DYNAMICS OF FERROCENE IN A THIOUREA INCLUSION MATRIX

57Fe Mössbauer and solid-state 2H NMR spectroscopy are used to investigate the orientation and onset of motion of Fe(C5D5)2 in the channels of the thiourea inclusion compound. 2H NMR spectra obtained by rotating a single crystal of Fe(C5D5)2·3(NH2)2CS about two orthogonal axes have been simulated and indicate that there are six different static ferrocene orientations at 140 K. At 140 K ∼55% of the ferrocenes are located in three positions related by the crystallographic C3 axis, where the ferrocene molecular axis is perpendicular to the channel axis of the thiourea host. The other three static ferrocene positions, which are interrelated by the C3 axis, are located with the ferrocene molecular axis ∼17° off the C3 axis. In each of the perpendicular and parallel sites there is a distribution of ferrocene sites due to static disorder. Abruptly at the 160 K phase transition the perpendicular ferrocenes start jumping between the three perpendicular sites and the others start jumping between the three essentially parallel sites. Simulations of the Mössbauer spectra show that in the 160-225 K range the rate of interconversion of ferrocenes between perpendicular and parallel orientations is slower than ∼105 s-1. The Mössbauer spectrum changes from two doublets in the 160-220 K region to become one doublet and then a single Lorentzian peak in the 220-300 K range. Spectra for a single crystal show that the 2H NMR spectrum collapses to a single doublet with a splitting of only 4.0 kHz with the field along the C3 axis. A model is proposed that accounts not only for the spectroscopic observables but also for the heat capacity results, which show an entropy gain of ΔS ≅ R In 4 at the 160 K transition and little entropy gain in the 180-300 K range where the Mössbauer and 2H NMR spectra change. © 1990, American Chemical Society. All rights reserved.