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.