Oxidized and reduced [2Fe-2S] clusters from an iron(I) synthon

[2Fe-2S] model compounds

This feature article reviews the synthesis, structural comparison, and physical properties of [2Fe-2S] model compounds, which serve as vital tools for understanding the structure and function of Fe-S clusters in biological systems. We explore various synthetic methods for constructing [2Fe-2S] cores, offering insights into their biomimetic relevance. A comprehensive analysis and comparison of Mössbauer spectroscopy data between model compounds and natural protein systems are provided, highlighting the structural and electronic parallels. Additionally, we discuss the redox potentials of synthetic [2Fe-2S] compounds, their deviation from biological systems, and potential strategies to align them with natural counterparts. The review concludes with a discussion of future research directions, particularly the development of models capable of mimicking biological processes such as catalysis and electron transfer reactions. This article serves as a valuable resource for researchers in inorganic chemistry, bioinorganic chemistry, biochemistry, and related fields, offering both fundamental insights and potential applications of [2Fe-2S] clusters.

Valence Delocalization and Metal-Metal Bonding in Carbon-Bridged Mixed-Valence Iron Complexes

The carbide ligand in the iron-molybdenum cofactor (FeMoco) in nitrogenase bridges iron atoms in different oxidation states, yet it is difficult to discern its ability to mediate magnetic exchange interactions due to the structural complexity of the cofactor. Here, we describe two mixed-valent diiron complexes with C-based ketenylidene bridging ligands, and compare the carbon bridges with the more familiar sulfur bridges. The ground state of the [Fe2 (μ-CCO)2 ]+ complex with two carbon bridges (4) is S=1 / 2 ${{ 1/2 }}$ , and it is valence delocalized on the Mössbauer timescale with a small thermal barrier for electron hopping that stems from the low Fe-C force constant. In contrast, one-electron reduction of the [Fe2 (μ-CCO)] complex with one carbon bridge (2) affords a mixed-valence species with a high-spin ground state (S=7 / 2 ${ 7/2 }$ ), and the Fe-Fe distance contracts by 1 Å. Spectroscopic, magnetic, and computational studies of the latter reveal an Fe-Fe bonding interaction that leads to complete valence delocalization. Analysis of near-IR intervalence charge transfer transitions in 5 indicates a very large double exchange constant (B) in the range of 780-965 cm-1 . These results show that carbon bridges are extremely effective at stabilizing valence delocalized ground states in mixed-valent iron dimers.

Mössbauer and Nuclear Resonance Vibrational Spectroscopy Studies of Iron Species Involved in N-N Bond Cleavage

Diketiminate-supported iron complexes are capable of cleaving the strong triple bond of N2 to give a tetra-iron complex with two nitrides (Rodriguez et al., Science, 2011, 334, 780-783). The mechanism of this reaction has been difficult to determine, but a transient green species was observed during the reaction that corresponds to a potential intermediate. Here, we describe studies aiming to identify the characteristics of this intermediate, using a range of spectroscopic techniques, including Mössbauer spectroscopy, electronic absorption spectroscopy, Raman spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and nuclear resonance vibrational spectroscopy (NRVS) complemented by density functional theory (DFT) calculations. We successfully elucidated the nature of the starting iron(II) species and the bis(nitride) species in THF solution, and in each case, THF breaks up the multiiron species. Various observations on the green intermediate species indicate that it has one N2 per two Fe atoms, has THF associated with it, and has NRVS features indicative of bridging N2. Computational models with a formally diiron(0)-N2 core are most consistent with the accumulated data, and on this basis, a mechanism for N2 splitting is suggested. This work shows the power of combining NRVS, Mössbauer, NMR, and vibrational spectroscopies with computations for revealing the nature of transient iron species during N2 cleavage.

Stabilization of intermediate spin states in mixed-valent diiron dichalcogenide complexes

The electronic structure and ground spin states, S, observed for mixed-valent iron–sulfur dimers (Fe^II-Fe^III) are typically determined by the Heisenberg exchange interaction, J, that couples the magnetic interaction of the two metal centres either ferromagnetically (J > 0, S = 9/2) or antiferromagnetically (J < 0, S = 1/2). In the case of antiferromagnetically coupled iron centres, stabilization of the high-spin S = 9/2 ground state is also feasible through a Heisenberg double-exchange interaction, B, which lifts the degeneracy of the Heisenberg spin states. This theorem also predicts intermediate spin states for mixed-valent dimers, but those have so far remained elusive. Herein, we describe the structural, electron paramagnetic resonance and Mössbauer spectroscopic, and magnetic characterization of a series of mixed-valent complexes featuring [Fe₂Q₂]⁺ (Q = S^2–, Se^2–, Te^2–), where the Se and Te complexes favour S = 3/2 spin states. The incorporation of heavier chalcogenides in this series reveals a delicate balance of antiferromagnetic coupling, Heisenberg double-exchange and vibronic coupling.

