The iron-sulfur cluster compounds Fe 4 S 4 (SR) 4 -2 [where —SR = —SCH 3 , —S—C(CH 3 ) 3 , and —S— CH 2 —CH(CH 3 ) 2 ] have been found to represent the base species of weak acids of pK a comparable to that of carboxylic acids. The acid species Fe 4 S 4 (SR) 4 H - is most subject to reaction with O 2 and to acid-catalyzed solvolysis, while the base species Fe 4 S 4 (SR) 4 -2 most readily undergoes ligand exchange. The kinetics for hydrolysis of the isobutyl mercaptide cluster salt has been investigated in detail and a mechanism involving the stepwise process [Formula: see text] has been proposed. The importance of the acid-base equilibria in determining the reactivity of the iron-sulfur clusters and its possible importance as a factor in the determination of the potentials of ferredoxins and high potential iron protein are discussed.

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The water-soluble tetranuclear iron-sulfur cluster ion Fe4S4(SCIgCH2COg)46- (II) has been prepared.The stability of II in water is sufficient to allow the spectrotitrimetric determination of the pKa of its Fe4S4 core as 7.4. In our hands the one-electron reductions of compounds I [Fe4S4SR)e-~, R = alkyl or aryl,] are thermodynamically irreversible with associated E1/2 values greater than those for one-electron reduction of ferredoxins. In contrast, the oneelectron 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-t Br-< HO-< CN-.Synthetic iron-sulfur cluster compounds of general structure I serve as analogs of the active sites of (Fe4S4)n ferredoxins and high potential iron proteins. Their synthesis as well as many valuable studies concerning their electrochemical properties, etc. are attributable to Richard H. Holm.In a previous communication (1), we described a kinetic study of the hydrolysis and oxidation of the Fe4S4 core of I [where R = -CH3, -CH2CH(CH3)2, -C(CH3)4] in the pH range 0 to 7. It was shown that compounds I are weak acids of PKa. e 3.9 (corrected to water solvent). The hydrolytic mechanism was established to involve preequilibrium protonation, a feature which also increased the rate of oxidation. In order to further our understanding of the chemistry of tetranuclear iron-sulfur clusters (I) we have now extended our studies into the basic pH region. We felt compelled to carry out these studies in solvent water where the distribution of lyate species is well understood. Towards this end a water-soluble, tetranuclear iron-sulfur cluster (II) was synthesized.MATERIALS AND METHODS Na5(n-C4Hg)4N[Fe4S4(SCH2CH2COO)4] was prepared in 80% yield via a modification of the method of Holm et al.. Synthetic details and x-ray structure confirmation will appear in a succeeding publication*. Polarograms were obtained and the time dependence of reactions followed on a Princeton Applied Research model 174 polarographic analyzer equipped with Hewlett Packard X-Y plotter and strip chart recorder. Computer fits and plotting were carried out on a Hewlett Packard 9820 calculator. Polarographic monitoring of the rate process is particularly suited to the present study since the ultimate products are insoluble, precluding study of the substitution reactions by optical means. Two modes of polarographically monitoring the reaction kinetics were employed: (1) fast reactions (t1/2 < 7 min) were followed in the dc sampled mode by setting the potential at -0.835 V (slightly more negative than the E1/2) and monitoring the decay of the diffusion current with time. A typical run followed first order kinetics over sev...

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