O−O Bond Formation and Liberation of Dioxygen Mediated by N5‐Coordinate Non‐Heme Iron(IV) Complexes

Formation of the O−O bond is considered the critical step in oxidative water cleavage to produce dioxygen. High‐valent metal complexes with terminal oxo (oxido) ligands are commonly regarded as instrumental for oxygen evolution, but direct experimental evidence is lacking. Herein, we describe the formation of the O−O bond in solution, from non‐heme, N5‐coordinate oxoiron(IV) species. Oxygen evolution from oxoiron(IV) is instantaneous once meta‐chloroperbenzoic acid is administered in excess. Oxygen‐isotope labeling reveals two sources of dioxygen, pointing to mechanistic branching between HAT (hydrogen atom transfer)‐initiated free‐radical pathways of the peroxides, which are typical of catalase‐like reactivity, and iron‐borne O−O coupling, which is unprecedented for non‐heme/peroxide systems. Interpretation in terms of [FeIV(O)] and [FeV(O)] being the resting and active principles of the O−O coupling, respectively, concurs with fundamental mechanistic ideas of (electro‐) chemical O−O coupling in water oxidation catalysis (WOC), indicating that central mechanistic motifs of WOC can be mimicked in a catalase/peroxidase setting.DFG, 12489635, SFB 658: Elementarprozesse in molekularen Schaltern auf OberflächenTU Berlin, Open-Access-Mittel - 201

Using nickel manganese oxide catalysts for efficient water oxidation

Nickel-manganese oxides with variable Ni : Mn ratios, synthesised from heterobimetallic single-source precursors, turned out to be efficient water oxidation catalysts. They were subjected to oxidant-driven, photo- and electro-catalytic water oxidation showing superior activity and remarkable stability. In addition, a structure-activity relation could be established.DFG, EXC 314, Unifying Concepts in CatalysisBMBF, 03IS2071D, Light2Hydroge

Optimized immobilization of ZnO:Co electrocatalysts realizes 5% efficiency in photoassisted splitting of water

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Correction: There is an error in Fig. 8 of the manuscript. The correct Fig. 8 is shown in the additional file. To cite the Correction refer to DOI:10.1039/c6ta90030e.Organic solvents with varied electrophoretic mobility have been employed for deposition of nanocrystalline ZnO: Co particles onto fluorinated tin oxide supports. Evaluation of the electrochemical activity for the oxygen evolution reaction proves a clear solvent-dependence with highest activity upon deposition from acetonitrile and lowest activity upon deposition from ethanol. Analysis of the resulting layer thickness and density attributes the improved electrochemical activity of acetonitrile-prepared samples to larger film thicknesses with lower film densities, i.e. to films with higher porosity. The findings suggest that the ZnO: Co films represent an initially nanocrystalline system where the catalytic activity is predominantly confined to a thin surface region rather than to comprise the entire volume. Closer inspection of this surface region proves successive in operando transformation of the nanocrystalline to an amorphous phase during evolution of oxygen. Furthermore, less active but highly transparent ZnO: Co phases, prepared from ethanol-containing suspensions, can be successfully employed in a stacking configuration with a low-cost triple-junction solar cell. Thereby, a solar-to-hydrogen efficiency of 5.0% in splitting of water at pH 14 could be realized. In contrast, highly light-absorbing acetonitrile/acetone-prepared samples limit the efficiency to about 1%, demonstrating thus the decisive influence of the used organic solvent upon electrophoretic deposition. Stability investigations over several days finally prove that the modular architecture, applied here, represents an attractive approach for coupling of highly active electrocatalysts with efficient photovoltaic devices.BMBF, 03IS2071F, Light2Hydrogen - Energien für die ZukunftDFG, SPP 1613, Regenerativ erzeugte Brennstoffe durch lichtgetriebene Wasserspaltung: Aufklärung der Elementarprozesse und Umsetzungsperspektiven auf technologische Konzept

Synthesis and application of cobalt- and nickel-substituted zinc oxide nanoparticles as precatalysts for water oxidation

