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 s-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 towards 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.