Cramping a Molecular Rollerball. Investigation of the Effect of Pressure on the Mössbauer Spectra of Three Cyclopentadienyl(arene)iron(II) Salts

The effect of pressure on the Mössbauer spectra of salts of [CpFePhH]+, [CpFePhF]+, and [CpFePhMe]+ has been examined over the range of 10-4 GPa to about 4 GPa. Dynamic reorientation of salts of the first two cations may be arrested on the Mössbauer time scale by the application of pressure (0.85 and 1.36 GPa). For these two cations, extrapolated quadrupole splitting values were obtained at 10-4 GPa at room temperature that help to verify and extend previous models of motion. A simplified theoretical model of the reorientation of the [CpFePhH]+ cation is presented. It is also shown that at pressures above those which arrest motion, the compounds manifest a gradual decrease of quadrupole splitting with increasing pressure. An explanation is discussed.