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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Low-Temperature NMR Studies of Zn Tautomerism and Hindered Rotations in Solid Zincocene Derivatives
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Low-Temperature NMR Studies of Zn Tautomerism and Hindered Rotations in Solid Zincocene Derivatives

机译：固态茂金属衍生物中锌互变异构和受阻旋转的低温NMR研究

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摘要

Using a combination of NMR methods we have detected and studied fluxional motions in the slip-sandwich structure of solid decamethylzincocene (I, [(η~5-C5Me5)Zn(η~1-C5Me5)]). For comparison, we have also studied the solid iminoacyl derivative [(η~2-C5Me5)Zn(η~1-C(NXyl)C5Me5)] (II). The variable temperature ~(13)C 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 η~1/η~5 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 η~5 state than Y. However, the interconversion is still fast and characterized by a reaction enthalpy of ΔH = 2.4 kJ mol~(-1) and a reaction entropy of ΔS = 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 "η~5 rotation", corresponding to 72° rotational jumps of the η~5 coordinated Cp~* ring. The latter motion becomes slow around 130 K. By line shape analysis, an activation energy of the η~5 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.
机译：使用NMR方法的组合，我们已经检测和研究了固体十甲基锌茂金属（I，[（η〜5-C5Me5）Zn（η〜1-C5Me5）]）的三明治结构中的通量运动。为了进行比较，我们还研究了固体亚氨基酰基衍生物[（η〜2-C5Me5）Zn（η〜1-C（NXyl）C5Me5）]（II）。 I的可变温度〜（13）C CPMAS NMR光谱表明，分子中的两个Cp〜*环快速旋转至156 K，并且在210 K附近存在有序-无序相变。这是由于锌互变异构现象的快速结合和分子简型相关的两个简并态A和B之间Cp〜*环的η〜1 /η〜5重组的快速结合而引起的。在有序阶段，A和B的简并性被提升;也就是说，两个环X和Y是不等价的，其中X在η〜5状态下的时间比Y大。但是，相互转换仍然很快，并且具有ΔH= 2.4 kJ mol〜（ -1）和反应熵ΔS= 4.9 JK〜（-1）mol〜（-1）。为了获得定量动力学信息，对I-d6和II-d6静态样品在300至100 K之间进行了可变温度〜2H NMR实验，其中每个环中的一个CH3均被一个CD3基团取代。对于II-d6，〜2H NMR线形表示CD3基团快速旋转和“η-5旋转”快速，对应于η〜5配位Cp〜*环的72°旋转跳变。后者的运动在130 K附近变慢。通过线形分析，η〜5旋转的活化能约为21 kJ mol〜（-1）。 I-d6的〜2H NMR线形分析表明在所有温度下CD3基团快速旋转。此外，在100 K和150 K之间，两个Cp〜*环的5倍旋转跳跃都观察到了从慢速交换到快速交换的转变，其活化能为18 kJ mol〜（-1）。该值通过〜2H NMR弛豫法得到证实，从中另外测定了CD3旋转和分子转化过程的活化能6.3 kJ mol〜（-1）和11.2 kJ mol〜（-1）。

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