Contributed by Richard H. Holm, March 5, 2018 (sent for review January 26, 2018; reviewed by Marcetta Y. Darensbourg, Douglas C. Rees, and Markus W. Ribbe)
The biosynthesis and mechanism of action of nitrogenase, an enzyme that converts dinitrogen to ammonia under ambient conditions, are problems of prominent significance in metallobiochemistry. Because the active centers of the enzyme are metal–ligand clusters, it is feasible that they are attainable by synthesis and as such are primary goals in the field of biomimetic inorganic chemistry. Here we present a ligand metathesis strategy utilizing the periodic near-identity of molybdenum and tungsten when incorporated into analogous compounds. The approach provides a pathway for constructing heterometal heteroleptic Fe–S clusters of presumed relevance to the active site. Based on cubane-type stereochemistry, clusters have been prepared allowing alterations in structure and ligand binding, and inclusion of a light core atom.
Molybdenum-dependent nitrogenases catalyze the transformation of dinitrogen into ammonia under ambient conditions. The active site (FeMo cofactor) is the structurally and electronically complex weak-field metal cluster [MoFe7S9C] built of Fe4S3 and MoFe3S3C portions connected by three sulfur bridges and containing an interstitial carbon atom centered in an Fe6 trigonal prism. Chemical synthesis of this cluster is a major challenge in biomimetic inorganic chemistry. One synthetic approach of core ligand metathesis has been developed based on the design and synthesis of unprecedented incomplete ([(Tp*)WFe2S3Q3]−) and complete ([(Tp*)WFe3S3Q4]2−) cubane-type clusters containing bridging halide (Q = halide). These clusters are achieved by template-assisted assembly in the presence of sodium benzophenone ketyl reductant; products are controlled by reaction stoichiometry. Incomplete cubane clusters are subject to a variety of metathesis reactions resulting in substitution of a μ2-bridging ligand with other bridges such as N3−, MeO−, and EtS−. Reactions of complete cubanes with Me3SiN3 and S8 undergo a redox metathesis process and lead to core ligand displacement and formation of [(Tp*)WFe3S3(μ3-Q)Cl3]− (Q = Me3SiN2−, S2−). This work affords entry to a wide variety of heteroleptic clusters derivable from incomplete and complete cubanes; examples are provided. Among these is the cluster [(Tp*)WFe3S3(μ3-NSiMe3)Cl3]−, one of the very few instances of a synthetic Fe–S cluster containing a light atom (C, N, O) in the core, which constitutes a close mimic of the [MoFe3S3C] fragment in FeMo cofactor. Superposition of them and comparison of metric information disclose a clear structural relationship [Tp* = tris(3,5-dimethyl-1-pyrazolyl)hydroborate(1−)].
Author contributions: X.-D.C. and R.H.H. designed research; G.X., Z.W., R.L., J.Z., and X.-D.C. performed research; G.X. and X.-D.C. analyzed data; and G.X., X.-D.C., and R.H.H. wrote the paper.
Reviewers: M.Y.D., Texas A&M University; D.C.R., Howard Hughes Medical Institute and California Institute of Technology; and M.W.R., University of California, Irvine.
The authors declare no conflict of interest.
Data deposition: The atomic coordinates and structure factors have been deposited in the Cambridge Structural Database (CSD) of the Cambridge Crystallographic Data Centre (CCDC), https://www.ccdc.cam.ac.uk/structures/ [CSD reference nos. 1587148 (2a), 1587149 (2b), 1587150 (3a), 1587151 (3b), 1587152 (5), 1587153 (4), and 1587154 (7)].
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1801025115/-/DCSupplemental.
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