Transformation of Acetylene to Ethenylidene, Carbene, Acetylide, Vinyl, and Olefin Groups with Cp*Fe(1,2-Cy 2 PC 6 H 4 S).

Tautomerization of C 2 H 2 at half-sandwich compound Cp*Fe(1,2-Cy 2 PC 6 H 4 S) exclusively produces an iron ethenylidene, Cp*Fe(=C=CH 2 )(1,2-Cy 2 PC 6 H 4 S) ( 2 ). Protonation of the ethenylidene causes nucleophilic attack of the C α by sulfur, affording a sulfur-tethered carbene complex, [Cp*Fe=C(CH 3 )SC 6 H 4 PCy 2 ] + ( 3 + ). This Fischer-type carbene complex undergoes an unusual isomerization by migrating a hydrogen atom from the β-CH 3 group to the α-C, leading to the formation of an olefin complex [Cp*Fe(η 4 -CH=CH 2 SC 6 H 4 PCy 2 ] + ( 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(η 3 -C(=CH 2 )SC 6 H 4 PCy 2 ] + ( 6 + ) upon 1e - oxidation. The interconversion between the vinyl and acetylide complexes can be realized through protonation/deprotonation reactions.