Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)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(12CO)6 with PMe3 in the α-cages of a Na56Y zeolite host proceed cleanly to form cis-Mo(CO)4(PMe3)2 by what are essentially pseudo-first-order dissociative and associative processes. Reactions with 13CO proceed only by the dissociative path, which is 103 times faster than corresponding reactions in homogeneous solution. Dissociative substitution by PMe3 is retarded by applied pressures of 12CO, as expected, but it is also retarded to a significant extent by increased Mo(CO)6 and PMe3 loadings. Substitution by 13CO is similarly retarded by increasing pressures of the entering 13CO. The activation parameters, ΔHd‡ = 61 ± 5 kJ mol-1 and ΔSd‡ = -139 ± 15 J K-1 mol-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 α-cages of Na56Y provide precisely defined activating environments of a 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(12CO)6 and PMe3, 13CO, or 12CO, housed within the α-cage of Na56Y, anchored to extraframework Na+ cations, and subject to loading-dependent cooperative interactions. These interactions appear to control the extent of activation of Mo(12CO)6 through the degree of ordering (lock and key) of the {Mo(12CO)5⋯12CO)‡ dissociative transition state. All involve α-cage Na+ anchoring interactions, probably together with some influence of oxygens of the six-rings. Substitution by PMe3 can also occur by two associative paths, one that involves PMe3 molecules chemisorbed to NaII + ions in the nanoreactors, and another that we believe involves PMe3 molecules physisorbed in the region of the 12-ring windows of the α-cages. Strikingly, these associative reactions are somewhat slower than corresponding reactions in homogeneous solution, and they are much less subject to loading effects than the dissociative reactions.115412151230Sponsor: FAPESP; São Paulo Research FoundationFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)