Pressure-Tuned Conformers and Their Populations in Ferrocene

In ferrocene (FeCp2, Cp is C5H5–), the dynamically disordered cyclopentadienyl rings (Cp) result in a distribution of staggered, rotated (axially chiral), and eclipsed conformations, strongly affected by pressure. Under normal conditions, about 54% of the molecules are staggered, 23% are rotated left, and 23% are rotated right, whereas at 2.2 GPa, the population of staggered conformers is 69% and that of eclipsed ones is 31%, while the rotated conformers are absent. The monotonic compression of phase I at 293 K gradually modifies the disorder angle between the Cp-ring sites about the pseudo-D5 axis and site-occupation factors (SOFs). In the population of rotated conformers, the twisting angle gradually changes between ±8° at 0.1 MPa, ±13° at 1.5 GPa, 0° (the eclipsed conformer) at 2.2 GPa, and 36° (the staggered conformer) at 3.24 GPa when phase I isostructurally transforms to the ordered phase I′, built exclusively of the staggered conformers. The structural models of ferrocene phase I have been refined de novo on high-pressure synchrotron X-ray diffraction data, and the molecular conformation has been described as a convolution of the sites of disordered Cp rings. The disordered Cp-ring sites are displaced, which generates a molecular dipole moment for the rotated and eclipsed conformers, consistent with the dielectric response of the ferrocene crystals.