cross-coupling reactivity in four-coordinate nickel complexes by using geometric control
    Petuker, A. and Merten, C. and Apfel, U.-P.
    European Journal of Inorganic Chemistry 2015 (2015)
    Herein, we present the synthesis of nickel complexes with tripodal phosphine ligands, CH<inf>3</inf>Si(CH<inf>2</inf>PPh<inf>2</inf>)<inf>3</inf> and CH<inf>3</inf>C(CH<inf>2</inf>PPh<inf>2</inf>)<inf>3</inf>, and their application as catalysts in Sonogashira cross-coupling reactions in water. Although both types of nickel complexes are based on similar tripodal ligands, the Si-derived compounds adopt stable tetrahedral coordination geometries, whereas the C-derived counterparts adopt a square-planar coordination environment. This structural and electronic difference has an important effect on the catalytic properties of the complexes. Our study demonstrates that C-derived complexes are catalytically inactive, whereas the complexes [CH<inf>3</inf>Si(CH<inf>2</inf>PPh<inf>2</inf>)<inf>3</inf>NiX<inf>2</inf>] (X = Cl-, Br-) are competent catalysts for cross-coupling reactions of aryl halides with phenylacetylenes. This investigation reveals the importance of structural tuning on catalysis and strongly supports the theory that tetrahedral (PR<inf>3</inf>)<inf>2</inf>NiCl<inf>2</inf> complexes are the active species in Sonogashira cross-coupling reactions. Two tripodal phosphines CH<inf>3</inf>E(CH<inf>2</inf>PPh<inf>2</inf>)<inf>3</inf> (E = C, Si) are applied as ligands in Ni-catalyzed cross-coupling reactions; various coordination modes at the Ni center can be achieved. For the first time, experimental evidence is provided to prove that tetrahedral instead of planar (PR<inf>3</inf>)<inf>2</inf>NiCl<inf>2</inf> moieties are required to perform effective Sonogashira coupling reactions. Copyright © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/ejic.201500148
  • Modulation of extrasynaptic NMDA receptors by synaptic and tonic zinc
    Anderson, C.T. and Radford, R.J. and Zastrow, M.L. and Zhang, D.Y. and Apfel, U.-P. and Lippard, S.J. and Tzounopoulos, T.
    Proceedings of the National Academy of Sciences of the United States of America 112 (2015)
    Many excitatory synapses contain high levels of mobile zinc within glutamatergic vesicles. Although synaptic zinc and glutamate are coreleased, it is controversial whether zinc diffuses away from the release site or whether it remains bound to presynaptic membranes or proteins after its release. To study zinc transmission and quantify zinc levels, we required a high-affinity rapid zinc chelator as well as an extracellular ratiometric fluorescent zinc sensor. We demonstrate that tricine, considered a preferred chelator for studying the role of synaptic zinc, is unable to efficiently prevent zinc from binding low-nanomolar zinc-binding sites, such as the high-affinity zinc-binding site found in NMDA receptors (NMDARs). Here, we used ZX1, which has a 1 nM zinc dissociation constant and second-order rate constant for binding zinc that is 200-fold higher than those for tricine and CaEDTA. We find that synaptic zinc is phasically released during action potentials. In response to short trains of presynaptic stimulation, synaptic zinc diffuses beyond the synaptic cleft where it inhibits extrasynaptic NMDARs. During higher rates of presynaptic stimulation, released glutamate activates additional extrasynaptic NMDARs that are not reached by synaptically released zinc, but which are inhibited by ambient, tonic levels of nonsynaptic zinc. By performing a ratiometric evaluation of extracellular zinc levels in the dorsal cochlear nucleus, we determined the tonic zinc levels to be low nanomolar. These results demonstrate a physiological role for endogenous synaptic as well as tonic zinc in inhibiting extrasynaptic NMDARs and thereby fine tuning neuronal excitability and signaling. © 2015, National Academy of Sciences. All rights reserved.
    view abstract10.1073/pnas.1503348112
  • Versatile reactivity of a solvent-coordinated diiron(II) compound: Synthesis and dioxygen reactivity of a mixed-valent FeIIFe III species
    Majumdar, A. and Apfel, U.-P. and Jiang, Y. and Moënne-Loccoz, P. and Lippard, S.J.
