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Fe-Mediated Nitrogen Fixation with a Metallocene Mediator: Exploring pK_a Effects and Demonstrating Electrocatalysis

Chalkley, Matthew J. and Del Castillo, Trevor J. and Matson, Benjamin D. and Peters, Jonas C. (2018) Fe-Mediated Nitrogen Fixation with a Metallocene Mediator: Exploring pK_a Effects and Demonstrating Electrocatalysis. Journal of the American Chemical Society, 140 (19). pp. 6122-6129. ISSN 0002-7863. http://resolver.caltech.edu/CaltechAUTHORS:20180418-164723702

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Abstract

Substrate selectivity in reductive multi-electron/proton catalysis with small molecules such as N_2, CO_2, and O_2 is a major challenge for catalyst design, especially where the competing hydrogen evolution reaction (HER) is thermodynamically and kinetically competent. In this study, we investigate how the selectivity of a tris(phosphine)borane iron(I) catalyst, P_3^BFe^+, for catalyzing the nitrogen reduction reaction (N_2RR, N_2-to-NH_3 conversion) versus HER changes as a function of acid pK_a. We find that there is a strong correlation between pKa and N_2RR efficiency. Stoichiometric studies indicate that the anilinium triflate acids employed are only compatible with the formation of early stage intermediates of N_2 reduction (e.g., Fe(NNH) or Fe(NNH_2)) in the presence of the metallocene reductant Cp*_2Co. This suggests that the interaction of acid and reductant is playing a critical role in N–H bond forming reactions. DFT studies identify a protonated metallocene species as a strong PCET donor and suggest that it should be capable of forming the early stage N–H bonds critical for N_2RR. Furthermore, DFT studies also suggest that the observed pK_a effect on N_2RR efficiency is attributable to the rate and thermodynamics, of Cp*_2Co protonation by the different anilinium acids. Inclusion of Cp*_2Co^+ as a co-catalyst in controlled potential electrolysis experiments leads to improved yields of NH_3. The data presented provide what is to our knowledge the first unambiguous demonstration of electrocatalytic nitrogen fixation by a molecular catalyst (up to 6.7 equiv NH_3 per Fe at −2.1 V vs Fc^(+/0)).


Item Type:Article
Related URLs:
URLURL TypeDescription
https://dx.doi.org/10.1021/jacs.8b02335DOIArticle
https://pubs.acs.org/doi/10.1021/jacs.8b02335PublisherArticle
ORCID:
AuthorORCID
Peters, Jonas C.0000-0002-6610-4414
Alternate Title:Fe-Mediated Nitrogen Fixation with a Metallocene Mediator: Exploring pKa Effects and Demonstrating Electrocatalysis
Additional Information:© 2018 American Chemical Society. Received: February 28, 2018 Published: April 18, 2018 Publication Date (Web): April 18, 2018. This work was supported by the NIH (GM-075757) and the Resnick Sustainability Institute at Caltech. MJC, TJDC, and BDM are grateful for NSF Graduate Research Fellowships and MJC acknowledges a Caltech Environment Microbial Interactions (CEMI) Fellowship. This work made use of the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by the NSF Grant ACI-1053575. We also thank Pakpoom Buabthong for technical assistance with XPS measurements.
Group:Resnick Sustainability Institute, Caltech Center for Environmental Microbial Interactions (CEMI)
Funders:
Funding AgencyGrant Number
NIHGM-075757
Resnick Sustainability InstituteUNSPECIFIED
NSF Graduate Research FellowshipUNSPECIFIED
Caltech Center for Environmental Microbial Interactions (CEMI)UNSPECIFIED
NSFACI-1053575
Record Number:CaltechAUTHORS:20180418-164723702
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20180418-164723702
Official Citation:Fe-Mediated Nitrogen Fixation with a Metallocene Mediator: Exploring pKa Effects and Demonstrating Electrocatalysis Matthew J. Chalkley, Trevor J. Del Castillo, Benjamin D. Matson, and Jonas C. Peters Journal of the American Chemical Society Just Accepted Manuscript DOI: 10.1021/jacs.8b02335
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:85963
Collection:CaltechAUTHORS
Deposited By: George Porter
Deposited On:19 Apr 2018 14:55
Last Modified:16 May 2018 23:23

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