Paddle Ball Dynamics during Conversion of a Rh–Methyl Hydride Complex to a Rh–Methane σ‑Complex through Reductive Coupling

We used quasiclassical direct dynamics simulations to examine reaction pathways for protonation-induced reductive coupling of a pincer (PONOP)­Rh–methyl hydride complex. These dynamics simulations revealed that the majority of trajectories (>90%) emerging from a vibrationally averaged velocity distribution of the Rh–methyl hydride reductive coupling transition state led to the Rh–methane σ-complex. Only a few trajectories (<10%) dynamically deviated and did not sample this σ-complex structure and resulted in direct methane reductive elimination/dissociation. This indicates that the Rh–methane σ-complex is directly dynamically connected to the Rh–methyl hydride reductive coupling transition state and is not the result of methane rebound due to a solvent cage. Unexpectedly, trajectories that resulted in a Rh–methane σ-complex occurred with paddle ball motion where methane began to leave the Rh coordination sphere but then returned to the σ-complex, and so the distinction between reductive coupling and elimination is blurred. Trajectories also revealed that immediately after reductive coupling, there is relatively fast methane tumbling motion that generally sampled η1-C,H- and η2-C,H-type structures, and this motion is correlated with the paddle ball motion.