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.
Iron sulfur (FeS) proteins perform a wide range of biological functions including electron transfer and catalysis. Understanding the complex reactivity of these systems requires a detailed understanding of their electronic properties, which are encoded in the low-energy d-d excited states. Here we demonstrate that iron L-edge 2p3d resonant inelastic X-ray scattering (RIXS) can measure d-d excitation spectra in a series of monomeric, dimeric, and tetrameric FeS model complexes. RIXS provides advantages over traditional optical spectroscopies, because it is capable of measuring low-energy electronic excitations (0-10 000 cm) and spin-flip transitions. RIXS reveals the dense manifold of d-d excited states in dimeric [2Fe-2S] and tetrameric [MFeS] (M = V or Mo) complexes resulting from covalency and exchange coupling. These results support recent ab initio theoretical predictions that FeS clusters possess a much greater number of low-lying excited states than predicted by model Hamiltonians.
CpFe(CO) 2 (CH 2 ) 6 PPh 2 (FpC6P) is synthesized as a monomer for bulk migration insertion polymerization (MIP). Due to the presence of a longer alkyl spacer in FpC6P, intramolecular cyclization reactions, which are observed during the MIP of CpFe(CO) 2 (CH 2 ) 3 PPh 2 (FpC3P), were completely suppressed under the conditions of the polymerization. The resultant PFpC6P is soluble in common organic solvents and stable enough for GPC analysis. However, the polymer degrades in solution over a few days because the PFpC6P chain is constructed from relatively weak metal coordination bonds. In the presence of an oxidant, e.g.H 2 O 2 , degradation of the polymers was accelerated. When the solution was exposed to LED light (wavelength: 400-410 nm), degradation was completed in 20 minutes with CO release first followed by the dissociation of phosphine. DSC analysis indicates that PFpC6P has a T g of ca. 68°C, which is lower than that of PFpC3P (ca. 99°C). TGA analysis reveals that PFpC6P is thermally stable up to 170°C and leaves a ca. 35.2% char yield at 676°C after three stages of weight loss. CV data shows that the polymer in DMF has reversible redox activity, with an oxidation peak at 0.74 V and a reduction peak at 0.56 V (relative to a Ag electrode). † Electronic supplementary information (ESI) available: Photo images of PFpC6P solutions in organic solvents, 31 P NMR spectra for degraded PFpC6P. See
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