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.
hexacarbonylmolybdenum(0); Na56Y zeolite; intrazeolite kinetics; PMe3 substitution
