Solvent and Pressure Effects on the Motions of Encapsulated Guests: Tuning the Flexibility of a Supramolecular Host

Abstract

The supramolecular host assembly [Ga 4 L 6 ] 12-(1; L = 1,5-bis[2,3-dihydroxybenzamido]naphthalene) contains a flexible, hydrophobic interior cavity that can encapsulate cationic guest molecules and catalyze a variety of chemical transformations. The Ph-CH 2 bond rotational barrier for encapsulated ortho-substituted benzyl phosphonium guest molecules is sensitive to the size and shape of the host interior space. Here we examine how changes in bulk solvent (water, methanol, or DMF) or applied pressure (up to 150 MPa) affect the rotational dynamics of encapsulated benzyl phosphonium guests, as a way to probe changes in host cavity size or flexibility. When host 1 is dissolved in organic solvents with large solvent internal pressures (∂U/∂V) T , we find that the free energy barrier to Ph-CH 2 bond rotation increases by 1 -2 kcal/mol, compared with aqueous solution. Likewise, when external pressure is applied to the host-guest complex in solution, the bond rotational rates for the encapsulated guests decrease. The magnitude of these rate changes and the volumes of activation obtained using either solvent internal pressure or applied external pressure, are very similar. NOE distance measurements reveal shorter average host-guest distances (~0.3 Å) in organic versus aqueous solution. These experiments demonstrate that increasing solvent internal pressure or applied external pressure reduces the host cavity size or flexibility, resulting in more restricted motions for encapsulated guest molecules. Changing bulk solvent or external pressure might therefore be used to tune the physical properties or reactivity of guest molecules encapsulated in a flexible supramolecular host.

Untangling the Diverse Interior and Multiple Exterior Guest Interactions of a Supramolecular Host by the Simultaneous Analysis of Complementary Observables

“…11−13 The metal−ligand framework of the host generates a large and hydrophobic interior cavity capable of encapsulating guest molecules of appropriate size 14 and a highly anionic exterior surface which promotes external association of cationic molecules, and renders the host water-soluble. 15 −17 The interior microenvironment of host 1, which differs dramatically from the bulk solvent environment, has been successfully used to modify the physical properties and reactivity of encapsulated guest molecules 18,19 and to catalyze chemical transformations 20−22 with rate accelerations of up 2 × 10 7 .…”

Iterative design of a helically folded aromatic oligoamide sequence for the selective encapsulation of fructose

“…2a). This type of molecular container has a smaller cavity than capsules formed by metal-or hydrogen-bondmediated self-assembly of organic building blocks but has the advantage of having no required symmetry [33][34][35] , a desirable property for selectively binding to complex guests. Similar to other folded aromatic oligoamides, local conformational preferences at aryl-amide linkages and intramolecular π−π interactions between aromatic monomers combine to endow the folded conformation with high stability and favourable crystallogenesis 12,13 .…”

“…The + or − signs indicate whether a P or M helicity is induced by the guest, respectively. Absolute assignment of helix handedness has been determined previously 35 Capsule backbones are represented in grey. Guests are shown in a thick tube representation.…”

A Triphasic Sorting System: Coordination Cages in Ionic Liquids

“…As seen in Figure 1, the affinity of each cage (2)(3)(4)f or its designated layer was visually conspicuous. This change resulted in amarked increase in the solubility of 4 in [emim][NTf 2 ], and cage 4 was therefore used in the sorting system of Scheme 3.…”