The Uptake and Assembly of Alkanes within a Porous Nanocapsule in Water: New Information about Hydrophobic Confinement.

In Nature, enzymes provide hydrophobic cavities and channels for sequestering small alkanes or long-chain alkyl groups from water. Similarly, the porous metal oxide capsule [{Mo(VI) 6 O21 (H2 O)6 }12 {(Mo(V) 2 O4 )30 (L)29 (H2 O)2 }](41-) (L=propionate ligand) features distinct domains for sequestering differently sized alkanes (as in Nature) as well as internal dimensions suitable for multi-alkane clustering. The ethyl tails of the 29 endohedrally coordinated ligands, L, form a spherical, hydrophobic "shell", while their methyl end groups generate a hydrophobic cavity with a diameter of 11 Šat the center of the capsule. As such, C7 to C3 straight-chain alkanes are tightly intercalated between the ethyl tails, giving assemblies containing 90 to 110 methyl and methylene units, whereas two or three ethane molecules reside in the central cavity of the capsule, where they are free to rotate rapidly, a phenomenon never before observed for the uptake of alkanes from water by molecular cages or containers.

Amplified Rate Acceleration by Simultaneous Up-Regulation of Multiple Active Sites in an Endo-Functionalized Porous Capsule.

Using the hydrolysis of epoxides in water as a model reaction, the effect of multiple active sites on Michaelis-Menten compliant rate accelerations in a porous capsule is demonstrated. The capsule is a water-soluble Ih-symmetry Keplerate-type complex of the form, [{Mo(VI)6O21(H2O)6}12{Mo(V)2O4(L)}30](42-), in which 12 pentagonal "ligands," {(Mo(VI))Mo(VI)5O21(H2O)6}(6-), are coordinated to 30 dimolybdenum sites, {Mo(V)2O4L}(1+) (L = an endohedrally coordinated η(2)-bound carboxylate anion), resulting in 20 Mo9O9 pores. When "up-regulated" by removal of ca. one-third of the blocking ligands, L, an equal number of dimolybdenum sites are activated, and the newly freed-up space allows for encapsulation of nearly twice as many substrate guests, leading to a larger effective molarity (amplification), and an increase in the rate acceleration (k(cat)/k(uncat)) from 16,000 to an enzyme-like value of 182,800.

A regioselective Huisgen reaction inside a Keplerate polyoxomolybdate nanoreactor.

A 1,3-dipolar cycloaddition reaction taking place quantitatively between propiolic acid "guests" and azide functions previously attached to binding sites within the cavity of a {Mo(132)}-type Keplerate reproducibly gives a 2 : 1 ratio of 1,4- and 1,5-triazoles.

Catalysis in a porous molecular capsule: activation by regulated access to sixty metal centers spanning a truncated icosahedron.

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.

Flexible pores of a metal oxide-based capsule permit entry of comparatively larger organic guests.

In zeolites and other rigid solid-state oxides, substrates whose sizes exceed the pore dimensions of the material are rigorously excluded. Now, using a porous 3 nm diameter capsule-like oxomolybdate complex [{Mo(VI)(6)O(21)(H(2)O)(6)}(12){(Mo(V)(2)O(4))(30)(OAc)(21)(H(2)O)(18)}](33-) as a water-soluble analogue of solid-state oxides (e.g., as a soluble analogue of 3 A molecular sieves), we show that carboxylates (RCO(2)(-)) can negotiate passage through flexible Mo(9)O(9) pores in the surface of the capsule and that the rates follow the general trend R = 1 degree >> 2 degrees > 3 degrees >> phenyl (no reaction). Surprisingly, the branched alkanes (R = iso-Pr and tert-Bu) enter the capsule even though they are larger than the crystallographic dimensions of the Mo(9)O(9) pores. Four independent lines of spectroscopic and kinetic evidence demonstrate that these organic guests enter the interior of the capsule through its Mo(9)O(9) apertures and that no irreversible changes in the metal oxide framework are involved. This unexpected phenomenon likely reflects the greater flexibility of molecular versus solid-state structures and represents a sharp departure from traditional models for diffusion through porous solid-state (rigid) oxides.