The FeMoco of nitrogenase is an iron-sulfur cluster with exceptional bond-reducing abilities. ENDOR studies have suggested that E4, the state that binds and reduces N2, contains bridging hydrides as part of the active-site iron-sulfide cluster. However, there are no examples of any isolable iron-sulfide cluster with a hydride, which would test the feasibility of such a species. Here, we describe a diiron sulfide hydride complex that is prepared using a mild method involving C-S cleavage of added thiolate. Its reactions with nitrogenase substrates show that the hydride can act as a base or nucleophile, and that reduction can cause the iron atoms to bind N2. These results add experimental support to hydride-based pathways for nitrogenase.

The use of hydride species for substrate reductions avoids strong reductants, and may enable nitrogenase to reduce multiple bonds without unreasonably low redox potentials. In this work, we explore the N=N bond cleaving ability of a high-spin iron(II) hydride dimer with concomitant release of H2. Specifically, this diiron(II) reacts with azobenzene (PhN=NPh) to perform the four-electron reduction to two imido groups, where two electrons come from H2 reductive elimination and the other two come from iron oxidation. The rate law of the H2 releasing reaction indicates that diazene binding occurs prior to H2 elimination, and the negative entropy of activation and inverse kinetic isotope effect indicate that H-H bond formation is the rate-limiting step. Thus, substrate binding causes reductive elimination of H2 that formally reduces the metals, and the metals use the additional two electrons to cleave the N-N multiple bond.

Small, coordinatively unsaturated iron-sulfur clusters are conceived as building blocks for the diverse set of shapes for iron-sulfur clusters in biological and synthetic chemistry. Here, we describe a synthetic method for preparing [2Fe-1S] clusters containing two iron(II) ions, which are supported by a relatively unhindered β-diketiminate supporting ligand. The [2Fe-1S] cluster can be isolated in the presence of trimethylphosphine, and the compound with one PMe 3 on each iron(II) ion is crystallographically characterized. The PMe 3 ligands may be removed with B(C 6 F 5) 3 to give a spectroscopically characterized species with solvent ligands. This species is a versatile synthon for [2Fe-2S], [4Fe-3S], and [10Fe-8S] clusters. Crystallographic characterization of the 10Fe cluster shows that it has all iron(II) ions, and the core has two [4Fe-4S] cubes that share a face in a novel arrangement. This cluster also has two iron sites that are coordinated to solvent donors, suggesting the potential for using this type of cluster for reactivity in the future.

Iron-sulfide complexes with hydride ligands provide an experimental precedent for spectroscopically detected hydride species on the iron-sulfur FeMoco of nitrogenase. In this contribution, we expand upon our recent synthesis of the first iron sulfide hydride complex from an iron hydride and a sodium thiolate (Arnet, N. A.; Dugan, T. R.; Menges, F. S.; Mercado, B. Q.; Brennessel, W. W.; Bill, E.; Johnson, M. A.; Holland, P. L., J. Am. Chem. Soc. 2015, 137, 13220–13223). First, we describe the isolation of an analogous iron sulfide hydride with a smaller diketiminate supporting ligand, which benefits from easier preparation of the hydride precursor and easier isolation of the product. Second, we describe mechanistic studies on the C-S bond cleavage through which the iron sulfide hydride product is formed. In a key experiment, use of cyclopropylmethanethiolate as the sulfur precursor leads to products from cyclopropane ring opening, implicating an alkyl radical as an intermediate. Combined with the results of isotopic labeling studies, the data are consistent with a mechanism in which homolytic C-S bond cleavage is followed by rebound of the alkyl radical to abstract a hydrogen atom from the iron to give the observed alkane and iron-sulfide products.

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