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Density Functional Characterization of Methane Metathesis with Cp*2MR (M = Sc, Y, Lu; R = Me, tBuCH2). Structural and Kinetic Consequences of Alkyl Steric Bulk

Woodrum, N. L.; Cramer, C. J.
Organometallics 2006, 25, 68.

The uni- and bimolecular C-H bond metathesis reactions of Cp*2ML (Cp* = pentamethylcyclopentadienide; L = CH3, CH2C(CH3)3; M = Sc, Y, Lu) were modeled with the MPW1K density functional and a relativistic effective-core-potential basis set. This level of theory, when combined with a one-dimensional tunneling model, provides enthalpies of activation that are in good agreement with experiment for known bimolecular reactions of methane with Cp*2ScCH2C(CH3)3 and Cp*2LuCH3. Analysis of theoretical trends as a function of metal and ligand indicates that bimolecular reactions dominate in every case under typical experimental conditions. However, unimolecular reactions proceeding through tuck-in complexes become increasingly competitive with increased steric bulk of the metal alkyl and also with metals having smaller ionic radii. For all of the C-H bond metatheses, quantum mechanical tunneling is predicted to increase overall reaction rates by one to three orders of magnitude over the temperature range 284-323 K.

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