Figure 1: Manipulation of highly air-sensitive compounds by Schlenk-, glove-box and high-vac line techniques in our lab.

Figure 2: Schematic relation of hydrogen atom transfer (HAT), proton transfer (PT) and electron transfer (ET) for a nickel complex with a redox-active and cooperating ligand.

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.

[1] L. Alig, M. Fritz, S. Schneider, "First-Row Transition Metal (De)hydrogenation Catalysis based on Functional Pincer Ligands", Chem. Rev., 2019, 119, 2681-2751..

[2] 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.

Figure 3: Conversion of dinitrogen into organic products via electrochemical reduction, photochemical N2 splitting and thermal nitride transfer.[4]

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.

[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.

Figure 4: Control of CO2 insertion selectivity by switching from thermal to photolytic conditions.[2]

Figure 5: Photo-driven reverse water gas shift reaction (left) and examination of the nickel hydride excited state dynamics by pump-probe IR (top right) and UV-Vis (bottom right) spectroscopy.

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.

[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.

Figure 6: Laminar flow ALD reactor developed in our group.

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.