Thermal decomposition of (eta6-Benzene)tricarbonylchromium(0) inside the a-cages of the Na56Y zeolite

Abstracts

We present here the first results that show the different thermal stabilities of the complex (eta6-benzene)tricarbonylchromium(0), [Cr(eta6-C6H6)(CO) 3], when anchored at different sites inside the alpha-cages of zeolite Na56Y, sites 1 and 2. When the system is heated under dynamic vacuum, the complex anchored at site 1, in which the interactions C6H6-Na+ and Na+-OC are stronger, decomposes slower than the complex anchored at site 2, in which such interactions are weaker. When the system is heated under static vacuum, the rates of decomposition of the complex anchored in both sites are comparable. The complex inside Na56Y is less stable than the same complex in the solid state.

(eta6-benzene)tricarbonylchromium(0); Na56Y zeolite; intrazeolite; thermal decomposition; FTIR spectra

Apresentamos aqui os primeiros resultados que mostram a diferença de estabilidade térmica do complexo (eta6-benzeno)tricarbonilcromo(0), [Cr(eta6-C6H6)(CO) 3], quando ancorado em sítios diferentes dentro das cavidades alfa do zeólito Na56Y, sítios 1 e 2. Quando o sistema é aquecido sob vácuo dinâmico, o complexo ancorado no sitio 1, onde as interações C6H6-Na+ e Na+-OC são mais fortes, decompõe mais rapidamente que o complexo ancorado no sitio 2, onde tais interações são mais fracas. Sob vácuo estático, a velocidade de decomposição do complexo nos dois sítios é quase a mesma. O complexo no interior de Na56Y é menos estável que o mesmo complexo no estado sólido.

COMMUNICATION

Thermal decomposition of (h⁶-Benzene)tricarbonylchromium(0) inside the a-cages of the Na₅₆Y zeolite

Ricardo Forner; Eduardo J. S. Vichi^*; Heloise O. Pastore

Instituto de Química, Universidade Estadual de Campinas, CP 6154, 13084-971 Campinas - SP, Brazil

ABSTRACT

We present here the first results that show the different thermal stabilities of the complex (h⁶-benzene)tricarbonylchromium(0), [Cr(h⁶-C₆H₆)(CO) ₃], when anchored at different sites inside the a-cages of zeolite Na₅₆Y, sites 1 and 2. When the system is heated under dynamic vacuum, the complex anchored at site 1, in which the interactions C₆H₆-Na⁺ and Na⁺-OC are stronger, decomposes slower than the complex anchored at site 2, in which such interactions are weaker. When the system is heated under static vacuum, the rates of decomposition of the complex anchored in both sites are comparable. The complex inside Na₅₆Y is less stable than the same complex in the solid state.

Keywords: (h⁶-benzene)tricarbonylchromium(0), Na₅₆Y zeolite, intrazeolite, thermal decomposition, FTIR spectra

RESUMO

Apresentamos aqui os primeiros resultados que mostram a diferença de estabilidade térmica do complexo (h⁶-benzeno)tricarbonilcromo(0), [Cr(h⁶-C₆H₆)(CO) ₃], quando ancorado em sítios diferentes dentro das cavidades a do zeólito Na₅₆Y, sítios 1 e 2. Quando o sistema é aquecido sob vácuo dinâmico, o complexo ancorado no sitio 1, onde as interações C₆H₆-Na⁺ e Na⁺-OC são mais fortes, decompõe mais rapidamente que o complexo ancorado no sitio 2, onde tais interações são mais fracas. Sob vácuo estático, a velocidade de decomposição do complexo nos dois sítios é quase a mesma. O complexo no interior de Na₅₆Y é menos estável que o mesmo complexo no estado sólido.

Introduction

One of the most studied host-guest systems involves a wide range of zeolite hosts and organometallic guests, because of their unique capacity to provide the shape selectivity of zeolites simultaneously with the reactivity of metal centers. ^1,2 In particular, the incorporation of a variety of transition metal carbonyl complexes is relevant, considering the modifications of the structure and reactivity of these molecules after occlusion in the zeolite cavities. A comprehensive study of intrazeolite group VI metal hexacarbonyls and sub-carbonyls topotaxy was published by Özkar et al.,³ followed by the reports of the first kinetic study of an intrazeolite reaction.⁴ In the case of the complex [Mo(h⁶- C₆H₆)(CO)₃], in faujasitic zeolites, Brémard et al.⁵ proposed two different sorption sites based on molecular simulations carried out at 300 K and at fixed loading. The first evidenced sorption site, 1, corresponds to the p-electron density of the C₆H₆ moiety interacting with one extraframework Na⁺ cation. Considering that the four Na⁺ cations are located in the vertices of a tetrahedron inside the a-cage, in the site 1, each of the other three Na⁺ cations are interacting with the oxygen end of each CO ligand. This leads to a decrease in the symmetry of the complex from C_3v to C_s or C₁.^3,6,7 In the second site, 2, the C₆H₆ moiety is centered in the 12-ring window of the a-cavity, and the ligands CO are far from the Na⁺ cations, and the Cr(CO)₃ fragment maintain its C_3v symmetry.

