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

Schild, Dirk J. and Peters, Jonas C. (2019) Light Enhanced Fe-Mediated Nitrogen Fixation: Mechanistic Insights Regarding H₂ Elimination, HER, and NH₃ Generation. ACS Catalysis, 9 (5). pp. 4286-4295. ISSN 2155-5435. PMCID PMC6715299. doi:10.1021/acscatal.9b00523.

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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.

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URLURL TypeDescription Information CentralArticle
Peters, Jonas C.0000-0002-6610-4414
Alternate Title:Light Enhanced Fe-Mediated Nitrogen Fixation: Mechanistic Insights Regarding H2 Elimination, HER, and NH3 Generation
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.
Group:Resnick Sustainability Institute
Funding AgencyGrant Number
Dow Next Generation Educator FundUNSPECIFIED
Resnick Sustainability InstituteUNSPECIFIED
Subject Keywords:ammonia synthesis, nitrogen fixation, N2RR, nitrogenase, hydrogen evolution, HER, iron catalysis, photocatalysis
Issue or Number:5
PubMed Central ID:PMC6715299
Record Number:CaltechAUTHORS:20190423-160445593
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Official Citation:Light Enhanced Fe-Mediated Nitrogen Fixation: Mechanistic Insights Regarding H2 Elimination, HER, and NH3 Generation. Dirk J. Schild and Jonas C. Peters. ACS Catalysis 2019 9 (5), 4286-4295 DOI: 10.1021/acscatal.9b00523
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:94908
Deposited By: Tony Diaz
Deposited On:23 Apr 2019 23:17
Last Modified:15 Feb 2022 22:58

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