Synthesis,Structure And Reactivity Of High Oxidation VB Transition Metal N-Heterocyclic Carbene Complexes

In this thesis, we synthesized a series of imidazolium salts as precursors of N-Heterocyclic carbenes. Three free N-Heterocyclic carbene complexes:1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene (IMes),1,3-di-tertbutyl-imidazol-2-ylidene (I’Bu) and1,3-bis(2,6-diisoproylphenyl)imidazol-2-ylidene (IPr) were obtained by proton elimination method. These imidazolium salts and free N-Heterocyclic carbene complexes were confirmed by NMR or single crystal X-ray diffraction analysis.Stable N-Heterocyclic carbene complexes IPrMCl5(M=Ta, Nb) were obtained by the reaction of monodentate1,3-diisopropylphenylimidazol-2-ylidene (IPr) and MCl5(M=Ta, Nb). The complexes were structurally characterized by NMR, elemental analysis and single-crystal X-ray diffraction. To our knowledge, the synthesis of Nb N-Heterocyclic carbene complex was firstly reported. Density functional theory (DFT) studies of IPrMCl5(M=Ta, Nb) indicate that the strong interaction between chloride lone-pair electron density and the formally vacant molecular orbitals situated on the Ccarbene likely stabilize the complexes. NBO studies show that IPrTaCl5is more stable than IPrNbCl5. IPrMCl5(M=Ta, Nb) is very sensitive to moisture and form IPrHMCl6(M=Ta, Nb). These complexes were identified by NMR and single crystal structure analysis. IPrMCl5(M=Ta, Nb) can catalyze the reaction of1,2-propylene oxide and CO2to form the only product of propylene carbonate under mild condition. IPrTaCl5is a better catalyst than IPrNbCl5since it is more stable. With IPrTaCl5as catalyst,100%conversion was reached for the reaction of1,2-propylene oxide and CO2at20℃,1MPa CO2pressure and stirred for10h. In addition, IPrCO2was obtained by the reaction of IPr and CO2. Unexpected complex [(IPrCO2)2TaCl3][TaCl6]2was obtained in situ by the reaction of IPrCO2and TaCl5(1:1.5).[(IPrCO2)2TaCl3][TaCl6]2can also catalyze the reaction of1,2-propylene oxide and CO2with high selectivity but the conversion is less that IPrTaCl5.For the synthesis of N-Heterocyclic carbene tantalum imide complexes, we perform the reaction of the free N-Heterocyclic carbene IPr and tantalum imide. We looked forward to get a stable N-Heterocyclic carbene tantalum imide complxes, but unexpectedly obtained a unique structure imine tantalum anion with imidazolium cation complexes [IPrH]+[RN=TaCl4py]-.The complxes were structurally characterized by NMR, elemental analysis and single-crystal X-ray diffraction. The fluorescene spectroscopy studies reveal that the maximum emission wavelength is mainly due to interaction of imide ligand and metal center, other than chlorine ion, pyridine ligand and imidazolium salt.