Catalytic conversion of nitrogen to ammonia by an iron model complex

Catalysis of the reduction of nitrogen to ammonia under mild conditions by a tris(phosphine)borane-supported iron complex indicates that a single iron site may be capable of stabilizing the various NxHy intermediates generated during catalytic ammonia formation. Industrial nitrogen fixation is performed on a vast scale by the Haber–Bosch process, which uses a solid-state iron catalyst at very high temperatures and pressures. Synthetic chemists have searched for decades for small metal-containing complexes to catalyse the transformation of nitrogen into ammonia in less extreme conditions, taking their lead from the nitrogenases found in plants and bacteria. To that end Jonas Peters and colleagues describe a tris(phosphine)borane-supported iron complex that catalyses the reduction of nitrogen into ammonia under mild conditions with reasonable efficiency. This suggests that a single iron site is sufficient for mediating nitrogen fixation, in line with recent biochemical and spectroscopic data that point to iron rather than the molybdenum also present in the FeMo cofactor or nitrogenase as the site of nitrogen binding and activation. The reduction of nitrogen (N2) to ammonia (NH3) is a requisite transformation for life1. Although it is widely appreciated that the iron-rich cofactors of nitrogenase enzymes facilitate this transformation2,3,4,5, how they do so remains poorly understood. A central element of debate has been the exact site or sites of N2 coordination and reduction6,7. In synthetic inorganic chemistry, an early emphasis was placed on molybdenum8 because it was thought to be an essential element of nitrogenases3 and because it had been established that well-defined molybdenum model complexes could mediate the stoichiometric conversion of N2 to NH3 (ref. 9). This chemical transformation can be performed in a catalytic fashion by two well-defined molecular systems that feature molybdenum centres10,11. However, it is now thought that iron is the only transition metal essential to all nitrogenases3, and recent biochemical and spectroscopic data have implicated iron instead of molybdenum as the site of N2 binding in the FeMo-cofactor12. Here we describe a tris(phosphine)borane-supported iron complex that catalyses the reduction of N2 to NH3 under mild conditions, and in which more than 40 per cent of the proton and reducing equivalents are delivered to N2. Our results indicate that a single iron site may be capable of stabilizing the various NxHy intermediates generated during catalytic NH3 formation. Geometric tunability at iron imparted by a flexible iron–boron interaction in our model system seems to be important for efficient catalysis13,14,15. We propose that the interstitial carbon atom recently assigned in the nitrogenase cofactor may have a similar role16,17, perhaps by enabling a single iron site to mediate the enzymatic catalysis through a flexible iron–carbon interaction18.