Entropically Driven Self-Assembly of Alcohols in Water via Nano-Encapsulation

Porous icosahedral-symmetry molybdenum-oxide nanocapsules, {Mo₁₃₂}, are comprised of twelve pentagonal [(Mo^VI)Mo^VI₅] units and 30 {Mo^V₂O₄L}¹⁺ linkers, where L is an endohedrally coordinated η²-bound carboxylate ligand. These building blocks form 20 Mo₉O₉ openings that allow exchange of ligands, L, with carboxylate anions in bulk water outside the capsule. Using L = formate, the capsule, [[{Mo^VI₆O₂₁(H₂O)₆}₁₂{(Mo^V₂O₄)₃₀(H₂O)₁₆}]^28-], was used for the uptake of a polyol guest, L-glycerate, CH₂(OH)CH(OH)CO₂^–, whose population inside the capsule was quantified by ¹H-NMR spectroscopy. As the pH decreased from pH 5.4 to 2.6, the population of L-glycerate ligands inside the capsule increased dramatically. Van’t Hoff’s plots obtained by variable-temperature ¹H-NMR revealed that at all pH values, uptake was entropically favorable and enthalpically unfavorable. At lower pH values, however, the unfavorable enthalpy is attenuated by the dissolution of H⁺ in the alcoholic environment inside the capsule, such that the favorable entropy becomes dominant. Proton uptake was indirectly supported by single-crystal X-ray structural analysis, which revealed water-molecule bridges between L-glycerate –OH groups inside the capsule. The findings show that under the right conditions, the assembly of polyols in water is entropically driven, with an energetic signature identical to that normally associated with the uptake of hydrocarbon hydrophobes.

Figure 1. L-glycerate uptake via ligand exchange inside a {Mo₁₃₂} capsule.

References:

1. Sourav Chakraborty^†, Alina Grego^†, Somenath Garai and Ira A. Weinstock*, Manuscript under preparation.

2. Alina Grego, Achim Müller, and Ira A. Weinstock, Angew. Chem. Int. Ed. 2013, 52, 8358 –8362.