Die Komplexe (I) bzw. (V) reagieren mit den Benzoylhalogeniden (II) in AcCN glatt zu den neuen 2‐ bzw. 4kernigen Dianionen (III) bzw. (VII), die in Form ihrer R4 N‐Salze isolierbar sind.

The tetramer [Fe4S4(S-/-Bu)4]2-in acetonitrile solution at ambient temperature undergoes facile ligand substitution reactions with thiols R'SH yielding [Fe4S4(S-i-Bu)4-"(SR')"]2-. The reactions have been monitored by electronic spectral and pmr studies of equilibrium solutions. In the cases of R' = p-C8H4NMe2 and p-tolyl centers have been mainly confined to intramolecular (

Selenium Substitution in [Fe4S4(SR)4]2: Synthesis and Comparative Properties of [Fe4X4(YC6H5)4]2" (X, Y = S, Se) and the Structure of [(CH3)4N]2[Fe4Se4(SC6H5)4]

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The complex [Fe4S4(S-z-Bu)4]2-in DMSO solution reacts readily with Ac-L-Cys-NHMe and the lcysteinyl peptides z-BOC-Gly-Cys-Gly-Gly-Cys-Gly-Gly-Cys-Gly-NH2 and z-BOC-Gly-Cys-Gly-Gly-Cys-Gly-Gly-Cys-Gly-Gly-Cys-Gly-NH, to afford the species [Fe4S4(S-Cys(Ac) ]2-(5) isolated as their tetraphenylarsonium salts. Reaction of 3 with Ac-L-Cys-NHMe affords the salt of [Fe4S4(9-peptide)(S-Cys(Ac)NHMe)]2-(4). The pmr and electronic spectra and voltammetry of 2-5 were determined in DMSO or 80% DMSO-20% H26 solutions. Contact shifted cysteinyl methylene proton resonances were observed at ca. 10-14 ppm downfield of TMS and provide further confirmation of signal assignments in oxidized ferredoxin (Fdox) and reduced high-potential (HPred) proteins. Electronic spectra and 2-/3-redox potentials of 2-5 more closely resemble those of the proteins in aqueous solution than is the case for simple alkylthiolate tetramers such as [Fe4S4(SEt)4]2-. However, potentials for the synthetic species are estimated to be > 300 mV more negative than that usually found for Fd,,x/Fd",:. Significantly, spectra of 2-5 and that of the unfolded form of Chromatium HProd in 80% DMSO are very similar and their redox potentials approach the upper limit estimated for the protein. These results indicate that denaturation of the protein affords the active site in a condition such that features inherent to isolated [Fe4S4(S-Cys)4] clusters, as exemplified by 2-5, are developed. Resultsreported in preceding parts of this series have . demonstrated that the synthetic tetranuclear dianions, 1, of Did stereochemistry serve as close reprela. R = Z-Bu b. R = Ph sentations of the [Fe4S4(S-Cys)4] active centers found in 4-Fe and 8-Fe bacterial proteins,2 which include the "high-potential" (HP) and ferredoxin (Fd) types.Evidence has been presented which shows that [Fe4S4-(SR)4]2-, HPred, and Fdox possess a common total oxidation level3-6 and that the structures of the R = CH2-Ph3•7 and Ph8 tetramers are very similar to those of Peptococcus aerogenes Fdox9'10 and Chromatium HPred•10 (1) Part VIII.