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

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.

Stepwise Construction of Mo–Fe–S Clusters Using a LEGO Strategy

“…All of these Mo−Fe−S clusters, together with their W analogs, were subjected to cyclic voltammetry studies to investigate their redox properties. Clusters 3, 5, 6, and 7 and their W analogs each exhibits one well-defined reversible redox process, as shown in Figure 3 35 This is indicative that carbene ligands terminally bound to Fe contribute more to the stability of the cubane cluster as compared to the halide ligands. It is also worthy mentioning that the reversibility of the redox processes for clusters 3, 5, 6, and 7 is better than that of their W analogs, indicating that Mo and W could contribute quite differently to the electrochemical properties of the M−Fe−S clusters (M = Mo or W).…”

“…One choice is the treatment of [(Tp*)-MoS(S 4 )] − with PPh 3 to deprive the additional S source to generate pyramidal ⟨MoS 3 ⟩ similar to the ⟨WS 3 ⟩ template frame. 35 Whereas we are looking at developing a strategy with full synthetic control such as playing with LEGO bricks (Figure 1), we made a second choice to build up the series of Mo−Fe−S clusters using a stepwise strategy. Instead of the direct synthesis of the halide-bridged incomplete and complete cubane clusters using multiple equivalents of an iron source such as that for the [(Tp*)WS 3 ] − precursor, only 1 equiv of FeCl 2 was introduced to react with [(Tp*)MoS(S 4 )] − , where a ⟨Fe⟩ brick was mounted onto the ⟨MoS 3 ⟩ template frame, resulting in the formation of a ⟨MoFeS 3 ⟩ cluster core in 2 (eq 1).…”

“…34 These synthetic strategies have very recently been applied to the analogous W/ Mo−Fe−S cluster system to build a series of carbene-ligated W/ Mo−Fe−S clusters incorporating 2p donors, including a carbyne donor at the core bridging site to mimic the FeMo-co partial structure. 35 With this rational progress to incorporate light atoms into Fe−S or M−Fe−S clusters, the synthetic iron− sulfur chemistry has been greatly advanced. However, rational synthetic strategies with full control of the reaction processes and production are still urgently needed to further explore the nitrogenase-related iron−sulfur cluster chemistry with respect to both structure-mimicking and mechanism studies of biological nitrogen fixation.…”

“…In our laboratory, we have recently developed controlled synthetic strategies named core ligand metathesis and core ligand redox metathesis, with which one can incorporate an N or O atom into the core of W–Fe–S clusters . These synthetic strategies have very recently been applied to the analogous W/Mo–Fe–S cluster system to build a series of carbene-ligated W/Mo–Fe–S clusters incorporating 2p donors, including a carbyne donor at the core bridging site to mimic the FeMo-co partial structure . With this rational progress to incorporate light atoms into Fe–S or M–Fe–S clusters, the synthetic iron–sulfur chemistry has been greatly advanced.…”

An Fe6C Core in All Nitrogenase Cofactors

“…As these states are reached under turnover condition, these studies imply a stabilizing function for the carbide. Meanwhile, electrochemical measurements on carbyne‐ligated complexes suggested that the carbide renders the cofactor more reducing, thereby potentially more reactive towards N 2 [12] …”