Evaluating Diazene to N Interconversion at Iron-Sulfur Complexes.

Biological N reduction occurs at sulfur-rich multiiron sites, and an interesting potential pathway is concerted double reduction/ protonation of bridging N through PCET. Here, we test the feasibility of using synthetic sulfur-supported diiron complexes to mimic this pathway. Oxidative proton transfer from μ-η : η-diazene (HN=NH) is the microscopic reverse of the proposed N fixation pathway, revealing the energetics of the process. Previously, Sellmann assigned the purple metastable product from two-electron oxidation of [{Fe(PPr)L}(μ-η : η-NH)] (L=tetradentate SSSS ligand) at -78 °C as [{Fe(PPr)L}(μ-η : η-N)], which would come from double PCET from diazene to sulfur atoms of the supporting ligands. Using resonance Raman, Mössbauer, NMR, and EPR spectroscopies in conjunction with DFT calculations, we show that the product is not an N complex. Instead, the data are most consistent with the spectroscopically observed species being the mononuclear iron(III) diazene complex [{Fe(PPr)L}(η-NH)]. Calculations indicate that the proposed double PCET has a barrier that is too high for proton transfer at the reaction temperature. Also, PCET from the bridging diazene is highly exergonic as a result of the high Fe redox potential, indicating that the reverse N protonation would be too endergonic to proceed. This system establishes the "ground rules" for designing reversible N/NH interconversion through PCET, such as tuning the redox potentials of the metal sites.