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, (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