Research

Modern molecular inorganic chemistry combines chemical synthesis and catalysis with advanced physical methods, such as spectroscopy, magnetometry, electrochemistry, calorimetry, crystallography and computations, to rationalize chemical bonding and reactivity (Figure 1). Our group applies these methods to develop (electronic) structure / reactivity relationships as guidelines for thermal, photochemical and electrochemical reactions that are key steps in catalytic processes mediated by d-block metals. Particular focus is put on (a) the transformation of comparatively inert substrates, such as N₂ or CO₂, and (b) the facilitation of the more abundant base metals in catalysis. A general approach that we follow is the utilization of functional ligands that allow for reversible storage of reduction equivalents and/or protons to enable selective multielectron and proton redox reactions.^[1] We mainly utilize rigid, tridentate 'pincer'-type ligands with basic amide groups and redox-active moieties, that allow for metal-ligand cooperative or purely ligand centered proton coupled electron transfer (PCET). This is exemplified by a nickel(II) complex that undergoes C–H activation of benzylic substrates by PCET without metal redox change (Figure 2). Besides the use of redox-active ligands for the storage of redox equivalents, we examine the electronic structure and reactivity of radical ligands that are relevant to group transfer reactions. As a current example, we examine electron rich, open shell nitride and oxo complexes with nitrogen- and oxygen-centered radical character.^[2] Such species are important, transient intermediates, e.g. in catalytic oxygenation reactions and water oxidation. This work is carried out within the DFG Research Training Group RTG 2455.

The thermodynamic stability and kinetic inertness of highly ubiquitous N₂ renders the fixation of dinitrogen to more reactive nitrogen compounds a challenging task. The nitrogenase enzymes catalyze N₂ fixation in nature at ambient conditions, yet at high energetic cost (16 ATP per N₂), and the industrial Haber-Bosch process consumes about 2 % of the global energy production. Our group examines the activation and functionalization of N₂ at mild conditions. We are particularly interested in developing transition metal catalysts that enable the direct transformation of N₂ into nitrogen-containing molecules beyond ammonia. This approach aims at bypassing NH₃ as an intermediate for both low-valent (e.g. amines, nitriles) and high-valent (e.g. nitro) nitrogen products. Metal-mediated, full splitting of dinitrogen into nitride complexes is examined as a key step in N₂ functionalization. We have developed transition metal platforms (rhenium, molybdenum, tungsten) that form nitride complexes by N₂ splitting via an external stimulus, such as reduction, protonation or photolysis. Kinetic examination by spectroscopic, electrochemical and computational methods gave a detailed mechanistic picture for both N₂ splitting and reverse nitride coupling.^[1,2] (Electro-)chemical nitride coupling is examined in the context of ammonia oxidation and its use as fuel. The splitting of N₂ into nitrides was exploited to develop quasi-catalytic model cycles for the synthesis of nitrogen compounds directly from N₂. As a recent example, nitriles and amides were obtained via electrochemical reduction, photolytic N₂ splitting and thermal N-transfer (Figure 3).^[3] This example also emphasizes the role of the auxiliary ligand in the nitride transfer reaction as a reservoir for 2e^–/2H⁺ reduction of the M≡N moiety. This work is supported by a Consolidator Grant of the European Research Council.

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[1] M. G. Scheibel, B. Askevold, F. Heinemann, E. I. Reijerse, B. de Bruin, S. Schneider, "Closed-Shell and Open-Shell, square-planar Iridium Nitrido Complexes", Nature Chem 2012, 4, 552.

[2] B. M. Lindley, R. S. van Alten, M. Finger, F. Schendzielorz, C. Würtele, A. J. M. Miller, I. Siewert, S. Schneider, "Mechanism of Chemical and Electrochemical N₂ Splitting by a Rhenium Pincer Complex", J. Am. Chem. Soc., 2018, 140, 7922-7935.

[3] F. Schendzielorz, M. Finger, J. Abbenseth, C. Würtele, V. Krewald, S. Schneider, "Metal-Ligand Cooperative Synthesis of Benzonitrile via Electrochemical Reduction and Photolytic Splitting of Dinitrogen", Angew. Chem. Int. Ed., 2019, 58, 830-834.

The high gravimetric hydrogen density renders compounds like formic acid (> 4 %) and ammonia borane (BH₃NH₃; > 19 %) interesting target molecule as H₂ vector for chemical energy storage. Our group develops highly active 3d catalysts for hydrogen release from polar substrates based on metal-ligand cooperativity (MLC). Besides hydrogen storage applications, this approach is applied in the development of synthetic methodologies, e.g. to polymeric ammonia borane dehydrocoupling products or to esters by selective acceptorless alcohol dehydrogenation.^[1] Our MLC catalysts are also utilized for hydrogenation reactions, such as halosilane hydrogenolysis or CO₂ reduction. These reactions are challenging both due to their unfavorable thermochemistry and intricate control of the selectivity. Photochemical approaches are employed to drive reactions under mild conditions and MLC reactivity could be introduced into the photochemical regime. An example from our recent work is the inversion of CO₂ insertion into a nickel hydride bond under photolytic conditions that relies on photolytic N–H reductive elimination (Figure 4). This reaction defines the key step that controls the CO₂ reduction selectivity.^[2] Transient spectroscopy is applied in collaboration to elucidate excited state dynamics as mechanistic basis. This reactivity enabled the development of a light-driven reverse water-gas-shift reaction at room temperature (Figure 5). This work is carried out within the DFG funded collaborative research center CRC 1073.

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[1] E. A. Bielinski, P. O. Lagaditis, Y. Zhang, B. Q. Mercado, C. Würtele, W. H. Bernskoetter, N. Hazari, S. Schneider, "Lewis Acid-Assisted Formic Acid Dehydrogenation Using a Pincer-Supported Iron Catalyst", J. Am. Chem. Soc. 2014, 136, 10234-10237.

[2] F. Schneck, J. Ahrens, M. Finger, A. C. Stückl, C. Würtele, D. Schwarzer S. Schneider, "The Elusive Abnormal CO₂ Insertion Enabled by Metal-Ligand Cooperative Photochemical Selectivity Inversion", Nat. Commun. 2018 9, 1161.

Atomic Layer Deposition (ALD) is a method for the gas phase deposition of ultrathin films on strongly textured surfaces. The layer-by-layer growth relies on a self-limiting mechanism that is defined by the chemical reactivity at the interface. Our group designs organometallic ALD precursors, studies the chemical basis for controlled film growth and develops new deposition protocols for nanostructured films and composites with diverse applications. The targeted materials currently include metal / metal oxide multilayers with phononic bandgaps (within CRC 1073) and transition metal chalcogenide and pnictide films as electrocatalysts.