Tridentate Thiolate Ligands: Application to the Synthesis of the Site-Differentiated [4Fe-4S] Cluster having a Hydrosulfide Ligand at the Unique Iron Center

We have designed new trithiols Temp(SH)3 and Tefp(SH)3 that can be synthesized conveniently in short steps and are useful for preparation of crystalline [3:1] site-differentiated [4Fe-4S] clusters suitable for X-ray structural analysis. The ethanethiolate clusters (PPh4)2[Fe4S4(SEt)(TempS3)] (4?a) and (PPh4)2[Fe4S4(SEt)(TefpS3)] (4?b) were prepared as precursors, and the unique iron sites were then selectively substituted. Upon reaction with H2S, (PPh4)2[Fe4S4(SH)(TempS3)] (6?a) and (PPh4)2[Fe4S4(SH)(TefpS3)] (6?b), which model the [4Fe-4S] cluster in the beta subunit of (R)-2-hydroxyisocaproyl-CoA dehydratase, were synthesized. Clusters 6?a and 6?b were further converted to the sulfido-bridged double cubanes (PPh4)4[{Fe4S4(TempS3)}2(mu 2-S)] (7?a) and (PPh4)4[{Fe4S4(TefpS3)}2(mu 2-S)] (7?b), respectively, via intermolecular condensation with the release of H2S. Conversely, addition of H2S to 7?a,b afforded the hydrosulfide clusters 6?a,b. The molecular structures of the clusters reported herein were elucidated by X-ray crystallographic analysis. Their redox properties were investigated by cyclic voltammetry.