X-ray Absorption and Emission Spectroscopic Studies of [L₂Fe₂S₂]ⁿ Model Complexes: Implications for the Experimental Evaluation of Redox States in Iron–Sulfur Clusters

Abstract

Herein, a systematic study of [L2Fe2S2]n model complexes (where L = bis(benzimidazolato) and n = 2-, 3-, 4-) has been carried out using iron and sulfur K-edge X-ray absorption (XAS) and iron Kβ and valence-to-core X-ray emission spectroscopies (XES). These data are used as a test set to evaluate the relative strengths and weaknesses of X-ray core level spectroscopies in assessing redox changes in iron–sulfur clusters. The results are correlated to density functional theory (DFT) calculations of the spectra in order to further support the quantitative information that can be extracted from the experimental data. It is demonstrated that due to canceling effects of covalency and spin state, the information that can be extracted from Fe Kβ XES mainlines is limited. However, a careful analysis of the Fe K-edge XAS data shows that localized valence vs delocalized valence species may be differentiated on the basis of the pre-edge and K-edge energies. These findings are then applied to existing literature Fe K-edge XAS data on the iron protein, P-cluster, and FeMoco sites of nitrogenase. The ability to assess the extent of delocalization in the iron protein vs the P-cluster is highlighted. In addition, possible charge states for FeMoco on the basis of Fe K-edge XAS data are discussed. This study provides an important reference for future X-ray spectroscopic studies of iron–sulfur clusters.

A Practical Guide to High‐resolution X‐ray Spectroscopic Measurements and their Applications in Bioinorganic Chemistry

“…In contrast to the mainlines, the VtC region has only limited sensitivity to metal spin and oxidation states ,. Instead, as transitions from the ligand‐localized orbitals, the VtC region is primarily sensitive to the ligands bound to a metal center; extensive computational studies have established that the VtC region can serve as a way to “map” the electronic structure of the ligands ,.…”

“…A notable example of this is a recent XES study of iron‐sulfur dimers, which range from diferric to mixed‐valent to diferrous. Despite the change in oxidation state across this series, all the Kβ mainline spectra were superimposable – a clear demonstration of the canceling effects of spin state and covalency …”

QM/MM Study of the Nitrogenase MoFe Protein Resting State: Broken-Symmetry States, Protonation States, and QM Region Convergence in the FeMoco Active Site

“…We also note that the minority-spin electrons in these mixed-valence pairs are not always completely delocalized as can be seen in the localized orbitals (see supporting information) and are furthermore dependent on the QM-region and density functional used. Finally we note that XAS edges of mixed-valence Fe dimers are not fully understood as discussed by DeBeer et al 83 Figure 5. Our current understanding of the electronic structure of FeMoco, featuring the [MoFe 7 S 9 C] 1-charge and the BS7-235 broken-symmetry solution based on an analysis of the localized orbitals (see supporting information).…”

Cubane‐Type [Mo3S4M] Clusters with First‐Row Groups 4–10 Transition‐Metal Halides Supported by C5Me5 Ligands on Molybdenum

“…An Fe II state was further supported by the Mçssbauer spectrum andt he theoretical analysiso f6a as discussed below.F or 7 and 8,C o II and Ni II states were proposed based on ac omparison of the CoÀCl distance between 7 (2.2238(7) )a nd Tm Dip CoCl (2.242(2) ) [25] as wella st he NiÀBr distance between 8 (2.3550(6) )a nd Cl Tm tBu NiBr (2.3660 (17) ; Cl Tm = chlorotris(mercaptoidazolyl)borate). [27] Even though these MÀCl(Br) distances give some indications of the oxida-tion states of M, an assessment based exclusively on structural data is not unequivocal.F urther spectroscopica nalyses,s uch as X-ray emission spectroscopy (XES), [28] would be required for clarification. In this context, it should be noted that ad etailed analysiswas conducted for [Mo 3 S 4 Fe] cluster 6a on the basis of its 57 Fe Mçssbauer spectrum and DFT calculations (see below).…”