Ferrocene and Dicarbonylcyclopentadienylcobalt in Faujasite-Type Zeolites:  A Study of Molecular Motion

The anisotropic molecular motion of Fe(C5H5)2 and Co(C5H5)(CO)2 molecules in the supercages of faujasite-type zeolites has been examined by NMR and by Mössbauer spectroscopy. Static 2H quad-echo and {H-}13C CP NMR techniques show that below 225 K the Fe(C5H5)2 molecules have no translational freedom, the only motion being rapid rotation of the cyclopentadienyl rings about their 5-fold axes. This is indicated by an axially symmetric powder pattern (δiso = 69.7 ppm, Ω = 75.0 ppm) in the {1H-}13C CP NMR spectrum and a broad Pake-type powder pattern (QCC = 97.3 kHz) in the 2H NMR spectrum. As the temperature is raised the molecules gain translational freedom, and at temperatures above 358 K isotropic molecular motion is identified as the only type of molecular motion. A model is proposed suggesting that the translational, isotropic motion is mainly caused by intracage, SII→SII jumps of the Fe(C5H5)2 molecules. Based on this model activation energies and diffusion coefficients were calculated from the NMR parameters. The molecular motion of intrazeolite Fe(C5H5)2 depends on the Si/Al ratio of the Na-faujasite host as well, being the highest for Na-faujasites with the lowest Si/Al ratio. The higher amount of sodium cations in the supercages probably causes a decrease in the energy barriers for site-to-site hopping. {1H-}13C CP NMR experiments show that Co(C5H5)(CO)2 molecules get firmly fixed in the zeolite at 183 K. This observation enabled the study of the OC−Co−CO bite angle, φ, by use of 13C Hahn-echo NMR experiments on enriched Co(C5H5)(13CO)2. The presence of an inverted axially symmetric powder pattern with span, Ω, of 127 ppm and a second powder pattern with Ω = 287 ppm indicate changes in the bite angle.