Homogeneous catalysis: Faster formation of fuels and fertilizers

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.