Theoretical Study of N–H σ‑Bond Activation by Nickel(0) Complex: Reaction Mechanism, Electronic Processes, and Prediction of Better Ligand

N–H σ-bond activation of alkylamine by Ni­(PCy3) was investigated using density functional theory (DFT) calculations. When simple alkylamine NHMe2 is a reactant, both concerted oxidative addition in Ni­(PCy3)­(NHMe2) and ligand-to-ligand H transfer reaction in Ni­(PCy3)­(C2H4)­(NHMe2) are endergonic and need a high activation energy. When NH­(Me)­(Bs) (Bs = SO2Ph, a model of tosyl group used in experiments) is a reactant, both reactions are exergonic and occur easily with a much smaller activation energy. The much larger reactivity of NH­(Me)­(Bs) than that of NHMe2 results from the stronger Ni–N­(Me)­(Bs) bond than the Ni–NMe2 bond and the presence of the Ni–O bonding interaction between the Bs group and the Ni atom in the product. N-Heterocyclic carbene, 1,3-bis­(2,6-diisopropylphenyl)­imidazol-2-ylidene (IPr), is computationally predicted to be better than PCy3 because the Ni–NMe2 and Ni–N­(Me)­(Bs) bonds in the IPr complex are stronger, respectively, than those of the PCy3 complex. The introduction of the electron-withdrawing Bs group to the N atom of amine and the use of IPr as a ligand are recommended for the N–H σ-bond activation. The C–H σ-bond activations of benzene via the oxidative addition and the ligand-to-ligand H transfer reaction were also investigated here for comparison with the N–H σ-bond activation. The differences between the C–H σ-bond activation of benzene and the N–H σ-bond activation of these amines are discussed in terms of the N–H, C–H, Ni–Ph, and Ni–NMe2, and Ni–N­(Me)­(Bs) bond energies and back-donation to benzene from the Ni atom.