Theoretical Analysis of Competing Pathways for Carbon-Hydrogen Activation of Cyclopentadienyl-Triphenylphosphine-Iridium in Benzene.
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Density functional theory (DFT) calculations are used to evaluate alternative reaction mechanisms when the photochemically produced (5-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 -bound intermediates, 2-C6H5X (X = H or PPh2), with the intermolecular -intermediate 2.0 kcal/mol more stable in free energy than the intramolecular -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 (5-C5Me5)Ir(PPh3)(Ph)H has a barrier of 15.9 kcal/mol. We propose that exchange between the -intermediates is faster than the oxidative additions, so the dominant early intermediate is the intermolecular -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 -intermediates that then proceed toward their own products. Further calculations on the less sterically crowded (5-C5H5)IrPPh3 predict that the intermolecular product should dominate the ratio.
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