Für die Nutzung von Wasser als nicht fossile Ressource zur Erzeugung solarer Brennstoffe sind aktive, stabile und aus häufig vorkommenden Elementen bestehende Wasseroxidationskatalysatoren (WOCs) vonnöten. In dieser Arbeit wurden über eine neue Syntheseroute CoO, NiO sowie hochgradig mit Cobalt (41%) und Nickel (9%) substituierte zinkoxidische Nanopartikel hergestellt (Co:ZnO, Ni:ZnO). Dies gelang durch die Verwendung von homo- bzw. heterobimetallischen Komplexverbindungen des Di-2-pyridylmethandiolat (dpdH; [M4-xZnx (dpdH)4(OAc)3(H2O)]ClO4; M = Co, Ni; x = 0 - 4) als metallorganische Einkomponentenvorstufen (SSP) mit prädefinierten M-µO-M/Zn Bindungen. Durch Solvolyse in Benzylamin konnten die Nanopartikel trotz der rigiden, tris-koordinierten dpdH Liganden im SSP bei niedrigen Temperaturen erhalten werden. In den Reaktionsmischungen konnten homoleptische Komplexverbindungen des Di-2-pyridylmethanamin (dpNH2, [Zn(dpNH2)2](ClO4)2, [Ni(dpNH2)2](ClO4)2, [Co(dpNH2)2](ClO4)3), die heteroleptischen Komplexe [Zn(OAc)2(NH2Bn)2, [Ni(OAc)2(NH2Bn)4] und [Ni4(dpdH)4(OAc)2(NH2Bn)2](ClO4)2 und N-Benzylidenbenzylamin nachgewiesen werden. Auf Basis dieser Beobachtungen konnten mechanistische Überlegungen zu den Initialschritten der Nanokristallitbildung aus den SSPs angestellt werden. Die heterobimetallischen Nanopartikel wurden in chemischen, elektrochemischen und photochemischen OER Experimenten gegenüber homometallischen Referenzmaterialien (CoO, NiO, Co3O4) als WOCs getestet. Durch die Berechnung von Kennzahlen (Rate der Sauerstoffentwicklung, Aktivität normiert auf Masse, Aktivität normiert auf spezifische Oberfläche, Überpotential, TOF) zeigte sich eine den homometallischen Referenzmaterialien gegenüber erhöhte katalytische Aktivität. Die Präkatalysatoren durchliefen dabei einen Selbstaktivierungsprozess, in welchem sich Nanokomposite aus Kern-Schale Strukturen bildeten. Diese bestanden aus Kernen der kristallinen Partikel des Präkatalysators und einer amorphen, oxyhydroxidischen Schale. Diese in-situ erzeugten Strukturen zeigten im Gegensatz zu den homometallischen Referenzen keine Desaktivierung in den Tests zur katalytischen Aktivität, obwohl die homometallischen Referenzen analoge Nanokomposite bildeten. Die beobachtete Stabilität und Aktivität wurde mit den ladungstrennenden und PCET vermittelnden Eigenschaften der hexagonalen Wurtzitphasen des Präkatalysatormaterials im Komposit, sowie der begünstigten Bildung und Stabilisierung von katalytisch aktiveren Modifikationen der oxyhydroxidischen Phasen diskutiert.To use water as a non-fossil resource for the generation of solar fuels, water oxidation catalysts (WOCs) that are active, stable and composed out of earth abundant elements are needed. In this work, CoO, NiO as well as highly cobalt (41%) and nickel (9%) substituted zinc oxide nanoparticles were fabricated (Co:ZnO, Ni:ZnO). This was achieved by employing homo- and heterobimetallic tetranuclear complexes of di-2-pyridylmethanediolate (dpdH; [M4-xZnx(dpdH)4(OAc)3(H2O)]ClO4 ; M = Co, Ni; x = 0 - 4) as metal organic precursors with pre-defined M-µO-M/Zn bonds. Solvolysis of these metal organic compounds in benzylamine afforded the targeted homo- and heterobimetallic nanoparticles. The materials were formed at low temperatures albeit the rigid, triscoordinated dpdH Ligand. Homoleptic complexes of di-2-pyridylmethaneamine (dpNH2, [Zn(dpNH2)2](ClO4)2, [Ni(dpNH2)2]ClO4)2, [Co(dpNH2)2](ClO4)3) were isolated from these reaction mixtures, as well as the heteroleptic complexes [Zn(OAc)2(NH2Bn)2], [Ni(OAc)2(NH2Bn)4] and [Ni4(dpdH)4(OAc)2(NH2Bn)2](ClO4)2 and N-benzylidenebenzylamine. Based on these findings, mechanistic aspects for the initial steps of nanocrystallite formation from the SSPs were discussed. The heterobimetallic nanoparticles were tested as WOCs in chemically, electrochemically and photochemically driven OER experiments and compared to homometallic references (CoO, NiO, Co3O4). By calculating operating figures (rate of oxygen evolution, activity normalized to mass, activity normalized to surface area, overpotential, TOF), a higher catalytic activity as compared to the reference materials was found. The precatalysts were self-activated during the experiments, forming nanocomposites composed of core-shell structures. The cores were the remnants of the crystalline precatalyst particles, while the shell was composed of an amorphous oxyhydroxous layer. These in-situ formed composites showed no deactivation during testing their catalytic activity. Although the homometallic reference materials formed similar nanocomposites, the catalysts deactivated quickly. This enhanced stability was linked to the charge-separating and PCET-mediating properties of the hexagonal precatalyst wurtzite-phase in the composite and to the favored formation and stabilization of catalytically more active modifications of the oxyhydroxidic phases.BMBF, 03IS2071D, Light2Hydrogen (L2H

