Spectroscopic Characterization Of Organometallic Reagents And Catalytically Active Organometallic Intermediates

In the past decades, the synthesis of novel organometallic compounds and the development of organometallic catalysis have tremendous contribution to the development of organic chemistry, as well as polymer chemistry, materials chemistry and pharmaceutical chemistry. Compared with the synthetic methodology development which huge effort has been put into, mechanistic investigation has attracted much less attention leading to very limited understanding towards the reaction mechanism, as well as the structure and reactivity of catalytically active intermediates.In this thesis, a very brief introduction of physical organic chemistry and organometallic chemistry was first provided, followed by a brief summary of methods commonly used in mechanistic investigation. A more detailed introduction of X-ray absorption spectroscopy (XAS) was given including its application in organometallic chemistry and homogeneous catalysis. The research in this thesis focuses on the application of in situ spectroscopic investigation into the structure and reactivity of organozinc reagents, as well as catalytically active intermediates in C-H functionalization reactions. The results are summarized below:1. X-ray absorption near edge structure and extended X-ray absorption fine structure investigation into the phenylzinc reagents prepared from phenyllithium and various zinc halides provided the structural information of the phenylzinc species including the coordination geometry, types of coordination atoms, coordination number and bond distances. The structures of the three phenylzinc reagents were determined as [PhZnl2(THF)]Li,[PhZnBr2(THF)]Li, and [PhZnCl2(THF)]Li, in which all the Zn cations adopt distorted tetrahedral geometry. Mainly because of the a interaction between Zn(II) cation and halide anion, the Zn-C bond distance follows the following trend:[PhZnl2(THF)]Li>[PhZnBr2(THF)]Li>[PhZnCl2(THF)]Li. In situ IR kinetic study showed that the transmetallation rate of the phenylzinc reagents in the Negishi-type oxidative homocoupling reaction also follows the same trend, indicating that longer Zn-C bond distance leads to higher transmetallation rate.2. In situ XAS and in situ electron paramagnetic resonance (EPR) spectroscopy were utilized to investigate the mechanism of the reduction of Cu(II) to Cu(I) by terminal alkynes in the presence of tetramethylethylenediamine (TMEDA). Based on the information provided by XAS including oxidation state, types of coordination atoms, coordination number and bond distance, the reduction mechanism was determined as follows:TMEDA coordinates to CuCl2forming a mononuclear complex,(TMEDA) CuCl2.(TMEDA) CuCl2coordinates to phenylacetylene and thus facilitates the deprotonation of the terminal C-H bond and the formation of Cu(II) acetylide intermediate. Homolytic cleavage of the Cu(II)-C bond forms the dinuclear Cu(Ⅰ) complex,[(TMEDA)CuCl]2, as well as the homocoupling product of phenylacetylene. These results provided new understanding toward the mechanism of Eglinton reaction, Glaser-Hay reaction, as well as Cu-acetylene chemistry.3. The reaction mechanism of the Cu-catalyzed aerobic hydroxylation reaction of benzothiazole was investigated using nuclear magnetic resonance (NMR), EPR, in situ XAS and IR spectroscopies. Monitoring the stoichiometric and catalytic reactions provided structural and kinetic information of several catalytically active species and catalytically non-active species establishing the following Cu(I)-Cu(II) catalytic cycle.[CuⅠ(O’Bu)2]Na is generated either by reduction of CuCl2via a radical process in the presence of tBuONa and DMF, or by salt metathesis of CuCl with tBuONa.[Cut(O’Bu)2]Na readily cleaves the C-H bond of benzothiazole and produces [ArCut(OtBu)]Na. Reaction of [ArCu’(O’Bu)]Na with O2provides [Ar(’BuO)CunOOCun(O’Bu)Ar]i/2Na, followed by rapid decomposition via homolysis of Cu(II)-Ar and O-O bonds to generate [ArOCu’(O’Bu)]Na which releases ArONa as the product completing the catalytic cycle and regenerating [Cu’(0’Bu)2]Na in the presence of’BuONa. The results shed light on the understanding of Cu-catalyzed C-H activation and hydroxylation of electron-poor aromatics.4. Three SiO2-supported Ga catalysts were synthesized and applied in propane dehydrogenation reaction at550癈with high selectivity. The selectivity remains above97%when the conversion reaches25%which is close to the equilibrium conversion,31%. XAS investigation showed that Ga remained as four-coordination Ga(III) under both hydrogenation and dehydrogenation reaction conditions, which is different from zeolite-supported Ga catalysts being reduced to Ga(I). UV-Raman spectroscopic results suggest Ga(III)-H is an active intermediate, and density functional theory calculation suggest a heterolytic bond dissociation of alkane C-H bonds to form a Ga-alkyl and a proton on the adjacent Ga-O-Si surface bond as the mechanism for alkane activation. The rate determining step is|3-H elimination of the Ga-alkyl with formation of a Ga-H.