NH₃ formation from N₂ and H₂ mediated by molecular tri-iron complexes

Living systems carry out the reduction of N₂ to ammonia (NH₃) through a series of protonation and electron transfer steps under ambient conditions using the enzyme nitrogenase. In the chemical industry, the Haber-Bosch process hydrogenates N₂ but requires high temperatures and pressures. Both processes rely on iron-based catalysts, but molecular iron complexes that promote the formation of NH₃ on addition of H₂ to N₂ have remained difficult to devise. Here, we isolate the tri(iron)bis(nitrido) complex [(Cp'Fe)₃(μ₃-N)₂] (in which Cp' = η⁵-1,2,4-(Me₃C)₃C₅H₂), which is prepared by reduction of [Cp'Fe(μ-I)]₂ under an N₂ atmosphere and comprises three iron centres bridged by two μ₃-nitrido ligands. In solution, this complex reacts with H₂ at ambient temperature (22 °C) and low pressure (1 or 4 bar) to form NH₃. In the solid state, it is converted into the tri(iron)bis(imido) species, [(Cp'Fe)₃(μ₃-NH)₂], by addition of H₂ (10 bar) through an unusual solid-gas, single-crystal-to-single-crystal transformation. In solution, [(Cp'Fe)₃(μ₃-NH)₂] further reacts with H₂ or H⁺ to form NH₃.