Electrostatic Presolvation Enables Selectivity in Photoinduced Ligand Exchange

Functional group compatibility remains a central challenge in photochemical C–H activation with metal complexes. Strongly donating groups preferentially coordinate to the reactive unsaturated metal and suppress weaker C–H coordination and σ-complex formation required for C–H bond activation. Controlling the initial coordination in photoinduced ligand exchange is therefore one approach to achieving selectivity in bond activation. Here, we resolve the elementary steps of photoinduced CO to acetonitrile ligand substitution for Rh(acac)(CO)2 with femtosecond-resolution X-ray absorption spectroscopy at the Rh L3-edge. We directly observe the formation of a hitherto undetected transient acetonitrile C–H σ-complex and its subsequent rearrangement to the thermodynamically favored nitrogen-coordinated solvent complex. Molecular dynamics simulations reveal that the initial C–H coordination is enabled by electrostatic presolvation, which orients the acetonitrile methyl groups toward the metal center prior to CO dissociation. Our results point to a general strategy to direct selectivity in C–H activation by controlling coordination through presolvation.