A 10‐Fold Enhancement in N2‐Binding Affinity of an Fe2(μ-H)2 Core upon Reduction to a Mixed-Valence FeFe 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 Fe (μH)2Fe I is described that displays a 10-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(μH)Fe(N2) sub-structure featured herein was previously unknown. This subunit may be relevant to consider in nitrogenases during turnover. T intimate mechanism of biological nitrogen fixation at the FeMo-cofactor (FeMoco) of nitrogenase enzymes is a fascinating, unsolved problem. One or two of the central iron atoms of the FeMoco have been highlighted as plausible N2 binding sites. Yet, despite a supporting body of biochemical and spectroscopic data, in addition to synthetic Fe−N2 model chemistry establishing that N2 reduction to NH3 is possible at an iron center, there remains the question as to how an iron site (or sites) that resides within a sulfide-rich environment would facilitate N2 binding. To date, there is little synthetic precedent for Fe−N2 complexes that feature sulfur within the immediate iron coordination sphere, and there are no synthetic Fe−N2 species where only sulfur ligates the iron center. An interesting possibility to consider is whether iron hydrides, either terminal or bridging, might facilitate better N2 binding at an iron site of FeMoco by increasing its πbasicity, owing to the relatively strong ligand field exerted by a hydride. Hydride ligands could be installed under the electronloading phase, prior to N2 uptake, a phase that is presumably leveled by the concomitant delivery of protons to iron and/or sulfur sites. It is noteworthy in this context that a bridging hydride form of the FeMoco (i.e., Fe(μ-H)Fe) has been proposed based on spectroscopic data under turnover conditions. While elimination of H2 upon N2 binding has been suggested as a way to rationalize presumed obligatory H2 elimination during N2 reduction to two equiv NH3, 11,12 an additional possibility to consider is that cofactor bound hydrides afford a sufficiently covalent Fe−N2 interaction to render N2 binding and reduction at an iron site (or sites) favorable. Because bridging rather than terminal hydrides have been implicated as observable intermediates, we sought to prepare Fe(μ-H)Fe model complexes that have an affinity for N2. Terminal Fe−H complexes are now well established to bind N2, 13,14 even with sulfur in the immediate iron coordination sphere, but to our knowledge there are no synthetic Fe−N2 complexes featuring hydrides bridging another Fe center. This situation exists despite the fact that there are hundreds of crystallographically characterized complexes featuring bridging hydride Fe(μ-H)Fe subunits. Herein we present a new binucleating scaffold that accommodates two bridging hydrides to afford a Fe(μ-H)2Fe subunit. This structural motif is shown to be compatible with N2 binding and reductive protonation to release a substoichiometric but nevertheless substantial amount of NH3. In addition, reversible coordination of N2 to the second iron site is observed, and the N2 binding affinity is shown to increase by 6 orders of magnitude upon further reduction of the N2−Fe(μH)2Fe subunit by one electron (Scheme 1). The S = 1/2 form, {N2−Fe(μ-H)2Fe−N2}, shows strong hyperfine coupling through the bridging hydride ligands. ■ RESULTS Synthesis and Structure of 3-N2. To support an unsaturated Fe(μ-H)Fe unit, we synthesized the bulky hexadentate ligand, [SiP2O]H2 (2). Compound 2 is synthesized by the controlled hydrolysis of previously reported bis(oReceived: July 21, 2014 Published: September 3, 2014 Article