Daniel L. M. Suess

Imido complexes of high-spin 3d metals can feature N subvalency, wherein configurations with hole character at N contribute significantly to the ground state electronic structure. Reported herein are the synthesis and characterization of a series of Fe-S cluster terminal imido complexes isolated in four charge states, [(IMes)₃Fe₄S₄(NAr)]ⁿ (IMes, 1,3-dimesitylimidazol-2-ylidene; Ar = 2,4,6-tri-tert-butylphenyl; n = 1-, 0, 1+, 2+), in which the weak, trigonal ligand field of the sulfides enforces high-spin configurations at the Fe sites. It is shown that whereas the ground states of the anionic and neutral members of the series feature dominant imide ([NAr]^2-) character, the mono- and dicationic clusters have significant imidyl ([NAr]^•-) and/or triplet nitrene (³NAr) character. This was demonstrated experimentally using variable-temperature ¹H NMR spectroscopy to reveal the spin topology of the [Fe(NAr)] unit across the series. Additional structural, spectroscopic, and computational analysis revealed the extent to which electron density changes occur at the imido-bound Fe versus the peripheral Fe sites. These results establish the preference for high-spin Fe-imido sites in Fe-S clusters and the possibility for such sites to have multiconfigurational electronic structures with imide, imidyl, and/or nitrene character.

The Mo nitrogenase is more efficient for N₂ reduction than the alternative nitrogenases for reasons that are not fully understood. A set of related hypotheses center on the Mo ion facilitating electron delocalization, which could aid in substrate activation and/or provide enhanced stability during turnover, among other possibilities. To understand how the incorporation of Mo or other 'heterometals' influences the electronic structures of Fe-S clusters, we prepared and characterized an isostructural and isoelectronic series of [MFe₃S₄] clusters (M = V, Cr, Mo, W) that includes the first example of a [CrFe₃S₄] cluster. Comparison of the clusters' structural, spectroscopic, and computed properties reveals that the Mo- and W-containing clusters have greater M-Fe delocalization than those containing only 3d metals. Although the [CrFe₃S₄] cluster adopts the same ground spin state as the other clusters (S = 3/2), the smaller Cr atom engages in substantially attenuated─though not fully obliterated─M-Fe delocalization. Moreover, electrochemical and Mössbauer spectroscopic studies reveal that the clusters with more substantial M-Fe delocalization have more electron-rich Fe sites. Such trends, if they apply to the nitrogenase cofactors, could account for the greater efficiency of the Mo nitrogenase in biological nitrogen fixation.

Iron-sulfur proteins perform a wide variety of reactions central to the metabolisms of all living organisms. Foundational to their reaction chemistry are the rich electronic structures of their constituent Fe-S clusters, which differ in important ways from the active sites of mononuclear Fe enzymes. In this perspective, we summarize the essential electronic structure features that make Fe-S clusters unique, and point to the need for studies aimed at understanding the electronic basis for their reactivity under physiological conditions. Specifically, at ambient temperature, both the ground state and a large number of excited states are thermally populated, and thus a complete understanding of Fe-S cluster reactivity must take into account the properties, energies, and reactivity patterns of these excited states. We highlight prior research toward characterizing the low-energy excited states of Fe-S clusters that has established what is now a consensus model of these excited state manifolds and the bonding interactions that give rise to them. In particular, we discuss the low-energy alternate spin states and valence electron configurations that occur in Fe-S clusters of varying nuclearities, and finally suggest that there may be unrecognized functional roles for these states. Graphical abstract

