Activation of H-H, C-H, C-C and C-Cl Bonds by Pd and PdCl(-). Understanding Anion Assistance in C-X Bond Activation.

To understand the mechanism of anion assistance in palladium-catalyzed H-H, C-H, C-C and C-Cl bond activation, several mechanistic pathways for oxidative addition of Pd and PdCl(-) to H2 (H-H), CH4 (C-H), C2H6 (C-C and C-H) and CH3Cl (C-Cl) were studied uniformly at the ZORA-BP86/TZ(2)P level of relativistic nonlocal density functional theory (DFT). Oxidative addition of the neutral, uncoordinated Pd atom proceeds, as reported earlier, via direct oxidative insertion (ΔH(⧧)298 is -22 to 10 kcal/mol), whereas straight SN2 substitution (yielding, e.g., PdCH3(+) + X(-)) is highly endothermic (144-237 kcal/mol) and thus not competitive. Anion assistance (i.e., going from Pd to PdCl(-)) lowers all activation barriers and increases the exothermicity of all model reactions studied. The effect is however selective:  it favors the highly endothermic SN2 mechanism over direct oxidative insertion (OxIn). Activation enthalpies ΔH(⧧)298 for oxidative insertion of PdCl(-) increase along C-H (-14.0 and -13.5 kcal/mol for CH4 and C2H6) ≈ C-Cl (-11.2 kcal/mol) < C-C (6.4 kcal/mol), i.e., essentially in the same order as for neutral Pd. Interestingly, in case of PdCl(-) + CH3Cl, the two-step mechanism of SN2 substitution followed by leaving-group rearrangement becomes the preferred mechanism for oxidative addition. The highest overall barrier of this pathway (-20.2 kcal/mol) drops below the barrier for direct oxidative insertion (-11.2 kcal/mol). The effect of anion assistance is analyzed using the Activation Strain model in which activation energies ΔE(⧧) are decomposed into the activation strain ΔE(⧧)strain of and the stabilizing transition state (TS) interaction ΔE(⧧)int between the reactants in the activated complex:  ΔE(⧧) = ΔE(⧧)strain + ΔE(⧧)int. For each type of activated bond and reaction mechanism, the activation strain ΔE(⧧)strain adopts characteristic values which differ only moderately, within a relatively narrow range, between corresponding reactions of Pd and PdCl(-). The lowering of activation barriers through anion assistance is caused by the TS interaction ΔE(⧧)int becoming more stabilizing.