Metal and Ligand Effects on Coordinated Methane pK_a: 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 pK_a of coordinated methane and the free energy barriers (ΔG^⧧) to subsequent H₃C-H activation. The few outliers to this relationship indicated other salient factors (such as thermodynamic stability of the product and ligand-metal coordination type) that impacted the methane activation barrier. High variations in the activation barriers and pK_a values were found with a range of 34.8 kcal/mol for the former and 28.6 pK_a units for the latter. Clear trends among specific metals and ligands were also derived; metal ions such as Co^I, as well as Lewis acids and π-acids, consistently yielded higher acidity for ligated methane and hence lower ΔG^⧧.