Carbon-hydrogen bond activation: Two, three, or more mechanisms?

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 G-bond metathesis (sigma BM) and oxidative addition/reductive elimination (OA/RE); the sigma BM 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 Ir-v 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 sigma BM (MA sigma BM), oxidative hydrogen migration (OHM), and oxidatively added transition state (OATS)/sigma-complex assisted metathesis (sigma-CAM), were analyzed with AIM analysis, and the bonding patterns were compared to sigma BM and OA/RE. This analysis revealed a spectrum of mechanisms with a variety of changing bonding patterns between the two classic extremes.