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

摘要

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-N_2 complexes of terradentate P_3~E ligands (E = B, C) generate catalytic yields of NH_3 under an atmosphere of N_2 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 NH_3 for each Fe catalyst has been realized (up to 64 equiv of NH_3 produced per Fe for P_3~B and up to 47 equiv for P_3~C) by increasing acid/reductant loading with highly purified acid. Cyclic voltammetry shows the apparent onset of catalysis at the P_3~BFe-N_2/P_3~BFe-N_2~- couple and controlled-potential electrolysis of P_3~BFe~+ at -45 ℃ demonstrates that electrolytic N_2 reduction to NH_3 is feasible. Kinetic studies reveal first-order rate dependence on Fe catalyst concentration (P_3~B), consistent with a single-site catalyst model. An isostructural system (P_3~(Si)) is shown to be appreciably more selective for hydrogen evolution. In situ fireeze-quench Moessbauer spectroscopy during turnover reveals an iron-borohydrido-hydride complex as a likely resting state of the P_3~BFe catalyst system. We postulate that hydrogen-evolving reaction activity may prevent iron hydride formation from poisoning the P_3~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.