Nitrogen is essential for life on earth. Both natural and industrial processes rely on iron catalysts, yet no isolated, homogenous iron complexes have been able to cleave N2, and the current industrial process requires harsh conditions. Professor Patrick L. Holland and his graduate student Meghan M. Rodriguez have reported an iron diketimidate complex capable of cleaving dinitrogen at normal temp and pressure (Science, 2011, 334(6057), pp.780-783). "Reduction of molecular nitrogen to ammonia under ambient conditions has been a dream of inorganic chemists for decades," says Professor John F. Berry of the University of Wisconsin-Madison.
Nitrogen gas (N2) is the primary component of our atmosphere and the nitrogen atom is found in all basic building blocks of all life forms on Earth. While the supply of this gas is abundant in our atmosphere, we have relied on nature to convert this inert (unreactive) gas to a usable form called ammonia (NH3) for millions of years through enzymatic processes in bacteria through utilization of metals such as iron. This process, referred to as "nitrogen fixation", breaks the three bonds between the bonded nitrogen atoms with the use of hydrogen (H2) gas to produce ammonia. Industrial nitrogen fixation is accomplished by the Haber-Bosch Process, which involves the reaction of dinitrogen with hydrogen facilitated by an iron catalyst. While this method of nitrogen fixation is effective, the process involves a reaction performed at high temperatures and pressures. Professor Nilay Hazari of Yale University comments, "The current method of producing ammonia is energy intensive and for over fifty years scientists have attempted to find a cheap catalyst that could make ammonia at ambient temperature and pressure." This is because, as Professor Holland adds, "About half of the nitrogen that comes into us is from the Haber-Bosch process rather than natural nitrogen fixation."
Professor Berry adds, "It's amazing that Holland's new iron complexes are able to do this. It's a long way from displacing the Haber process, but is a major fundamental advance in iron chemistry." So if the complex is not going to change ammonia production, where is its value? The answer lies in the mechanism by which iron catalyzes the conversion of nitrogen into ammonia during the Haber process. Scientists can hypothesize what is happening on the molecular level, but nobody has been able to study the process, and exactly how many iron or potassium atoms are involved is still unknown. What makes Holland and Rodriguez's complex so special is that the complex bound to nitrogen can be isolated and studied. Other than giving new insight into the mechanism of the Haber-Bosch process, the complex has significant potential to be used in synthetic laboratories. "A lot of people are interested in the structure of the complex I made," says Rodriguez. "At every conference Pat goes to people ask him if you can fit sodium in there instead of potassium, so we still have a lot of work to do on this complex. We are also interested in trying to tune the catalyst to control how activated the nitrogen bond is. We would like to be able to reduce it to a double or single bond and use that to do other chemistry." This range of activation could give synthetic chemists the ability to use dinitrogen in ways they never have before. "There are lots of pharmaceutical compounds that have nitrogen in them," says Holland, "and if you can get that from atmospheric nitrogen that would be a nice starting material because, well, it's everywhere."
The fact that the catalyst is soluble allowed William Brennessel, also at the University of Rochester, to obtain a crystal structure of this complex. The distance Brennessel observed between the two nitrogen atoms indicated there is no more bond between them. This is possible by a 6-electron reduction of the triple bond to give two N3- atoms, called nitrides. As can be seen from the crystal structure, it takes two potassium ions and four iron atoms to accomplish this bond cleavage. "Although iron complexes containing nitrides have always been a possibility in the Holland group research, no evidence of such were seen prior to our obtaining that initial crystal structure," says Brennessel. "And then we had to be sure that it was truly a nitride. From the standpoint of crystallography, I had to convince myself and the others that the bridging ligands were nitrides, and not something considerably less exciting like hydroxides or oxides." All of this will help scientists understand more about how the Haber process works. It should also be noted that Mössbauer experiments were done to determine the electronic structure of the catalyst and 15N2 labeling experiments were done to make sure that the ammonia being produced by the catalyst originated from the nitrogen going into the catalyst.
The first complexes that bound nitrogen had big groups around the iron atoms and although these complexes were able to bind to nitrogen, they were not able to cleave it. Realizing that giving iron the ability to move in closer to the nitrogen molecule, the new catalysts had less bulk and the iron atoms were able to arrange in a way that cleaved the N-N bond. When I first started here I had an idea for a completely different way of going about trying to solve nitrogen fixation, which didn't work at all. Then something else that we were doing turned out to bind nitrogen in an interesting way and that's what got us rolling."
"Recent results from the Holland group at the University of Rochester represent a significant advance in the search for energy efficient iron catalysts for ammonia production," says Hazari. "They have shown for the first time that two of the proposed steps in the conversion of dinitrogen to ammonia are possible using an iron complex." Holland and his group will continue to study the conversion of dinitrogen into ammonia. "We have a fixation on fixation," says Holland. As with most scientific explorations, every new discovery poses more questions, and the potential of this complex has yet to be realized.