Light-Driven Hydride Transfer from Iridium Hydride Complexes Public Deposited

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  • March 20, 2019
Creator
  • Barrett, Seth
    • Affiliation: College of Arts and Sciences, Department of Chemistry
Abstract
  • Light-triggered hydride transfer is an emerging class of chemical transformations that is increasingly being applied to systems previously promoted with elevated reaction temperatures. The excited state of [Cp*Ir(bpy)(H)]+ can drive chemical reactivity. The strength of the Ir-H bond can be determined through hydricity in both the ground and excited states to predict reactivity with substrates. Using thermochemical cycles, the excited state hydride is predicted to become a much stronger hydride donor, with nearly a 20 kcal·mol–1 enhancement in the experimentally measured excited state hydricity compared to the ground state hydricity. Using [Cp*Ir(bpy)(H)]+ and 460 nm light, hydride transfer to methylnicotinamide results in the formation of singly and doubly reduced products. Notably, the doubly reduced product is traditionally formed using harsh reaction conditions. Instead, the doubly reduced product can be exclusively formed using visible light. Moving beyond stoichiometric hydride transfer, [Cp*Ir(bpy)(H)]+ excited state reactivity can be applied to photocatalytic dehydrogenation of formic acid. By the careful study of a new [Cp*Ir(bpy-OMe)(H)]+ photocatalyst, the reaction mechanism can be better understood in order to improve catalytic performance. Deactivation pathways, including light-induced ligand loss, are reduced in order to achieve more than 500 TON of H2. Additionally, the hydride catalyst is stable across a wide pH range, promoting the reaction at both acidic and basic pH. Between pH 7 – 11, 96% pure H2 is collected by trapping evolved CO2 in solution. Catalytic hydrodehalogenation of dichloromethane is also promoted using the excited state reactivity of [Cp*Ir(bpy)(H)]+. A biphasic reaction setup allows for the in situ formation of the hydride catalyst, which is irradiated to form chloromethane-d2. Formate regenerates the active hydride to promote catalytic turnover, with greater than 3 TON of chloromethane. UV-Vis reaction monitoring and kinetic analysis using the method of initial rates determines the hydrodehalogenation to be second order in iridium. A proposed mechanism consistent with this observation is discussed herein. In hydrodehalogenation of dichloromethane, light-induced hydride transfer is harnessed towards another chemical transformation and contributes to a better mechanistic understanding of excited state reactivity.
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  • In Copyright
Advisor
  • Schauer, Cynthia
  • Brookhart, Maurice
  • Warren, Scott
  • Gagne, Michel
  • Miller, Alexander
Degree
  • Doctor of Philosophy
Degree granting institution
  • University of North Carolina at Chapel Hill Graduate School
Graduation year
  • 2016
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