UvA chemists isolate elusive nitrogen particle

‘Nitridyl radical’ might offer new approach for sustainable synthesis of fertilizers

23 May 2014

Chemists of the Van 't Hoff Institute for Molecular Sciences (HIMS) at the University of Amsterdam (UvA) are the first to have isolated an elusive ‘nitridyl radical’. Their research has been published this week by the international journal Angewandte Chemie. The reactive nitrogen particle, captured between two rhodium atoms, may well play an important role in new processes for converting molecular nitrogen into useful compounds such as fertilizers.

In 2012 and 2013 the UvA team already had the ‘nitridyl radicals’ in sight. In those years they spectroscopically detected comparable atomic nitrogen radicals bound to iridium and rhodium. Now they managed to isolate the reactive species - a world's first.

With chemical ingenuity they were able to 'capture' the radical bivalent nitrogen ions (•N2- ions) between two rhodium atoms. Furthermore they could investigate the electronic structure and the initial reactivity of this unique compound. They expect the new nitrogen radical to open up new pathways for capturing atmospheric nitrogen.

Nitridyl radical

The nitridyl radical, sandwiched between to rhodium atoms within a protective ligand environment.

Fertilizer

Chemists use nitrogen from air as a starting material for the synthesis of all kinds of chemicals, notably fertilizers.  It's very abundant - almost 80% of the Earth’s atmosphere consists of elemental nitrogen. On the other hand the dinitrogen molecules don't react very easily, making nitrogen fixation an inefficient process that consumes lots of energy. The industrial Haber-Bosch process, which combines nitrogen under high pressure and at high temperature to form ammonia, consumes 3-5% of the of the world's natural gas production and 1-2% of the global annual energy supply. Hence, a more efficient way to convert elemental nitrogen in useful chemicals would be quite welcome.

The nitridyl radicals may well play an important role to achieve this. The UvA-HIMS chemists not only isolated the reactive particles but were also able to do some chemistry, combining them to dinitrogen. The chemical principle of ‘micro-reversibility’ now suggests that reversing this reaction should also be possible. The idea now is to activate dinitrogen between two metals, resulting in the formation of two nitridyl radical complexes which can be used to synthesize nitrogen containing compounds.

This will still require energy, but hopefully much less than in the Haber-Bosch process. The scientists even see possibilities in using (sun)light for splitting elemental nitrogen. This would make production of fertilizers more sustainable. Furthermore this could be used to store a temporary excess of (electrical or solar) energy in the form of chemical energy.

Breaking the nitrido wall

Graphic to appear on the inside cover of the print issue of Angewante Chemie International Edition.

Protective environment

It took the team of Bas de Bruin at the UvA all their chemical ingenuity to capture the nitridyl radicals. They used two rhodium atoms combined with a protecting ligand environment, shielding  the ‘nitridyl radical’ efficiently from its environment. The fact that rhodium can be used to stabilize ‘nitridyl radicals’ is actually quite remarkable, since negatively charged atomic nitrogen species usually bind only very weakly  to late transition metals such as rhodium. This is because  the electrons of the nitrogen atom and those in the filled d-orbitals of the metal repel each other strongly.

Among scientists active in this field this observation is called the ‘nitrido wall’. Hence, the isolation of a nitridyl-radical bridged Rh=N•-Rh complex also represents a breach in the ‘nitrido wall’.

The protecting ligand and the use of two rhodium atoms instead of only one made it possible to isolate such a unique species and to characterize it in detail (despite the unusual combination with rhodium).

However, the fact that the species can be isolated does not mean that it is not reactive. As soon as another molecule (such as carbon monoxide) binds to one of the rhodium atoms the complex falls apart, after which the ‘nitridyl ligand’ reveals its typical radical character. The rhodium-nitridyl species generated then react in a characteristic manner with themselves to form elemental nitrogen (N2).

Nitridyl radical

Elemental nitrogen (N2) is formed from two nitridyl radicals. This ‘radical coupling’ occurs as soon as carbon monoxide binds to one of the two rhodium atoms causing the complex to dissociate.

Part of the research of the UvA team was carried out in collaboration with scientists from Leiden and Göttingen. The research was funded by the Netherlands Organisation for Scientific Research NWO (VICI grant Bas de Bruin and NOW-FOM funding) and the European Union (ERC grant Bas de Bruin and COST action CARISMA).

Publication

Gloaguen, Y., Rebreyend, C., Lutz, M., Kumar, P., Huber, M., van der Vlugt, J. I., Schneider, S. and de Bruin, B. (2014), An Isolated Nitridyl Radical-Bridged {Rh(N•)Rh} Complex. Angew. Chem. Int. Ed.. doi: 10.1002/anie.201403445

Published by  HIMS