Synthesis, Structure, Ligand Dynamics, and Catalytic Activity of Cationic [Pd(η3-allyl)(κ2(E,N)-EN-chelate)]+ (E = P, O, S, Se) Complexes

A series of cationic palladium allyl complexes of the type [Pd(η3-allyl)(κ2(E,N)-EN-chelate)]+ containing several heterodifunctional EN (E = P, O, S, Se) ligands based on N-(2-pyridinyl)aminophosphines and oxo, thio, and seleno derivatives thereof are prepared. These complexes are studied by one- and two-dimensional NMR techniques together with X-ray and DFT calculations. Variable-temperature and phase-sensitive 1H,1H NOESY NMR measurements reveal both allyl and EN ligand dynamics. In the case of palladium, PN complexes' η3 to η1 isomerization takes place by opening the η3-allyl group selectively at the trans position with respect to the phosphorus center, while for EN (E = O, S, Se) complexes an “apparent” allyl rotation is observed proceeding with Pd–E and Pd–N bond breaking. DFT calculations indicate that both isomerization processes are solvent assisted, in agreement with the NMR data. In addition, the use of the new palladium allyl complexes has been examined as catalysts for Suzuki–Miyaura coupling of various aryl bromides and arylboronic acids. [Pd(η3-CHPhCHCH2)(ON-Ph)]+, bearing an η3-cinnamyl ligand, is one of the most efficient catalysts, converting aryl bromides and arylboronic acids at 80 °C with a catalyst loading of 0.1 mol % quantitatively into the expected biaryl products.本研究合成了一系列通式为[Pd(η3-烯丙基)(κ2(E,N)-EN-螯合配体)]+的阳离子钯烯丙基配合物，该类配合物包含多款基于N-(2-吡啶基)氨基膦及其氧代、硫代、硒代衍生物的异双功能EN（E=P、O、S、Se）配体。采用一维、二维核磁共振（NMR）技术，结合X射线衍射与密度泛函理论（DFT）计算对该系列配合物开展了系统研究。变温相敏型1H-1H核Overhauser效应光谱（NOESY）核磁共振测试结果表明，烯丙基配体与EN配体均存在动态行为。对于含磷的PN型配合物，其η3到η1异构化过程通过相对于磷中心的反式位点选择性开环η3-烯丙基实现；而对于E=O、S、Se的EN型配合物，则观察到表观烯丙基旋转过程，该过程伴随Pd-E与Pd-N键的断裂。密度泛函理论计算结果显示，两种异构化过程均受溶剂辅助，与核磁共振实验数据相符。此外，本研究还考察了这类新型钯烯丙基配合物作为催化剂，用于各类芳基溴化物与芳基硼酸的铃木-宫浦（Suzuki-Miyaura）偶联反应的催化性能。其中，[Pd(η3-CHPhCHCH2)(ON-Ph)]+配合物（带有η3-肉桂基配体）是催化性能最优的催化剂之一，该催化剂在80℃、催化剂负载量为0.1 mol%的条件下，可将芳基溴化物与芳基硼酸定量转化为目标联芳基产物。