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(C5D5)2]PF6. An axially symmetric powder pattern is seen in the solid-state H-2 NMR spectrum from 188 to 344 K with the residual quadrupole splitting ranging from 67 (2) to 62 (2) kHz, respectively. In this temperature range the C5D5- rings are rotating rapidly about local C5 axes, and the [Fe(C5D5)2]+ cation at the higher temperatures is also experiencing some small-amplitude libration of its molecular axis. At the 347 K first-order phase transition the 62-kHz residual quadrupole splitting is abruptly and reversibly reduced to essentially zero. The ferrocenium cation is tumbling in the solid as if it were in solution. In order to explain the entropy gain observed by adiabatic calorimetry, a model incorporating correlated motion of the cation and anion which starts at the 347 K phase transition and is present in the high-temperature plastic phase of ferrocenium hexafluorophosphate is developed. As the temperature is increased above the 347 K phase transition, the ferrocenium cation converts from being in four different lattice orientations to a dynamic disorder between twelve orientations. The PF6- anion converts from one orientation below 347 K to a dynamic disorder unequally distributed between four orientations above 347 K. An analysis of the crystallographic results shows that [Fe(C5H5)2]PF6 experiences orientational frustration in its plastic high-temperature phase. When the ferrocenium cation is in nine of its twelve available positions above 347 K, the eight neareSt PF6- anions cannot be in their three low-occupancy sites. On the basis of this model, the entropy gain occurring in the 347 K phase transition is theoretically expected to be DELTAS = R In 5.25, which compares favorably with the value of DELTAS = R ln 5.38 determined by heat capacity measurements.