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

The anisotropic molecular motion of Fe(C 5 H 5 ) 2 and Co(C 5 H 5 )(CO) 2 molecules in the supercages of faujasite-type zeolites has been examined by NMR and by Mössbauer spectroscopy. Static 2 H quad-echo and { 1 H-} 13 C CP NMR techniques show that below 225 K the Fe(C 5 H 5 ) 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 (δ iso = 69.7 ppm, Ω = 75.0 ppm) in the { 1 H-} 13 C CP NMR spectrum and a broad Pake-type powder pattern (QCC = 97.3 kHz) in the 2 H 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 intra cage, SII→SII jumps of the Fe(C 5 H 5 ) 2 molecules. Based on this model activation energies and diffusion coefficients were calculated from the NMR parameters. The molecular motion of intrazeolite Fe(C 5 H 5 ) 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. { 1 H-} 13 C CP NMR experiments show that Co(C 5 H 5 )(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 13 C Hahn-echo NMR experiments on enriched Co(C 5 H 5 )( 13 CO) 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.