A hydride out of hiding

J. Am. Chem. Soc. 137, 13220–13223 (2015)

Credit: JACS

Nitrogenase uses an iron-sulfur cluster–based cofactor to convert dinitrogen into ammonia, but the full details of the mechanism have proven elusive. One topical question is whether hydride ions are part of the reaction mechanism, and recent experimental data does provide evidence for Fe-H bonds. However, there are no inorganic complexes of iron sulfides with hydride ligands, limiting the available knowledge about expected structures and raising questions as to the chemical feasibility of such an unusual intermediate. To enable further study, Arnet et al. now report the synthesis and analysis of such a model complex. The authors began with a known iron hydride complex, [L^MeFeH]₂, in which L^Me is a bulky bivalent ligand. A heated reaction with sodium dodecanethiolate followed by treatment with a metal chelator led to the replacement of one hydrogen atom with a sulfur atom, as demonstrated through X-ray diffraction analysis, Mössbauer analysis and mass spectrometry. With the complex in hand, the authors examined its reactivity. Nitrogenase is capable of reducing alkynes; however, the addition of an alkyne substrate to the chelated complex led to a bound alkyne product and loss of H₂ gas, leading the authors to speculate that the hydride is a better base than a nucleophile. How this translates to the nitrogenase mechanism, and whether steric effects unique to the ligand environment influenced the result, will require further study. Nitrogenase is known to reduce CO₂ to CO, CH₂O₂, or CH₄, but the initial reaction step is unknown; exposing the synthetic complex to CO₂ led to the formation of a CHO₂ ligand, suggesting that formate may be the first step along the pathway. Finally, the complex was capable of binding N₂ upon reduction. These results provide new opportunities to test and explore nitrogenase's function.