CaltechAUTHORS
  A Caltech Library Service

Light Enhanced Fe-Mediated Nitrogen Fixation: Mechanistic Insights Regarding H_2 Elimination, HER, and NH_3 Generation

Schild, Dirk J. and Peters, Jonas C. (2019) Light Enhanced Fe-Mediated Nitrogen Fixation: Mechanistic Insights Regarding H_2 Elimination, HER, and NH_3 Generation. ACS Catalysis, 9 (5). pp. 4286-4295. ISSN 2155-5435. http://resolver.caltech.edu/CaltechAUTHORS:20190423-160445593

[img] PDF (Experimental procedures and compound characterization) - Supplemental Material
See Usage Policy.

2906Kb
[img] Crystallographic Info File (CIF) (X-ray data) - Supplemental Material
See Usage Policy.

19Mb
[img] Other (Cartesian Coordinates) - Supplemental Material
See Usage Policy.

37Kb

Use this Persistent URL to link to this item: http://resolver.caltech.edu/CaltechAUTHORS:20190423-160445593

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 N_2-to-NH_3 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 N_2RR 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 (P_2P^(Ph))Fe(N_2)_2, which itself is unreactive with acids at low temperature. N_2 functionalization does occur with acids and silyl electrophiles for the reduced species [(P_2P^(Ph))Fe(N_2)]^− and [(P_2P^(Ph))Fe(N_2)]^(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 N_2RR 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) N_2RR catalysts.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1021/acscatal.9b00523DOIArticle
https://pubs.acs.org/doi/suppl/10.1021/acscatal.9b00523PublisherSupporting Information
ORCID:
AuthorORCID
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
Funders:
Funding AgencyGrant Number
NIHGM-075757
NSFMRI-153194
Dow Next Generation Educator FundUNSPECIFIED
Resnick Sustainability InstituteUNSPECIFIED
Subject Keywords:ammonia synthesis, nitrogen fixation, N2RR, nitrogenase, hydrogen evolution, HER, iron catalysis, photocatalysis
Record Number:CaltechAUTHORS:20190423-160445593
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20190423-160445593
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
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:23 Apr 2019 23:17
Last Modified:09 May 2019 16:16

Repository Staff Only: item control page