Low-Temperature NMR Studies of Zn Tautomerism and Hindered Rotations in Solid Zincocene Derivatives
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- 作者：Juan Miguel Lopez del Amo ; Gerd Buntkowsky ; Hans-Heinrich Limbach ; Irene Resa ; Rafael Ferná ; ndez ; Ernesto Carmona
- 刊名：Journal of Physical Chemistry A
- 出版年：2008
- 出版时间：April 24, 2008
- 年：2008
- 卷：112
- 期：16
- 页码：3557 - 3565
- 全文大小：225K
- 年卷期：v.112,no.16(April 24, 2008)
- ISSN：1520-5215
文摘
Using a combination of NMR methods we have detected and studied fluxional motions in the slip-sandwichstructure of solid decamethylzincocene (I, [(
<SUP>5-C5Me5)Zn(1-C5Me5)]). For comparison, we have also studiedthe solid iminoacyl derivative [(5-C5Me5)Zn(1-C(NXyl)C5Me5)] (II). The variable temperature 13C CPMASNMR spectra of I indicate fast rotations of both Cp* rings in the molecule down to 156 K as well as thepresence of an order-disorder phase transition around 210 K. The disorder is shown to be dynamic arisingfrom a fast combined Zn tautomerism and 1/5 reorganization of the Cp* rings between two degeneratestates 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 5 state thanY. However, the interconversion is still fast and characterized by a reaction enthalpy of 
H = 2.4 kJ mol-1and a reaction entropy of S = 4.9 J K-1 mol-1. In order to obtain quantitative kinetic information, variabletemperature 2H NMR experiments were performed on static samples of I-d6 and II-d6 between 300 and 100K, where in each ring one CH3 is replaced by one CD3 group. For II-d6, the 2H NMR line shapes indicate fastCD3 group rotations and a fast "5 rotation", corresponding to 72
rotational jumps of the 5 coordinated Cp*ring. The latter motion becomes slow around 130 K. By line shape analysis, an activation energy of the 5rotation of about 21 kJ mol-1 was obtained. 2H NMR line shapes analysis of I-d6 indicates fast CD3 grouprotations at all temperatures. Moreover, between 100 and 150 K, a transition from the slow to the fast exchangeregime is observed for the 5-fold rotational jumps of both Cp* rings, exhibiting an activation energy of 18kJ mol-1. This value was corroborated by 2H NMR relaxometry from which additionally the activation energies6.3 kJ mol-1 and 11.2 kJ mol-1 for the CD3 rotation and the molecular inversion process were determined.
<SUP>5-C5Me5)Zn(1-C5Me5)]). For comparison, we have also studiedthe solid iminoacyl derivative [(5-C5Me5)Zn(1-C(NXyl)C5Me5)] (II). The variable temperature 13C CPMASNMR spectra of I indicate fast rotations of both Cp* rings in the molecule down to 156 K as well as thepresence of an order-disorder phase transition around 210 K. The disorder is shown to be dynamic arisingfrom a fast combined Zn tautomerism and 1/5 reorganization of the Cp* rings between two degeneratestates 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 5 state thanY. However, the interconversion is still fast and characterized by a reaction enthalpy of H = 2.4 kJ mol-1and a reaction entropy of S = 4.9 J K-1 mol-1. In order to obtain quantitative kinetic information, variabletemperature 2H NMR experiments were performed on static samples of I-d6 and II-d6 between 300 and 100K, where in each ring one CH3 is replaced by one CD3 group. For II-d6, the 2H NMR line shapes indicate fastCD3 group rotations and a fast "5 rotation", corresponding to 72 rotational jumps of the 5 coordinated Cp*ring. The latter motion becomes slow around 130 K. By line shape analysis, an activation energy of the 5rotation of about 21 kJ mol-1 was obtained. 2H NMR line shapes analysis of I-d6 indicates fast CD3 grouprotations at all temperatures. Moreover, between 100 and 150 K, a transition from the slow to the fast exchangeregime is observed for the 5-fold rotational jumps of both Cp* rings, exhibiting an activation energy of 18kJ mol-1. This value was corroborated by 2H NMR relaxometry from which additionally the activation energies6.3 kJ mol-1 and 11.2 kJ mol-1 for the CD3 rotation and the molecular inversion process were determined.