Research in my group focuses on the synthesis of small molecular model complexes for spectroscopic investigations. Systematic variations of structural and electronic features in a series of compounds of similar composition allows to correlate the spectroscopic response with structure. Our samples are typically analyzed by standard methods and their structure determined before they are further investigated (Mössbauer, EPR, SQUID, XAS, etc.). The combination of exact structural data with high- quality spectroscopic measurements of model compounds makes it possible to test and improve quantum chemical methods which in return help to interpret data obtained from catalytically active systems.
We have been working on modeling the catalytically active site of nitrogenase for some time. This bacterial enzyme catalyzes the conversion of atmospheric nitrogen to NH3 under physiological conditions. Ammonia is the primary source for the biosynthesis of essential nitrogen compounds in these bacteria and of plants which live in symbiosis with such organisms.
The chemical activation of nitrogen is extremly challenging since the triple bond of molecular nitrogen is the most inert small molecule one could think of. We have learned a lot about the chemistry and structure of nitro- genase but the exact electronic structure in the reaction cycle, the catalytic mechanism and the function of the interstitial carbon atom in the molybdenum cofactor remain unclear. The metallic cofactor FeMoco in nitro- genase has been identified to be the active site of the enzyme where nitrogen binds and ammonia is released. It is basically composed of seven iron-, a molybdenum-center, nine sulfides, and a central carbide ion (see structure of framework in Fig. 1).
We are particulary interested in the chemical function of the carbide ion in the cluster and have therefore tried to synthesize model systems that show similarities to the natural example.
We have investigated a series of carbon bridged dinuclear iron complexes which are accessible by reaction of bis(diphenylthiophosphinoyl)methanediide2- with FeCl2 in THF.  Figure 1 shows the composition and structure of complex 1 and demonstrates resemblance with FeMoco.
Compound 1 can be reversibly electrochemically or chemi-cally one-electron oxidized to form the mixed-valent Fe(II)Fe(III)-complex 2. Figure 2 shows the zero-field 57Fe-Mössbauer spectra of 1 and 2 which clearly show that the iron centers in the complexes are high-spin Fe(II)Fe(II) and Fe(II)Fe(III) ions, respectively.
Complex 1 can bind suitable substrates by expansion of the coordination number from 4 to 5 at one iron site. Figure 3 shows the change of the UV/vis spectrum upon addition of dimethylaminopyridine (DMAP). Spectro- metric titration and x-ray structure anaylsis of the product reveal that 1 can exactly bind one molecule DMAP to form compound 3. The 57Fe-Mössbauer spectrum shown in Figure 2 (right) clearly indicates the inequi- valence of the two Fe(II) sites in 3.
Knowledge of the precise three-dimensional structure of the model compounds and catalytically active compounds studied in our departments is of great importance to understand the properties and reactivity of these molecules. The determined structure of the metal complexes also forms the basis of quantum mechanical calculations on these substances, which in turn serve to understand the reactivity and spectroscopic properties that are investigated in the department “Inorganic Spectroscopy” and the "Joint Workspace".
The work in my group focuses on the structure elucidation of small and medium-sized molecules using X-ray diffraction and NMR spectroscopy. Research on mole- cular transition metal compounds for catalysis relies on structure determination by means of X-ray diffraction on single crystals, since self-organization effects often dominate coordination chemistry and targeted synthesis of compounds is only possible to a limited extent. It is often not easily possible to determine the structure using other methods, such as NMR spectroscopy, since high-resolution NMR spectra cannot normally be obtained in the presence of paramagnetic metal centers. Independently of this, the X-ray structure analysis of the single crystal provides highly precise information about the three-dimensional arrangement of atoms and produces precise bond lengths and bond angles, which are of fundamental importance for understanding the chemical properties of a compound. However, the prerequisite for determining the structure using this method are single crystals, which are not always easy to obtain, sometimes not at all. NMR spectroscopy is very well suited for deter- mining the structure in solution. It provides detailed information about the connectivity of atoms in molecules in diamagnetic compounds, but cannot determine exact values for bond lengths and angles. However, it is an excellent and indispensable tool, especially for the rapid assessment of synthetic intermediates and for the characterization of target compounds.
 a) Lancaster, K. M.; Roemelt, M.; Ettenhuber, P.; Hu, Y.; Ribbe, M. W.; Neese, F.; Bergmann, U.; DeBeer, S., X-ray Emission Spectroscopy Evidences a Central Carbon in the Nitrogenase Iron-Molybdenum Cofactor. Science 2011, 334 (6058), 974. b) Spatzal, T.; Aksoyoglu, M.; Zhang, L.; Andrade, S. L. A.; Schleicher, E.; Weber, S.; Rees, D. C.; Einsle, O., Evidence for Interstitial Carbon in Nitrogenase FeMo Cofactor. Science 2011, 334 (6058), 940-940.
 a) Yogendra, S.; Wilson, D.; Hahn, A.; Weyhermüller, T.; van Stappen, C.; Holland, P.; DeBeer, S., Sulfur-Ligated [2Fe-2C]-Clusters as Synthetic Model Systems for Nitrogenase subm. Inorg. Chem. 2022 under review b) Fustier-Boutignon, M.; Heuclin, H.; Le Goff, X. F.; Mezailles, N., Transmetalation of a nucleophilic carbene fragment: from early to late transition metals. Chem. Commun., 2012, 48 (27), 3306-8.