Partial synthetic models of FeMoco with sulfide and carbyne ligands: Effect of interstitial atom in nitrogenase active site



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Significance

Nitrogen, a common component of biomolecules, is sourced from abundant dinitrogen in the atmosphere through conversion to ammonia. Organisms capable of fixing N2 employ nitrogenases, metalloproteins that display metal-sulfide clusters that facilitate electron transfers and substrate activation. The site of N2 conversion to NH3 is FeMco (M = Mo, Fe, or V), a cluster of notable complexity in bioinorganic chemistry, which displays an unusual interstitial carbon ligand. The function of this bridging ligand remains unclear, and systematic structure-function studies with bridging C-donors are challenged by a lack of synthetic methods for analogous clusters. Herein, we report the first synthetic cluster that models a cubane moiety of FeMco bearing a chelating carbyne ligand and related structure-property studies.

Abstract

Nitrogen-fixing organisms perform dinitrogen reduction to ammonia at an Fe-M (M = Mo, Fe, or V) cofactor (FeMco) of nitrogenase. FeMco displays eight metal centers bridged by sulfides and a carbide having the MFe7S8C cluster composition. The role of the carbide ligand, a unique motif in protein active sites, remains poorly understood. Toward addressing how the carbon bridge affects the physical and chemical properties of the cluster, we isolated synthetic models of subsite MFe3S3C displaying sulfides and a chelating carbyne ligand. We developed synthetic protocols for structurally related clusters, [Tp*M’Fe3S3X]n−, where M’ = Mo or W, the bridging ligand X = CR, N, NR, S, and Tp* = Tris(3,5-dimethyl-1-pyrazolyl)hydroborate, to study the effects of the identity of the heterometal and the bridging X group on structure and electrochemistry. While the nature of M’ results in minor changes, the chelating, μ3-bridging carbyne has a large impact on reduction potentials, being up to 1 V more reducing compared to nonchelating N and S analogs.

Footnotes

    • Accepted September 29, 2021.
  • Author contributions: L.N.V.L., G.A.B., A.G.S., and T.A. designed research; L.N.V.L., G.A.B., and A.G.S. performed research; L.N.V.L., G.A.B., A.G.S., and T.A. analyzed data; L.N.V.L., G.A.B., A.G.S., and T.A. wrote the paper; and T.A. mentored researchers.

  • The authors declare no competing interest.

  • This article is a PNAS Direct Submission.

  • This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2109241118/-/DCSupplemental.

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