Lopez del Amo Juan Miguel, Buntkowsky Gerd, Limbach Hans-Heinrich, Resa Irene, Fernandez Rafael, Carmona Ernesto
Institut für Chemie und Biochemie der Freien Universität Berlin, Takustrasse 3, D-14195, Berlin, Germany.
J Phys Chem A. 2008 Apr 24;112(16):3557-65. doi: 10.1021/jp711504g. Epub 2008 Mar 27.
Using a combination of NMR methods we have detected and studied fluxional motions in the slip-sandwich structure of solid decamethylzincocene (I, [(eta5-C5Me5)Zn(eta1-C5Me5)]). For comparison, we have also studied the solid iminoacyl derivative [(eta5-C5Me5)Zn(eta1-C(NXyl)C5Me5)] (II). The variable temperature 13C CPMAS NMR spectra of I indicate fast rotations of both Cp* rings in the molecule down to 156 K as well as the presence of an order-disorder phase transition around 210 K. The disorder is shown to be dynamic arising from a fast combined Zn tautomerism and eta1/eta5 reorganization of the Cp* rings between two degenerate states A and B related by a molecular inversion. In the ordered phase, the degeneracy of A and B is lifted; that is, the two rings X and Y are inequivalent, where X exhibits a larger fraction of time in the eta5 state than Y. However, the interconversion is still fast and characterized by a reaction enthalpy of DeltaH = 2.4 kJ mol-1 and a reaction entropy of DeltaS = 4.9 J K-1 mol-1. In order to obtain quantitative kinetic information, variable temperature 2H NMR experiments were performed on static samples of I-d6 and II-d6 between 300 and 100 K, where in each ring one CH3 is replaced by one CD3 group. For II-d6, the 2H NMR line shapes indicate fast CD3 group rotations and a fast "eta5 rotation", corresponding to 72 degrees rotational jumps of the eta5 coordinated Cp* ring. The latter motion becomes slow around 130 K. By line shape analysis, an activation energy of the eta5 rotation of about 21 kJ mol-1 was obtained. 2H NMR line shapes analysis of I-d6 indicates fast CD3 group rotations at all temperatures. Moreover, between 100 and 150 K, a transition from the slow to the fast exchange regime is observed for the 5-fold rotational jumps of both Cp* rings, exhibiting an activation energy of 18 kJ mol-1. This value was corroborated by 2H NMR relaxometry from which additionally the activation energies 6.3 kJ mol-1 and 11.2 kJ mol-1 for the CD3 rotation and the molecular inversion process were determined.
我们使用多种核磁共振方法，检测并研究了固态十甲基二茂锌（I，[(η⁵-C₅Me₅)Zn(η¹-C₅Me₅)]）滑移夹心结构中的动态过程。为作比较，我们还研究了固态亚氨基酰基衍生物[(η⁵-C₅Me₅)Zn(η¹-C(NXyl)C₅Me₅)]（II）。I的变温¹³C交叉极化魔角旋转核磁共振谱表明，分子中的两个Cp环在低至156 K时快速旋转，并且在210 K左右存在有序-无序相变。结果表明，无序是由快速的锌互变异构以及Cp环在通过分子反转相关的两个简并态A和B之间的η¹/η⁵重排引起的动态过程。在有序相中，A和B的简并性消除；也就是说，两个环X和Y不等价，其中X在η⁵态的时间占比大于Y。然而，相互转化仍然很快，反应焓ΔH = 2.4 kJ·mol⁻¹，反应熵ΔS = 4.9 J·K⁻¹·mol⁻¹。为了获得定量动力学信息，在300至100 K之间对I-d₆和II-d₆的静态样品进行了变温²H核磁共振实验，其中每个环中的一个CH₃被一个CD₃基团取代。对于II-d₆，²H核磁共振谱线形状表明CD₃基团快速旋转以及快速的“η⁵旋转”，对应于η⁵配位的Cp环72度的旋转跳跃。后一种运动在130 K左右变慢。通过谱线形状分析，得到η⁵旋转的活化能约为21 kJ·mol⁻¹。I-d₆的²H核磁共振谱线形状分析表明，CD₃基团在所有温度下均快速旋转。此外，在100至150 K之间，观察到两个Cp环的5重旋转跳跃从慢速交换 regime 转变为快速交换 regime，活化能为18 kJ·mol⁻¹。²H核磁共振弛豫测量证实了该值，从中还确定了CD₃旋转和分子反转过程的活化能分别为6.3 kJ·mol⁻¹和11.2 kJ·mol⁻¹。
