C–H Activation of Methane by Nickel–Methoxide Complexes: A Density Functional Theory Study

Density functional
theory is used to analyze methane C–H
activation by neutral and cationic nickel–methoxide complexes.
This research seeks to identify strategies to reduce high barriers
through evaluation of supporting ligand modifications, solvent polarity,
overall charge of complex, metal identity, and counterion effects.
A Ni–OMe pincer complex was substituted at the para positions of the phenyl and pyridine rings with different electron-donating
and -withdrawing groups, and the results showed that resonance effects
did not significantly change the ΔG⧧act and ΔGrxn compared to the reference complex, which
was confirmed by frontier orbitals plots and Hammett graphs. The effect
of solvent polarity is greater upon the thermodynamics more than transition
state energies. Among modeled supporting ligands, bipyridine was the
most promising. Overall, neutral complexes are calculated to have
lower activation barriers than the cationic complexes. For both neutral
and cationic complexes, the methane C–H activations proceed
via a σ-bond metathesis rather than an oxidative addition/reductive
elimination pathway. Barrier free energies for Ni complexes and its
precious metal Pt complex congener are comparable; the thermodynamics
for the latter are closer to thermoneutral than the Ni complexes.
Neutralizing the cationic catalyst models by a counterion, BF4–, has a considerable impact on reducing
the methane activation barrier free energy. Computed Ni–C bond
dissociation free energies suggest radical pathways are likely competitive.