Theoretical Measurements of Quantitative Effects Caused by Spectator Ligands on Palladium-catalyzed C-H Activation

Ligands can definitely influence C-H activation at the metal center. A ligand not directly participating in the reaction is called a spectator ligand. We attempt to quantitatively characterize the effects of diverse spectator ligands on C-H activation at palladium. We designed a model palladium catalyst and selected an array of spectator ligands, such as methoxyl, amide, methyl, phenyl, cyanide, fluorine, chlorine, and several neutral ligands, and performed density functional theory calculations on the mechanism and energetics of C-H activation reactions of benzene with different catalysts. Univalent ligands have substantially larger effects than neutral ligands, and strongly sigma-donating ligands (e.g., methyl and phenyl) severely hinder the C-H activation in progress. A ligand trans to the reaction site influences C-H activation more than that cis to the reaction site, indicating electronic effects to be at work. For example, the existence of a methyl ligand raises the barrier height of C-H activation by 6.4 or 14.4 kcal/mol when it is placed at the position cis or trans to the C-H activation site. The effects of poorly sigma-donating ligands are not significant and similar to those of the kappa(1)-acetate ligand. Some sigma-donating and pi-accepting ligands, such as cyanide and isonitrile, hinder the C-H activation trans to them but appear to facilitate the C-H activation cis to them. On the basis of molecular orbital analyses, a chemical model is proposed to understand the observed ligand effects. Lastly, the conclusions are applied to explain the plausible mechanism of the dehydrogenative Heck coupling.

Automated summary

Ligands can have a significant influence on C-H activation at the metal center. Univalent ligands and strongly sigma-donating ligands hinder the C-H activation process, while poorly sigma-donating ligands have minimal effects. Ligands trans to the reaction site have a greater impact on C-H activation than those cis to the reaction site.