Publikationen

Photocatalysis enables valuable reactions such as synthetic transformations or energy conversion processes like water splitting. To rationally improve photocatalytic reactions, mechanistic insights are required. These can be obtained with kinetic measurements, which are, however, difficult to obtain for a large enough number of reaction conditions to provide systematic and valuable insights. To this end, we present a system for performing photocatalytic reactions that combines time-resolved in situ measurements with a fully automated process for liquid handling, light source control, dynamic feedback and automated kinetic data evaluation, enabled by an open-source Python framework. The system is applied to study photocatalytic water oxidation using [Ru(bpy)3]2+ as both photosensitizer and water oxidation catalyst, with oxygen formation being measured in situ. Using this benchmark reaction, we investigate the effect of different reaction parameters (catalyst and sacrificial oxidant concentration, irradiance, pH-value) on the rate of oxygen evolution. The results show that the automated process enables highly reproducible experiments while obtaining full time-concentration curves with a high temporal resolution (seconds) for each experiment. Using the obtained data we derive a chemical reaction network and rate constants to describe the detailed mechanism of the photocatalytic reaction. This kinetic model reveals an unexpected light-driven step to convert [Ru(bpy)3]3+ and water to hydrogen peroxide (which ultimately disproportionates to form oxygen) as well as two competing deactivation pathways, unimolecular and bimolecular decomposition, the latter of which is autocatalytic. These results demonstrate how the combination of in situ kinetic measurements and automation unlocks the data- driven exploration of the chemical space, producing novel mechanistic insights.

The chiral ruthenium(II)bis-SINpEt complex is a versatile and powerful catalyst for the hydrogenation of a broad range of heteroarenes. This study aims to provide understanding of the active form of this privileged catalyst as well as the reaction mechanism, and to identify the factors which control enantioselectivity. To this end we used computational methods and in situ NMR spectroscopy to study the hydrogenation of 2-methylbenzofuran promoted by this system. The high flexibility and conformational freedom of the carbene ligands in this complex lead to the formation of a chiral pocket interacting with the substrate in a “lock-and-key” fashion. The non-covalent stabilization of the substrate in this particular pocket is an exclusive feature of the major enantiomeric pathway and is preserved throughout the mechanism. Substrate coordination leading to the minor enantiomer inside this pocket is inhibited by steric repulsion. Rather, the catalyst exhibits a “flat” interaction surface with the substrate in the minor enantiomer pathway. We probe this concept by computing transition states of the rate determining step of this reaction for a series of different substrates. Our findings open up a new approach for the rational design of chiral catalysts.

Recently, chemoselective methods for the hydrogenation of fluorinated, silylated, and borylated arenes have been developed, providing direct access to previously unattainable, valuable products. Herein, a comprehensive study on the employed rhodium-cyclic (alkyl)(amino)carbene (CAAC) catalyst precursor is disclosed. Mechanistic experiments, kinetic studies, and surface-spectroscopic methods revealed supported rhodium(0) nanoparticles (NP) as the active catalytic species. Further studies suggest that CAAC-derived modifiers play a key role in determining the chemoselectivity of the hydrogenation of fluorinated arenes, thus offering an avenue for further tuning of the catalytic properties.

A highly porous photocatalyst (copper on TiO2 aerogel) was synthesized and applied in aqueous CO2 reduction without using external sacrificial electron donors. For the first time, complete selectivity toward CO and improved catalyst productivity are observed in the presence of oxygen. The optimal activity is achieved in a feed containing 0.5 vol% O2 in CO2. In situ XAS, EPR, and UV-vis measurements suggest, among different Cu oxidation states, Cu(I) to be the most active species in photocatalytic CO2 reduction. Oxygen sensing of the catalyst in the presence of O2/CO2 mixtures indicated an unexpected photoadsorption of oxygen on the titania surface. We propose photooxidation of surface hydroxyl groups to be the electron source for CO2 reduction, which is supported by hydroxyl group consumption, detection of hydroxyl radicals using in situ EPR, and detection of surface peroxide species after the reaction.

The production of primary benzylic and aliphatic amines, which represent essential feed- stocks and key intermediates for valuable chemicals, life science molecules and materials, is of central importance. Here, we report the synthesis of this class of amines starting from carbonyl compounds and ammonia by Ru-catalyzed reductive amination using H2. Key to success for this synthesis is the use of a simple RuCl2(PPh3)3 catalyst that empowers the synthesis of >90 various linear and branched benzylic, heterocyclic, and aliphatic amines under industrially viable and scalable conditions. Applying this catalyst, −NH2 moiety has been introduced in functionalized and structurally diverse compounds, steroid derivatives and pharmaceuticals. Noteworthy, the synthetic utility of this Ru-catalyzed amination protocol has been demonstrated by upscaling the reactions up to 10 gram-scale syntheses. Further- more, in situ NMR studies were performed for the identification of active catalytic species. Based on these studies a mechanism for Ru-catalyzed reductive amination is proposed.

Herein we describe the first homogeneous non-noble metal catalyst for the hydrogenation of CO₂ to methanol. The catalyst is formed in situ from [Co(acac)₃], Triphos, and HNTf₂ and enables the reaction to be performed at 100 °C without a decrease in activity. Kinetic studies suggest an inner-sphere mechanism, and in situ NMR and MS experiments reveal the formation of the active catalyst through slow removal of the acetylacetonate ligands.

Grüner Wasserstoff ist ein Hauptakteur in der Energiewende. Während im öffentlichen Diskurs der Fokus auf grünem Wasserstoff liegt, der via Photovoltaik oder Windenergie und Wasserelektrolyse produziert wird, informiert dieser Beitrag über den aktuellen Stand bei der Erforschung der direkten Wasserspaltung mit Licht. Dabei wird eingegangen auf die grundlegenden Konzepte und Prozesse in der Wasserspaltung, bisherige Umsetzungen sowie aktuelle Herausforderungen, um diese Technologie in die Anwendung zu bringen. Um die noch anstehenden Herausforderungen zu überwinden ist auch die Einbindung dieser Thematik in den Chemieunterricht notwendig. Es wird auf Experimente, digitale Medien, und Lehrkonzepte, die dafür entwickelt wurden, hingewiesen.

Gibt es Wege, Wasser mit Licht zu spalten, die nicht die natürliche Photosynthese zum Vorbild haben? Jacob Schneidewind ist ihnen auf der Spur: Er entwickelt Photokatalysatoren dafür.