Visible Light and Divalent Lanthanides in Photoredox Catalysis

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In this project funded by the Chemical Catalysis program of the Chemistry Division, Professor Matthew J. Allen, Chemistry Department, Wayne State University, is studying new lanthanide systems that catalytically perform reductions and bond-forming reactions when exposed to visible light. Catalysts are substances that permit chemical reactions to be run rapidly with minimum energy consumption. They are used in a variety of industrial processes. In this project, catalysts are being developed that utilize and can be switched on and off by visible light. In addition, they operate under conditions that are not compatible with current catalysts. Several important processes including reactions that form carbon-carbon bonds are studied. Graduate, undergraduate, and high school students are trained to become skilled and ethical scientists. High school students, who are from groups underrepresented in science, have an opportunity to participate in research, leading to a more educated and diverse scientific workforce.This project focuses on new lanthanide-containing complexes that can drive visible-light-promoted photoredox catalysis. Specifically, the project tests the hypothesis that if trivalent lanthanide-containing azacryptates are exposed to a sacrificial reductant and visible light, they perform catalysis at redox potentials more negative than divalent lanthanide salts alone. Testing this hypothesis simultaneously addresses two long-standing challenges in redox chemistry: (1) the need for photoredox catalyts tunable across a range of potentials and wavelengths for improved chemoselectivity and (2) the need for air-stable, non-toxic alternatives to samarium(II) hexamethyl phosphoramide. The research includes the synthesis of a series of complexes of divalent and trivalent europium and ytterbium. These complexes are characterized by UV/visible and fluorescence spectra, electrochemical potentials, thermodynamic and kinetic stabilities, crystal structures, solubilities, and propensities for transmetallation. Additionally, the complexes are studied with respect to their ability to catalyze important classes of photoredox reactions including reductions, carbon-carbon bond formations, and ring closures. Finally, students are trained in the scientific method and the responsible conduct of research.