Dipole-Moment Modulation in New Incommensurate Ferrocene

Area of Science:

• Solid-state chemistry
• Crystallography
• Materials science

Background:

• Metallocenes, particularly ferrocene (FeCp₂), have been extensively studied for 70 years.
• Unresolved aspects of ferrocene's phase diagram and its dielectric response persist.
• The molecular 5-fold symmetry of ferrocene is inconsistent with its observed dielectric behavior.

Purpose of the Study:

• To investigate the unresolved phase behavior of ferrocene.
• To elucidate the relationship between molecular conformation, intermolecular interactions, and electric permittivity.
• To reconcile the molecular symmetry of ferrocene with its crystal's dielectric response.

Main Methods:

• Discovery and characterization of a new ferrocene phase (phase I″).
• Analysis of structural transformations between 172.8 and 163.5 K.
• Description of the crystal structure using (3+2)-dimensional superspace.

Main Results:

• Identification of phase I″ in ferrocene, occurring between 172.8 and 163.5 K.
• Observation of conformational disorder transforming into incommensurate modulation.
• Detailed structural description involving modulated Cp ring conformations, tilts, cation displacements, and CH···π bonds.

Conclusions:

• The new phase I″ explains the dielectric response of ferrocene crystals.
• Geometric changes lead to FeCp₂ bending distortion, breaking the 5-fold symmetry.
• This distortion generates waves of molecular dipole moments, resolving the symmetry-dielectric discrepancy.