Tuning Electronic Properties of Biomimetic [2Fe-2S] Clusters by Ligand Variations

Electronic effects of coordinated thiolate ligands on biomimetic [2Fe-2S] cluster cores have been examined by using three 2,2'-dithiobiphenyl chelate ligands that differ by the backbone substituents (Cl, H, tBu). While the X-ray crystallographic analyses show little structural variation and Mossbauer spectra reveal only subtle differences for the three complexes 1-3, superconducting quantum interference device (SQUID) magnetic data and UV/Vis spectroscopy in solution demonstrate that the different ligand environments cause significant electronic changes in the cluster cores. In particular, potentials for the one-electron reduction differ drastically over almost 0.5 V among the series. Due to the tight chelate arrangement, the reduced mixed-valent species are relatively stable on the electrochemical time scale, which is unusual for all-S-ligated [2Fe-2S] systems. The set of new complexes 1-3 thus provides a valuable platform to investigate reactivity patterns and the effect of electronic variations in biomimetic [2Fe-2S] clusters. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)