Ruthenium-mediated cyclotrimerization of alkynes utilizing the cationic complex [RuCp(CH3CN)3]PF6

The substitutionally labile cationic complex [RuCp(CH3CN)3]+ (as the PF6− salt) was tested with respect to its ability to catalyze the cyclotrimerization of terminal alkynes and diynes to afford benzene derivatives. Whereas [RuCp(CH3CN)3]+ is in fact promoting the catalytic cyclotrimerization of alkynes, the formation of stable and inert sandwich complexes of the type [RuCp(η6-arene)]+ deactivates the catalyst and thus quenches the catalytic cycle. All new sandwich complexes were isolated and characterized spectroscopically. A proposal for a plausible catalytic cycle including possible degradation pathways of the catalyst is presented based on DFT calculations. As key intermediates several novel carbene complexes have been identified including metallacyclopentatriene and metallaheptatetraene species.