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|Type:||Artigo de periódico|
|Title:||Zeolite Activation Of Organometallics: Anchoring And Decarbonylation Kinetics Of Mo(co) 6 In Dehydrated Na 56y Zeolite|
|Abstract:||Quantitative temperature-dependent kinetic studies are reported of thermal reactions of Mo(CO) 6 that has been newly encapsulated in the α-cages of a Na 56Y zeolite host. Under static vacuum, Mo(CO) 6 undergoes parallel anchoring and decarbonylation reactions, the latter yielding (OC) 3Mo-Na 56Y directly and the former resulting in the development of a characteristic six-band spectrum in the C-O stretching region. Kinetic and spectroscopic evidence was obtained for there being two different forms of (OC) 3Mo-Na 56Y. The activation parameters are unusual, with very low enthalpies and extremely negative entropies, viz. δH‡ = 40 ± 3 kJ mol -1 and δS‡ = -185 ± 9 J K -1 mol -1 for anchoring and decarbonylation. This contrasts with the typically high enthalpic and positive entropie factors reported for corresponding CO dissociative reactions in xylene (δH‡ = 133 ± 6 kJ mol -1 and δS‡ = +28 ± 16 J K -1 mol -1) or even the quite unusual values δH‡ = 61 ± 5 kJ mol -1 and δS‡ = -139 ± 13 J K -1 mol -1 reported previously for 13CO exchange in Mo(CO) 6-Na 56Y. It is concluded that the inner surface of Na 56Y provides different activating environments for different reactions, behaving as multidentate anionic "zeolate" nucleophiles, with varying numbers of O 2- ions participating in highly ordered transition states. A common intermediate, with three O 2- ions participating in the anchoring and decarbonylation process, is proposed, one with two O 2- ions being proposed for the 13CO exchange reaction. The importance of temperature-dependent time-resolved infrared spectroscopy in elucidating such intimate mechanistic details is emphasized.|
|Appears in Collections:||Unicamp - Artigos e Outros Documentos|
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