Iron-sulfur clusters II: Kinetics of ligand exchange studied on a water-soluble Fe(4)S(4)(SR)(4) cluster

The water-soluble tetranuclear iron-sulfur cluster ion Fe₄S₄(SCH₂CH₂CO₂^-)₄^6- (II) has been prepared. The stability of II in water is sufficient to allow the spectrotitrimetric determination of the pK_a of its Fe₄S₄ core as 7.4. In our hands the one-electron reductions of compounds I Fe₄S₄(SR)₄^2-, R = alkyl or aryl,] are thermodynamically irreversible with associated E_1/2 values greater than those for one-electron reduction of ferredoxins. In contrast, the one-electron reduction of II is thermodynamically reversible and the associated potential (-0.58 V versus hydrogen electrode) approaches closely that of the ferredoxins. The kinetics for ligand exchange of II as a function of pH and thiol concentration are in accord with four reversible mercaptan/lyate species exchange reactions followed by product formation via specific acid and base catalysis. Preliminary experiments indicate the nucleophilic order towards II to be Cl^- unk] Br^- < HO^- < CN^-.