Here we report on the results of the thermal decomposition of [Cr(h⁶-C₆H₆)(CO) ₃] inside the a-cages of the zeolite Na₅₆Y.

Results and Discussion

The organometallic complex was sublimed into a zeolite wafer inside the vacuum chamber of an IR cell similar to the one described by Pastore et al.⁸ The spectrum at 393 K, shown in Figure 1a, is consistent with the theoretical proposal⁵ of anchoring in two different sites 1 and 2, with loading of ca. one molecule per a - cage.

The spectrum A in Figure 1a is typical of the newly loaded complex: no decomposition occurred during the first 40 min at 393 K. The assignment of the two differently anchored species was made considering the position of the n_CO bands corresponding to the unsymmetrical stretching, E. The bands at lower wavenumbers (1860 and 1834 cm^-1) were attributed to the site were the interaction Na⁺-OC is stronger (site 1). The band located at higher wave number (1887 cm^-1) was related to the site were the interaction Na⁺-OC is weaker (site 2).

According to this interpretation, from room temperature up to 393 K both sites are equally favored judging by the intensities of the bands at 1946, 1860, and 1834 cm^-1, assuming that the molar absorptivities are similar. Thermal treatment at temperatures higher than 393 K, in dynamic vacuum, showed that the complex at sites 1 and 2 has different stabilities. In Figure 1a, the intensity of the stretching n_CO bands, corresponding to both sites, decrease as the thermal treatment progresses. However, it is observed that after heating for 2 hours at 433 K the band at 1887 cm^-1 disappears almost completely while the other bands, at 1946, 1860 and 1834 cm^-1, attributed to stretching modes A₁ and E, remain ca.55% of the original absorbances. Since the band at 1887 cm^-1 was attributed to site 2, it means that the organometallic anchored to this site decarbonilates or detaches from the site faster than when anchored to site 1, which show, bands at 1860, and 1834 cm^-1. The profile of the band at 1946 cm^-1 shows a little change when compared to the bands belonging to mode E. The shoulder close to 1980 cm^-1 becomes more apparent. Probably it is associated with the complex in site 1. If it were associated to site 2 it would vanish after thermal treatment at 433 K. This attribution is in agreement with the reduction of the symmetry of the complex after anchoring to site 1. Since the band at 1946 cm^-1 is related to both anchoring sites, it is difficult to carry out a detailed analysis, with unequivocal assignments. The band at ca. 3078 cm^-1, attributed to the stretching n_CH of C₆H₆ in [Cr(h⁶-C₆H₆)(CO) ₃]-Na₅₆Y, is shown in Figure 1b. Like the n_CO bands the intensity of the n_CH band decrease throughout thermal treatment but, contrary to the observed for the n_CO bands, the profile of the n_CH band does not change very much. The spectra in Figure 1 suggest that the organomettalic species initially anchored in sites 1 and 2 decomposes when submitted to temperatures higher than 393 K in dynamic vacuum. The decomposition is complete after eight hours, at 433 K. In these conditions both free CO and C₆H₆ are removed completely from the zeolite. The presence of chromium metal in the wafer was detected by X-ray fluorescence, showing that the thermal decomposition of the complex occur inside the a-cages.

One of the most interesting features of the thermal treatment of the [Cr(h⁶-C₆H₆)(CO) ₃]-Na₅₆Y system is that, contrary to the observed in the zeolite NaX,⁶ the anchored Cr(CO)₃ moiety is not formed. When the [Cr(h⁶-C₆H₆)(CO) ₃]-NaX system is submitted to temperatures around 423 K, the complex loose the C₆H₆, and the resulting Cr(CO)₃ moiety remains anchored in the oxygen atoms of the zeolite network.⁶ This sub-carbonyl is also formed from the thermal treatment of complex [Cr(CO)₆] in both NaY and NaX. In the zeolite NaY, the anchored Cr(CO)₃ is the product of the thermolysis of [Cr(CO)₆] in the temperature interval of 373 – 423 K³ while in zeolite NaX, the anchored sub-carbonyl forms at ca. 303 K.⁶ An explanation for the different temperatures of decarbonylation of [Cr(CO)₆] in zeolites NaX and NaY is that the sub-carbonyls are stabilized by the Lewis basicity of the framework oxygen atoms to which they are bonded. Since zeolite NaX is more basic⁶ the sub-carbonyls are formed at lower temperatures when compared to the zeolite NaY. Further decarbonylation of Cr(CO)₃ starts at 423 K in the zeolite Y while in the zeolite NaX it starts at 500 K. Therefore the anchored Cr(CO)₃ sub-carbonyl is thermally more stable in NaX than in NaY.

