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

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 KC8 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 S3C 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 eta(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.