A fundamental laboratory advance has made it possible to break, at room temperature and pressure, two of the strongest types of chemical bonds in order to make common industrial compounds. In doing so, researchers at Cornell University have taken an important first step toward less-energy-intensive processes for making nitrogen-containing organic compounds.
“The nitrogen-carbon bond is the backbone of almost all top-selling pharmaceuticals,” says Paul Chirik, professor of chemistry at Cornell. Nitrogen-carbon bonds are found in nylon, fertilizer, insecticide, and in every protein. Bringing together carbon and nitrogen, though, typically requires large amounts of energy because chemists use ammonia as a nitrogen source. Chirik has developed a new reaction that uses carbon monoxide and molecular nitrogen to make these bonds. Such a reaction also typically requires large amounts of energy. This work is described this week in the journal Nature Chemistry.
In its naturally occurring form, molecular nitrogen, which is made up of two nitrogen atoms held together by a triple bond, is one of the most stable molecules that exists. “It has no negative or positive ends, so it’s very hard to make it react,” says Chirik. Other chemists are working on mimicking biological enzymes that “fix” molecular nitrogen to make ammonia that could be used as the feedstock for organic chemicals. Chirik’s lab, in contrast, is developing a reaction for breaking the nitrogen bond not to make ammonia but to make organic-nitrogen compounds directly.
The key to the Cornell reaction, which takes two steps to break the nitrogen bonds, is a complex containing the metal hafnium. In the first step, two metal complexes surround each nitrogen molecule, caging it in. The hafnium complexes react with the nitrogen, breaking two of the bonds and creating an intermediate molecule. Then carbon monoxide is added to the mixture. Carbon monoxide is also a very stable compound and would not react with molecular nitrogen. But carbon monoxide will react with the nitrogen-hafnium intermediate, breaking the final nitrogen bond to form an organic molecule called oxamide that is released from the hafnium complex by the addition of acid.
“People producing organo-nitrogen compounds today have to make ammonia first,” says Christopher Cummins, professor of chemistry at MIT. The nice thing about the new Cornell technique, he says, is that “they are developing reactions to make nitrogen into organo-nitrogens directly.” Cummins points out that the only company to do this industrially, American Cyanamid, used hydropower produced by Niagara Falls to make an electrical arc powerful enough to drive the reaction.
The Cornell chemistry isn’t ready for industrial use yet. So far, the reaction they’ve developed isn’t catalytic, and therefore isn’t practical. The hafnium complex makes it possible for the reaction to proceed in ambient conditions, but it gets used up during the reaction. Chirik is working on “how to get the pieces off the metal” so that it can be reused. The Cornell researchers are also trying to determine how general the reaction is. They’ve used it to make a fertilizer; further work will tell if this type of reaction will work for a myriad of organo-nitrogen compounds. Chirik says he’s also trying to determine whether other metals can be used to speed the reaction. Hafnium is effective, but it’s rare.
“This is a window into something for the future,” says Cummins. “The basic reaction chemistry of simple molecules like nitrogen and carbon monoxide is still being uncovered.”