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 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.