Pressure-Tuned\nConformers and Their Populations in\nFerrocene

In ferrocene (FeCp<sub>2</sub>, Cp is C<sub>5</sub>H<sub>5</sub><sup>–</sup>), the dynamically disordered cyclopentadienyl\nrings (Cp) result in a distribution of staggered, rotated (axially\nchiral), and eclipsed conformations, strongly affected by pressure.\nUnder normal conditions, about 54% of the molecules are staggered,\n23% are rotated left, and 23% are rotated right, whereas at 2.2 GPa,\nthe population of staggered conformers is 69% and that of eclipsed\nones is 31%, while the rotated conformers are absent. The monotonic\ncompression of phase I at 293 K gradually modifies the disorder angle\nbetween the Cp-ring sites about the pseudo-<i>D</i><sub>5</sub> axis and site-occupation factors (SOFs). In the population\nof rotated conformers, the twisting angle gradually changes between\n±8° at 0.1 MPa, ±13° at 1.5 GPa, 0° (the\neclipsed conformer) at 2.2 GPa, and 36° (the staggered conformer)\nat 3.24 GPa when phase I isostructurally transforms to the ordered\nphase I′, built exclusively of the staggered conformers. The\nstructural models of ferrocene phase I have been refined <i>de\nnovo</i> on high-pressure synchrotron X-ray diffraction data,\nand the molecular conformation has been described as a convolution\nof the sites of disordered Cp rings. The disordered Cp-ring sites\nare displaced, which generates a molecular dipole moment for the rotated\nand eclipsed conformers, consistent with the dielectric response of\nthe ferrocene crystals.