The molecular and electronic structure of carbon–hydrogen bond activation and transition metal assisted hydrogen transfer

Following the optimization of the geometry by density functional theory (DFT), we analyze midpoint species along the reaction coordinate for hydrogen transfer in the reaction R′–H + M–R → R–H + M–R′ with Bader’s “Atoms in Molecules” (AIM) analysis. The two “classic” mechanisms for hydrogen transfer, σ-bond metathesis (σBM) and oxidative addition/reductive elimination (OA/RE), are well represented by the reaction of CH4 with [Cp2ScCH3] and [CpIr(PH3)(CH3)]+, respectively (Cp = η5-C5H5). The midpoint species for these two pathways are the transition state [Cp2Sc(CH3)2H]‡ and the OA intermediate, [CpIr(PH3)(CH3)2H]+. Bond (B) critical points (CP) were located along the Sc–C and C–H coordinates of the four-center geometry of [Cp2Sc(CH3)2H]‡; a ring (R) CP was located in the center. For the intermediate [CpIr(PH3)(CH3)2H]+, bond critical points (BCPs) were located only along the Ir–C and Ir–H coordinates of the four-center geometry. Most of the new mechanisms that have been proposed are characterized by single transition states with short M–H distances connecting reactants with products. Representative models of a variety of alternative mechanisms were analyzed; in all, seven different bonding patterns were identified that describe seven unique mechanisms for hydrogen transfer. In general, those metal centers that have d electrons available to participate in hydrogen transfer interact with the transferring hydrogen during this reaction as the hydrogen atom is nearly unique in being able to support omni-directional bridging bonds. Basis sets and density functional effects were investigated with [Cp2Sc(CH3)2H]‡ as the model. Seventeen basis sets were assigned to scandium and the densities at the Sc–C BCP increased slightly up to the quadruple-ζ level in the basis. The densities at the C–H BCPs are constant, but the densities at the ring critical point (RCP) also increased slightly. Ten density functionals were tested and the densities at the BCPs and RCP are constant, but the Sc–C BCP moved slightly farther from the scandium as the amount of exact exchanged admixed into the functional increased.