Depolymerization of end-of-life poly(dimethylsilazane) with boron trifluoride diethyl etherate to produce difluorodimethylsilane as useful commodity

<p>A straightforward protocol for the depolymerization of end-of-life poly(dimethylsilazane) using boron trifluoride diethyl etherate as depolymerization reagent to convert the Si-N to Si-F bonds was set-up. The application of the depolymerization reagent affords difluorodimethylsilane as major products, which can be a suitable synthon for the synthesis of new polymers (e.g., poly(dimethylsiloxanes) and allow an overall recycling of the [Me₂Si]-unit. </p

Ligand-Modulated Chemical and Structural Implications in Four-, Five-, and Six-fold Coordinated Aluminum Heteroaryl Alkenolates

Synthesis and characterization (gas phase, solution, and solid-state) of a series of four-, five- and six-fold coordinated heteroaryl-alkenolato aluminum complexes were performed to demonstrate the delicate interplay of structural and chemical influences of ligands in the design of new precursors for chemical vapor deposition. We are investigating the properties of heteroaryl alkenols as ON chelating ligands [where ON is 3,3,3-trifluoro(pyridin-2-yl)propen-2-ol (H-PyTFP), 3,3,3-trifluoro(1,3-benzimidazol-2-yl)propen-2-ol (H-BITFP), 3,3,3-trifluoro(dimethyl-1,3-oxazol-2-yl)propen-2-ol (H-DMOTFP), 3,3,3-trifluoro(1,3-benzoxazol-2-yl)propen-2-ol (H-BOTFP), 3,3,3-trifluoro(1,3-benzthiazol-2-yl)propen-2-ol (H-BTTFP), and 3,3,3-trifluoro(dimethyl-1,3-thiazol-2-yl)propen-2-ol (H-DMTTFP)] to prepare volatile and air-stable compounds. All three methyl groups in highly reactive AlMe3 could be replaced by H-PyTFP, H-BITFP, H-DMOTFP, and H-BOTFP yielding octahedral complexes of the type Al(ON)(3); under similar conditions H-BTTFP and H-DMTTFP produced heteroleptic MeAl(ON)(2) compounds with five-fold coordinated aluminum centers. Various attempts to obtain tris-alkenolato derivatives by choosing higher temperatures and prolonged reaction times were not successful. The reaction of H-PyTFP with [Al(OtBu)(3)](2) produced the dimeric heteroleptic [Al(PyTFP)(OtBu)(2)](2) complex with Al atoms present in both octahedral (Oh) and tetrahedral (Td) coordination in a single molecular unit. The introduction of the chelating ligand H-PyTFP in the dimeric framework of [Al(OtBu)(3)](2) enhanced the stability against hydrolyses significantly. The tendency of Al(III) centers to preferably coordinate in Td or Oh environment was elucidated by hydrolysis studies of monomeric Al(PyTFP)(3), Al(BOTFP)(3), and MeAl(BTTFP)(2) that produced hydroxo-bridged dimers to retain the octahedral environment for Al atoms. Surprisingly, hydrolysis of monomeric MeAl(DMTTFP)(2) yielded an oxo-bridged dimer with two five-fold coordinated aluminum centers. The structural features of all new complexes were investigated in solution, vapor, and solid state by multinuclear NMR spectroscopy, EI-MS spectrometry, and single-crystal X-ray diffraction analyses, respectively

