Trajectory-Based Time-Resolved Mechanism for Benzene Reductive Elimination from Cyclopentadienyl Mo/W Phenyl Hydride Complexes

Calculated potential energy structures
and landscapes are very
often used to define the sequence of reaction steps in an organometallic
reaction mechanism and interpret kinetic isotope effect (KIE) measurements.
Underlying most of this structure-to-mechanism translation is the
use of statistical rate theories without consideration of atomic/molecular
motion. Here we report direct dynamics simulations for an organometallic
benzene reductive elimination reaction, where nonstatistical intermediates
and dynamic-controlled pathways were identified. Specifically, we
report single spin state as well as mixed spin state quasiclassical
direct dynamics trajectories in the gas phase and explicit solvent
for benzene reductive elimination from Mo and W bridged cyclopentadienyl
phenyl hydride complexes ([Me2Si(C5Me4)2]M(H)(Ph), M = Mo and W). Different from the energy
landscape mechanistic sequence, the dynamics trajectories revealed
that after the benzene C–H bond forming transition state (often
called reductive coupling), σ-coordination and π-coordination
intermediates are either skipped or circumvented and that there is
a direct pathway to forming a spin flipped solvent caged intermediate,
which occurs in just a few hundred femtoseconds. Classical molecular
dynamics simulations were then used to estimate the lifetime of the
caged intermediate, which is between 200 and 400 picoseconds. This
indicates that when the η2-π-coordination intermediate
is formed, it occurs only after the first formation of the solvent-caged
intermediate. This dynamic mechanism intriguingly suggests the possibility
that the solvent-caged intermediate rather than a coordination intermediate
is responsible (or partially responsible) for the inverse KIE value
experimentally measured for W. Additionally, this dynamic mechanism
prompted us to calculate the kH/kD KIE value for the C–H bonding forming
transition states of Mo and W. Surprisingly, Mo gave a normal value,
while W gave an inverse value, albeit small, due to a much later transition
state position.