The free-living diazotroph Azotobacter vinelandii produces three genetically distinct but functionally and mechanistically similar nitrogenase isozymes, designated as Mo-dependent, V-dependent, and Fe-only. They respectively harbor nearly identical catalytic cofactors that are distinguished by a heterometal site occupied by Mo (FeMo-cofactor), V (FeV-cofactor), or Fe (FeFe-cofactor). Completion of FeMo-cofactor and FeV-cofactor formation occurs on molecular scaffolds prior to delivery to their catalytic partners. In contrast, completion of FeFe-cofactor assembly occurs directly within its cognate catalytic partner. Because hybrid nitrogenase species that contain the incorrect cofactor type cannot reduce N ₂ to support diazotrophic growth there must be a way to prevent misincorporation of an incorrect cofactor when different nitrogenase isozyme systems are produced at the same time. Here, we show that fidelity of the Fe-only nitrogenase is preserved by blocking the misincorporation of either FeMo-cofactor or FeV-cofactor during its maturation. This protection is accomplished by a two-domain protein, designated AnfO. It is shown that the N-terminal domain of AnfO binds to an immature form of the Fe-only nitrogenase and the C-terminal domain, tethered to the N-terminal domain by a flexible linker, has the capacity to capture FeMo- and FeV-cofactor. AnfO does not prevent the normal activation of Fe-only nitrogenase because completion of FeFe-cofactor assembly occurs within its catalytic partner and, therefore, is never available for capture by AnfO. These results support a post-translational mechanism involving the molecular sorting of structurally similar metallocofactors that involve both protein-protein interactions and metallocofactor binding while exploiting differential pathways for nitrogenase associated catalytic cofactor assembly.

Although Fe-S clusters are privileged metallocofactors for the reduction of N2, CO, and other pi-acidic substrates, their constituent metal ions-high-spin Fe2+ and Fe3+-are typically not amenable to binding and activating strong pi-acids. Here, we demonstrate that [Fe4S4]+ clusters can overcome this limitation by adopting a noncanonical electronic structure. Specifically, we report the synthesis and characterization of a series of 3:1 site-differentiated [Fe4S4]+ clusters in which the unique Fe site is bound by one of 10 electronically variable arylisocyanide ligands. Rather than being continuously tuned as a function of the arylisocyanides' electronic properties (e.g., as quantified by linear free energy relationships), the structures of the clusters are divided into two groups: (i) those with moderately pi-acidic isocyanides, which adopt a "typical" structure characterized by standard bond metrics and geometric distortions from tetrahedral symmetry, and (ii) those with more strongly pi-acidic isocyanides, which adopt a "contracted" structure with an unusually symmetric geometry and a compressed cluster core. Computational studies revealed that although the "typical" structure has a canonical electronic structure, the "contracted" structure has a noncanonical arrangement of spin density, with a full complement of pi-backbonding electrons and more substantial Fe-Fe delocalization. These features of the "contracted" structure enable substantial C-N bond weakening of the strongest pi-acceptors in the series. More generally, the experimental characterization of the "contracted" electronic isomer suggests that other noncanonical electronic structures of Fe-S clusters remain to be discovered.

Attempts to generate open coordination sites for N₂ binding at synthetic Fe-S clusters often instead result in cluster oligomerization. Recently, it was shown for Mo-Fe-S clusters that such oligomerization reactions can be prevented through the use of sterically protective supporting ligands, thereby enabling N₂ complex formation. Here, this strategy is extended to Fe-only Fe-S clusters. One-electron reduction of (IMes)₃Fe₄S₄Cl (IMes = 1,3-dimesitylimidazol-2-ylidene) forms the transiently stable edge-bridged double cubane (IMes)₆Fe₈S₈, which loses two IMes ligands to form the face-bridged double-cubane, (IMes)₄Fe₈S₈. The finding that the three supporting IMes ligands do not confer sufficient protection to curtail cluster oligomerization prompted the design of a new N-heterocyclic carbene, SIAr^Me,iPr (1,3-bis(3,5-diisopropyl-2,6-dimethylphenyl)-2-imidazolidinylidene; abbreviated as SIAr), that features bulky groups strategically placed in remote positions. When the reduction of (SIAr)₃Fe₄S₄Cl or [(SIAr)₃Fe₄S₄(THF)]⁺ is conducted in the presence of SIAr, the formation of (SIAr)₄Fe₈S₈ is indeed suppressed, permitting characterization of the reduced [Fe₄S₄]⁰ product. Surprisingly, rather than being an N₂ complex, the product is simply (SIAr)₃Fe₄S₄: a cluster with a three-coordinate Fe site that adopts an unusually pyramidalized geometry. Although (SIAr)₃Fe₄S₄ does not coordinate N₂ to any appreciable extent under the surveyed conditions, it does bind CO to form (SIAr)₃Fe₄S₄(CO). This finding demonstates that the binding pocket at the unique Fe is not too small for N₂; instead, the exceptionally weak affinity for N₂ can be attributed to weak Fe-N₂ bonding. The differences in the N₂ coordination chemistry between sterically protected Mo-Fe-S clusters and Fe-only Fe-S clusters are discussed.