Kβ X‐Ray Emission Spectroscopic Study of a Second‐Row Transition Metal (Mo) and Its Application to Nitrogenase‐Related Model Complexes

• Published in: Angewandte Chemie International Edition 2020-07, Vol.59 (31), p.12965-12975
• Main Authors: Castillo, Rebeca G., Henthorn, Justin T., McGale, Jeremy, Maganas, Dimitrios, DeBeer, Serena
• Format: Article
• Language: English
• Subjects: Catalysis; Chemists; Cofactors; Electronic structure; Emission analysis; Emission spectroscopy; Heavy metals; Ligands; Metals; Molybdenum; Nitrogenase; Oxidation; Spectroscopy; Theoretical analysis; Transition metals; Valence; valence-to-core; X-ray emission spectroscopy
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.202003621

In recent years, X‐ray emission spectroscopy (XES) in the Kβ (3p‐1s) and valence‐to‐core (valence‐1s) regions has been increasingly used to study metal active sites in (bio)inorganic chemistry and catalysis, providing information about the metal spin state, oxidation state and the identity of coordinated ligands. However, to date this technique has been limited almost exclusively to first‐row transition metals. In this work, we present an extension of Kβ XES (in both the 4p‐1s and valence‐to‐1s [or VtC] regions) to the second transition row by performing a detailed experimental and theoretical analysis of the molybdenum emission lines. It is demonstrated in this work that Kβ2 lines are dominated by spin state effects, while VtC XES of a 4d transition metal provides access to metal oxidation state and ligand identity. An extension of Mo Kβ XES to nitrogenase‐relevant model complexes shows that the method is sufficiently sensitive to act as a spectator probe for redox events that are localized at the Fe atoms. Mo VtC XES thus has promise for future applications to nitrogenase, as well as a range of other Mo‐containing biological cofactors. Further, the clear assignment of the origins of Mo VtC XES features opens up the possibility of applying this method to a wide range of second‐row transition metals, thus providing chemists with a site‐specific tool for the elucidation of 4d transition metal electronic structure. The first systematic Kβ XES study for a second‐row transition metal, including a thorough analysis of the transitions responsible for Kβ2 (4p→1s) and valence‐to‐core (valence→1s) emission lines, is presented. Mo–Fe–S cubane clusters are also explored as a practical example of the efficiency of this technique for elucidating the nature of not only Mo but also any other second‐row TM center in biological systems.