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|Type:||Artigo de periódico|
|Title:||Zeolite Activation Of Organometallics: Revisiting Substitution Kinetics Of [mo(co)6] With Chemisorbed Pme3 In Dehydrated Na 56y Zeolite|
|Abstract:||Reactions of [Mo(CO)6] under vacuum in α-cages of Na 56Y zeolite fully loaded with chemisorbed PMe3 form cis-[Mo(CO)4(PMe3)2] but mixtures of [Mo(CO)5(PMe3)] and cis-[Mo(CO)4(PMe 3)2] are formed under CO. Reactions under vacuum exhibit low enthalpic and very negative entropic factors (ΔH‡ = 71.4 ± 3.5 kJ mol-1 and ΔS‡ = -102 ± 11 J K-1 mol-1) compared with much higher enthalpic and positive entropic factors for CO dissociative reactions with P(n-Bu)3 in xylene observed elsewhere. Reaction at 66 °C under vacuum is ca.103 times faster than "spontaneous" CO dissociative reactions in solution. Intrazeolite substitution is concluded to occur by a "zeolite assisted" mechanism in which two oxide ions in the cavity walls simultaneously displace two neighbouring CO ligands from the [Mo(CO)6]. This contrasts with even lower enthalpy, and more negative entropy values, for simultaneous displacement of three neighbouring CO ligands in thermal decarbonylation reactions. The α-cages behave as multidentate anionic "zeolate" ligands with varying numbers of O2- ions participating to create highly ordered transition states. These results emphasize the high degree to which such kinetic studies can reveal intimate details of the nature of these activating effects. ©2008 Sociedade Brasileira de Química.|
|Appears in Collections:||Unicamp - Artigos e Outros Documentos|
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