VISIBLE-LIGHT-DRIVEN REACTIONS FROM A SIX-COORDINATE NICKEL(II) COMPLEX

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