Despite extensive investigations of mixed-valence complexes, molecules with intermediate spin states have remained elusive. Now, selenium- and tellurium-bridged mixed-valent iron dimers have been prepared in which a balance of Heisenberg exchange and double-exchange coupling of the unpaired electron, combined with moderate vibronic contributions, stabilizes S = 3/2 ground spin states.

[2Fe-2S] Cluster Supported by Redox-Active o-Phenylenediamide Ligands and Its Application toward Dinitrogen Reduction

As prevalent cofactors in living organisms, iron-sulfur clusters participate in not only the electron-transfer processes but also the biosynthesis of other cofactors. Many synthetic iron-sulfur clusters have been used in model studies, aiming to mimic their biological functions and to gain mechanistic insight into the related biological systems. The smallest [2Fe-2S] clusters are typically used for one-electron processes because of their limited capacity. Our group is interested in functionalizing small iron-sulfur clusters with redox-active ligands to enhance their electron storage capacity, because such functionalized clusters can potentially mediate multielectron chemical transformations. Herein we report the synthesis, structural characterization, and catalytic activity of a diferric [2Fe-2S] cluster functionalized with two o-phenylenediamide ligands. The electrochemical and chemical reductions of such a cluster revealed rich redox chemistry. The functionalized diferric cluster can store up to four electrons reversibly, where the first two reduction events are ligand-based and the remainder metal-based. The diferric [2Fe-2S] cluster displays catalytic activity toward silylation of dinitrogen, affording up to 88 equiv of the amine product per iron center.

A Molecular Low-Coordinate [Fe-S-Fe] Unit in Three Oxidation States

A [Fe-S-Fe] subunit with a single sulfide bridging two low-coordinate iron ions is the supposed active site of the iron-molybdenum co-factor (FeMoco) of nitrogenase. Here we report a dinuclear monosulfido bridged diiron(II) complex with a similar complex geometry that can be oxidized stepwise to diiron(II/III) and diiron(III/III) complexes while retaining the [Fe-S-Fe] core. The series of complexes has been characterized crystallographically, and electronic structures have been studied using, inter alia, 57 Fe Mössbauer spectroscopy and SQUID magnetometry. Further, cleavage of the [Fe-S-Fe] unit by CS2 is presented.

Cleavage of cluster iron-sulfide bonds in cyclophane-coordinated FenSm complexes

Reaction of the tri(μ-sulfido)triiron(iii) tris(β-diketiminate) cyclophane complex, Fe3S3LEt/Me (1), or of the di(μ-sulfido)diiron(iii) complex Fe2S2HLEt/Me (5), with the related tri(bromide)triiron(ii) complex Fe3Br3LEt/Me (2) results in electron and ligand redistribution to yield the mixed-ligand multiiron complexes, including Fe3Br2SLEt/Me (3) and Fe2Br2SHLEt/Me (4). The cleavage and redistribution observed in these complexes is reminiscent of necessary Fe-S bond cleavage for substrate activation in nitrogenase enzymes, and provides a new perspective on the lability of Fe-S bonds in FeS clusters.

The influences of carbon donor ligands on biomimetic multi-iron complexes for N2 reduction

The active site clusters of nitrogenase enzymes possess the only examples of carbides in biology. These are the only biological FeS clusters that are capable of reducing N₂ to NH₄⁺, implicating the central carbon and its interaction with Fe as important in the mechanism of N₂ reduction. This biological question motivates study of the influence of carbon donors on the electronic structure and reactivity of unsaturated, high-spin iron centers. Here, we present functional and structural models that test the impacts of carbon donors and sulfide donors in simpler iron compounds. We report the first example of a diiron complex that is bridged by an alkylidene and a sulfide, which serves as a high-fidelity structural and spectroscopic model of a two-iron portion of the active-site cluster (FeMoco) in the resting state of Mo-nitrogenase. The model complexes have antiferromagnetically coupled pairs of high-spin iron centers, and sulfur K-edge X-ray absorption spectroscopy shows comparable covalency of the sulfide for C and S bridged species. The sulfur-bridged compound does not interact with N₂ even upon reduction, but upon removal of the sulfide it becomes capable of reducing N₂ to NH₄⁺ with the addition of protons and electrons. This provides synthetic support for sulfide extrusion in the activation of nitrogenase cofactors.

High-spin diiron alkylidenes give insight into the electronic structure and functional relevance of carbon in the FeMoco active site of nitrogenase.

