Date of Award
Summer 2024
Document Type
Thesis
Degree Name
Master of Science (MS)
Department
Chemistry
First Advisor
Dian Wang
Second Advisor
Chae S. Yi
Third Advisor
James Gardinier
Abstract
Transition metal complexes capable of absorbing visible light can be excited upon light irradiation and facilitate chemical bond formation and cleavage. In this approach, a transition metal-substrate complex is formed in the ground state, followed by photoinduced substrate activation via an inner-sphere mechanism. Unlike well-established photoredox catalysis that does not involve substrate binding, the ground-state interaction here is important for substrate activation. Among the 3d transition metals, nickel is considered a cost-effective alternative to Ru- and Ir-based photocatalysts due to its ability to accommodate different oxidation states. However, the application of Ni photocatalysis in synthesis had remained largely underexplored. In 2018, the Doyle group showcased a light-induced Ni(II) aryl bond homolysis as a key step in a Ni-catalyzed C-O cross-coupling reaction. This discovery ignited significant interest in advancing the use of nickel in visible-light-driven organic synthesis.
In this study, we synthesized six-coordinate Ni(eba)2(bpy) and Ni(eaa)2(bpy) complexes (eba: ethyl benzoyl acetate, eaa: ethyl acetoacetate, bpy: 2,2’-bipyridine). For further studies, we also prepared a series of Ni(eba)2(bpy) complexes by electronically varying the 4,4’-substituents of bipyridine (H, OCH3, tert-butyl, Br) and the para substituent of the phenyl group in the eba (OCH3, Br). These Ni complexes were characterized by spectroscopic methods (UV/Vis, NMR, IR), elemental analysis, melting point measurement, and X-ray crystallography. In efforts to test the photoreactivity of the eaa/eba ligand, both complexes were found to be reactive with PhSSPh under light conditions. Two possible reaction pathways include the formation of a Ni-SPh bond and excited-state hydrogen atom transfer to form thiophenol. Light-driven ligand dissociation has also been investigated for Ni(eba)2(bpy). Finally, the photoreactivity with different nitrogen and oxygen nucleophiles was tested, and C-O and C-N bond-forming products were observed. These findings will likely advance our fundamental understanding of Ni-based visible-light-driven reactions and provide a mechanistic foundation for developing catalytic carbon-heteroatom bond-forming reactions. Transition metal complexes capable of absorbing visible light can be excited upon light irradiation and facilitate chemical bond formation and cleavage. In this approach, a transition metal-substrate complex is formed in the ground state, followed by photoinduced substrate activation via an inner-sphere mechanism. Unlike well-established photoredox catalysis that does not involve substrate binding, the ground-state interaction here is important for substrate activation. Among the 3d transition metals, nickel is considered a cost-effective alternative to Ru- and Ir-based photocatalysts due to its ability to accommodate different oxidation states. However, the application of Ni photocatalysis in synthesis had remained largely underexplored. In 2018, the Doyle group showcased a light-induced Ni(II) aryl bond homolysis as a key step in a Ni-catalyzed C-O cross-coupling reaction. This discovery ignited significant interest in advancing the use of nickel in visible-light-driven organic synthesis.
In this study, we synthesized six-coordinate Ni(eba)2(bpy) and Ni(eaa)2(bpy) complexes (eba: ethyl benzoyl acetate, eaa: ethyl acetoacetate, bpy: 2,2’-bipyridine). For further studies, we also prepared a series of Ni(eba)2(bpy) complexes by electronically varying the 4,4’-substituents of bipyridine (H, OCH3, tert-butyl, Br) and the para substituent of the phenyl group in the eba (OCH3, Br). These Ni complexes were characterized by spectroscopic methods (UV/Vis, NMR, IR), elemental analysis, melting point measurement, and X-ray crystallography. In efforts to test the photoreactivity of the eaa/eba ligand, both complexes were found to be reactive with PhSSPh under light conditions. Two possible reaction pathways include the formation of a Ni-SPh bond and excited-state hydrogen atom transfer to form thiophenol. Light-driven ligand dissociation has also been investigated for Ni(eba)2(bpy). Finally, the photoreactivity with different nitrogen and oxygen nucleophiles was tested, and C-O and C-N bond-forming products were observed. These findings will likely advance our fundamental understanding of Ni-based visible-light-driven reactions and provide a mechanistic foundation for developing catalytic carbon-heteroatom bond-forming reactions.