Fe-Mediated Nitrogen Fixation with a Metallocene Mediator: Exploring pK_a Effects and Demonstrating Electrocatalysis
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)).
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.Attached Files
Accepted Version - nihms974382.pdf
Supplemental Material - ja8b02335_si_001.pdf
Supplemental Material - ja8b02335_si_002.mol
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Additional details
- Alternative title
- Fe-Mediated Nitrogen Fixation with a Metallocene Mediator: Exploring pKa Effects and Demonstrating Electrocatalysis
- PMCID
- PMC6071328
- Eprint ID
- 85963
- DOI
- 10.1021/jacs.8b02335
- Resolver ID
- CaltechAUTHORS:20180418-164723702
- NIH
- GM-075757
- Resnick Sustainability Institute
- NSF Graduate Research Fellowship
- Caltech Center for Environmental Microbial Interactions (CEMI)
- NSF
- ACI-1053575
- Created
-
2018-04-19Created from EPrint's datestamp field
- Updated
-
2022-03-10Created from EPrint's last_modified field
- Caltech groups
- Resnick Sustainability Institute, Caltech Center for Environmental Microbial Interactions (CEMI)