Mechanistic Molecular Motion of Transition-Metal Mediated Beta-Hydride Transfer: Quasiclassical Trajectories Reveal Dynamically Ballistic, Dynamically Unrelaxed, Two Step, and Concerted Mechanisms

2020-04-23T05:30:20Z (GMT) by Josh Wheeler Ryan Carlsen Daniel Ess
The transfer of a -hydrogen from a metal-alkyl group to ethylene is a fundamental
organometallic transformation. Previously proposed mechanisms for this transformation involve either a
two-step -hydrogen elimination and migratory insertion sequence with a metal hydride intermediate
or a one-step concerted pathway. Here, we report density functional theory (DFT) quasiclassical direct
dynamics trajectories that reveal new dynamical mechanisms for the -hydrogen transfer of
[Cp*RhIII(Et)(ethylene)]
Despite the DFT energy landscape showing a two-step mechanism with a Rh-H
intermediate, quasiclassical trajectories commencing from the -hydrogen elimination transition state
revealed complete dynamical skipping of this intermediate. The skipping occurred either extremely fast
(typically <100 femtoseconds (fs)) through a dynamically ballistic mechanism or slower through a
dynamically unrelaxed mechanism. Consistent with trajectories begun at the transition state, all
trajectories initiated at the Rh-H intermediate show continuation along the reaction coordinate. All of
these trajectory outcomes are consistent with the Rh-H intermediate <1 kcal/mol stabilized relative to
the -hydrogen elimination and migratory insertion transition states. For Co, which on the energy
landscape is a one-step concerted mechanism, trajectories showed extremely fast traversing of the
transition-state zone (<50 fs), and this concerted mechanism is dynamically different than the Rh
ballistic mechanism. In contrast to Rh, for Ir, in addition to dynamically ballistic and unrelaxed
mechanisms, trajectories also stopped at the Ir-H intermediate. This is consistent with an Ir-H
intermediate that is stabilized by ~3 kcal/mol relative to the -hydrogen elimination and migratory
insertion transition states. Overall, comparison of Rh to Co and Ir provides understanding of the
relationship between the energy surface shape and resulting dynamical mechanisms of an
organometallic transformation.