Iron L₂,₃-Edge X-ray Absorption and X-ray Magnetic Circular Dichroism Studies of Molecular Iron Complexes with Relevance to the FeMoco and FeVco Active Sites of Nitrogenase

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. For mononuclear systems, the experimental data are correlated with atomic multiplet calculations in order to provide insights into the experimental observations. Finally, XMCD is applied to the tetranuclear heterometal-iron-sulfur clusters [MFe₃S₄]3+/2+ (M = Mo, V), which serve as structural analogues of the FeMoco and FeVco active sites of nitrogenase. It is demonstrated that the XMCD data can be utilized to obtain information on the oxidation state distribution in complex clusters that is not readily accessible for the Fe L₂,₃-edge XAS data alone. The advantages of XMCD relative to standard K-edge and L₂,₃-edge XAS are highlighted. This study provides an important foundation for future XMCD studies on complex (bio)inorganic systems.