A 10(6)-Fold Enhancement in N(2)-Binding Affinity of an Fe(2)(μ-H)(2) Core upon Reduction to a Mixed-Valence Fe(II)Fe(I) State

[Image: see text] 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 N(2) to NH(3) is unknown. One role of these ligands may be to facilitate N(2) 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 N(2) depending on the redox state of the Fe(2)(μ-H)(2) unit. An unusual example of a mixed-valent Fe(II)(μ-H)(2)Fe(I) is described that displays a 10(6)-fold enhancement of N(2) 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 NH(3) are produced upon exposure to proton and electron equivalents. The Fe(μ-H)Fe(N(2)) sub-structure featured herein was previously unknown. This subunit may be relevant to consider in nitrogenases during turnover.