| Abstract
| - The dimerization, unimolecular methane ejection, and bimolecular methane metathesisreactions of L2MCH3 species where L = H, Cl, Cp, and Cp* and M = Sc, Y, and Lu aremodeled at the density functional level (B3LYP) using a relativistic effective core potentialbasis set. Results for cases with H or Cl ligands are in poor quantitative agreement withanalogous results for cases with Cp* ligands; in some instances, Cp ligands provide resultsin good agreement with those for Cp*, but in the case of methane metathesis, activationenthalpies are underestimated by 3−4 kcal mol-1 with the unmethylated ligand. Unimolecular methane ejection via formation of a tuck-in complex versus bimolecular methanemetathesis is predicted potentially to be a competitive process for Sc, but to be comparativelytoo high in energy for Y and Lu to be thermodynamically significant under typical sets ofreaction conditions. The difference is ascribable to the shorter metal−ligand distancesobserved for Sc. For (Cp*)2LuCH3, quantum mechanical tunneling is predicted to increasethe overall rate of methane metathesis by factors of 4−93 over the temperature range 300−400 K. When tunneling is accounted for in the experimentally measured rate constants, asemiclassical enthalpy of activation of 19.2 kcal mol-1 is predicted for the methane metathesisreaction, in good agreement with a direct prediction from density functional theory of 20.3kcal mol-1.
- Accurate modeling of the kinetics of methane metathesis catalyzed by Cp*2MCH3 metallocenes requires complete representation of the Cp* rings and accounting for quantum mechanical tunneling in the activated complex. While a bimolecular metathesis reaction is strongly favored for M = Y and Lu, a two-step process involving a tuck-in complex intermediate is predicted to be energetically accessible for M = Sc.
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