Synthesis and Characterization of Palladium Pincer Bis(carbene) CCC Complexes

The metalation of the ^DIPPCCC (^DIPPCCC = bis(diisopropylphenyl-imidazol-2-ylidene)phenyl) ligand platform with Pd was achieved under mild conditions by reacting [H₃(^DIPPCCC)]Cl₂ with Pd(OAc)₂ at room temperature in the presence of 3.1 equiv of LiN(SiMe₃)₂. The resulting complexes (^DIPPCCC)PdX (X = Cl or Br) were oxidized by two-electron oxidants PhICl₂, Br₂, and BTMABr₃. All the complexes were crystallographically characterized, and analysis of structural parameters around the ligand scaffold show no evidence of a ligand-centered radical, rendering the metal center in the oxidized species, (^DIPPCCC)PdX₃ (X = Cl or Br), a formal Pd^IV oxidation state. Unlike their Ni^IV analogues, these Pd^IV complexes are stable to air and moisture. The addition of styrene to (^DIPPCCC)PdBr₃ resulted in the clean reduction of Pd^IV to Pd^II, along with the formation of the halogenated alkane. The oxidation to Pd^IV and subsequent return to Pd^II upon reduction, as opposed to formation of Pd^III species, showcases the accessibility of high-valent palladium ^DIPPCCC complexes.