Direct Dynamics Trajectories Reveal Nonstatistical Coordination Intermediates and Demonstrate that σ and π‑Coordination Are Not Required for Rhenium(I)-Mediated Ethylene C–H Activation

The C–H activation reaction
between Cp­(PMe3)2Re and ethylene results in
kinetic selectivity for the Re-vinyl
hydride I over the thermodynamically more stable Cp­(PMe3)2Re­(η2-ethylene) π-complex II. While transition-state and variational transition-state
structures were located for individual pathways leading to I and II, DFT and CCSD­(T) energies predict a large kinetic
selectivity of 102–104, which is incompatible
with the experimental 10:1 ratio. DFT direct quasiclassical trajectories
revealed that the transition
states do not provide a qualitatively correct reaction mechanism or
a quantitatively correct selectivity due to a nonstatistical σ-CH
coordination intermediate that precedes the transition states for
C–H activation and π coordination. Using metadynamics
and quasiclassical direct dynamics, we show that trajectories for
the reaction between Cp­(PMe3)2Re and ethylene
result in direct formation of either the Re-vinyl hydride I or the π-complex II. Trajectories leading to
the Re-vinyl hydride skip σ-coordination and do not require
π-coordination. Consistent with experiments, trajectory selectivity
provides a relatively small kinetic selectivity for the Re-vinyl hydride.

Cp(PMe₃)₂Re与乙烯之间的C–H活化（C–H activation）反应，对Re-乙烯基氢化物I的动力学选择性高于热力学更稳定的Cp(PMe₃)₂Re(η²-乙烯) π配合物（π-complex）II。尽管针对生成I和II的各条反应路径，已分别确定了过渡态（transition state）与变分过渡态（variational transition state）结构，但密度泛函理论（DFT）与耦合簇CCSD(T)（CCSD(T)）的能量计算预测的动力学选择性可达10²–10⁴，这与实验测得的10:1比例并不相符。DFT直接准经典轨迹（quasiclassical trajectories）模拟结果表明，由于在C–H活化与π配位的过渡态之前存在非统计σ-CH配位（σ-CH coordination）中间体，过渡态理论无法定性正确描述反应机理，也无法定量准确预测选择性。通过元动力学（metadynamics）与准经典直接动力学模拟，本研究发现Cp(PMe₃)₂Re与乙烯的反应轨迹可直接生成Re-乙烯基氢化物I或π配合物II。生成Re-乙烯基氢化物的反应轨迹会跳过σ配位步骤，且无需经历π配位过程。与实验结果一致，轨迹模拟给出的Re-乙烯基氢化物动力学选择性相对较低，与实验测得的10:1比例相符。