Iron replaces pricey metal

You’ve heard the song about a girl with diamonds on the soles of her shoes, but did you know that you have platinum in your sneakers? It also can turn up in shampoo, denim jeans, envelope glue and even beer.

The high-priced metal is an important industrial catalyst, but it is hard to remove from the final product and miniscule amounts are embedded within consumer products, costing manufacturers millions of dollars in lost platinum a year.

Now chemist Paul Chirik, Princeton’s Edwards S. Sanford Professor of Chemistry, and his research team have found that they can replace platinum with a much more common substance — iron. The switch to iron could save money and benefit the environment through a reduction in mining precious metals.

Chirik’s approach essentially coaxes iron into acting in a manner that is similar to pricier metals. Platinum can catalyze efficiently because it can transfer two electrons at a time between molecules during a chemical reaction. Iron, in contrast, typically transfers only one electron at a time. Because electrons like to travel in pairs, this one-electron transfer leaves the surrounding molecules with unpaired electrons, or free radicals, that react with other nearby molecules. As a result, iron can spur the creation of various chemical products other than the intended product.

“The fundamental question is how do you take a metal that wants to do one-electron chemistry and shut that inclination off so that it does twoelectron chemistry,” Chirik said.

The solution pioneered by Chirik and then-graduate student Aaron Tondreau, now a postdoctoral researcher at the Swiss Federal Institute of Technology in Zurich, was to pack a bulky molecule that also engages in radical chemistry around the iron. Thus, both the iron and the surrounding molecule participate in radical chemistry, giving the net result of transferring two electrons.

The team’s early results attracted the attention of Albany-based Momentive Performance Materials, which makes silicone for, among other things, envelope glue. In collaboration with Momentive scientists, Chirik’s team began to tinker with the size of the surrounding molecule on the iron. Through trial and error they found the supporting molecules that were just the right size needed to make silicone without making unwanted byproducts. They published the results in Science in February 2012. Chirik and Princeton graduate student Crisita Atienza are now working to make a hardier version of the catalyst.

The reactions are elegantly simple, Chirik said. “It is just good old-fashioned chemistry that makes what you want,” he said.

The work was supported by the National Science Foundation, the Air Force Office of Scientific Research, the Army Research Office, the Office of Naval Research, the David and Lucile Packard Foundation, and the Alfred P. Sloan Foundation.