A new series of Pd-catalysts based on an N-heterocyclic carbene PEPPSI-type ligand (PEPPSI = pyridine enhanced precatalyst preparation stabilization and initiation) with the general formula [Pd(II)Br2(NHC)(pyridine)] was synthesized and fully characterized via spectroscopic analytical methods. Further, the structural characterization of 3a, 3b, and 3d was performed via single-crystal X-ray diffraction, which supported the proposed structures and offered a more detailed structural characterization. Additionally, the electronic, vibrational, thermodynamic, and optical properties of 3a, as a representative molecule, were confirmed utilizing density functional theory (DFT) calculations. The experimental molecular geometry of the ground state of complex 3a was compared with the minimized structure obtained from DFT calculations. The B3LYP functional in conjunction with the LANL2DZ basis set for the palladium atom and the 6-311G(d,p) basis set for the hydrogen, carbon, nitrogen, and bromine atoms were used for all calculations. The frontier molecular orbitals and molecular electrostatic potential were also analyzed and discussed. Due to the significant interest in halo-substituted arylated heteroarenes in organic chemistry, the catalytic activity of these Pd(II)–NHC PEPPSI–type complexes were evaluated via the direct arylation of five-membered heteroaromatics such as thiophene, furan, and thiazole derivatives with various (hetero)aryl bromides in the presence of 1 mol% catalyst. The results showed that all new Pd–NHC complexes are effective catalysts, which exhibited very good catalytic activity and gave C-H activation selectively at the C(5)-position of 2-acetyl furan and 2-acetyl thiophene.

中文翻译：

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五元杂芳烃与新型钯-N-杂环卡宾PEPPSI型配合物的芳基溴化物直接芳基化的合成、结构、DFT计算和催化应用

合成了一系列基于 N-杂环卡宾 PEPPSI 型配体（PEPPSI = 吡啶增强预催化剂制备稳定和引发）的新系列 Pd 催化剂，通式为 [Pd( II )Br2(NHC)(吡啶)]其特征在于通过分光分析方法。此外，3a、3b和3d的结构表征是通过单晶 X 射线衍射进行的，这支持了所提出的结构并提供了更详细的结构表征。此外，3a的电子、振动、热力学和光学性质作为代表性分子，利用密度泛函理论 (DFT) 计算得到了确认。将配合物3a基态的实验分子几何形状与从 DFT 计算获得的最小化结构进行比较。B3LYP 泛函结合钯原子的 LANL2DZ 基组和氢、碳、氮和溴原子的 6-311G(d,p) 基组用于所有计算。还分析和讨论了前沿分子轨道和分子静电势。由于在有机化学卤素取代的芳基化的杂芳烃兴趣显著，这些钯（的催化活性II）-NHC PEPPSI型配合物进行了评价通过在 1 mol% 催化剂存在下，五元杂芳烃如噻吩、呋喃和噻唑衍生物与各种（杂）芳基溴化物的直接芳基化。结果表明，所有新的 Pd-NHC 配合物都是有效的催化剂，表现出非常好的催化活性，并在 2-乙酰基呋喃和 2-乙酰基噻吩的 C(5) 位选择性地活化 CH。