Timing and Structures of σ‑Bond Metathesis C–H Activation Reactions from Quasiclassical Direct Dynamics Simulations

Metal-mediated σ-bond metathesis
and σ-complex-assisted
metathesis (σ-CAM) reactions represent a major class of alkane
C–H activation reactions. Here, we present quasiclassical direct
dynamics trajectories that analyze the structures and lifetimes of
transition states and intermediates during Lu, Ir, and W metathesis
reactions. This provides insight into the influence of atomic momentum
on reaction mechanisms and whether there are nonintrinsic reaction
coordinate (non-IRC) and nonstatistical pathways. For the archetype σ-bond
metathesis between methane and (Cp*)2Lu­(CH3),
trajectories showed this reaction to be an example of a highly concerted
process with extremely rapid traversal of the transition-state region
without significant Lu–H vibration. Despite our locating a
(Cp*)2Lu­(CH3)­(CH4) σ-complex
on the potential-energy surface, this structure is always dynamically
skipped, consistent with early speculation about this reaction. For
the reaction of methane with (acac)2Ir­(CH3),
in contrast to previous DFT studies, a two-step oxidative cleavage/reductive
coupling set of transition states were located. However, in contrast
to the IRC motion for these reaction steps, trajectories reveal complete
skipping of the Ir–H intermediate, although with a few Ir–H
oscillations. This reaction generally, but not always, forms a σ-complex,
so the term σ-CAM is a reasonable description. For the reaction
of methane and (Cp*)­(CO)2W­(BCat), only about half of the
trajectories show a methane σ-complex prior to C−H bond
cleavage. After the single metathesis transition state a highly fluxional
HBCat coordination intermediate is formed.