Molecular Rotation of Bis(η-cyclopentadienyl)iron(II) Derivatives Enclosed in α- and β-Cyclodextrin Molecule by the MOssbauer Spectroscopy

Abstract

The ⁵⁷Fe Mössbauer spectra of the title clathrates have been recorded at various temperatures in the range of 78-320 K. At low temperatures the spectrum consists of a simple quadrupole doublet, but upon heating it brings about an inward collapse into a single peak with of much reduced intensity(Fig.3). The variation of the spectrum with temperature has been interpreted by the model (Figs.1 and 2) that (η-formylcyclopentadienyl) (η-cyclopentadienyl)iron(II) (FFc) molecule reorientates in the cavity of α-cyclodextrin (α-CyD) with an activation energy of 14.3 kJ/mol (Figs.5 and 9). The reorientation of a FFc molecule became faster with increasing temperature and the isotropic relaxation time was 7.2 × 10^-8 s at 320 K (Fig.4). The variation of the Mössbauer spectra of α-CyD(η-methylcyclopentadienyl) (η-cyclopentadienyl) iron(II) (Fig.6) and β CyD-FFc clathrates (Fig.7) with temperature were recorded and the reorientation of the guest molecule was also observed. It has been concluded that the larger space in the cavity of CyD does not always give fast reorientation of a guest molecule.