A Synthetic Single-Site Fe Nitrogenase: High Turnover, Freeze-Quench ⁵⁷Fe Mössbauer Data, and a Hydride Resting State

The mechanisms of the few known molecular nitrogen-fixing systems, including nitrogenase enzymes, are of much interest but are not fully understood. We recently reported that Fe-N₂ complexes of tetradentate P₃^E ligands (E = B, C) generate catalytic yields of NH₃ under an atmosphere of N₂ with acid and reductant at low temperatures. Here we show that these Fe catalysts are unexpectedly robust and retain activity after multiple reloadings. Nearly an order of magnitude improvement in yield of NH₃ for each Fe catalyst has been realized (up to 64 equiv of NH₃ produced per Fe for P₃^B and up to 47 equiv for P₃^C) by increasing acid/reductant loading with highly purified acid. Cyclic voltammetry shows the apparent onset of catalysis at the P₃^BFe-N₂/P₃^BFe-N₂^- couple and controlled-potential electrolysis of P₃^BFe⁺ at -45 °C demonstrates that electrolytic N₂ reduction to NH₃ is feasible. Kinetic studies reveal first-order rate dependence on Fe catalyst concentration (P₃^B), consistent with a single-site catalyst model. An isostructural system (P₃^Si) is shown to be appreciably more selective for hydrogen evolution. In situ freeze-quench Mössbauer spectroscopy during turnover reveals an iron-borohydrido-hydride complex as a likely resting state of the P₃^BFe catalyst system. We postulate that hydrogen-evolving reaction activity may prevent iron hydride formation from poisoning the P₃^BFe system. This idea may be important to consider in the design of synthetic nitrogenases and may also have broader significance given that intermediate metal hydrides and hydrogen evolution may play a key role in biological nitrogen fixation.