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 Delta G double dagger(act) and Delta G(rxn), 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 sigma-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.