Catalysis in a Porous Molecular Capsule: Activation by Regulated Access to Sixty Metal Centers Spanning a Truncated Icosahedron

Abstract

The 30 cationic {Mo(V)2O4(acetate)}(+) units linking 12 negatively charged pentagonal "ligands," {(Mo(VI))Mo(VI)5O21(H2O)6}(6-) of the porous metal-oxide capsule, [{Mo(VI)6O21(H2O)6}12{Mo(V)2O4(acetate)}30](42-) provide active sites for catalytic transformations of organic "guests". This is demonstrated using a well-behaved model reaction, the fully reversible cleavage and formation of methyl tert-butyl ether (MTBE) under mild conditions in water. Five independent lines of evidence demonstrate that reactions of the MTBE guests occur in the ca. 6 × 10(3) Å(3) interior of the spherical capsule. The Mo atoms of the {Mo(V)2O4(acetate)}(+) linkers--spanning an ca. 3-nm truncated icosahedron--are sterically accessible to substrate, and controlled removal of their internally bound acetate ligands generates catalytically active {Mo(V)2O4(H2O)2}(2+) units with labile water ligands, and Lewis- and Brønsted-acid properties. The activity of these units is demonstrating by kinetic data that reveal a first-order dependence of MTBE cleavage rates on the number of acetate-free {Mo(V)2O4(H2O)2}(2+) linkers. DFT calculations point to a pathway involving both Mo(V) centers, and the intermediacy of isobutene in both forward and reverse reactions. A plausible catalytic cycle--satisfying microscopic reversibility--is supported by activation parameters for MTBE cleavage, deuterium and oxygen-18 labeling studies, and by reactions of deliberately added isobutene and of a water-soluble isobutene analog. More generally, pore-restricted encapsulation, ligand-regulated access to multiple structurally integral metal-centers, and options for modifying the microenvironment within this new type of nanoreactor, suggest numerous additional transformations of organic substrates by this and related molybdenum-oxide based capsules.

Tunable Keplerate Type‐Cluster “Mo132” Cavity with Dicarboxylate Anions

“…showed that regioselective Huisgen reaction is able to take place inside the Keplerate anions,25 while Weinstock et al. reported that the porous capsule provides internal active sites for catalytic transformations 26…”

Encapsulated Water Inside Mo132 Capsules: The Role of Long-Range Correlations of about 1 nm

“…This particular finding, related to commensurability between the tridimensional water network and the container, is the major outcome of this study. Such a dual behavior of confined water may have potential applications related to selectivity of ions 47 and specific catalysis, 48 51. In both capsules 100 water molecules were trapped inside the cavity together with 72 coordinated water molecules covering the inner side.…”

A Unique Fluoride Nanocontainer: Porous Molecular Capsules Can Accommodate an Unusually High Number of “Rather Labile” Fluoride Anions

“…26 , respectively ( Figure S4), that most probably include the 15 remaining fluoride ions (see formula of 3 above) besides water molecules.Though the distinction between fluorine and oxygen is problematical in the present case,itshould be noted that af luoride ion can play the topological role of aw ater molecule as shown for example by the comparison of cyclic [F(H 2 O)] 4 4À [15] and (H 2 O) 8 [23] assemblies.T he largest shell is attracted towards the spherical ligand wall as shown by the corresponding hydrogen-bonding distances;f or details,s ee Figure S4. 26 , respectively ( Figure S4), that most probably include the 15 remaining fluoride ions (see formula of 3 above) besides water molecules.Though the distinction between fluorine and oxygen is problematical in the present case,itshould be noted that af luoride ion can play the topological role of aw ater molecule as shown for example by the comparison of cyclic [F(H 2 O)] 4 4À [15] and (H 2 O) 8 [23] assemblies.T he largest shell is attracted towards the spherical ligand wall as shown by the corresponding hydrogen-bonding distances;f or details,s ee Figure S4.…”

“…[26] We are currently exploring the reactivity of the fluoride balls 3a and 4a to further assess the potential of Keplerates as nanoreactors.Apreliminary result refers to hydrolysis of methyl acetate;the related complete results will be reported as soon as possible. [26] We are currently exploring the reactivity of the fluoride balls 3a and 4a to further assess the potential of Keplerates as nanoreactors.Apreliminary result refers to hydrolysis of methyl acetate;the related complete results will be reported as soon as possible.…”