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 nonnative 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, (SiP3)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 calculatoins. Reductive protonation reactions with these compounds converge at stable examples of unusual, formally iron(IV) and iron(V) carbyne complexes, as in diamagnetic (SiP3)Fe≡CCH3 and the paramagnetic cation S = 1/2 [(SiP3)Fe≡CCH3] +. Both alkylcarbyne compounds possess short Fe-C triple bonds (approx. 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-CH3 protons with iron indicates hyperconjugation with the spin/hole character on the Fe≡C-CH3 unit. The electronic structures of (SiP3)Fe≡CCH3 and [(SiP3)Fe≡CCH3] + are discussed in comparison to previously characterized, but hetero-substituted, iron carbynes, and also a hypothetical nitride species, (SiP3)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.

Electrochemical Conversion of CO2 to CO by a Competent FeI Intermediate Bearing a Schiff Base Ligand

“…A further evidence of reactivity and oxidation of Fe I in the presence of CO 2 came from electron paramagnetic resonance (EPR) spectroscopy. For a frozen sample of the electrogenerated Fe I state, a signal at 3450 G (g � 2) is observed at 80 K, which is consistent with a S = 1 = 2 for a d 7 low-spin Fe I , [59] ( Figure S8); in the sample treated with CO 2 , the signal above disappears, while the appearance of a broad band at ca. 3330 G and the raising of a feature at 1600 G (g � 4.…”

Closed Shell Iron(IV) Oxo Complex with an Fe–O Triple Bond: Computational Design, Synthesis, and Reactivity

“…There are well‐characterized complexes with nitride ligands [22, 23, 25–27] . Fe‐X triple bonds also exist with carbyne ligands [28] and mass‐spectrometry has suggested existence of triple bonds with heavier group 14 and 15 elements, [29, 30] or even the existence of the Fe‐B quadruple bond [31] . However, these bonds have not been characterized by means of vibrational spectroscopy.…”

Closed Shell Iron(IV) Oxo Complex with an Fe–O Triple Bond: Computational Design, Synthesis, and Reactivity

“…There are well-characterized complexes with nitride ligands. [22,23,[25][26][27] Fe-X triple bonds also exist with carbyne ligands [28] and mass-spectrometry has suggested existence of triple bonds with heavier group 14 and 15 elements, [29,30] or even the existence of the Fe-Bq uadruple bond. [31] However,t hese bonds have not been characterized by means of vibrational spectroscopy.…”