Metal and Ligand Effects on Coordinated Methane pKa. Direct Correlation with the Methane Activation Barrier
DFT and coupled cluster methods were used to investigate the impact of 3d metals and ligands upon the acidity and activation of coordinated methane C–H bonds. A strong, direct relationship was established between the pKa of coordinated methane and the subsequent free energy barriers to H3C–H activation. The few outliers to this relationship indicated other factors– such as thermodynamic stability of the product and ligand-metal coordination type – also impacted the methane activation barrier (dG‡). High variations in the activation barriers and pKa values were found with a range of 34.8 kcal/mol for the former and 28.6 pKa units for the latter. Clear trends among specific metals and ligands were also derived; specific metals, such as CoI, as well as Lewis and p-acids consistently yielded higher acidity for the ligated methane and hence lower dG‡.