Evaluating Diazene to N2 Interconversion at Iron‐Sulfur Complexes

Abstract

AbstractBiological N2 reduction occurs at sulfur‐rich multiiron sites, and an interesting potential pathway is concerted double reduction/ protonation of bridging N2 through PCET. Here, we test the feasibility of using synthetic sulfur‐supported diiron complexes to mimic this pathway. Oxidative proton transfer from μ‐η1 : η1‐diazene (HN=NH) is the microscopic reverse of the proposed N2 fixation pathway, revealing the energetics of the process. Previously, Sellmann assigned the purple metastable product from two‐electron oxidation of [{Fe2+(PPr3)L1}2(μ‐η1 : η1‐N2H2)] (L1=tetradentate SSSS ligand) at −78 °C as [{Fe2+(PPr3)L1}2(μ‐η1 : η1‐N2)]2+, 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 N2 complex. Instead, the data are most consistent with the spectroscopically observed species being the mononuclear iron(III) diazene complex [{Fe(PPr3)L1}(η2‐N2H2)]+. 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 Fe3+/2+ redox potential, indicating that the reverse N2 protonation would be too endergonic to proceed. This system establishes the “ground rules” for designing reversible N2/N2H2 interconversion through PCET, such as tuning the redox potentials of the metal sites.