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Published May 3, 2019 | Supplemental Material + Accepted Version
Journal Article Open

Light Enhanced Fe-Mediated Nitrogen Fixation: Mechanistic Insights Regarding H₂ Elimination, HER, and NH₃ Generation

Abstract

Despite their proposed accumulation at the Fe sites of the FeMo-cofactor of MoFe-nitrogenase, the presence of hydride ligands in molecular model systems capable of the nitrogen reduction reaction (N_2RR) appears to diminish catalytic N₂-to-NH₃ conversion. We find that, for an iron-based system bearing the trisphosphine ligand P_2P^(Ph), a dramatic difference in yields is observed for N2RR catalyzed by precatalysts with zero, one, or two hydride ligands; however, irradiating the three different catalysts with a mercury lamp results in similar NH₃ yields. Although the efficacy for N₂RR versus the hydrogen evolution reaction (HER) is modest for this system by comparison to certain iron (and other metal) catalysts, the system provides an opportunity to study the role of hydrides in the selectivity for N₂RR versus HER, which is a central issue in catalyst design. Stoichiometric reactions with hydride containing precatalysts reveal a hydrogen evolution cycle in which no nitrogen fixation occurs. Irradiation of the dihydride precatalysts, observed during turnover, results in H2 elimination and formation of (P₂P^(Ph))Fe(N₂)₂, which itself is unreactive with acids at low temperature. N₂ functionalization does occur with acids and silyl electrophiles for the reduced species [(P₂P^(Ph))Fe(N₂]⁻ and [(P₂P^(Ph))Fe(N₂)]²⁻, which have been characterized independently. The requirement of accessing such low formal oxidation states explains the need for strong reductants. The low selectivity of the system for functionalization at Nβ versus Fe creates off-path hydride species that participate in unproductive HER, helping to explain the low selectivity for N₂RR over HER. The data presented here thus lend further insight into the growing understanding of the selectivity, activity, and reductant strengths relevant to iron (and other) N₂RR catalysts.

Additional Information

© 2019 American Chemical Society. Received: February 4, 2019; Revised: March 21, 2019; Published: March 26, 2019. Dr. Michael Takase, Larry Henling, and Dr. Marcus Drover are acknowledged for their assistance with crystallographic studies. Dr. Niklas Thompson is thanked for assisting with fitting the variable temperature NMR data. We are grateful to the NIH (GM-075757) for support of this research, and also to the National Science Foundation for support of the Caltech EPR Facility via the NSF-MRI Grant NSF-153194, and to the Dow Next Generation Educator Fund. D.J.S. acknowledges the support of the Resnick Sustainability Institute at Caltech for a Graduate Fellowship. The authors declare no competing financial interest.

Attached Files

Accepted Version - nihms-1036680.pdf

Supplemental Material - cs9b00523_si_001.pdf

Supplemental Material - cs9b00523_si_002.cif

Supplemental Material - cs9b00523_si_003.mol

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Additional details

Created:
August 19, 2023
Modified:
October 20, 2023