Mechanistic molecular motion of transition-metal mediated β-hydrogen transfer: quasiclassical trajectories reveal dynamically ballistic, dynamically unrelaxed, two step, and concerted mechanisms.

The transfer of a β-hydrogen from a metal-alkyl group to ethylene is a fundamental organometallic transformation. Previously proposed mechanisms for this transformation involve either a two-step β-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 β-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 β-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-1 stabilized relative to the β-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-1 relative to the β-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.