Chemical activation of inert small molecules like CO2, CH4, N2 is a key problem in energy research. In the future, energy from renewable sources will be used on a big scale to transform these abundant materials to chemicals for industry and agriculture. Metal catalysts are needed to make such transformations energetically and chemically efficient and selective. We strongly believe that a deep understanding of mechanistic functionality and electronic structure of catalytic systems vastly supports the process of developing better catalysts. It is our approach to combine in-house spectroscopic methods (EPR, MCD, Mössbauer, Resonance Raman, X-Ray methods, etc.) with quantum theory to shine light on the chemical and electronic structure of catalytically active centers. The combination of spectroscopy and theory allows to interpret even very complicated spectroscopic data and to extract the desired chemical information.
Research in my group focuses on the synthesis of small molecular metal complexes for spectroscopic investigations. Directed variation of structural and electronic parameters in a series of compounds allows to systematically studying their spectroscopic response. Our samples are typically analyzed by standard methods (elemental analysis, IR, UV/vis, NMR, XRD) before they are further investigated as mentioned above. Such compounds with known molecular structure provide a reliable basis to collect high quality spectroscopic data. In the following, two examples of recent projects are given.
Nitrogenase is a bacterial enzyme which catalyzes the conversion of nitrogen from air to ammonia, an essential source for the biosynthesis of nitrogen containing compounds like peptides or nucleobases. The activation of nitrogen is very challenging since it is probably the most inert small molecule one could think of. We have learned a lot about the chemistry and structure of nitrogenase but the exact electronic structure, the catalytic mechanism and the function of an interstitial carbon atom in the molybdenum cofactor remains extremely challenging.
One of the two metal containing cofactors in nitrogenase, namely FeMoco, 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 I in Figure 1).
After we have recently worked on model complexes of FeMoco to shine light on the oxidation state of the Mo ion in the resting state we started a project to synthesize iron clusters with bridging carbon ligands to model the central carbon atom in FeMoco. Very few examples of such complexes have been reported in literature and synthetic strategies allowing the introduction of C-based ligands bonded to more than one Fe atom (μ2-6-C-based ligands) are very rare, explaining the lack of suitable model systems.
We felt that ylides could be suitable ligands to build up carbon bridged complexes and investigated the reaction of ylides with Fe(II) diamido species [Fe(N(SiMe3)2)2] which, in a first step, formed mononuclear higly reactive three coordinate iron(II) complexes[3a] of type A or more general E (Figure 1).
Further experiments showed that E can undergo a self-protolysis reaction at elevated temperatures since a carbon bound proton of the ylid is in close proximity to a strong base L which allows formation of doubly yldiide-bridged diiron(II) complexes of type F and HL.
Complexes 1 and 2 represent the first examples of dinuclear ylid-supported Fe2C2 iron diamond cores (Figure 2). Fe-C-Fe angles are found to be very acute at about 78.5° and the Fe…Fe distances are very short at ~2.58 Å. Mössbauer and x-ray absorption spectra in combination with magnetic susceptibility studies showed that the complexes are strongly antiferromagnetically coupled high-spin iron(II) dimers. Density functional calculations (DFT) reproduce the experimental data well and exclude a direct metal-metal bond.
We are continuing this project with sulfur containing ligands of a similar type which better model the sulfur-carbon ligation environment of the iron centers in FeMoco and have successfully isolated diiron complexes containing a distorted tetrahedral C2S2 environment and trigonal bipyramidal C2S2N coordination shell. All full spectroscopic characterization and DFT study is on the way.[3b]
In close collaboration with the x-ray diffraction facility of the MPI für Kohlenforschung, my group provides service for the x-ray determination of compounds produced in the MPI CEC. Research on molecular transition metal compounds for catalysis or spectroscopic investigations heavily relies on single crystal structure determinations since self-assembly often dominates in coordination chemistry and directed synthesis to obtain target compounds is in many cases limited. X-ray structure analysis delivers highly precise information about the three-dimensional arrangement of atoms, thereby providing bond length and bond angles, which are of enormous importance in understanding chemical properties. Since it is our aim to correlate experimental features and functional properties with structure, X-ray structure analysis is vital to this area of research.
As an example, Figure 3 displays two crystal structures from a recent paper of the department of Molecular Catalysis (Prof. Leitner) in which a precatalyst 1 forms a reaction intermediate 2 upon addition of pinacol borane in KOtBu/THF solution. The system is highly active and catalyses the reductive hydroboration of various aliphatic and aromatic carboxylic acids and even CO2.
 Benson, E. E.; Kubiak, C. P.;Sathrum, A. J.; Smieja, J. M. Chem. Soc.Rev. 2009, 38, 89-99.
 (a) Costentin, C.;Drouet, S.; Passard, G.; Robert, M.; Saveant, J. M. J. Am. Chem. Soc. 2013, 135, 9023-9031. (b) Costentin, C.; Passard, G.; Robert, M.; Saveant, J. M. J. Am. Chem. Soc. 2014, 136, 11821–11829.
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 Bjornsson, R.; Lima, F.A.;Weyhermüller, T.; Glatzel, P.; Spatzal, T.; Einsle, O.; Bill, E.; Neese, F.;DeBeer, S.; Chem. Sci. 2014, 5,3096-3103
 K. Weber, T. Krämer, H.Shafaat, T. Weyhermüller, E. Bill, M. van Gastel, F. Neese, W. Lubitz: Afunctional [NiFe]-hydrogenase model compound that undergoes biologicallyrelevant reversible thiolate protonation. J. Am. Chem. Soc. 2012, 134, 20745-20755