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Low-temperature NMR studies of Zn tautomerism and hindered rotations in solid zincocene derivatives

Lopez del Amo, J. M. and Buntkowsky, G. and Limbach, H. H. and Resa, I. and Fernandez, R. and Carmona, E. (2008):
Low-temperature NMR studies of Zn tautomerism and hindered rotations in solid zincocene derivatives.
112, In: Journal of Physical Chemistry A, (16), pp. 3557-3565, [Online-Edition: http://apps.webofknowledge.com/full_record.do?product=WOS&se...],
[Article]

Abstract

Using a combination of NMR methods we have detected and studied fluxional motions in the slip-sandwich structure of solid decamethylzincocene (I, (eta(5)-C(5)Me(5))Zn(eta(1)-C(5)Me(5))). For comparison, we have also studied the solid iminoacyl derivative (eta(5)-C(5)Me(5))Zn(eta(1)-C(NXyl)C(5)Me(5)) (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 eta(1)/eta(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 eta(5) state than Y. However, the interconversion is still fast and characterized by a reaction enthalpy of delta H = 2.4 kJ mol(-1) and a reaction entropy of delta S = 4.9 J K(-1) mol(-1). In order to obtain quantitative kinetic information, variable temperature (2)H NMR experiments were performed on static samples of I-d(6) and II-d(6) between 300 and 100 K, where in each ring one CH(3) is replaced by one CD(3) group. For II-d(6), the (2)H NMR line shapes indicate fast CD(3) group rotations and a fast "eta(5) rotation", corresponding to 72 degrees rotational jumps of the eta(5) coordinated Cp* ring. The latter motion becomes slow around 130 K. By line shape analysis, an activation energy of the eta(5) rotation of about 21 kJ mol(-1) was obtained. (2)H NMR line shapes analysis of I-d(6) indicates fast CD(3) 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 (2)H NMR relaxometry from which additionally the activation energies 6.3 kJ mol(-1) and 11.2 kJ mol(-1) for the CD(3) rotation and the molecular inversion process were determined.

Item Type: Article
Erschienen: 2008
Creators: Lopez del Amo, J. M. and Buntkowsky, G. and Limbach, H. H. and Resa, I. and Fernandez, R. and Carmona, E.
Title: Low-temperature NMR studies of Zn tautomerism and hindered rotations in solid zincocene derivatives
Language: English
Abstract:

Using a combination of NMR methods we have detected and studied fluxional motions in the slip-sandwich structure of solid decamethylzincocene (I, (eta(5)-C(5)Me(5))Zn(eta(1)-C(5)Me(5))). For comparison, we have also studied the solid iminoacyl derivative (eta(5)-C(5)Me(5))Zn(eta(1)-C(NXyl)C(5)Me(5)) (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 eta(1)/eta(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 eta(5) state than Y. However, the interconversion is still fast and characterized by a reaction enthalpy of delta H = 2.4 kJ mol(-1) and a reaction entropy of delta S = 4.9 J K(-1) mol(-1). In order to obtain quantitative kinetic information, variable temperature (2)H NMR experiments were performed on static samples of I-d(6) and II-d(6) between 300 and 100 K, where in each ring one CH(3) is replaced by one CD(3) group. For II-d(6), the (2)H NMR line shapes indicate fast CD(3) group rotations and a fast "eta(5) rotation", corresponding to 72 degrees rotational jumps of the eta(5) coordinated Cp* ring. The latter motion becomes slow around 130 K. By line shape analysis, an activation energy of the eta(5) rotation of about 21 kJ mol(-1) was obtained. (2)H NMR line shapes analysis of I-d(6) indicates fast CD(3) 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 (2)H NMR relaxometry from which additionally the activation energies 6.3 kJ mol(-1) and 11.2 kJ mol(-1) for the CD(3) rotation and the molecular inversion process were determined.

Journal or Publication Title: Journal of Physical Chemistry A
Volume: 112
Number: 16
Uncontrolled Keywords: nuclear-magnetic-resonance state nmr electron-diffraction dynamics chemistry beryllocene bis(pentamethylcyclopentadienyl)zinc benzene-d(6) spectroscopy complexes
Divisions: 07 Department of Chemistry
07 Department of Chemistry > Physical Chemistry
Date Deposited: 27 Oct 2014 20:45
Official URL: http://apps.webofknowledge.com/full_record.do?product=WOS&se...
Additional Information:

290DZ Times Cited:1 Cited References Count:49

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