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

Abstract

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)^2Fe^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(N2_) sub-structure featured herein was previously unknown. This subunit may be relevant to consider in nitrogenases during turnover.