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.