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.