Chain-link molecules form squishy crystals

An elastic crystal built from interlocking molecular rings could pave the way for squeezable materials that can capture and release gases such as CO₂ (Nature 2021, DOI: 10.1038/s41586-021-03880-x).

The crystal is a type of metal-organic framework (MOF), a family of porous materials built from a scaffold of organic molecules connected by metal-based nodes. Unusually, the organic struts of the elastic MOF are made of catenanes—pairs of interlocked molecular rings. These linked loops can slide toward each other when the crystal is compressed, making it remarkably squishy.

Hiroshi Sato at the Riken Center for Emergent Matter Science and colleagues built the MOF from catenanes containing two identical cyclic molecules that carry benzylamide and carboxylic acid groups. Mixed with cobalt nitrate, they assembled into thin, rectangular, dark green crystals. The catenanes’ carboxylic acid groups coordinate to cobalt ions, while hydrogen bonds between the benzylamide groups initially help to give the MOF some rigidity.

But soaking the crystals in a solvent like dimethylformamide (DMF) disrupts those hydrogen bonds and allows the molecular rings to move more freely. This makes the crystal much easier to compress, giving it a low stiffness comparable to polypropylene.

“It’s really impressive,” says Stephen J. Loeb at the University of Windsor, who develops MOFs containing other kinds of mechanically interlocked molecules. “I wish I’d done it.”

Loeb points out that other MOFs can flex by reconfiguring double bonds, for example. “But this is completely different from other flexible crystals,” he says, because the catenanes readily spring back into position once pressure is removed. To demonstrate this, Sato’s team showed that pushing the tip of a diamond into the MOF’s surface left no indentation, unlike with typical crystalline materials.

MOFs are riddled with pores that can accommodate small molecules, and they are being used for gas storage and separation, as drug-delivery vehicles, and even to harvest water from the air. For example, MOFs could be used to capture CO₂ from the flues of power stations and other industrial plants, which would allow the greenhouse gas to be stored or used to make chemicals.

Sato thinks that future catenane-based MOFs could help to solve a key problem with such porous materials: the better they adsorb CO₂, the tougher it is to get those molecules out again and regenerate the MOF. “So we need an additional trick to achieve both strong uptake and easy release,” Sato says.

Previous efforts to release gases from MOFs have often relied on heat, but Sato says that in principle, simply squeezing a catenane-based MOF should be enough to free the CO₂ trapped in its pores, which may require less energy than thermal methods. Although the new MOF has only a modest ability to adsorb CO₂, the researchers plan to tweak its catenanes to improve uptake, and test whether the gas can indeed be squeezed out.