[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.

Model of the MitoNEET [2Fe-2S] Cluster Shows Proton Coupled Electron Transfer

MitoNEET is an outer membrane protein whose exact function remains unclear, though a role of this protein in redox and iron sensing as well as in controlling maximum mitochondrial respiratory rates has been discussed. It was shown to contain a redox active and acid labile [2Fe-2S] cluster which is ligated by one histidine and three cysteine residues. Herein we present the first synthetic analogue with biomimetic {SN/S2} ligation which could be structurally characterized in its diferric form, 52-. In addition to being a high fidelity structural model for the biological cofactor, the complex is shown to mediate proton coupled electron transfer (PCET) at the {SN} ligated site, pointing at a potential functional role of the enzyme's unique His ligand. Full PCET thermodynamic square schemes for the mitoNEET model 52- and a related homoleptic {SN/SN} capped [2Fe-2S] cluster 42- are established, and kinetics of PCET reactivity are investigated by double-mixing stopped-flow experiments for both complexes. While the N-H bond dissociation free energy (BDFE) of 5H2- (230 ± 4 kJ mol-1) and the free energy ΔG°PCET for the reaction with TEMPO (-48.4 kJ mol-1) are very similar to values for the homoleptic cluster 4H2- (232 ± 4 kJ mol-1, -46.3 kJ mol-1) the latter is found to react significantly faster than the mitoNEET model (data for 5H2-: k = 135 ± 27 M-1 s-1, ΔH‡ = 17.6 ± 3.0 kJ mol-1, ΔS‡ = -143 ± 11 J mol-1 K-1, and ΔG‡ = 59.8 kJ mol-1 at 293 K). Comparison of the PCET efficiency of these clusters emphasizes the relevance of reorganization energy in this process.

Electron transfer and proton-coupled electron transfer reactivity and self-exchange of synthetic [2Fe-2S] complexes: models for Rieske and mitoNEET clusters

This report describes the thermochemistry, proton-coupled electron transfer (PCET) reactions and self-exchange rate constants for a set of bis-benzimidazolate-ligated [2Fe–2S] clusters. These clusters serve as a model for the chemistry of biological Rieske and mitoNEET clusters. PCET from [Fe₂S₂(^Prbbim)(^PrbbimH)]^2– (4) and [Fe₂S₂(^Prbbim)(^PrbbimH₂)]^1– (5) to TEMPO occurs via concerted proton–electron transfer (CPET) mechanisms (^PrbbimH₂ = 4,4-bis-(benzimidazol-2-yl)heptane). Intermolecular electron transfer (ET) self-exchange between [Fe₂S₂(^Prbbim)₂]^2– (1) and [Fe₂S₂(^Prbbim)₂]^3– (2) occurs with a rate constant of (1.20 ± 0.06) × 10⁵ M^–1 s^–1 at 26 °C. A similar self-exchange rate constant is found for the related [2Fe–2S] cluster [Fe₂S₂(SArO)₂]^2–/3–, SArO^2– = thiosalicylate. These are roughly an order of magnitude slower than that reported for larger [4Fe–4S] clusters and 1 order of magnitude faster than that reported for N-ligated high-spin iron complexes. These results suggest that the rate of intermolecular ET to/from [Fe–S] clusters is modulated by cluster size. The measured PCET self-exchange rate constant for 1 and 4 at −30 °C is (3.8 ± 0.7) × 10⁴ M^–1 s^–1. Analysis of rate constants using the Marcus cross-relation suggests that this process likely occurs via a concerted proton–electron transfer (CPET) mechanism. The implications of these findings to biological systems are also discussed, including the conclusion that histidine-ligated [2Fe–2S] clusters should not have a strong bias to undergo concerted e^–/H⁺ transfers.

[Fe₂S₂(^Prbbim)(^PrbbimH_x)]^y- clusters have been generated in multiple redox and protonation states. Their PCET and ET thermochemistry and reactivity are described. The PCET self-exchange reaction occurs by concerted e⁻/H⁺ exchange, and the ET self-exchange barriers for different clusters are shown to scale with [Fe−S] cluster size. The implications of these results for the reactivity of biochemical imidazole-ligated clusters is discussed.

