Syntheses And Characterization Of Metal Complexes Containing Chelating Imidazole Or Triazole N-Heterocyclic Carbene Ligands

N-Heterocyclic carbenes (NHCs) have been established as universal ligands in the coordination chemistry and homogeneous catalysis since they are excellent alternatives to phosphine ligands due to their strong a-donor capacity.2,2'-Bipyridine (bipy), one of the most common classic bidentate ligands, has been widely used to coordinate many metal ions in coordination chemistry. NHC analogues of bipy, bistriazol-2-ylidene (bitz) containing two triazole-2-ylidene groups with a direct N-N bond linking two triazole rings, have attracted considerable attention. Rhodium, iridium, ruthenium and palladium complexes with bistriazol-2-ylidene ligands have been studied. Novel coordination modes have been observed in one of the Rh complexes of bitz, in which the rhodium atoms adopt an octahedral geometry with two bridging bitz ligands forming two six-membered metallacycle rings. It is surprising that no iron, cobalt with bistriazol-2-ylidene ligands have been reported yet. We are interested in extending the coordination chemistry of this type of ligands to iron, cobalt, and ruthenium to form novel complexes.Recently Stagni and our group have independently investigated the luminescent properties of fac-[Re(CO)3(C^N)(X)] complexes with pyridine-N-heterocyclic carbenes as electron-rich C,N bidentate ligands. These studies showed that the introduction of an N-heterocyclic carbene moiety instead of an imine group in the ligand shift the emission wavelength to the blue-green region. With the aim of further tuning luminescent properties of Re(I)-NHC complexes, pyrimidine-functionalized N-heterocyclic carbene rhenium carbonyl complexes have been explored.The research presented in this thesis focused on the synthesis, characterization and properties of bistriazol-2-ylidene ruthenium complexes, ring opening and in situ generation of a tridentate ligand with iron or cobalt, and rhenium complexes with pyrimidine-functionalized N-heterocyclic carbenes. This thesis contains the following three sections:1. Ruthenium(?) carbonyl complexes with bistriazol-2-ylidene ligandTreatment of bistriazol-2-ylidene ligand precursor 1,1'-dimethyl-4,4'-bi-1,2,4-triazolium diiodide (H2L1·2I) and 1,1'-diethyl-4,4'-bi-1,2,4-triazolium dibromide (H2L2·2Br) with Ag2O afforded Ag carbenes in situ, which was then reacted with [Ru(CO)2Cl2]n to afford ruthenium(II) carbonyl complexes [Ru(L1)(CO)2Cl2] (1), [Ru(L2)(CO)2Cl2] (2), and [Ru(L2)(CO)2ClBr] (3), respectively. These complexes have been characterized by 1H and C NMR spectroscopies, infrared spectroscopy and elemental analyses. The molecular structures of 1,2 and 3 have been established by single-crystal X-ray diffraction, confirming that the ruthenium(II) ion is in a distorted octahedral environment. The bite angles of these three complexes,77.58(12)°,77.13(14)°and 77.14(11)°, are small as a consequence of the chelation of the bitz ligands.2. Formation of stable iron/cobalt NHC complexes via unexpected ring opening and in situ generation of a tridentate ligand.The reactions of FeCl2 and CoCl2 with a bistriazolium salt yield NHC complexes [Fe?(L3)2]I·H2O(4), [Fe?(L4)2]PF6-CH3CN(5) and [CoIII(L3)2]I·0.5CH3CN(6), respectively, through an unusual ring opening of one of the triazoyl rings, leading to the formation of C,N,O tridentate ligands L3 or L4. Furthermore, treatment of 4 with KBH4 yields a Fe(II) species [Fe?(L3)2]·CH2Cl2 (7) in good yield. These complexes have been characterized by H and 13C NMR spectroscopies, infrared spectroscopy, ESI-MS, cyclic voltammetry, magnetic susceptibility, and elemental analyses. The molecular structures of 4-7 have been established by single-crystal X-ray diffraction, which unambiguously show that unexpected C-N cleveage of the triazole rings occurs in the reactions.3. Synthesis, structures and luminescent properties of rhenium(?) carbonyl complexes containing pyrimidine-functionalized N-heterocyclic carbenes.Treatment of pyrimidine-functionalized N-heterocyclic carbene ligand precursor 3-alkyl(or aryl)-2-pyrimidine(imidazolium) chloride with Ag2O afforded Ag carbenes in situ, which were then reacted with [Re(CO)5Cl] to afford a series of neutral fac-[Re(CO)3(C^N)Cl] complexes 8-11.These complexes have been characterized by 1H and 13C NMR spectroscopies, IR spectroscopy, and elemental analysis. The molecular structures of 9 and 11 have been established by single-crystal X-ray diffraction. Their electrochemical and photophysical behaviors are also reported. In both degassed CH2Cl2 solutions and solid state at room temperature, complexes 8-11 exhibit the emissions at 515-570 nm.