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] [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] 可催化炔烃环三聚生成芳烃。将该铁配合物与1当量二苯基乙炔反应，可得到双金属桥联炔烃配合物，同时伴随IPr从铁中心解离。在高温条件下，该桥联炔烃配合物发生氧化偶联，生成双金属环戊二烯配合物，形式上每个金属中心均发生单电子氧化。各配合物均可催化二苯基乙炔的环三聚反应。动力学研究显示其反应速率对双金属配合物呈一级动力学依赖，为催化循环中该类物种的存在提供了进一步佐证。密度泛函理论(DFT)计算验证了实验得到的机理数据，并表明催化循环的完成路径为：炔烃结合至二烯配合物，随后经插入反应生成己三烯物种，该物种再通过环闭合形成反夹心配合物[DIPP(Me₃Si)N]Fe(η⁶-芳烃)Fe[N(SiMe₃)DIPP]。最终芳烃产物被炔烃置换，从而完成整个催化循环。