Effect of Oxidation and Protonation States on [2Fe–2S] Cluster Nitrosylation Giving {Fe(NO)$_{2}$}$^{9}$ Dinitrosyl Iron Complexes (DNICs)

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 (Fe$^{II}$Fe$^{III}$, 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 12– 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 $^{57}$Fe 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