The all-ferric ½Fe 4 S 4 4þ cluster ½Fe 4 S 4 fNðSiMe 3 Þ 2 g 4 1 and its oneelectron reduced form ½1 − serve as convenient precursors for the synthesis of 3∶1-site differentiated ½Fe 4 S 4 clusters and highpotential iron-sulfur protein (HiPIP) model clusters. The reaction of 1 with four equivalents (equiv) of the bulky thiol HSDmp (Dmp¼ 2,6-ðmesitylÞ 2 C 6 H 3 , mesityl ¼2,4,6-Me 3 C 6 H 2 ) followed by treatment with tetrahydrofuran (THF) resulted in the isolation of ½Fe 4 S 4 ðSDmpÞ 3 ðTHFÞ 3 2. Cluster 2 contains an octahedral iron atom with three THF ligands, and its FeðSÞ 3 ðOÞ 3 coordination environment is relevant to that in the active site of substrate-bound aconitase. An analogous reaction of ½1 − with four equiv of HSDmp gave ½Fe 4 S 4 ðSDmpÞ 4 − 3, which models the oxidized form of HiPIP. The THF ligands in 2 can be replaced by tetramethyl-imidazole (Me 4 Im) to give ½Fe 4 S 4 ðSDmpÞ 3 ðMe 4 ImÞ 4 modeling the ½Fe 4 S 4 ðCysÞ 3 ðHisÞ cluster in hydrogenases, and its one-electron reduced form ½4 − was synthesized from the reaction of 3 with Me 4 Im. The reversible redox couple between 3 and ½3 − was observed at E 1∕2 ¼−820 mV vs. Ag∕Ag þ , and the corresponding reversible couple for 4 and ½4 − is positively shifted by þ440 mV. The cyclic voltammogram of 3 also exhibited a reversible oxidation couple, which indicates generation of the all-ferric ½Fe 4 S 4 4þ cluster, ½Fe 4 S 4 ðSDmpÞ 4 .Fe4S4 cluster | thiolates C uboidal ½Fe 4 S 4 clusters are ubiquitous metal-centers in proteins, expediting electron transfer and enzymatic reactions. These ½Fe 4 S 4 cores are usually bound to four cysteinyl thiolates (Cys) as found in the high-potential iron-sulfur proteins (HiPIP) and widely distributed ferredoxins (Fd). Some ½Fe 4 S 4 clusters carrying an N-or O-donor ligand and three Cys ligands are also known, for example the ½Fe 4 S 4 ðCysÞ 3 ðHisÞ cluster (His ¼ histidinyl imidazole) in [NiFe] and [FeFe] hydrogenases ( Fig. 1) (1-6), and the ½Fe 4 S 4 ðCysÞ 3 ðO-donorÞ cluster in aconitase (7-9) and protochlorophyllide reductase (10). All of these ½Fe 4 S 4 clusters are present in three oxidation states, ½Fe 4 S 4 3þ (HiPIP ox ), ½Fe 4 S 4 2þ (HiPIP red ∕Fd ox ), and ½Fe 4 S 4 þ (Fd red ), while the ½Fe 4 S 4 0 state has been suggested for the cluster in the Fe-protein of nitrogenase (11,12). To date, no ½Fe 4 S 4 4þ cluster has been found in proteins.