IRON-SULFUR DIMERS WITH BENZIMIDAZOLATE-THIOLATE, BENZIMIDAZOLATE-PHENOLATE OR BIS(BENZIMIDAZOLATE) TERMINAL CHELATING LIGANDS - MODELS FOR RIESKE-TYPE PROTEINS

Complexes [FeS2L2]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 [H-2L1 = 2-(2-mercaptophenyl)benzimidazole; H-2L2 = 2-(2-hydroxyphenyl)benzimidazole; H-2L3 = 2-(2-hydroxyphenyl)methylbenzimidazole; H-2L4 = 2,2-bis(benzimidazol-2-yl)propane; H-2L5 = 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, H-1 NMR and Mossbauer spectroscopies. Lack of heterogeneity in the H-1 NMR spectrum of the iron(II)-iron(III) complex 6 reveals valence averaging. On the Mossbauer 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 almost-equal-to 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 types of this series, suggesting a requirement for oxygen co-ordination.