Max Planck Institute for Chemical Energy Conversion

Despite the vast number of reports on benzimidazoles, 1,2-substituted N-amido benzimidazoles remain an underrepresented and scarcely accessible compound class with promising pharmacological relevance. We present a safe, reliable electrochemical protocol that provides easy access to those structures. The reaction exhibits broad functional group tolerance and affords the target compounds in ≤89% yields.

Molecular electron spin qubits show great potential for quantum information storage and processing but require paramagnetic systems with slow relaxation, where coherence is limited by spin-phonon coupling. Understanding this coupling remains challenging due to scarce observables and conflicting models, making direct experimental insights crucial. Vibrational spectroscopy (IR and Raman) under magnetic fields provides a promising approach. Here, we present the first comprehensive simulation protocol to extract spin-phonon coupling parameters from THz EPR/Magnetic IR spectra. Different spectral features are illustrated using a one-phonon model, emphasizing cases where the coupling is weaker than the line width. Using a tetrahedral high-spin Co(II) complex, we validate the method and benchmark it against quantum-chemical calculations. The dominant coupling is attributed to a twisting mode of the first coordination sphere. Excellent agreement among experiment, simulation, and computation demonstrates the power of THz EPR/Magnetic IR spectroscopy as a direct probe of spin-phonon coupling.

Metal nitrene transfer catalysis is an efficient method for the direct formation of C-N bonds, but currently requires pre-functionalized, hazardous nitrene precursors. To expand the use of metal nitrene transfer catalysis, a method that generates the desired metal nitrene intermediate from an unfunctionalized nitrogen source is imperative. In this work, we demonstrate that a cobalt nitrene intermediate can be generated from 4-phenylbutylamine under electrochemically oxidative conditions, supported by electron paramagnetic resonance (EPR) and high-resolution mass spectrometry (HR-MS) measurements. Constant potential electrolysis allowed C-H amination of 4-phenylbutylamine to 2-phenylpyrroline, a cyclic imine, formed via oxidation of 2-phenylpyrrolidine. This method provides a proof of principle to unlock the use of unfunctionalized amines as nitrene precursor alternatives, thereby extending the applicability of metal nitrene transfer catalysis.

Small variations in Ru loading on Ni nanowires control OER activity through Ru morphology. Dispersed sub-nanometric Ru clusters stabilise high-valent Ni and boost performance, whereas Ru nanoparticles leach and yield Ni-like activity.

Electrochemical hydrogenationof aza-arenes is an appealing strategyto gain access to privileged saturated heterocycles for drug discovery,overcoming the limitations of classical hydrogenations that oftensuffer from energy-intensive conditions and safety hazards. Herein,we demonstrate an operationally simple, sustainable, and general electrochemicalhydrogenation of aza-arenes with commercialized Ni foam electrodesand setup. With water as the hydrogen donor under acidic conditions,the reaction proceeds at ambient temperature and pressure to deliverbroad substrate generality, excellent functional group tolerance,and excellent selectivity. The method tolerates a wide range of aza-arenesincluding(iso)­quinolines, quinoxalines, pyridines, and their nium saltshighlightingits generality and robustness. Synthetic utility was showcased throughthe preparation of bioactive molecules, while scalability was achievedup to 25 g of product, highlighting the method’s technicalapplicability with stable 22 h operation without changes in the cellvoltage or significant electrode degradation. Extensive mechanisticinvestigations using a combination of cyclic and RDE linear sweepvoltammetry suggest two plausible routes based on the substrate’sredox properties: hydrogenation by chemisorbed hydrogen (Hads) or initial substrate reduction followed by Hads transfer. This work sets a clean, practical,and versatile platform for aza-arene dearomatization, bridging academicinterest with industrial targets in electrochemical hydrogenation.

Two key characteristics differentiate molybdenum-dependent(MoFe)and iron-only (FeFe) nitrogenases: their efficacy for N2 fixation and their product distributions for CO2 reduction,yielding HCO2– and CH4, respectively.Despite their divergent properties, prior research argues that thedistinct cofactors share a common mechanism and equivalent structureswith the addition of “n” electronsand protons at their En catalytic states.The proposed equivalence between the cofactors was foundational inthe assignment of an Fe-hydride at their E1 states basedon interpretation of FeFeco’s photolabile and thermolabileE1 isomers (Inorg. Chem. 2022, 61, 5459–5464). However, the E0 state crystalstructures of FeMoco and FeFeco have sharp, previously unaddresseddistinctions in their metal–metal distances and proximal residueidentities at their respective octahedral M-sites (M = Mo or Fe).Herein, we study QM/MM models of the E0 and E1 states of FeMoco and FeFeco to distinguish their geometric and electronicstructures. Our analysis shows diminished metal–metal bondingand increased hydrogen bonding at FeFeco’s M-site, supportingthe presence of two energetically low-lying E1 states differentiatedby protonation of a μ3- or μ2-sulfide.The calculated thermodynamic and kinetic properties of FeFeco’ssulfide-protonated states agree with experimental data, without invokingFe-hydride formation. Unlike FeFeco, FeMoco’s E1 state strictly favors μ2-sulfide protonation. FeFeco’sdistinct μ3-sulfide-protonated E1 isomerhas a five-coordinate, reduced M-site that could explain its divergentreactivity relative to FeMoco.

