Stir bar contamination may inadvertently catalyze reactions

The humble stir bar is a ubiquitous workhorse of the chemistry lab that has swirled its way into researchers’ hearts. But it seems that magnetic stirrers have also been mixing things up in a different way—by smuggling rogue metals into chemists’ reaction flasks (ACS Catal. 2019, DOI: 10.1021/acscatal.9b00294).

Stir bars are typically coated with polytetrafluoroethylene (PTFE), a durable and inert polymer. But they can quickly become discolored, especially when exposed to catalytic metals such as palladium. So Valentine P. Ananikov of the N. D. Zelinsky Institute of Organic Chemistry gathered 60 used stir bars from other labs in his institute and took a closer look at what had caused the stains.

Electron microscopy revealed that the bars’ surfaces were littered with scratches, dents, and cracks that often contained a tangle of polymer filaments. These filaments trapped metal nanoparticles or microparticles, including palladium, gold, platinum, cobalt, or iron, which Ananikov’s team identified by X-ray spectroscopy. “It appears that almost all used stir bars in labs doing intensive catalysis and synthesis experiments are contaminated to varying degrees,” Ananikov says.

The researchers also found that these metals could leach into solution and interfere with reactions. Ananikov’s team tested used stir bars in the palladium-catalyzed Suzuki-Miyaura reaction, which couples aryl halides with boronic acids. First they used a palladium catalyst and a fresh stir bar to carry out a series of these reactions; then they repeated the experiments with no catalyst and a contaminated stir bar. In several cases, the dirty stir bar alone delivered a significant amount of the coupling product, and in one case it even matched the yield from the palladium catalyst. Running the reaction with no catalyst and a brand new stir bar gave no products at all.

Chemists who work with palladium catalysts generally know that dirty stir bars can affect their reactions, but it’s important to understand the extent of the problem, says synthetic chemist Mimi Hii of Imperial College London. “It’s been talked about anecdotally for a while, so it’s really nice to see a systematic study on it.” Because Hii’s group has found palladium to be active at concentrations as low as 5 parts per million, the researchers cleanse their stir bars of palladium with aqua regia, a mixture of nitric acid and hydrochloric acid. Others are not so thorough—indeed, many chemists clean their stir bars with nothing more than a quick scrub and a rinse with acetone and water, Ananikov says.

“I was surprised that there was enough metal there to catalyze reactions,” says Vladimir Gevorgyan, an organic chemist at the University of Illinois at Chicago. He points out that trace amounts of metal can sometimes interact with other metal catalysts to alter their reaction mechanism, so the problem is unlikely to be restricted to palladium chemistry. “I think it’s more general than that,” he says.

Significant stir bar damage can appear within weeks of use, and Ananikov’s theoretical calculations suggest that damaged PTFE binds metal particles much more strongly than pristine PTFE. He advises chemists to use new stir bars when they report reactions that apparently need very low concentrations of metal catalysts, or even none at all. “It’s a cautionary tale for anyone working in catalysis,” Hii says.