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

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.