Ferrocenium hexafluorophosphate: molecular dynamics in the solid state

The nature of the molecular dynamics associate with the cation of ferrocenium hexafluorophosphate is probed with solid-state H-2 NMR, Fe-57 Mosbauer and single-crystal X-ray diffraction techniques. At room temperature [Fe-(C5H5)2]PF6 crystallizes in the monoclinic space group P2(1)/c which at 299 K has a = 13.408 (6) angstrom, b = 9.530 (2) angstrom, c = 9.482 (2) angstrom, beta = 93.17 (3)-degrees, Z = 4, and d(calc) = 818 g cm-3. Least squares refinement with 1429 observed reflections led to R = 0.067 and R(w) = 0.085. There are two crystallographically independent [Fe(C5H5)2]+ cations. Together with the two 2(1) symmetry-related cations there are four unique orientations of the ferrocenium cations in the solid state at 299 K. Only one orientation of the PF6- anion exists at 299 K. When the complex is heated to 360 K, the structure of [Fe(C5H5)2]PF6 changes from the monoclinic room-temperature setting to a cubic setting with a = 6.806 (4) angstrom, Z = 1, and d(calc) = 1.743 g cm-3 A unique solution to the structure at 360 K could not be determined due to the uncertainity in Laue symmetry and the limited amount of unique diffraction peaks observable above the diffuse scattering inherent in the plastic phase. However, one possible solution to the structure is presented. Least-squares refinement with 149 observed reflections in Pm3BAR led to R = 0.079 and R(w) = 0.05 1. In this space group the structure at 360 K consists of a pseudo-body-centered CsCl-type cubic lattice with orientationally disordered ferrocenium cations and PF6- anions. The ferrocenium cations are disordered in four orientations with the C5 axis at an angle of 18.3-degrees about each of three mutually perpendicular axes to give a total of twelve different orientations at 360 K. The PF6- anion is disordered in a total of four orientations, one of which is the orientation seen at 299 K with an occupancy of 0.66 and the other three of which (each with 0.11 occupancy) result from a rotation of the anion from this original orientation by 45-degrees about three mutually perpenicular F-P-F bond axes. Mossbauer spectra recorded from 120 to 375 K for [Fe(C5H5)2]PF6 show that the single quadrupole-split doublet in the spectrum undergoes a dramatic reduction in intensity upon heating the salt from 343 to 355 K. The suggested onset of dynamics associated with the ferrocenium cation is verified and delineated by variable-temperature (185-355 K) H-2 NMR spectra run for a polycrystalline sample of [Fe(C