Origin of Intramolecular versus Intermolecular C-H Arene Activation Selectivity by...

Photolysis of (η⁵-C₅Me₅)Ir(PPh₃)(H)₂ in benzene generates the 16-electron (16e^-) complex (η⁵-C₅Me₅)Ir(PPh₃) that undergoes competitive intramolecular ortho-metalation with a phenyl group from -PPh₃ and intermolecular C-H activation with benzene. Previous density functional theory (DFT) studies identified the intramolecular π-complex and the intermolecular 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 intramolecular versus intermolecular pathway selectivity has been established. Here, we characterized the open-shell 16e^- iridium species and extensively mapped out the energy landscape for intramolecular ortho-metalation of -PPh₃ versus intermolecular benzene C-H activation. Also, we performed DFT-based direct dynamics simulations, and the results suggest that the intramolecular versus intermolecular 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.