A triple bond is the strongest

03/15/2021 Calcium cracks molecular nitrogen

Metal breaks highly stable triple bonds

Chemists around the world are constantly on the lookout for the elemental nitrogen, or N for short, in the air2to make them available for chemical reactions with simple means. That turns out to be difficult, because nitrogen is a less reactive gas with a triple bond, which is one of the strongest known chemical bonds. A research team with the participation of the Marburg chemist Professor Dr. Gernot Frenking has now shown that the metal calcium, which occurs frequently in nature, is able to break the highly stable nitrogen bond - even at minus 60 degrees Celsius. This is important in two respects: The research group is thus providing new insights into the bond-breaking abilities of calcium, which has hitherto been ridiculed in this regard. In addition, their results can serve as a basis for the development of future industrial processes.

As the main component of air, nitrogen is available in unlimited quantities everywhere. Due to its low chemical reactivity, it is used, among other things, as a protective gas for storing food. Nitrogen keeps pre-baked rolls fresh for months; plants need it to grow. However, they cannot use the nitrogen from the air directly. The big challenge is to find the highly stable diatomic molecule N2 to convert into useful chemicals. In the early 1900s, two German chemists succeeded in doing this. They developed the Haber-Bosch process named after them, the N2 in ammonia (NH3) converts. Ammonia was originally used to make explosives, but is now mainly used as a fertilizer. In the Haber-Bosch process, a transition metal catalyst sets the chemical reaction in motion. In addition, the conversion of the highly stable nitrogen into ammonia requires high pressures and temperatures. This so-called bread-from-air process is therefore very energy-intensive.

In order to simplify this chemical process, among other things, chemists are looking for other ways of breaking the strong N≡N triple bond. The research team around Professor Dr. Sjoerd Harder from the Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) was able to show that the main group element calcium can accomplish this feat. Calcium is a metal that occurs mainly in limestone rocks and thus in nature, and which up until now has hardly been trusted to break strong chemical bonds. In contrast to the often poisonous transition metals, calcium is generally not able to use d orbitals - a wave function of particular symmetry that facilitates bond breaking reactions.

In their search for calcium atoms in the unusual oxidation state + I, the working group found out by chance that the metal reacts with nitrogen - ironically, in this experiment the nitrogen should actually only be an unreactive protective gas. Harder and his team isolated a molecule in which nitrogen was trapped between two calcium atoms and demonstrated its further conversion to hydrazine. In contrast to the very stable nitrogen, hydrazine is used as a highly reactive rocket fuel. Together with Frenking and other theoretical chemists from the University of Nanjing in China, Harder's group discovered that, unexpectedly, d orbitals play an essential role in nitrogen activation on calcium. This controversial but important finding breaks the dogma that d orbitals are irrelevant for main group metals - metals that are assigned to one of the main groups in the periodic table.

While the process is neither catalytic nor economical, it does provide fundamental new and important insights into bond-breaking reactions with calcium. These findings will not only change student textbooks, but could also contribute to the development of simplified processes in industry. (Press text: FAU)