Low-Temperature NMR Studies of Zn Tautomerism and Hindered Rotations in Solid Zincocene Derivatives.

Using a combination of NMR methods we have detected and studied fluxional motions in the slip-sandwich structure of solid decamethylzincocene (I, (5-C5Me5)Zn(1-C5Me5)). For comparison, we have also studied the solid iminoacyl derivative (5-C5Me5)Zn(1-C(NXyl)C5Me5) (II). The variable temperature 13C CPMAS NMR spectra of Iindicate 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 1/5reorganization 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 5state than Y. However, the interconversion is still fast and characterized by a reaction enthalpy of H2.4 kJ mol-1and a reaction entropy of S4.9 J K-1mol-1. In order to obtain quantitative kinetic information, variable temperature 2H NMR experiments were performed on static samples of I-d6and II-d6between 300 and 100 K, where in each ring one CH3is replaced by one CD3group. For II-d6, the 2H NMR line shapes indicate fast CD3group rotations and a fast “5rotation”, corresponding to 72° rotational jumps of the 5coordinated 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-1was obtained. 2H NMR line shapes analysis of I-d6indicates fast CD3group 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-1and 11.2 kJ mol-1for the CD3rotation and the molecular inversion process were determined. [ABSTRACT FROM AUTHOR]