Dinuclear Palladium Complexes of Pyrazole-Bridged Bis(NHC) Ligands: A Delicate Balance between Normal and Abnormal Carbene Coordination

Pyrazole-bridged bis­(imidazolium) salts [H3LR]­(PF6)2 (R = Et, nBu, C6H2Me3-2,4,6, and tBu), which are precursors to the corresponding pyrazole-bridged bis­(N-heterocyclic carbene) ligands, were found to give, upon reaction with Pd­(OAc)2, dinuclear palladium complexes with either normal or abnormal binding modes of the two NHC groups: via C2 in [LR2Pd2]­(PF6)2 (except R = tBu) and via C4/5 in [aLR2Pd2]­(PF6)2. It has been shown that the course of the reaction crucially depends on the amount of NH4OAc added, suggesting an acetate-assisted pathway leading to [LR2Pd2]­(PF6)2. Further reaction of [LR2Pd2]­(PF6)2 and [aLR2Pd2]­(PF6)2 with PdCl2 and NEt4Cl gave the corresponding neutral dinuclear complexes LEtPd2Cl3 and aLEtPd2Cl3 selectively, without any normal/abnormal rearrangement occurring during transmetalation. Only aLtBuPd2Cl3 is accessible directly from [H4LtBu]­Cl3 and Pd­(OAc)2. All complexes have been characterized by NMR spectroscopy and elemental analysis, and several of them have also been characterized by ESI mass spectrometry and single-crystal X-ray diffraction. The observed binding modes and structural features have been rationalized by density functional theory calculations, which evidence that for a given complex the thermodynamically favored conformer is found in the solid state.

吡唑桥联双(咪唑鎓)盐[H₃LR](PF₆)₂（R = 乙基Et、正丁基nBu、2,4,6-三甲基苯基C₆H₂Me₃以及叔丁基tBu）作为对应吡唑桥联双(N-杂环卡宾)配体的前体，研究发现其与乙酸钯(Pd(OAc)₂)反应后，可生成具有两种NHC基团结合模式的双核钯配合物：当NHC通过C2位配位时，得到[LR₂Pd₂](PF₆)₂（R = tBu除外）；当通过C4/5位配位时，则得到[aLR₂Pd₂](PF₆)₂。研究表明，反应进程关键取决于所添加的乙酸铵(NH₄OAc)的用量，这提示存在一条由乙酸根辅助的、生成[LR₂Pd₂](PF₆)₂的反应路径。将[LR₂Pd₂](PF₆)₂与[aLR₂Pd₂](PF₆)₂分别与氯化钯(PdCl₂)及四乙基氯化铵(NEt₄Cl)进行进一步反应，可高选择性地得到对应的中性双核配合物LEtPd₂Cl₃与aLEtPd₂Cl₃，且在转金属化过程中未发生正常/异常配位模式的重排。仅aLtBuPd₂Cl₃可直接由[H₄LtBu]Cl₃与乙酸钯反应制得。所有配合物均通过核磁共振波谱(Nuclear Magnetic Resonance, NMR)与元素分析完成了结构表征，其中部分配合物还通过电喷雾电离质谱(Electrospray Ionization Mass Spectrometry, ESI-MS)及单晶X射线衍射进行了表征。观测到的配位模式与结构特征可通过密度泛函理论(Density Functional Theory, DFT)计算得到合理解释，计算结果证实，对于任一配合物而言，其热力学优势构象均存在于固态样品中。