Comparative Isotropic Shifts, Redox Potentials, and Ligand Binding Propensities of [1:3] Site-Differentiated Cubane-Type [Fe₄Q₄]²⁺Clusters (Q = S, Se)

Abstract

The [1:3] site-differentiated cubane-type clusters [Fe4Q4(LS3)Cl]2- (Q = S, Se; LS3 = 1,3,5-tris((4,6-dimethyl-3-mercaptophenyl)thio)-2,4,6-tris(p-tolylthio)benzenate(3−)) undergo substitution reactions at the unique iron site with a variety of ligands including thiolates, phenolates, cyclic triamines and a trisulfide, imidazoles, and tertiary phosphines. Reactions are readily followed because of the extreme sensitivity of isotropically shifted resonances to the nature of ligand L‘ in the product clusters [Fe4Q4(LS3)L‘]2-,-. Isotropic shifts and redox potentials are reported for over 40 clusters, including many cluster pairs differing only in the core chalcogenide atom. In this way, comparative properties of sulfide and selenide clusters can be elicited. It is shown that the larger isotropic shifts consistently observed for selenide clusters and dominantly contact in nature arise from larger magnetic susceptibilities, which indicate a lesser extent of antiferromagnetic coupling. It is further demonstrated that, without exception, redox potentials of selenide clusters are more positive than those of sulfide clusters, usually by 20−60 mV, at parity of ligation. The difference in potentials of ca. 300 mV between the [Fe4S4]2+,+ couples of [Fe4S4(LS3)(SEt)]2- and [Fe4S4(LS3)(Im)]- (Im = imidazole) clusters is the best available estimate of the intrinsic potential difference between protein-bound [Fe4S4(Cys-S)4] and [Fe4S4(Cys-S)3(His-N)] clusters. The ligand LS3 undergoes spontaneous transfer between [Fe4S4]2+, [Fe4Se4]2+, and [MoFe3S4]3+ cores in reactions whose equilibrium constants are near the statistical value. The ligand binding affinity order PhS- > PhO- > CN- ≫ Cl- for [Fe4Q4]2+ cores was established. When taken together with earlier results from this laboratory, a comprehensive picture of ligand binding to single iron sites in [Fe4Q4]2+ clusters emerges.

Spectroscopic and Computational Investigations of Ligand Binding to IspH: Discovery of Non‐diphosphate Inhibitors

“…So, IspH, SoHydG (the 4Fe-4S cluster) as well as the (wild type) ferredoxin all appear to bind CN − to the 4 th Fe, forming a tetrahedral species. It should be noted, however, that CN − is actually a very poor IspH inhibitor (IC 50 >1 mM) and, as noted by Suess et al [25] , CN − binds only weakly to other biological [26] as well as synthetic [27] 4Fe-4S clusters, and cysteine displaces CN − from HydG [25] .…”

Synthetic analogues of [Fe 4 S 4 (Cys) 3 (His)] in hydrogenases and [Fe 4 S 4 (Cys) 4 ] in HiPIP derived from all-ferric [Fe 4 S 4 {N(SiMe 3 ) 2 } Ohki1, Tanifuji2, Yamada3 et al. 2011Proc. Natl. Acad. Sci. U.S.A.431290View full textAdd to dashboardCite

“…The generation of one unique iron site in a ½Fe 4 S 4 3þ cubane cluster by reduction of 1 is intriguing, and this reaction offers a new synthetic route to 3∶1-site differentiated ½Fe 4 S 4 clusters without invoking the use of tridentate thiolate auxiliaries (19,27,28 (29). Cluster 2 was structurally identified by X-ray crystallographic analysis.…”

N‐Coordinated [2Fe–2S] and [4Fe–4S] Clusters: Synthesis, Structures and Spectroscopic Characterisation

“…A synthetic N-coordinated [4Fe-4S] 2+ cluster with prolinato ligands has been characterised by 1 H NMR spectroscopy, but has not been isolated. [10] In addition there have been several approaches to site-differentiated clusters with one exchangeable ligand including imidazoles and other N-donor ligands, [11,12] yet no other fully N-coordinated [4Fe-4S] cluster has been reported so far, to the best of the authors' knowledge. Furthermore, there is only one other example of a bioinspired [4Fe-4S] cluster resents the first all-N-ligated mixed-valent [4Fe-4S] complex, rational syntheses for 3 as well as for 2 were then developed.…”