Effect of Oxidation and Protonation States on [2Fe-2S] Cluster Nitrosylation Giving {Fe(NO)₂}⁹ Dinitrosyl Iron...

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^IIFe^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 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 1H₂ 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 (Δ E_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.