The interconversion of N₂H₂ and N₂ is probed using μ-N₂H₂ iron-sulfur complexes. An intermediate trapped at low temperature was formerly presumed to be a μ-N₂ species, but new in situ spectroscopic characterization shows a new assignment. Our results are used to predict requirements to observe iron- and sulfur-based reversible proton-coupled electron transfer (PCET) for N₂ fixation.

Abstract

Biological 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 Fe^3+/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.