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.