PerspectiveHost-Guest Chemistry

Supramolecular cages trap pesky anions

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Science  12 Jul 2019:
Vol. 365, Issue 6449, pp. 124-125
DOI: 10.1126/science.aax9369

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The design of cryptands—organic molecules that can capture simple inorganic and organic ions from solution through multiple weak interactions—can be quite challenging, especially when the task is to capture a specific ion with high affinity. In many instances, targeted recognition is necessary to remove an unwanted species from some site because of its overabundance, to remove contamination, or even to capture it for its economic value (e.g., gold from seawater) (1). Negatively charged ions, especially smaller anions such as Cl, are especially troublesome. Greater energies are generally needed to peel away their more tightly held hydration shells relative to larger anions of the same charge, such as I, which has a more diffuse charge cloud. Likewise, anions possess higher free energies of hydration than cations of similar size and charge. On page 159 of this issue, Liu et al. (2) report the design of a bicyclic cryptand that specifically recognizes Cl ions with high affinity. Their results build on a long history of increasingly complex cryptand cages specifically designed to target anions (see the figure).