Although the major use of NH3 is in fertilizers, the molecule is also being touted as a fuel option. The industrial synthesis of NH3 involves high-pressure N2 hydrogenation, an alternative to which is N2 electroreduction. Of course, the success of the latter approach rests on having cathodic catalysts that can efficiently process N2, protons and electrons (not to mention a suitable anodic catalyst to mediate an accompanying reaction such as H2O oxidation). Nishibayashi and co-workers now report a base metal N2 reduction catalyst featuring Mo supported by a PCP-donor ligand — a pincer bearing an N-heterocyclic carbene flanked by two phosphine groups.

Credit: Adapted from Eizawa, A. et al. (2017), CC-BY-4.0.

As described in Nature Communications, the new complex acts on N2, the acid 2,6-lutidinium and the reductant decamethylchromocene to give NH3, with only traces of H2 by-product being observed. In contrast, less selective catalysts sometimes ignore N2 and instead couple protons and electrons to make H2. The Mo catalyst, which exists as an N2-bridged dimer, is not an electrocatalyst because it sources its electrons from a sacrificial reductant instead of an electrode. The approach is also not particularly fast but it is mild and robust — operating under 1 atmosphere of N2 at ambient temperature, each dimer can produce 230 molecules of NH3. However, such success in this multielectron reduction must be placed in context, and the real test will be how the system can be adapted to milder reductants and, eventually, electrocatalytic conditions.