Zerovalent Palladium and Nickel Complexes of Heterocyclic Carbenes: Oxidative Addition of Organic Halides, Carbon−Carbon Coupling Processes, and the Heck Reaction

Abstract

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 PdII derivatives Pd(tmiy)2(Ph)I (4), Pd(tmiy)2I2 (5), Pd(tmiy)2(4-nitrophenyl)I (6), and Pd(tmiy)2Br2 (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)2I2 (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 coordi...