Proton-coupled electron transfer (PCET) is essentialfor [FeFe]-hydrogenaseto operate reversibly with high activity at a minimal overpotential.However, whether the reduction of the [4Fe4S] cluster is proton-coupledduring the catalytic cycle remains under debate. Here, we employ indiumtin oxide (ITO)-functionalized nanostructured electrodes to resolvethe elusive non-turnover redox signals of all three [4Fe4S] clustersin Desulfovibrio desulfuricans [FeFe]-hydrogenase(DdHydAB). Analysis of the redox properties of the[4Fe4S] clusters revealed a moderate pH dependence (∼30 mV/pH),suggesting that the reduction potential is modulated by second-coordination-sphereeffects rather than direct protonation of the coordinating cysteineresidues. Additionally, integration of the non-turnover signals yieldedan accurate quantification of the enzyme’s concentration onthe electrode surface. In combination with in situ maturation, this allowed us to quantitatively determine the turnoverfrequency of the enzyme on the electrode surface.

Cobalt oxide (Co3O4) is a promising alternative due its electro- and thermocatalytic activity in oxidation reactions. To identify the active sites within Co3O4 and gain deeper insights into its catalytic properties, model systems are required. In this study, hexagonal Co3O4 platelets with predominantly <111> orientation were investigated. Vanadium ions were integrated into the Co3O4 crystal structure via Pulsed Laser Defect Engineering in Liquid (PUDEL), a modification known to influence catalytic activity. A combination of established nanostructure characterization techniques (SEM, XRD, XPS STEM-EDX, ICP-MS, EELS, HAADF, SAEM, and EPR spectroscopy) confirmed and verified a successful incorporation of the vanadium cations into the Co3O4 lattice with pulse-by-pulse control over the amount and depth of doped vanadium. The platelet-like morphology, particle size, and phase purity of the Co3O4 spinel nanoparticles were maintained in all cases. XPS and EDX analysis both revealed a linear increase of Vanadium content with higher number of applied laser pulses and an average vanadium oxidation state of +4 and +5. Electron energy loss spectroscopy (EELS) and electron paramagnetic resonance (EPR) further confirm that the doped Co3O4 is partially reduced by the inserted vanadium and could potentially facilitate the formation of HS Co2+ at Oh-site. The vanadium-doped Co3O4 platelets were shown to enhance the oxidative conversion of cinnamyl alcohol (CA) and to lower the electrocatalytic overpotential of the oxygen evolution reaction (OER) that is reduced by ~40 mV to ~50 mV with increasing number of applied laser pulses during laser processing. Density functional theory (DFT) approximation reveal that a high *OH coverage on the Co3O4 surface leads to an increase in the OER activity. The DFT study also verified that vanadium incorporation further activates the Co3O4 surface for OER, in line with the experimental observations. The investigation of vanadium-doped Co3O4 spinel’s opens new avenues for further doping experiments with other oxides as well as thin-films instead of nanoparticles, to systematically evaluate the influence of individually oriented facets on activity.

Facilitating the cyclotrimerization of acetylene to benzene in the gas-phase on the surface of solid catalysts has captivated researchers, both theoretically and experimentally, for several decades. Coupled with acetylene production from renewable feedstocks, this reaction offers a promising pathway for the direct, low-temperature synthesis of renewable benzene. Recognizing the physical limitations of typical solid catalysts, which adsorb benzene too strongly, this study investigates various high-valent early transition metal chlorides─potent molecular cyclotrimerization catalysts─as solid catalysts in this gas-phase conversion. Alongside catalytic assessment under industrially relevant conditions, various analytical techniques such as 93Nb solid-state NMR spectroscopy, X-ray emission spectroscopy, and quasi in situ X-ray photoelectron spectroscopy were applied to reveal that the catalytic functioning proceeds through the in situ formation of catalytically active reduced species as reported for a homogeneous reaction environment. Investigating deactivation phenomena, various catalyst precursors and reaction conditions led to the design of an immobilized molecular catalyst via chemical grafting, dispersion, and spatial isolation of active catalytic species on a mesoporous silica gel support. The resulting NbClx-silica gel catalyst exhibits an average benzene selectivity of up to 70% at an acetylene conversion level between 30% and 70% and a lifetime at ≥90% acetylene conversion of up to 8.5 h under industrially relevant conditions (10 vol % C2H2, 3 bar, 180 °C), outperforming previously reported solid catalysts by orders of magnitude.

Identifying active sites in FeNC catalysts for oxygen reduction reactions (ORR) and their alteration during preparation, storage, and electrochemical cycling are key challenges in the quest for improved catalysts. In this work, high-resolution transmission electron microscopy (TEM) is combined with 57Fe Mössbauer and electron paramagnetic resonance (EPR) spectroscopies to investigate iron centers in high-performance FeNC catalysts with regard to their structure, coordination and their oxidation and spin states. Reversible and irreversible changes in these properties during the preparation of FeNC electrodes and their use in electrochemical cells are investigated by complementary spectroelectrochemical Mössbauer and EPR measurements. Microscopy of the as prepared FeNC materials reveals iron to be evenly distributed in isolated sites or few iron atoms containing sites. Mössbauer and EPR identify strongly and weakly magnetically coupled high-spin Fe(III) in oxygenic coordination environment or superparamagnetic clusters, high-spin Fe(II) six coordinated in iron oxides and intermediate-spin Fe(II) in square planar coordination. Upon oxygen exposure a notable oxidation state change from Fe(II) to Fe(III) is observed, the amount of isolated and clustered iron are reduced and larger iron oxide nanoparticles are formed. It is noted that for this catalyst, before and after oxygen exposure most of the iron is bound in iron oxide structures. Under applied potential, Fe(III) is partially reduced to Fe(II) in clustered and isolated or weakly coupled sites. This change is mostly reversible, suggesting structural retention of the majority of the catalyst. All except the intermediate-spin Fe(II) in square planar coordination were found to undergo significant changes during electrochemical treatment. The observed structural variability and in particular the reversible redox activity of iron oxide structures may indicate that they also contribute to the ORR activity of FeNCs.