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

Abstract

The metalation of the DIPP CCC ( DIPP CCC = bis(diisopropylphenyl-imidazol-2-ylidene)phenyl) ligand platform with Pd was achieved under mild conditions by reacting [H 3 ( DIPP CCC)]Cl 2 with Pd(OAc) 2 at room temperature in the presence of 3.1 equiv of LiN(SiMe 3 ) 2 . The resulting complexes ( DIPP CCC)PdX (X = Cl or Br) were oxidized by two-electron oxidants PhICl 2 , Br 2 , and BTMABr 3 . 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, ( DIPP CCC)PdX 3 (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 ( DIPP CCC)PdBr 3 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 DIPP CCC complexes.

An N-heterocyclic carbene-based pincer system of palladium and its versatile reactivity under oxidizing conditions

“…First, NHCs are known to be strong electron donors, which have enabled the isolation of a handful of high-oxidation-state organopalladium( iv ) species. 4,5 In addition, pincers, as mer -tridentate chelators, are generally believed to enhance the thermostability of the corresponding complexes of transition metals in various oxidation states, 6 including Pd II and Pd IV . 7…”