    Inorganic Chemistry 53 (2014)
    A new, DMF-coordinated, preorganized diiron compound [Fe 2(N-Et-HPTB)(DMF)4](BF4)3 (1) was synthesized, avoiding the formation of [Fe(N-Et-HPTB)](BF4) 2 (10) and [Fe2(N-Et-HPTB)(μ-MeCONH)](BF 4)2 (11), where N-Et-HPTB is the anion of N,N,N′,N′-tetrakis[2-(1-ethylbenzimidazolyl)]-2-hydroxy-1, 3-diaminopropane. Compound 1 is a versatile reactant from which nine new compounds have been generated. Transformations include solvent exchange to yield [Fe2(N-Et-HPTB)(MeCN)4](BF4)3 (2), substitution to afford [Fe2(N-Et-HPTB)(μ-RCOO)](BF 4)2 (3, R = Ph; 4, RCOO = 4-methyl-2,6-diphenyl benzoate]), one-electron oxidation by (Cp2Fe)(BF4) to yield a Robin-Day class II mixed-valent diiron(II,III) compound, [Fe 2(N-Et-HPTB)(μ-PhCOO)(DMF)2](BF4) 3 (5), two-electron oxidation with tris(4-bromophenyl)aminium hexachloroantimonate to generate [Fe2(N-Et-HPTB)Cl 3(DMF)](BF4)2 (6), reaction with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl to form [Fe5(N-Et-HPTB) 2(μ-OH)4(μ-O)(DMF)2](BF4) 4 (7), and reaction with dioxygen to yield an unstable peroxo compound that decomposes at room temperature to generate [Fe4(N-Et- HPTB)2(μ-O)3(H2O)2](BF 4)·8DMF (8) and [Fe4(N-Et-HPTB)2(μ-O) 4](BF4)2 (9). Compound 5 loses its bridging benzoate ligand upon further oxidation to form [Fe2(N-Et-HPTB)(OH) 2(DMF)2](BF4)3 (12). Reaction of the diiron(II,III) compound 5 with dioxygen was studied in detail by spectroscopic methods. All compounds (1-12) were characterized by single-crystal X-ray structure determinations. Selected compounds and reaction intermediates were further examined by a combination of elemental analysis, electronic absorption spectroscopy, Mössbauer spectroscopy, EPR spectroscopy, resonance Raman spectroscopy, and cyclic voltammetry. © 2013 American Chemical Society.
    view abstract10.1021/ic4019585
  • [FeFe] Hydrogenase Models: An Overview
    Apfel, U. and Pétillon, F.Y. and Schollhammer, P. and Talarmin, J. and Weigand, W.
    Bioinspired Catalysis: Metal-Sulfur Complexes (2014)
    This chapter focuses on models for the mixed-valent Hox state as well as protonation reactions of Hred models and on some significant insights into proton reduction chemistry. Although numerous [FeFe] hydrogenase model complexes exist, synthetic access to iron-chalcogenolato compounds is limited to substitution reactions and oxidative additions. During the past two decades, significant development in modeling the active site of [FeFe] hydrogenases allowed to reveal the fundamental modes of action at the molecular level. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA. All rights reserved.
    view abstract10.1002/9783527664160.ch4
  • [FeFe]-hydrogenase models assembled into vesicular structures
    Menzel, K. and Apfel, U.-P. and Wolter, N. and Rüger, R. and Alpermann, T. and Steiniger, F. and Gabel, D. and Förster, S. and Weigand, W. and Fahr, A.
