Iron Complex Turns Nitrogen And Hydrogen Into Ammonia

An iron complex that can turn dinitrogen and dihydrogen into ammonia provides long-sought mechanistic clues as to how the Haber-Bosch process produces NH₃ industrially (Science, DOI: 10.1126/science.1211906).

Ammonia’s main use in industry is to make fertilizer, but it is also used in cleaners, as a refrigerant, and as a building block for other nitrogen-containing compounds.

Fritz Haber and Carl Bosch developed the process to make ammonia from N₂ and H₂ in the early 1900s. The reaction can be catalyzed by a number of different metals, but the most common is iron with potassium added as a promoter. Studies of that system show that the iron surface chemisorbs N₂, with the N≤N bond cleaving to give surface-bound nitride (N^3–). But key mechanistic details—such as the number of iron atoms involved and the role of potassium—have remained unclear. Model complexes could provide insight, but until now no one had been able to reproduce the reaction using iron.

The new complex is an Fe(II) β-diketiminate chloride compound in which two Fe(II) atoms are bridged by two chloride ions. It was prepared and characterized by a group led by University of Rochester graduate student Meghan M. Rodriguez and chemistry professor Patrick L. Holland.

Two of these Fe(II) complexes react with N₂ and potassium graphite, a strong reductant, to produce a four-iron bis(nitride) complex. In the bis(nitride) complex, one nitride is bound to two Fe(III) and one Fe(II) ions. The other nitride is bound to the same two Fe(III) ions and the fourth iron, an Fe(II) ion, through bridging potassium and chloride ions.

Rodriguez, Holland, and colleagues propose that the potassium graphite reduces the iron atoms in the reagent complex to Fe(I). The Fe(I) ions then donate charge to N₂, weakening the N≤N bond and allowing it to break to form the bis(nitride) complex.

Adding hydrogen to the bis(nitride) complex results in NH₃ formation with 42% yield. One of the Fe(II) ions in the bis(nitride) complex is three coordinate, and the researchers suggest that it is this iron that reacts with H₂ to break the H–H bond and enable NH₃ formation.

Unlike the Haber-Bosch system, the new complex is not catalytic. Aside from NH₃, the reaction produces an Fe(II) β-diketiminate hydride compound that does not repeat the chemistry. Holland and coworkers are working to uncover additional details of the reaction mechanism, as well as to design a ligand that will produce a catalytic complex, Holland says.

Previous experiments with similar ligands that provided more bulk around the iron atoms did not produce bis(nitride) complexes, suggesting that allowing multiple iron atoms to interact simultaneously is key to cleaving N₂. Such cooperativity has been proposed for Haber-Bosch catalysis but not demonstrated experimentally. “To actually demonstrate it with a model system is very interesting because it makes the argument for cooperativity stronger,” says David R. Tyler, a chemistry professor at the University of Oregon. A cooperative mechanism is also proposed for nitrogenase enzymes, which incorporate iron-sulfur clusters and an iron-molybdenum cofactor to turn N₂ into NH₃.

The chemistry of the Fe(II) β-diketiminate complex may also lead to new synthetic schemes for nitrogen-containing compounds, says Paul J. Chirik, a chemistry professor at Princeton University. “Hopefully, this system will continue to be an exciting platform to manipulate the reactivity of cleaved nitrogen to form other interesting molecules,” he says.