Observation of a Short-Lived Triplet Precursor in CpCo(CO)-Catalyzed Alkyne Cyclotrimerization

The picosecond dynamics of CpCo(CO), a catalytic intermediate in the cyclotrimerization of alkynes, have been studied using time-resolved infrared spectroscopy and density functional theory calculations. In neat 1-hexyne or 1-hexene solution, the first intermediate to form is a triplet η2-coordinated species, which then converts to a singlet η2 species in ca. 30–40 ps. The η2 triplet is the only solvent-coordinated species observed at early times, suggesting that this is the dominant mechanistic pathway for coordination of alkynes and alkenes to the CpCo(CO) catalyst. These new results complement the recent discovery that CpCo(CO) can coordinate to certain solvent molecules in both singlet and triplet spin states and further support previous studies implicating triplet intermediates in cobalt-catalyzed cyclotrimerization and cyclo-oligomerization reaction mechanisms.