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

Abstract

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.

How van der Waals bonds orient molecules in zeolites

“…Yet the reactivity of the guest (ferrocene in the present case) is dramatically increased when it is in the zeolite. 2,3 The popular concept is that the increased reactivity of the guest arises from changes in its electronic structure is incorrect in the present case, and it is the orientation in which the guest ferrocene molecule is held that renders it vulnerable to attack. Analyses of this type in which we separate the electronic and steric effects may have a general impact on the study of catalytic systems.…”

Localization of ferrocene in NaY zeolite by powder x-ray and neutron diffraction

“…We find that the number of ferrocene molecules as found from the neutron pattern is less than the number of ferrocene molecules as determined from the preparation. Also, it has been shown before by means of Mössbauer experiments 4 that at elevated temperatures a small fraction of the encaged ferrocene molecules diffuses out of the zeolite cages and forms a nonhomogeneous phase on the surface of the zeolite crystals. This could well have happened to our sample during storage.…”

“…Because of these future prospects of applications in heterogeneous catalysis confinement of organometallics in the voids of zeolite host frameworks has gained considerable attention over the last years. [1][2][3][4][5] For a fundamental understanding of these systems knowledge of the interaction between the organometallic guest molecules and the zeolite host framework is desired. We use the relatively simple organometallic ferrocene, Fe͑C 5 H 5 ) 2 , adsorbed in the supercages of NaY zeolite, Na x (AlO 2 ) x (SiO 2 ) 192Ϫx , as a model system.…”

“…The inclusion of ferrocene in the voids of zeolites has been investigated before ͑see Ref. 4, and references therein͒. One remarkable effect found is that unlike pure ferrocene in the solid or gas-phase state, ferrocene in zeolite Y oxidizes quite easily.…”

“…One remarkable effect found is that unlike pure ferrocene in the solid or gas-phase state, ferrocene in zeolite Y oxidizes quite easily. 1,4 To fully understand the interaction between the ferrocene molecules and the zeolite framework, knowledge of the location of ferrocene in the zeolite host is required. In the present study we determine the location of ferrocene in NaY zeolite by means of powder x-ray and neutron diffraction at a temperature Tϭ10 K and Tϭ1.5 K, respectively, for loadings of one and two ferrocene molecules per supercage.…”