Origin of Intramolecular versus Intermolecular C–H Arene Activation Selectivity by Cyclopentadienyl–Triphenylphosphine Iridium

Photolysis of (η⁵-C₅Me₅)Ir(PPh₃)(H)₂ in benzene generates the 16-electron (16e^-) complex (η⁵-C₅Me₅)Ir(PPh₃) that undergoes competitive <i>intra</i>molecular ortho-metalation with a phenyl group from -PPh₃ and <i>inter</i>molecular C-H activation with benzene. Previous density functional theory (DFT) studies identified the <i>intra</i>molecular π-complex and the <i>inter</i>molecular benzene π-complex intermediates and their corresponding C-H activation transition states. However, neither the mechanism of interconversion between these intermediates nor the origin of <i>intra</i>molecular versus <i>inter</i>molecular pathway selectivity has been established. Here, we characterized the open-shell 16e^- iridium species and extensively mapped out the energy landscape for <i>intra</i>molecular ortho-metalation of -PPh₃ versus <i>inter</i>molecular benzene C-H activation. Also, we performed DFT-based direct dynamics simulations, and the results suggest that the <i>intra</i>molecular versus <i>inter</i>molecular pathway selectivity is determined dynamically within picoseconds as the 16e^- iridium species evolves into a coordinatively saturated structure. During this process, the π-complexes are formed concurrently with, instead of prior to, the iridium hydrides, which could not be explained by kinetic models that assume C-H cleavage as the rate-limiting step. These findings demonstrate that dynamics simulations in addition to DFT calculations are needed for a more complete mechanistic understanding of photoinduced C-H activation reactions, of which the product selectivity can be influenced by atomic motion.