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

Abstract

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.

The Electronics of CH Activation by Energy Decomposition Analysis: From Transition Metals to Main-Group Metals

“…The Bickelhaupt group has applied the ADF-EDA scheme to a number of model alkane CH activation reactions (for select examples, see [61][62][63][64]). In one report, insertion transition states were analyzed for model CH activation reactions between methane and zero-valent group 2 (Be, Mg, and Ca) and group 12 (Zn and Cd) metals [61][62][63][64].…”

“…In one report, insertion transition states were analyzed for model CH activation reactions between methane and zero-valent group 2 (Be, Mg, and Ca) and group 12 (Zn and Cd) metals [61][62][63][64]. One motivation of this study was to provide an understanding based on transition state interactions why the lowest oxidation state of s-block main-group elements compared to Pd(0) have dramatically larger activation barriers for methane CH bond insertion.…”

“…The Bickelhaupt group also examined model CH activation reactions between methane and low-oxidation state group-11 transition-metal cations Cu(I), Ag(I), and Au(I), with a comparison to Pd(0) [61][62][63][64]. This study revealed that at the beginning of the reaction pathway the low-oxidation state group II cations are more effective electrophiles than Pd(0) due to the much lower energy of the 4s, 5s, or 6s LUMOs.…”

Computational study of the transmetalation process in the Suzuki–Miyaura cross-coupling of aryls

“…The whole mechanism could be described in three main steps [2,23]: (a) oxidative addition, (b) transmetalation, and (c) reductive elimination. Contrary to steps (a) and (c), that have been the subject of several experimental [24][25][26][27][28][29][30] and theoretical [31][32][33][34][35][36][37][38][39][40] studies less is known about the transmetalation process [41][42][43][44][45][46][47][48][49][50]. This is thus the most characteristic step of the cross-coupling reactions.…”

A theoretical study of the dihydrogen molecule confined inside carbon nanotubes

“…It is reported by Diefenbach et al that activated HAH bond length is equal to 1.383 Å . [41] Lows et al [42] and Rabaii et al [43] indicated that the length of HAH bond activation can reach even 2.14 and 2.79 Å , respectively. One would intuitively think that HAH bond length will decrease monotonously as the diameter of the nanotube increases and reach its equilibrium value inside the tube of larger diameter.…”