Transformation of Acetylene to Ethenylidene, Carbene, Acetylide, Vinyl, and Olefin Groups with Cp*Fe(1,2-Cy₂PC₆H₄S)

Tautomerization of C2H2 at half-sandwich compound Cp*Fe(1,2-Cy2PC6H4S) exclusively produces an iron ethenylidene, Cp*Fe(=C=CH2)(1,2-Cy2PC6H4S) (2). Protonation of the ethenylidene causes nucleophilic attack of the C-alpha by sulfur, affording a sulfur-tethered carbene complex, [Cp*Fe=C(CH3)SC6H4PCy2](+) (3(+)). This Fischer-type carbene complex undergoes an unusual isomerization by migrating a hydrogen atom from the beta-CH3 group to the alpha-C, leading to the formation of an olefin complex [Cp*Fe(eta(4)-CH=CH2SC6H4PCy2](+) (4(+)). Compound 2 also displays diverse redox reactivities. It transforms to a neutral acetylide ferric complex (5) when reacting with free radical scavengers and to a cationic vinyl complex [Cp*Fe(eta(3)-C(=CH2)SC6H4PCy2](+) (6(+)) upon 1e(-) oxidation. The interconversion between the vinyl and acetylide complexes can be realized through protonation/deprotonation reactions.