Recycling Concept for End-of-Life Silicones: Boron Trifluoride Diethyl Etherate as Depolymerization Reagent to Produce Difluorodimethylsilane as Useful Commodity

The current waste management system is based primarily on landfills, thermal recycling and down-cycling. On the other hand, only a small portion of the end-of-life materials is recycled by depolymerization. In more detail, low-molecular weight commodities/monomers are produced by this process, which can be polymerized in a second step to produce new high quality polymers. Importantly, by applying these depolymerization/polymerization methodologies, contributions to more a sustainable, resource-conserving and environmental-benign society can be made. In this regard, we have set up a capable low-temperature (100 °C) protocol for the depolymerization of poly­(dimethylsiloxane)­s. In more detail, boron trifluoride diethyl etherate was applied as a depolymerization reagent in the conversion of numerous silicones to obtain difluorodimethylsilane and 1,3-difluoro-1,1,3,3-tetramethyldisiloxane as well-defined products under solvent-free conditions in yields up to 85%. Moreover, it was demonstrated that difluorodimethylsilane and 1,3-difluoro-1,1,3,3-tetramethyldisiloxane are useful starting materials for the synthesis of new silicones (polymerization). In consequence a recycling of silicones is feasible. The principle of operation of the recycling concept was demonstrated in the depolymerization/polymerization of 500 g of a polysiloxane

Establishing causation in climate litigation: admissibility and reliability

Pfrommer T, Goeschl T, Proelss A, et al. Establishing causation in climate litigation: admissibility and reliability. CLIMATIC CHANGE. 2019;152(1):67-84.Climate litigation has attracted renewed interest as a governance tool. A key challenge in climate litigation is to assess the factual basis of causation. Extreme weather attribution, specifically the Fraction of Attributable Risk (FAR), has been proposed as a way to tackle this challenge. What remains unclear is how attribution science interacts with the legal admissibility of evidence based on climate models. While evidence has to be legally admissible in order to be considered in a trial, it has to be reliable in order for the court to arrive at a legally correct conclusion. Since parties to the trial have incentives to produce evidence favorable to their case, admissibility requirements and the reliability of the evidence brought forward are linked. We provide a specific proposal for how to accommodate FAR estimates in admissibility standards by modifying an existing set of admissibility criteria, the Daubert criteria. We argue that two of the five Daubert criteria are unsuitable for dealing with such evidence and that replacing those criteria with ones directly addressing the reliability of FAR estimates is adequate. Lastly, we highlight the dependence of courts on both the existence and accessibility of a framework to determine the reliability of FAR estimates in executing such criteria

In Situ Studies of the Electrochemical Reduction of a Supported Ultrathin Single-Crystalline RuO2RuO_{2} (110) Layer in an Acidic Environment

With in situ surface X-ray diffraction (SXRD)and X-ray reflectivity (XRR) in combination with ex situcharacterization by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and cyclic voltamme-try, the electrochemical reduction of an ultrathin (1.66 nmthick) single-crystalline RuO$_2$(110) layer supported onRu (0001) is studied in an acidic environment, providingclear-cut evidence and mechanistic details for the trans-formation of RuO$_2$ to hydrous RuO$_2$ and metallic Ru. The reduction process proceeds via proton insertion into the RuO$_2$ (110)lattice. For electrode potentials (0 to−50 mV vs standard hydrogen electrode), the layer spacing of RuO2(110) increased,maintaining the octahedral coordination of Ru (SXRD). Continuous proton insertion at−100 to−150 mV leads to thetransformation of the lattice oxygen of RuO$_2$ to OH and water, which destroys the connectivity among the Ru-O6octahedronsand eventually leads to the loss of crystallinity (SXRD) in the RuO$_2$ (110)film at−200 mV accompanied by a swelling of thelayer with a well-defined thickness (XRR). During the protonation process, soluble Ru complexes may form. With XPS thetransformation of RuO$_2$ (110) to a hydrous RuO$_2$ layer is followed, a process that proceedsfirst homogeneously and at highercathodic potentials heterogeneously by re-deposition of previously electrochemically dissolved Ru complexes