The iron(II) complex 1 of a macrocyclic tetracarbene binds NO to form a low-spin (S = (1)/2) {FeNO}(7) complex (2) with a linear FeNO unit and a short Fe-NO bond. IR, electron paramagnetic resonance, and Mössbauer spectroscopies as well as density functional theory calculations suggest some Fe(I)NO(+) character and reveal that the singly occupied molecular orbital of 2, resulting from the σ-antibonding interaction of Fe dz(2) and the NO lone pair, is largely iron-based. Reduction yields a quite stable {FeNO}(8) species (3); both 2 and 3 feature very low Mössbauer isomer shifts (∼0.0 mm·s(-1)).

Starting from their six‐coordinate iron(II) precursor complexes [L8RFe(MeCN)]2+, a series of iron(III) complexes of the known macrocyclic tetracarbene ligand L8H and its new octamethylated derivative L8Me, both providing four imidazol‐2‐yliden donors, were synthesized. Several five‐ and six‐coordinate iron(III) complexes with different axial ligands (Cl−, OTf−, MeCN) were structurally characterized by X‐ray diffraction and analyzed in detail with respect to their spin state variations, using a bouquet of spectroscopic methods (NMR, UV/Vis, EPR, and 57Fe Mößbauer). Depending on the axial ligands, either low‐spin (S=1/2) or intermediate‐spin (S=3/2) states were observed, whereas high‐spin (S=5/2) states were inaccessible because of the extremely strong in‐plane σ‐donor character of the macrocyclic tetracarbene ligands. These findings are reminiscent of the spin state patterns of topologically related ferric porphyrin complexes. The ring conformations and dynamics of the macrocyclic tetracarbene ligands in their iron(II), iron(III) and μ‐oxo diiron(III) complexes were also studied.

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−1s−1, ΔH‡ = 17.6 ± 3.0 kJ mol−1, ΔS‡ = −143 ± 11 J mol−1 K−1, 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.

Ligand exchange plays an important role in the biogenesis of Fe/S clusters, most prominently during cluster transfer from a scaffold protein to its target protein. Although in vivo and in vitro studies have provided some insight into this process, the microscopic details of the ligand exchange steps are mostly unknown. In this work, the kinetics of the ligand rearrangement in a biomimetic [2Fe-2S] cluster with mixed S/N capping ligands have been studied. Two geometrical isomers of the cluster are present in solution, and mechanistic insight into the isomerization process was obtained by variable-temperature H NMR spectroscopy. Combined experimental and computational results reveal that this is an associative process that involves the coordination of a solvent molecule to one of the ferric ions. The cluster isomerizes at least two orders of magnitude faster in its protonated and mixed-valent states. These findings may contribute to a deeper understanding of cluster transfer and sensing processes occurring in Fe/S cluster biogenesis.

The nitrosylation of biological Fe/S clusters to give protein-bound dinitrosyl iron complexes (DNICs) is physiologically important. Biomimetic studies on the reaction of synthetic [2Fe–2S] clusters with NO have so far been limited to diferric model complexes. This work now compares the nitrosylation of [2Fe–2S] clusters with SN- or NN-chelating benzimidazolate/thiophenolate or bis­(benzimidazolate) capping ligands in their diferric (1 2– and 2 2– ) and mixed-valent (FeIIFeIII, 1 3– , and 2 3– ) forms. Furthermore, the effect of protonation of the imidazole part of the SN ligand has been probed on both the nitrosylation reaction and properties of the resulting DNIC. The reaction of 1 2– and 2 2– with 4 equiv NO yields the new anionic {Fe­(NO)2}9 DNICs 3 – and 4 – , respectively, which have been comprehensively characterized, including X-ray crystallography of their PPN+ salts. Nitrosylation of mixed-valent [2Fe–2S] clusters 1 3– and 2 3– first leads to slow oxidation to the corresponding diferric congeners, followed by core degradation and DNIC formation. In the case of 2 3– , a second diferric intermediate very similar to 2 2– is detected by UV–vis spectroscopy, but could not be further identified. Nitrosylation of 1H 2 gives the neutral, N-protonated DNIC 3H, and acid/base titrations show that interconversion between 3 – and 3H is reversible. Peripheral ligand protonation leads to a blue shift of the NO stretching vibrations by about 23 cm–1 and a significant shift of the reduction potential to less negative values (ΔE 1/2 = 0.26 V), but no effect on 57Fe Mössbauer parameters is observed. Density functional theory calculations based on the structure of 3 – indicate that the electronic ground-state properties of 3 – and 3H are similar, although the NO­(π*) → Fe 3d π-donation is slightly increased and π-backbonding is slightly decreased upon protonation. As a result, protonation has a significant effect on the NO stretching frequencies, but only minor effects on the Fe–(NO)2 modes. This is confirmed by nuclear inelastic scattering of 3 – and 3H, which shows no clear influence of protonation on the energy of the Fe–(NO)2 bending and stretching modes occurring in the range 400–600 cm–1, but characteristic changes below 350 cm–1 that reflect perturbation of free rotary motion of the thiophenolate and benzimidazole ring systems of the capping ligand after N-protonation. These findings add to the understanding of [2Fe–2S] cluster nitrosylation and will help to identify DNICs resulting from the reaction of NO with Fe/S cofactors featuring alternative, proton-responsive histidine ligands such as the Rieske and mitoNEET [2Fe–2S] clusters.