Zerovalent palladium and nickel complexes of heterocyclic carbenes: Oxidative addition of organic halides, carbon-carbon coupling processes, and the Heck reaction

Zerovalent carbene complexes of Pd containing the 1,3,4,5-tetramethylimidazol-2-ylidene ligand (tmiy) have been synthesized. Pd(COD)(alkene) (COD = cyclooctadiene) reacts with the nucleophilic carbene tmiy to produce the complexes Pd(tmiy)(2)(alkene) (alkene = maleic anhydride (MAH) (2), tetracyanoethylene (TCNE) (3)). Spectroscopic studies on the complexes provide strong evidence of the almost purely donor nature of the carbene ligand. Oxidative addition of hydrocarbyl halide and dihalide substrates to 2 and 3 yield the Pd(II) derivatives Pd(tmiy)(2)(Ph)I (4), Pd(tmiy)(2)I(2) (5), Pd(tmiy)(2)(4-nitrophenyl)I (6), and Pd(tmiy)(2)Br(2) (7). The zerovalent Ni complex Ni(tmiy)(2) was produced in situ from the reaction of Ni(COD)(2) with tmiy, and the oxidative addition of organic halides yields Ni(tmiy)(2)(o-tolyl)Br (8), Ni(tmiy)(2)(Me)I (9), and Ni(tmiy)(2)I(2) (10). X-ray crystal structures of complexes 8 and 10 are reported which reveal square-planar coordination with the carbene ligands inclined at significant angles to the coordination planes of the complexes. Metal to ligand bond distances are all indicative of the electron density induced on the metal center by the donor carbene ligands. Halide abstraction from 6 in the presence of n-butyl acrylate leads to migratory insertion of the olefin and elimination of the Heck coupling product, Under stoichiometric reaction conditions hydrocarbyl-imidazolium ions are also produced as byproducts. Selected complexes were tested as catalysts for the Heck reaction (Pd) and Suzuki coupling (Pd, Ni) and were found to be highly active. The observed reaction behavior is interpreted, and a mechanism for the Heck coupling is provided.