Densely Packed Hydrophobic Clustering: Encapsulated Valerates Form a High‐Temperature‐Stable {Mo₁₃₂} Capsule System

Abstract

Porous molecular nanocontainers of {Mo132 }-type Keplerates offer unique opportunities to study a wide variety of relevant phenomena. An impressive example is provided by the highly reactive {Mo132 -CO3 } capsule, the reaction of which with valeric acid results in the very easy release of carbon dioxide and the uptake of 24 valerate ions/ligands that are integrated as a densely packed aggregate, thus indicating the unique possibility of hydrophobic clustering inside the cavity. Two-dimensional NMR techniques were used to demonstrate the presence of the 24 valerates and the stability of the capsule up to ca. 100 °C. Increasing the number of hydrophobic parts enhances the stability of the whole system. This situation also occurs in biological systems, such as globular proteins or protein pockets.

Nanoconfinement Inside Molecular Metal Oxide Clusters: Dynamics and Modified Encapsulation Behavior

“…The slowing down of the rotation for the confined ligand is probably due to the strong interaction between the neighboring ligands. In fact, as shown in Figure b, the crowded distribution of the ligands inside the cluster and the curvature of the sphere lead to a significant interaction between the terminal methyl groups …”

“…These modifications provide adaptability of the clusters to broad types of encapsulations and reactions . When the size of the internal ligand becomes large, saturated occupation of the internal surface can be achieved through almost direct contact with their neighbors . These ligands form a network that defines the pathway for the entrance of a guest molecule .…”

“…When the size of the internal ligand becomes large, saturated occupation of the internal surface can be achieved through almost direct contact with their neighbors . These ligands form a network that defines the pathway for the entrance of a guest molecule . The ligands could play decisive roles in regulating the entering/exiting of guest molecules besides the gating effect of the interfacial pores defined by the metal oxide framework .…”

“…The internal surface can be functionalized maximally with 30 ligands via the coordination between ligands and the {Mo V 2 } units (red polyhedron in Figure ) . The clusters with butyric acid (formula: [Mo VI 72 Mo V 60 O 372 (C 3 H 7 COO) 24 (H 2 O) 84 ] 36− ; abbreviation, {C 4 Mo 132 }) and valeric acid (formula: [Mo VI 72 Mo V 60 O 372 (C 4 H 9 COO) 24 (H 2 O) 84 ] 36− ; abbreviation, {C 5 Mo 132 }) as internal ligands, respectively, are selected as models based on the following considerations: 1) The ligands are bulky and in close contact with their neighbors, as determined from structural analysis and 2) the internal area has a void space for possible encapsulation of guest molecules . There are a total of 20 hexagonal pores with a diameter of ∼0.8 nm on the cluster surface, which are partially covered by the internal ligands from the inside view.…”

“…The {C 5 Mo 132 } sample was also studied under the same experimental conditions in order to explore the possible effect of chain length to the dynamics of nanoconfined ligands (see Figures S1 and S2 in the Supporting Information). Given the fact that the confined space is defined by the rigid inorganic shell and these two clusters have an identical number of ligands inside them, the degree of close packing of the ligands is only determined by the sizes of their alkyl chain fragments . Therefore, it is expected that the ligands (valeric acid) with longer backbones in {C 5 Mo 132 } are more confined than that of {C 4 Mo 132 } and thus their dynamics should be even slower.…”