Theoretical Analysis of Competing Pathways for Carbon–Hydrogen Activation of Cyclopentadienyl–Triphenylphosphine–Iridium in Benzene

Density functional theory (DFT) calculations are used
to evaluate
alternative reaction mechanisms when the photochemically produced
(η5-C5Me5)­IrPPh3 oxidatively adds a C–H bond from either a benzene solvent
molecule or a phenyl group of the phosphine ligand. Experimentally,
the ortho-metalated complexes produced from intramolecular C–H
activation and the hydridophenyl complexes produced from intermolecular
C–H activation form in a ratio of 53:47 (Janowicz, A. H.; Bergman,
R. G. J. Am. Chem. Soc. 1982, 104, 352–354). Both products are predicted to be
thermodynamically stable such that the back reaction, reductive elimination,
is predicted to be exceedingly unfavorable. Thus, the product ratio
must be under kinetic control. The DFT calculations predict the initial
formation of π-bound intermediates, η2-C6H5X (X = H or PPh2), with the intermolecular
π-intermediate 2.0 kcal/mol more stable in free energy than
the intramolecular π-intermediate. From these intermediates,
the two competing reactions have slightly different free-energy barriers.
The intramolecular activation of a phenyl C–H bond to yield
ortho-metalated complexes has a barrier of 13.4 kcal/mol, and the
intermolecular oxidative addition of solvent molecule C6H6 to form (η5-C5Me5)­Ir­(PPh3)­(Ph)H has a barrier of 15.9 kcal/mol. We propose
that exchange between the π-intermediates is faster than the
oxidative additions, so the dominant early intermediate is the intermolecular
π-intermediate. Hence, from this intermediate the two reaction
paths have free-energy barriers of 15.4 (intramolecular) and 15.9
(intermolecular) kcal/mol. Thus, within the accuracy of DFT, the free-energy
barriers for the intramolecular pathway and the intermolecular pathway
are very compatible with the 53:47 product ratio. However, the calculations
cannot completely exclude a higher interchange barrier, which would
mean that the final product ratio must result from the nearly equal
distribution (53:47) of the two π-intermediates that then proceed
toward their own products. Further calculations on the less sterically
crowded (η5-C5H5)­IrPPh3 predict that the intermolecular product should dominate the
ratio.