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.

双配位配合物(IPr)­Fe­[N­(SiMe3)­DIPP]（其中IPr=1,3-双(2,6-二异丙基苯基)咪唑啉-2-亚基；DIPP=2,6-二异丙基苯基）可催化炔烃的环三聚反应，生成芳烃类产物。将该铁配合物与1当量二苯乙炔进行反应，可得到双金属桥联炔烃配合物，同时IPr配体从铁中心解离。在高温环境下，该桥联炔烃配合物会发生氧化偶联反应，生成双金属环戊二烯配合物，形式上每个金属中心均经历单电子氧化过程。上述两种配合物均可催化二苯乙炔的环三聚反应。动力学研究显示，反应速率对双金属配合物呈一级动力学依赖关系，进一步佐证了此类物种参与催化循环的合理性。密度泛函理论（Density Functional Theory, DFT）计算结果支持实验得到的机理数据，并提出完整的催化循环路径为：炔烃结合至二烯配合物，随后发生插入反应生成己三烯中间体，该中间体再经环闭合形成反夹心配合物[DIPP­(Me3Si)­N]­Fe­(η6-芳烃)­Fe­[N­(SiMe3)­DIPP]。后续芳烃产物会被炔烃取代，从而完成整个催化循环。