About & Research

We are a young research group based at the Institute of Inorganic Chemistry, University of Heidelberg. Our work spans between hard-core inorganic chemistry (air sensitive synthesis, spectroscopy), computational chemistry (calculation of electronic structures, correlation with spectroscopy) and catalytic applications in fine organic chemistry (development of new catalysts, exporing substrate scopes, etc). Therefore, we are always looking for motivated researchers with core training in synthetic chemistry but also with an analytical mindset.
 
We are supported by the Fonds der Chemischen Industrie (FCI), the Deutsche Forschungsgemeinschaft (DFG) and the University of Heidelberg.
 

First row transition metals display rich redox chemistry and offer exciting opportunities in catalysis and in the activation of small molecules. Nevertheless, their reactivity can prove difficult to control. In order to address this challenge, our group focuses on employing new tools such as redox active ligands. In some cases, these ligands are used in synergy with secondary interactions, which are known to stabilise reactive complexes or certain productive transition states.

The new findings and modes of activation studied at a molecular level are then used in the discovery of new iron and cobalt based applications in organic chemistry methodology.
The knowledge gained by studying the scope of the reaction is then used in the redesign of the parent catalyst. The main research directions are summarised as follows:

1. π-acidic redox active ligands for catalytic applications

A new ligand design, based on a pyrimidinediimine scaffold, enables access to the first 1,3,5-selective alkyne trimerization reaction which is iron based. The increased π-acidity of the heterocycle is essential for this transformation, as it triggers the acceleration of the turnover determining step (reductive elimination). For the 1,3,5-selectivity, the alkyne insertion regioselectivity needs to be rigorously controlled throughout the catalytic cycle. Experimental evidence for the regioselectivity of the first insertion step could be obtained through stoichiometric studies. Further kinetic studies shed light on the reaction mechanism. The electronic structure of the isolated catalytically relevant species was studied by a combination of spectroscopy and computational methods.

2. Metal-ligand cooperativity in geometrically flexible PNN Systems

Metal-ligand coorperativity (MLC) is an attractive strategy to discover new reactivity patterns in first row transition metal chemistry, which can be then valorised in the discovery of new catalytic organic transformations. To date, most approaches focus on metal-carbonyl species, which control the spin state of the metal centres. Nevertheless, the same CO ligands are usually not substitutionally labile and therefore can block active sites at the metal centre. We therefore investigated chemical and redox MLC in the context of iron dinitrogen complexes, where ligand non-innocence and flexible changes between spin states facilitate new reaction pathways. For this, a PNN ligand platform is ideal, as it combines (i) a benzylic phosphine arm which can trigger pyridine dearomatisation by deprotonation, (ii) an imin functional group which can reversibly store electrons and (iii) a flexible pincer environment which can allow geometry changes at the iron centre, often accompanied by spin changes.

• Inorg. Chem. 2022, 61, 7426.

3. Heterobimetallic systems for redox-switchable catalysis

The field of metal-based redox switchable catalysis is dominated by metallocene-based redox switches. Metallocenes, especially ferrocene show obvious advantages – easy synthesis, tunability, robust and well-understood chemistry. Nevertheless, the potential window does not allow significant deviations from one of the ferrocene/ferrocenium couples. We therefore introduce a pincer-iron redox switch, based on a pyrazinediimine ligand, able to operate at potentials significantly different from ferrocene. The switch is synthesised by N-methylation followed by alpha deprotonation, to give rise to a mesoionic NHC, where Fe is part of the ligand backbone. Complexation with noble metals (e.g. rhodium) gives rise to heterobimetallic complexes, and allows the development of robust redox-switchable catalysts.