Direct Observation of Metal Ketenes Formed by Photoexcitation of a Fischer Carbene using Ultrafast Infrared Spectroscopy

Fischer carbenes are commonly used
as reagents in the synthesis
of new carbon–carbon bonds, a reaction made possible by the
unique chemistry of the formal metal–carbon double bond. Nevertheless,
the photoinduced reactions of these complexes are relatively poorly
understood. For instance, it has been postulated but not confirmed
that visible irradiation leads to photocarbonylation, in which a CO
ligand inserts into the metal–carbon bond to form a metal ketene
intermediate. Here, we report the first direct observation of this
intermediate following 400 nm photoexcitation of the model group 6
Fischer carbene Cr­(CO)5[CCH3(OCH3)]. Using ultrafast time-resolved infrared spectroscopy (TRIR), we
observe the formation of three distinct metal ketene structures, which
we assign as a singlet and two isoenergetic triplet excited-state
structures. The singlet relaxes to the ground state on a time scale
of ∼35 ps, whereas the two triplets are long-lived (>2 ns).
TRIR of the tungsten analogue yields no evidence for a metal ketene
structure, consistent with the limited reactivity of this complex.
The results directly elucidate the fundamental role of triplet metal
ketenes in the photoreactivity of Fischer carbene complexes.