    Journal of Liposome Research 24 (2014)
    Compartmentalization is a major prerequisite for the origin of life on earth according to Wächtershäuser "Iron-Sulfur-World". The hypothesis is mainly based on an autocatalytic inorganic energy reproducing redox system consisting of iron and sulfur as requirement for the subsequent synthesis of complex organic structures. Here, we modified [FeFe]-hydrogenase models by means of covalent coupling to either oleic acid or the amphiphilic block copolymer polybutadiene-polyethyleneoxide (PB-PEO) and incorporated those into the membranes of vesicles composed of phospholipids (liposomes) or the unmodified amphiphilic polymer (polymersomes). We employed a [2Fe-2S] cluster as a hydrogenase model, since these structures are known to be suitable catalysts for the generation of H2 in the presence of weak acids. Successful incorporation was confirmed by spectrophotometric iron quantification and the vesicles formed were characterized by size determination (photon correlation spectroscopy (PCS)), and zeta potential as well as by cryo-transmission electron microscopy (Cryo-TEM). The modified models could be incorporated into liposomes or polymersomes up to molar proportions of 3.15% and 28%, respectively. Due to the immobilization in vesicular bilayers the [FeFe]-hydrogenase models can even exhibit catalytic action under the particular conditions of the intravesicular microenvironment. Our results suggest that the vesicular systems described may be applied as a nanoreactor for the reduction of encapsulated substances by generating hydrogen and thus as a minimal cell model. © 2014 Informa Healthcare USA, Inc.
    view abstract10.3109/08982104.2013.833225
  • A novel [FeFe] hydrogenase model with a (SCH2)2Pi=O moiety
    Almazahreh, L.R. and Apfel, U.-P. and Imhof, W. and Rudolph, M. and Görls, H. and Talarmin, J. and Schollhammer, P. and El-Khateeb, M. and Weigand, W.
    Organometallics 32 (2013)
    A novel [FeFe]-hydrogenase model complex containing phosphine oxide in the dithiolato ligand, namely [Fe2(CO)6][(μ-SCH 2)2(Ph)Pi=O] (1), has been synthesized and characterized. Complex 1 was prepared via the reaction of equimolar quantities of (μ-LiS)2Fe2(CO)6 and Oi=P(Ph)(CH 2Cl)2. The protonation properties of complex 1 have been investigated by monitoring the changes in IR (in the ν(CO) region) and 31P{1H} NMR spectra upon addition of pyridinium tetrafluoroborate, [HPy][BF4], and BF4·Et 2O, suggesting protonation of the Pi=O functionality. In addition, high-level DFT calculations on the protonation sites of complex 1 in CH 2Cl2 have been performed and support our experimental observations that the Pi=O unit is protonated by HBF4·Et 2O. Cyclic voltammetric experiments on complex 1 showed an anodic shift of the oxidation peak upon addition of HBF4·Et 2O, suggesting a CE process. © 2013 American Chemical Society.
    view abstract10.1021/om4003544
  • A silicon-heteroaromatic system as photosensitizer for light-driven hydrogen production by hydrogenase mimics
    Goy, R. and Apfel, U.-P. and Elleouet, C. and Escudero, D. and Elstner, M. and Görls, H. and Talarmin, J. and Schollhammer, P. and González, L. and Weigand, W.
    European Journal of Inorganic Chemistry (2013)
    The utilization of light and inexpensive catalysts to afford hydrogen represents a huge challenge. Following our interest in silicon-containing [FeFe]-hydrogenase ([FeFe]-H2ase) mimics, we report a new model approach for a photocatalytic [FeFe]-H2ase mimic 1, which contains a 1-silafluorene unit as a photosensitizer. Thereby, the photoactive ligand is linked to the [2Fe2S] cluster through S-CH2-Si bridges. Photochemical H2 evolution experiments were performed and revealed a turnover number (TON) of 29. This is the highest reported photocatalytic efficiency for an [FeFe]-H2ase model complex in which the photosensitizer is covalently linked to the catalytic center. We report a viable synthetic pathway for the construction of a new photoactive model of the [FeFe]-hydrogenase ([FeFe]-H2ase) active site with a dithiolate bridge and 1-silafluorene as a photosensitizer. The [FeFe]-H2ase mimic represents a very compact, easily accessible, and inexpensive photocatalyst for hydrogen generation. Copyright © 2013 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstract10.1002/ejic.201300537
  • Aging-associated enzyme human clock-1: Substrate-mediated reduction of the diiron center for 5-demethoxyubiquinone hydroxylation
    Lu, T.-T. and Lee, S.J. and Apfel, U.-P. and Lippard, S.J.
