Synthesis And Characterization Of Nickel, Cobalt Complexes Supported By N-Heterocyclic Carbenes And Their Applications In Cross Coupling Reactions

The exploration of environmentally friendly, air-stable, and highly efficient metal catalysts is of practical importance in organic chemistry. Recently, N-heterocylic carbenes (NHCs) have become a very important class of ligands in organometallic chemistry and catalysis due to their excellentσ-donor properties, ease of synthesis, and variable steric bulk. We have synthesized a series of air-stable metal catalysts for cross-coupling reactions. These metal complexes based on multidentate NHC ligands were synthesized by carbene transfer reactions. This dissertation mainly includes three parts:First, triangular Ag3 and linear chain Ag4 complexes have been synthesized and structurally characterized via the simple deprotonation reaction of the pyridine-functionalized imidazolium salts with Ag2O. Short silver-silver contacts have been observed. The carbene transfer reactions of these silver complexes with Ni(DME)Cl2 afforded square-planar nickel(Ⅱ) complexes supported by pyridine-functionalized N-heterocylic carbene ligands. The Ni(II)-NHC system was demonstrated to be efficient and high yielding for the Suzuki coupling reactions of aryl chlorides and bromides under mild conditions. The Kumada cross-coupling reaction of a variety of unactivated aryl chlorides, vinyl chlorides, and heteroaryl chlorides catalyzed by these nickel (Ⅱ) complexes is also described. These catalysts are so active that allow the reactions proceed at room temperature in excellent yields.Second, we describe the first nickel-NHC catalyzed Negishi cross-coupling reaction of a variety of unactivated aryl chlorides, heterocyclic chlorides, aryl dichlorides, and vinyl chlorides. The mononuclear and binuclear nickel NHC complexes supported by heteroarene-functionalized NHC ligands are found to be highly efficient for the coupling of unactivated aryl chlorides and organozinc reagents leading to biaryls and terphenyls in good to excellent yields under mild conditions. For all aryl chlorides, the binuclear nickel catalysts show higher activities than mononuclear nickel complexes because of possible bimetallic cooperative effect.Finally, cobalt(Ⅲ)-NHC complexes have been prepared from the corresponding pyrimidine functionalized imidazolium salts via in situ generated silver carbene complexes under mild conditions. These cobalt complexes were characterzied by 1H and 13C NMR spectroscopy and X-ray diffraction analysis. The cobalt complexes have been found to be good catalyst precursors for Kumada-Corriu cross-coupling reactions of aryl halides and Grignard reagents at room temperature.