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/a-bond metathesis reaction between methane and cationic Cp*(PMe3)M-III(CH3) complexes, DFT energy landscapes only define either a two-step oxidative addition/reductive elimination mechanism with an intervening M-V-H intermediate (M = Ir and Rh) or a one-step concerted mechanism (M = Co). Reported here, quasiclassical direct dynamics trajectory simulations reveal that for Jr 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 Ir-V-H intermediate (dynamically ballistic mechanism) or slower skipping of the Ir-V-H intermediate (dynamically unrelaxed mechanism). For Rh, despite a Rh-V-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 Rh-V-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-CH(3)symmetrical bending. Propagating trajectories at the M-V-H intermediate with the transition-state energy revealed that the dynamically unrelaxed mechanism results from the lack of intramolecular vibrational energy redistribution.