Iron–sulfur dimers with benzimidazolate–thiolate, –phenolate or bis(benzimidazolate) terminal chelating ligands. Models for Rieske-type proteins

Abstract

Complexes [Fe₂S₂L₂]^2–1–5 having terminal co-ordination by the ligands L¹–L⁵ have been prepared and studied by a range of physical techniques as models for the Rieske-type proteins [H₂L¹= 2-(2-mercaptophenyl)benzimidazole; H₂L²= 2-(2-hydroxyphenyl)benzimidazole; H₂L³= 2-(2-hydroxyphenyl)methylbenzimidazole; H₂L⁴= 2,2-bis(benzimidazol-2-yl)propane; H₂L⁵= 4,4-bis(benzimidazol-2-yl)heptane]. A preparation is described for [NEt₄]₃[Fe₂S₂L⁴₂]6, the first [2Fe–2S]⁺ reduced complex to be isolated. The complexes have been characterised by UV/VIS, ¹H NMR and Mössbauer spectroscopies. Lack of heterogeneity in the ¹H NMR spectrum of the iron(II)–iron(III) complex 6 reveals valence averaging. On the Mössbauer time-scale the complex becomes valence-trapped as the temperature is lowered. The first reduction processes for 1–4 occur at potentials in the range –0.82 to –1.13 V. The positive shift produced by benzimidazolate co-ordination compared to thiolate is comparable to that for the Rieske-type proteins relative to tetracysteinyl ferredoxins. Benzimidazolate co-ordination is also found considerably to stabilise the trianionic chemical reduction products. These display frozen-solution ESR spectra with a range of lineshapes, analysis of which, taken together with previous data for [2Fe–2S]⁺ complexes, indicates the ordering SS > NS > NN > OS > NO ≈ OO in ligand type as correlated with the magnitude of the lowest two g values. The Rieske-type proteins have g values which fit best with the last two ligand type of this series, suggesting a requirement for oxygen co-ordination.