Transition-Metal-Free One-Pot Synthesis of Ammonium Chloride

The reduction of dinitrogen (N₂) to ammonia (NH₃) is an essential transformation for our life due to its presence in amino acids. However, it’s extremely stable elemental form of dinitrogen (N₂) which makes difficult to utilize nitrogen either biologically or synthetically. The industrial Haber-Bosch process using a solid-state Fe catalyst at very high temperatures and pressures is the major source of ammonium fertilizer and other synthetic nitrogen compounds worldwide [1]. A facile Fe₂O₃ and TiO₂-based nanoparticles are synthesized via the precipitation/calcination route and applied them in a water photo-splitting setup to photosynthesize ammonia (Figure 1) [2].

Figure 1. Photocatalytic N₂ conversion to ammonia using efficient nanostructured solar-energy-materials in aqueous media [2].

In 2018, Professor Braunschweig has reported the reaction of a p-block molecule with N₂ under transition-metal-free conditions by taking advantage of the emerging concept of main-group metallomimetics: p-block compounds can able to react with transition metal complexes. A year later they have used a similar system to demonstrate the first coupling of two nitrogen molecules via transient dicoordinate borylene.  He is says that the key in both of these discoveries was the use of boron, the fifth lightest element, as the atom to which the nitrogen binds. After these two discoveries, it was clear that they have a pretty special system on our hands.

The breakthrough came from the simplest reagents: Traces of water left behind in a sample were enough to promote a sequential reaction that brought the team only a single step away from the target of ammonium. It was later discovered the one-pot, two-step chemical conversion of N₂ to ammonium without transition metals, and the isolation of the intermediate products of every two-electron/two-proton step in the process. These key reactions could be done using a solid acid, allowing the reactions to occur sequentially in a single reaction flask, all at room temperature [3].

This dinitrogen-to-ammonium process proceeds by a stepwise reduction–protonation mechanism via an end-on bridging N₂ species. It should be noted that the B(OH)₃ used a protonating agent in this work is very mild compared with those used in transition-metal-based catalytic processes for N₂ conversion, which are often relatively strong organic acids, such as 2,6-lutidinium salts, triflic acid or tetra fluoroaryl borate superacids, indicating the existence of highly nucleophilic nitrogen atoms throughout in the process. The success of these reactions can thus be attributed to the unwillingness of the electron-rich boron atoms in our system to accept further electrons (in contrast with many transition metal complexes), forcing the reduction steps to occur predominantly on the N₂ moiety and thereby facilitating N–N cleavage and protonation. In this regard, the limited metallomimicry of the borylene system is a distinct advantage, promoting a facile cascade mechanism with reduction events largely centered on nitrogen. The one-pot chemical conversion of N₂, while not a catalytic process, is a convincing demonstration of the ability of main-group elements in low oxidation states not only to activate very inert small molecules but also to mediate their conversion through cascades of redox and protonation processes (Figure 2).

Figure 2. The conversion of dinitrogen to ammonium chloride at a main-group element at room-temperature [3]. 

While exciting, the reaction is still far from being a truly practical process for industrially producing ammonium. Ideally, finding a way to re-form the active species will be needed to make the process energy efficient and economical. Nevertheless, the discovery is an exciting demonstration that the lighter elements can tackle even the biggest challenges in chemistry. There is much left to be done here, but boron and the other lighter elements have already surprised us so many times and are clearly capable of much more.

References

1. Chemistry Habers process chemistry. India: Arihant publications, 264 (2018). ISBN 9789313163039.

2. M. Lashgari, P. Zeinalkhani,, Appl. Catal. A 529, 91 (2017).

3. M. André Légaré, G. Bélanger-Chabot, M. Rang, R. D. Dewhurst, I. Krummenacher, R. Bertermann, H. Braunschweig. Nature Chemistry, 2020, DOI: 10.1038/s41557-020-0520-6.

Blog Written By

Dr. S. Chandrasekar 

Institute for Advanced Study

Shenzhen University, China

Editors
Dr. A. S. Ganeshraja
Dr. K. Rajkumar
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Dr. Y. Sasikumar
Dr. S. Thirumurugan
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