Iron centers in [FeFe] hydrogenase and biomimetic coordination complexes studied by XAS/XES and DFT: hydrogen catalysis and oxygen inhibition

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Understanding the mechanism of molecular hydrogen (H2) production at iron centers and inhibition of this reaction by oxygen (O2) is an important topic in sustainable energy research. [FeFe] hydrogenase proteins are the most efficient biological H2 catalysts, which contain a six-iron active site ([4Fe4S]-2FeH) denoted as H-cluster. Synthetic chemistry has produced coordination complexes, which mimic structural features of the diiron sub-cluster in the enzymes, but often show unsatisfying activity and stability. Mechanistic bottlenecks of the reactions in [FeFe] hydrogenase and diiron model complexes will be studied, employing high-resolution X-ray absorption and emission spectroscopy (XAS/XES) in combination with density functional theory calculations (DFT). Intermediates in the H2 cleavage/production and O2 modification processes are characterized using XAS/XES to determine their molecular structure (metal-ligand bond lengths; metal-metal distances; ligation changes; substrate interactions; reactive oxygen species formation) and electronic configuration (metal oxidation state; spin state; molecular orbitals structure, occupancy, and energy; HOMO-LUMO energy gap). Site-selective XAS/XES methods are developed and applied for the discrimination of individual iron species in the polynuclear complexes. By DFT the spectroscopic results are integrated in geometry-optimized structural models and quantitatively interpreted in terms of the electronic structure, which are related to the H2 and O2 reaction pathways.Comparative investigations on [FeFe] hydrogenase and model complexes will lead to an advanced understanding of the specific prerequisites of H2 turnover and O2 inhibition at the molecular level. In addition, the XAS/XES-DFT approach for the study of metal centers in biology and chemistry is further developed. General insights into routes towards more efficient H2 formation and increased O2 tolerance will be obtained, which will be tested in genetically engineered hydrogenase and tailored synthetic diiron catalysts.