Zeolite activation of organometallics: revisiting substitution kinetics of [Mo(CO)6] with chemisorbed PMe3 in dehydrated Na56Y zeolite

摘要

Reactions of [Mo(CO)6] under vacuum in α-cages of Na56Y 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(PMe3)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.