Intrazeolite metal carbonyl kinetics: substitution reactions of molybdenum carbonyl (Mo(12CO)6) in sodium zeolite Y

Abstract

The first quantitative and detailed kinetic study of simple thermal reactions of a molecule encapsulated in a zeolite cavity is reported. Substitution reactions of Mo([sup 12]CO)[sub 6] with PMe[sub 3] in the [alpha]-cages of a Na[sub 56] Y zeolite host proceed cleanly to form cis-Mo(CO)[sub d](PMe[sub 3])[sup 2] by what are essentially pseudo-first-order dissociative and associative processes. Reactions with [sup 13]CO proceed only by the dissociative path, which is 10[sup 3] times faster than corresponding reactions in homogeneous solution. Dissociative substitution by PMe[sub 3] is retarded by applied pressures of [sup 12]CO, as expected, but it is also retarded to a significant extent by increased Mo(CO)[sub 6] and PMe[sub 3] loadings. Substitution by [sup 13]CO is similarly retarded by increasing pressures of the entering [sup 13]CO. The activation parameters, [Delta]H[sub d] = 61 [plus minus] 5 kJ mol[sup [minus]1] and [Delta]S[sub d] = -139 [plus minus] 15 J K[sup [minus]1] mol[sup [minus]1]/, are unusual for a dissociative process but can be interpreted on the basis of structural information concerning pertinent reactant and product guests obtained mainly from FT-mid-IR, EXAFS, and DOR-MAS-NMR analytical methods. The kinetics data reveal that the [alpha]-cages of Na[sub 56]Y provide precisely defined activating environments of amore » unique kind. From a coordination chemistry point of view, these nanoreactors appear to behave as macrospheroidal multidentate multisite anionic ligands (which we call zeolates) toward extraframework charge-balancing cations to which metal carbonyl reactant, product, and ligand guests can become attached. The best model that emerges from this study pictures a supramolecular assembly of Mo([sup 12]CO)[sub 6] and PMe[sub 3], [sup 13]CO, or [sup 12]CO, housed within the [alpha]-cage of Na[sub 56]Y, anchored to extraframework Na[sup +] cations, and subject to loading-dependent cooperative interactions. 38 refs., 13 figs., 8 tabs.« less