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 formation of σ-complexes, in which a C–H σ-bond coordinates to the metal and is ultimately broken by specific charge-transfer interactions. Since metal-alkane σ-complexes are short-lived transient intermediates and because their reactivity is determined by electronic-structure effects, direct experimental characterization is challenging and understanding of their reactivity is limited. Here, we demonstrate how optical pump and metal-specific X-ray probe spectroscopy reveals the decisive nature of charge-transfer interactions in σ-complexes photochemically prepared with three different Rh-carbonyl complexes. We use femtosecond-resolution X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) at the Rh L3-edge to probe with orbital-level accuracy how the charge-transfer interactions between the Rh centers, the ancillary ligands and the alkane C–H σ-bonds vary with varying reactivity towards C–H activation. Supported by theoretical calculations, we reveal the importance of specific occupied molecular orbitals with specific symmetry in modulating the reactivity of the σ-complexes by providing, channeling and directing charge 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.