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A new approach to anion recognition that uses electrostatic and hydrogen-bonding interactions has been developed by chemists at the University of Missouri, Columbia. GOTCHA Tetramethylammonium cation (yellow) traps chloride anion (green) inside molecular capsule. COURTESY OF UNIVERSITY OF MISSOURI, COLUMBIA "For the first time, we have utilized a single molecule to completely encapsulate an ion pair in polar media," chemistry professor Jerry L. Atwood tells C&EN. "We envision that resins incorporating these capsules could be used in anion sensing in environmental applications." Atwood and postdoc Agnieszka Szumna embedded a tetramethylammonium cation in the pocket of a resorcin[4]arene molecule functionalized with bulky amide substituents. The complex selectively binds to a chloride anion in solvents such as methanol [Chem. Comm., 2003, 940]. "The cavity into which the cation is bound may be entered in only one way," Atwood continues. "The usually nondirectional electrostatic attraction is thus directionalized. This means that the entrance to the cavity functions as a selectivity gate for the anion. Further, the entrance is easy to functionalize, so one may easily fine-tune the anion selectivity." The molecular host capsules are robust, Atwood and Szumna note. The cation binds to the interior of the capsule by C–H ··· interactions with eight surrounding aromatic rings. Though not covalently bound, it remains "remarkably stable" over a variety of conditions. The guest chloride anion is bound by hydrogen bonding to the hydrogen atoms of the host molecule's amide groups. "The tetramethylammonium ion is the 'honey in the trap' for the chloride anion, which in turn seals the cation into the receptor cavity," observes Philip A. Gale, senior lecturer in chemistry at Southampton University, in England. "The synergy between cation and anion stabilizes the complex, allowing chloride complexation even in polar hydrogen-bond-donating solvents such as methanol. This is a great idea and an elegant design." |
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