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⁺
Abstract
Iron diazenido species (Fe(NNH)) have been proposed as the earliest intermediates of catalytic N₂-to-NH₃ conversion (N₂RR) mediated by synthetic iron complexes and relatedly as intermediates of N₂RR 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(NNSiR₃), and also of further downstream intermediates (e.g., Fe(NNH₂)), 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-N₂– species, supported by a bulky trisphosphinoborane ligand (^(Ar)P₃^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)P₃^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(NNH₂)]⁺ in the presence of additional acid, the latter being related to a previously characterized [Fe(NNH₂)]⁺ 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, N₂-to-NH₃ 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.
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.Attached Files
Accepted Version - jacs.8b12082
Accepted Version - nihms-1036679.pdf
Supplemental Material - ja8b12082_si_001.pdf
Supplemental Material - ja8b12082_si_002.cif
Supplemental Material - ja8b12082_si_003.mol
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Additional details
- Alternative 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+
- PMCID
- PMC6636918
- Eprint ID
- 95208
- DOI
- 10.1021/jacs.8b12082
- Resolver ID
- CaltechAUTHORS:20190503-131410286
- NIH
- R01-070757
- NSF
- MRI-153194
- Dow Next Generation Educator Fund
- Created
-
2019-05-03Created from EPrint's datestamp field
- Updated
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2022-03-01Created from EPrint's last_modified field