    Biochemistry 52 (2013)
    The mitochondrial membrane-bound enzyme Clock-1 (CLK-1) extends the average longevity of mice and Caenorhabditis elegans, as demonstrated for Δclk-1 constructs for both organisms. Such an apparent impact on aging and the presence of a carboxylate-bridged diiron center in the enzyme inspired this work. We expressed a soluble human CLK-1 (hCLK-1) fusion protein with an N-terminal immunoglobulin binding domain of protein G (GB1). Inclusion of the solubility tag allowed for thorough characterization of the carboxylate-bridged diiron active site of the resulting GB1-hCLK-1 by spectroscopic and kinetic methods. Both UV-visible and Mössbauer experiments provide unambiguous evidence that GB1-hCLK-1 functions as a 5-demethoxyubiquinone-hydroxylase, utilizing its carboxylate-bridged diiron center. The binding of DMQn (n = 0 or 2) to GB1-hCLK-1 mediates reduction of the diiron center by nicotinamide adenine dinucleotide (NADH) and initiates O2 activation for subsequent DMQ hydroxylation. Deployment of DMQ to mediate reduction of the diiron center in GB1-hCLK-1 improves substrate specificity and diminishes consumption of NADH that is uncoupled from substrate oxidation. Both Vmax and k cat/KM for DMQ hydroxylation increase when DMQ0 is replaced by DMQ2 as the substrate, which demonstrates that an isoprenoid side chain enhances enzymatic hydroxylation and improves catalytic efficiency. © 2013 American Chemical Society.
    view abstract10.1021/bi301674p
  • Biomimetic assembly of the [FeFe] hydrogenase: Synthetic mimics in a biological shell
    Apfel, U.-P. and Weigand, W.
    ChemBioChem 14 (2013)
    Combining synthetic chemistry and biology: A new method that allows the incorporation of synthetic [FeFe] hydrogenase mimics into the apo-hydrogenase is highlighted. Azadithiolato-functionalized model complexes showed similar activity to wild-type enzymes when implemented into the protein. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cbic.201300523
  • Detection of nitric oxide and nitroxyl with benzoresorufin-based fluorescent sensors
    Apfel, U.-P. and Buccella, D. and Wilson, J.J. and Lippard, S.J.
    Inorganic Chemistry 52 (2013)
    A new family of benzoresorufin-based copper complexes for fluorescence detection of NO and HNO is reported. The copper complexes, CuBRNO1-3, elicit 1.5-4.8-fold emission enhancement in response to NO and HNO. The three sensors differ in the nature of the metal-binding site. The photophysical properties of these sensors are investigated with assistance from density functional theory calculations. The fluorescence turn-on observed upon reaction with HNO is an unexpected result that is discussed in detail. The utility of the new sensors for detecting HNO and NO in HeLa cells and RAW 264.7 macrophages is demonstrated. © 2013 American Chemical Society.
    view abstract10.1021/ic302793w
  • Triptycene-based, carboxylate-bridged biomimetic diiron(II) complexes
    Li, Y. and Soe, C.M.M. and Wilson, J.J. and Tuang, S.L. and Apfel, U.-P. and Lippard, S.J.
    European Journal of Inorganic Chemistry (2013)
    A triptycene-based bis(benzimidazole) ester ligand, L3, was designed to enhance the electron-donating ability of the heterocyclic nitrogen atoms relative to those of the first-generation bis(benzoxazole) analogs, L1 and L2. A convergent synthesis of L3 was designed and executed. Three-component titration experiments using UV/Vis spectroscopy revealed that the desired diiron(II) complex could be obtained with a 1:2:1 ratio of L3/Fe(OTf)2(MeCN) 2/external carboxylate reactants. X-ray crystallographic studies of two diiron complexes derived in this manner from L3 revealed their formulas to be [Fe2L3(μ-OH)(μ-O2CR)(OTf)2], where R = 2,6-bis(p-tolyl)phenyl (7) or triphenylmethyl (8). The structures are similar to that of a diiron complex derived from L1, [Fe2L1(μ-OH)(μ- O2CArTol)(OTf)2] (9), a notable difference being that, in 7 and 8, the geometry at iron more closely resembles square-pyramidal than trigonal-bipyramidal. Mössbauer spectroscopic analyses of 7 and 8 indicate the presence of high-spin diiron(II) cores. These results demonstrate the importance of substituting benzimidazole for benzoxazole for assembling biomimetic diiron complexes with syn disposition of two N-donor ligands, as found in O2-activating carboxylate-bridged diiron centers in biological systems. Copyright © 2013 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstract10.1002/ejic.201201387
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