Dinuclear Palladium(II) and -(III) Compounds with O,O-Chelating Ligands. Room-Temperature Direct 2‑Phenylation of 1‑Methylindole

New dinuclear palladium­(III) compounds of general formula Pd2[(C6H4)­PPh2]2[O–O]2Cl2, O–O being chelating phenolates C6H4OC­(O)­R (R = CH3, 3a; R = C2H5, 3b; R = OPh, 3c) or acetylacetonates RC­(O)­CHC­(O)­R (R = CH3, 4a; R = CF3, 4b; R = C­(CH3)3, 4c), have been obtained by oxidation with PhICl2 of the corresponding palladium­(II) compounds. The stability of the new compounds has been studied by 31P NMR spectroscopy from 200 to 298 K. DFT calculations of the stability of the complexes have also been performed. In agreement with these calculations, only compound Pd2[(C6H4)­PPh2]2[(CF3C­(O)­CHC­(O)­CF3]2Cl2, 6b, showed the highest thermal stability. 6b was characterized by X-ray diffraction methods, presenting the longest Pd–Pd distance, 2,6403(6) Å, observed among the already described discrete Pd26+ compounds. The isolated palladium­(II) and -(III) compounds have been tested at room temperature in the catalytic 2-phenylation of 1-methylindole with [Ph2I]­PF6. With 3a as precatalyst the reaction was completed in 2 h with a 93% isolated yield. The results were compared with those obtained with other orthometalated dinuclear and mononuclear palladium compounds.

本研究通过二氯碘苯(PhICl₂)氧化相应的钯(II)前驱配合物，成功合成了通式为Pd₂[(C₆H₄)PPh₂]₂[O–O]₂Cl₂的新型双核钯(III)配合物；其中O–O为螯合酚盐配体C₆H₄OC(O)R（R=CH₃，对应3a；R=C₂H₅，对应3b；R=OPh，对应3c）或乙酰丙酮类配体RC(O)CHC(O)R（R=CH₃，对应4a；R=CF₃，对应4b；R=C(CH₃)₃，对应4c）。该类配合物的稳定性通过31P核磁共振波谱法在200至298 K的温度范围内被考察，同时开展了其稳定性的密度泛函理论(DFT)计算。计算结果与实验观测一致，仅配合物Pd₂[(C₆H₄)PPh₂]₂[(CF₃C(O)CHC(O)CF₃)]₂Cl₂（编号6b）表现出最优的热稳定性。6b经X射线衍射法表征，其Pd-Pd键长为2.6403(6) Å，是已报道的分立型Pd₂⁶+配合物中所观测到的最长Pd-Pd键长。本研究在室温条件下测试了分离得到的钯(II)与钯(III)配合物对1-甲基吲哚与[Ph₂I]PF₆的2-苯基化反应的催化性能；以3a作为预催化剂时，反应可在2小时内完成，分离收率达93%。研究还将该催化结果与其他邻金属化双核及单核钯配合物的催化表现进行了对比。