Understanding of aqueous solutions
is of significance to a wide
range of fields. Crown macrocycles emerge as a new generation of model
hosts for studying the chemistry of water and aqueous supramolecular
chemistry. Herein, we present the oxacrown ether cyanobenzo-21-crown-7
ether (C7CN), and its selenium-containing counterpart
selencrown ether C7SeCN possess only one chalcogen atom
diverse in structure but exhibits dramatic difference in shape, solvation
(e.g., hydration), and consequent thermodynamics of guest binding
experimentally. The hydrogen bond strength of Se···H
is similar to that of O···H, but theoretical calculations
pointed out that there is a prominent electrostatic potential change
for the entire molecule caused by Se substitution, which leads to
the decrease of the interactions between water clusters and crown
macrocycles thermodynamically and kinetically. Results established
that C7CN and C7SeCN provide an easy-to-access
model pair to exclusively probe water–solute interaction and
host pocket wettability change caused by one atom substitution.
Organic selenides are famous for their coordination and catalytic functions in the organic phase, albeit challenging for aqueous medium. Herein, the combination of a hydrophilic body of crown ether and substitution of one oxygen atom with a selenium one provides a new type of design route for organic selenide entities with charming functions in aqueous solution. The selenacrown ether C9Se presented here intrinsically shows an amphiphile-like property. Its nanosphere structure in water readily expands the catalysis of organic selenide to aqueous substrates in thiol/disulfide conversion.
Supramolecular assemblies based on oligo(ethylene glycol) (OEG) building blocks are well-known for their neutral chemical property and thermal-responsive behavior. Here, the cyclic "CLOSED" and linear "OPEN" typologies of OEGs led to dramatic difference in the sensitivity to guanidinium-containing species. From thermodynamic studies, the association constant (K a ) between the "CLOSED" form amphiphile and guanidinium salt was determined to be 28.7 M −1 , whereas there was no detectable binding affinity for the "OPEN" form. Therefore, considering ion specificity, the present results establish that crown ether derivatives with "CLOSED" and "OPEN" topologies provide an easy-to-access model pair with designed ion-recognition sites and special functional moieties and geometries (like the binding pockets of enzymes or ion channels in cellular members) that allow the manipulation of the intercrossed relationship between supramolecular solutes, waters, and guanidinium salts. These supramolecular forces in aqueous solution offered an alternative strategy to fabricate thermal-responsive systems in ionic medium.
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