Abnormal-NHC Palladium(II) Complexes: Rational Synthesis, Structural Elucidation, and Catalytic Activity

Reaction of a C2-arylated imidazolium iodide (IPrPh)I (1) (IPrPh = 1,3-bis­(2,6-diisopropylphenyl)-2-phenyl-imidazolium) with PdCl2 in the presence of Ag2O affords abnormal N-heterocyclic carbene (aNHC) palladium complexes (aIPrPh)­PdCl2 (2) and (aIPrPh)2PdCl2 (3) (aIPrPh = 1,3-bis­(2,6-diisopropylphenyl)-2-phenyl-imidazol-4-ylidene). Treatment of 2 with a pyridine gives Pd-PEPPSI-type complexes (aIPrPh)­PdCl2(L) (L = pyridine (py), 5; L = 3-chloropyridine (3Cl-py), 6). Compounds 5 and 6 are also accessible by a one-pot reaction of 1, PdCl2, and Ag2O in a pyridine solvent. While the use of a conventional base K2CO3 leads to the formation of mixed halide complexes (aIPrPh)­Pd­(Cl)­I­(L) (7, L = py; 8, L = 3Cl-py), iodide derivatives (aIPrPh)­PdI2(L) (9, L = py; 10, L = 3Cl-py) can be selectively prepared with addition of an excess of KI to the reaction mixture. Albeit in a low yield, a putative transmetalation agent {(aIPrPh)2Ag}­AgI2 (4) has been isolated and characterized. Compounds 2–10 are air stable crystalline solids and have been characterized by elemental analysis, mass spectrometry, and NMR spectroscopic studies. Molecular structures of 2–10 have been established by single crystal X-ray diffraction analyses. Catalytic activity of three representative compounds 2, 5, and 6 has been tested for the Suzuki-Miyaura cross-coupling reactions.

将C2-芳基化咪唑鎓碘化物（C2-arylated imidazolium iodide）(IPrPh)I (1) [其中IPrPh = 1,3-双(2,6-二异丙基苯基)-2-苯基咪唑鎓]与氯化钯（PdCl2）在氧化银（Ag2O）存在下反应，可得到反常N-杂环卡宾（abnormal N-heterocyclic carbene, aNHC）钯配合物(aIPrPh)PdCl2 (2)与(aIPrPh)2PdCl2 (3) [其中aIPrPh = 1,3-双(2,6-二异丙基苯基)-2-苯基咪唑-4-亚基]。将化合物2与吡啶类化合物反应，可得到Pd-PEPPSI型配合物(aIPrPh)PdCl2(L)，其中L=吡啶（py）时为5，L=3-氯吡啶（3Cl-py）时为6。化合物5与6也可通过1、PdCl2与Ag2O在吡啶溶剂中进行一锅法反应制备得到。当使用常规碱碳酸钾（K2CO3）时，反应会生成混合卤化物配合物(aIPrPh)Pd(Cl)I(L)（7，L=py；8，L=3Cl-py）；而向反应体系中加入过量碘化钾（KI）时，则可选择性制备得到碘代衍生物(aIPrPh)PdI2(L)（9，L=py；10，L=3Cl-py）。尽管产率较低，但我们成功分离并表征了一种疑似转金属化试剂（transmetalation agent）{(aIPrPh)2Ag}AgI2 (4)。化合物2~10均为空气稳定的结晶固体，通过元素分析、质谱（mass spectrometry）分析与核磁共振（NMR）波谱学研究完成了表征。化合物2~10的分子结构通过单晶X射线衍射（single crystal X-ray diffraction）分析得以确定。我们测试了3种代表性化合物2、5与6在铃木-宫浦交叉偶联反应（Suzuki-Miyaura cross-coupling reactions）中的催化活性。