Veiros Luis F
Centro de Química Estrutural, Instituto Superior Técnico, 1049-001 Lisboa, Portugal.
Chemistry. 2005 Apr 8;11(8):2505-18. doi: 10.1002/chem.200401235.
The mechanisms of three closely related reactions were studied in detail by means of DFT/B3 LYP calculations with a VDZP basis set. Those reactions correspond to 1) the reductive elimination of methane from [Zr(eta5-Ind)2(CH3)(H)] (Ind=C9H7-, indenyl), 2) the formation of the THF adduct, [Zr(eta5-Ind)(eta6-Ind)(thf)] and 3) the interconversion between the two indenyl ligands in the Zr sandwich complex, [Zr(eta5-Ind)(eta9-Ind)], which forms the link between the two former reactions. An analysis of the electronic structure of this species indicates a saturated 18-electron complex. A full understanding of the indenyl interchange process required the characterisation of several isomers of the Zr-bis(indenyl) species, corresponding to different spin states (S=0 and S=1), different coordination modes of the two indenyl ligands (eta5/eta9, eta5/eta5 and eta6/eta9), and three conformations for each isomer (syn, anti, and gauche). The fluxionality observed was found to occur in a mechanism involving bis(eta5-Ind) intermediates, and the calculated activation energy (11-14 kcal mol(-1)) compares very well with the experimental values. Two alternative mechanisms were explored for the reductive elimination of methane from the methyl/hydride complex. In the more favourable one, the initial complex, [Zr(eta5-Ind)2(CH3)(H)], yields [Zr(eta5-Ind)2] and methane in one crucial step, followed by a smooth transition of the Zr intermediate to the more stable eta5/eta9-species. The overall activation energy calculated (Ea=29 kcal mol(-1)) compares well with experimental values for related species. The formation of the THF adduct follows a one step mechanism from the appropriate conformer of the [Zr(eta5-Ind)(eta9-Ind)] complex, producing easily (Ea=6.5 kcal mol(-1)) the known product, [Zr(eta5-Ind)(eta6-Ind)(thf)], a species previously characterised by X-ray crystallography. This complex was found to be trapped in a potential well that prevents it from evolving to the 3.4 kcal mol(-1) more stable isomer, [Zr(eta5-Ind)2(thf)], with both indenyl ligands in a eta5-coordination mode and a spin-triplet state (S=1).
通过采用VDZP基组的DFT/B3 LYP计算方法，对三个密切相关反应的机理进行了详细研究。这些反应分别为：1）从[Zr(η⁵-Ind)₂(CH₃)(H)]（Ind = C₉H₇⁻，茚基）中还原消除甲烷；2）形成THF加合物[Zr(η⁵-Ind)(η⁶-Ind)(thf)]；3）Zr夹心配合物[Zr(η⁵-Ind)(η⁹-Ind)]中两个茚基配体之间的相互转化，该转化在前面两个反应之间起到了连接作用。对该物种电子结构的分析表明它是一个饱和的18电子配合物。要全面理解茚基交换过程，需要对Zr-双（茚基）物种的几种异构体进行表征，这些异构体对应不同的自旋态（S = 0和S = 1）、两个茚基配体的不同配位模式（η⁵/η⁹、η⁵/η⁵和η⁶/η⁹），并且每种异构体有三种构象（顺式、反式和gauche式）。发现观察到的分子内重排发生在一个涉及双（η⁵-Ind）中间体的机理中，计算得到的活化能（11 - 14 kcal mol⁻¹）与实验值非常吻合。对于从甲基/氢化物配合物中还原消除甲烷，探索了两种替代机理。在更有利的一种机理中，可以从初始配合物[Zr(η⁵-Ind)₂(CH₃)(H)]一步生成[Zr(η⁵-Ind)₂]和甲烷，随后Zr中间体平稳地转变为更稳定的η⁵/η⁹物种。计算得到的总活化能（Ea = 29 kcal mol⁻¹）与相关物种的实验值相当。THF加合物的形成是从[Zr(η⁵-Ind)(η⁹-Ind)]配合物的适当构象通过一步机理进行的，很容易生成（Ea = 6.5 kcal mol⁻¹）已知产物[Zr(η⁵-Ind)(η⁶-Ind)(thf)]，该物种先前已通过X射线晶体学进行了表征。发现该配合物被困在一个势阱中，这阻止了它演变为能量低3.4 kcal mol⁻¹的更稳定异构体[Zr(η⁵-Ind)₂(thf)]，其中两个茚基配体均处于η⁵配位模式且为自旋三重态（S = 1）。
