Study On Pyrazolyl-functionalized NHC-Metal Complexes And A DFT Investigation Of Formal S_N2’ Substitution Of Imido Metal Complexes With Allylic Electrophiles

This thesis mainly involves the studies on the synthesis and catalytic activity of pyrazolyl-functionalized N-heterocyclic carbenes (NHCs) transition metal complexes, as welll as the DFT investigation on formal Sn2’substitution of imido metal complexes with allylic electrophiles with recently developed M06method. The results are as follows:1. A series of mono-and dinuclear N-heterocyclic carbene silver complexes based on1-[2-(pyrazol-1-yl)phenyl]imidazole have been synthesized and characterized by elemental analysis and NMR spectroscopy, and their structures have been confirmed by single crystal X-ray diffraction. The N-functionalized carbene ligands exhibit versatile coordination modes in these silver complexes. For example, N-[2-(3,5-dimethylpyrazol-1-yl)phenyl]-N-benzylimidazol-2-ylidene (L) and N-[2-(pyrazol-1-yl)phenyl]-N-benzylimidazol-2-ylidene (L’) act as monodentate ligands through the carbene carbon in mononuclear LAgCl and L’AgCl. While in dinuclear L’2Ag2(PF6)2and L2Ag2(PF6)2, L’ and L act as bridging bidentate ligands to two silver atoms through the pyrazolyl nitrogen and the carbene carbon atoms. Furthermore, these two silver atoms have different coordination environments. In the former, one silver atom coordinates with two carbene carbons, the other coordinates with two pyrazolyl nitrogen atoms. In the latter, each silver atom coordinates with one carbene carbon and one pyrazolyl nitrogen atom, respectively. The dinuclear macrocyclic structure is observed in (L)CH2C6H4CH2(L)Ag2(PF6)2and (L)CH2C6H4CH2(L)Ag2(BF4)2, in which the coordination mode of carbene ligands is similar with that in L2Ag2(PF6)2.Preliminary catalytic tests show all these complexes exhibit highly effective catalytic activity in the three-component coupling reaction of alkyne, aldehyde and amine forming propargylamines.Two pyrazolyl-functionalized NHC-Pd complexes were obtained by the carbene transfer reaction of the corresponding NHC-Ag complexes with Pd(CH3CN)2Cl2, while another NHC-Pd complex coordinated by acetate ligand was obtained by the reaction of N-[2-(3,5-dimethylpyrazol-1-yl)phenyl]-N’-isopropyl imidazolium iodide with Pd(OAc)2in situ. These three complexes have been characterized by elemental analyses and NMR spectroscopy and their structures have been confirmed by single crystal X-ray diffraction. A seven-membered metallocycle with boat-like conformation was observed and the palladium atom exhibits a four-coordinate slightly distorted square-planar coordination geometry. The pyrazolyl-functionalized NHCs act as chelating N,C-bidentate ligands in these three complexes. Catalytic tests show that these complexes exhibit highly effective catalytic activity for the Suzuki-Miyaura and Mizoroki-Heck coupling reactions in water or aqueous/organic media under air atmosphere. The substituents on the pyrazolyl ring show different influences on the catalytic activity of complexes in these coupling reactions.Reaction of NHC-Ag complexes as transfer regants with W(CO)5THF and Mo(CO)5THF in refluxing THF produces NHC-Mo/W complexes. In most of these complexes a seven-membered metallocycle with boat-like conformation was observed and the molybdenum and tungsten atoms exhibit a six-coordinate slightly distorted octahedral geometry. The pyrazolyl-functionalized NHCs act as monodentate and chelating N,C-bidentate ligands in these complexes. Preliminery catalytic tests show that these complexes exhibit highly effective chemical selectivity for the oxidation of styrene in acetonitrile media under air atmosphere. The production of benzaldehyde was obtained with H2O2as the oxygen source, while t-BuOOH as the oxygen source gave2,5-diphenyl-1,4-dioxane. A tetranuclear ruthenium carbonyl clusters has been synthesized by transfer reaction with LAgCl and characterized by1H NMR spectroscopy, and its structure has been confirmed by single crystal X-ray diffraction. The pyrazolyl-functionalized NHC also acts as a chelating N,C-bidentate ligand in this complex. No intramolecular C-H activation was observed during the reaction.2. A DFT investigation on two SN2’ substitution reactions with different central metals (zirconium and titanium) and types of allylic sources was carried out with recently developed M06method and revealed that in both cases, a [2+2]/retro-[2+2] pathway is energetically favorable. This is in contrast with the usually accepted [3,3] sigmatropic rearrangement mechanism (see Figure1; TMS=trimethylsilyl).