Carbon−Hydrogen Bond Activation: Two, Three, or More Mechanisms?

Abstract

Classic and newly proposed mechanisms for metal-mediated hydrogen atom transfer (HAT) were analyzed with density functional theory (DFT) and Bader's atoms in molecules (AIM) analysis. The two classic mechanisms for HAT are σ-bond metathesis (σBM) and oxidative addition/reductive elimination (OA/RE); the σBM mechanism was modeled by methane addition to [Cp2Sc(CH3)], and the transition state (TS) for HAT is characterized by two Sc−C and C−H bond critical points (BCP) and one ring critical point (RCP) in the center of the four-centered geometry of the TS (1). The OA/RE mechanism, modeled by methane addition to [Cp*Ir(PMe3)(CH3)]+, proceeds through an IrV intermediate (2) that is characterized by two Ir−C, BCPs, one Ir−H BCP, and no C−H interactions. Newly proposed mechanisms of intermediate character, such as metal-assisted σBM (MAσBM), oxidative hydrogen migration (OHM), and oxidatively added transition state (OATS)/σ-complex assisted metathesis (σ-CAM), were analyzed with AIM analysis, and the bonding patterns were compared to σBM and OA/RE. This analysis revealed a spectrum of mechanisms with a variety of changing bonding patterns between the two classic extremes.