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

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, (SiP3)­Fe,
to give Fe­(I) and Fe­(II) side-on adducts, including S = 1/2 FeI(η2-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
(SiP3)­FeCCH3 and the paramagnetic cation S = 1/2 [(SiP3)­FeCCH3]+. 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-CH3 protons with iron indicates hyperconjugation with the spin/hole
character on the FeCCH3 unit. The electronic structures
of (SiP3)­FeCCH3 and [(SiP3)­FeCCH3]+ are discussed in comparison
to previously characterized, but heterosubstituted, iron carbynes,
as well as 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.