Decomposition of a Mixed-Valence [2Fe-2S] Cluster to Linear Tetra-Ferric and Ferrous Clusters

Despite the ease of preparing di-ferric [2Fe-2S] clusters, preparing stable mixed-valence analogues remains a challenge, as these clusters have limited thermal stability. Herein we identify two decomposition products of the mixed-valence thiosalicylate-ligated [2Fe-2S] cluster, [Fe₂S₂(SArCOO)₂]^3- ((SArCOO)^2- = thiosalicylate).

Protonation and concerted proton-electron transfer reactivity of a bis-benzimidazolate ligated [2Fe-2S] model for Rieske clusters

A model system for biological Rieske clusters that incorporates bis-benzimidazolate ligands ((Pr)bbim)(2-) has been developed ((Pr)bbimH(2) = 4,4-bis(benzimidazol-2-yl)heptane). The diferric and mixed-valence clusters have been prepared and characterized in both their protonated and deprotonated states. The thermochemistry of interconversions of these species has been measured, and the effect of protonation on the reduction potential is in good agreement to that observed in the biological systems. The mixed-valence and protonated congener [Fe(2)S(2)((Pr)bbim)((Pr)bbimH)](Et(4)N)(2) (4) reacts rapidly with TEMPO or p-benzoquinones to generate diferric and deprotonated [Fe(2)S(2)((Pr)bbim)(2)](Et(4)N)(2) (1) and 1 equiv of TEMPOH or 0.5 equiv of p-benzohydroquinones, respectively. The reaction with TEMPO is the first well-defined example of concerted proton-electron transfer (CPET) at a synthetic ferric/ferrous [Fe-S] cluster.

Relatively stable N-ligated [2Fe2S]2+ clusters with dipyrromethane capping ligands

[2Fe2S] clusters with terminal N-ligation (His, Arg) and unique functions are increasingly recognized in biological systems. In this work three new [2Fe2S] clusters 1-3 with different 1,1'-dipyrrolmethane derivatives as chelating terminal ligands have been prepared and fully characterized, including by X-ray crystallography: (NEt(4))(2)[L(2)Fe(2)(mu-S)(2)] with L=Me(2)C(C(4)H(3)N)(2) (1), Ph(2)C(C(4)H(3)N)(2) (2), (CH(2))(5)C(C(4)H(3)N)(2) (3). These systems represent rare examples of synthetic [2Fe2S] complexes with N-donor capping ligands. While geometric parameters as well as spectroscopic and electrochemical characteristics of the new complexes are as anticipated, the chelating nature of the terminal ligands in 1-3 imparts a relatively high stability that will be advantageous for reactivity studies of the [2Fe2S] core.

Identification of amino acid residues associated with the [2Fe-2S] cluster of the 25 kDa (NQO2) subunit of the proton-translocating NADH-quinone oxidoreductase of Paracoccus denitrificans

In order to identify the ligand residues of the [2Fe-2S] cluster of the 25 kDa (NQO2) subunit of the proton-translocating NADH-quinone oxidoreductase of Paracoccus denitrificans, we mutated individually all seven cysteine residues (C61, C96, C101, C104, C113, C137, and C141) and one conserved histidine residue (H92) to Ser or Ala and expressed them in E. coli. After purification of the mutated 25 kDa subunits, the effect of mutations on the iron-sulfur cluster were characterized by chemical analyses and UV-visible and EPR spectroscopy. All mutated subunits, especially mutants of conserved cysteines, contained lower amounts of non-heme iron than wild-type. The subunits of three non-conserved cysteine residues (C61, C104, and C113) mutated to Ser and a histidine residue (H92) mutated to Ala exhibited essentially the same spectroscopic properties as those of the wild-type subunit. In contrast, mutation of the four conserved cysteine residues (C96, C101, C137, and C141) to Ser or Ala considerably altered the UV-visible and EPR spectra from the wild-type subunit. These results indicate that the four conserved cysteine residues coordinate the [2Fe-2S] cluster in the P. denitrificans 25 kDa subunit.