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 [Pd-2(dba)(3)]. Reaction of I a 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-C-rNHC bonds and subsequent dimerization. In contrast, both isocyanides reacted with the dimers [PdI2(Me-2-indy)](2) (2) and [PdI2(Me-2-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 a-nitrogen versus those with one or two a-nitrogen atoms, as a result of their decreasing donor abilities.