Iron L_2,3-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

Abstract

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.

X‐ray Magnetic Circular Dichroism Spectroscopy Applied to Nitrogenase and Related Models: Experimental Evidence for a Spin‐Coupled Molybdenum(III) Center

“…In both the protein and the cubane data, the Fe L 3 -edge XMCD has an egative sign, indicating that the iron sites comprise the majority spin contribution to the S = 3/2 spin state in both FeMoco and the cubane model. [14,16] This observation was first made by Cramer and co-workers in Ref. [17],a nd has been subsequently observed in our studies on iron-sulfur and ironheterometallic systems.…”

“…[13] We have shown in our previous study based on aset of molecular iron compounds that Fe L 2,3 -edge XMCD serves as as pin-sensitive technique even in highly covalent systems,a nd thus allows insight regarding iron electronic spin distribution. [14] XMCD experiments thus provide am eans to selectively probe the S = 3/2 FeMoco in the presence of S = 0P -clusters.W en ote that while XMCD intensity can still theoretically arise for S = 0systems through the Faraday effect, these contributions are expected to be very minor at low temperatures relative to those arising from S > 0systems.T his has been demonstrated by measurements of S = 0i ron sulfur model complexes,w hich show no appreciable XMCD intensity [14] (see also Figure S1 in the Supporting Information). Furthermore,wehave also utilized the element (and spin) selectivity of XMCD to measure both the Mo L 3 -edge and Fe L 2,3 -edge XMCD of a[ Mo III Fe 3 S 4 ] 3+ cubane.T hese experiments allow us to very selectively probe the local Mo electronic structure and determine the nature of the coupling between the Fe and Mo sites.…”

“…[17],a nd has been subsequently observed in our studies on iron-sulfur and ironheterometallic systems. [14] From Figure 2, it can also be seen that the MoFep rotein Fe L 2,3 -edge XMCD data are noisy,h ighlighting the chal- [14].T he MoFeprotein Fe L 2,3 -edge XMCD spectrum was smoothed using the Savitzky-Golay algorithm (2nd order polynomial with 9points window) in order to obtain better peak definition.…”

X‐ray Magnetic Circular Dichroism Spectroscopy Applied to Nitrogenase and Related Models: Experimental Evidence for a Spin‐Coupled Molybdenum(III) Center

“…Meanwhile, all systems with S >0 will have a difference between the CL and CR absorption process and result in XMCD intensity . We have shown in our previous study based on a set of molecular iron compounds that Fe L 2,3 ‐edge XMCD serves as a spin‐sensitive technique even in highly covalent systems, and thus allows insight regarding iron electronic spin distribution . XMCD experiments thus provide a means to selectively probe the S =3/2 FeMoco in the presence of S =0 P‐clusters.…”

“…We note that while XMCD intensity can still theoretically arise for S =0 systems through the Faraday effect, these contributions are expected to be very minor at low temperatures relative to those arising from S >0 systems. This has been demonstrated by measurements of S =0 iron sulfur model complexes, which show no appreciable XMCD intensity (see also Figure S1 in the Supporting Information). Furthermore, we have also utilized the element (and spin) selectivity of XMCD to measure both the Mo L 3 ‐edge and Fe L 2,3 ‐edge XMCD of a [Mo III Fe 3 S 4 ] 3+ cubane.…”