A 106‑Fold Enhancement in N2‑Binding Affinity of an Fe2(μ-H)2 Core upon Reduction to a Mixed-Valence FeIIFeI State

Transient hydride ligands bridging two or more iron centers purportedly accumulate on the iron–molybdenum cofactor (FeMoco) of nitrogenase, and their role in the reduction of N2 to NH3 is unknown. One role of these ligands may be to facilitate N2 coordination at an iron site of FeMoco. Herein, we consider this hypothesis and describe the preparation of a series of diiron complexes supported by two bridging hydride ligands. These compounds bind either one or two molecules of N2 depending on the redox state of the Fe2(μ-H)2 unit. An unusual example of a mixed-valent FeII(μ-H)2FeI is described that displays a 106-fold enhancement of N2 binding affinity over its oxidized congener, quantified by spectroscopic and electrochemical techniques. Furthermore, these compounds show promise as functional models of nitrogenase as substantial amounts of NH3 are produced upon exposure to proton and electron equivalents. The Fe­(μ-H)­Fe­(N2) sub-structure featured herein was previously unknown. This subunit may be relevant to consider in nitrogenases during turnover.