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 N2 binding and reduction in nature. We recently reported the first
example of N2 binding at a mononuclear iron site supported
by only S and C donors. In this work, we report experiments that examine
the mechanism of N2 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
Mössbauer, 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 η6-arene
species which reacts with N2. Thus, in this system the
loss of thiolate and binding of N2 require reduction beyond
the iron­(I) level to the iron(0) level. Further reduction of the iron(0)–N2 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 N2 at a synthetic
Fe–S–C site. Catalytic N2 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.