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Published June 15, 2022 | Submitted + Supplemental Material
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Photoinduced transfer hydrogenation of nitrogen to ammonia using a Mo-catalyst and a Hantzsch ester donor is demonstrated with and without an Ir-photoredox co-catalyst

Abstract

Whereas photoredox catalysis using molecular systems enjoys considerable utility in small molecule transformations and reactions relevant to organic synthesis, to date there are no related examples of photodriven catalytic nitrogen fixation. We wondered whether a photoinduced transfer hydrogenation strategy might provide a viable pathway toward such a reaction. Hantzsch esters (and related organic structures) offer an opportunity for catalysis design in this context as they can behave as photoreductants, though to our knowledge they had yet to be shown to be compatible with such a redox intensive process (6 e⁻/6 H⁺). In the present study we demonstrate that fully reduced Hantzsch esters (abbreviated as HEH2) successively deliver stored H₂-equivalents to N₂, producing NH₃ catalytically, in the presence of a molecular precatalyst (Mo) under blue-light irradiation but otherwise ambient conditions. While not required for the observed photocatalysis, the addition of a photoredox catalyst (Ir) to the reaction mixture enhances both the rate and turnover number of the net transformation. Encouraging with respect to future studies toward recycling the donor, electrochemically or via hydrogenation, other N-heterocycle H₂-donors are also compatible with catalysis in the presence of the photoredox catalyst. The reduction of N₂ to NH₃ by HEH₂ or H₂ are thermodynamically very similar (ΔΔG_f(NH₃) = 1.8 kcal mol⁻¹ in acetonitrile). However, whereas the combination of H₂ with N₂ to produce NH₃ is accomplished via high temperature and pressure over a metal catalyst, the needed overpotential to drive the reduction of N₂ by HEH₂ can instead be derived from light. This study hence illustrates a promising photoredox catalysis approach toward deep reduction of robust small molecule substrates via photoinduced transfer hydrogenation, with the complete reduction of the triple bond of N₂ providing a vivid example.

Additional Information

The content is available under CC BY NC ND 4.0 License. We thank the Dow Next Generation Educator Fund and Instrumentation Grants for their support of the NMR facility at Caltech. The Beckman Institute Laser Resource Center and Jay R. Winkler are acknowledged for providing support with steady-state luminescence experiments. We also thank the Resnick Sustainability Institute Water and 5 Environment Laboratory (WEL) at Caltech for the use of their instrumentation. Funding: National Institutes of Health (R01 GM-075757) EAB acknowledges the support of the National Science Foundation for a Graduate Research Fellowship under Grant No. DGE‐1745301 Authors declare that they have no competing interests. Data and materials availability: All data are available in the main text or the supporting information.

Attached Files

Submitted - photoinduced-transfer-hydrogenation-of-nitrogen-to-ammonia-using-a-mo-catalyst-and-a-hantzsch-ester-donor-is-demonstrated-with-and-without-an-ir-photoredox-co-catalyst.pdf

Supplemental Material - supporting-information.pdf

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photoinduced-transfer-hydrogenation-of-nitrogen-to-ammonia-using-a-mo-catalyst-and-a-hantzsch-ester-donor-is-demonstrated-with-and-without-an-ir-photoredox-co-catalyst.pdf

Additional details

Created:
August 20, 2023
Modified:
October 24, 2023