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···π 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 × 10^-30 C·m.