ORGANIC CHEMISTRY IN A POROUS METAL-OXIDE NANOCAPSULE

Over the past several years, we have used porous icosahedral-symmetry metal-oxide capsules, [{Mo^VI₆O₂₁(H₂O)₆}₁₂{Mo^V₂O₄(L)}₃₀]^42– {Mo₁₃₂} (L = an endohedrally coordinated η²-bound carboxylate anion), to investigate organic reactions within nanoconfined domains in water. Topics investigated include: 1) diffusion through flexible pores, 2) catalysis, and 3) self-assembly.

Flexible pores. Using the capsule as a soluble analogue of porous solid-state (rigid) oxides, we showed that branched-alkane carboxylate “guests” could enter the capsule’s interior by negotiating passage through flexible subnanometer Mo₉O₉ apertures whose geometrical dimensions were smaller than the entering species themselves.

Catalysis. Nano-containers in which structurally integral metal centers serve as catalytic sites for encapsulated substrates are relatively rare, and the {Mo₁₃₂} capsules provide a unique opportunity for exploring this class of reactions. These have included the cleavage of methyl tert-butyl ether under mild conditions in water, and for the Michaelis-Menten compliant hydrolysis of epoxides, an enzyme-like rate acceleration (k_cat / k_uncat) of 182,800, the largest yet reported for a cage or container at room temperature in water.

Self-assembly. Here, a {Mo₁₃₂} capsule was used to reveal the energetics of individual growth steps in the formation of a “micelle”-like organic aggregate of n-butyrate ions. In other work, two distinct host domains within the propionate-ligand form of the capsule are preferentially populated as a function of alkane size, with n-butane guests between the propionate ligands, and ethane molecules rotating rapidly in the 11-Å diameter (700 ų) hydrophobic cavity at the capsule’s center. Finally, H-bonding between alcohols alone does not give rise to self-assembly in bulk water. This is not the case, however, for H-bonded self-assembly within the nanoconfined domain inside the capsule, where a new type of self-assembly has recently been identified.