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.