Breaking the N≡N triple bond with calcium

FAU chemists report in Science how to activate nitrogen with the main group metal calcium (Ca)

Molecular nitrogen (N₂) is a highly stable gas with a N≡N triple bond that is among one of the strongest known chemical bonds. It reacts with hardly anything and is used as a protective gas for food storage. Being the main component of air, it is also used by plants as a nitrogen source. However, in order to take up nitrogen, the fully inert N₂ must first be converted in water soluble ammonium or nitrite and nitrate ions. Plants do this by complicated enzymes but the process is extremely slow. While nature has time, mankind converts N₂ to ammonia (NH₃) by the Haber-Bosch process. The bulk production of NH₃ needs high temperatures and pressures (500 °C, 300 bar). Initially, the process was invented for the production of explosives but nowadays enormous amounts of NH₃ are needed for fertilizers to grow crops for a rapidly increasing world population. This industrial “bread from air” process consumes up to 2% of all our energy.

Both, nature and industry, use a transition metal catalyst for N₂ conversion. The partially filled d-orbitals on the metal are the key to N₂ activation. Now Sjoerd Harder, professor of Inorganic and Organometallic Chemistry, and graduate student Bastian Rösch discovered N₂ activation by the main group metal Ca at very low temperatures.

Bastian Rösch: “It was actually a complete surprise to us. Together with my colleague Thomas Gentner we tried to make complexes of Ca in the unusual oxidation state +I. We found that these complexes reduced the aromatic solvents. For instance, benzene became anti-aromatic by picking up two electrons. So we switched to alkanes as solvent and found that Ca(+I) is even able to reduce N₂ which we ironically used as a protective gas!”

The Science article can be found at: https://science.sciencemag.org/content/371/6534/1125

Sjoerd Harder: “We found that the activated N₂ can be converted to hydrazine (N₂H₄) which in contrast to N₂ is highly reactive and used as rocket fuel. That Ca can do this trick is highly surprising and we started to think that d-orbitals on Ca may be important. Comprehensive calculations by theory groups from Marburg and Nanjing confirmed this possibility. Although highly controversial, this will likely change chemistry text books and maybe initiate industrial research. Our discovery will certainly not replace Haber-Bosch but the fact that the Haber-Bosch catalyst contains not only iron but also calcium oxide is food for thought. Also the presence of Ca in many enzymes or in the Frank-Caro process, the reaction between Ca carbide and N₂, may not be coincidental. Time will tell what else Ca can do. Our group will certainly do its best to uncover further secrets of the metal Ca.”

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