Synthesis And Catalytic Applications Of Transition Metal Complexes Of Multidentate N-Heterocyclic Carbenes

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. The development of environmentally friendly, air-stable, and highly efficient metal catalysts is of practical importance in organic chemistry. We have synthesized a series of air-stable transition metal complexes supported by multidentate N-heterocyclic carbenes. These metal complexes display excellent catalysis activities in organic synthesis. This dissertation mainly includes four parts:(1) A series of palladium and nickel complexes, [Pd(Ll)Cl(PF6)] (2.1), [Ni(L1)Cl(PF6)] (2.2), and [Ni(Ll)2(PF6)2] (2.3), containing 3-butyl-1-(1,10-phenanthrolin-2-yl)imidazolylidene (L1) have been synthesized and fully characterized by NMR and ESI-MS spectroscopy and X-ray diffraction analysis. The unsymmetrical NNC pincer complexes 2.1 and 2.2 are square planar with a chloride trans to the internal nitrogen atom of phenanthroline. Nickel complex 2.3 is a paramagnetic octahedral complex with its central metal surrounded by two pincer ligands. A catalytic study shows that palladium carbene complex [Pd(L1)Cl](PF6) (2.1) is highly active in copper-free Sonogashira cross-coupling reactions of aryl iodides at room temperature and aryl bromides at 80℃in neat water. The nickel complex [Ni(L1)Cl](PF6) (2.2) exhibits good activity in Kumada Cross-Coupling reaction of aryl chlorides at room temperature.(2) A series of mononuclear and polynuclear complexes, [Pd2(L2)2Cl2(PF6)2] (3.1), [Pd3(L2)2Cl4(PF6)2] (3.2), [PdAg(L2)2(PF6)3] (3.3), [PdCu(L2)2(PF6)3] (3.4), [Ni(L2)2(PF6)2] (3.5), [PtCl(L2)(PF6)] (3.6), and [Co(L2)2(PF6)3] (3.7), containing 3-(1,10-phenanthrolin-2-yl)-1-(pyridin-2-ylmethyl)imidazolylidene (L2) have been synthesized and fully characterized by NMR, elemental analysis, and X-ray crystallography. Complexes 3.3 and 3.4 are isostructural to each other in which the palladium ions are surrounded by two pyridines and two imidazolylidenes and Ag(I) or Cu(I) is coordinated by two 1,10-phenanthroline moieties. In the trinuclear palladium complex 3.2, one palladium ion has an identical coordination mode as in 3.3 and 3.4, and the other two palladium ions are bonded to the 1,10-phenanthroline. In complexes 3.5-3.7, the ligand binds to metals in a pincer NNC fashion with the pyridine group uncoordinated. The heterobimetallic complex 3.4 exhibits excellent catalytic activity for the tandem Click/Sonogashira reaction of 1-(bromomethyl)-4-iodobenzene, NaN3, and ethynylbenzene in which three C-N bonds and one C-C bond are formed in a single flask.(3) A series of mononuclear and dinuclear ruthenium carbene complexes, [Ru(L2)Cl(CH3CN)2(PF6)](4.1), [Ru(L2)Cl(CH3CN)(PPh3)(PF6)] (4.2), [Ru(L2)Cl(CO)2(PPh3)(PF6)](4.3), [Ru(L2)Cl(CO)(PPh3)(PF6)] (4.4), [Ru(L2)(CH3CN)3(PF6)2](4.6), and [Ru2Ag2(L2)2Cl2(CH3CN)6(PF6)4](4.5), containing 3-(1,10-phenanthrolin-2-yl)-1-(pyridin-2-ylmethyl)imidazolylidene (L2) have been synthesized and structurally characterized. Novel dinuclear ruthenium complex 4.5 is linked by the bridged Ag to form a novel zigzag chain with a folded chair conformation.(4) A series of imidazolium salts bearing triazole groups are synthesized via a copper catalyzed click reaction, and the silver, palladium, and platinum complexes of their N-heterocyclic carbenes are studied. [Ag4(L)4(PF6)4](5.4b), [Pd(L)Cl(PF6)](5.5b), [Pt(L)Cl (PF6)] (5.5c)(L= 3-((1-benzyl-1H-1,2,3-triazol-4-yl)methyl)-1-(pyrimidin-2-yl)-1 H-imidazolylidene), [Pd2(L')2CI2(PF6)2] (5.5d), and [Pd(L')2](PF6)2(5.5e) (L'=1-butyl-3-((1-(pyridin-2-yl)-1H-1,2,3-triazol-4-yl)methyl) imidazolylidene) have been synthesized and fully characterized by NMR, elemental analysis, and X-ray crystallography. The silver complex [Ag4(L)4](PF6)4(5.4b) consists of a Ag4 zigzag chain. The complexes [Pd(L)Cl(PF6)] (5.5b) and [Pt(L)Cl(PF6)](5.5c), containing a nonsymmetrical NCN' pincer ligand, are square planar with a chloride trans to the carbene donor. [Pd2(L')2Cl2](PF6)2 (5.5d) consists of two palladium centers with CN2Cl coordination mode, whereas the palladium in [Pd(L')2](PF6)2(5.5e) is surrounded by two carbene and two triazole groups with two uncoordinated pyridines. The palladium compounds are highly active for Suzuki-Miyaura cross coupling reactions of aryl bromides and 1,1-dibromo-1-alkenes in neat water under an air atmosphere. These complexes are all stable towards air and moisture, and such properties make them promising catalyst precursors for C-C formation reactions.