Computational Evaluation of the σ‑Complex Lifetime and Origin of the Inverse Kinetic Isotope Effect for Methane Reductive Elimination from Cationic Cp₂Re(H)(CH₃)

Metal-alkane σ-complexes are often key intermediates used to rationalize experimental observations, such as kinetic isotope effects (KIEs). Reductive elimination of methane from [Cp2Re(H)(CH3)]+ (1) has been proposed to occur through a σ-complex intermediate and has a measured inverse KIE. Density functional theory (DFT) calculations and explicit solvent molecular dynamics simulations were used to examine the origin and lifetime of the σ-complex intermediate and inverse KIE. While the methane σ-complex is a stationary-point structure, inclusion of zero-point energy (ZPE) suggests that the σ-complex is not an intermediate, and dynamics trajectories showed an extremely short lifetime. Trajectories also revealed that the transition state for C–H bond formation results in either extremely fast reformation of 1 or fast hydrogen exchange coupled with reforming 1. This means that a stable methane σ-complex intermediate is not required to rationalize relative rates of hydrogen exchange versus methane elimination or the inverse KIE value. The calculated KIE value using the methane dissociation transition state was inverse and close to the experimental value. Analysis of the KIE components, although dependent on which density functional used, suggests that in addition to ZPE, thermal excitation energy of deuterium vibrational modes drives the KIE to an inverse value.金属-烷烃σ配合物（σ-complex）常作为关键中间体，用于阐释诸如动力学同位素效应（KIEs）等实验现象。从[Cp₂Re(H)(CH₃)]⁺（1）中还原消除甲烷的过程，被认为经由σ配合物中间体进行，且实验测得其具有逆动力学同位素效应。本研究采用密度泛函理论（DFT）计算与显式溶剂分子动力学模拟，对该σ配合物中间体的起源、寿命以及逆动力学同位素效应的成因展开探究。尽管甲烷σ配合物属于驻点结构，但零点能（ZPE）修正结果表明，该σ配合物并非真正的中间体；同时动力学轨迹模拟显示其寿命极短。轨迹分析还显示，C-H键形成的过渡态要么会快速重新生成化合物1，要么伴随快速氢交换后重新生成化合物1。这意味着，要解释氢交换与甲烷消除的相对速率，抑或是逆动力学同位素效应的数值，并不需要稳定的甲烷σ配合物中间体。基于甲烷解离过渡态计算得到的动力学同位素效应数值为逆效应，且与实验值较为接近。尽管动力学同位素效应的组分分析结果依赖于所选用的密度泛函，但研究表明，除零点能外，氘振动模式的热激发能同样会推动动力学同位素效应呈现逆值。