Scientists synthesize molecular "cage" to trap chloride

Researchers at Indiana University have created a powerful new molecule to extract salt from a liquid. This work could help increase the amount of drinking water on Earth.

Constructed with the help of chemical bonds considered up to then too weak, the new molecule is about ten billion times more efficient compared to a similar structure created more than 10 years ago at IU. The design of the molecules was reported on May 23 in the newspaper Science.

"If you drop a millionth of a gram of this molecule into a ton of water, 100% of them will still be able to capture a salt," said Yun Liu, who led the study. as a doctorate. student in the laboratory of Amar Flood, chemistry professor James F. Jackson and professor Luther Dana Waterman at the Chemistry Department of the IU Bloomington College of Arts and Sciences.

The molecule is designed to capture the chloride, which is formed when the chlorine element couples with another element to gain an electron. The most known chloride salt is sodium chloride, or common table salt. The other chloride salts are potassium chloride, calcium chloride and ammonium chloride.

As the human population continues to grow, the infiltration of salt into freshwater systems reduces access to clean water around the world. In the United States alone, the US Geological Survey estimates 271 tonnes of dissolved solids, including salts, entering freshwater streams each year. Contributing factors include the chemical processes involved in oil extraction, the use of road salts and water softeners, as well as the natural aging of rocks. Just a teaspoon of salt will permanently pollute five gallons of water.

The new salt extraction molecule created at IU is made up of six "motifs" of triazole – five-membered rings composed of nitrogen, carbon and hydrogen – which together form a "cage" "three-dimensional perfectly formed to trap chloride. In 2008, the Flood laboratory created a two-dimensional, donut-shaped molecule containing four triazoles. The two additional triazoles confer on the new molecule its three-dimensional shape and an efficiency multiplied by ten.

The molecule is also unique in that it binds chloride using carbon-hydrogen bonds, previously thought to be too weak to create stable interactions with chloride compared to the traditional use of nitrogen-hydrogen bonds. Despite expectations, the researchers found that the use of triazoles created a cage so rigid that it formed a vacuum in the center that sucked in chloride ions.

On the other hand, cages with nitrogen-hydrogen bonds are often more flexible and the similar vacuum-like center required for chloride capture requires energy input, which reduces their efficiency compared to a triazole cage. .

"If you had to take our molecule and superimpose it on other cages that use [stronger] Flood. This study really shows that rigidity is underestimated in the design of molecular cages. "

Rigidity also allows the molecule to retain its shape after the loss of central chloride, compared to other models that collapse under the same circumstances because of their flexibility. This provides the molecule with increased efficiency and versatility.

The work is also reproducible. The synthesis of the first molecule took almost a year, said Liu, who was shocked to discover the crystals needed to confirm the unique structure of the molecule formed after the experiment was left alone in the laboratory for several years. month. monitoring.

The crystal formation represented a "eureka" moment, proving that the unique design of the molecule was truly viable. Later, Wei Zhao, a postdoctoral researcher in Flood's lab, was able to recreate the molecule in several months.