The active sites of nitrogenase are made of unprecedented iron(molybdenum) sulfide clusters, namely P-cluster and FeMo-co, which have been long-standing challenging targets of chemical synthesis. In this presentation the structural/electronic properties of P-cluster inorganic core are discussed, based on our synthetic models, specifically addressing the following questions: 1) How rigid or flexible are the P-cluster core geometries? 2) What are the stable electronic configurations? We have developed new synthetic routes to iron sulfide clusters using non-polar solvents and bulky thiolates. Thus the [8Fe-7S] inorganic core of nitrogenase PN-cluster (1) was isolated from the reaction of Fe{N(SiMe3)2}2, tetramethylthiourea (tmtu), 2, 4, 6-tri-isopropylbenzenethiol (HS-tip), and S8. A better P-cluster model (2), where Tbt thiolates bridges iron atoms, has been synthesized from Fe{N(SiMe3)2}2, HS-tbt, and S8, via all ferric cuboidal Fe4S4(S-tbt)4! On the other hand, we have isolated a [8Fe-7S] cluster modeling the oxidized form of P-cluster (3), in which an Fe-S bond of the cluster core is cleaved. Interestingly, addition of tmtu to (3) facilitates formation of the Fe-S bond with the core (4) being reduced by one electron, resulting in the geometry of the reduced form of P-cluster (PN). Our model clusters demonstrate that the [8Fe-7S] inorganic core of P-cluster assume a variety of oxidation states, which are all higher than that of nitrogenase PN. This study also indicates that the P-cluster core can be generated from all-ferric [4Fe-4S] clusters, and that a desulfurization process must be a key to the transformation. Perhaps it is time to seek for new enzymes showing a yet-unknown desulfurization function.

Chemistry of the nitrogenase P-cluster: Structural and electronic flexibility

254th National Meeting (2017)

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