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Synthesis

Molecular knot ties up anions to catalyze reactions

Chemists find a practical purpose for an aesthetically-appealing molecule

by Bethany Halford
June 27, 2016 | APPEARED IN VOLUME 94, ISSUE 26

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Credit: David Leigh
The Leigh group’s pentafoil molecular knot; purple = Zn, blue = N, gray = C.
Credit: David Leigh
The Leigh group’s pentafoil molecular knot; purple = Zn, blue = N, gray = C.

With their tantalizingly tangled structures, molecular knots have undeniable aesthetic appeal. But they haven’t found any particular practical use—until now. Chemists at the University of Manchester led by David A. Leigh have discovered a molecular knot that catalyzes reactions via anion abstraction (Science 2016, DOI: 10.1126/science.aaf3673). Last year, Leigh’s group reported a pentafoil molecular knot with a central cavity so good at sequestering chloride it plucked the anion out of glassware (J. Am. Chem. Soc. 2015, DOI: 10.1021/jacs.5b06340). “That made us think that maybe we could use this very strong, selective anion binding for something useful,” Leigh says. The insight led his group to discover it can use the same knot to pull halide ions off of certain substituents to form carbocations. The knot structure is crucial to the reaction, he says, because it creates the perfect anion-binding pocket. Leigh notes that silver salts do this same type of halide abstraction, but not with any selectivity. The molecular knot offers far better control. “Will people use these?” Leigh wonders. “I don’t know. It depends if we can find some reactions that can’t be done in other kinds of ways.”

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