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

了解铁中心分子氢(H2)产生的机理以及氧(O2)对该反应的抑制作用是可持续能源研究中的一个重要课题。[FeFe]氢酶蛋白是最有效的生物H2催化剂，它含有一个六铁活性中心([4Fe4S]-2FeH)，记为H-簇。合成化学已经产生了配位络合物，它们模仿了酶中二铁亚簇的结构特征，但往往表现出不令人满意的活性和稳定性。采用高分辨X射线吸收和发射光谱(XAS/XES)和密度泛函理论计算(DFT)相结合的方法，研究[FeFe]氢酶和双铁模型络合物中反应的机理瓶颈。用XAS/XES对H2裂解/生成和O2修饰过程中的中间体进行了表征，以确定它们的分子结构(金属-配体键长；金属-金属距离；连接变化；底物相互作用；活性氧物种的形成)和电子构型(金属氧化态；自旋态；分子轨道结构、占位和能量；HOMO-LUMO能隙)。发展了位置选择性XAS/XES方法，并将其应用于多核络合物中单个铁物种的鉴别。通过DFT，光谱结果被整合到几何优化的结构模型中，并用与H2和O2反应途径相关的电子结构进行定量解释。对[FeFe]氢酶和模型络合物的比较研究将在分子水平上加深对H2周转和O2抑制的具体先决条件的理解。此外，还进一步发展了XAS/XES-DFT方法研究生物和化学中的金属中心。将在基因工程氢酶和定制的合成二铁催化剂中进行测试，从而获得对更有效的氢气形成和更高的O2耐受性途径的总体见解。

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.