Nitrogenase-Relevant Reactivity of a Synthetic Iron–Sulfur–Carbon Site

Abstract

Simple synthetic compounds with only S and C donors offer a ligation environment similar to the active site of nitrogenase (FeMoco) and thus demonstrate reasonable mechanisms and geometries for N 2 binding and reduction in nature. We recently reported the first example of N 2 binding at a mononuclear iron site supported by only S and C donors. In this work, we report experiments that examine the mechanism of N 2 binding in this system. The reduction of an iron(II) tris(thiolate) complex with 1 equiv of KC 8 leads to a thermally unstable intermediate, and a combination of Mossbauer, EPR, and X-ray absorption spectroscopies identifies it as a high-spin (S = 3/2) iron(I) species that maintains coordination of all three sulfur atoms. DFT calculations suggest that this iron(I) intermediate has a pseudotetrahedral geometry that resembles the S 3 C iron coordination environment of the belt iron sites in the resting state of the FeMoco. Further reduction to the iron(0) oxidation level under argon causes the dissociation of one of the thiolate donors and gives an η 6 -arene species which reacts with N 2 . Thus, in this system the loss of thiolate and binding of N 2 require reduction beyond the iron(I) level to the iron(0) level. Further reduction of the iron(0)−N 2 complex gives a reactive, formally iron(−I) species. Treatment of the putative iron(−I) complex with weak acids gives low yields of ammonia and hydrazine, demonstrating that these nitrogenase products can be generated from N 2 at a synthetic Fe−S−C site. Catalytic N 2 reduction is not observed, which is attributed to protonation of the supporting ligand and degradation of the complex via ligand dissociation. Identification of the challenges in this system gives insight into the design features needed for functional biomimetic complexes.

Ultra‐High Initial Coulombic Efficiency Induced by Interface Engineering Enables Rapid, Stable Sodium Storage

“…Lower I D /I G ratio of FeS 2 /N,S-C (1.03) than that of N,S-C (1.09) in Raman spectrum also represents fewer defects of FeS 2 /N,S-C (Figure 2c). [21] Above results suggest that FeS 2 and defects are intimately linked together (Figure 2d). [22,23] In addition, the content of FeS 2 in FeS 2 /N,S-C (48.60 wt %) is obtained from Thermal gravimetric analysis result (Figure S11).…”

Cluster Models of FeMoco with Sulfide and Carbyne Ligands: Effect of Interstitial Atom in Nitrogenase Active Site

“…This parallels the results from experiments using synthetic iron catalysts for N2 reduction, many of which require strong external reducing agents like KC8. 8,53 Considering the potential impact of replacing the bridging carbyne with a more biologically inexpensive sulfide, a much less reducing cluster ( 8) is generated for the same redox state, (MFe3) 10+ , and even an additional reducing equivalent in (MFe3) 9+ does not match with the (MFe3) 10+ carbyne system. Therefore, an interstitial sulfide may not provide sufficient reducing power to efficiently convert N2 into NH3, leading instead to the preference for the unusual bridging carbide motif.…”

Structure, reactivity, and spectroscopy of nitrogenase-related synthetic and biological clusters

“…2) which modelled the Fe-S/C coordination in FeMo-cofactor. 58,59 The group focused on the hypothesis that one of the Fe-S bonds dissociates upon reduction/protonation to provide an open coordination at the iron site for substrate binding. 37,60 [L(Fe)-N 2 ] 2À could be synthesized by reduction of [L(Fe)-SAr*] À (compound 5, Fig.…”