A [2Fe-1S] Complex That Affords Access to Bimetallic and Higher-Nuclearity Iron-Sulfur Clusters

Small, coordinatively unsaturated iron-sulfur clusters are conceived as building blocks for the diverse set of shapes of iron-sulfur clusters in biological and synthetic chemistry. Here we describe a synthetic method for preparing [2Fe-1S] clusters containing two iron(II) ions, which are supported by a relatively unhindered β-diketiminate supporting ligand. The [2Fe-1S] cluster can be isolated in the presence of trimethylphosphine, and the compound with one PMe₃ on each iron(II) ion has been crystallographically characterized. The PMe₃ ligands may be removed with B(C₆F₅)₃ to give a spectroscopically characterized species with solvent ligands. This species is a versatile synthon for [2Fe-2S], [4Fe-3S], and [10Fe-8S] clusters. Crystallographic characterization of the 10Fe cluster shows that it has all iron(II) ions, and the core has two [4Fe-4S] cubes that share a face in a novel arrangement. This cluster also has two iron sites that are coordinated to solvent donors, suggesting the potential for using this type of cluster for reactivity in the future.

Density Functional Calculations for Prediction of 57Fe Mössbauer Isomer Shifts and Quadrupole Splittings in β-Diketiminate Complexes

The relative ease of Mössbauer spectroscopy and of density functional theory (DFT) calculations encourages the use of Mössbauer parameters as a validation method for calculations, and the use of calculations as a double check on crystallographic structures. A number of studies have proposed correlations between the computationally determined electron density at the iron nucleus and the observed isomer shift, but deviations from these correlations in low-valent iron β-diketiminate complexes encouraged us to determine a new correlation for these compounds. The use of B3LYP/def2-TZVP in the ORCA platform provides an excellent balance of accuracy and speed. We provide here not only this new correlation and a clear guide to its use but also a systematic analysis of the limitations of this approach. We also highlight the impact of crystallographic inaccuracies, DFT model truncation, and spin states, with intent to assist experimentalists to use Mössbauer spectroscopy and calculations together.

Reactivity studies on [Cp'Fe(μ-I)]2: nitrido-, sulfido- and diselenide iron complexes derived from pseudohalide activation

Facile pseudohalide activation occurs in the reaction of SCN^–, SeCN^– and N₃^– with the iron half-sandwich [Cp′Fe(μ-I)]₂.

The iron half-sandwich [Cp′Fe(μ-I)]₂ (Cp′ = 1,2,4-(Me₃C)₃C₅H₂, 1) reacts with the pseudohalides NCO^–, SCN^–, SeCN^– and N₃^– to give [Cp′Fe(μ-NCO)]₂ (2), [Cp′Fe(μ-S)]₂ (3), [Cp′Fe(μ-Se₂)]₂ (4) and [Cp′Fe(μ-N)]₂ (5), respectively. Various spectroscopic techniques including X-ray diffraction, solid-state magnetic susceptibility studies and ⁵⁷Fe Mössbauer spectroscopy were employed in the characterization of these species. Mössbauer spectroscopy shows a decreasing isomer shift with increasing formal oxidation state, ranging from Fe(ii) to Fe(iv), in complexes 1 to 5. The sulfido-bridged dimer 3 exhibits strong antiferromagnetic coupling between the Fe(iii) centers. This leads to temperature-independent paramagnetism (TIP) at low temperature, from which the energy gap between the ground and the excited state can be estimated to be 2J = ca. 700 cm^–1. The iron(iv) nitrido complex [Cp′Fe(μ-N)]₂ (5) shows no reactivity towards H₂ (10 atm), but undergoes clean reactions with CO (5 bar) and XylNC (Xyl = 2,6-Me₂C₆H₃) to form the diamagnetic isocyanate and carbodiimide complexes [Cp′Fe(CO)₂(NCO)] (7) and [Cp′Fe(CNXyl)₂(NCNXyl)] (8), respectively. All compounds were fully characterized, and density functional theory (DFT) computations provide useful insights into their formation and the electronic structures of complexes 3 and 5.

Diazoalkanes in Low-Coordinate Iron Chemistry: Bimetallic Diazoalkyl and Alkylidene Complexes of Iron(II)

The addition of (trimethylsilyl)diazomethane and its conjugate base to iron β-diketiminate precursors gives novel dinuclear complexes in which the bridges are either diazomethane derivatives or an alkylidene. One product is an unusual bridging alkylidene complex containing two three-coordinate iron(II) centers. On the other hand, syntheses using the deprotonated diazomethane give two bridging diazomethyl species with binding modes that have not been observed in iron complexes previously. In the presence of a coordinating tetrahydrofuran solvent, a diiron(II) compound with μ-N bridges rearranges to a more stable isomer with μ-N,C bridges, a process that is accompanied by a 1,3-shift of a silyl group.