Sensitivity of X‑ray Core Spectroscopy to Changes in Metal Ligation: A Systematic Study of Low-Coordinate, High-Spin Ferrous Complexes

In order to assess the sensitivity and complementarity of X-ray absorption and emission spectroscopies for determining changes in the metal ligation sphere, a systematic experimental and theoretical study of iron model complexes has been carried out. A series of high-spin ferrous complexes, in which the ligation sphere has been varied from a three-coordinate complex, [L^tBuFe­(SPh)] (<b>1</b>) (where L^tBu = bulky β-diketiminate ligand; SPh = phenyl thiolate) to four-coordinate complexes [L^tBuFe­(SPh)­(X)] (where X = CN^tBu (<b>2</b>); 1-methylimidazole (<b>3</b>); or <i>N</i>,<i>N</i>-dimethylformamide (DMF) (<b>4</b>)), has been investigated using a combination of Fe K-edge X-ray absorption (XAS) and Kβ X-ray emission (XES) spectroscopies. The Fe K XAS pre-edge and edge of all four complexes are consistent with a high-spin ferrous assignment, with the largest differences in the pre-edge intensities attributed to changes in covalency of the fourth coordination site. The X-ray emission spectra show pronounced changes in the valence to core region (V2C) as the identity of the coordinated ligand is varied. The experimental results have been correlated to density functional theory (DFT) calculations, to understand key molecular orbital contributions to the observed absorption and emission features. The calculations also have been extended to a series of hypothetical high-spin iron complexes to understand the sensitivity of XAS and XES techniques to different ligand protonation states ([L^tBuFe^II(SPh)­(NH_<i>n</i>)]^3–<i>n</i> (<i>n</i> = 3, 2, 1, 0)), metal oxidation states [L^tBuFe­(SPh)­(N)]^<i>n</i>− (<i>n</i> = 3, 2, 1), and changes in the ligand identity [L^tBuFe^IV(SPh)­(X)]^<i>n</i>− (X = C^4–, N^3–, O^2–; <i>n</i> = 2, 1, 0). This study demonstrates that XAS pre-edge data have greater sensitivity to changes in oxidation state, while valence to core (V2C) XES data provide a more sensitive probe of ligand identity and protonation state. The combination of multiple X-ray spectroscopic methods with DFT results thus has the potential to provide for detailed characterization of complex inorganic systems in both chemical and biological catalysis

Identification of a Single Light Atom within a Multinuclear Metal Cluster Using Valence-to-Core X-ray Emission Spectroscopy

Iron valence-to-core Fe Kβ X-ray emission spectroscopy (V2C XES) is established as a means to identify light atoms (C, N, O) within complex multimetallic frameworks. The ability to distinguish light atoms, particularly in the presence of heavier atoms, is a well-known limitation of both crystallography and EXAFS. Using the sensitivity of V2C XES to the ionization potential of the bound ligand, energetic shifts of ∼10 eV in the ligand 2s ionization energies of bound C, N, and O may be observed. As V2C XES is a high-energy X-ray method, it is readily applicable to samples in any physical form. This method thus has great potential for application to multimetallic inorganic frameworks involved in both small molecule storage and activation