Deciphering the Cu(I):O2 to Cu(II):O2- Transition along the Binding Pathway through Computational Transient X-ray Absorption Spectra

Oxidation state (OS) is a fundamental concept for understanding chemical compounds, especially coordination complexes. Determining the OS of a central metal can be challenging in the presence of non-innocent ligands like O2/O2-/O2 2-, as multiple OSs may arise from metal-ligand distance changes. Here, we demonstrate that transient X-ray absorption spectroscopy (TXAS) can precisely distinguish OS changes in a copper cation along its dioxygen binding pathway, by computing spectra of the CuO2+ cation at decreasing Cu-O distances, from gas- to crystal-phase geometries. The Cu(I):O2 to Cu(II):O2- transition was deciphered by comparing spectra of complexes with clearly characterized OSs, consistently on both the O K-edge and the Cu L2,3-edge. Our multiconfigurational calculations agree well with experiments, allowing reliable peak-by-peak assignments across all snapshots. This enables us to track the evolution of electronic excitations and metal-to-ligand charge transfer during the ultrafast structural dynamics. This work showcases TXAS as a powerful tool for determining OS changes, highlighting the power of precise computational TXAS spectroscopy against reaction coordinate snapshots. Our findings clarify the intrinsic electronic structure change along the Cu-O2 binding pathway, illuminating more complex biological and catalytic processes involving Cu:O2 or analogous interactions.