DFT Mechanistic Insights into the Alkyne Insertion Reaction Affording Diiron μ-Vinyliminium Complexes and New Functionalization Pathways

Insertion of propyne or 2-butyne into the Fe-carbyne bond belonging to the fragment [Fe2Cp2(CO)(μ-CO)μ-CNMe(R)]+ (R = Me or Xyl = 2,6-C6H3Me2) was investigated via density functional theory (DFT), and plausible intermediates were identified along the formation of the vinyliminium complexes [Fe2Cp2(CO)(μ-CO)μ-η1:η3-C(Me)C(R″)CN(Me)(R)]SO3CF3, [2a-d]+, thus allowing us to explain regio- and stereochemical features. The X-ray structure of [2a]SO3CF3 (R = Me, R″ = H) was determined by single crystal X-ray diffraction. Novel C-C and C-S bond forming pathways involving the vinyliminium ligand were then explored. Thus, [2b]SO3CF3 (R = Xyl, R″ = H) reacted with cyclopentadiene (or cyclopentene), triphenylphosphonium methylide, and benzyl bromide, in tetrahydrofuran in the presence of sodium hydride, respectively, to give [Fe2Cp2(CO)(μ-CO)μ-η1:η2-C(Me)CC(CH)4CN(Me)(Xyl)], 3, [Fe2Cp2(CO)(μ-CO)μ-η1:η2-C(Me)C(CH2)CN(Me)(Xyl)], 4, and [Fe2Cp2(CO)(μ-CO)μ-η1:η3-C(Me)C(CH2Ph)CN(Me)(Xyl)]Br, [5]Br, in good yields. Unstable complex 4 (detected by IR spectroscopy) readily converted into [2c]SO3CF3 (R = Xyl, R″ = Me) upon HSO3CF3 protonation of the methylide function. [5]Br was obtained as E/Z isomeric mixture, which was then quantitatively converted into the most stable Z form, by heating in methanol solution at 50 °C. The reactions of [2c,d]SO3CF3 (R = Me, Xyl, R″ = Me) with PhSSPh/NaH selectively yielded the aminoalkylidyne species [Fe2Cp2(SPh)(CO)(μ-CO)μ-CN(Me)(Xyl)], 6, and the bis-alkylidene [Fe2Cp2(CO)(μ-CO)μ-η1:η2-C(Me)C(Me)(SPh)CN(Me)2], 7, respectively, probably via the intermediacy of radical compounds 2c,d. The structures of 3-7 and 2c,d were elucidated by DFT calculations, and the isolated products were characterized by analytical and spectroscopic methods.