Chemical Synthesis of an Asymmetric Mimic of the Nitrogenase Active Site

The synthetic inorganic chemistry of metal-sulfur (M-S, M = metals) clusters has played an important, complementary role to the biochemical analyses of nitrogenase toward a better understanding of the enzyme active site. The active site of nitrogenase (designated the M-cluster) can be extracted from the protein in a solvent-stabilized form, [(cit)MoFe₇S₉C] (cit = (R)-homocitrate). One important finding of the extracted M-cluster is its catalytic activity toward the reduction of C₁-substrates (CN^-, CO, CO₂) into C₁-C₅ hydrocarbons in solution. This catalytic property poses challenges for chemists to reproduce the function with synthetic mimics, not only because of the biochemical interests but also due to the potential significance in green chemistry and catalysis research. In this context, our successful synthesis of an asymmetric Mo-Fe-S cluster, [Cp*MoFe₅S₉(SH)]^3-, is one of the recent important achievements in synthetic M-S chemistry, as this cluster catalyzes the reduction of C₁-substrates in a similar manner to the extracted M-cluster. Even though the synthetic protocol for this cluster has been described in the literature, there are plenty of pitfalls for researchers unfamiliar with synthetic M-S chemistry. In this chapter, we provide general precautionary statements and detailed protocols for the synthesis of [Cp*MoFe₅S₉(SH)]^3-, with a brief discussion of the experimental tips based on the authors' experience in both biochemical and synthetic chemical fields.