Ring Rotation in Ferrocene and Ferrocene-containing Polymers

Ferrocene is an organometallic molecular sandwich complex with an iron atom coordinated between two cyclopentadienyl rings. The reorientation of these rings in a process of rotational jump diffusion between multiple equilibrium sites on a circle is investigated using quasielastic neutron time of flight and backscattering spectroscopy experiments. Existing results on the ring rotation in bulk, crystalline ferrocene are extended, and the study is widened to oxidized ferrocenium cations in the triiodide complex FcI3, and to ferrocene containing polymers like poly(vinylferrocene) PVFc. Emphasis is put on a robust data analysis of neutron scattering data, including corrections for multiple scattering and simultaneous analysis of many data sets taken on different spectrometers. It is shown that the 5-fold rotational jump diffusion model needs to be extended to a non-equivalent sites model to account for rotational disorder in the monoclinic room temperature phase of ferrocene which is metastable down to 164 K. In the triclinic phase below 164 K, the combination of time of flight and backscattering spectroscopy enables to separate two dynamical processes due to crystallographically different molecules in the unit cell. In the triiodide complex of ferrocenium cations, FcI3, the barrier to rotation is found to be significantly lower than in bulk ferrocene. Moreover, a hitherto unknown phase transition is found at 85 K which causes a discontinuity in the temperature dependence of the correlation time of the ring rotation in FcI3, very similar to the triclinic-monoclinic transition in ferrocene. The ring rotation above 85 K is closer to continuous rotation due to its low barrier, while 5-fold jumps are favored below 85 K. In the polymer PVFc, where ferrocene units are laterally attached to a polymer chain, it is shown that the ring rotation is still active, but the correlation times are broadly distributed. The neutron scattering data can be described very well by a rotation rate distribution model over the large temperature range from 80 K to 350 K. The average activation energy of the motion is 9.61(2) kJ mol−1, with a distribution having circa one third of this value as second moment. Moreover, a vibrational study has been performed on ferrocene, FcI3, and various ferrocene containing polymers. It turns out that the vibrational fingerprint modes of the molecule are mildly affected when the ferrocene unit is laterally attached to a polymer chain, but more severely if it is incorporated into the polymer backbone. Finally, measurements under external magnetic fields did not reveal any field dependence of the ring rotation dynamics in oxidized PVFc, where the oxidation leads to magnetic moments on the ferrocene units. Even though unrelated to the main topic of ring rotation, these experiments nicely demonstrated inelastic magnetic neutron scattering on Zeeman split levels of the electronic ground state, and high resolution measurements allowed to directly observe nuclear hyperfine splitting in external magnetic fields.