Reactivity Differences of Palladium(II) Dimers Bearing Heterocyclic Carbenes with Two, One, or No α-Nitrogen Atoms toward Isocyanides

Pd(II) dimers [PdI2(rNHC)]2 (1a,b) of pyrazole-based remote N-heterocyclic carbenes (rNHCs) have been synthesized through oxidative addition of 4-iodopyrazolium iodides (A, B) to [Pd2(dba)3]. Reaction of 1a with aromatic (CN-Xyl) or aliphatic (CN-Cy) isocyanides led to the template-assisted formation of novel Pd(II) dimers 8 and 9 bearing betainic C-imino ligands via isocyanide insertion into Pd−CrNHC bonds and subsequent dimerization. In contrast, both isocyanides reacted with the dimers [PdI2(Me2-indy)]2 (2) and [PdI2(Me2-bimy)]2 (3) bearing indazolin-3-ylidenes and benzimidazolin-2-ylidenes with formation of mononuclear mixed carbene/isocyanide complexes 4−7. Notably, only dimer 8 underwent further bridge cleavage with excess isocyanide, yielding the mixed C-imino/CN-Xyl complex 10, while dimer 9 remained intact. These results highlight the uniquely different reactivities of complexes with carbenes having no α-nitrogen versus those with one or two α-nitrogen atoms, as a result of their decreasing donor abilities.

基于吡唑的远程氮杂环卡宾（remote N-heterocyclic carbenes，rNHCs）的二聚钯(II)配合物[PdI₂(rNHC)]₂（1a、1b），通过4-碘吡唑鎓碘化物（A、B）与三(二亚苄基丙酮)二钯([Pd₂(dba)₃])的氧化加成反应合成得到。将配合物1a分别与芳香族异氰化物（CN-Xyl）或脂肪族异氰化物（CN-Cy）反应，经由异氰化物插入Pd−rNHC配位键并随后发生二聚化，通过模板辅助策略得到了带有甜菜碱型C-亚氨基配体（betainic C-imino ligands）的新型二聚钯(II)配合物8和9。与之形成对比的是，两种异氰化物均与分别带有吲唑啉-3-亚基（indazolin-3-ylidenes）和苯并咪唑啉-2-亚基（benzimidazolin-2-ylidenes）的二聚钯(II)配合物[PdI₂(Me₂-indy)]₂（2）及[PdI₂(Me₂-bimy)]₂（3）反应，生成了单核混合卡宾/异氰化物配合物（mononuclear mixed carbene/isocyanide complexes）4−7。值得注意的是，仅配合物8可与过量异氰化物进一步发生桥键断裂（bridge cleavage），得到混合C-亚氨基/CN-Xyl配合物10，而配合物9则保持结构完整。上述结果表明，不含α-氮原子的卡宾配合物与带有一个或两个α-氮原子的卡宾配合物，其反应活性存在显著差异，这源于后者给电子能力（donor abilities）逐渐减弱。