Salt-Enhanced Oxidative Addition of Iodobenzene to Pd: An Interplay Between Cation, Anion, and Pd–Pd Cooperative Effects

Halide salts facilitate the oxidative addition of organic halides to Pd(0). This phenomenon originates from a combination of anionic, cationic, and Pd–Pd cooperative effects. Exhaustive computational exploration at the density functional theory level of the complexes obtained from [Pd0(PPh3)2] and a salt (NMe4Cl or LiCl) showed that chlorides promote phosphine release, leading to a mixture of mononuclear and dinuclear Pd(0) complexes. Anionic Pd(0) dinuclear complexes exhibit a cooperativity between Pd(0) centers, which favors the oxidative addition of iodobenzene. The higher activity of Pd(0) dimers toward oxidative addition rationalizes the previously reported kinetic laws. In the presence of Li+, the oxidative addition to mononuclear [Pd0L­(Li2Cl2)] is estimated barrierless. LiCl coordination polarizes Pd(0), enlarging both the electrophilicity and the nucleophilicity of the complex, which promotes both coordination of the substrate and the subsequent insertion into the C–I bond. These conclusions are paving the way to the rational use of the salt effects in catalysis for the activation of more challenging bonds.卤化物盐可促进有机卤化物向零价钯（Pd(0)）的氧化加成反应。该现象源于阴离子效应、阳离子效应以及钯-钯协同效应的共同作用。针对由三苯基膦合钯(0)（[Pd⁰(PPh₃)₂]）与四甲基氯化铵（NMe₄Cl）或氯化锂（LiCl）形成的配合物，研究人员通过密度泛函理论（Density Functional Theory, DFT）层面的详尽计算发现，氯离子可促进膦配体解离，进而生成单核与双核零价钯配合物的混合体系。阴离子型双核零价钯配合物中，两个Pd(0)中心间存在协同作用，这一作用可促进碘苯的氧化加成反应。零价钯二聚体更高的氧化加成活性，可对此前报道的动力学规律作出合理解释。在锂离子（Li⁺）存在的条件下，单核配合物[Pd⁰L(Li₂Cl₂)]的氧化加成过程被估算为无势垒反应。氯化锂的配位作用可极化零价钯中心，同时增强该配合物的亲电性与亲核性，既促进底物的配位过程，也推动后续碳碘键的插入反应。上述研究结论为在催化领域合理利用盐效应以活化更具挑战性的化学键铺平了道路。