Relationship Between Energy Landscape Shape and Dynamics Trajectory Outcomes for Methane C–H Activation by Cationic Cp*(PMe3)Ir/Rh/Co(CH3)

Density functional theory (DFT) calculations are routinely used to determine organometallic reaction mechanisms. However, these calculations only represent an average structure and lack mechanistic details from dynamical motion. For the C–H activation/σ-bond metathesis reaction between methane and cationic Cp*­(PMe3)­MIII(CH3) complexes, DFT energy landscapes only define either a two-step oxidative addition/reductive elimination mechanism with an intervening MV–H intermediate (M = Ir and Rh) or a one-step concerted mechanism (M = Co). Reported here, quasiclassical direct dynamics trajectory simulations reveal that for Ir there is both a two-step mechanism as well as a dynamically concerted mechanism. The dynamically concerted trajectories show either extremely fast bypassing of the IrV–H intermediate (dynamically ballistic mechanism) or slower skipping of the IrV–H intermediate (dynamically unrelaxed mechanism). For Rh, despite a RhV–H intermediate on the DFT energy landscape, all trajectories skip this intermediate between 15 and 200 fs, and this reaction should be considered a dynamical one-step mechanism. The timing of the Rh reaction to progress past the RhV–H intermediate is longer than the concerted reaction mechanism for Co, where all trajectories pass beyond the transition-state zone within 15 fs. Statistical analysis revealed that the origin of the dynamically ballistic mechanism results from reaction coordinate motion coupled with excited CH3–Ir–CH3 symmetrical bending. Propagating trajectories at the MV–H intermediate with the transition-state energy revealed that the dynamically unrelaxed mechanism results from the lack of intramolecular vibrational energy redistribution.