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Characterization of the earliest intermediate of Fe-N_2 protonation: CW and Pulse EPR detection of an Fe-NNH species and its evolution to Fe-NNH_2^+

Nesbit, Mark A. and Oyala, Paul H. and Peters, Jonas C. (2019) Characterization of the earliest intermediate of Fe-N_2 protonation: CW and Pulse EPR detection of an Fe-NNH species and its evolution to Fe-NNH_2^+. Journal of the American Chemical Society, 141 (20). pp. 8116-8127. ISSN 0002-7863. PMCID PMC6636918.

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Iron diazenido species (Fe(NNH)) have been proposed as the earliest intermediates of catalytic N_2-to-NH_3 conversion (N_2RR) mediated by synthetic iron complexes and relatedly as intermediates of N_2RR by nitrogenase enzymes. However, direct identification of such iron species, either during or independent of catalysis, has proven challenging owing to their high degree of instability. The isolation of more stable silylated diazenido analogues, Fe(NNSiR_3), and also of further downstream intermediates (e.g., Fe(NNH_2)), nonetheless points to Fe(NNH) as the key first intermediate of protonation in synthetic systems. Herein we show that low-temperature protonation of a terminally bound Fe-N_2– species, supported by a bulky trisphosphinoborane ligand (^(Ar)P_3^B), generates an S = 1/2 terminal Fe(NNH) species that can be detected and characterized by continuous-wave (CW) and pulse EPR techniques. The ^1H-hyperfine for ^(Ar)P_3^BFe(NNH) derived from the presented ENDOR studies is diagnostic for the distally bound H atom (a_(iso) = 16.5 MHz). The Fe(NNH) species evolves further to cationic [Fe(NNH_2)]+ in the presence of additional acid, the latter being related to a previously characterized [Fe(NNH_2)]+ intermediate of N2RR mediated by a far less encumbered iron tris(phosphine)borane catalyst. While catalysis is suppressed in the present sterically very crowded system, N_2-to-NH_3 conversion can nevertheless be demonstrated. These observations in sum add support to the idea that Fe(NNH) plays a central role as the earliest intermediate of Fe-mediated N2RR in a synthetic system.

Item Type:Article
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URLURL TypeDescription CentralArticle
Nesbit, Mark A.0000-0002-5642-9303
Oyala, Paul H.0000-0002-8761-4667
Peters, Jonas C.0000-0002-6610-4414
Alternate Title:Characterization of the earliest intermediate of Fe-N2 protonation: CW and Pulse EPR detection of an Fe-NNH species and its evolution to Fe-NNH2+
Additional Information:© 2019 American Chemical Society. Received: November 9, 2018; Published: May 2, 2019. This work was supported by the NIH (R01-070757). The EPR facility at the California Institute of Technology has been supported by the NSF via its MRI program (NSF-1531940) and the DOW Next Generation Educator Fund. We thank Dirk Schild for performing a BDFE_(N–H) calculation on trans-(H)(DMeOPrPE)_2Fe(N═NH) and Dr. Jonathan Rittle for contributions to the ligand synthesis. The authors declare no competing financial interest.
Funding AgencyGrant Number
Dow Next Generation Educator FundUNSPECIFIED
Issue or Number:20
PubMed Central ID:PMC6636918
Record Number:CaltechAUTHORS:20190503-131410286
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Official Citation:Characterization of the Earliest Intermediate of Fe-N2 Protonation: CW and Pulse EPR Detection of an Fe-NNH Species and Its Evolution to Fe-NNH2+. Mark A. Nesbit, Paul H. Oyala, and Jonas C. Peters. Journal of the American Chemical Society 2019 141 (20), 8116-8127. DOI: 10.1021/jacs.8b12082
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:95208
Deposited By: George Porter
Deposited On:03 May 2019 20:30
Last Modified:04 Apr 2020 00:06

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