Switchable Catalysts

Catalysts don’t usually come with an on-off switch. But catalytically active supramolecular complexes can be designed to controllably flip back and forth between configurations that selectively mediate or inhibit a reaction, according to researchers at Northwestern University (Science 2010, 330, 66). The study suggests new strategies for designing flexible catalysts that can be regulated by simple chemical means.

In response to specific chemical stimuli, such as the presence of certain ions, allosteric enzymes undergo conformational changes that can regulate enzyme activity. These conformational changes are typically triggered by binding of the ion or other chemical stimuli to a site other than the enzyme’s active site.

If the same type of response could be elicited from metal-containing organic molecules, then allosterism could serve as a new type of handle with which to control the variety of reactions catalyzed by that broad class of compounds. But engineering molecules that mimic nature in that way has remained challenging.

The Northwestern team, which includes chemists Hyo Jae Yoon, Junpei Kuwabara, Jun-Hyun Kim, and Chad A. Mirkin, turned to supramolecular synthesis methods to overcome those challenges.

The researchers synthesized multipart molecular assemblies that feature two large inert multi-phenyl-ring units and a catalytically active unit containing an Al(III)-salen moiety. The multiring sections serve as blocking layers or flaps that, depending on conformation, either expose or hide the metal center, which functions as a polymerization catalyst.

In a proof-of-concept demonstration, the team showed that the triple-layer assembly could mediate ring-opening polymerization of ε-caprolactone. Treating the closed and inactive form of the structure with chloride ions caused the flaps to open, thereby exposing the catalytic center and triggering polymerization. Adding a small amount of a chloride-abstracting agent caused the assembly to fold shut and terminated the polymerization. By exploiting this reversible process, the team tuned the polymers’ molecular weights.

“This is a beautiful example of how the flexibility of supramolecular chemistry can be used to mimic nature,” says University of North Carolina, Chapel Hill, chemistry professor Wenbin Lin. With this approach to turning chemistry on and off, it’s easy to envision many exciting applications, he adds.