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Published September 25, 2019 | Accepted Version + Supplemental Material
Journal Article Open

Mononuclear Fe(I) and Fe(II) Acetylene Adducts and their Reductive Protonation to Terminal Fe(IV) and Fe(V) Carbynes

Abstract

The activity of nitrogenase enzymes, which catalyze the conversion of atmospheric dinitrogen to bioavailable ammonia, is most commonly assayed by the reduction of acetylene gas to ethylene. Despite the practical importance of acetylene as a substrate, little is known concerning its binding or activation in the iron-rich active site. "Fischer–Tropsch" type coupling of non-native C1 substrates to higher-order C_(≥2) products is also known for nitrogenase, though potential metal–carbon multiply bonded intermediates remain underexplored. Here we report the activation of acetylene gas at a mononuclear tris(phosphino)silyl-iron center, (SiP_3)Fe, to give Fe(I) and Fe(II) side-on adducts, including S = 1/2 Fe^I(η^2-HCCH); the latter is characterized by pulse EPR spectroscopy and DFT calculations. Reductive protonation reactions with these compounds converge at stable examples of unusual, formally iron(IV) and iron(V) carbyne complexes, as in diamagnetic (SiP_3)Fe≡CCH_3 and the paramagnetic cation S = 1/2 [(SiP_3)Fe≡CCH_3]^+. Both alkylcarbyne compounds possess short Fe–C triple bonds (approximately 1.7 Å) trans to the anchoring silane. Pulse EPR experiments, X-band ENDOR and HYSCORE, reveal delocalization of the iron-based spin onto the α-carbyne nucleus in carbon p-orbitals. Furthermore, isotropic coupling of the distal β-CH_3 protons with iron indicates hyperconjugation with the spin/hole character on the Fe≡CCH_3 unit. The electronic structures of (SiP_3)Fe≡CCH_3 and [(SiP_3)Fe≡CCH_3]^+ are discussed in comparison to previously characterized, but heterosubstituted, iron carbynes, as well as a hypothetical nitride species, (SiP_3)Fe≡N. Such comparisons are germane to the consideration of formally high-valent, multiply bonded Fe≡C and/or Fe≡N intermediates in synthetic or biological catalysis by iron.

Additional Information

© 2019 American Chemical Society. Received: July 2, 2019; Published: August 20, 2019. This work was supported by the National Institutes of Health (General Medical Sciences, grant GM070757). C.C. acknowledges the NIH Ruth L. Kirschstein National Service Fellowship for financial support. Additional support has been provided by the Caltech EPR Facility, supported via NSF-1531940, and the Dow Next Generation Educator Fund. We also acknowledge the Beckman Institute for use of the its X-ray facility and thank Larry M. Henling and Dr. David G. VanderVelde for technical assistance with X-ray and NMR experiments, respectively. The authors declare no competing financial interest.

Attached Files

Accepted Version - nihms-1054561.pdf

Supplemental Material - ja9b06987_si_001.pdf

Supplemental Material - ja9b06987_si_002.cif

Supplemental Material - ja9b06987_si_003.txt

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August 19, 2023
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