Bimetallic Mechanism for Alkyne Cyclotrimerization with a Two-Coordinate Fe Precatalyst

The two-coordinate compound (IPr)­Fe­[N­(SiMe3)­DIPP] (IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazolin-2-ylidene;
DIPP = 2,6-diisopropylphenyl) catalyzes the cyclotrimerization of
alkynes to arenes. Treatment of the Fe complex with 1 equiv of diphenylacetylene
results in the formation of a bimetallic bridging alkyne complex,
along with dissociation of IPr from Fe. At elevated temperatures,
the bridging alkyne complex undergoes oxidative coupling to form a
dimetallacyclopentadiene complex, formally by a one-electron oxidation
at each metal center. Each complex catalyzes the cyclotrimerization
of diphenylacetylene. Kinetic studies exhibit first-order dependence
on the bimetallic complexes, providing further support for the presence
of these species in the catalytic cycle. DFT calculations support
the experimental mechanistic data and suggest that the catalytic cycle
is completed by binding of an alkyne to the diene complex, followed
by insertion to form a hexatriene species that then undergoes ring
closure to form an inverse sandwich complex, [DIPP­(Me3Si)­N]­Fe­(η6-arene)­Fe­[N­(SiMe3)­DIPP]. The arene product is then
displaced by alkyne to close the catalytic cycle.