Dipole-Moment Modulation in New Incommensurate Ferrocene

Despite 70 years of research on metallocenes and their applications, there are still unresolved regions in its phase diagram of the prototypic sandwich compound, ferrocene Fe2+[C5H5](-) (2) (FeCp2), and its molecular 5-fold symmetry cannot be reconciled with the dielectric response of this crystal. We found a new phase I '' of ferrocene, which reveals the relationships between the molecular conformation, intermolecular interactions, and electric permittivity of this compound. Between 172.8 and 163.5 K, the conformational disorder of ferrocene molecules transforms into the incommensurate modulation. The structure of phase I '' is described in the (3+2)-dimensional superspace, where the molecular conformations, rotations and inclinations of the Cp rings, molecular tilts, and displacements of the Fe2+ cations, as well as the CH center dot center dot center dot pi bonds in the crystal environment, are modulated. These geometric changes combine into the FeCp2 bending distortion, breaking the 5-fold symmetry and generating waves of molecular dipole moments with their amplitudes approaching 4 x 10(-30) C center dot m.

Automated summary

A new phase I" of ferrocene has been discovered, revealing the relationship between molecular conformation, intermolecular interactions, and electric permittivity. The conformational disorder of ferrocene molecules transforms into incommensurate modulation between 172.8 and 163.5 K. The structure of phase I" is described in the (3+2)-dimensional superspace, with modulation of molecular conformations, rotations and inclinations, Fe2+ cation displacements, and CH...π bonds in the crystal environment, leading to the bending distortion of FeCp2 and generation of molecular dipole moment waves.