Iron L23-Edge X-ray Absorptionand X-ray Magnetic Circular Dichroism Studies of Molecular Iron Complexeswith Relevance to the FeMoco and FeVco Active Sites of Nitrogenase

摘要

Herein, a systematic study of a series of molecular iron model complexes has been carried out using Fe L2,3-edge X-ray absorption (XAS) and X-ray magnetic circular dichroism (XMCD) spectroscopies. This series spans iron complexes of increasing complexity, starting from ferric and ferrous tetrachlorides ([FeCl4]^−/2–), to ferric and ferrous tetrathiolates ([Fe(SR)4]^−/2–), to diferric and mixed-valent iron–sulfur complexes [Fe2S2R4]^2–/3–. This test set of compounds is used to evaluate the sensitivity of both Fe L2,3-edge XAS and XMCD spectroscopy to oxidation state and ligation changes. It is demonstrated that the energy shift and intensity of the L2,3-edge XAS spectra depends on both the oxidation state and covalency of the system; however, the quantitative information that can be extracted from these data is limited. On the other hand, analysis of the Fe XMCD shows distinct changes in the intensity at both L3 and L2 edges, depending on the oxidation state of the system. It is also demonstrated that the XMCD intensity is modulated by the covalency of the system. Formononuclear systems, the experimental data are correlated with atomicmultiplet calculations in order to provide insights into the experimentalobservations. Finally, XMCD is applied to the tetranuclear heterometal–iron–sulfurclusters [MFe3S4]^3+/2+ (M = Mo, V),which serve as structural analogues of the FeMoco and FeVco activesites of nitrogenase. It is demonstrated that the XMCD data can beutilized to obtain information on the oxidation state distributionin complex clusters that is not readily accessible for the Fe L2,3-edge XAS data alone. The advantages of XMCD relative tostandard K-edge and L2,3-edge XAS are highlighted. Thisstudy provides an important foundation for future XMCD studies oncomplex (bio)inorganic systems.