We speculate that in the system [Cr(h⁶-C₆H₆)(CO) ₃]-Na₅₆Y the formation of Cr(CO)₃ might occur at temperatures above 413 K. However, according to Özkar et al.³ decarbonilation of Cr(CO)₃ obtained from [Cr(CO)₆] occurs at around 420 K. Presumably, the first step of the decomposition of [Cr(h⁶-C₆H₆)(CO) ₃] occluded in NaY is dissociation of C₆H₆, followed by immediate decarbonilation of the resulting sub-carbonyl.

Experiments in static vacuum show that the species (h⁶-C₆H₆ )Cr-Na₅₆Y is not formed during the decomposition. The spectrum B in Figure 2b obtained after the thermal treatment, compared to the spectra A and C, indicates the presence of free benzene in the zeolite wafer.

Under static vacuum, contrary to the observed under dynamic vacuum, the complexes in both sites (schematic 1 and 2) decompose at comparable rates as shown by the FTIR spectra in Figure 2a. In static vacuum the system is closed and free benzene is observed inside the cavities as shown by the FTIR spectra in Figure 2b. Huang and Poissant⁷ mixed mechanically the zeolite NaY with crystalline [Cr(h⁶-C₆H₆)(CO) ₃] in a closed vessel, and obtained a different metal carbonyl species, [Cr(CO)₆]. In our system, the presence of free benzene, and some CO, inside the cavities, retard the rate of decomposition in site 2.

Acknowledgements

We thank the Conselho Nacional de Pesquisa e Desenvolvimento, CNPq, Project Pronex, and the Fundação de Amparo à Pesquisa do Estado de São Paulo, Fapesp, for financial support.

References

1. For important reviews, see: Gates, B.C.; Guczi, L.; Knozinger, H; Metal Clusters in Catalysis, Elsevier: Amsterdam, 1986; Ozin, G.A.; Gil, C.; Chem. Rev. 1989, 89, 1749; Ozin, G.A.; Kuperman, A.; Stein, A.; Angew. Chem. Int. Ed. 1989, 28, 359; Stucky, G.D.; Macdougall, J.E.; Science 1990, 247, 433; Zecchina, A.; Otero-Arean, C.; Catal. Rev.-Sci. Eng. 1993, 35, 261; Bein T. In Comprehensive Supramolecular Chemistry; Lehn, J.-M.; Atwood, J.L.; Davies, J.E.D.; MacNicol, D.D.; Vögtle, F., eds.; Pergamon: Oxford, 1996, vol. 7, ch. 20; Psaro, R.; Recchia, S.; Catal. Today 1998, 41, 139; Brémard, C.; Coord. Chem. Rev. 1998, 180, 1647.

2. Shen G.C., Liu, A.M.; Ichikawa, M.J.; J. Chem. Soc., Faraday Trans. 1998, 94, 1353; Muller, B.R.; Calzaferri, G.; Microporous Mesoporous Mater. 1998, 21, 59.

3. Özkar, S.; Ozin, G.A.; Moller, K. Bein, T.; J. Am. Chem. Soc. 1990, 112, 9575.

4. The reaction studied was the substitution of CO by PMe₃, in the well defined system Mo(CO)₆-M₅₆Y (M = Li, Na, K, Rb and Cs). See Ozin, G.H.; Özkar, S.; Pastore, H.O.; Pöe, A. J.; Vichi, E. J. S.; ACS Symp. Series 1992, 499, 314.

5. Brémard, C.; Ginestet, G.; Le Maire, M.; J. Am. Chem. Soc. 1996, 118, 12724.

6. Shirley, W.; Scoville, S.P.; Microporous Mesoporous Mater. 2000, 37, 271.

7. Huang, Y.; Poissant, R. R.; Langmuir 2002, 18, 5487.

8. Pastore, H.O.; Ozin, G.A.; Pöe, A.J.; J. Am. Chem. Soc. 1993, 115, 1215.

Received: June 17, 2003

Published on the web: November 6, 2003

FAPESP helped in meeting the publication costs of this article.

Publication Dates

• Publication in this collection
  25 May 2004
• Date of issue
  Feb 2004

History

• Accepted
  06 Nov 2003
• Received
  17 June 2003