Straub Bernd F, Gollub Caroline
Department Chemie der Ludwig-Maximilians-Universität, München, Butenandtstr. 5-13, (Haus F), 81377 München, Germany.
Chemistry. 2004 Jun 21;10(12):3081-90. doi: 10.1002/chem.200305666.
In this B3 LYP model study, homoleptic nickel(0) ethyne complexes have been predicted as the catalyst resting state for the title reaction. Ethyne ligand coupling of Ni(C(2)H(2))(3) yields monoethyne nickelacyclopentadiene in the rate-determining step. Ethyne coordination is followed by insertion of an ethyne ligand into the Ni--C sigma bond. A highly strained monoethyne trans-nickelacycloheptatriene is formed. This trans intermediate is unable to reductively eliminate benzene without prior isomerization to a cis-structure. Instead, it rapidly collapses to a nickelacyclononatetraene. Ethyne coordination induces reductive elimination to the cyclooctatetraene complex Ni(eta(2)-C(2)H(2))(eta(2)-C(8)H(8)), followed by facile ligand exchange. Other ethyne coupling pathways have been computed to be less favored. The cyclooctatetraene ligand binds significantly weaker to nickel(0) than ethyne, both for mononuclear, and for dinuclear species. For this reason, C--C bond formation steps at Ni(2)(micro-cot) fragments have been predicted to feature prohibitively high overall reaction barriers.
在这项B3 LYP模型研究中，已预测同配镍（0）乙炔配合物是该标题反应的催化剂静止状态。在速率决定步骤中，Ni(C₂H₂)₃的乙炔配体偶联生成单乙炔镍环戊二烯。乙炔配位之后是一个乙炔配体插入Ni-C σ键。形成了一个高度张力的单乙炔反式镍环庚三烯。这个反式中间体在没有预先异构化为顺式结构的情况下无法还原消除苯。相反，它迅速分解为镍环壬四烯。乙炔配位诱导还原消除生成环辛四烯配合物Ni(η²-C₂H₂)(η²-C₈H₈)，随后是容易的配体交换。已计算出其他乙炔偶联途径不太有利。对于单核和双核物种，环辛四烯配体与镍（0）的结合明显弱于乙炔。因此，已预测在Ni₂(μ-cot)片段处的C-C键形成步骤具有过高得难以进行的总反应势垒。
