Time-Resolved Resonant Inelastic X-ray Scattering Reveals How Orbital Symmetry Alignment Enables C-H Activation

Photochemical C-H activation reactions mediated by transition metal complexes often proceed via the formation of σ-complexes. In these intermediates, the metal center coordinates to a C-H σ-bond, and metal-ligand donation and back-donation interactions ultimately lead to C-H bond cleavage. Because metal-alkane σ-complexes are weakly bond and short-lived, current experimental methods provide limited access to the transient electronic-structure effects that control their reactivity. Here, we photochemically prepared three different types of Rh-alkane σ-complexes in solution and demonstrate how optical pump and X-ray probe spectroscopy gives access to their decisive valence-electron interactions. With femtosecond-resolution and Rh specific X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) at the Rh L₃-edge we access the orbital interactions between the Rh centers, the ancillary ligands and the alkane C-H σ-bonds of the Rh-alkane σ-complexes. Supported by theoretical calculations, we identify spectral fingerprints of Rh-alkane donation and back-donation interactions and find trends for the reactivity of the σ-complexes toward C-H activation. We uncover the particular importance of specific occupied molecular orbitals with specific symmetry in modulating reactivity by providing, channeling and directing electron density via the metal to and from the C-H bond. Elucidating the electronic factors that facilitate C-H bond activation provides a basis for future ligand substitutions aimed at enhancing the reactivity of metal-alkane σ-complexes with enhanced efficiency of the activation step.